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High-Throughput Electrical Characterization of Nanomaterials from Room to Cryogenic Temperatures

  • Luke W. Smith*
    Luke W. Smith
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    *E-mail: [email protected]. Phone: +44 (0)1223 766130.
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    Orcidhttp://orcid.org/0000-0003-2194-4708
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  • Jack O. Batey
    Jack O. Batey
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
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  • Jack A. Alexander-Webber
    Jack A. Alexander-Webber
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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    Orcidhttp://orcid.org/0000-0002-9374-7423
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  • Ye Fan
    Ye Fan
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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  • Yu-Chiang Hsieh
    Yu-Chiang Hsieh
    Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
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  • Shin-Jr Fung
    Shin-Jr Fung
    Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
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  • Dimitars Jevtics
    Dimitars Jevtics
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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    Orcidhttp://orcid.org/0000-0002-6678-8334
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  • Joshua Robertson
    Joshua Robertson
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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  • Benoit J. E. Guilhabert
    Benoit J. E. Guilhabert
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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  • Michael J. Strain
    Michael J. Strain
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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    Orcidhttp://orcid.org/0000-0002-9752-3144
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  • Martin D. Dawson
    Martin D. Dawson
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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  • Antonio Hurtado
    Antonio Hurtado
    Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
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    Orcidhttp://orcid.org/0000-0002-4448-9034
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  • Jonathan P. Griffiths
    Jonathan P. Griffiths
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
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  • Harvey E. Beere
    Harvey E. Beere
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
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  • Chennupati Jagadish
    Chennupati Jagadish
    Department of Electronic Materials Engineering and Australian Research Council Centre of Excellence on Tranformative Meta-Optical Systems, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia
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  • Oliver J. Burton
    Oliver J. Burton
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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  • Stephan Hofmann
    Stephan Hofmann
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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    Orcidhttp://orcid.org/0000-0001-6375-1459
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  • Tse-Ming Chen
    Tse-Ming Chen
    Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
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  • David A. Ritchie
    David A. Ritchie
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
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  • Michael Kelly
    Michael Kelly
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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  • Hannah J. Joyce
    Hannah J. Joyce
    Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
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    Orcidhttp://orcid.org/0000-0002-9737-680X
  • , and 
  • Charles G. Smith
    Charles G. Smith
    Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
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Cite this: ACS Nano 2020, 14, 11, 15293–15305
Publication Date (Web):October 26, 2020
https://doi.org/10.1021/acsnano.0c05622

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY.

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Abstract

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We present multiplexer methodology and hardware for nanoelectronic device characterization. This high-throughput and scalable approach to testing large arrays of nanodevices operates from room temperature to milli-Kelvin temperatures and is universally compatible with different materials and integration techniques. We demonstrate the applicability of our approach on two archetypal nanomaterials—graphene and semiconductor nanowires—integrated with a GaAs-based multiplexer using wet or dry transfer methods. A graphene film grown by chemical vapor deposition is transferred and patterned into an array of individual devices, achieving 94% yield. Device performance is evaluated using data fitting methods to obtain electrical transport metrics, showing mobilities comparable to nonmultiplexed devices fabricated on oxide substrates using wet transfer techniques. Separate arrays of indium-arsenide nanowires and micromechanically exfoliated monolayer graphene flakes are transferred using pick-and-place techniques. For the nanowire array mean values for mobility μFE = 880/3180 cm2 V–1 s–1 (lower/upper bound), subthreshold swing 430 mV dec–1, and on/off ratio 3.1 decades are extracted, similar to nonmultiplexed devices. In another array, eight mechanically exfoliated graphene flakes are transferred using techniques compatible with fabrication of two-dimensional superlattices, with 75% yield. Our results are a proof-of-concept demonstration of a versatile platform for scalable fabrication and cryogenic characterization of nanomaterial device arrays, which is compatible with a broad range of nanomaterials, transfer techniques, and device integration strategies from the forefront of quantum technology research.

KEYWORDS:
For technologies to be realized from the many intriguing nanomaterial devices that are being developed in research laboratories, it is essential to have scalable approaches (1) that lean toward mass production, measuring device repeatability, and optimizing designs. We present a high-throughput testing technique for electrical characterization of nanotechnology device arrays. In this work, its specific applicability to 2D materials and nanowires and compatibility with different material fabrication/transfer schemes are demonstrated. The variability of nanoscale electronic device performance is a significant issue for the fabrication of complex circuit designs, particularly those that rely on quantum phenomenon and require mounting and cooling in cryogenic systems. Devices are tested individually, and multiple repetitions must be fabricated and measured to find working examples. This is time-consuming, and samples cannot be later integrated into circuit designs if they perform well. By introducing a multiplexer system, many devices can be tested in a single experimental run.
We study arrays of devices from graphene grown by chemical vapor deposition (CVD), mechanically exfoliated graphene devices, and InAs nanowire devices. The CVD graphene is wet transferred as a continuous 2D material film, and exfoliated flakes and nanowires are transferred by pick-and-place techniques. Each device is individually measured using a multiplexing technique that can operate from room to milli-Kelvin temperatures. This scheme requires no modification of current experimental apparatus, which can be costly and time-consuming, does not suffer from carrier freeze-out, which can render commercial alternatives inoperative, and is capable of operating at high magnetic fields. The multiplexer and array are integrated on a single chip of similar size to chips used for nonmultiplexed devices and does not require integration or testing of off-the-shelf components.
Beyond 2D materials and nanowires, cases where this technique might be beneficially applied for statistical analysis and high-throughput testing include arrays of inkjet-printed transistor circuits, (2−4) molecular transistor arrays created by plating colloidal particles in nanogaps, (5,6) transistor arrays such as carbon nanotubes deposited using electrophoresis techniques, (7) molecules in graphene nanogaps, (8,9) and possibly even biological applications such as multiplexing DNA microarrays. (10) The multiplexer will also enable studies such as investigating the role of damage/disorder and modification of device properties (11,12) by varying the amount of irradiation across the array, for example using a focused ion beam, or comparing ensemble averaging with single-device averaging techniques used to study phase coherent properties in 2D graphene, (13,14) graphene nanoribbons, (15) and nanowires. (16−20)
The GaAs heterostructure is capable of operating over a wide temperature range, allowing prechecking at room temperature before testing of devices in cryogenic conditions where quantum effects are exploited for quantum technology and functional electronics. Since exotic phenomena in quantum research are often only found in the best of a batch of devices, scaling up measurement will speed up this search and provide large-scale statistics of how reproducible phenomena are. (21−24) Device geometries can be varied across the array to systematically assess impact on performance (25) and optimize designs. Multiplexing can also be used to reveal relationships between the physical and electrical parameters, as seen in arrays of split gate transistors. (26) Additionally, there is the possibility of one or two devices in the array possessing unique properties, e.g., due to being the narrowest or shortest in a distribution of dimensions, which allows opportunistic study of unusual phenomena. So far, the multiplexer has been used for devices fabricated within the two-dimensional electron gas (2DEG) itself, (21,25−27) limiting its applicability. Here, we take the approach of integrating arbitrary nanomaterials using transfer techniques, which allows the platform to be used to study different materials and physics.
Arrays contain 2(t–3)/2 devices, where t is the total number of electrical contacts, including input, output, addressing gates, and back gate for local density control. Arrays of up to 16 devices are measured as a proof-of-principle, requiring 11 contacts. Very few additional contacts are required to measure much larger arrays; for example 19 or 21 contacts are needed for 256 or 512 devices, respectively. In the following sections the multiplexer operation is described, followed by a demonstration of room-temperature addressing. The results are organized in two categories, multiplexing arrays fabricated from large-area 2D materials (for which we test CVD-grown monolayer graphene) or pick-and-place techniques (for which InAs nanowires and mechanically exfoliated monolayer graphene flakes are tested).

Results and Discussion

ARTICLE SECTIONS
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A single multiplexer design is used throughout this study. A schematic layout with eight channels is shown in Figure 1(a). The conduction path is defined in a 2DEG of a GaAs heterostructure. Multiplexer outputs are selected using addressing gates, labeled A1 to A6. A negative voltage is applied to half the addressing gates, depleting the 2DEG below. White (yellow) represents when a depletion voltage is (is not) applied. In this example voltages are applied to A2, A4, and A6, to select the left-most channel. In certain locations the addressing gates cross channels that must remain open when an addressing voltage is applied. An additional insulating layer is added beneath the gate at these points to prevent the 2DEG depleting. A back gate is defined adjacent to multiplexer outputs and covered with Al2O3 deposited by atomic layer deposition (ALD). Nanomaterials are transferred on top of the insulator and connected via source/drain electrodes to multiplexer outputs and the common drain. The location of the nanomaterials is indicated by the gray rectangles in Figure 1(a).

Figure 1

Figure 1. (a) Schematic diagram of a multiplexer with one source input (S) connected to eight output channels. Addressing gates are labeled A1 to A6. In this example channel 1 is selected by applying voltages to several addressing gates. White (yellow) gates indicate a voltage is (is not) applied. The white arrow shows the current path through the multiplexer. Insulated regions depicted by green rectangles below the gates, such as at ‘R’, prevent depletion of the 2DEG when addressing gates are biased. Gray rectangles show the location of nanomaterials at the multiplexer output, one per channel, connected to a common drain output. (b) Equivalent circuit of the top two levels of the multiplexer. Dashed lines indicate switches that are controlled by the same addressing gate. All switches are open in (i), corresponding to addressing voltages applied to all gates. Certain switches are closed in (ii) to address channel 1. (c) Cross section through two branches of the multiplexer, corresponding to the dotted line in (a). For all multiplexers in this study the branches corresponding to ‘R’ are covered by a layer of Al2O3. A thinner upper layer covers both branches for the multiplexed CVD graphene array.

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Figure 1(b) shows an equivalent circuit of the top two multiplexer levels. In (i) all switches are open, corresponding to a depletion voltage applied to all addressing gates. In (ii) the voltage on several gates is set to zero (the switches are closed), to select channel 1. Figure 1(c) shows a cross section through two adjacent arms of the multiplexer, as along the black dotted line in Figure 1(a). All multiplexers are covered with an Al2O3 layer deposited by ALD, which is removed over specific channels by HF etching. This creates a voltage range where the 2DEG can be depleted wherever the lower layer has been removed, such as at L, but not at R [Figure 1(c)]. The multiplexer used for CVD graphene also has a second, thinner layer of Al2O3 covering both channels, to prevent current leaking from surface gates to the GaAs at room temperature. Figure 2(e) shows the 16-channel multiplexer fabricated for this study. The branching structure outlines the etched mesa containing the 2DEG. Ohmic contacts are defined at the multiplexer source and at each output (labeled 1 → 16).

Room-Temperature Operation

To demonstrate addressing gate functionality, Figure 2(a) and (b) show conductance G as a function of addressing gate voltage VA,i at room temperature and T = 4.2 K, respectively, where i is the addressing gate index. A multiplexed array of CVD graphene is used in this example, with lower and upper Al2O3 gate dielectric thicknesses of ∼110 and ∼15 nm, respectively (the total gate oxide thickness under the CVD graphene ≈ 125 nm). The number of working devices can therefore be determined prior to cool down. The differential conductance is measured using a two-terminal constant source–drain voltage Vsd at 77 Hz, typically Vsd = 100 μV. Measurements at T = 4.2 K are performed with the sample immersed in liquid helium. Addressing gates A5 (black trace) and A6 (red trace) are swept in the negative direction consecutively. The 2DEG is first depleted beneath A5 (blocking half the channels), then beneath A6 (blocking the remainder); VA,i = 0 V for all other gates. The behavior is the same at room temperature and T = 4.2 K, aside from changes in depletion voltage and overall conductance. Figure 2(c) and (d) show examples of addressing a specific channel (channel 9) at room temperature and T = 4.2 K, respectively. The current path is illustrated pictorially in Figure 2(e), where red bars indicate depletion of the 2DEG.

Figure 2

Figure 2. Room-temperature and T = 4.2 K operation. (a and b) Source–drain conductance as a function of addressing gate voltage VA,i, where i is the gate index. Gates A5 (black trace) and A6 (red trace) are swept sequentially as indicated by labels 1 and 2, with VA,i = 0 V for all other gates. The voltage is maintained on A5 while A6 is swept (i.e., the gate is “on”). Each multiplexer output is connected to a single graphene device, all of which share a common drain contact. (c and d) Addressing channel 9 at room temperature and T = 4.2 K, respectively. Gates A1, A4, A6, and A8 are swept sequentially as indicated by labels 1 to 4, and voltages are maintained on each gate after sweeping. Channel 9 addressing is illustrated on the multiplexer in (e), where the white arrow indicates the current path. Gates A1, A4, A6, and A8 deplete the 2DEG at the red bars. The lighter color branching structure indicates the mesa containing the 2DEG.

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The current ratio as addressing gates are swept [Figure 2(c) and (d)] is governed by the resistance of each channel, which includes the 2DEG, device, and contact resistance. The 2DEG resistance can be estimated for each channel by considering the area in terms of the aggregated length-to-width aspect ratio N = L/W and calculating sheet resistance R = L/σW where σ = neμ. For the device measured in Figure 2, the 2DEG sheet density and electron mobility are measured on a separate Hall bar device as n = 1.53 × 1011 cm–2 and μ = 8.17 × 105 cm2 V–1 s–1, respectively. Here N = 10 for channels 1, 2, 15, and 16, N = 9 for channels 3, 4, 7, 8, 9, 10, 13, and 14, and N = 8 for channels 5, 6, 11, and 12, corresponding to R = 500, 450, and 400 Ω, respectively. The maximum path-dependent difference in 2DEG resistance (≈100 Ω) is much smaller than variations in contact resistance for the arrays measured here (on the order at least kΩ); therefore device variability is the largest factor determining the current ratio.

CVD-Grown Graphene

Monolayer graphene films are grown by CVD on Cu and transferred to the multiplexer by etching the Cu in a (NH4)2S2O8 solution with poly(methyl methacrylate) (PMMA) protection of graphene. (28) Source and drain electrodes (Ti/Au) are created prior to transfer, Figure 3(a). Graphene channels 10 μm wide are defined at outputs of the multiplexer, and graphene is removed from unwanted areas by etching in an O2 plasma. Source–drain contacts are separated by 10 μm. Further fabrication details are described in Methods. Figure 3(b) shows a false-color scanning electron microscope (SEM) image of a single graphene device. Blue, yellow, and green indicate graphene, source/drain contacts, and the back gate, respectively. Figure 3(c) shows a cross section from source to drain.

Figure 3

Figure 3. Multiplexed CVD graphene. (a) Two multiplexer outputs and drain contacts prior to graphene transfer. (b) False-color SEM of a single device. Green, yellow, and blue indicate the back gate, contact electrodes, and graphene, respectively. (c) Cross section through an individual device. (d) Transfer curves for each graphene device at T = 0.28 K. Black lines are fits to the data using eq 1. Channel numbers are given by each panel. Channel 11 did not conduct (G < 0.25 μS at all VG).

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Measurements are performed in a 3He cryostat with a base temperature of T = 0.28 K. Typical Dirac behavior is observed for 15 of 16 devices, corresponding to 94% yield (channel 11 did not conduct). This compares favorably to yields of more industrially produced devices (29,30) and to multiplexed devices created within the 2DEG. (27)Figure 3(d) shows transfer curves for all devices. The two-terminal differential conductance is measured as a function of back gate voltage (VG). The total device resistance is modeled using (31,32)
(1)
where μ is the carrier mobility, n0 is the residual carrier density, L and W are the length and width, respectively, n is the back-gate-dependent carrier density, and RP is the total parasitic resistance. The carrier density is given by n = CG(VG – VCNP)/e, where VCNP is the charge-neutrality-point voltage, CG is the gate capacitance per unit area (CG = ϵ0ϵ/d), ϵ0 is the free space permittivity, ϵ is the oxide permittivity, and d is the oxide thickness. Separate mobilities and parasitic resistances are estimated for electron and hole carriers such that μ = μhole and RP = RP,hole for V < VCNP and μ = μelectron and RP = RP,electron for V > VCNP. The VCNP is found by first fitting with a single density-independent μ and RP. Capacitance CG is estimated using high magnetic field measurements performed on two example channels (13 and 14), Figure 4(a) and (b), discussed below, giving a mean CG ≈ 34 nF cm–2 from the gate dependence of the ν = ±2 quantum Hall states, where ν is the filling factor, using n = νeB/h.

Figure 4

Figure 4. (a and b) Device resistance ΔR(VG) = R(VG) – R(8) as a function of VG and magnetic field for channels 13 and 14, respectively. Labels ±2 refer to filling factors ν = ±2, respectively. Arrows highlight the location and gradient of Landau levels −1 and +1.

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Estimated electronic parameters are listed in Table 1, where devices are categorized as “pristine” or “unique” according to whether μm scale process-induced defects or significant transfer residues are visible in SEM images, shown in Supporting Information. Eight devices are classified as unique, where imperfections include holes/gaps and whether graphene appears multilayer, folded, or larger than intended. The total parasitic resistance includes contact electrodes to the device under test (RC), the multiplexer 2DEG, ohmic contacts, and circuit/cryostat wiring. It is likely that RC is by far the dominant term, since separate measurements estimate that the multiplexer and wiring contribute an average of ∼469 Ω per channel, as discussed in Methods. Equation 1 overestimates RP since it assumes the resistance at high density is only due to the contact resistance and does not take into account any other effects. (32) Therefore values in Table 1 are an upper bound. The model does not result in a good fit for channel 8, which appears in the SEM image to be a mixture of different parallel areas with varying thicknesses indicated by a varying contrast between areas of graphene (33) (Supporting Information). The device conductance is therefore a nontrivial addition of these areas, and attempting to fit the transfer curve–which shows a significant increase in resistance (35-fold from VG = −10 V to the charge neutrality point)–leads to the unrealistic RP < 0.1 Ω. For future devices improvements must be made to reduce contact resistance, since there is the possibility of interlayer contamination from the transfer process as source–drain electrodes connecting the graphene are fabricated prior to transfer. Nevertheless, the high device yield (15/16) and similarity of electronic parameters compared to nonmultiplexed devices illustrate the versatility and power of the multiplexed approach. The multiplexer is amenable to more sophisticated contacting methods which achieve lower contact resistance, such as using edge contacts, (34,35) fabricating contacts post-transfer, using electron-beam (e-beam) lithography to define graphene areas to avoid use of UV photoresists, cleaning contact areas using an oxygen plasma (36,37) or UV ozone (38) prior to metallization, or thermal annealing techniques. (37,39,40)
Table 1. Parasitic Resistances, Charge Neutrality Point (CNP) Voltage, Residual Carrier Density, and Mobilites Extracted from Fits to Transfer Curves in Figure 3(d) with Asymmetric Hole and Electron Mobilities and Parasitic Resistancesa
channelL/WRP,hole (kΩ)RP,electron (kΩ)VCNP (V)n0 (× 1011 cm–2)μhole (cm2V–1s–1)μelectron (cm2V–1s–1)lhole (nm)lelectron (nm)
Pristine
2131.522.61.351.97205408.25
7131.927.71.971.4111093011.98.8
9119.2202.5512602703.42.8
12143.142.92.321.26406307.66
14133.336.72.711.43503603.63.7
15128.933.62.91.65906706.36.9
1611617.93.082.51780227016.819.9
mean 29.128.82.411.67808108.27.6
std 9.19.30.60.55206804.85.8
Unique
1140.528.1–0.460.81601501.71.9
3136.432–1.110.81101101.31.4
41.17100.999.3–1.0118107808.87.3
5152.659.11.4711301301.61.1
61.25238.3282.12.141.11501802.61
80.44004.640.670701.11
10111.19.94.4655104705.36
130.5126.225.81.811.310301000119.5
mean 63.2671.491.43703604.23.6
std 7792.22.271.43703503.83.4
a

The mean free paths are estimated using eq 2 at nhole and nelectron = 6 × 1011 cm–2. Devices are categorized as pristine or unique depending on whether cracks/folds in the graphene or significant transfer residues are evident in SEM images.

Data from pristine devices allow assessment of the variability in transport parameters across a single CVD graphene sheet. The estimated residual carrier densities are similar to values estimated for graphene grown by CVD on Ni (41) on Si/SiO2 substrates and for exfoliated graphene. (31) The mobilities of pristine devices vary from 260 to 2270 cm2 V–1 s–1 and are similar to values achieved for wet-transferred CVD graphene on an oxide substrate, (42) although separate high magnetic field measurements imply the mobility may be much higher than values estimated using eq 1, up to μ ≈ 4000 cm2 V–1 s–1 for channel 13 (discussed below). The lowest mobilities occur for devices in the unique category, where cracks, residues, or debris may be responsible. Grain boundaries can lead to lower mobility due to preferential adsorption of residues which dope graphene and scatter carriers, although grain sizes for similar CVD graphene have been measured at >100 μm, (43) larger than our device area of 10 × 10 μm2. Therefore, each device is not likely to contain more than one grain boundary, so these are unlikely to be a significant factor here. Another possibility is chemical adsorption from transfer residues and exposure to atmospheric conditions, which can be mitigated against by encapsulation of the graphene. (44−47) All pristine and most unique channels show positive VCNP, indicating slight p doping. These factors may contribute to asymmetries of electron and hole characteristics, which arise from scattering from adsorbates, (48) as well as from metal-induced doping at the contacts (48−52) causing a pinning of the Fermi energy in the contacts at a different value from the channel.
A small hysteresis (ΔVCNP = VCNP(down) – VCNP(up)) is observed between transfer curves for VG swept in the up and down directions (data presented in Figure 3 and Table 1 are for VG swept in the up direction). For pristine/unique devices the mean ΔVCNP = 0.49/0.47 V, corresponding to a carrier trap density (46) of 1.6/1.5 × 1011 cm–2 using a capacitive model, Δn = CGΔVCNP/e. The gate voltage is swept slowly (dVG/dt = 100 V h–1), to estimate maximum hysteresis. (46)
Device mobilities can also be estimated from quantum Hall measurements. Compatibility of the multiplexer with magnetic field is important as it allows for magnetotransport measurements, which are a useful tool for probing physics in quantum devices. When the magnetic field is high enough, the 2D density of states becomes quantized in Landau levels (53,54) and electrons undergo cyclotron motion. Figure 4(a) and (b) show resistance, ΔR(VG) = R(VG) – R(8), as a function of VG and magnetic field for channels 13 and 14, respectively, where R(8) denotes the resistance at VG = 8 V. This background resistance at high carrier concentration is subtracted at each B to improve the visibility of Landau levels. The darker areas indicate quantum Hall plateau regions with filling factors ν = ±2. For channel 13, Landau level separation is visible for B ≳ 2.25 T. We use the approximation that τ ≈ 1/ωc and μ ≈ 1/B at the minimum B when Landau levels start to appear, since τωc ≫ 1 or μB ≫ 1 is required for Landau levels to be clearly observed. (55) Here , τ is the scattering time, and vF is the Fermi velocity. This corresponds to μ = 1/B = 4444 cm2 V–1 s–1 and mean free path l = 12 nm using l = vFτ and vF = 1 × 106 m s–1. The gate capacitance is estimated using dn/dB = νe/h for a capacitive model en = CG(VG – VCNP), where VCNP is the charge neutrality point voltage, giving CG ≈ 37 and 31 nF cm–2 for channels 13 and 14, respectively (for channel 14 Landau levels are less distinct, therefore this value is only approximate). A mean value of CG ≈ 34 nF cm–2 is used in our analysis.
Finally, we calculate the mean free path from device resistivity using the Drude model:
(2)
where ρ is sheet resistivity ρ = R(W/L), and R is the device resistance after correcting for series resistance. Mean free path values at nhole and nelectron = 6 × 1011 cm–2 are given in Table 1 for each channel. For pristine devices, mean values of l = 8.2 and 7.6 nm are estimated for holes and electrons, respectively. Plots of l as a function of n are provided in Supporting Information. Uncertainties in these values may be introduced from the estimates of RP and length-to-width ratio and for unique devices the assumption that each device is single-layer graphene. For comparison, a mean free path of nearly 40 nm at similar density is reported for CVD graphene wet transferred onto hexagonal boron nitride, (56) at T = 80 K.
We note that further studies such as testing reproducibility of electronic properties from cool down to cool down and performing magnetoresistance measurements are all possible with the multiplexer. A wide variety of 2D materials can be grown by CVD (57,58) and transferred in a similar way; therefore the method presented here makes it possible to study large arrays of devices from many different 2D materials.

Nanowires

Wurtzite (WZ) InAs nanowires are grown via metal organic vapor phase epitaxy using 50-nm-diameter Au nanoparticles to drive anisotropic nanowire growth. Growth is performed in an Aixtron 200/4 reactor using trimethylindium (1.2 × 10–5 mol/min) and arsine (3.3 × 10–5 mol/min) precursors at a growth temperature of 500 °C. Further growth details can be found in ref (59). Nanowires are spread on a donor substrate (PDMS) and individually transfer printed (60−63) to another location on the PDMS surface, to create a 1D array. Selected nanowires are then transfer printed on top of the ≃50-nm-thick Al2O3 oxide covering the back gate on the multiplexer, using a flat-tip polymer microstamp. (60)Figure 5(a) shows a nanowire post-transfer, prior to contact fabrication. Nanowires are aligned parallel to the multiplexer output. Source/drain electrodes are defined by e-beam lithography and metallized with ≃70 nm sputtered Ni after transfer, Figure 5(b). Full details on the nanowire transfer printing technique can be found in Methods and in refs (60)– () () (63).

Figure 5

Figure 5. Multiplexed nanowire arrays. (a) Nanowires are placed at every multiplexer output, above a back gate covered with ≃50 nm thick Al2O3. (b) Top contacts defined by electron-beam lithography connect nanowires to multiplexer output channels and the common drain. (c) Scanning electron micrographs of an example single nanowire and nanowire pair in a multiplexed array. Contact electrodes are shown in false color (yellow). (d) Differential conductance as a function of back gate voltage for L = 401 nm (left column) and L = 606 nm (right column) nanowires, before subtraction of series and contact resistance. Red and blue arrows indicate the direction of the gate voltage sweep.

