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Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors
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Functional Inorganic Materials and Devices

Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors
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  • Aniello Pelella
    Aniello Pelella
    Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
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    Orcidhttp://orcid.org/0000-0002-3831-0210
  • Osamah Kharsah
    Osamah Kharsah
    Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
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  • Alessandro Grillo
    Alessandro Grillo
    Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
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    Orcidhttp://orcid.org/0000-0002-8909-9865
  • Francesca Urban
    Francesca Urban
    Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
    INFN—Gruppo Collegato di Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
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    Orcidhttp://orcid.org/0000-0003-2109-1370
  • Maurizio Passacantando
    Maurizio Passacantando
    Department of Physical and Chemical Sciences, University of L’Aquila, and CNR-SPIN L’Aquila, via Vetoio, Coppito, L’Aquila 67100, Italy
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    Orcidhttp://orcid.org/0000-0002-3680-5295
  • Filippo Giubileo
    Filippo Giubileo
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
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    Orcidhttp://orcid.org/0000-0003-2233-3810
  • Laura Iemmo
    Laura Iemmo
    Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
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  • Stephan Sleziona
    Stephan Sleziona
    Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
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  • Erik Pollmann
    Erik Pollmann
    Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
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    Orcidhttp://orcid.org/0000-0002-3961-0426
  • Lukas Madauß
    Lukas Madauß
    Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
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    Orcidhttp://orcid.org/0000-0003-2556-5967
  • Marika Schleberger
    Marika Schleberger
    Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
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    Orcidhttp://orcid.org/0000-0002-5785-1186
  • Antonio Di Bartolomeo*
    Antonio Di Bartolomeo
    Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy
    CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-3629-726X
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Cite this: ACS Appl. Mater. Interfaces 2020, 12, 36, 40532–40540
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https://doi.org/10.1021/acsami.0c11933
Published August 10, 2020

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Abstract

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Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing.

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Introduction

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Molybdenum disulfide (MoS2) is one of the most studied transition metal dichalcogenides, owing to its layered structure and useful mechanical, chemical, electronic, and optoelectronic properties. (1−4) A molybdenum (Mo) atomic plane sandwiched between two sulfur (S) planes constitutes the monolayer that is bonded to other monolayers by weak van der Waals forces to form the bulk material. MoS2 is a semiconductor suitable for several applications, (5−9) having a 1.2 eV indirect band gap in the bulk form that widens up to 1.8–1.9 eV and becomes direct in the monolayer. (3) Despite the lower field-effect mobility than graphene, (10,11) ranging from few tenths to hundreds (12−15) of cm2 V–1 s–1, MoS2 field-effect transistors (FETs) have recently become very popular as alternatives to graphene FETs (12−17) for next-generation electronics based on 2D materials. (18−25)
The fabrication and characterization of devices based on 2D materials greatly rely on the application of electron beam (e-beam) lithography or focused ion beam processing and on scanning electron microscopy (SEM) or transmission electron microscopy, which imply irradiation by charged particles. The exposure to low-energy electrons and/or ions can modify the electronic properties of the 2D materials or their interfaces. (9,17,26) Indeed, structural defects can locally modify the band structure and behave as charge traps, thereby changing the device characteristics both in the case of e-beam (27,28) and ion beam irradiation. (29,30) Conversely, electron beam, ion irradiation, or plasma treatments can be intentionally used for nanoincisions, (31) for pores, (32) or to purposely create defects, for instance, to reduce the contact resistance. (33−35) Choi et al. reported the effects of 30 keV electron beam irradiation of monolayer MoS2 FETs, showing that irradiation-induced defects act as trap sites by reducing the carrier mobility and concentration and shifting the threshold voltage. (36) A study of point defects in MoS2 using SEM imaging and first-principles calculations, by Zhou et al., demonstrated that vacancies are created by e-beam irradiation at low energies, (37) below 30 keV. Durand et al. studied the effects of e-beam on the MoS2-based FET, reporting an increase in carrier density and a decrease in mobility explained as irradiation-induced generation of intrinsic defects in MoS2 and as Coulomb scattering by charges at the MoS2–SiO2 interface, respectively. (38) Giubileo et al. reported a negative threshold voltage shift and a carrier mobility enhancement under 10 keV electron irradiation of few-layer MoS2 FETs attributed to beam-induced positive charge trapped in the SiO2 gate oxide. (27)
In this paper, we present the spectroscopic and electrical characterization of monolayer MoS2-based FETs, with Schottky Ti/Au contacts, focusing on the effects of low-energy e-beam irradiation. We show that the long exposure of the metal contacts to 10 keV e-beam in a SEM chamber enhances the transistor’s on-current. We explain such an improvement by radiation-induced lowering of the Schottky barrier at the metal contacts. We perform Monte Carlo simulation to track the e-beam through the device and show that when the beam is focused onto the contacts, most of the beam energy is absorbed within the metal. The local heat can induce atomic diffusion and interfacial reactions that change the chemical composition and structure of the metal–MoS2 interface or can generate or release tensile strain. Both effects cause the lowering of the Schottky barrier and the consequent increase in transistor current.
Our study shows that electron beam exposure during SEM imaging has non-negligible effects on MoS2 devices; however, it also highlights that a suitable exposure, with the e-beam focused on the contact region, can be conveniently exploited to reduce the contact resistance of the transistor. Compared to thermal annealing, our finding provides a way to improve the contact resistance by local conditioning, which avoids the exposure of the entire wafer to a high thermal budget.