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On channels 1–8 the source and drain contacts are shorted with Ni, and their resistance is used to estimate the series resistance of the multiplexer and circuit. The mean series resistance is RS = 3.77 kΩ and includes the ohmic/Ni interface and Ni contacts themselves. The mean length of channels 9–12 (13–16) is L = 606 (401) nm. These sets of four are defined to be the same length, set by the separation between source–drain contacts. The average diameter is W ≈ 65 nm for all nanowires. For channels 10 and 16 single nanowires are transferred. Pairs of nanowires in parallel are transferred to channels 9, 11, 12, 13, and 14. The nanowire at channel 15 became detached during subsequent fabrication. Figure 5(c) shows SEM images of single and pairs of nanowires. In the future, prescreening nanowires using a multistage transfer-printing process (64) or SEM/atomic force microscopy techniques will enable selection of purely single nanowires.
Figure 5(e) shows G as a function of back gate voltage VG for L = 401 and 606 nm nanowires, in the left- and right-hand columns, respectively. Measurements are performed at T = 4.2 K with the device immersed in liquid helium. The two-terminal differential conductance is measured using a constant source–drain voltage Vsd = 10 μV rms at 77 Hz. At high VG the conductance curve flattens as the total resistance R = RNW + 2RC + RS becomes dominated by the series resistance RS and contact resistance RC between the nanowires and metal electrodes. Although the nanowire resistance is not negligible, (65−67) this gives an upper limit of RC from ∼0.5 to 7.4 kΩ across the array.
We estimate the electron density n from the total charge e(πr2L)n = CNWΔV, where ΔV is the change in voltage from saturation to threshold using the average threshold voltage Vt for gate sweeps in the up and down directions, and CNW is the gate capacitance from a cylinder-on-plane model (68,69)
(3)
For oxide thickness d = 50 nm and nanowire radius r = 32.5 nm, CNW = 0.16 and 0.10 fF for channels 9–12 and 13–16, respectively. The mean density is n = 40.8 × 1017 cm–3, corresponding to an electron–electron spacing of ∼6 nm. We neglect capacitance from surface/interface states. (70)
The field-effect mobility of a nanowire on a planar substrate is given by μFE = gmL2/CNWVsd, where gm = ∂Isd/∂VG is the transconductance, Isd = GVsd is the source–drain current, and Vsd is the rms source–drain voltage. This gives μFE from 1140 to 4490 cm2 V–1 s–1 after subtracting for RS and RC, with a mean of 3180 cm2 V–1 s–1. This is an upper bound, since RC is overestimated. Simply taking the raw data without subtracting for either RC or RS provides lower bounds of μFE from 340 to 1630 cm2 V–1 s–1, with a mean of 880 cm2 V–1 s–1.
Two further parameters for quantifying nanowire behavior are the subthreshold swing (SS = (∂(log10Isd)/∂VG)−1) and the on/off ratio, taken here as the number of orders of magnitude by which Isd changes between threshold and saturation. Resistance subtraction makes little difference on a log scale near threshold; therefore very similar values for SS are found when resistance is/is not subtracted. The higher number of SS is given as a conservative estimate of device behavior. Table 2 summarizes the calculated parameters for the seven multiplexed nanowires. While data points are too few for statistical comparison, it is a useful demonstration that nanowires can be successfully integrated with the multiplexer without any reduction in quality, since parameters compare favorably to nonmultiplexed WZ nanowires, (69) which showed Vt = −7.4 V, on/off ratio = 3.4 dec, SS = 2320 mV dec–1, n = 4.7 × 1017 cm–3, and μFE = 340 cm2 V s–1. These measurements were performed at room temperature, which accounts for the lower mobility and higher SS (71) compared to our devices. The carrier density is larger in our measurements since an ammonium sulfide etch is performed prior to contact deposition to remove native oxide. Although a patterned resist is used to selectively etch only in the contact region, there can be unintentional etching away from the contact region underneath the resist, up to a couple of hundred nm into the nanowire channel. For nanowires with contacts separated by >1 μm a greater proportion of the channel is unaffected (nanowires in ref (69) are several microns long). However, in short channels (<500 nm), it appears that most of the nanowire can be exposed to ammonium sulfide, which can increase surface accumulation, leading to higher carrier densities. (72)
Table 2. Contact Resistance, Carrier Density, Mobility, Sub-threshold Swing, on/off Ratio, Threshold Voltages in the up/down Sweep Directions, and Hysteresis ΔVt = Vt(down) – Vt(up), Extracted from Data in Figure 5(e)a
channelRC (kΩ)n (× 1017 cm–3)μFE (cm2 V–1s–1)SS (mV dec–1)on/off (dec)Vt(up) (V)Vt(down) (V)ΔVt (V)
90.7939.41630–44904703.3–7.24–7.010.23
102.0738.3860–20001403.1–7.1–6.710.39
110.5142.21340–44004503.3–7.82–7.610.21
122.0339.9810–40503003.1–7.78–6.711.07
132.9744.2480–32005003–8.48–7.780.7
141.4143.9710–29803203.2–8.63–7.51.13
167.3637.7340–11408502.7–7.16–6.420.74
mean2.4540.8880–31804303.1–7.74–7.110.64
std2.322.6460–12602200.20.620.530.38
a

The mean length of nanowires 9–12 (13–16) is L = 606 (401) nm. Lower and upper bounds for μFE are given by considering data before and after subtracting for series and contact resistance, respectively.

Previously, nanowires are studied either individually or in parallel arrays. (19,20) Multiplexing allows both in a single array. This enables a range of studies, for example the comparison of techniques used to suppress conductance fluctuations in magnetotransport measurements, which can be studied either using gate voltage averaging techniques in individual nanowires (16−18) or by measuring nanowires in parallel. (19,20)

Exfoliated Graphene

An array of mechanically exfoliated graphene flakes is assembled by using a polymer-based dry transfer technique, (35,73) as described in Methods. This is presented as a proof of concept of compatibility with exfoliated 2D materials and van der Waals heterostructures (multilayer 2D material stacks), (74) which are created using this transfer technique and used to research a wide variety of quantum phenomena.
Eight monolayer graphene flakes are transferred to the multiplexer. A close-up of one device is shown in Figure 6(a), with graphene highlighted in blue (false color). Source–drain contacts (≈100 nm Cr/Au) connecting the graphene to multiplexer output channels and the common drain are fabricated after transfer. Figure 6(b) shows the device array; arrows indicate channels with graphene attached. A 75% yield is achieved since channels 1 and 3 show open-circuit behavior. The proximity of flakes is set by the size of the polymer stamp, here ∼2 × 2 mm. Larger arrays can be created by reducing the stamp size, using a flat-tip microstamp (60) or a stamp formed by a drop of Elvacite/anisole solution. (75) Alternatively, the distance between multiplexer outputs may be increased.

Figure 6

Figure 6. Multiplexed arrays of exfoliated graphene. (a) Exfoliated graphene device connected to channel 1. Source–drain electrodes connect to the multiplexer output and common drain. The device active area is highlighted in blue (false color), for clarity. (b) Graphene flakes are connected to eight multiplexer channels, indicated by the arrows. (c) Resistance as a function of back gate voltage (VG) for channels 6, 7, 11, 12, 15, and 16 (left-to-right); devices on channels 1 and 3 do not conduct. Arrows indicate the gate voltage sweep direction.

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Figure 6(c) shows the resistance as a function of VG for six devices. Red (blue) show gate voltage sweeps in the up (down) direction, for measurements at T = 4.2 K. DC measurements are performed with a large DC bias Vdc = −500 mV since IV curves showed nonlinear behavior as Vdc is swept, similar to when a Schottky barrier exists between the graphene and contacts. (76,77) We estimate contact resistivity ρC = 1.27, 0.31, 3.51, 0.06, 1.37, 1.79 × 107 Ω μm for devices on channels 6, 7, 11, 12, 15, and 16, respectively, using ρC = RCW, with W = 12, 7.5, 3.7, 0.7, 7.8, and 12.2 μm and RC = 1.06, 0.42, 9.47, 0.79, 1.76, and 1.47 MΩ. Equation 1 is used to find RP, and we approximate RC = RP/2, since the contact resistance dominates over all other series resistance terms. Previously, resistivities between 103 and 106 Ω μm have been reported for Cr/Au contacts on graphene without specifically mentioned contact optimization, (78) and reasonable variability is not unexpected. (79) The high resistivity of our devices and nonlinearity of dc bias suggest the existence of an unintentional interlayer between the metal and the graphene, such as polymer residues, and likely arises from contamination during the transfer procedure. Our graphene flakes are picked up directly by the polymer stamp and transferred to the multiplexer, in contrast to typical dry transfer, where a hBN encapsulation layer is picked up prior to the graphene to create a stack (35,73) and prevents the graphene from being exposed to transfer materials and chemicals, resulting in much higher device performance and clean contact interfaces. For comparison ρC for our CVD pristine devices varies from ∼1 to 2 × 105 Ω μm, with the same assumption that RC = RP/2.
Lower contact resistances are achievable, and a comprehensive list of contact resistivities for different metals/processes for CVD and exfoliated graphene is available in ref (80). Resistivities as low as 23 Ω μm have been reported (81,82) by optimizing contact metals and methodologies. There is no intrinsic reason why low resistance cannot be achieved with the multiplexer by improving the contact fabrication and transfer, using techniques such as edge contacts to 2D materials encapsulated in hexagonal boron nitride, (35) cleaning of contact areas with an oxygen plasma (36,37) or UV ozone (38) prior to metallization, thermal annealing, (37,39,40) different contact metals, (80) patterning holes or cuts in the graphene underneath the contacts, (34,82) fabrication in a glovebox environment, (75,83) n-doping graphene combined with edge contacts, (81) or shadow mask evaporation. (84) Since our emphasis is on high-throughput testing, techniques that can be globally applied during fabrication give the most scalable solution. This would include options such as glovebox fabrication in which graphene is not exposed to ambient conditions, shadow mask evaporation, which avoids use of resists, or plasma/ozone cleaning.
In Figure 6(c) the Dirac point occurs at positive VG for most devices, reflecting unintentional p doping from moisture and oxygen adsorption after exposure to ambient conditions. The hysteresis in gate voltage sweeps, ΔVCNP = VCNP(down) – VCNP(up), is used to estimate a carrier trap density at the graphene–Al2O3 interface from the corresponding change in carrier density Δn = CGΔVCNP/e, (46) giving Δn = 3, 3.5, 4.2, 5.9, 2.6, and 3.2 × 1012 cm–2 for channels 6, 7, 11, 12, 15, and 16, respectively. The large hysteresis reflects the quality of the graphene–Al2O3 interface, post-transfer processing, and exposure to ambient conditions, which was not mitigated against for these devices.
With improvement the multiplexer will become a useful tool for testing arrays of many types of exfoliated 2D materials and van der Waals heterostructures, which are created using the same pick-up-and-place transfer system as used here. (35,73) Different material configurations and geometries can be tested side-by-side. Automated assembly (75) may also allow rapid creation of large arrays. More exotic devices can also be combined with the multiplexer, for example arrays of single electron/single molecular transistors using nanoparticles in nanogaps. (5,6) The potential for the multiplexing geometry is significant in the fabrication of quantum circuits, where adding a multiplexer creates built-in redundancy by selecting one device from an array. This is particularly important for devices with low yield, such as those with quantum effects that are only realized in critical geometric combinations. As initial next innovations to reduce standard deviation and increase yield for all types of arrays studied here we suggest investigating encapsulation (44−47) and thermal annealing (37,39,40) to achieve higher mobilities and lower contact resistances and to reduce exposure to environmental conditions. Edge contact methods should also be investigated for the 2D materials. (34,35)
The multiplexer can be used from room temperature to cryogenic temperatures, which allows the study of temperature-dependent behavior of transistors and quantum devices. In this case the temperature-dependence of the 2DEG resistance is also relevant. (85) For many devices, such as those presented in this paper, the series resistance can be extracted at each temperature by analyzing transfer characteristics. Two further options are possible with future design innovations, either to measure the 2DEG resistance directly or circumvent the series resistance altogether using four probe measurements. First, there is much benefit to fabricating future multiplexers with one channel shorted to the common drain and using this channel to measure the 2DEG resistance directly. This term can then be used in analysis of multiplexed devices on that chip. A second, important innovation is to achieve four probe measurements of multiplexed devices, for example by adding an insulating layer on top of the multiplexer to allow metal gates to be routed into the nanodevice area, which can be connected to individual devices.

Conclusions

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A single multiplexer design is presented to measure individual devices within arrays of nanomaterials. Three different arrays are measured—CVD-grown graphene, InAs nanowires, and mechanically exfoliated graphene flakes—with devices transferred by wet and dry processes. The multiplexer can be operated at room temperature to check devices are connected prior to cooling to milli-Kelvin temperatures. Key parameters are extracted to quantify electronic performance including mobility, residual carrier density, and parasitic/contact resistance for CVD graphene devices and density, mobility, subthreshold swing, and on/off ratio for nanowires. Values similar to nonmultiplexed devices are found, showing integration with the multiplexer is not detrimental to performance. Mechanically exfoliated monolayer graphene flakes transferred using a polymer-based dry transfer technique are measured as a proof of concept that 2D material flakes and van der Waals heterostructure devices can also be multiplexed. Low-resistance contact fabrication techniques are required for future arrays; data are presented here without optimized contact fabrication as a proof of concept.
The multiplexer technique applied to 2D materials, to nanowires, and potentially to colloidal particles in nanogaps and molecules in graphene nanogaps is a shift from individual testing of devices and multiple repetitions of fabrication/measurement to a scalable and efficient approach that measures many devices in a single experimental run. The multiplexer is universally compatible to many kinds of nanoelectronic devices beyond those presented here. Large data sets can be gathered for ensemble assessment, as well as speeding up the search for individual devices with exotic phenomena occurring when precise/opportunistic fabrication conditions are met.

Methods

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GaAs Multiplexer

The multiplexer conduction path is defined in a GaAs high electron mobility transistor, and the mesa is defined by chemical etching. Multiplexers for CVD and exfoliated graphene are fabricated on Cavendish wafer V832, and the nanowire multiplexer is fabricated on Cavendish wafer V684. The 2DEG forms 90/70 nm below the surface for wafers V832/V684. For V832, the mean sheet density and electron mobility are n = 1.53 × 1011 cm–2 and μ = 8.17 × 105 cm2 V–1 s–1, respectively, measured on a separate Hall bar device. For V684, Hall bar measurements gave n = 2.49 × 1011 cm–2 and μ = 4.20 × 105 cm2 V–1 s–1. Ohmic contacts are created at the source and 16 outputs, and a 10/40 nm thick Ti/Au back gate is defined adjacent to the outputs. An Al2O3 gate dielectric is deposited by ALD, and the dielectric thickness is 110, 94, and 50 nm for multiplexers of CVD-grown graphene, exfoliated graphene, and nanowires, respectively. Windows are etched over bond pads, ohmic contacts, and multiplexer channels as necessary, e.g., above channel L in Figure 1(c). A second Al2O3 layer of 15 nm thick is deposited on the CVD graphene multiplexer, and windows are etched above bond pads and ohmic contacts, which allows this multiplexer to be tested at room temperature. Addressing gates (Ti/Au) are deposited at a slight rota-tilt angle (10–15°) to facilitate continuity over the etched mesa edge.
The total parasitic resistance in the circuit is RP = 2RC + R2DEG + Rohmic + Rwiring, where resistances of the multiplexer 2DEG, ohmic contacts, circuit/cryostat wiring, and contact electrodes to the device under test are denoted by R2DEG, Rohmic, Rwiring, and RC, respectively. A multiplexer with outputs wire bonded to the common drain is measured in a He-3 cryostat with a base temperature of T = 0.28 K. The mean resistance of 15 channels is R = 469 Ω (one channel was not connected). Assuming negligible resistance from the wire bonds (i.e., RC = 0), this is an estimate of the upper limit of resistances R2DEG + Rohmic + Rwiring. The largest component is likely to be R2DEG, since the expected channel-dependent 2DEG resistance through the multiplexer is R2DEG ≤ 500 Ω, estimated using carrier density n = 1.53 × 1011 cm–2 for this wafer (V832).

Wet Transfer of CVD Graphene

Monolayer graphene films are grown by chemical vapor deposition on Cu. The graphene/Cu substrate is coated on one side by PMMA, and graphene is removed from the unprotected side by reactive-ion etching (RIE) with an oxygen plasma. The PMMA–graphene–Cu stack is floated on the surface of a 1.2% solution of ammonium persulfate etchant (NH4)2S2O8, with the Cu side in contact with the (NH4)2S2O8. A 1 × 1 cm2 stack is etched for approximately 6 h to ensure the Cu is dissolved. The PMMA–graphene is rinsed by transferring to the surface of a DI water bath for 1 min and to a second DI water bath for 1 h. The PMMA–graphene stack is lifted from the DI water using the target substrate and dipped in isopropyl alcohol (IPA) for a few seconds, before drying in air overnight. The sample is baked at 125 °C prior to removing the PMMA to improve adhesion of graphene to the target substrate. For this study CVD graphene devices are defined by photolithography and are contacted from below by electrodes fabricated prior to transfer. Graphene is removed from the rest of the chip using an oxygen reactive ion etch, 30 W power for 1 min.

Exfoliated Graphene Arrays

Graphene is exfoliated onto a SiO2 substrate cleaned by reactive ion etching in an O2 plasma. Monolayer flakes are picked up and transferred to the multiplexer using a polymer stack of polydimethylsiloxane (PDMS) block/polycarbonate (PC) film on a glass slide. Flakes are transferred one at a time. After four flakes are transferred the multiplexer chip is immersed in chloroform to dissolve the PC film and then rinsed in IPA. Four more flakes are transferred followed by a second immersion in chloroform and IPA. Contacts are defined by e-beam lithography; ∼100 nm total thickness Cr/Au is deposited using electron beam evaporation.

Nanowire Transfer

Given the ultrasmall dimensions of the InAs nanowires (diameter ∼65 nm, length ∼600 nm), the transfer-printing process for their integration onto the multiplexer is done in three stages: (1) InAs NWs are captured in bulk from their growth substrate using a large, ∼1 mm2, PDMS block and applying shear motion against the NW direction on the growth substrate. The captured NWs are subsequently transferred in bulk onto an intermediate PDMS surface. (2) The bulk-transferred InAs NWs are then visually assessed, and selected NW devices arranged in 1D arrays against a reference point in the intermediate PDMS sample. The 1D arrays of InAs NWs are mapped using a high-resolution optical microscope to ensure they are undamaged and in suitable condition for final integration in the multiplexer. (3) The final selected InAs NWs are captured from their location in the formed 1D NW arrays by means of a polymeric (PDMS) μ-stamp (surface area 10 × 30 μm) and precisely transfer-printed onto the target locations on the multiplexer back gate with controlled orientation.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.0c05622.

  • Scanning electron microscope images of CVD graphene devices and mean free path calculations (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Jack O. Batey - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    • Jack A. Alexander-Webber - Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.;  Orcidhttp://orcid.org/0000-0002-9374-7423
    • Ye Fan - Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
    • Yu-Chiang Hsieh - Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
    • Shin-Jr Fung - Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
    • Dimitars Jevtics - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.;  Orcidhttp://orcid.org/0000-0002-6678-8334
    • Joshua Robertson - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
    • Benoit J. E. Guilhabert - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
    • Michael J. Strain - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.;  Orcidhttp://orcid.org/0000-0002-9752-3144
    • Martin D. Dawson - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.
    • Antonio Hurtado - Institute of Photonics, Department of Physics, University of Strathclyde, Technology and Innovation Centre, 99 George Street, G1 1RD, Glasgow, U.K.;  Orcidhttp://orcid.org/0000-0002-4448-9034
    • Jonathan P. Griffiths - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    • Harvey E. Beere - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    • Chennupati Jagadish - Department of Electronic Materials Engineering and Australian Research Council Centre of Excellence on Tranformative Meta-Optical Systems, Research School of Physics, The Australian National University, Canberra, ACT 2601, Australia
    • Oliver J. Burton - Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
    • Stephan Hofmann - Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.;  Orcidhttp://orcid.org/0000-0001-6375-1459
    • Tse-Ming Chen - Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
    • David A. Ritchie - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
    • Michael Kelly - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.;  Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.
    • Hannah J. Joyce - Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, U.K.;  Orcidhttp://orcid.org/0000-0002-9737-680X
    • Charles G. Smith - Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
  • Notes
    The authors declare no competing financial interest.

    The data set for this article is available at: https://doi.org/10.17863/CAM.58559.

Acknowledgments

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This work is supported by the Engineering and Physical Sciences Research Council Grant No. EP/R029075/1. The authors thank G. Stefanou for SEM imaging of CVD graphene devices. J.A.-W. acknowledges the support of his Research Fellowship from the Royal Commission for the Exhibition of 1851 and Royal Society Dorothy Hodgkin Research Fellowship. Y.F. and S.H. acknowledge funding from EPSRC (EP/P005152/1). O.J.B. acknowledges an EPSRC Doctoral Training Award (EP/M508007/1). C.J. thanks the Australian Research Council for financial support and Australian National Fabrication Facility, ACT node, for facility support. The Strathclyde team acknowledges support by the European Commission (Grant 828841-ChipAI-H2020-FETOPEN-2018-2020) and the UK’s EPSRC (EP/N509760, EP/R03480X/1, and EP/P013597/1). L.W.S., Y.-C.H., S.-J.F., and T.M.C. acknowledge support from the Ministry of Science and Technology (Taiwan).

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    A review. In an attempt to give a brief introduction to carbon nanotube inkjet printing, this review paper discusses the issues that come along with prepg. and printing carbon nanotube ink. Carbon nanotube inkjet printing is relatively new, but it has great potential for broad applications in flexible and printable electronics, transparent electrodes, electronic sensors, and so on due to its low cost and the extraordinary properties of carbon nanotubes. In addn. to the formulation of carbon nanotube ink and its printing technologies, recent progress and achievements of carbon nanotube inkjet printing are reviewed in detail with brief discussion on the future outlook of the technol.
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  4. 4
    Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits. Science 2000, 290, 2123– 2126,  DOI: 10.1126/science.290.5499.2123
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    High-resolution ink-jet printing of all-polymer transistor circuits
    Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P.
    Science (Washington, D. C.) (2000), 290 (5499), 2123-2126CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Direct printing of functional electronic materials may provide a new route to low-cost fabrication of integrated circuits. However, to be useful it must allow continuous manufg. of all circuit components by successive soln. deposition and printing steps in the same environment. The authors demonstrate direct ink-jet printing of complete transistor circuits, including via-hole interconnections based on soln.-processed polymer conductors, insulators, and self-organizing semiconductors. The use of substrate surface energy patterning to direct the flow of H2O-based conducting polymer ink-jet droplets enables high-resoln. definition of practical channel lengths of 5 μm. High mobilities of 0.02 cm2/V-s and on-off current switching ratios of 105 were achieved.
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    Choi, Y. Y.; Teranishi, T.; Majima, Y. Robust Pt-Based Nanogap Electrodes with 10 nm Scale Ultrafine Linewidth. Appl. Phys. Express 2019, 12, 025002,  DOI: 10.7567/1882-0786/aafb20
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    Robust Pt-based nanogap electrodes with 10 nm scale ultrafine linewidth
    Choi, Yoon Young; Teranishi, Toshiharu; Majima, Yutaka
    Applied Physics Express (2019), 12 (2), 025002/1-025002/5CODEN: APEPC4; ISSN:1882-0786. (IOP Publishing Ltd.)
    Robust Pt-based nanogap electrodes with a 10 nm scale ultrafine linewidth were fabricated on SiO2/Si substrates by electron-beam lithog. The structures of the fabricated electrodes remained unchanged up to temps. of 773 K. The robust thermal stability of the fabricated electrodes in comparison with that of Au-based nanogap electrodes was discussed based on Rayleigh instability. A chem. assembled singleelectron transistor was prepd. by chemisorbing a 6 nm sized Au nanoparticle between the Pt-based nanogap electrodes, and the transistor showed clear and ideal Coulomb diamonds up to 180 K.
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    Lee, S. J.; Kim, J.; Tsuda, T.; Takano, R.; Shintani, R.; Nozaki, K.; Majima, Y. Single-Molecule Single-Electron Transistor (SM-SET) Based on π-Conjugated Quinoidal-Fused Oligosilole and Heteroepitaxial Spherical Au/Pt Nanogap Electrode. Appl. Phys. Express 2019, 12, 125007,  DOI: 10.7567/1882-0786/ab56e5
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    Single-molecule single-electron transistor (SM-SET) based on π-conjugated quinoidal-fused oligosilole and heteroepitaxial spherical Au/Pt nanogap electrodes
    Lee, Seung Joo; Kim, Jaeyeon; Tsuda, Tomohiro; Takano, Ryo; Shintani, Ryo; Nozaki, Kyoko; Majima, Yutaka
    Applied Physics Express (2019), 12 (12), 125007CODEN: APEPC4; ISSN:1882-0786. (IOP Publishing Ltd.)
    A single-mol. single-electron transistor (SM-SET) consisting of a π-conjugated quinoidal-fused oligosilole deriv., Si-2, of radius 0.75 nm and heteroepitaxial spherical (HS) Au/Pt nanogap electrodes is demonstrated. Si-2 with two inner silicon atoms and two ethylenethiol groups at both ends is chemisorbed by an S-Au bond between HS-Au/Pt nanogap electrodes fabricated by combining electron-beam lithog. and electroless Au plating. An ideal Coulomb staircase, Coulomb oscillation, and Coulomb diamond were obsd. using the 1.5 nm Si-2 SM-SET, which were supported by theor. results based on the orthodox model.
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    Shekhar, S.; Stokes, P.; Khondaker, S. I. Ultrahigh Density Alignment of Carbon Nanotube Arrays by Dielectrophoresis. ACS Nano 2011, 5, 1739– 1746,  DOI: 10.1021/nn102305z
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    Ultrahigh Density Alignment of Carbon Nanotube Arrays by Dielectrophoresis
    Shekhar, Shashank; Stokes, Paul; Khondaker, Saiful I.
    ACS Nano (2011), 5 (3), 1739-1746CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The authors report ultrahigh d. assembly of aligned single-walled carbon nanotube (SWNT) two-dimensional arrays via a.c. dielectrophoresis using high-quality surfactant-free and stable SWNT solns. After optimization of frequency and trapping time, the authors can reproducibly control the linear d. of the SWNT between prefabricated electrodes from 0.5 SWNT/μm to >30 SWNT/μm by tuning the concn. of the nanotubes in the soln. The authors' max. d. of 30 SWNT/μm is the highest for aligned arrays via any soln. processing technique reported so far. Further increase of SWNT concn. results in a dense array with multiple layers. How the orientation and d. of the nanotubes vary with concns. and channel lengths are discussed. Elec. measurement data show that the densely packed aligned arrays have low sheet resistances. Selective removal of metallic SWNTs via controlled elec. breakdown produced field-effect transistors with high current on-off ratio. Ultrahigh d. alignment reported here will have important implications in fabricating high-quality devices for digital and analog electronics.
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    Prins, F.; Barreiro, A.; Ruitenberg, J. W.; Seldenthuis, J. S.; Aliaga-Alcalde, N.; Vandersypen, L. M. K.; van der Zant, H. S. J. Room-Temperature Gating of Molecular Junctions Using Few-Layer Graphene Nanogap Electrodes. Nano Lett. 2011, 11, 4607– 4611,  DOI: 10.1021/nl202065x
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    Room-Temperature Gating of Molecular Junctions Using Few-Layer Graphene Nanogap Electrodes
    Prins, Ferry; Barreiro, Amelia; Ruitenberg, Justus W.; Seldenthuis, Johannes S.; Aliaga-Alcalde, Nuria; Vandersypen, Lieven M. K.; van der Zant, Herre S. J.
    Nano Letters (2011), 11 (11), 4607-4611CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors report on a method to fabricate and measure gateable mol. junctions that are stable at room temp. The devices are made by depositing mols. inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have sepns. ∼1-2 nm as estd. from a Simmons model for tunneling. The mol. junctions display gateable I-V-characteristics at room temp.
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    Limburg, B.; Thomas, J. O.; Holloway, G.; Sadeghi, H.; Sangtarash, S.; Hou, I. C.-Y.; Cremers, J.; Narita, A.; Müllen, K.; Lambert, C. J.; Briggs, G. A. D.; Mol, J. A.; Anderson, H. L. Anchor Groups for Graphene-Porphyrin Single-Molecule Transistors. Adv. Funct. Mater. 2018, 28, 1803629,  DOI: 10.1002/adfm.201803629
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    Bumgarner, R. Overview of DNA Microarrays: Types, Applications, and Their Future. Curr. Protoc. Mol. Biol. 2013,  DOI: 10.1002/0471142727.mb2201s101 .
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    Krasheninnikov, A. V.; Nordlund, K. Ion and Electron Irradiation-Induced Effects in Nanostructured Materials. J. Appl. Phys. 2010, 107, 071301,  DOI: 10.1063/1.3318261
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    Ion and electron irradiation-induced effects in nanostructured materials
    Krasheninnikov, A. V.; Nordlund, K.
    Journal of Applied Physics (2010), 107 (7), 071301/1-071301/70CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    A review. A common misconception is that the irradn. of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addn. to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent expts. show that irradn. can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphol. in a controllable manner, and tailor their mech., electronic, and even magnetic properties. Harnessing irradn. as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect prodn. and annealing in nanotargets. In this article, the authors review recent progress in the understanding of effects of irradn. on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the two-dimensional nanosystem graphene due to its similarity with carbon nanotubes. We dwell on both theor. and exptl. results and discuss at length not only the physics behind irradn. effects in nanostructures but also the tech. applicability of irradn. for the engineering of nanosystems. (c) 2010 American Institute of Physics.
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    Teweldebrhan, D.; Balandin, A. A. Modification of Graphene Properties Due to Electron-Beam Irradiation. Appl. Phys. Lett. 2009, 94, 013101,  DOI: 10.1063/1.3062851
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    Modification of graphene properties due to electron-beam irradiation
    Teweldebrhan, D.; Balandin, A. A.
    Applied Physics Letters (2009), 94 (1), 013101/1-013101/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors report micro-Raman investigation of changes in the single and bilayer graphene crystal lattice induced by the low and medium energy electron-beam irradn. (5-20 keV). It was found that the radiation exposures result in the appearance of the strong disorder D band around 1345 cm-1, indicating damage to the lattice. The D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests graphene's transformation to the nanocryst. and then to amorphous form. The results have important implications for graphene characterization and device fabrication, which rely on the electron microscopy and focused ion beam processing. (c) 2009 American Institute of Physics.
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    Gorbachev, R. V.; Tikhonenko, F. V.; Mayorov, A. S.; Horsell, D. W.; Savchenko, A. K. Weak Localization in Bilayer Graphene. Phys. Rev. Lett. 2007, 98, 176805,  DOI: 10.1103/PhysRevLett.98.176805
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    Weak Localization in Bilayer Graphene
    Gorbachev, R. V.; Tikhonenko, F. V.; Mayorov, A. S.; Horsell, D. W.; Savchenko, A. K.
    Physical Review Letters (2007), 98 (17), 176805/1-176805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We have performed the first exptl. investigation of quantum interference corrections to the cond. of a bilayer graphene structure. A neg. magnetoresistance-a signature of weak localization-is obsd. at different carrier densities, including the electroneutrality region. It is very different, however, from the weak localization in conventional two-dimensional systems. We show that it is controlled not only by the dephasing time, but also by different elastic processes that break the effective time-reversal symmetry and provide intervalley scattering.
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    Tikhonenko, F. V.; Horsell, D. W.; Gorbachev, R. V.; Savchenko, A. K. Weak Localization in Graphene Flakes. Phys. Rev. Lett. 2008, 100, 056802,  DOI: 10.1103/PhysRevLett.100.056802
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    Weak localization in graphene flakes
    Tikhonenko, F. V.; Horsell, D. W.; Gorbachev, R. V.; Savchenko, A. K.
    Physical Review Letters (2008), 100 (5), 056802/1-056802/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We show that the manifestation of quantum interference in graphene is very different from that in conventional 2D systems. Because of the chiral nature of charge carriers, it is not only sensitive to inelastic, phase-breaking scattering, but also to a no. of elastic scattering processes. We study weak localization in different samples and at different carrier densities, including the Dirac region, and find the characteristic rates that det. it. We show how the shape and quality of graphene flakes affect the values of the elastic and inelastic rates and discuss their phys. origin.
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    Minke, S.; Bundesmann, J.; Weiss, D.; Eroms, J. Phase Coherent Transport in Graphene Nanoribbons and Graphene Nanoribbon Arrays. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 155403,  DOI: 10.1103/PhysRevB.86.155403
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    Phase coherent transport in graphene nanoribbons and graphene nanoribbon arrays
    Minke, S.; Bundesmann, J.; Weiss, D.; Eroms, J.
    Physical Review B: Condensed Matter and Materials Physics (2012), 86 (15), 155403/1-155403/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We have exptl. investigated quantum interference corrections to the cond. of graphene nanoribbons at temps. down to 20 mK studying both weak localization (WL) and universal conductance fluctuations (UCFs). Since in individual nanoribbons at milli-Kelvin temps. the UCFs strongly mask the weak localization feature we employ both gate averaging and ensemble averaging to suppress the UCFs. This allows us to ext. the phase coherence length from both WL and UCF at all temps. Above 1 K the phase coherence length is suppressed due to Nyquist scattering, whereas at low temps. we observe a satn. of the phase coherence length at a few hundred nanometers, which exceeds the ribbon width, but stays below values typically found in bulk graphene. To better describe the expts. at elevated temps., we extend the formula for one-dimensional (1D) weak localization in graphene, which was derived in the limit of strong intervalley scattering, to include all elastic scattering rates.
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    Petersen, G.; Hernández, S. E.; Calarco, R.; Demarina, N.; SchäPers, T. Spin-Orbit Coupling and Phase-Coherent Transport in InN Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 125321,  DOI: 10.1103/PhysRevB.80.125321
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    Spin-orbit coupling and phase-coherent transport in InN nanowires
    Petersen, G.; Hernandez, S. Estevez; Calarco, R.; Demarina, N.; Schaepers, Th.
    Physical Review B: Condensed Matter and Materials Physics (2009), 80 (12), 125321/1-125321/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The low-temp. quantum transport properties of gated InN nanowires were investigated. Magnetic-field-dependent as well as gate-dependent measurements of universal conductance fluctuations were performed to gain information on the phase coherence in the electron transport. We found a pronounced decrease in the variance of the conductance by about a factor of 2 in gate-dependent fluctuation measurements if a magnetic field is applied. This effect is explained by the suppression of the Cooperon channel of the electron correlation contributing to the conductance fluctuations. Despite the fact that the diam. of the nanowire is less than 100 nm a clear weak antilocalization effect is found in the averaged magnetoconductance being in strong contrast to the suppression of weak antilocalization for narrow quantum wires based on planar 2D electron gases. The unexpected robustness of the weak antilocalization effect obsd. here is attributed to the tubular topol. of the surface electron gas in InN nanowires.
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    Estévez Hernández, S.; Akabori, M.; Sladek, K.; Volk, C.; Alagha, S.; Hardtdegen, H.; Pala, M. G.; Demarina, N.; Grützmacher, D.; SchäPers, T. Spin-Orbit Coupling and Phase Coherence in Inas Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 235303,  DOI: 10.1103/PhysRevB.82.235303
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    Spin-orbit coupling and phase coherence in InAs nanowires
    Estevez Hernandez, S.; Akabori, M.; Sladek, K.; Volk, Ch.; Alagha, S.; Hardtdegen, H.; Pala, M. G.; Demarina, N.; Grutzmacher, D.; Schapers, Th.
    Physical Review B: Condensed Matter and Materials Physics (2010), 82 (23), 235303/1-235303/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We investigated the magnetotransport of InAs nanowires grown by selective-area metal-org. vapor phase epitaxy. In the temp. range between 0.5 and 30 K reproducible fluctuations in the conductance upon variation in the magnetic field or the backgate voltage are obsd., which are attributed to electron interference effects in small disordered conductors. From the correlation field of the magnetoconductance fluctuations the phase-coherence length lφ is detd. At the lowest temps. lφ is found to be at least 300 nm while for temps. exceeding 2 K a monotonous decrease in lφ with temp. is obsd. A direct observation of the weak antilocalization effect indicating the presence of spin-orbit coupling is masked by the strong magnetoconductance fluctuations. However, by averaging the magnetoconductance over a range of gate voltages a clear peak in the magnetoconductance due to the weak antilocalization effect was resolved. By comparison of the exptl. data to simulations based on a recursive two-dimensional Green's-function approach a spin-orbit scattering length of approx. 70 nm was extd., indicating the presence of strong spin-orbit coupling.
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    Roulleau, P.; Choi, T.; Riedi, S.; Heinzel, T.; Shorubalko, I.; Ihn, T.; Ensslin, K. Suppression of Weak Antilocalization in InAs Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 155449,  DOI: 10.1103/PhysRevB.81.155449
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    Suppression of weak antilocalization in InAs nanowires
    Roulleau, P.; Choi, T.; Riedi, S.; Heinzel, T.; Shorubalko, I.; Ihn, T.; Ensslin, K.
    Physical Review B: Condensed Matter and Materials Physics (2010), 81 (15), 155449/1-155449/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We investigate the crossover between weak localization and weak antilocalization in InAs nanowires of different diams. (75 nm-140 nm-217 nm). For a magnetic field applied perpendicularly to the nanowire axis, we ext. the spin orbit and coherence lengths using a quasi-1D model of the conductance. We find a spin-orbit length inversely proportional to the width of the nanowire. When a parallel magnetic field is applied, we observe that the weak-antilocalization contribution is less affected by the magnetic field than in the perpendicular case.
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    Hansen, A. E.; Björk, M. T.; Fasth, C.; Thelander, C.; Samuelson, L. Spin Relaxation in InAs Nanowires Studied by Tunable Weak Antilocalization. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 205328,  DOI: 10.1103/PhysRevB.71.205328
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    Spin relaxation in InAs nanowires studied by tunable weak antilocalization
    Hansen, A. E.; Bjork, M. T.; Fasth, C.; Thelander, C.; Samuelson, L.
    Physical Review B: Condensed Matter and Materials Physics (2005), 71 (20), 205328/1-205328/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We report on a low-temp. magnetoconductance study to characterize the elec. and spin transport properties of n-type InAs nanowires grown by chem. beam epitaxy. A gate-controlled crossover from weak localization to weak antilocalization is obsd. The measured magnetoconductance data agrees well with theory for one-dimensional quasi-ballistic systems and yields a spin relaxation length which decreases with increasing gate voltage.
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    Lehnen, P.; SchäPers, T.; Kaluza, N.; Thillosen, N.; Hardtdegen, H. Enhanced Spin-Orbit Scattering Length in Narrow AlxGa1–xN/GaN Wires. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 205307,  DOI: 10.1103/PhysRevB.76.205307
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    Enhanced spin-orbit scattering length in narrow AlxGa1-xN/GaN wires
    Lehnen, Patrick; Schapers, Thomas; Kaluza, Nicoleta; Thillosen, Nicolas; Hardtdegen, Hilde
    Physical Review B: Condensed Matter and Materials Physics (2007), 76 (20), 205307/1-205307/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The magnetotransport in a set of identical parallel AlxGa1-xN/GaN quantum wire structures was studied. The width of the wires ranges between 1110 and 340 nm. For all sets of wires, clear Shubnikov-de Haas oscillations are obsd. The electron concn. and mobility are approx. the same for all wires, confirming that the electron gas in the AlxGa1-xN/GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires, the weak antilocalization effect is clearly obsd., indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective elec. width <250 nm, the weak antilocalization effect is suppressed. By comparing the exptl. data to a theor. model for quasi-one-dimensional structures, the authors come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.
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    Smith, L. W.; Al-Taie, H.; Sfigakis, F.; See, P.; Lesage, A. A. J.; Xu, B.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Statistical Study of Conductance Properties in One-Dimensional Quantum Wires Focusing on the 0.7 Anomaly. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 045426,  DOI: 10.1103/PhysRevB.90.045426
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    Statistical study of conductance properties in one-dimensional quantum wires focusing on the 0.7 anomaly
    Smith, L. W.; Al-Taie, H.; Sfigakis, F.; See, P.; Lesage, A. A. J.; Xu, B.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
    Physical Review B: Condensed Matter and Materials Physics (2014), 90 (4), 045426CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The properties of conductance in one-dimensional (1D) quantum wires are statistically investigated using an array of 256 lithog. identical split gates, fabricated on a GaAs/AlGaAs heterostructure. All the split gates are measured during a single cooldown under the same conditions. Electron many-body effects give rise to an anomalous feature in the conductance of a one-dimensional quantum wire, known as the "0.7 structure" (or "0.7 anomaly"). To handle the large data set, a method of automatically estg. the conductance value of the 0.7 structure is developed. Large differences are obsd. in the strength and value of the 0.7 structure [from 0.63 to 0.84 × (2e2/h)], despite the const. temp. and identical device design. Variations in the 1D potential profile are quantified by estg. the curvature of the barrier in the direction of electron transport, following a saddle-point model. The 0.7 structure appears to be highly sensitive to the specific confining potential within individual devices.
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  22. 22
    Al-Taie, H.; Smith, L. W.; Lesage, A. A. J.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Spatial Mapping and Statistical Reproducibility of an Array of 256 One-Dimensional Quantum Wires. J. Appl. Phys. 2015, 118, 075703,  DOI: 10.1063/1.4928615
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    Spatial mapping and statistical reproducibility of an array of 256 one-dimensional quantum wires
    Al-Taie, H.; Smith, L. W.; Lesage, A. A. J.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
    Journal of Applied Physics (Melville, NY, United States) (2015), 118 (7), 075703/1-075703/5CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We utilize a multiplexing architecture to measure the conductance properties of an array of 256 split gates. We investigate the reproducibility of the pinch off and one-dimensional definition voltage as a function of spatial location on two different cooldowns, and after illuminating the device. The reproducibility of both these properties on the two cooldowns is high, the result of the d. of the two-dimensional electron gas returning to a similar state after thermal cycling. The spatial variation of the pinch-off voltage reduces after illumination; however, the variation of the one-dimensional definition voltage increases due to an anomalous feature in the center of the array. A technique which quantifies the homogeneity of split-gate properties across the array is developed which captures the exptl. obsd. trends. In addn., the one-dimensional definition voltage is used to probe the d. of the wafer at each split gate in the array on a micron scale using a capacitive model. (c) 2015 American Institute of Physics.
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  23. 23
    Lesage, A. A. J.; Smith, L. W.; Al-Taie, H.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Assisted Extraction of the Energy Level Spacings and Lever Arms in Direct Current Bias Measurements of One-Dimensional Quantum Wires, Using an Image Recognition Routine. J. Appl. Phys. 2015, 117, 015704,  DOI: 10.1063/1.4905484
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    23
    Assisted extraction of the energy level spacings and lever arms in direct current bias measurements of one-dimensional quantum wires, using an image recognition routine
    Lesage, A. A. J.; Smith, L. W.; Al-Taie, H.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
    Journal of Applied Physics (Melville, NY, United States) (2015), 117 (1), 015704/1-015704/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    A multiplexer technique is used to individually measure an array of 256 split gates on a single GaAs/AlGaAs heterostructure. This gave large vols. of data, which requires the development of automated data anal. routines. An algorithm is developed to find the spacing between discrete energy levels, which form due to transverse confinement from the split gate. The lever arm, which relates split gate voltage to energy, is also found from the measured data. This reduces the time spent on the anal. Comparison with ests. obtained visually shows that the algorithm returns reliable results for subband spacing of split gates measured at 1.4 K. The routine is also used to assess d.c. bias spectroscopy measurements at lower temps. (50 mK). This technique is versatile and can be extended to other types of measurements. For example, it is used to ext. the magnetic field at which Zeeman-split 1D subbands cross 1 another. (c) 2015 American Institute of Physics.
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  24. 24
    Puddy, R. K.; Smith, L. W.; Al-Taie, H.; Chong, C. H.; Farrer, I.; Griffiths, J. P.; Ritchie, D. A.; Kelly, M. J.; Pepper, M.; Smith, C. G. Multiplexed Charge-Locking Device for Large Arrays of Quantum Devices. Appl. Phys. Lett. 2015, 107, 143501,  DOI: 10.1063/1.4932012
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    Multiplexed charge-locking device for large arrays of quantum devices
    Puddy, R. K.; Smith, L. W.; Al-Taie, H.; Chong, C. H.; Farrer, I.; Griffiths, J. P.; Ritchie, D. A.; Kelly, M. J.; Pepper, M.; Smith, C. G.
    Applied Physics Letters (2015), 107 (14), 143501/1-143501/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses an on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the no. of wires available in cryostat measurement systems. The device architecture that we describe here utilizes a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total no. exceeds that of the available elec. contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate such devices on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approx. one Coulomb oscillation per h. (c) 2015 American Institute of Physics.
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  25. 25
    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Hamilton, A. R.; Kelly, M. J.; Smith, C. G. Dependence of the 0.7 Anomaly on the Curvature of the Potential Barrier in Quantum Wires. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 235402,  DOI: 10.1103/PhysRevB.91.235402
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    Dependence of the 0.7 anomaly on the curvature of the potential barrier in quantum wires
    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Hamilton, A. R.; Kelly, M. J.; Smith, C. G.
    Physical Review B: Condensed Matter and Materials Physics (2015), 91 (23), 235402/1-235402/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Ninety-eight one-dimensional channels defined using split gates fabricated on a GaAs/AlGaAs heterostructure are measured during one cooldown at 1.4 K. The devices are arranged in an array on a single chip and are individually addressed using a multiplexing technique. The anomalous conductance feature known as the "0.7 structure" is studied using statistical techniques. The ensemble of data shows that the 0.7 anomaly becomes more pronounced and occurs at lower values as the curvature of the potential barrier in the transport direction decreases. This corresponds to an increase in the effective length of the device. The 0.7 anomaly is not strongly influenced by other properties of the conductance related to d. The curvature of the potential barrier appears to be the primary factor governing the shape of the 0.7 structure at a given T and B.
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  26. 26
    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Effect of Split Gate Size on the Electrostatic Potential and 0.7 Anomaly within Quantum Wires on a Modulation-Doped GaAs/AlGaAs Heterostructure. Phys. Rev. Appl. 2016, 5, 044015,  DOI: 10.1103/PhysRevApplied.5.044015
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    Effect of split gate size on the electrostatic potential and 0.7 anomaly within quantum wires on a modulation-doped GaAs/AlGaAs heterostructure
    Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
    Physical Review Applied (2016), 5 (4), 044015/1-044015/10CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
    We study 95 split gates of different size on a single chip using a multiplexing technique. Each split gate defines a one-dimensional channel on a modulation-doped GaAs/AlGaAs heterostructure, through which the conductance is quantized. The yield of devices showing good quantization decreases rapidly as the length of the split gates increases. However, for the subset of devices showing good quantization, there is no correlation between the electrostatic length of the one-dimensional channel (estd. using a saddle-point model) and the gate length. The variation in electrostatic length and the one-dimensional subband spacing for devices of the same gate length exceeds the variation in the av. values between devices of different lengths. There is a clear correlation between the curvature of the potential barrier in the transport direction and the strength of the "0.7 anomaly": the conductance value of the 0.7 anomaly reduces as the barrier curvature becomes shallower. These results highlight the key role of the electrostatic environment in one dimensional systems. Even in devices with clean conductance plateaus, random fluctuations in the background potential are crucial in detg. the potential landscape in the active device area such that nominally identical gate structures have different characteristics.
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  27. 27
    Al-Taie, H.; Smith, L. W.; Xu, B.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Cryogenic on-Chip Multiplexer for the Study of Quantum Transport in 256 Split-Gate Devices. Appl. Phys. Lett. 2013, 102, 243102,  DOI: 10.1063/1.4811376
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    Cryogenic on-chip multiplexer for the study of quantum transport in 256 split-gate devices
    Al-Taie, H.; Smith, L. W.; Xu, B.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
    Applied Physics Letters (2013), 102 (24), 243102/1-243102/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors present a multiplexing scheme for the measurement of large nos. of mesoscopic devices in cryogenic systems. The multiplexer was used to contact an array of 256 split gates on a GaAs/AlGaAs heterostructure, in which each split gate can be measured individually. The low-temp. conductance of split-gate devices is governed by quantum mechanics, leading to the appearance of conductance plateaus at intervals of 2e2/h. A fabrication-limited yield of 94% is achieved for the array, and a quantum yield is also defined, to account for disorder affecting the quantum behavior of the devices. The quantum yield rose from 55% to 86% after illuminating the sample, explained by the corresponding increase in carrier d. and mobility of the two-dimensional electron gas. The multiplexer is a scalable architecture, and can be extended to other forms of mesoscopic devices. It overcomes previous limits on the no. of devices that can be fabricated on a single chip due to the no. of elec. contacts available, without the need to alter existing exptl. set ups. (c) 2013 American Institute of Physics.