Fabrication and Experimental Methods

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The MoS2 monolayer flakes were grown via chemical vapor deposition in a three-zone split tube furnace, purged with 500 Ncm3/min Ar gas for 15 min to minimize the O2 content. The growth SiO2/Si substrate was spin-coated with a 1% sodium cholate solution; then, a saturated ammonium heptamolybdate (AHM) solution was first annealed at 300 °C under ambient conditions to turn AHM into MoO3 to be used as the source for molybdenum. The target material was placed in a three-zone tube furnace along with 50 mg of S powder, positioned upstream in a separate heating zone. The zones containing S and AHM were heated to 150 °C and 750 °C, respectively. After 15 min of growth, the process was stopped, and the sample was cooled rapidly.
We realized FETs using the SiO2/Si substrate (thickness of the dielectric: 285 nm) as the back gate and evaporating the drain and source electrodes on selected MoS2 flakes through standard photolithography and lift-off processes. The contacts were made of Ti (10 nm) and Au (40 nm) used as adhesion and cover layers, respectively. Ti was deposited in high vacuum, which could not exclude the formation of TiO2, contributing to the resistance and Schottky barrier at the contacts. Figure 1a,b shows the SEM top view of a typical device and its schematic layout and measurement setup. The channel is made up from a monolayer flake [as confirmed by Raman and photoluminescence (PL), see below] of width and length of 20 and 4 μm, respectively, and a nominal thickness of 0.7 nm. Atomic force microscope (AFM) images (Figure 1c,d,e) show that the flake has an average height of 1.2±0.3 nm (which is typical for single-layer MoS2 measured in air by AFM) and appears to be extremely flat (roughness rms < 0.25 nm) and structurally intact. There are some contaminants because of the lithography process, which are weakly bound and can be swept by the AFM tip. Contacted and noncontacted flake areas do not differ with respect to contamination density—spectroscopic data should thus be comparable.

Figure 1

Figure 1. (a) SEM image of the MoS2 device and contact labels. (b) MoS2 FET layout and schematic of the common source configuration used for electrical characterization. (c) AFM image of the MoS2 flake between the electrical contacts, which appear here in white as the scale has been adjusted to properly image the MoS2 flake. (d) Zoom-in into the upper region of (c), showing that the flake is flat and structurally intact. The rms roughness is 0.221 nm for the SiO2 substrate and 0.237 nm for MoS2. (e) Height distribution taken from image (d), yielding a flake height of ∼1.2 nm.