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    Suk, J. W.; Kitt, A.; Magnuson, C. W.; Hao, Y.; Ahmed, S.; An, J.; Swan, A. K.; Goldberg, B. B.; Ruoff, R. S. Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates. ACS Nano 2011, 5, 6916– 6924,  DOI: 10.1021/nn201207c
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    28
    Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates
    Suk, Ji Won; Kitt, Alexander; Magnuson, Carl W.; Hao, Yufeng; Ahmed, Samir; An, Jinho; Swan, Anna K.; Goldberg, Bennett B.; Ruoff, Rodney S.
    ACS Nano (2011), 5 (9), 6916-6924CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Reproducible dry and wet transfer techniques were developed to improve the transfer of large-area monolayer graphene grown on copper foils by CVD. The techniques reported here allow transfer onto three different classes of substrates: substrates covered with shallow depressions, perforated substrates, and flat substrates. A novel dry transfer technique was used to make graphene-sealed microchambers without trapping liq. inside. The dry transfer technique uses a polydimethylsiloxane frame that attaches to the poly(Me methacrylate) spun over the graphene film, and the monolayer graphene was transferred onto shallow depressions with 300 nm depth. The improved wet transfer onto perforated substrates with 2.7 μm diam. holes yields 98% coverage of holes covered with continuous films, allowing the ready use of Raman spectroscopy and TEM to study the intrinsic properties of CVD-grown monolayer graphene. Addnl., monolayer graphene transferred onto flat substrates has fewer cracks and tears, as well as lower sheet resistance than previous transfer techniques. Monolayer graphene films transferred onto glass had a sheet resistance of ∼980 Ω/sq and a transmittance of 97.6%. These transfer techniques open up possibilities for the fabrication of various graphene devices with unique configurations and enhanced performance.
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  29. 29
    Graphene Field-Effect Transistor Chip: S10; Graphenea, 2020; Datasheet 05–25–2020.
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    Mzali, S.; Montanaro, A.; Xavier, S.; Servet, B.; Mazellier, J.-P.; Bezencenet, O.; Legagneux, P.; Piquemal-Banci, M.; Galceran, R.; Dlubak, B.; Seneor, P.; Martin, M.-B.; Hofmann, S.; Robertson, J.; Cojocaru, C.-S.; Centeno, A.; Zurutuza, A. Stabilizing a Graphene Platform toward Discrete Components. Appl. Phys. Lett. 2016, 109, 253110,  DOI: 10.1063/1.4972847
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    Stabilizing a graphene platform toward discrete components
    Mzali, Sana; Montanaro, Alberto; Xavier, Stephane; Servet, Bernard; Mazellier, Jean-Paul; Bezencenet, Odile; Legagneux, Pierre; Piquemal-Banci, Maelis; Galceran, Regina; Dlubak, Bruno; Seneor, Pierre; Martin, Marie-Blandine; Hofmann, Stephan; Robertson, John; Cojocaru, Costel-Sorin; Centeno, Alba; Zurutuza, Amaia
    Applied Physics Letters (2016), 109 (25), 253110/1-253110/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We report on statistical anal. and consistency of elec. performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chem. vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithog. process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance min. related to the Dirac point. While no conductance min. was obsd. in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance min. and low hysteresis. The max. of the device no. distribution corresponds to a residual doping below 5 × 1011 cm-2 (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhibiting small hysteresis in the transfer characteristics can be envisaged for discrete components, with even further potential for low power driven electronics. (c) 2016 American Institute of Physics.
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    Kim, S.; Nah, J.; Jo, I.; Shahrjerdi, D.; Colombo, L.; Yao, Z.; Tutuc, E.; Banerjee, S. K. Realization of a High Mobility Dual-Gated Graphene Field-Effect Transistor with Al2O3 Dielectric. Appl. Phys. Lett. 2009, 94, 062107,  DOI: 10.1063/1.3077021
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    Realization of a high mobility dual-gated graphene field-effect transistor with Al2O3 dielectric
    Kim, Seyoung; Nah, Junghyo; Jo, Insun; Shahrjerdi, Davood; Colombo, Luigi; Yao, Zhen; Tutuc, Emanuel; Banerjee, Sanjay K.
    Applied Physics Letters (2009), 94 (6), 062107/1-062107/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors fabricate and characterize dual-gated graphene field-effect transistors using Al2O3 as top-gate dielec. The authors use a thin Al film as a nucleation layer to enable the at. layer deposition of Al2O3. The authors' devices show mobility values of over 8000 cm2/V s at room temp., a finding which indicates that the Top-gate stack does not significantly increase the carrier scattering and consequently degrade the device characteristics. The authors propose a device model to fit the exptl. data using a single mobility value. (c) 2009 American Institute of Physics.
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    Zhong, H.; Zhang, Z.; Xu, H.; Qiu, C.; Peng, L.-M. Comparison of Mobility Extraction Methods Based on Field-Effect Measurements for Graphene. AIP Adv. 2015, 5, 057136,  DOI: 10.1063/1.4921400
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    Comparison of mobility extraction methods based on field-effect measurements for graphene
    Zhong, Hua; Zhang, Zhiyong; Xu, Haitao; Qiu, Chenguang; Peng, Lian-Mao
    AIP Advances (2015), 5 (5), 057136/1-057136/8CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
    Carrier mobility extn. methods for graphene based on field-effect measurements are explored and compared according to theor. anal. and exptl. results. A group of graphene devices with different channel lengths were fabricated and measured, and carrier mobility is extd. from those elec. transfer curves using three different methods. Accuracy and applicability of those methods were compared. Transfer length method (TLM) can obtain accurate d. dependent mobility and contact resistance at relative high carrier d. based on data from a group of devices, and then can act as a std. method to verify other methods. As two of the most popular methods, direct transconductance method (DTM) and fitting method (FTM) can ext. mobility easily based on transfer curve of a sole graphene device. DTM offers an underestimated mobility at any carrier d. owing to the neglect of contact resistances, and the accuracy can be improved through fabricating field-effect transistors with long channel and good contacts. FTM assumes a const. mobility independent on carrier d., and then can obtain mobility, contact resistance and residual d. stimulations through fitting a transfer curve. However, FTM tends to obtain a mobility value near Dirac point and then overestimates carrier mobility of graphene. Comparing with the DTM and FTM, TLM could offer a much more accurate and carrier d. dependent mobility, that reflects the complete properties of graphene carrier mobility. (c) 2015 American Institute of Physics.
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    Li, H.; Wu, J.; Huang, X.; Lu, G.; Yang, J.; Lu, X.; Xiong, Q.; Zhang, H. Rapid and Reliable Thickness Identification of Two-Dimensional Nanosheets Using Optical Microscopy. ACS Nano 2013, 7, 10344– 10353,  DOI: 10.1021/nn4047474
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    Rapid and Reliable Thickness Identification of Two-Dimensional Nanosheets Using Optical Microscopy
    Li, Hai; Wu, Jumiati; Huang, Xiao; Lu, Gang; Yang, Jian; Lu, Xin; Xiong, Qihua; Zhang, Hua
    ACS Nano (2013), 7 (11), 10344-10353CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The phys. and electronic properties of ultrathin 2-dimensional (2D) layered nanomaterials are highly related to their thickness. The rapid and accurate identification of single- and few- to multilayer nanosheets is essential to their fundamental study and practical applications. Here, a universal optical method was developed for simple, rapid, and reliable identification of single- to quindecuple-layer (1L-15L) 2D nanosheets, including graphene, MoS2, WSe2, and TaS2, on Si substrates coated with 90 or 300 nm SiO2. The optical contrast differences between the substrates and 2D nanosheets with different layer nos. were collected and tabulated, serving as a std. ref., from which the layer no. of a given nanosheet can be readily and reliably detd. without using complex calcn. or expensive instrument. The general optical identification method will facilitate the thickness-dependent study of various 2D nanomaterials and expedite their research toward practical applications.
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    Smith, J. T.; Franklin, A. D.; Farmer, D. B.; Dimitrakopoulos, C. D. Reducing Contact Resistance in Graphene Devices through Contact Area Patterning. ACS Nano 2013, 7, 3661– 3667,  DOI: 10.1021/nn400671z
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    Reducing contact resistance in graphene devices through contact area patterning
    Smith, Joshua T.; Franklin, Aaron D.; Farmer, Damon B.; Dimitrakopoulos, Christos D.
    ACS Nano (2013), 7 (4), 3661-3667CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Performance of graphene electronics is limited by contact resistance assocd. with the metal-graphene (M-G) interface, where unique transport challenges arise as carriers are injected from a 3-dimensional metal into a 2-dimensional graphene sheet. In this work, enhanced carrier injection is exptl. achieved in graphene devices by forming cuts in the graphene within the contact regions. These cuts are oriented normal to the channel and facilitate bonding between the contact metal and carbon atoms at the graphene cut edges, reproducibly maximizing "edge-contacted" injection. Despite the redn. in M-G contact area caused by these cuts, the authors find that a 32% redn. in contact resistance results in Cu-contacted, 2-terminal devices, while a 22% redn. is achieved for top-gated graphene transistors with Pd contacts as compared to conventionally fabricated devices. The crucial role of contact annealing to facilitate this improvement is also elucidated. This simple approach provides a reliable and reproducible means of lowering contact resistance in graphene devices to bolster performance. Importantly, this enhancement requires no addnl. processing steps.
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    Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R. One-Dimensional Electrical Contact to a Two-Dimensional Material. Science 2013, 342, 614,  DOI: 10.1126/science.1244358
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    One-Dimensional Electrical Contact to a Two-Dimensional Material
    Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R.
    Science (Washington, DC, United States) (2013), 342 (6158), 614-617CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal B nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality elec. contact. Here, the authors report a contact geometry in which the authors metalize only the 1-dimensional edge of a 2-dimensional graphene layer. In addn. to outperforming conventional surface contacts, the edge-contact geometry allows a complete sepn. of the layer assembly and contact metalization processes. In graphene heterostructures, this enables high electronic performance, including low-temp. ballistic transport over distances longer than 15 μm, and room-temp. mobility comparable to the theor. phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2-dimensional materials.
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    Choi, M. S.; Lee, S. H.; Yoo, W. J. Plasma Treatments to Improve Metal Contacts in Graphene Field Effect Transistor. J. Appl. Phys. 2011, 110, 073305,  DOI: 10.1063/1.3646506
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    Plasma treatments to improve metal contacts in graphene field effect transistor
    Choi, Min Sup; Lee, Seung Hwan; Yoo, Won Jong
    Journal of Applied Physics (2011), 110 (7), 073305/1-073305/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Graphene formed via chem. vapor deposition was exposed to various plasmas (Ar, O2, N2, and H2) in order to examine its effects on the bonding properties of graphene to metal. After exposing patterned graphene to Ar plasma, the subsequently deposited metal electrodes remained intact, enabling the successful fabrication of field effect transistor arrays. The effects of the enhanced adhesion between graphene and metals were more evident from the O2 plasma than the Ar, N2, and H2 plasmas, suggesting that a chem. reaction of O radicals imparts hydrophilic properties to graphene more effectively than the chem. reaction of H and N radicals or the phys. bombardment of Ar ions. The elec. measurements (drain current vs. gate voltage) of the field effect transistors before and after Ar plasma exposure confirmed that the plasma treatment is quite effective in controlling the graphene to metal bonding accurately without the need for buffer layers. (c) 2011 American Institute of Physics.
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    Robinson, J. A.; LaBella, M.; Zhu, M.; Hollander, M.; Kasarda, R.; Hughes, Z.; Trumbull, K.; Cavalero, R.; Snyder, D. Contacting Graphene. Appl. Phys. Lett. 2011, 98, 053103,  DOI: 10.1063/1.3549183
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    Contacting graphene
    Robinson, Joshua A.; LaBella, Michael; Zhu, Mike; Hollander, Matt; Kasarda, Richard; Hughes, Zachary; Trumbull, Kathleen; Cavalero, Randal; Snyder, David
    Applied Physics Letters (2011), 98 (5), 053103/1-053103/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We present a robust method for forming high quality ohmic contacts to graphene, which improves the contact resistance by nearly 6000 times compared to untreated metal/graphene interfaces. The optimal specific contact resistance for treated Ti/Au contacts is found to av. <10-7 Ω cm2. Addnl., we examine Al/Au, Ti/Au, Ni/Au, Cu/Au, Pt/Au, and Pd/Au contact metalizations and find that most metalizations result in similar specific contact resistances in this work regardless of the work function difference between graphene and the metal overlayer. The results presented in this work serve as a foundation for achieving ultralow resistance ohmic contacts to graphene for high speed electronic and optoelectronic applications. (c) 2011 American Institute of Physics.
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    Wei Chen, C.; Ren, F.; Chi, G.-C.; Hung, S.-C.; Huang, Y. P.; Kim, J.; Kravchenko, I. I.; Pearton, S. J. UV Ozone Treatment for Improving Contact Resistance on Graphene. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2012, 30, 060604,  DOI: 10.1116/1.4754566
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    Malec, C. E.; Elkus, B.; Davidović, D. Vacuum-Annealed Cu Contacts for Graphene Electronics. Solid State Commun. 2011, 151, 1791– 1793,  DOI: 10.1016/j.ssc.2011.08.025
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    Vacuum-annealed Cu contacts for graphene electronics
    Malec, C. E.; Elkus, B.; Davidovic, D.
    Solid State Communications (2011), 151 (23), 1791-1793CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
    We present transfer length method measurements of the contact resistance between Cu and graphene, and a method to significantly reduce the contact resistance by vacuum annealing. Even in samples with heavily contaminated contacts, the contacts display very low contact resistance post annealing. Due to the common use of Cu, and its low chem. reactivity with graphene, thermal annealing will be important for future graphene devices requiring non-perturbing contacts with low contact resistance.
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    Balci, O.; Kocabas, C. Rapid Thermal Annealing of Graphene-Metal Contact. Appl. Phys. Lett. 2012, 101, 243105,  DOI: 10.1063/1.4769817
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    Rapid thermal annealing of graphene-metal contact
    Balci, Osman; Kocabas, Coskun
    Applied Physics Letters (2012), 101 (24), 243105/1-243105/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    High quality graphene-metal contacts are desirable for high-performance graphene based electronics. Process related factors result large variation in the contact resistance. A post-processing method is needed to improve graphene-metal contacts. We studied rapid thermal annealing (RTA) of graphene-metal contacts. We present results of a systematic investigation of device scaling before and after RTA for various metals. RTA provides a convenient technique to reduce contact resistance, thus to obtain reproducible device operation. (c) 2012 American Institute of Physics.
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    Huang, B.-C.; Zhang, M.; Wang, Y.; Woo, J. Contact Resistance in Top-Gated Graphene Field-Effect Transistors. Appl. Phys. Lett. 2011, 99, 032107,  DOI: 10.1063/1.3614474
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    Contact resistance in top-gated graphene field-effect transistors
    Huang, Bo-Chao; Zhang, Ming; Wang, Yanjie; Woo, Jason
    Applied Physics Letters (2011), 99 (3), 032107/1-032107/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The parasitic resistance of different source/drain metals for top-gated graphene field-effect transistors was extd. by fitting the measured ID-VG data with a resistance model and was found to be a significant part of the total resistance of graphene field-effect transistors. The results show that Ti/Au gives relatively large contact resistance, about 7500 Ω·μm. Ni/Au contact shows better result compared to Ti/Au, which is around 2100 Ω·μm. The lowest contact resistance was given by Ti/Pd/Au, which is around 750 Ω·μm. The contact resistivity for Ti/Pd/Au source/drain contact is around 2 × 10-6 Ω·cm2, close to state of the art GaAs technol. (c) 2011 American Institute of Physics.
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    Jauregui, L. A.; Cao, H.; Wu, W.; Yu, Q.; Chen, Y. P. Electronic Properties of Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition. Solid State Commun. 2011, 151, 1100– 1104,  DOI: 10.1016/j.ssc.2011.05.023
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    Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition
    Jauregui, Luis A.; Cao, Helin; Wu, Wei; Yu, Qingkai; Chen, Yong P.
    Solid State Communications (2011), 151 (16), 1100-1104CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
    We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycryst. Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede elec. transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.
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    Van Veldhoven, Z. A.; Alexander-Webber, J. A.; Sagade, A. A.; Braeuninger-Weimer, P.; Hofmann, S. Electronic Properties of CVD Graphene: The Role of Grain Boundaries, Atmospheric Doping, and Encapsulation by ALD. Phys. Status Solidi B 2016, 253, 2321– 2325,  DOI: 10.1002/pssb.201600255
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    Electronic properties of CVD graphene: The role of grain boundaries, atmospheric doping, and encapsulation by ALD
    Van Veldhoven, Zenas A.; Alexander-Webber, Jack A.; Sagade, Abhay A.; Braeuninger-Weimer, Philipp; Hofmann, Stephan
    Physica Status Solidi B: Basic Solid State Physics (2016), 253 (12), 2321-2325CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Grain boundaries and unintentional doping can have profound effects on graphene-based devices. Here we study these in detail for CVD grown poly-cryst. monolayer graphene with two significantly different grain size distributions centered around 10-25 μm and 100-400 μm. Although the two types of graphene are processed under identical conditions after growth, they show distinct transport properties in field effect transistor devices. While all as-fabricated samples showed similar p-type doping, the smaller grain size type graphene with larger no. of grain boundaries exhibit lower av. mobility. In order to sep. out the effects of grain boundaries and doping from ambient exposure on the transport properties, the devices were encapsulated with Al2O3 by at. layer deposition. The encapsulation of large grain samples thereby showed drastic improvements in the performance with negligible doping while the small grain samples are largely intolerant to this process. We discuss the implications of our data for the integrated manufg. of graphene-based device platforms.
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    De Fazio, D.; Purdie, D. G.; Ott, A. K.; Braeuninger-Weimer, P.; Khodkov, T.; Goossens, S.; Taniguchi, T.; Watanabe, K.; Livreri, P.; Koppens, F. H. L.; Hofmann, S.; Goykhman, I.; Ferrari, A. C.; Lombardo, A. High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition. ACS Nano 2019, 13, 8926– 8935,  DOI: 10.1021/acsnano.9b02621
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    High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
    De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
    ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
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    Alexander-Webber, J. A.; Groschner, C. K.; Sagade, A. A.; Tainter, G.; Gonzalez-Zalba, M. F.; Di Pietro, R.; Wong-Leung, J.; Tan, H. H.; Jagadish, C.; Hofmann, S.; Joyce, H. J. Engineering the Photoresponse of InAs Nanowires. ACS Appl. Mater. Interfaces 2017, 9, 43993– 44000,  DOI: 10.1021/acsami.7b14415
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    Engineering the Photoresponse of InAs Nanowires
    Alexander-Webber, Jack A.; Groschner, Catherine K.; Sagade, Abhay A.; Tainter, Gregory; Gonzalez-Zalba, M. Fernando; Di Pietro, Riccardo; Wong-Leung, Jennifer; Tan, H. Hoe; Jagadish, Chennupati; Hofmann, Stephan; Joyce, Hannah J.
    ACS Applied Materials & Interfaces (2017), 9 (50), 43993-44000CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either neg. or pos. photocond. (NPC or PPC) are reported. The NPC photoresponse time and magnitude is highly tunable by varying the nanowire diam. under controlled growth conditions. Using hysteresis characterization, the obsd. photoexcitation-induced hot electron trapping was decoupled from conventional elec. field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Surface passivation without chem. etching was demonstrated which both enhances the field-effect mobility of the nanowires by approx. an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an intrinsic pos. photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications.
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    Alexander-Webber, J. A.; Sagade, A. A.; Aria, A. I.; Van Veldhoven, Z. A.; Braeuninger-Weimer, P.; Wang, R.; Cabrero-Vilatela, A.; Martin, M.-B.; Sui, J.; Connolly, M. R.; Hofmann, S. Encapsulation of Graphene Transistors and Vertical Device Integration by Interface Engineering with Atomic Layer Deposited Oxide. 2D Mater. 2017, 4, 011008,  DOI: 10.1088/2053-1583/4/1/011008
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    Encapsulation of graphene transistors and vertical device integration by interface engineering with atomic layer deposited oxide
    Alexander-Webber, Jack A.; Sagade, Abhay A.; Aria, Adrianus I.; Van Veldhoven, Zenas A.; Braeuninger-Weimer, Philipp; Wang, Ruizhi; Cabrero-Vilatela, Andrea; Martin, Marie-Blandine; Sui, Jinggao; Connolly, Malcolm R.; Hofmann, Stephan
    2D Materials (2017), 4 (1), 011008/1-011008/9CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    We demonstrate a simple, scalable approach to achieve encapsulated graphene transistors with negligible gate hysteresis, low doping levels and enhanced mobility compared to as-fabricated devices. We engineer the interface between graphene and at. layer deposited (ALD) Al2O3 by tailoring the growth parameters to achieve effective device encapsulation while enabling the passivation of charge traps in the underlying gate dielec. We relate the passivation of charge trap states in the vicinity of the graphene to conformal growth of ALD oxide governed by in situ gaseous H2O pretreatments. We demonstrate the long term stability of such encapsulation techniques and the resulting insensitivity towards addnl. lithog. steps to enable vertical device integration of graphene for multi-stacked electronics fabrication.
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    Mayorov, A. S.; Gorbachev, R. V.; Morozov, S. V.; Britnell, L.; Jalil, R.; Ponomarenko, L. A.; Blake, P.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K. Micrometer-Scale Ballistic Transport in Encapsulated Graphene at Room Temperature. Nano Lett. 2011, 11, 2396– 2399,  DOI: 10.1021/nl200758b
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    Micrometer-scale ballistic transport in encapsulated graphene at room temperature
    Mayorov, Alexander S.; Gorbachev, Roman V.; Morozov, Sergey V.; Britnell, Liam; Jalil, Rashid; Ponomarenko, Leonid A.; Blake, Peter; Novoselov, Kostya S.; Watanabe, Kenji; Taniguchi, Takashi; Geim, A. K.
    Nano Letters (2011), 11 (6), 2396-2399CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Devices made from graphene encapsulated in hexagonal BN exhibit pronounced neg. bend resistance and an anomalous Hall effect, which are a direct consequence of room-temp. ballistic transport at a micrometer scale for a wide range of carrier concns. The encapsulation makes graphene practically insusceptible to the ambient atm. and, simultaneously, allows the use of BN as an ultrathin top gate dielec.
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    Robinson, J. P.; Schomerus, H.; Oroszlány, L.; Fal’ko, V. I. Adsorbate-Limited Conductivity of Graphene. Phys. Rev. Lett. 2008, 101, 196803,  DOI: 10.1103/PhysRevLett.101.196803
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    Adsorbate-limited conductivity of graphene
    Robinson, John P.; Schomerus, Henning; Oroszlany, Laszlo; Fal'ko, Vladimir I.
    Physical Review Letters (2008), 101 (19), 196803/1-196803/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We present a theory of electronic transport in graphene in the presence of randomly placed adsorbates. Our anal. predicts a marked asymmetry of the cond. about the Dirac point, as well as a neg. weak-localization magnetoresistivity. In the region of strong scattering, renormalization group corrections drive the system further towards insulating behavior. These results explain key features of recent expts., and are validated by numerical transport computations.
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    Farmer, D. B.; Golizadeh-Mojarad, R.; Perebeinos, V.; Lin, Y.-M.; Tulevski, G. S.; Tsang, J. C.; Avouris, P. Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices. Nano Lett. 2009, 9, 388– 392,  DOI: 10.1021/nl803214a
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    Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices
    Farmer, Damon B.; Golizadeh-Mojarad, Roksana; Perebeinos, Vasili; Lin, Yu-Ming; Tulevski, George S.; Tsang, James C.; Avouris, Phaedon
    Nano Letters (2009), 9 (1), 388-392CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors study poly(ethylene imine) and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these mols. exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequil. Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.
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    Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; van den Brink, J.; Kelly, P. J. Doping Graphene with Metal Contacts. Phys. Rev. Lett. 2008, 101, 026803,  DOI: 10.1103/PhysRevLett.101.026803
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    Doping graphene with metal contacts
    Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; van den Brink, J.; Kelly, P. J.
    Physical Review Letters (2008), 101 (2), 026803/1-026803/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Making devices with graphene necessarily involves making contacts with metals. We use d. functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au, and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by ∼0.5 eV. At equil. sepns., the crossover from p-type to n-type doping occurs for a metal work function of ∼5.4 eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple anal. model which characterizes the metal solely in terms of its work function, greatly extending their applicability.
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    Huard, B.; Stander, N.; Sulpizio, J. A.; Goldhaber-Gordon, D. Evidence of the Role of Contacts on the Observed Electron-Hole Asymmetry in Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 121402,  DOI: 10.1103/PhysRevB.78.121402
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    Evidence of the role of contacts on the observed electron-hole asymmetry in graphene
    Huard, B.; Stander, N.; Sulpizio, J. A.; Goldhaber-Gordon, D.
    Physical Review B: Condensed Matter and Materials Physics (2008), 78 (12), 121402/1-121402/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We perform elec. transport measurements in graphene with several sample geometries. In particular, we design "invasive" probes crossing the whole graphene sheet as well as "external" probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge d. and is sym. between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sublinear conductance as a function of d. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge d. below the metal, which thereby forms a p-n or p-p junction, depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sub-linearity obsd. in conductance as a function of d. in a large no. of expts. on graphene, which has generally been attributed to short-range scattering only.
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    Liu, W.; Wei, J.; Sun, X.; Yu, H. A Study on Graphene—Metal Contact. Crystals 2013, 3, 257– 274,  DOI: 10.3390/cryst3010257
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    A study on graphene-metal contact
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    Crystals (2013), 3 (), 257-274CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
    A review. The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. Reviewed are the authors's recent studies on the graphene-metal contact characteristics from the following viewpoints: (i) metal prepn. method; (ii) asym. conductance; (iii) annealing effect; (iv) interfaces impact.
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    Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666– 669,  DOI: 10.1126/science.1102896
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    Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.
    Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
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    Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201– 204,  DOI: 10.1038/nature04235
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    Zhang, Yuanbo; Tan, Yan-Wen; Stormer, Horst L.; Kim, Philip
    Nature (London, United Kingdom) (2005), 438 (7065), 201-204CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    When electrons are confined in two-dimensional materials, quantum-mech. enhanced transport phenomena such as the quantum Hall effect can be obsd. Graphene, consisting of an isolated single at. layer of graphite, is an ideal realization of such a two-dimensional system. However, its behavior is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality. Indeed, a distinctive half-integer quantum Hall effect was predicted theor., as has the existence of a nonzero Berry's phase (a geometric quantum phase) of the electron wavefunction-a consequence of the exceptional topol. of the graphene band structure. Recent advances in micromech. extn. and fabrication techniques for graphite structures now permit such exotic two-dimensional electron systems to be probed exptl. Here the authors report an exptl. study of magneto-transport in a high-mobility single layer of graphene. Adjusting the chem. potential using the elec. field effect, the authors observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these expts. is confirmed by magneto-oscillations. In addn. to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.
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    Arjmandi-Tash, H.; Kalita, D.; Han, Z.; Othmen, R.; Nayak, G.; Berne, C.; Landers, J.; Watanabe, K.; Taniguchi, T.; Marty, L.; Coraux, J.; Bendiab, N.; Bouchiat, V. Large Scale Graphene/h-BN Heterostructures Obtained by Direct CVD Growth of Graphene Using High-Yield Proximity-Catalytic Process. J. Phys. Mater. 2018, 1, 015003,  DOI: 10.1088/2515-7639/aac66e
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    Cai, Z.; Liu, B.; Zou, X.; Cheng, H.-M. Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures. Chem. Rev. 2018, 118, 6091– 6133,  DOI: 10.1021/acs.chemrev.7b00536
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    Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures
    Cai, Zhengyang; Liu, Bilu; Zou, Xiaolong; Cheng, Hui-Ming
    Chemical Reviews (Washington, DC, United States) (2018), 118 (13), 6091-6133CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chem. properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite no. of materials and show exotic phys. phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable prepn. of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chem. vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition metal dichalcogenides. We provide insight into the growth mechanisms of single crystal 2D domains and the key technologies used to realize wafer-scale growth of continuous and homogeneous 2D films which are important for practical applications. Meanwhile, strategies to design and grow various kinds of 2D material based heterostructures are thoroughly discussed. The applications of CVD-grown 2D materials and their heterostructures in electronics, optoelectronics, sensors, flexible devices, and electrocatalysis are also discussed. Finally, we suggest solns. to these challenges and ideas concerning future developments in this emerging field.
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    Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6, 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
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    CVD-Enabled Graphene Manufacture and Technology
    Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
    Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
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    Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Phase Perfection in Zinc Blende and Wurtzite III-IV Nanowires Using Basic Growth Parameters. Nano Lett. 2010, 10, 908– 915,  DOI: 10.1021/nl903688v
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    Phase Perfection in Zinc Blende and Wurtzite III-V Nanowires Using Basic Growth Parameters
    Joyce, Hannah J.; Wong-Leung, Jennifer; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati
    Nano Letters (2010), 10 (3), 908-915CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Phase-perfect nanowires, of arbitrary diam., can be prepd. by tailoring basic growth parameters: temp. and V/III ratio. Phase purity is achieved without sacrificing important specifications of diam. and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temp. coupled with a high V/III ratio. A high growth temp. coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. A comprehensive nucleation model is presented to explain the formation of these markedly different crystal phases under these growth conditions. Crit. to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temp. and V/III ratio.
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    Guilhabert, B.; Hurtado, A.; Jevtics, D.; Gao, Q.; Tan, H. H.; Jagadish, C.; Dawson, M. D. Transfer Printing of Semiconductor Nanowires with Lasing Emission for Controllable Nanophotonic Device Fabrication. ACS Nano 2016, 10, 3951– 3958,  DOI: 10.1021/acsnano.5b07752
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    Transfer Printing of Semiconductor Nanowires with Lasing Emission for Controllable Nanophotonic Device Fabrication
    Guilhabert, Benoit; Hurtado, Antonio; Jevtics, Dimitars; Gao, Qian; Tan, Hark Hoe; Jagadish, Chennupati; Dawson, Martin D.
    ACS Nano (2016), 10 (4), 3951-3958CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Accurate positioning and organization of InP nanowires (NWs) with lasing emission at room temp. is achieved using a nanoscale transfer printing (TP) technique. The NWs retained their lasing emission after their transfer to targeted locations on different receiving substrates (e.g., polymers, SiO2, and metal surfaces). The NWs were also organized into complex spatial patterns, including 1D and 2D arrays, with a controlled no. of elements and dimensions. The developed TP technique enables the fabrication of bespoke nanophotonic systems using NW lasers and other NW devices as building blocks.
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  61. 61
    Jevtics, D.; McPhillimy, J.; Guilhabert, B.; Alanis, J. A.; Tan, H. H.; Jagadish, C.; Dawson, M. D.; Hurtado, A.; Parkinson, P.; Strain, M. J. Characterization, Selection, and Microassembly of Nanowire Laser Systems. Nano Lett. 2020, 20, 1862– 1868,  DOI: 10.1021/acs.nanolett.9b05078
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    Characterization, Selection, and Microassembly of Nanowire Laser Systems
    Jevtics, Dimitars; McPhillimy, John; Guilhabert, Benoit; Alanis, Juan A.; Tan, Hark Hoe; Jagadish, Chennupati; Dawson, Martin D.; Hurtado, Antonio; Parkinson, Patrick; Strain, Michael J.
    Nano Letters (2020), 20 (3), 1862-1868CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Semiconductor nanowire (NW) lasers are a promising technol. for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy d., and subsequently printed in arrays onto a secondary substrate. Statistical anal. of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Anal. on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.
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  62. 62
    Jevtics, D.; Hurtado, A.; Guilhabert, B.; McPhillimy, J.; Cantarella, G.; Gao, Q.; Tan, H. H.; Jagadish, C.; Strain, M. J.; Dawson, M. D. Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate. Nano Lett. 2017, 17, 5990– 5994,  DOI: 10.1021/acs.nanolett.7b02178
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    Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate
    Jevtics, Dimitars; Hurtado, Antonio; Guilhabert, Benoit; McPhillimy, John; Cantarella, Giuseppe; Gao, Qian; Tan, Hark Hoe; Jagadish, Chennupati; Strain, Michael J.; Dawson, Martin D.
    Nano Letters (2017), 17 (10), 5990-5994CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Nanowire lasers are integrated with planar waveguide devices using a high positional accuracy microtransfer printing technique. Direct nanowire to waveguide coupling is demonstrated, with coupling losses as low as -17 dB, dominated by mode mismatch between the structures. Coupling is achieved using both end-fire coupling into a waveguide facet, and from nanowire lasers printed directly onto the top surface of the waveguide. In-waveguide peak powers up to 11.8 μW are demonstrated. Basic photonic integrated circuit functions such as power splitting and wavelength multiplexing are presented. Finally, devices are fabricated on a mech. flexible substrate to demonstrate robust coupling between the on-chip laser source and waveguides under significant deformation of the system.
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    Xu, W.-Z.; Ren, F.-F.; Jevtics, D.; Hurtado, A.; Li, L.; Gao, Q.; Ye, J.; Wang, F.; Guilhabert, B.; Fu, L.; Lu, H.; Zhang, R.; Tan, H. H.; Dawson, M. D.; Jagadish, C. Vertically Emitting Indium Phosphide Nanowire Lasers. Nano Lett. 2018, 18, 3414– 3420,  DOI: 10.1021/acs.nanolett.8b00334
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    Vertically Emitting Indium Phosphide Nanowire Lasers
    Xu, Wei-Zong; Ren, Fang-Fang; Jevtics, Dimitars; Hurtado, Antonio; Li, Li; Gao, Qian; Ye, Jiandong; Wang, Fan; Guilhabert, Benoit; Fu, Lan; Lu, Hai; Zhang, Rong; Tan, Hark Hoe; Dawson, Martin D.; Jagadish, Chennupati
    Nano Letters (2018), 18 (6), 3414-3420CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Semiconductor nanowire (NW) lasers have attracted considerable research effort given their excellent promise for nanoscale photonic sources. NW lasers currently exhibit poor directionality and high threshold gain, issues critically limiting their prospects for on-chip light sources with extremely reduced footprint and efficient power consumption. A new design is proposed and a vertically emitting InP NW laser structure showing high emission directionality and reduced energy requirements for operation is exptl. demonstrated. The structure of the laser combines an InP NW integrated in a cat's eye (CE) antenna. Thanks to the antenna guidance with broken asymmetry, strong focusing ability, and high Q-factor, the designed InP CE-NW lasers exhibit a higher degree of polarization, narrower emission angle, enhanced internal quantum efficiency, and reduced lasing threshold.
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  64. 64
    Peng, K.; Jevtics, D.; Zhang, F.; Sterzl, S.; Damry, D. A.; Rothmann, M. U.; Guilhabert, B.; Strain, M. J.; Tan, H. H.; Herz, L. M.; Fu, L.; Dawson, M. D.; Hurtado, A.; Jagadish, C.; Johnston, M. B. Three-Dimensional Cross-Nanowire Networks Recover Full Terahertz State. Science 2020, 368, 510– 513,  DOI: 10.1126/science.abb0924
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    Three-dimensional cross-nanowire networks recover full terahertz state
    Peng, Kun; Jevtics, Dimitars; Zhang, Fanlu; Sterzl, Sabrina; Damry, Djamshid A.; Rothmann, Mathias U.; Guilhabert, Benoit; Strain, Michael J.; Tan, Hark H.; Herz, Laura M.; Fu, Lan; Dawson, Martin D.; Hurtado, Antonio; Jagadish, Chennupati; Johnston, Michael B.
    Science (Washington, DC, United States) (2020), 368 (6490), 510-513CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    THz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to IR light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of THz light, yet to date, most time-domain THz detectors are sensitive to just 1 polarization component. A nanotechnol.-based semiconductor detector using cross-nanowire networks that records the full polarization state of THz pulses is demonstrated. The monolithic device allows simultaneous measurements of the orthogonal components of the THz elec. field vector without cross-talk. The capabilities of the detector for the study of metamaterials are demonstrated.
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    Sourribes, M. J. L.; Isakov, I.; Panfilova, M.; Warburton, P. A. Minimization of the Contact Resistance between InAs Nanowires and Metallic Contacts. Nanotechnology 2013, 24, 045703,  DOI: 10.1088/0957-4484/24/4/045703
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    Minimization of the contact resistance between InAs nanowires and metallic contacts
    Sourribes, M. J. L.; Isakov, I.; Panfilova, M.; Warburton, P. A.
    Nanotechnology (2013), 24 (4), 045703CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    We investigate different processes for optimizing the formation of Ohmic contacts to InAs nanowires. The nanowires are grown via mol. beam epitaxy without the use of metal catalysts. Metallic contacts are attached to the nanowires by using an electron beam lithog. process. Before deposition of the contacts, the InAs nanowires are treated either by wet etching in an ammonium polysulfide (NH4)2Sx soln. or by an argon milling process in order to remove a surface oxide layer. Two-point elec. measurements show that the resistance of the ammonium polysulfide-treated nanowires is two orders of magnitude lower than that of the untreated nanowires. The nanowires that are treated by the argon milling process show a resistance which is more than an order of magnitude lower than that of those treated with ammonium polysulfide. Four-point measurements allow us to ext. an upper bound of 1.4 × 10-7 Ω cm2 for the contact resistivity of metallic contacts on nanowires treated by the argon milling process.
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    Shi, T.; Fu, M.; Pan, D.; Guo, Y.; Zhao, J.; Chen, Q. Contact Properties of Field-Effect Transistors Based on Indium Arsenide Nanowires Thinner Than 16 nm. Nanotechnology 2015, 26, 175202,  DOI: 10.1088/0957-4484/26/17/175202
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    Contact properties of field-effect transistors based on indium arsenide nanowires thinner than 16 nm
    Shi Tuanwei; Fu Mengqi; Pan Dong; Guo Yao; Zhao Jianhua; Chen Qing
    Nanotechnology (2015), 26 (17), 175202 ISSN:.
    With the scaling down of field effect transistors (FETs) to improve performance, the contact between the electrodes and the channel becomes more and more important. Contact properties of FETs based on ultrathin InAs NWs (with the diameter ranging from sub-7 nm to 16 nm) are investigated here. Chromium (Cr) and nickel (Ni) are proven to form ohmic contact with the ultrathin InAs NWs, in contrast to a recent report (Razavieh A et al ACS Nano 8 6281). Furthermore, the contact resistance is found to depend on the NW diameter and the contact metals, which between Cr and InAs NWs increases more rapidly than that between Ni and InAs NWs when the NW diameter decreases. The origins of the contact resistance difference for the two kinds of metals are studied and NixInAs is believed to play an important role. Based on our results, it is advantageous to use Ni as contact metal for ultrathin NWs. We also observe that the FETs are still working in the diffusive regime even when the channel length is scaled down to 50 nm.
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    Fan, D.; Kang, N.; Ghalamestani, S. G.; Dick, K. A.; Xu, H. Q. Schottky Barrier and Contact Resistance of InSb Nanowire Field-Effect Transistors. Nanotechnology 2016, 27, 275204,  DOI: 10.1088/0957-4484/27/27/275204
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    Schottky barrier and contact resistance of InSb nanowire field-effect transistors
    Fan, Dingxun; Kang, N.; Ghalamestani, Sepideh Gorji; Dick, Kimberly A.; Xu, H. Q.
    Nanotechnology (2016), 27 (27), 275204/1-275204/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Understanding of the elec. contact properties of semiconductor nanowire (NW) field-effect transistors (FETs) plays a crucial role in the use of semiconducting NWs as building blocks for future nanoelectronic devices and in the study of fundamental physics problems. Here, we report on a study of the contact properties of Ti/Au, a widely used contact metal combination, when contacting individual InSb NWs via both two-probe and four-probe transport measurements. We show that a Schottky barrier of height ΦSB ∼ 20 meV is present at the metal-InSb NW interfaces and its effective height is gate-tunable. The contact resistance (Rc) in the InSb NWFETs is also analyzed by magnetotransport measurements at low temps. It is found that Rc in the on-state exhibits a pronounced magnetic field-dependent feature, namely it is increased strongly with increasing magnetic field after an onset field Bc. A qual. picture that takes into account magnetic depopulation of subbands in the NWs is provided to explain the observation. Our results provide solid exptl. evidence for the presence of a Schottky barrier at Ti/Au-InSb NW interfaces and can be used as a basis for design and fabrication of novel InSb NW-based nanoelectronic devices and quantum devices.
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    Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Single- and Multi-Wall Carbon Nanotube Field-Effect Transistors. Appl. Phys. Lett. 1998, 73, 2447– 2449,  DOI: 10.1063/1.122477
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    Single- and multi-wall carbon nanotube field-effect transistors
    Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph.
    Applied Physics Letters (1998), 73 (17), 2447-2449CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temp., it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors.
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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXms1eqsrc%253D&md5=ad460a19a39b79259317e14d353072de
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    Ullah, A. R.; Joyce, H. J.; Tan, H. H.; Jagadish, C.; Micolich, A. P. The Influence of Atmosphere on the Performance of Pure-Phase WZ and ZB InAs Nanowire Transistors. Nanotechnology 2017, 28, 454001. DOI: 10.1088/1361-6528/aa8e23
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    The influence of atmosphere on the performance of pure-phase WZ and ZB InAs nanowire transistors
    Ullah, A. R.; Joyce, H. J.; Tan, H. H.; Jagadish, C.; Micolich, A. P.
    Nanotechnology (2017), 28 (45), 454001/1-454001/8CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    We compare the characteristics of phase-pure MOCVD grown ZB and WZ InAs nanowire transistors in several atmospheres: air, dry pure N2 and O2, and N2 bubbled through liq. H2O and alcs. to identify whether phase-related structural/surface differences affect their response. Both WZ and ZB give poor gate characteristics in dry state. Adsorption of polar species reduces off-current by 2-3 orders of magnitude, increases on-off ratio and significantly reduces sub-threshold slope. The key difference is the greater sensitivity of WZ to low adsorbate level. We attribute this to facet structure and its influence on the sepn. between conduction electrons and surface adsorption sites. We highlight the important role adsorbed species play in nanowire device characterization. WZ is commonly thought superior to ZB in InAs nanowire transistors. We show this is an artifact of the moderate humidity found in ambient lab. conditions: WZ and ZB perform equally poorly in the dry gas limit yet equally well in the wet gas limit. We also highlight the vital role d.-lowering disorder has in improving gate characteristics, be it stacking faults in mixed-phase WZ or surface adsorbates in pure-phase nanowires.
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    Dayeh, S. A.; Aplin, D. P. R.; Zhou, X.; Yu, P. K. L.; Yu, E. T.; Wang, D. High Electron Mobility InAs Nanowire Field-Effect Transistors. Small 2007, 3, 326– 332,  DOI: 10.1002/smll.200600379
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    High electron mobility InAs nanowire field-effect transistors
    Dayeh, Shadi A.; Aplin, David P. R.; Zhou, Xiaotian; Yu, Paul K. L.; Yu, Edward T.; Wang, Deli
    Small (2007), 3 (2), 326-332CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Single-crystal InAs nanowires (NWs) were synthesized using metal-org. CVD (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO2/n+-Si substrate with a global n+-Si back-gate and sputtered SiOx/Au underlap top-gate. For top-gate NWFETs, the authors have developed a model that allows accurate estn. of characteristic NW parameters, including carrier field-effect mobility and carrier concn. by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temp. field-effect mobility ≤6580 cm2 V-1 s-1, which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.
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    Holloway, G. W.; Haapamaki, C. M.; Kuyanov, P.; LaPierre, R. R.; Baugh, J. Electrical Characterization of Chemical and Dielectric Passivation of InAs Nanowires. Semicond. Sci. Technol. 2016, 31, 114004,  DOI: 10.1088/0268-1242/31/11/114004
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    Electrical characterization of chemical and dielectric passivation of InAs nanowires
    Holloway, Gregory W.; Haapamaki, Chris M.; Kuyanov, Paul; LaPierre, Ray R.; Baugh, Jonathan
    Semiconductor Science and Technology (2016), 31 (11), 114004/1-114004/8CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
    The native oxide at the surface of III-V nanowires, such as InAs, can be a major source of charge noise and scattering in nanowire-based electronics, particularly for quantum devices operated at low temps. Surface passivation provides a means to remove the native oxide and prevent its regrowth. Here, we study the effects of surface passivation and conformal dielec. deposition by measuring elec. conductance through nanowire field effect transistors treated with a variety of surface prepns. By extg. field effect mobility, subthreshold swing, threshold shift with temp., and the gate hysteresis for each device, we infer the relative effects of the different treatments on the factors influencing transport. It is found that a combination of chem. passivation followed by deposition of an aluminum oxide dielec. shell yields the best results compared to the other treatments, and comparable to untreated nanowires. Finally, it is shown that an entrenched, top-gated device using an optimally treated nanowire can successfully form a stable double quantum dot at low temps. The device has excellent electrostatic tunability owing to the conformal dielec. layer and the combination of local top gates and a global back gate.
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    Petrovykh, D. Y.; Yang, M. J.; Whitman, L. J. Chemical and Electronic Properties of Sulfur-Passivated InAs Surfaces. Surf. Sci. 2003, 523, 231– 240,  DOI: 10.1016/S0039-6028(02)02411-1
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    Chemical and electronic properties of sulfur-passivated InAs surfaces
    Petrovykh, D. Y.; Yang, M. J.; Whitman, L. J.
    Surface Science (2003), 523 (3), 231-240CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
    Treatment with ammonium sulfide ((NH4)2Sx) solns. is used to produce model passivated InAs(0 0 1) surfaces with well-defined chem. and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aq. solns., as characterized by XPS, at. force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the assocd. downward band bending.
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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXmslWk&md5=3e02099b1e49a1c425ca72a712a21461
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    Nature Communications (2016), 7 (), 11894CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled prodn. of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.
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    A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
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    Nature communications (2018), 9 (1), 1413 ISSN:.
    Van der Waals heterostructures are comprised of stacked atomically thin two-dimensional crystals and serve as novel materials providing unprecedented properties. However, the random natures in positions and shapes of exfoliated two-dimensional crystals have required the repetitive manual tasks of optical microscopy-based searching and mechanical transferring, thereby severely limiting the complexity of heterostructures. To solve the problem, here we develop a robotic system that searches exfoliated two-dimensional crystals and assembles them into superlattices inside the glovebox. The system can autonomously detect 400 monolayer graphene flakes per hour with a small error rate (<7%) and stack four cycles of the designated two-dimensional crystals per hour with few minutes of human intervention for each stack cycle. The system enabled fabrication of the superlattice consisting of 29 alternating layers of the graphene and the hexagonal boron nitride. This capacity provides a scalable approach for prototyping a variety of van der Waals superlattices.
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    Scientific Reports (2016), 6 (), 27549CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Carrier multiplication (i.e. generation of multiple electron-hole pairs from a single high-energy electron, CM) in graphene has been extensively studied both theor. and exptl., but direct application of hot carrier multiplication in graphene has not been reported. Here, taking advantage of efficient CM in graphene, we fabricated graphene/TiO2 Schottky nanodiodes and found CM-driven enhancement of quantum efficiency. The unusual photocurrent behavior was obsd. and directly compared with Fowler's law for photoemission on metals. The Fowler's law exponent for the graphene-based nanodiode is almost twice that of a thin gold film based diode; the graphene-based nanodiode also has a weak dependence on light intensity-both are significant evidence for CM in graphene. Furthermore, doping in graphene significantly modifies the quantum efficiency by changing the Schottky barrier. The CM phenomenon obsd. on the graphene/TiO2 nanodiodes can lead to intriguing applications of viable graphene-based light harvesting.
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    Over the past decade graphene devices have inspired the progress of future electronic and optoelectronic technologies. The unique combination of fast carrier dynamics and intrinsic quantum capacitance of graphene is a fertile ground for implementing novel device architectures. Here, we report on a novel device architecture comprising graphene Schottky diode varactors and assess the potential applications of this type of new device in optoelectronics. We show that graphene varactor diodes exhibit significant advantages compared with existing graphene photodetectors including elimination of high dark currents and enhancement of the external quantum efficiency (EQE). Our devices demonstrate a large photoconductive gain and EQE of up to 37%, fast photoresponse, and low leakage currents at room temp.
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    The contact properties between metal and graphene were examd. The elec. measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The anal. using the cross-bridge Kelvin (CBK) structure suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of -1 μm. The contact resistivity for Ni was measured as -5 × 10-6 Ω cm2 using the CBK. A simple calcn. suggests that a contact resistivity less than 10-9 Ω cm2 is required for miniaturized graphene field effect transistors. (c) 2010 American Institute of Physics.
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    Applied Physics Letters (2014), 104 (1), 013114/1-013114/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Arrays of nearly identical graphene devices on Si/SiO2 exhibit a substantial device-to-device variation, even in case of a high-quality CVD or mech. exfoliated graphene. We propose that such device-to-device variation could provide a platform for highly selective multisensor electronic olfactory systems. We fabricated a multielectrode array of CVD graphene devices on a Si/SiO2 substrate and demonstrated that the diversity of these devices is sufficient to reliably discriminate different short-chain alcs.: methanol, ethanol, and isopropanol. The diversity of graphene devices on Si/SiO2 could possibly be used to construct similar multisensor systems trained to recognize other analytes as well. (c) 2014 American Institute of Physics.
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    The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extn. of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technol. Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the exptl. and theor. activity that in the last decade has been focusing on the redn. of the contact resistance in graphene transistors. We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.
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    Park, H.-Y.; Jung, W.-S.; Kang, D.-H.; Jeon, J.; Yoo, G.; Park, Y.; Lee, J.; Jang, Y. H.; Lee, J.; Park, S.; Yu, H.-Y.; Shin, B.; Lee, S.; Park, J.-H. Extremely Low Contact Resistance on Graphene through n-Type Doping and Edge Contact Design. Adv. Mater. 2016, 28, 864– 870,  DOI: 10.1002/adma.201503715
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    The effects of graphene n-doping on metal-graphene contact resistance in combination with edge contacts, presenting a record contact resistance of 23 Ω μm at room temp. (19 Ω at 100 K), was investigated. The graphene n-doping was achieved through the charge transfer from a poly(4-vinyl phenol)/poly(melamine-co-formaldehyde) (PVP/PMF) insulator with triazine functional groups which are electron-rich arom. mols. Though n-doping the graphene by 400% PVP/PMF layer, about a 2-fold lower contact resistance value was obtained compared to the 2-dimensional surface contact resistance of pristine graphene (initially, p-type) under the same gate bias. However, after the n-doping, the 2-dimensional contacted metals did not affect the Fermi level of graphene similarly to the previous pristine graphene. Addnl., compared to the 2-dimensional contact sample (RC=1.4 kΩ μm), the authors confirmed about a 3-fold lower contact resistance (RC=484 kΩ) in the edge 3 pattern with 1 μm wide periodic lines/gaps. Finally, the authors applied this contact scheme to a graphene-perovskite hybrid photodetector for performance improvement and also minimized the contact resistance to 23 Ω μm.
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    Contact resistance is one of the main factors limiting performance of short-channel graphene fieldeffect transistors (GFETs), preventing their use in low-voltage applications. Here we investigated the contact resistance between graphene grown by chem. vapor deposition (CVD) and different metals, and found that etching holes in graphene below the contacts consistently reduced the contact resistance, down to 23 ω · μm with Au contacts. This low contact resistance was obtained at the Dirac point of graphene, in contrast to previous studies where the lowest contact resistance was obtained at the highest carrier d. in graphene (here 200 ω · μm was obtained under such conditions). The 'holey' Au contacts were implemented in GFETs which exhibited an av. transconductance of 940 S m-1 at a drain bias of only 0.8 V and gate length of 500 nm, which out-perform GFETs with conventional Au contacts.
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    Cao, Y.; Mishchenko, A.; Yu, G. L.; Khestanova, E.; Rooney, A. P.; Prestat, E.; Kretinin, A. V.; Blake, P.; Shalom, M. B.; Woods, C.; Chapman, J.; Balakrishnan, G.; Grigorieva, I. V.; Novoselov, K. S.; Piot, B. A.; Potemski, M.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K. Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere. Nano Lett. 2015, 15, 4914– 4921,  DOI: 10.1021/acs.nanolett.5b00648
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    Many layered materials can be cleaved down to individual at. planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decomp. in air, which has severely hindered their study and potential applications. Here the authors introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atm. To illustrate the technol., the authors choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. The authors' field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of exptl. accessible two-dimensional crystals and their heterostructures.
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    To pattern elec. metal contacts, electron beam lithog. or photolithog. are commonly utilized, and these processes require polymer resists with solvents. During the patterning process the graphene surface is exposed to chems., and the residue on the graphene surface was unable to be completely removed by any method, causing the graphene layer to be contaminated. A lithog. free method can overcome these residue problems. In this study, we use a micro-grid as a shadow mask to fabricate a graphene based field-effect-transistor (FET). Elec. measurements of the graphene based FET samples are carried out in air and vacuum. It is found that the Dirac peaks of the graphene devices on SiO2 or on hexagonal boron nitride (hBN) shift from a pos. gate voltage region to a neg. region as air pressure decreases. In particular, the Dirac peaks shift very rapidly when the pressure decreases from ∼2 × 10-3 Torr to ∼5 × 10-5 Torr within 5 min. These Dirac peak shifts are known as adsorption and desorption of environmental gases, but the shift amts. are considerably different depending on the fabrication process. The high gas sensitivity of the device fabricated by shadow mask is attributed to adsorption on the clean graphene surface.
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    A modulation-doped Al0.35Ga0.65As/GaAs single interface structure with a 700 Å undoped setback grown by solid-source MBE shows a Hall mobility of 11.7 × 106 cm2/V s at a carrier d. of 2.4 × 1011 electrons/cm2 measured in van der Pauw geometry after exposure to light at 0.35 K. This is the highest carrier mobility ever measured in a semiconductor. Similar Al0.32Ga0.68As/Ga/as structures with 1000-2000 Å setbacks show Hall mobilities in the dark as 0.35 K as high as 4.9 × 106 cm2/V s for carrier densities of 5.4 × 1010 electrons/cm2 and lower.
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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic diagram of a multiplexer with one source input (S) connected to eight output channels. Addressing gates are labeled A1 to A6. In this example channel 1 is selected by applying voltages to several addressing gates. White (yellow) gates indicate a voltage is (is not) applied. The white arrow shows the current path through the multiplexer. Insulated regions depicted by green rectangles below the gates, such as at ‘R’, prevent depletion of the 2DEG when addressing gates are biased. Gray rectangles show the location of nanomaterials at the multiplexer output, one per channel, connected to a common drain output. (b) Equivalent circuit of the top two levels of the multiplexer. Dashed lines indicate switches that are controlled by the same addressing gate. All switches are open in (i), corresponding to addressing voltages applied to all gates. Certain switches are closed in (ii) to address channel 1. (c) Cross section through two branches of the multiplexer, corresponding to the dotted line in (a). For all multiplexers in this study the branches corresponding to ‘R’ are covered by a layer of Al2O3. A thinner upper layer covers both branches for the multiplexed CVD graphene array.