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A total of seven MoS2 channels of identically prepared FETs have been characterized by Raman and PL spectroscopy just after processing. The measurements were performed with a Renishaw InVia Raman microscope at the Interdisciplinary Center for Analytics on the Nanoscale (ICAN). The excitation laser wavelength was 532 nm, and the power density was kept below 0.1 mW/μm2 to avoid damage to the MoS2 flake. Exemplary spectra of Raman characterization are shown in Figure 2. The chosen reference measurements are spectra obtained from MoS2 flakes on the same substrate, which were also in contact with the photoresist and various solvents during the processing and lift-off for the production of the FETs, but are not in contact with metal electrodes themselves. The shape of the PL spectra (Figure 2a) and the difference of the Raman modes (Figure 2b) differ significantly. The PL intensity (sum of all excitons and trions) for noncontacted MoS2 flakes is higher by a factor of 1.7 ± 0.8 than that for contacted MoS2. The mode differences for noncontacted and contacted MoS2 are 21.3 ± 0.7 cm–1 and 19.7 ± 0.7 cm–1, respectively. Both the changes in PL and Raman mode difference can be associated with built-in strain or changes in the electronic properties and the band structure of the MoS2 sheets. (39−43) From the linear dependencies of Raman mode positions on doping and strain, (39,40) we find a reduction of tensile strain by (0.46 ± 0.28) % and an increase in electron doping of 0.44 ± 0.36 × 1013 electrons per cm2 for the contacted 2D material in comparison with noncontacted MoS2 (details of the calculation method can be found in ref (44)). Hence, the significant alterations in the spectroscopic pre-characterization of the MoS2 channels can be clearly attributed to electronic and structural changes at the metal contact.

Figure 2

Figure 2. (a) PL and (b) Raman spectrum of monolayer MoS2 after FET processing. Blue: contacted MoS2 monolayer flake and red: noncontacted monolayer MoS2 flake.

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In the following, most of the electrical characterization refers to the transistor between the contacts labeled C2 and C3 in Figure 1a. The contact C3 was used as the drain and C2 as the grounded source. The electrical measurements were carried out inside a SEM chamber (LEO 1530, Zeiss), endowed with two metallic probes with nanometer positioning capability, connected to a Keithley 4200 SCS (source measurement units, Tektronix Inc.), at room temperature and a pressure of about 10–6 mbar. The e-beam of SEM, set to 10 keV and 10 pA, was used for the time-controlled irradiation of specific parts of the device.

Results and Discussion

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The output (Ids–Vds) and the transfer (Ids–Vgs) characteristics of the transistor are shown in Figure 3a,b, respectively. The output curve shows rectification with the forward current appearing at negative Vds, typical of a p-type Schottky diode, while the transfer characteristic shows an n-type transistor. This apparently contradictory behavior has been previously reported for MoS2 and WSe2 transistors and explained by the formation of two back-to-back and possibly asymmetric Schottky barriers at the contacts. (45,46) The forward current at negative Vds is caused by the different contact areas and by the image force barrier lowering of the forced junction (i.e., the drain, C3, in our case), while the reverse current at Vds > 0 V is limited by the grounded junction at the source (C2) contact. As the barrier lowering is more effective on the forced junction, the voltage being directly applied to it, the negative bias gives rise to the higher (apparently forward) current.

Figure 3

Figure 3. Output (a) and transfer (b) characteristics of the device between C2 and C3 contacts, with C3 used as the drain and C2 as the grounded source.

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After the initial electrical characterization, we performed two sets of exposures to the SEM electron beam. Each exposure lasted 300 s, corresponding to a fluence of ∼180 e–/nm2, over a surface area of ∼100 μm2. The two sets of irradiations were carried out first on the drain contact (C3) and then on the grounded source contact (C2). A final exposure of the MoS2 channel to the e-beam was performed as well.
Figure 4 summarizes the obtained results. The Ids–Vds curves were measured at the end of each irradiation, ∼120 s after the blanking of the e-beam, to allow cooling down. Starting from the bottom (black) line in Figure 4a, representing the output curve of the unexposed device, the current increases with the e-beam exposures. We note two major discontinuities in the sequence of Ids–Vds curves, corresponding to the start of the two irradiations sets. These gaps are likely due to the uncontrolled exposure of the whole device during the selection of the drain (C3) and grounded source (C2) contact areas for the respective irradiation sets.