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    Figure 2

    Figure 2. Room-temperature and T = 4.2 K operation. (a and b) Source–drain conductance as a function of addressing gate voltage VA,i, where i is the gate index. Gates A5 (black trace) and A6 (red trace) are swept sequentially as indicated by labels 1 and 2, with VA,i = 0 V for all other gates. The voltage is maintained on A5 while A6 is swept (i.e., the gate is “on”). Each multiplexer output is connected to a single graphene device, all of which share a common drain contact. (c and d) Addressing channel 9 at room temperature and T = 4.2 K, respectively. Gates A1, A4, A6, and A8 are swept sequentially as indicated by labels 1 to 4, and voltages are maintained on each gate after sweeping. Channel 9 addressing is illustrated on the multiplexer in (e), where the white arrow indicates the current path. Gates A1, A4, A6, and A8 deplete the 2DEG at the red bars. The lighter color branching structure indicates the mesa containing the 2DEG.

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    Figure 3

    Figure 3. Multiplexed CVD graphene. (a) Two multiplexer outputs and drain contacts prior to graphene transfer. (b) False-color SEM of a single device. Green, yellow, and blue indicate the back gate, contact electrodes, and graphene, respectively. (c) Cross section through an individual device. (d) Transfer curves for each graphene device at T = 0.28 K. Black lines are fits to the data using eq 1. Channel numbers are given by each panel. Channel 11 did not conduct (G < 0.25 μS at all VG).

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    Figure 4

    Figure 4. (a and b) Device resistance ΔR(VG) = R(VG) – R(8) as a function of VG and magnetic field for channels 13 and 14, respectively. Labels ±2 refer to filling factors ν = ±2, respectively. Arrows highlight the location and gradient of Landau levels −1 and +1.

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    Figure 5

    Figure 5. Multiplexed nanowire arrays. (a) Nanowires are placed at every multiplexer output, above a back gate covered with ≃50 nm thick Al2O3. (b) Top contacts defined by electron-beam lithography connect nanowires to multiplexer output channels and the common drain. (c) Scanning electron micrographs of an example single nanowire and nanowire pair in a multiplexed array. Contact electrodes are shown in false color (yellow). (d) Differential conductance as a function of back gate voltage for L = 401 nm (left column) and L = 606 nm (right column) nanowires, before subtraction of series and contact resistance. Red and blue arrows indicate the direction of the gate voltage sweep.

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    Figure 6

    Figure 6. Multiplexed arrays of exfoliated graphene. (a) Exfoliated graphene device connected to channel 1. Source–drain electrodes connect to the multiplexer output and common drain. The device active area is highlighted in blue (false color), for clarity. (b) Graphene flakes are connected to eight multiplexer channels, indicated by the arrows. (c) Resistance as a function of back gate voltage (VG) for channels 6, 7, 11, 12, 15, and 16 (left-to-right); devices on channels 1 and 3 do not conduct. Arrows indicate the gate voltage sweep direction.

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    This article references 85 other publications.

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      Spin relaxation in InAs nanowires studied by tunable weak antilocalization
      Hansen, A. E.; Bjork, M. T.; Fasth, C.; Thelander, C.; Samuelson, L.
      Physical Review B: Condensed Matter and Materials Physics (2005), 71 (20), 205328/1-205328/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We report on a low-temp. magnetoconductance study to characterize the elec. and spin transport properties of n-type InAs nanowires grown by chem. beam epitaxy. A gate-controlled crossover from weak localization to weak antilocalization is obsd. The measured magnetoconductance data agrees well with theory for one-dimensional quasi-ballistic systems and yields a spin relaxation length which decreases with increasing gate voltage.
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    20. 20
      Lehnen, P.; SchäPers, T.; Kaluza, N.; Thillosen, N.; Hardtdegen, H. Enhanced Spin-Orbit Scattering Length in Narrow AlxGa1–xN/GaN Wires. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 205307,  DOI: 10.1103/PhysRevB.76.205307
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      Enhanced spin-orbit scattering length in narrow AlxGa1-xN/GaN wires
      Lehnen, Patrick; Schapers, Thomas; Kaluza, Nicoleta; Thillosen, Nicolas; Hardtdegen, Hilde
      Physical Review B: Condensed Matter and Materials Physics (2007), 76 (20), 205307/1-205307/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The magnetotransport in a set of identical parallel AlxGa1-xN/GaN quantum wire structures was studied. The width of the wires ranges between 1110 and 340 nm. For all sets of wires, clear Shubnikov-de Haas oscillations are obsd. The electron concn. and mobility are approx. the same for all wires, confirming that the electron gas in the AlxGa1-xN/GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires, the weak antilocalization effect is clearly obsd., indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective elec. width <250 nm, the weak antilocalization effect is suppressed. By comparing the exptl. data to a theor. model for quasi-one-dimensional structures, the authors come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.
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      Smith, L. W.; Al-Taie, H.; Sfigakis, F.; See, P.; Lesage, A. A. J.; Xu, B.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Statistical Study of Conductance Properties in One-Dimensional Quantum Wires Focusing on the 0.7 Anomaly. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 045426,  DOI: 10.1103/PhysRevB.90.045426
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      Statistical study of conductance properties in one-dimensional quantum wires focusing on the 0.7 anomaly
      Smith, L. W.; Al-Taie, H.; Sfigakis, F.; See, P.; Lesage, A. A. J.; Xu, B.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
      Physical Review B: Condensed Matter and Materials Physics (2014), 90 (4), 045426CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The properties of conductance in one-dimensional (1D) quantum wires are statistically investigated using an array of 256 lithog. identical split gates, fabricated on a GaAs/AlGaAs heterostructure. All the split gates are measured during a single cooldown under the same conditions. Electron many-body effects give rise to an anomalous feature in the conductance of a one-dimensional quantum wire, known as the "0.7 structure" (or "0.7 anomaly"). To handle the large data set, a method of automatically estg. the conductance value of the 0.7 structure is developed. Large differences are obsd. in the strength and value of the 0.7 structure [from 0.63 to 0.84 × (2e2/h)], despite the const. temp. and identical device design. Variations in the 1D potential profile are quantified by estg. the curvature of the barrier in the direction of electron transport, following a saddle-point model. The 0.7 structure appears to be highly sensitive to the specific confining potential within individual devices.
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      Al-Taie, H.; Smith, L. W.; Lesage, A. A. J.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Spatial Mapping and Statistical Reproducibility of an Array of 256 One-Dimensional Quantum Wires. J. Appl. Phys. 2015, 118, 075703,  DOI: 10.1063/1.4928615
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      Spatial mapping and statistical reproducibility of an array of 256 one-dimensional quantum wires
      Al-Taie, H.; Smith, L. W.; Lesage, A. A. J.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
      Journal of Applied Physics (Melville, NY, United States) (2015), 118 (7), 075703/1-075703/5CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We utilize a multiplexing architecture to measure the conductance properties of an array of 256 split gates. We investigate the reproducibility of the pinch off and one-dimensional definition voltage as a function of spatial location on two different cooldowns, and after illuminating the device. The reproducibility of both these properties on the two cooldowns is high, the result of the d. of the two-dimensional electron gas returning to a similar state after thermal cycling. The spatial variation of the pinch-off voltage reduces after illumination; however, the variation of the one-dimensional definition voltage increases due to an anomalous feature in the center of the array. A technique which quantifies the homogeneity of split-gate properties across the array is developed which captures the exptl. obsd. trends. In addn., the one-dimensional definition voltage is used to probe the d. of the wafer at each split gate in the array on a micron scale using a capacitive model. (c) 2015 American Institute of Physics.
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      Lesage, A. A. J.; Smith, L. W.; Al-Taie, H.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Assisted Extraction of the Energy Level Spacings and Lever Arms in Direct Current Bias Measurements of One-Dimensional Quantum Wires, Using an Image Recognition Routine. J. Appl. Phys. 2015, 117, 015704,  DOI: 10.1063/1.4905484
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      Assisted extraction of the energy level spacings and lever arms in direct current bias measurements of one-dimensional quantum wires, using an image recognition routine
      Lesage, A. A. J.; Smith, L. W.; Al-Taie, H.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
      Journal of Applied Physics (Melville, NY, United States) (2015), 117 (1), 015704/1-015704/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      A multiplexer technique is used to individually measure an array of 256 split gates on a single GaAs/AlGaAs heterostructure. This gave large vols. of data, which requires the development of automated data anal. routines. An algorithm is developed to find the spacing between discrete energy levels, which form due to transverse confinement from the split gate. The lever arm, which relates split gate voltage to energy, is also found from the measured data. This reduces the time spent on the anal. Comparison with ests. obtained visually shows that the algorithm returns reliable results for subband spacing of split gates measured at 1.4 K. The routine is also used to assess d.c. bias spectroscopy measurements at lower temps. (50 mK). This technique is versatile and can be extended to other types of measurements. For example, it is used to ext. the magnetic field at which Zeeman-split 1D subbands cross 1 another. (c) 2015 American Institute of Physics.
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      Puddy, R. K.; Smith, L. W.; Al-Taie, H.; Chong, C. H.; Farrer, I.; Griffiths, J. P.; Ritchie, D. A.; Kelly, M. J.; Pepper, M.; Smith, C. G. Multiplexed Charge-Locking Device for Large Arrays of Quantum Devices. Appl. Phys. Lett. 2015, 107, 143501,  DOI: 10.1063/1.4932012
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      Multiplexed charge-locking device for large arrays of quantum devices
      Puddy, R. K.; Smith, L. W.; Al-Taie, H.; Chong, C. H.; Farrer, I.; Griffiths, J. P.; Ritchie, D. A.; Kelly, M. J.; Pepper, M.; Smith, C. G.
      Applied Physics Letters (2015), 107 (14), 143501/1-143501/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses an on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the no. of wires available in cryostat measurement systems. The device architecture that we describe here utilizes a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total no. exceeds that of the available elec. contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate such devices on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approx. one Coulomb oscillation per h. (c) 2015 American Institute of Physics.
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    25. 25
      Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Hamilton, A. R.; Kelly, M. J.; Smith, C. G. Dependence of the 0.7 Anomaly on the Curvature of the Potential Barrier in Quantum Wires. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 235402,  DOI: 10.1103/PhysRevB.91.235402
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      Dependence of the 0.7 anomaly on the curvature of the potential barrier in quantum wires
      Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Hamilton, A. R.; Kelly, M. J.; Smith, C. G.
      Physical Review B: Condensed Matter and Materials Physics (2015), 91 (23), 235402/1-235402/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Ninety-eight one-dimensional channels defined using split gates fabricated on a GaAs/AlGaAs heterostructure are measured during one cooldown at 1.4 K. The devices are arranged in an array on a single chip and are individually addressed using a multiplexing technique. The anomalous conductance feature known as the "0.7 structure" is studied using statistical techniques. The ensemble of data shows that the 0.7 anomaly becomes more pronounced and occurs at lower values as the curvature of the potential barrier in the transport direction decreases. This corresponds to an increase in the effective length of the device. The 0.7 anomaly is not strongly influenced by other properties of the conductance related to d. The curvature of the potential barrier appears to be the primary factor governing the shape of the 0.7 structure at a given T and B.
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      Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Effect of Split Gate Size on the Electrostatic Potential and 0.7 Anomaly within Quantum Wires on a Modulation-Doped GaAs/AlGaAs Heterostructure. Phys. Rev. Appl. 2016, 5, 044015,  DOI: 10.1103/PhysRevApplied.5.044015
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      Effect of split gate size on the electrostatic potential and 0.7 anomaly within quantum wires on a modulation-doped GaAs/AlGaAs heterostructure
      Smith, L. W.; Al-Taie, H.; Lesage, A. A. J.; Thomas, K. J.; Sfigakis, F.; See, P.; Griffiths, J. P.; Farrer, I.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
      Physical Review Applied (2016), 5 (4), 044015/1-044015/10CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
      We study 95 split gates of different size on a single chip using a multiplexing technique. Each split gate defines a one-dimensional channel on a modulation-doped GaAs/AlGaAs heterostructure, through which the conductance is quantized. The yield of devices showing good quantization decreases rapidly as the length of the split gates increases. However, for the subset of devices showing good quantization, there is no correlation between the electrostatic length of the one-dimensional channel (estd. using a saddle-point model) and the gate length. The variation in electrostatic length and the one-dimensional subband spacing for devices of the same gate length exceeds the variation in the av. values between devices of different lengths. There is a clear correlation between the curvature of the potential barrier in the transport direction and the strength of the "0.7 anomaly": the conductance value of the 0.7 anomaly reduces as the barrier curvature becomes shallower. These results highlight the key role of the electrostatic environment in one dimensional systems. Even in devices with clean conductance plateaus, random fluctuations in the background potential are crucial in detg. the potential landscape in the active device area such that nominally identical gate structures have different characteristics.
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    27. 27
      Al-Taie, H.; Smith, L. W.; Xu, B.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G. Cryogenic on-Chip Multiplexer for the Study of Quantum Transport in 256 Split-Gate Devices. Appl. Phys. Lett. 2013, 102, 243102,  DOI: 10.1063/1.4811376
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      Cryogenic on-chip multiplexer for the study of quantum transport in 256 split-gate devices
      Al-Taie, H.; Smith, L. W.; Xu, B.; See, P.; Griffiths, J. P.; Beere, H. E.; Jones, G. A. C.; Ritchie, D. A.; Kelly, M. J.; Smith, C. G.
      Applied Physics Letters (2013), 102 (24), 243102/1-243102/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The authors present a multiplexing scheme for the measurement of large nos. of mesoscopic devices in cryogenic systems. The multiplexer was used to contact an array of 256 split gates on a GaAs/AlGaAs heterostructure, in which each split gate can be measured individually. The low-temp. conductance of split-gate devices is governed by quantum mechanics, leading to the appearance of conductance plateaus at intervals of 2e2/h. A fabrication-limited yield of 94% is achieved for the array, and a quantum yield is also defined, to account for disorder affecting the quantum behavior of the devices. The quantum yield rose from 55% to 86% after illuminating the sample, explained by the corresponding increase in carrier d. and mobility of the two-dimensional electron gas. The multiplexer is a scalable architecture, and can be extended to other forms of mesoscopic devices. It overcomes previous limits on the no. of devices that can be fabricated on a single chip due to the no. of elec. contacts available, without the need to alter existing exptl. set ups. (c) 2013 American Institute of Physics.
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      Suk, J. W.; Kitt, A.; Magnuson, C. W.; Hao, Y.; Ahmed, S.; An, J.; Swan, A. K.; Goldberg, B. B.; Ruoff, R. S. Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates. ACS Nano 2011, 5, 6916– 6924,  DOI: 10.1021/nn201207c
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      28
      Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates
      Suk, Ji Won; Kitt, Alexander; Magnuson, Carl W.; Hao, Yufeng; Ahmed, Samir; An, Jinho; Swan, Anna K.; Goldberg, Bennett B.; Ruoff, Rodney S.
      ACS Nano (2011), 5 (9), 6916-6924CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Reproducible dry and wet transfer techniques were developed to improve the transfer of large-area monolayer graphene grown on copper foils by CVD. The techniques reported here allow transfer onto three different classes of substrates: substrates covered with shallow depressions, perforated substrates, and flat substrates. A novel dry transfer technique was used to make graphene-sealed microchambers without trapping liq. inside. The dry transfer technique uses a polydimethylsiloxane frame that attaches to the poly(Me methacrylate) spun over the graphene film, and the monolayer graphene was transferred onto shallow depressions with 300 nm depth. The improved wet transfer onto perforated substrates with 2.7 μm diam. holes yields 98% coverage of holes covered with continuous films, allowing the ready use of Raman spectroscopy and TEM to study the intrinsic properties of CVD-grown monolayer graphene. Addnl., monolayer graphene transferred onto flat substrates has fewer cracks and tears, as well as lower sheet resistance than previous transfer techniques. Monolayer graphene films transferred onto glass had a sheet resistance of ∼980 Ω/sq and a transmittance of 97.6%. These transfer techniques open up possibilities for the fabrication of various graphene devices with unique configurations and enhanced performance.
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      Graphene Field-Effect Transistor Chip: S10; Graphenea, 2020; Datasheet 05–25–2020.
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      Mzali, S.; Montanaro, A.; Xavier, S.; Servet, B.; Mazellier, J.-P.; Bezencenet, O.; Legagneux, P.; Piquemal-Banci, M.; Galceran, R.; Dlubak, B.; Seneor, P.; Martin, M.-B.; Hofmann, S.; Robertson, J.; Cojocaru, C.-S.; Centeno, A.; Zurutuza, A. Stabilizing a Graphene Platform toward Discrete Components. Appl. Phys. Lett. 2016, 109, 253110,  DOI: 10.1063/1.4972847
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      Stabilizing a graphene platform toward discrete components
      Mzali, Sana; Montanaro, Alberto; Xavier, Stephane; Servet, Bernard; Mazellier, Jean-Paul; Bezencenet, Odile; Legagneux, Pierre; Piquemal-Banci, Maelis; Galceran, Regina; Dlubak, Bruno; Seneor, Pierre; Martin, Marie-Blandine; Hofmann, Stephan; Robertson, John; Cojocaru, Costel-Sorin; Centeno, Alba; Zurutuza, Amaia
      Applied Physics Letters (2016), 109 (25), 253110/1-253110/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We report on statistical anal. and consistency of elec. performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chem. vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithog. process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance min. related to the Dirac point. While no conductance min. was obsd. in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance min. and low hysteresis. The max. of the device no. distribution corresponds to a residual doping below 5 × 1011 cm-2 (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhibiting small hysteresis in the transfer characteristics can be envisaged for discrete components, with even further potential for low power driven electronics. (c) 2016 American Institute of Physics.
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      Kim, S.; Nah, J.; Jo, I.; Shahrjerdi, D.; Colombo, L.; Yao, Z.; Tutuc, E.; Banerjee, S. K. Realization of a High Mobility Dual-Gated Graphene Field-Effect Transistor with Al2O3 Dielectric. Appl. Phys. Lett. 2009, 94, 062107,  DOI: 10.1063/1.3077021
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      Realization of a high mobility dual-gated graphene field-effect transistor with Al2O3 dielectric
      Kim, Seyoung; Nah, Junghyo; Jo, Insun; Shahrjerdi, Davood; Colombo, Luigi; Yao, Zhen; Tutuc, Emanuel; Banerjee, Sanjay K.
      Applied Physics Letters (2009), 94 (6), 062107/1-062107/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The authors fabricate and characterize dual-gated graphene field-effect transistors using Al2O3 as top-gate dielec. The authors use a thin Al film as a nucleation layer to enable the at. layer deposition of Al2O3. The authors' devices show mobility values of over 8000 cm2/V s at room temp., a finding which indicates that the Top-gate stack does not significantly increase the carrier scattering and consequently degrade the device characteristics. The authors propose a device model to fit the exptl. data using a single mobility value. (c) 2009 American Institute of Physics.
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      Zhong, H.; Zhang, Z.; Xu, H.; Qiu, C.; Peng, L.-M. Comparison of Mobility Extraction Methods Based on Field-Effect Measurements for Graphene. AIP Adv. 2015, 5, 057136,  DOI: 10.1063/1.4921400
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      Comparison of mobility extraction methods based on field-effect measurements for graphene
      Zhong, Hua; Zhang, Zhiyong; Xu, Haitao; Qiu, Chenguang; Peng, Lian-Mao
      AIP Advances (2015), 5 (5), 057136/1-057136/8CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
      Carrier mobility extn. methods for graphene based on field-effect measurements are explored and compared according to theor. anal. and exptl. results. A group of graphene devices with different channel lengths were fabricated and measured, and carrier mobility is extd. from those elec. transfer curves using three different methods. Accuracy and applicability of those methods were compared. Transfer length method (TLM) can obtain accurate d. dependent mobility and contact resistance at relative high carrier d. based on data from a group of devices, and then can act as a std. method to verify other methods. As two of the most popular methods, direct transconductance method (DTM) and fitting method (FTM) can ext. mobility easily based on transfer curve of a sole graphene device. DTM offers an underestimated mobility at any carrier d. owing to the neglect of contact resistances, and the accuracy can be improved through fabricating field-effect transistors with long channel and good contacts. FTM assumes a const. mobility independent on carrier d., and then can obtain mobility, contact resistance and residual d. stimulations through fitting a transfer curve. However, FTM tends to obtain a mobility value near Dirac point and then overestimates carrier mobility of graphene. Comparing with the DTM and FTM, TLM could offer a much more accurate and carrier d. dependent mobility, that reflects the complete properties of graphene carrier mobility. (c) 2015 American Institute of Physics.
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      Li, H.; Wu, J.; Huang, X.; Lu, G.; Yang, J.; Lu, X.; Xiong, Q.; Zhang, H. Rapid and Reliable Thickness Identification of Two-Dimensional Nanosheets Using Optical Microscopy. ACS Nano 2013, 7, 10344– 10353,  DOI: 10.1021/nn4047474
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      Rapid and Reliable Thickness Identification of Two-Dimensional Nanosheets Using Optical Microscopy
      Li, Hai; Wu, Jumiati; Huang, Xiao; Lu, Gang; Yang, Jian; Lu, Xin; Xiong, Qihua; Zhang, Hua
      ACS Nano (2013), 7 (11), 10344-10353CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The phys. and electronic properties of ultrathin 2-dimensional (2D) layered nanomaterials are highly related to their thickness. The rapid and accurate identification of single- and few- to multilayer nanosheets is essential to their fundamental study and practical applications. Here, a universal optical method was developed for simple, rapid, and reliable identification of single- to quindecuple-layer (1L-15L) 2D nanosheets, including graphene, MoS2, WSe2, and TaS2, on Si substrates coated with 90 or 300 nm SiO2. The optical contrast differences between the substrates and 2D nanosheets with different layer nos. were collected and tabulated, serving as a std. ref., from which the layer no. of a given nanosheet can be readily and reliably detd. without using complex calcn. or expensive instrument. The general optical identification method will facilitate the thickness-dependent study of various 2D nanomaterials and expedite their research toward practical applications.
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      Smith, J. T.; Franklin, A. D.; Farmer, D. B.; Dimitrakopoulos, C. D. Reducing Contact Resistance in Graphene Devices through Contact Area Patterning. ACS Nano 2013, 7, 3661– 3667,  DOI: 10.1021/nn400671z
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      Reducing contact resistance in graphene devices through contact area patterning
      Smith, Joshua T.; Franklin, Aaron D.; Farmer, Damon B.; Dimitrakopoulos, Christos D.
      ACS Nano (2013), 7 (4), 3661-3667CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Performance of graphene electronics is limited by contact resistance assocd. with the metal-graphene (M-G) interface, where unique transport challenges arise as carriers are injected from a 3-dimensional metal into a 2-dimensional graphene sheet. In this work, enhanced carrier injection is exptl. achieved in graphene devices by forming cuts in the graphene within the contact regions. These cuts are oriented normal to the channel and facilitate bonding between the contact metal and carbon atoms at the graphene cut edges, reproducibly maximizing "edge-contacted" injection. Despite the redn. in M-G contact area caused by these cuts, the authors find that a 32% redn. in contact resistance results in Cu-contacted, 2-terminal devices, while a 22% redn. is achieved for top-gated graphene transistors with Pd contacts as compared to conventionally fabricated devices. The crucial role of contact annealing to facilitate this improvement is also elucidated. This simple approach provides a reliable and reproducible means of lowering contact resistance in graphene devices to bolster performance. Importantly, this enhancement requires no addnl. processing steps.
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    35. 35
      Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R. One-Dimensional Electrical Contact to a Two-Dimensional Material. Science 2013, 342, 614,  DOI: 10.1126/science.1244358
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      One-Dimensional Electrical Contact to a Two-Dimensional Material
      Wang, L.; Meric, I.; Huang, P. Y.; Gao, Q.; Gao, Y.; Tran, H.; Taniguchi, T.; Watanabe, K.; Campos, L. M.; Muller, D. A.; Guo, J.; Kim, P.; Hone, J.; Shepard, K. L.; Dean, C. R.
      Science (Washington, DC, United States) (2013), 342 (6158), 614-617CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal B nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality elec. contact. Here, the authors report a contact geometry in which the authors metalize only the 1-dimensional edge of a 2-dimensional graphene layer. In addn. to outperforming conventional surface contacts, the edge-contact geometry allows a complete sepn. of the layer assembly and contact metalization processes. In graphene heterostructures, this enables high electronic performance, including low-temp. ballistic transport over distances longer than 15 μm, and room-temp. mobility comparable to the theor. phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2-dimensional materials.
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      Choi, M. S.; Lee, S. H.; Yoo, W. J. Plasma Treatments to Improve Metal Contacts in Graphene Field Effect Transistor. J. Appl. Phys. 2011, 110, 073305,  DOI: 10.1063/1.3646506
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      Plasma treatments to improve metal contacts in graphene field effect transistor
      Choi, Min Sup; Lee, Seung Hwan; Yoo, Won Jong
      Journal of Applied Physics (2011), 110 (7), 073305/1-073305/6CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Graphene formed via chem. vapor deposition was exposed to various plasmas (Ar, O2, N2, and H2) in order to examine its effects on the bonding properties of graphene to metal. After exposing patterned graphene to Ar plasma, the subsequently deposited metal electrodes remained intact, enabling the successful fabrication of field effect transistor arrays. The effects of the enhanced adhesion between graphene and metals were more evident from the O2 plasma than the Ar, N2, and H2 plasmas, suggesting that a chem. reaction of O radicals imparts hydrophilic properties to graphene more effectively than the chem. reaction of H and N radicals or the phys. bombardment of Ar ions. The elec. measurements (drain current vs. gate voltage) of the field effect transistors before and after Ar plasma exposure confirmed that the plasma treatment is quite effective in controlling the graphene to metal bonding accurately without the need for buffer layers. (c) 2011 American Institute of Physics.
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      Robinson, J. A.; LaBella, M.; Zhu, M.; Hollander, M.; Kasarda, R.; Hughes, Z.; Trumbull, K.; Cavalero, R.; Snyder, D. Contacting Graphene. Appl. Phys. Lett. 2011, 98, 053103,  DOI: 10.1063/1.3549183
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      Contacting graphene
      Robinson, Joshua A.; LaBella, Michael; Zhu, Mike; Hollander, Matt; Kasarda, Richard; Hughes, Zachary; Trumbull, Kathleen; Cavalero, Randal; Snyder, David
      Applied Physics Letters (2011), 98 (5), 053103/1-053103/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We present a robust method for forming high quality ohmic contacts to graphene, which improves the contact resistance by nearly 6000 times compared to untreated metal/graphene interfaces. The optimal specific contact resistance for treated Ti/Au contacts is found to av. <10-7 Ω cm2. Addnl., we examine Al/Au, Ti/Au, Ni/Au, Cu/Au, Pt/Au, and Pd/Au contact metalizations and find that most metalizations result in similar specific contact resistances in this work regardless of the work function difference between graphene and the metal overlayer. The results presented in this work serve as a foundation for achieving ultralow resistance ohmic contacts to graphene for high speed electronic and optoelectronic applications. (c) 2011 American Institute of Physics.
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      Wei Chen, C.; Ren, F.; Chi, G.-C.; Hung, S.-C.; Huang, Y. P.; Kim, J.; Kravchenko, I. I.; Pearton, S. J. UV Ozone Treatment for Improving Contact Resistance on Graphene. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2012, 30, 060604,  DOI: 10.1116/1.4754566
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      Malec, C. E.; Elkus, B.; Davidović, D. Vacuum-Annealed Cu Contacts for Graphene Electronics. Solid State Commun. 2011, 151, 1791– 1793,  DOI: 10.1016/j.ssc.2011.08.025
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      Vacuum-annealed Cu contacts for graphene electronics
      Malec, C. E.; Elkus, B.; Davidovic, D.
      Solid State Communications (2011), 151 (23), 1791-1793CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
      We present transfer length method measurements of the contact resistance between Cu and graphene, and a method to significantly reduce the contact resistance by vacuum annealing. Even in samples with heavily contaminated contacts, the contacts display very low contact resistance post annealing. Due to the common use of Cu, and its low chem. reactivity with graphene, thermal annealing will be important for future graphene devices requiring non-perturbing contacts with low contact resistance.
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      Balci, O.; Kocabas, C. Rapid Thermal Annealing of Graphene-Metal Contact. Appl. Phys. Lett. 2012, 101, 243105,  DOI: 10.1063/1.4769817
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      Rapid thermal annealing of graphene-metal contact
      Balci, Osman; Kocabas, Coskun
      Applied Physics Letters (2012), 101 (24), 243105/1-243105/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      High quality graphene-metal contacts are desirable for high-performance graphene based electronics. Process related factors result large variation in the contact resistance. A post-processing method is needed to improve graphene-metal contacts. We studied rapid thermal annealing (RTA) of graphene-metal contacts. We present results of a systematic investigation of device scaling before and after RTA for various metals. RTA provides a convenient technique to reduce contact resistance, thus to obtain reproducible device operation. (c) 2012 American Institute of Physics.
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      Huang, B.-C.; Zhang, M.; Wang, Y.; Woo, J. Contact Resistance in Top-Gated Graphene Field-Effect Transistors. Appl. Phys. Lett. 2011, 99, 032107,  DOI: 10.1063/1.3614474
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      Contact resistance in top-gated graphene field-effect transistors
      Huang, Bo-Chao; Zhang, Ming; Wang, Yanjie; Woo, Jason
      Applied Physics Letters (2011), 99 (3), 032107/1-032107/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The parasitic resistance of different source/drain metals for top-gated graphene field-effect transistors was extd. by fitting the measured ID-VG data with a resistance model and was found to be a significant part of the total resistance of graphene field-effect transistors. The results show that Ti/Au gives relatively large contact resistance, about 7500 Ω·μm. Ni/Au contact shows better result compared to Ti/Au, which is around 2100 Ω·μm. The lowest contact resistance was given by Ti/Pd/Au, which is around 750 Ω·μm. The contact resistivity for Ti/Pd/Au source/drain contact is around 2 × 10-6 Ω·cm2, close to state of the art GaAs technol. (c) 2011 American Institute of Physics.
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      Jauregui, L. A.; Cao, H.; Wu, W.; Yu, Q.; Chen, Y. P. Electronic Properties of Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition. Solid State Commun. 2011, 151, 1100– 1104,  DOI: 10.1016/j.ssc.2011.05.023
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      Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition
      Jauregui, Luis A.; Cao, Helin; Wu, Wei; Yu, Qingkai; Chen, Yong P.
      Solid State Communications (2011), 151 (16), 1100-1104CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Ltd.)
      We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycryst. Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede elec. transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.
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      Van Veldhoven, Z. A.; Alexander-Webber, J. A.; Sagade, A. A.; Braeuninger-Weimer, P.; Hofmann, S. Electronic Properties of CVD Graphene: The Role of Grain Boundaries, Atmospheric Doping, and Encapsulation by ALD. Phys. Status Solidi B 2016, 253, 2321– 2325,  DOI: 10.1002/pssb.201600255
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      Electronic properties of CVD graphene: The role of grain boundaries, atmospheric doping, and encapsulation by ALD
      Van Veldhoven, Zenas A.; Alexander-Webber, Jack A.; Sagade, Abhay A.; Braeuninger-Weimer, Philipp; Hofmann, Stephan
      Physica Status Solidi B: Basic Solid State Physics (2016), 253 (12), 2321-2325CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Grain boundaries and unintentional doping can have profound effects on graphene-based devices. Here we study these in detail for CVD grown poly-cryst. monolayer graphene with two significantly different grain size distributions centered around 10-25 μm and 100-400 μm. Although the two types of graphene are processed under identical conditions after growth, they show distinct transport properties in field effect transistor devices. While all as-fabricated samples showed similar p-type doping, the smaller grain size type graphene with larger no. of grain boundaries exhibit lower av. mobility. In order to sep. out the effects of grain boundaries and doping from ambient exposure on the transport properties, the devices were encapsulated with Al2O3 by at. layer deposition. The encapsulation of large grain samples thereby showed drastic improvements in the performance with negligible doping while the small grain samples are largely intolerant to this process. We discuss the implications of our data for the integrated manufg. of graphene-based device platforms.
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      De Fazio, D.; Purdie, D. G.; Ott, A. K.; Braeuninger-Weimer, P.; Khodkov, T.; Goossens, S.; Taniguchi, T.; Watanabe, K.; Livreri, P.; Koppens, F. H. L.; Hofmann, S.; Goykhman, I.; Ferrari, A. C.; Lombardo, A. High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition. ACS Nano 2019, 13, 8926– 8935,  DOI: 10.1021/acsnano.9b02621
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      High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition
      De Fazio, Domenico; Purdie, David G.; Ott, Anna K.; Braeuninger-Weimer, Philipp; Khodkov, Timofiy; Goossens, Stijn; Taniguchi, Takashi; Watanabe, Kenji; Livreri, Patrizia; Koppens, Frank H. L.; Hofmann, Stephan; Goykhman, Ilya; Ferrari, Andrea C.; Lombardo, Antonio
      ACS Nano (2019), 13 (8), 8926-8935CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report high room-temp. mobility in single-layer graphene grown by chem. vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temp. and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepd. by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycryst. graphene, achieving room-temp. mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temp. mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.
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      Alexander-Webber, J. A.; Groschner, C. K.; Sagade, A. A.; Tainter, G.; Gonzalez-Zalba, M. F.; Di Pietro, R.; Wong-Leung, J.; Tan, H. H.; Jagadish, C.; Hofmann, S.; Joyce, H. J. Engineering the Photoresponse of InAs Nanowires. ACS Appl. Mater. Interfaces 2017, 9, 43993– 44000,  DOI: 10.1021/acsami.7b14415
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      Engineering the Photoresponse of InAs Nanowires
      Alexander-Webber, Jack A.; Groschner, Catherine K.; Sagade, Abhay A.; Tainter, Gregory; Gonzalez-Zalba, M. Fernando; Di Pietro, Riccardo; Wong-Leung, Jennifer; Tan, H. Hoe; Jagadish, Chennupati; Hofmann, Stephan; Joyce, Hannah J.
      ACS Applied Materials & Interfaces (2017), 9 (50), 43993-44000CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either neg. or pos. photocond. (NPC or PPC) are reported. The NPC photoresponse time and magnitude is highly tunable by varying the nanowire diam. under controlled growth conditions. Using hysteresis characterization, the obsd. photoexcitation-induced hot electron trapping was decoupled from conventional elec. field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Surface passivation without chem. etching was demonstrated which both enhances the field-effect mobility of the nanowires by approx. an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an intrinsic pos. photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications.
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      Alexander-Webber, J. A.; Sagade, A. A.; Aria, A. I.; Van Veldhoven, Z. A.; Braeuninger-Weimer, P.; Wang, R.; Cabrero-Vilatela, A.; Martin, M.-B.; Sui, J.; Connolly, M. R.; Hofmann, S. Encapsulation of Graphene Transistors and Vertical Device Integration by Interface Engineering with Atomic Layer Deposited Oxide. 2D Mater. 2017, 4, 011008,  DOI: 10.1088/2053-1583/4/1/011008
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      Encapsulation of graphene transistors and vertical device integration by interface engineering with atomic layer deposited oxide
      Alexander-Webber, Jack A.; Sagade, Abhay A.; Aria, Adrianus I.; Van Veldhoven, Zenas A.; Braeuninger-Weimer, Philipp; Wang, Ruizhi; Cabrero-Vilatela, Andrea; Martin, Marie-Blandine; Sui, Jinggao; Connolly, Malcolm R.; Hofmann, Stephan
      2D Materials (2017), 4 (1), 011008/1-011008/9CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      We demonstrate a simple, scalable approach to achieve encapsulated graphene transistors with negligible gate hysteresis, low doping levels and enhanced mobility compared to as-fabricated devices. We engineer the interface between graphene and at. layer deposited (ALD) Al2O3 by tailoring the growth parameters to achieve effective device encapsulation while enabling the passivation of charge traps in the underlying gate dielec. We relate the passivation of charge trap states in the vicinity of the graphene to conformal growth of ALD oxide governed by in situ gaseous H2O pretreatments. We demonstrate the long term stability of such encapsulation techniques and the resulting insensitivity towards addnl. lithog. steps to enable vertical device integration of graphene for multi-stacked electronics fabrication.
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      Mayorov, A. S.; Gorbachev, R. V.; Morozov, S. V.; Britnell, L.; Jalil, R.; Ponomarenko, L. A.; Blake, P.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K. Micrometer-Scale Ballistic Transport in Encapsulated Graphene at Room Temperature. Nano Lett. 2011, 11, 2396– 2399,  DOI: 10.1021/nl200758b
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      Micrometer-scale ballistic transport in encapsulated graphene at room temperature
      Mayorov, Alexander S.; Gorbachev, Roman V.; Morozov, Sergey V.; Britnell, Liam; Jalil, Rashid; Ponomarenko, Leonid A.; Blake, Peter; Novoselov, Kostya S.; Watanabe, Kenji; Taniguchi, Takashi; Geim, A. K.
      Nano Letters (2011), 11 (6), 2396-2399CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Devices made from graphene encapsulated in hexagonal BN exhibit pronounced neg. bend resistance and an anomalous Hall effect, which are a direct consequence of room-temp. ballistic transport at a micrometer scale for a wide range of carrier concns. The encapsulation makes graphene practically insusceptible to the ambient atm. and, simultaneously, allows the use of BN as an ultrathin top gate dielec.
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      Robinson, J. P.; Schomerus, H.; Oroszlány, L.; Fal’ko, V. I. Adsorbate-Limited Conductivity of Graphene. Phys. Rev. Lett. 2008, 101, 196803,  DOI: 10.1103/PhysRevLett.101.196803
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      Adsorbate-limited conductivity of graphene
      Robinson, John P.; Schomerus, Henning; Oroszlany, Laszlo; Fal'ko, Vladimir I.
      Physical Review Letters (2008), 101 (19), 196803/1-196803/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We present a theory of electronic transport in graphene in the presence of randomly placed adsorbates. Our anal. predicts a marked asymmetry of the cond. about the Dirac point, as well as a neg. weak-localization magnetoresistivity. In the region of strong scattering, renormalization group corrections drive the system further towards insulating behavior. These results explain key features of recent expts., and are validated by numerical transport computations.
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      Farmer, D. B.; Golizadeh-Mojarad, R.; Perebeinos, V.; Lin, Y.-M.; Tulevski, G. S.; Tsang, J. C.; Avouris, P. Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices. Nano Lett. 2009, 9, 388– 392,  DOI: 10.1021/nl803214a
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      Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices
      Farmer, Damon B.; Golizadeh-Mojarad, Roksana; Perebeinos, Vasili; Lin, Yu-Ming; Tulevski, George S.; Tsang, James C.; Avouris, Phaedon
      Nano Letters (2009), 9 (1), 388-392CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors study poly(ethylene imine) and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these mols. exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequil. Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.
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      Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; van den Brink, J.; Kelly, P. J. Doping Graphene with Metal Contacts. Phys. Rev. Lett. 2008, 101, 026803,  DOI: 10.1103/PhysRevLett.101.026803
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      Doping graphene with metal contacts
      Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; van den Brink, J.; Kelly, P. J.
      Physical Review Letters (2008), 101 (2), 026803/1-026803/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Making devices with graphene necessarily involves making contacts with metals. We use d. functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au, and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by ∼0.5 eV. At equil. sepns., the crossover from p-type to n-type doping occurs for a metal work function of ∼5.4 eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple anal. model which characterizes the metal solely in terms of its work function, greatly extending their applicability.
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      Huard, B.; Stander, N.; Sulpizio, J. A.; Goldhaber-Gordon, D. Evidence of the Role of Contacts on the Observed Electron-Hole Asymmetry in Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 121402,  DOI: 10.1103/PhysRevB.78.121402
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      Evidence of the role of contacts on the observed electron-hole asymmetry in graphene
      Huard, B.; Stander, N.; Sulpizio, J. A.; Goldhaber-Gordon, D.
      Physical Review B: Condensed Matter and Materials Physics (2008), 78 (12), 121402/1-121402/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We perform elec. transport measurements in graphene with several sample geometries. In particular, we design "invasive" probes crossing the whole graphene sheet as well as "external" probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge d. and is sym. between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sublinear conductance as a function of d. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge d. below the metal, which thereby forms a p-n or p-p junction, depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sub-linearity obsd. in conductance as a function of d. in a large no. of expts. on graphene, which has generally been attributed to short-range scattering only.