Figure 4

Figure 4. (a) Output characteristics at Vgs = 0 V of the transistor formed by contacts C2–C3 exposed to two sets of electron irradiations performed first on contact C3 and then on C2. (b) Rectification ratio and (c) maximum forward and reverse current, at Vds = ±5 V, as a function of the irradiation number. (d) Zero-bias Schottky barrier variation at the contacts C2 and C3 as a function of the irradiation number.

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A different behavior of the forward with respect to the reverse current can be observed in Figure 4a, and a distinction of the effects of the irradiations on the drain (C3) and the grounded source (C2) can be made. Although the irradiation of the drain increases both the forward and the reverse currents, keeping the rectification ratio almost constant (see Figure 4 b), the irradiation of the source augments the reverse current in a faster way, rendering the output curves more symmetric. Figure 4b shows that repeated irradiations of the drain contact (C3) do not change the rectification ratio (at Vds = ±5 V), while the irradiation of the grounded source contact (C2) dramatically decreases the rectification ratio. Figure 4c shows that the maximum reverse and forward currents, at Vds = ±5 V, have different variation rates when the irradiation is either on the drain or source. Noticeably, Figure 4c shows that the increase in both the reverse and forward currents is an exponential function of the fluence, which is proportional to and can be parametrized by the irradiation number.
As the shape and the current intensity of the output characteristics are related to the Schottky barrier heights at the contacts, the exponentially increasing current and the changing rectification ratio point to radiation-induced Schottky barrier lowering. The energy release in the metal contacts can modify the chemistry of the metal–MoS2 interface or create stress and defects that can lead to a lowering of the barrier and a consequent contact resistance reduction. We note that the reduction of contact resistance by chemical reactions between the metal contacts and MoS2 channel has been reported for the metal deposited under ultrahigh vacuum (47) and contact laser annealing. (48) A disordered, compositionally graded layer, composed of Mo and TixSy species, forms on the surface of the MoS2 crystal following the deposition of Ti, and thermal annealing in the 100–600 °C temperature range can cause Ti diffusion inducing further chemical and structural changes at the Ti–MoS2 interface. (49,50) It is also possible that diffusion of Au atoms to the interface with MoS2 occurs under the energetic electron beam irradiation. Au does not react with MoS2 but reduces the contact resistance and therefore the Schottky barrier height. (51)
Similarly, tensile strain has been demonstrated to induce considerable Schottky and tunneling barrier lowering. (52)
A Schottky barrier of ∼0.2 eV is formed by several metals on MoS2 because of Fermi level pinning below the MoS2 conduction band. (53−55) Density functional theory calculations have indicated that the pinning at the metal–MoS2 interface is different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder-induced gap states, which are applicable to traditional metal–semiconductor junctions. At metal–MoS2 interfaces, the Fermi level is pinned either by a metal work function modification due to interface dipole formation arising from the charge redistribution or by the production of gap states mainly of Mo d-orbitals, characterized by the weakened intralayer S–Mo bonding because of the interface metal–S interaction. (56,57) The observed decrease in the Schottky barrier by e-beam irradiation, up to its complete disappearance, supports the occurrence of interface modifications that cause Fermi level depinning.