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      Liu, W.; Wei, J.; Sun, X.; Yu, H. A Study on Graphene—Metal Contact. Crystals 2013, 3, 257– 274,  DOI: 10.3390/cryst3010257
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      A study on graphene-metal contact
      Liu, Wenjun; Wei, Jun; Sun, Xiaowei; Yu, Hongyu
      Crystals (2013), 3 (), 257-274CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
      A review. The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. Reviewed are the authors's recent studies on the graphene-metal contact characteristics from the following viewpoints: (i) metal prepn. method; (ii) asym. conductance; (iii) annealing effect; (iv) interfaces impact.
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      Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666– 669,  DOI: 10.1126/science.1102896
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      Electric Field Effect in Atomically Thin Carbon Films
      Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.
      Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84
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      Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201– 204,  DOI: 10.1038/nature04235
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      Experimental observation of the quantum Hall effect and Berry's phase in graphene
      Zhang, Yuanbo; Tan, Yan-Wen; Stormer, Horst L.; Kim, Philip
      Nature (London, United Kingdom) (2005), 438 (7065), 201-204CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      When electrons are confined in two-dimensional materials, quantum-mech. enhanced transport phenomena such as the quantum Hall effect can be obsd. Graphene, consisting of an isolated single at. layer of graphite, is an ideal realization of such a two-dimensional system. However, its behavior is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality. Indeed, a distinctive half-integer quantum Hall effect was predicted theor., as has the existence of a nonzero Berry's phase (a geometric quantum phase) of the electron wavefunction-a consequence of the exceptional topol. of the graphene band structure. Recent advances in micromech. extn. and fabrication techniques for graphite structures now permit such exotic two-dimensional electron systems to be probed exptl. Here the authors report an exptl. study of magneto-transport in a high-mobility single layer of graphene. Adjusting the chem. potential using the elec. field effect, the authors observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these expts. is confirmed by magneto-oscillations. In addn. to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtF2nsrnJ&md5=9e5c67d812c899a4f0ab95df50cd25b7
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      Foa Torres, L. E. F.; Roche, S.; Charlier, J.-C. Introduction to Graphene-Based Nanomaterials: From Electronic Structure to Quantum Transport; Cambridge University Press: Cambridge, UK, 2014.
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      Arjmandi-Tash, H.; Kalita, D.; Han, Z.; Othmen, R.; Nayak, G.; Berne, C.; Landers, J.; Watanabe, K.; Taniguchi, T.; Marty, L.; Coraux, J.; Bendiab, N.; Bouchiat, V. Large Scale Graphene/h-BN Heterostructures Obtained by Direct CVD Growth of Graphene Using High-Yield Proximity-Catalytic Process. J. Phys. Mater. 2018, 1, 015003,  DOI: 10.1088/2515-7639/aac66e
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      Cai, Z.; Liu, B.; Zou, X.; Cheng, H.-M. Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures. Chem. Rev. 2018, 118, 6091– 6133,  DOI: 10.1021/acs.chemrev.7b00536
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      Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures
      Cai, Zhengyang; Liu, Bilu; Zou, Xiaolong; Cheng, Hui-Ming
      Chemical Reviews (Washington, DC, United States) (2018), 118 (13), 6091-6133CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      Two-dimensional (2D) materials have attracted increasing research interest because of the abundant choice of materials with diverse and tunable electronic, optical, and chem. properties. Moreover, 2D material based heterostructures combining several individual 2D materials provide unique platforms to create an almost infinite no. of materials and show exotic phys. phenomena as well as new properties and applications. To achieve these high expectations, methods for the scalable prepn. of 2D materials and 2D heterostructures of high quality and low cost must be developed. Chem. vapor deposition (CVD) is a powerful method which may meet the above requirements, and has been extensively used to grow 2D materials and their heterostructures in recent years, despite several challenges remaining. In this review of the challenges in the CVD growth of 2D materials, we highlight recent advances in the controlled growth of single crystal 2D materials, with an emphasis on semiconducting transition metal dichalcogenides. We provide insight into the growth mechanisms of single crystal 2D domains and the key technologies used to realize wafer-scale growth of continuous and homogeneous 2D films which are important for practical applications. Meanwhile, strategies to design and grow various kinds of 2D material based heterostructures are thoroughly discussed. The applications of CVD-grown 2D materials and their heterostructures in electronics, optoelectronics, sensors, flexible devices, and electrocatalysis are also discussed. Finally, we suggest solns. to these challenges and ideas concerning future developments in this emerging field.
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      Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVD-Enabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6, 2714– 2721,  DOI: 10.1021/acs.jpclett.5b01052
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      CVD-Enabled Graphene Manufacture and Technology
      Hofmann, Stephan; Braeuninger-Weimer, Philipp; Weatherup, Robert S.
      Journal of Physical Chemistry Letters (2015), 6 (14), 2714-2721CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Integrated manufg. is arguably the most challenging task in the development of technol. based on graphene and other 2D materials, particularly with regard to the industrial demand for "electronic-grade" large-area films. To control the structure and properties of these materials at the monolayer level, their nucleation, growth and interfacing needs to be understood to a level of unprecedented detail compared to existing thin film or bulk materials. Chem. vapor deposition (CVD) has emerged as the most versatile and promising technique to develop graphene and 2D material films into industrial device materials and this Perspective outlines recent progress, trends, and emerging CVD processing pathways. A key focus is the emerging understanding of the underlying growth mechanisms, in particular on the role of the required catalytic growth substrate, which brings together the latest progress in the fields of heterogeneous catalysis and classic crystal/thin-film growth.
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      Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Phase Perfection in Zinc Blende and Wurtzite III-IV Nanowires Using Basic Growth Parameters. Nano Lett. 2010, 10, 908– 915,  DOI: 10.1021/nl903688v
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      Phase Perfection in Zinc Blende and Wurtzite III-V Nanowires Using Basic Growth Parameters
      Joyce, Hannah J.; Wong-Leung, Jennifer; Gao, Qiang; Tan, H. Hoe; Jagadish, Chennupati
      Nano Letters (2010), 10 (3), 908-915CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Phase-perfect nanowires, of arbitrary diam., can be prepd. by tailoring basic growth parameters: temp. and V/III ratio. Phase purity is achieved without sacrificing important specifications of diam. and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temp. coupled with a high V/III ratio. A high growth temp. coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. A comprehensive nucleation model is presented to explain the formation of these markedly different crystal phases under these growth conditions. Crit. to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temp. and V/III ratio.
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      Guilhabert, B.; Hurtado, A.; Jevtics, D.; Gao, Q.; Tan, H. H.; Jagadish, C.; Dawson, M. D. Transfer Printing of Semiconductor Nanowires with Lasing Emission for Controllable Nanophotonic Device Fabrication. ACS Nano 2016, 10, 3951– 3958,  DOI: 10.1021/acsnano.5b07752
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      Transfer Printing of Semiconductor Nanowires with Lasing Emission for Controllable Nanophotonic Device Fabrication
      Guilhabert, Benoit; Hurtado, Antonio; Jevtics, Dimitars; Gao, Qian; Tan, Hark Hoe; Jagadish, Chennupati; Dawson, Martin D.
      ACS Nano (2016), 10 (4), 3951-3958CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Accurate positioning and organization of InP nanowires (NWs) with lasing emission at room temp. is achieved using a nanoscale transfer printing (TP) technique. The NWs retained their lasing emission after their transfer to targeted locations on different receiving substrates (e.g., polymers, SiO2, and metal surfaces). The NWs were also organized into complex spatial patterns, including 1D and 2D arrays, with a controlled no. of elements and dimensions. The developed TP technique enables the fabrication of bespoke nanophotonic systems using NW lasers and other NW devices as building blocks.
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    61. 61
      Jevtics, D.; McPhillimy, J.; Guilhabert, B.; Alanis, J. A.; Tan, H. H.; Jagadish, C.; Dawson, M. D.; Hurtado, A.; Parkinson, P.; Strain, M. J. Characterization, Selection, and Microassembly of Nanowire Laser Systems. Nano Lett. 2020, 20, 1862– 1868,  DOI: 10.1021/acs.nanolett.9b05078
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      Characterization, Selection, and Microassembly of Nanowire Laser Systems
      Jevtics, Dimitars; McPhillimy, John; Guilhabert, Benoit; Alanis, Juan A.; Tan, Hark Hoe; Jagadish, Chennupati; Dawson, Martin D.; Hurtado, Antonio; Parkinson, Patrick; Strain, Michael J.
      Nano Letters (2020), 20 (3), 1862-1868CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Semiconductor nanowire (NW) lasers are a promising technol. for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy d., and subsequently printed in arrays onto a secondary substrate. Statistical anal. of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Anal. on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.
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    62. 62
      Jevtics, D.; Hurtado, A.; Guilhabert, B.; McPhillimy, J.; Cantarella, G.; Gao, Q.; Tan, H. H.; Jagadish, C.; Strain, M. J.; Dawson, M. D. Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate. Nano Lett. 2017, 17, 5990– 5994,  DOI: 10.1021/acs.nanolett.7b02178
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      Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate
      Jevtics, Dimitars; Hurtado, Antonio; Guilhabert, Benoit; McPhillimy, John; Cantarella, Giuseppe; Gao, Qian; Tan, Hark Hoe; Jagadish, Chennupati; Strain, Michael J.; Dawson, Martin D.
      Nano Letters (2017), 17 (10), 5990-5994CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Nanowire lasers are integrated with planar waveguide devices using a high positional accuracy microtransfer printing technique. Direct nanowire to waveguide coupling is demonstrated, with coupling losses as low as -17 dB, dominated by mode mismatch between the structures. Coupling is achieved using both end-fire coupling into a waveguide facet, and from nanowire lasers printed directly onto the top surface of the waveguide. In-waveguide peak powers up to 11.8 μW are demonstrated. Basic photonic integrated circuit functions such as power splitting and wavelength multiplexing are presented. Finally, devices are fabricated on a mech. flexible substrate to demonstrate robust coupling between the on-chip laser source and waveguides under significant deformation of the system.
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    63. 63
      Xu, W.-Z.; Ren, F.-F.; Jevtics, D.; Hurtado, A.; Li, L.; Gao, Q.; Ye, J.; Wang, F.; Guilhabert, B.; Fu, L.; Lu, H.; Zhang, R.; Tan, H. H.; Dawson, M. D.; Jagadish, C. Vertically Emitting Indium Phosphide Nanowire Lasers. Nano Lett. 2018, 18, 3414– 3420,  DOI: 10.1021/acs.nanolett.8b00334
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      63
      Vertically Emitting Indium Phosphide Nanowire Lasers
      Xu, Wei-Zong; Ren, Fang-Fang; Jevtics, Dimitars; Hurtado, Antonio; Li, Li; Gao, Qian; Ye, Jiandong; Wang, Fan; Guilhabert, Benoit; Fu, Lan; Lu, Hai; Zhang, Rong; Tan, Hark Hoe; Dawson, Martin D.; Jagadish, Chennupati
      Nano Letters (2018), 18 (6), 3414-3420CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Semiconductor nanowire (NW) lasers have attracted considerable research effort given their excellent promise for nanoscale photonic sources. NW lasers currently exhibit poor directionality and high threshold gain, issues critically limiting their prospects for on-chip light sources with extremely reduced footprint and efficient power consumption. A new design is proposed and a vertically emitting InP NW laser structure showing high emission directionality and reduced energy requirements for operation is exptl. demonstrated. The structure of the laser combines an InP NW integrated in a cat's eye (CE) antenna. Thanks to the antenna guidance with broken asymmetry, strong focusing ability, and high Q-factor, the designed InP CE-NW lasers exhibit a higher degree of polarization, narrower emission angle, enhanced internal quantum efficiency, and reduced lasing threshold.
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    64. 64
      Peng, K.; Jevtics, D.; Zhang, F.; Sterzl, S.; Damry, D. A.; Rothmann, M. U.; Guilhabert, B.; Strain, M. J.; Tan, H. H.; Herz, L. M.; Fu, L.; Dawson, M. D.; Hurtado, A.; Jagadish, C.; Johnston, M. B. Three-Dimensional Cross-Nanowire Networks Recover Full Terahertz State. Science 2020, 368, 510– 513,  DOI: 10.1126/science.abb0924
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      Three-dimensional cross-nanowire networks recover full terahertz state
      Peng, Kun; Jevtics, Dimitars; Zhang, Fanlu; Sterzl, Sabrina; Damry, Djamshid A.; Rothmann, Mathias U.; Guilhabert, Benoit; Strain, Michael J.; Tan, Hark H.; Herz, Laura M.; Fu, Lan; Dawson, Martin D.; Hurtado, Antonio; Jagadish, Chennupati; Johnston, Michael B.
      Science (Washington, DC, United States) (2020), 368 (6490), 510-513CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      THz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to IR light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of THz light, yet to date, most time-domain THz detectors are sensitive to just 1 polarization component. A nanotechnol.-based semiconductor detector using cross-nanowire networks that records the full polarization state of THz pulses is demonstrated. The monolithic device allows simultaneous measurements of the orthogonal components of the THz elec. field vector without cross-talk. The capabilities of the detector for the study of metamaterials are demonstrated.
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    65. 65
      Sourribes, M. J. L.; Isakov, I.; Panfilova, M.; Warburton, P. A. Minimization of the Contact Resistance between InAs Nanowires and Metallic Contacts. Nanotechnology 2013, 24, 045703,  DOI: 10.1088/0957-4484/24/4/045703
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      65
      Minimization of the contact resistance between InAs nanowires and metallic contacts
      Sourribes, M. J. L.; Isakov, I.; Panfilova, M.; Warburton, P. A.
      Nanotechnology (2013), 24 (4), 045703CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      We investigate different processes for optimizing the formation of Ohmic contacts to InAs nanowires. The nanowires are grown via mol. beam epitaxy without the use of metal catalysts. Metallic contacts are attached to the nanowires by using an electron beam lithog. process. Before deposition of the contacts, the InAs nanowires are treated either by wet etching in an ammonium polysulfide (NH4)2Sx soln. or by an argon milling process in order to remove a surface oxide layer. Two-point elec. measurements show that the resistance of the ammonium polysulfide-treated nanowires is two orders of magnitude lower than that of the untreated nanowires. The nanowires that are treated by the argon milling process show a resistance which is more than an order of magnitude lower than that of those treated with ammonium polysulfide. Four-point measurements allow us to ext. an upper bound of 1.4 × 10-7 Ω cm2 for the contact resistivity of metallic contacts on nanowires treated by the argon milling process.
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    66. 66
      Shi, T.; Fu, M.; Pan, D.; Guo, Y.; Zhao, J.; Chen, Q. Contact Properties of Field-Effect Transistors Based on Indium Arsenide Nanowires Thinner Than 16 nm. Nanotechnology 2015, 26, 175202,  DOI: 10.1088/0957-4484/26/17/175202
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      Contact properties of field-effect transistors based on indium arsenide nanowires thinner than 16 nm
      Shi Tuanwei; Fu Mengqi; Pan Dong; Guo Yao; Zhao Jianhua; Chen Qing
      Nanotechnology (2015), 26 (17), 175202 ISSN:.
      With the scaling down of field effect transistors (FETs) to improve performance, the contact between the electrodes and the channel becomes more and more important. Contact properties of FETs based on ultrathin InAs NWs (with the diameter ranging from sub-7 nm to 16 nm) are investigated here. Chromium (Cr) and nickel (Ni) are proven to form ohmic contact with the ultrathin InAs NWs, in contrast to a recent report (Razavieh A et al ACS Nano 8 6281). Furthermore, the contact resistance is found to depend on the NW diameter and the contact metals, which between Cr and InAs NWs increases more rapidly than that between Ni and InAs NWs when the NW diameter decreases. The origins of the contact resistance difference for the two kinds of metals are studied and NixInAs is believed to play an important role. Based on our results, it is advantageous to use Ni as contact metal for ultrathin NWs. We also observe that the FETs are still working in the diffusive regime even when the channel length is scaled down to 50 nm.
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      Fan, D.; Kang, N.; Ghalamestani, S. G.; Dick, K. A.; Xu, H. Q. Schottky Barrier and Contact Resistance of InSb Nanowire Field-Effect Transistors. Nanotechnology 2016, 27, 275204,  DOI: 10.1088/0957-4484/27/27/275204
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      67
      Schottky barrier and contact resistance of InSb nanowire field-effect transistors
      Fan, Dingxun; Kang, N.; Ghalamestani, Sepideh Gorji; Dick, Kimberly A.; Xu, H. Q.
      Nanotechnology (2016), 27 (27), 275204/1-275204/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      Understanding of the elec. contact properties of semiconductor nanowire (NW) field-effect transistors (FETs) plays a crucial role in the use of semiconducting NWs as building blocks for future nanoelectronic devices and in the study of fundamental physics problems. Here, we report on a study of the contact properties of Ti/Au, a widely used contact metal combination, when contacting individual InSb NWs via both two-probe and four-probe transport measurements. We show that a Schottky barrier of height ΦSB ∼ 20 meV is present at the metal-InSb NW interfaces and its effective height is gate-tunable. The contact resistance (Rc) in the InSb NWFETs is also analyzed by magnetotransport measurements at low temps. It is found that Rc in the on-state exhibits a pronounced magnetic field-dependent feature, namely it is increased strongly with increasing magnetic field after an onset field Bc. A qual. picture that takes into account magnetic depopulation of subbands in the NWs is provided to explain the observation. Our results provide solid exptl. evidence for the presence of a Schottky barrier at Ti/Au-InSb NW interfaces and can be used as a basis for design and fabrication of novel InSb NW-based nanoelectronic devices and quantum devices.
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      Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Single- and Multi-Wall Carbon Nanotube Field-Effect Transistors. Appl. Phys. Lett. 1998, 73, 2447– 2449,  DOI: 10.1063/1.122477
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      Single- and multi-wall carbon nanotube field-effect transistors
      Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph.
      Applied Physics Letters (1998), 73 (17), 2447-2449CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temp., it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations-in our case a collapsed tube-can make them operate as field-effect transistors.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXms1eqsrc%253D&md5=ad460a19a39b79259317e14d353072de
    69. 69
      Ullah, A. R.; Joyce, H. J.; Tan, H. H.; Jagadish, C.; Micolich, A. P. The Influence of Atmosphere on the Performance of Pure-Phase WZ and ZB InAs Nanowire Transistors. Nanotechnology 2017, 28, 454001. DOI: 10.1088/1361-6528/aa8e23
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      The influence of atmosphere on the performance of pure-phase WZ and ZB InAs nanowire transistors
      Ullah, A. R.; Joyce, H. J.; Tan, H. H.; Jagadish, C.; Micolich, A. P.
      Nanotechnology (2017), 28 (45), 454001/1-454001/8CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      We compare the characteristics of phase-pure MOCVD grown ZB and WZ InAs nanowire transistors in several atmospheres: air, dry pure N2 and O2, and N2 bubbled through liq. H2O and alcs. to identify whether phase-related structural/surface differences affect their response. Both WZ and ZB give poor gate characteristics in dry state. Adsorption of polar species reduces off-current by 2-3 orders of magnitude, increases on-off ratio and significantly reduces sub-threshold slope. The key difference is the greater sensitivity of WZ to low adsorbate level. We attribute this to facet structure and its influence on the sepn. between conduction electrons and surface adsorption sites. We highlight the important role adsorbed species play in nanowire device characterization. WZ is commonly thought superior to ZB in InAs nanowire transistors. We show this is an artifact of the moderate humidity found in ambient lab. conditions: WZ and ZB perform equally poorly in the dry gas limit yet equally well in the wet gas limit. We also highlight the vital role d.-lowering disorder has in improving gate characteristics, be it stacking faults in mixed-phase WZ or surface adsorbates in pure-phase nanowires.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitFWmtr7F&md5=02e9b271765c47c9b9432af609ec2205
    70. 70
      Dayeh, S. A.; Aplin, D. P. R.; Zhou, X.; Yu, P. K. L.; Yu, E. T.; Wang, D. High Electron Mobility InAs Nanowire Field-Effect Transistors. Small 2007, 3, 326– 332,  DOI: 10.1002/smll.200600379
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      High electron mobility InAs nanowire field-effect transistors
      Dayeh, Shadi A.; Aplin, David P. R.; Zhou, Xiaotian; Yu, Paul K. L.; Yu, Edward T.; Wang, Deli
      Small (2007), 3 (2), 326-332CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Single-crystal InAs nanowires (NWs) were synthesized using metal-org. CVD (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO2/n+-Si substrate with a global n+-Si back-gate and sputtered SiOx/Au underlap top-gate. For top-gate NWFETs, the authors have developed a model that allows accurate estn. of characteristic NW parameters, including carrier field-effect mobility and carrier concn. by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temp. field-effect mobility ≤6580 cm2 V-1 s-1, which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.
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    71. 71
      Holloway, G. W.; Haapamaki, C. M.; Kuyanov, P.; LaPierre, R. R.; Baugh, J. Electrical Characterization of Chemical and Dielectric Passivation of InAs Nanowires. Semicond. Sci. Technol. 2016, 31, 114004,  DOI: 10.1088/0268-1242/31/11/114004
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      71
      Electrical characterization of chemical and dielectric passivation of InAs nanowires
      Holloway, Gregory W.; Haapamaki, Chris M.; Kuyanov, Paul; LaPierre, Ray R.; Baugh, Jonathan
      Semiconductor Science and Technology (2016), 31 (11), 114004/1-114004/8CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)
      The native oxide at the surface of III-V nanowires, such as InAs, can be a major source of charge noise and scattering in nanowire-based electronics, particularly for quantum devices operated at low temps. Surface passivation provides a means to remove the native oxide and prevent its regrowth. Here, we study the effects of surface passivation and conformal dielec. deposition by measuring elec. conductance through nanowire field effect transistors treated with a variety of surface prepns. By extg. field effect mobility, subthreshold swing, threshold shift with temp., and the gate hysteresis for each device, we infer the relative effects of the different treatments on the factors influencing transport. It is found that a combination of chem. passivation followed by deposition of an aluminum oxide dielec. shell yields the best results compared to the other treatments, and comparable to untreated nanowires. Finally, it is shown that an entrenched, top-gated device using an optimally treated nanowire can successfully form a stable double quantum dot at low temps. The device has excellent electrostatic tunability owing to the conformal dielec. layer and the combination of local top gates and a global back gate.
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    72. 72
      Petrovykh, D. Y.; Yang, M. J.; Whitman, L. J. Chemical and Electronic Properties of Sulfur-Passivated InAs Surfaces. Surf. Sci. 2003, 523, 231– 240,  DOI: 10.1016/S0039-6028(02)02411-1
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      72
      Chemical and electronic properties of sulfur-passivated InAs surfaces
      Petrovykh, D. Y.; Yang, M. J.; Whitman, L. J.
      Surface Science (2003), 523 (3), 231-240CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
      Treatment with ammonium sulfide ((NH4)2Sx) solns. is used to produce model passivated InAs(0 0 1) surfaces with well-defined chem. and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aq. solns., as characterized by XPS, at. force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the assocd. downward band bending.
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    73. 73
      Pizzocchero, F.; Gammelgaard, L.; Jessen, B. S.; Caridad, J. M.; Wang, L.; Hone, J.; Bøggild, P.; Booth, T. J. The Hot Pick-Up Technique for Batch Assembly of van der Waals Heterostructures. Nat. Commun. 2016, 7, 11894,  DOI: 10.1038/ncomms11894
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      The hot pick-up technique for batch assembly of van der Waals heterostructures
      Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S.; Caridad, Jose M.; Wang, Lei; Hone, James; Boeggild, Peter; Booth, Timothy J.
      Nature Communications (2016), 7 (), 11894CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled prodn. of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.
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    74. 74
      Geim, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419– 425,  DOI: 10.1038/nature12385
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      Van der Waals heterostructures
      Geim, A. K.; Grigorieva, I. V.
      Nature (London, United Kingdom) (2013), 499 (7459), 419-425CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A review. Research on graphene and other two-dimensional at. crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated at. planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
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    75. 75
      Masubuchi, S.; Morimoto, M.; Morikawa, S.; Onodera, M.; Asakawa, Y.; Watanabe, K.; Taniguchi, T.; Machida, T. Autonomous Robotic Searching and Assembly of Two-Dimensional Crystals to Build van der Waals Superlattices. Nat. Commun. 2018, 9, 1413,  DOI: 10.1038/s41467-018-03723-w
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      75
      Autonomous robotic searching and assembly of two-dimensional crystals to build van der Waals superlattices
      Masubuchi Satoru; Morimoto Masataka; Morikawa Sei; Onodera Momoko; Asakawa Yuta; Machida Tomoki; Watanabe Kenji; Taniguchi Takashi
      Nature communications (2018), 9 (1), 1413 ISSN:.
      Van der Waals heterostructures are comprised of stacked atomically thin two-dimensional crystals and serve as novel materials providing unprecedented properties. However, the random natures in positions and shapes of exfoliated two-dimensional crystals have required the repetitive manual tasks of optical microscopy-based searching and mechanical transferring, thereby severely limiting the complexity of heterostructures. To solve the problem, here we develop a robotic system that searches exfoliated two-dimensional crystals and assembles them into superlattices inside the glovebox. The system can autonomously detect 400 monolayer graphene flakes per hour with a small error rate (<7%) and stack four cycles of the designated two-dimensional crystals per hour with few minutes of human intervention for each stack cycle. The system enabled fabrication of the superlattice consisting of 29 alternating layers of the graphene and the hexagonal boron nitride. This capacity provides a scalable approach for prototyping a variety of van der Waals superlattices.
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    76. 76
      Lee, Y. K.; Choi, H.; Lee, H.; Lee, C.; Choi, J. S.; Choi, C.-G.; Hwang, E.; Park, J. Y. Hot Carrier Multiplication on Graphene/TiO2 Schottky Nanodiodes. Sci. Rep. 2016, 6, 27549,  DOI: 10.1038/srep27549
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      Hot carrier multiplication on graphene/TiO2 Schottky nanodiodes
      Lee, Young Keun; Choi, Hongkyw; Lee, Hyunsoo; Lee, Changhwan; Choi, Jin Sik; Choi, Choon-Gi; Hwang, Euyheon; Park, Jeong Young
      Scientific Reports (2016), 6 (), 27549CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Carrier multiplication (i.e. generation of multiple electron-hole pairs from a single high-energy electron, CM) in graphene has been extensively studied both theor. and exptl., but direct application of hot carrier multiplication in graphene has not been reported. Here, taking advantage of efficient CM in graphene, we fabricated graphene/TiO2 Schottky nanodiodes and found CM-driven enhancement of quantum efficiency. The unusual photocurrent behavior was obsd. and directly compared with Fowler's law for photoemission on metals. The Fowler's law exponent for the graphene-based nanodiode is almost twice that of a thin gold film based diode; the graphene-based nanodiode also has a weak dependence on light intensity-both are significant evidence for CM in graphene. Furthermore, doping in graphene significantly modifies the quantum efficiency by changing the Schottky barrier. The CM phenomenon obsd. on the graphene/TiO2 nanodiodes can lead to intriguing applications of viable graphene-based light harvesting.
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    77. 77
      Levi, A.; Kirshner, M.; Sinai, O.; Peretz, E.; Meshulam, O.; Ghosh, A.; Gotlib, N.; Stern, C.; Yuan, S.; Xia, F.; Naveh, D. Graphene Schottky Varactor Diodes for High-Performance Photodetection. ACS Photonics 2019, 6, 1910– 1915,  DOI: 10.1021/acsphotonics.9b00811
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      77
      Graphene Schottky Varactor Diodes for High-Performance Photodetection
      Levi, Adi; Kirshner, Moshe; Sinai, Ofer; Peretz, Eldad; Meshulam, Ohad; Ghosh, Arnab; Gotlib, Noam; Stern, Chen; Yuan, Shaofan; Xia, Fengnian; Naveh, Doron
      ACS Photonics (2019), 6 (8), 1910-1915CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Over the past decade graphene devices have inspired the progress of future electronic and optoelectronic technologies. The unique combination of fast carrier dynamics and intrinsic quantum capacitance of graphene is a fertile ground for implementing novel device architectures. Here, we report on a novel device architecture comprising graphene Schottky diode varactors and assess the potential applications of this type of new device in optoelectronics. We show that graphene varactor diodes exhibit significant advantages compared with existing graphene photodetectors including elimination of high dark currents and enhancement of the external quantum efficiency (EQE). Our devices demonstrate a large photoconductive gain and EQE of up to 37%, fast photoresponse, and low leakage currents at room temp.
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    78. 78
      Nagashio, K.; Nishimura, T.; Kita, K.; Toriumi, A. Contact Resistivity and Current Flow Path at Metal/Graphene Contact. Appl. Phys. Lett. 2010, 97, 143514,  DOI: 10.1063/1.3491804
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      Contact resistivity and current flow path at metal/graphene contact
      Nagashio, K.; Nishimura, T.; Kita, K.; Toriumi, A.
      Applied Physics Letters (2010), 97 (14), 143514/1-143514/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The contact properties between metal and graphene were examd. The elec. measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The anal. using the cross-bridge Kelvin (CBK) structure suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of -1 μm. The contact resistivity for Ni was measured as -5 × 10-6 Ω cm2 using the CBK. A simple calcn. suggests that a contact resistivity less than 10-9 Ω cm2 is required for miniaturized graphene field effect transistors. (c) 2010 American Institute of Physics.
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    79. 79
      Lipatov, A.; Varezhnikov, A.; Augustin, M.; Bruns, M.; Sommer, M.; Sysoev, V.; Kolmakov, A.; Sinitskii, A. Intrinsic Device-To-Device Variation in Graphene Field-Effect Transistors on a Si/SiO2 Substrate as a Platform for Discriminative Gas Sensing. Appl. Phys. Lett. 2014, 104, 013114,  DOI: 10.1063/1.4861183
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      79
      Intrinsic device-to-device variation in graphene field-effect transistors on a Si/SiO2 substrate as a platform for discriminative gas sensing
      Lipatov, Alexey; Varezhnikov, Alexey; Augustin, Martin; Bruns, Michael; Sommer, Martin; Sysoev, Victor; Kolmakov, Andrei; Sinitskii, Alexander
      Applied Physics Letters (2014), 104 (1), 013114/1-013114/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Arrays of nearly identical graphene devices on Si/SiO2 exhibit a substantial device-to-device variation, even in case of a high-quality CVD or mech. exfoliated graphene. We propose that such device-to-device variation could provide a platform for highly selective multisensor electronic olfactory systems. We fabricated a multielectrode array of CVD graphene devices on a Si/SiO2 substrate and demonstrated that the diversity of these devices is sufficient to reliably discriminate different short-chain alcs.: methanol, ethanol, and isopropanol. The diversity of graphene devices on Si/SiO2 could possibly be used to construct similar multisensor systems trained to recognize other analytes as well. (c) 2014 American Institute of Physics.
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    80. 80
      Giubileo, F.; Di Bartolomeo, A. The Role of Contact Resistance in Graphene Field-Effect Devices. Prog. Surf. Sci. 2017, 92, 143– 175,  DOI: 10.1016/j.progsurf.2017.05.002
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      80
      The role of contact resistance in graphene field-effect devices
      Giubileo, Filippo; Di Bartolomeo, Antonio
      Progress in Surface Science (2017), 92 (3), 143-175CODEN: PSSFBP; ISSN:0079-6816. (Elsevier B.V.)
      The extremely high carrier mobility and the unique band structure, make graphene very useful for field-effect transistor applications. According to several works, the primary limitation to graphene based transistor performance is not related to the material quality, but to extrinsic factors that affect the electronic transport properties. One of the most important parasitic element is the contact resistance appearing between graphene and the metal electrodes functioning as the source and the drain. Ohmic contacts to graphene, with low contact resistances, are necessary for injection and extn. of majority charge carriers to prevent transistor parameter fluctuations caused by variations of the contact resistance. The International Technol. Roadmap for Semiconductors, toward integration and down-scaling of graphene electronic devices, identifies as a challenge the development of a CMOS compatible process that enables reproducible formation of low contact resistance. However, the contact resistance is still not well understood despite it is a crucial barrier towards further improvements. In this paper, we review the exptl. and theor. activity that in the last decade has been focusing on the redn. of the contact resistance in graphene transistors. We will summarize the specific properties of graphene-metal contacts with particular attention to the nature of metals, impact of fabrication process, Fermi level pinning, interface modifications induced through surface processes, charge transport mechanism, and edge contact formation.
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    81. 81
      Park, H.-Y.; Jung, W.-S.; Kang, D.-H.; Jeon, J.; Yoo, G.; Park, Y.; Lee, J.; Jang, Y. H.; Lee, J.; Park, S.; Yu, H.-Y.; Shin, B.; Lee, S.; Park, J.-H. Extremely Low Contact Resistance on Graphene through n-Type Doping and Edge Contact Design. Adv. Mater. 2016, 28, 864– 870,  DOI: 10.1002/adma.201503715
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      81
      Extremely low contact resistance on graphene through n-type doping and edge contact design
      Park, Hyung-Youl; Jung, Woo-Shik; Kang, Dong-Ho; Jeon, Jaeho; Yoo, Gwangwe; Park, Yongkook; Lee, Jinhee; Jang, Yun Hee; Lee, Jaeho; Park, Seongjun; Yu, Hyun-Yong; Shin, Byungha; Lee, Sungjoo; Park, Jin-Hong
      Advanced Materials (Weinheim, Germany) (2016), 28 (5), 864-870CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The effects of graphene n-doping on metal-graphene contact resistance in combination with edge contacts, presenting a record contact resistance of 23 Ω μm at room temp. (19 Ω at 100 K), was investigated. The graphene n-doping was achieved through the charge transfer from a poly(4-vinyl phenol)/poly(melamine-co-formaldehyde) (PVP/PMF) insulator with triazine functional groups which are electron-rich arom. mols. Though n-doping the graphene by 400% PVP/PMF layer, about a 2-fold lower contact resistance value was obtained compared to the 2-dimensional surface contact resistance of pristine graphene (initially, p-type) under the same gate bias. However, after the n-doping, the 2-dimensional contacted metals did not affect the Fermi level of graphene similarly to the previous pristine graphene. Addnl., compared to the 2-dimensional contact sample (RC=1.4 kΩ μm), the authors confirmed about a 3-fold lower contact resistance (RC=484 kΩ) in the edge 3 pattern with 1 μm wide periodic lines/gaps. Finally, the authors applied this contact scheme to a graphene-perovskite hybrid photodetector for performance improvement and also minimized the contact resistance to 23 Ω μm.
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    82. 82
      Anzi, L.; Mansouri, A.; Pedrinazzi, P.; Guerriero, E.; Fiocco, M.; Pesquera, A.; Centeno, A.; Zurutuza, A.; Behnam, A.; Carrion, E. A.; Pop, E.; Sordan, R. Ultra-Low Contact Resistance in Graphene Devices at the Dirac Point. 2D Mater. 2018, 5, 025014,  DOI: 10.1088/2053-1583/aaab96
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      82
      Ultra-low contact resistance in graphene devices at the Dirac point
      Anzi, Luca; Mansouri, Aida; Pedrinazzi, Paolo; Guerriero, Erica; Fiocco, Marco; Pesquera, Amaia; Centeno, Alba; Zurutuza, Amaia; Behnam, Ashkan; Carrion, Enrique A.; Pop, Eric; Sordan, Roman
      2D Materials (2018), 5 (2), 025014/1-025014/8CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Contact resistance is one of the main factors limiting performance of short-channel graphene fieldeffect transistors (GFETs), preventing their use in low-voltage applications. Here we investigated the contact resistance between graphene grown by chem. vapor deposition (CVD) and different metals, and found that etching holes in graphene below the contacts consistently reduced the contact resistance, down to 23 ω · μm with Au contacts. This low contact resistance was obtained at the Dirac point of graphene, in contrast to previous studies where the lowest contact resistance was obtained at the highest carrier d. in graphene (here 200 ω · μm was obtained under such conditions). The 'holey' Au contacts were implemented in GFETs which exhibited an av. transconductance of 940 S m-1 at a drain bias of only 0.8 V and gate length of 500 nm, which out-perform GFETs with conventional Au contacts.
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    83. 83
      Cao, Y.; Mishchenko, A.; Yu, G. L.; Khestanova, E.; Rooney, A. P.; Prestat, E.; Kretinin, A. V.; Blake, P.; Shalom, M. B.; Woods, C.; Chapman, J.; Balakrishnan, G.; Grigorieva, I. V.; Novoselov, K. S.; Piot, B. A.; Potemski, M.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K. Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere. Nano Lett. 2015, 15, 4914– 4921,  DOI: 10.1021/acs.nanolett.5b00648
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      83
      Quality Heterostructures from Two-Dimensional Crystals Unstable in Air by Their Assembly in Inert Atmosphere
      Cao, Y.; Mishchenko, A.; Yu, G. L.; Khestanova, E.; Rooney, A. P.; Prestat, E.; Kretinin, A. V.; Blake, P.; Shalom, M. B.; Woods, C.; Chapman, J.; Balakrishnan, G.; Grigorieva, I. V.; Novoselov, K. S.; Piot, B. A.; Potemski, M.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K.; Gorbachev, R. V.
      Nano Letters (2015), 15 (8), 4914-4921CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Many layered materials can be cleaved down to individual at. planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decomp. in air, which has severely hindered their study and potential applications. Here the authors introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atm. To illustrate the technol., the authors choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. The authors' field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of exptl. accessible two-dimensional crystals and their heterostructures.
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    84. 84
      Tien, D. H.; Park, J.-Y.; Kim, K. B.; Lee, N.; Seo, Y. Characterization of Graphene-Based FET Fabricated using a Shadow Mask. Sci. Rep. 2016, 6, 25050,  DOI: 10.1038/srep25050
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      84
      Characterization of Graphene-based FET Fabricated using a Shadow Mask
      Tien, Dung Hoang; Park, Jun-Young; Kim, Ki Buem; Lee, Naesung; Seo, Yongho
      Scientific Reports (2016), 6 (), 25050CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      To pattern elec. metal contacts, electron beam lithog. or photolithog. are commonly utilized, and these processes require polymer resists with solvents. During the patterning process the graphene surface is exposed to chems., and the residue on the graphene surface was unable to be completely removed by any method, causing the graphene layer to be contaminated. A lithog. free method can overcome these residue problems. In this study, we use a micro-grid as a shadow mask to fabricate a graphene based field-effect-transistor (FET). Elec. measurements of the graphene based FET samples are carried out in air and vacuum. It is found that the Dirac peaks of the graphene devices on SiO2 or on hexagonal boron nitride (hBN) shift from a pos. gate voltage region to a neg. region as air pressure decreases. In particular, the Dirac peaks shift very rapidly when the pressure decreases from ∼2 × 10-3 Torr to ∼5 × 10-5 Torr within 5 min. These Dirac peak shifts are known as adsorption and desorption of environmental gases, but the shift amts. are considerably different depending on the fabrication process. The high gas sensitivity of the device fabricated by shadow mask is attributed to adsorption on the clean graphene surface.
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    85. 85
      Pfeiffer, L.; West, K. W.; Stormer, H. L.; Baldwin, K. W. Electron Mobilities Exceeding 107 cm2/V s in Modulation-Doped GaAs. Appl. Phys. Lett. 1989, 55, 1888– 1890,  DOI: 10.1063/1.102162
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      Electron mobilities exceeding 107 cm2/V s in modulation-doped gallium arsenide
      Pfeiffer, Loren; West, K. W.; Stormer, H. L.; Baldwin, K. W.
      Applied Physics Letters (1989), 55 (18), 1888-90CODEN: APPLAB; ISSN:0003-6951.
      A modulation-doped Al0.35Ga0.65As/GaAs single interface structure with a 700 Å undoped setback grown by solid-source MBE shows a Hall mobility of 11.7 × 106 cm2/V s at a carrier d. of 2.4 × 1011 electrons/cm2 measured in van der Pauw geometry after exposure to light at 0.35 K. This is the highest carrier mobility ever measured in a semiconductor. Similar Al0.32Ga0.68As/Ga/as structures with 1000-2000 Å setbacks show Hall mobilities in the dark as 0.35 K as high as 4.9 × 106 cm2/V s for carrier densities of 5.4 × 1010 electrons/cm2 and lower.
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