As the forward current at Vds < 0 V is limited by the Schottky barrier at the drain contact (C3), while the reverse current at Vds > 0 V is limited by the Schottky barrier at the grounded source contact C2 (which are the reverse-biased junctions for negative and positive Vds, respectively), the output curves of Figure 4a, which correspond always to reverse current, can be used to extract the behavior of the Schottky barriers as a function of the fluence (i.e., the e-beam irradiation number). Let us consider the thermionic current through a reverse-biased Schottky barrier (58,59)
(1)
where φBn and Isn are the barrier height and the reverse saturation current at the n-th e-beam irradiation, S is the junction area, A2D* is the 2D Richardson constant, k is the Boltzmann constant, T is the temperature, n is the ideality factor, and Va is the negative voltage across the barrier that makes . Let us define I0 as the reverse saturation current before e-beam exposure, that is, associated to the maximum barrier height φB0. To avoid the effect of bias which can induce image-force barrier lowering, (60) both In and I0 are obtained by extrapolating the measured currents to zero bias. Then, eq 1 can be used to evaluate the variation of the Schottky barrier, ΔφBn = φBn – φB0, as a function of the irradiation number
(2)
The zero-bias Schottky barrier variation, ΔφBn, is shown in Figure 4d for both source (C2) and drain (C3) contacts. The overall reduction of both barriers is comparable to the expected initial barrier height based on Fermi level pinning, meaning that the long irradiation can completely remove the barriers. The plot indicates that the two barriers behave differently for the irradiation of C2 or C3. Although the beam irradiation of either contact results in a lowering of both Schottky barriers, the barrier decrease is faster for the irradiation of the grounded source. Besides, the Schottky barrier at the source contact is the most affected by the irradiation of the source.
To explain these results, we propose the model based on the energy band diagrams, shown in Figure 5. A negative (positive) voltage applied to the drain contact (C3) causes an upward (downward) shift of the energy bands in the drain region. Electron beam irradiation of the contact lowers the Schottky barrier and the relative built-in potential, as shown by the red dashed lines in Figure 5. The reduction of a Schottky barrier and of its associated built-in potential, at the irradiated contact, results also in the lowering of the unexposed barrier, which can experience a stronger potential drop because of the reduced contact resistance of the first contact. Figure 5a represents the situation in which the e-beam is focused on the biased drain contact (C3). At Vds < 0 V, the current is limited mainly by the drain contact barrier which is lowered by the successive irradiations, causing the exponential increase in maximum forward current. At Vds > 0 V, the current is limited by the un-irradiated source contact (C2) barrier, and its dependence on the irradiation cycle is caused by the lowering of the built-in potential at the drain (C3). As the barrier and built-in lowering are the same, the rectification ratio remains almost constant. For irradiation of the grounded source (C2, Figure 5b), the current increases because of a similar mechanism, with the difference that the drain contact barrier limits the current for Vds > 0 V to a lesser extent, having been already irradiation-lowered. Therefore, the reverse current increases faster with the repeated irradiation and the rectification ratio decreases.

Figure 5

Figure 5. Low-bias energy band diagrams (black) and their modification under electron irradiation (red) of C3 (a) and of C2 (b) contacts resulting in barrier lowering (φ̅B).

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The effect of irradiation on the transfer characteristic of the transistor is shown in Figure 6 and confirms the radiation-induced increase in channel current. Besides, Figure 6a shows that the e-beam, independent of onto which contact it is focused on, causes a left shift of the transfer curve. Such a shift corresponds to a decrease in threshold voltage, defined as the x-axis intercept of the linear fit of the transfer curve on the linear scale. The threshold voltage as a function of the irradiation is displayed in Figure 6b. Although the e-beam exposure of the contacts provokes a left shift (the transfer curves are taken at the end of the two irradiation sets on the drain (C3) and grounded source (C2)), further left shift of the threshold voltage is observed when two successive irradiations are performed in the channel region.

Figure 6

Figure 6. (a) FET transfer characteristics at Vds = −4 V before and after e-beam irradiations of contacts C3 and C2 and of the channel. (b) Left shift of the threshold voltage extrapolated from the transfer characteristics over the e-beam exposure.

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The observed negative shift of the threshold voltage has been reported and discussed before. (27) It can be explained by the pile-up of positive charge in trap states of the SiO2 gate dielectric or at the SiO2–Si interface. The e-beam exposures produce electron–hole pairs in the SiO2 gate oxide and in the Si substrate: although mobile electrons are easily swept by the applied bias, the positive charges can be stored for long times. (27) The positive charge storage acts as an extra gate (similarly to the gating effect under light irradiation (61,62)) and enhances the n-type doping of the channel.
Indeed, Figure 6 shows that there is a slight recovery of the threshold voltage after 12 h of annealing at room temperature. However, we highlight that, as demonstrated by Figure 6a, the maximum channel current, which is limited by the contact resistances, remains unchanged after annealing, demonstrating that the irradiation-induced improvement of the contacts is permanent.
To further confirm our model, we performed a Monte Carlo simulation to track the path of the electrons under the contacts and in the channel region (Figure 7a,b), using the CASINO software package. (63−65) We simulated a 10 keV beam with one million electrons and a radius beam of 10 nm. The cathodoluminescence spectrum (Figure 7c) shows that electrons lose their energy and are stopped (Figure 7d) mostly in the Ti/Au metal stack, while they reach and are absorbed in the Si substrate when the irradiation is on the channel. The high release of energy in the metal contacts, similarly to thermal annealing, (66,67) induces Ti–MoS2 reactions and creates contact with the reduced Schottky barrier and contact resistance. Conversely, when we directly irradiate the MoS2 channel, energy is prevalently adsorbed in the Si bulk and its effect manifests only through the positive charge traps generated in the SiO2 layer.

Figure 7

Figure 7. Monte Carlo simulation using CASINO v2 of e-beam irradiation of the device (a) contacts and (b) of the MoS2 channel. (c) Simulated cathodoluminescence intensity through the sample, with the e-beam focused onto the contacts and the flake. (d) Simulation of the electrons' penetration depth through the sample.

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Conclusions

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We investigated the effects of 10 keV electron beam irradiation of the Schottky metal contacts in MoS2-based FETs. Spectroscopic analysis by Raman and PL shows that the presence of metal contacts changes the properties of monolayer MoS2 with respect to strain and doping. The electrical measurements revealed that electron beam irradiation improves the device conductance, reduces the rectification of the output characteristic, and causes a left shift of the threshold voltage. To explain such a feature, we propose that the energy absorbed in the metal contacts induces atomic diffusion and interfacial reactions that lower the Schottky barrier at the contacts and improve the contact resistance. We corroborate our model by direct measurement of the Schottky barrier height variation and by simulation of the electron trajectories in the contact regions.

Author Information

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  • Corresponding Author
    • Antonio Di Bartolomeo - Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy;  Orcidhttp://orcid.org/0000-0002-3629-726X;  Email: [email protected]
  • Authors
    • Aniello Pelella - Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy;  Orcidhttp://orcid.org/0000-0002-3831-0210
    • Osamah Kharsah - Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
    • Alessandro Grillo - Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy;  Orcidhttp://orcid.org/0000-0002-8909-9865
    • Francesca Urban - Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy;  INFN—Gruppo Collegato di Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  Orcidhttp://orcid.org/0000-0003-2109-1370
    • Maurizio Passacantando - Department of Physical and Chemical Sciences, University of L’Aquila, and CNR-SPIN L’Aquila, via Vetoio, Coppito, L’Aquila 67100, Italy;  Orcidhttp://orcid.org/0000-0002-3680-5295
    • Filippo Giubileo - CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy;  Orcidhttp://orcid.org/0000-0003-2233-3810
    • Laura Iemmo - Department of Physics and Interdepartmental Centre NanoMates, University of Salerno, via Giovanni Paolo II, Fisciano 84084, Italy;  CNR-SPIN, via Giovanni Paolo II, Fisciano 84084, Italy
    • Stephan Sleziona - Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany
    • Erik Pollmann - Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany;  Orcidhttp://orcid.org/0000-0002-3961-0426
    • Lukas Madauß - Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany;  Orcidhttp://orcid.org/0000-0003-2556-5967
    • Marika Schleberger - Fakultät für Physik and CENIDE, Universität Duisburg-Essen, Lotharstrasse 1, Duisburg 47057, Germany;  Orcidhttp://orcid.org/0000-0002-5785-1186
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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A.D.B. acknowledges the financial support from MIUR—Italian Ministry of Education, University and Research (projects Pico & Pro ARS01_01061 and RINASCIMENTO ARS01_01088). M.S. acknowledges the financial support from DFG—German Research Foundation (project number 406129719). The authors thank ICAN—facility founded by the German Research Foundation (DFG, reference RI_00313)—for Raman and PL spectroscopy, and A. Lorke of University of Duisburg-Essen for providing access to the clean room facilities.

References

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This article references 67 other publications.

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    The current understanding of phys. principles governing electronic transport in graphene field effect transistors (GFETs) has reached a level where we can model quite accurately device operation and predict intrinsic frequency limits of performance. In this work, we use this knowledge to analyze DC and RF transport properties of bottom-gated graphene on boron nitride field effect transistors exhibiting pronounced velocity satn. by substrate hyperbolic phonon polariton scattering, including Dirac pinch-off effect. We predict and demonstrate a max. oscillation frequency exceeding 20 GHz. We discuss the intrinsic 0.1 THz limit of GFETs and envision plasma resonance transistors as an alternative for sub-THz narrow-band detection.
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    Journal of Physical Chemistry C (2018), 122 (23), 12193-12199CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    In this study, the poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane-co-cyclohexyl methacrylate) [p(V4D4-co-CHMA)] copolymer was developed for use as a gate dielec. in molybdenum disulfide (MoS2) field-effect transistors (FETs). The p(V4D4-co-CHMA) copolymer was synthesized via the initiated chem. vapor deposition (iCVD) of two types of monomers: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4) and cyclohexyl methacrylate (CHMA). Four vinyl groups of V4D4 monomers and cyclohexyl groups of CHMA monomers were introduced to enhance the elec. strength of gate dielecs. through the formation of a highly crosslinked network and to reduce the charge trap densities at the MoS2-dielec. interface, resp. The iCVD-grown p(V4D4-co-CHMA) copolymer films yielded a dielec. const. of 2.3 and a leakage current of 3.8 × 10-11 A/cm2 at 1 MV/cm. The resulting MoS2 FETs with p(V4D4-co-CHMA) gate dielecs. exhibited excellent elec. properties, including an electron mobility of 35.1 cm2/V s, a subthreshold swing of 0.2 V/dec, and an on-off current ratio of 2.6 × 106. In addn., the environmental and operational stabilities of MoS2 FETs with p(V4D4-co-CHMA) top-gate dielecs. were superior to those of devices with SiO2 back-gate dielecs. The use of iCVD-grown copolymer gate dielecs. as demonstrated in this study provides a novel approach to realizing next-generation 2-dimensional electronics.
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    Rasmussen, F. A.; Thygesen, K. S. Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides. J. Phys. Chem. C 2015, 119, 13169– 13183,  DOI: 10.1021/acs.jpcc.5b02950
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    21
    Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides
    Rasmussen, Filip A.; Thygesen, Kristian S.
    Journal of Physical Chemistry C (2015), 119 (23), 13169-13183CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin-orbit coupling are calcd. in the G0W0 approxn., and comparison is made with different d. functional theory descriptions. Pitfalls related to the convergence of GW calcns. for two-dimensional (2D) materials are discussed together with possible solns. The monolayer band edge positions relative to vacuum are used to est. the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice const. is analyzed and rationalized in terms of the electronic structure. Finally, the q-dependent dielec. functions and effective electron and hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to est. exciton binding energies. Throughout the paper we focus on trends and correlations in the electronic structure rather than detailed anal. of specific materials. All the computed data is available in an open database.
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  22. 22
    Di Bartolomeo, A.; Pelella, A.; Liu, X.; Miao, F.; Passacantando, M.; Giubileo, F.; Grillo, A.; Iemmo, L.; Urban, F.; Liang, S. J. Pressure-Tunable Ambipolar Conduction and Hysteresis in Thin Palladium Diselenide Field Effect Transistors. Adv. Funct. Mater. 2019, 29, 1902483,  DOI: 10.1002/adfm.201902483
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    Di Bartolomeo, A.; Luongo, G.; Iemmo, L.; Urban, F.; Giubileo, F. Graphene–Silicon Schottky Diodes for Photodetection. IEEE Trans. Nanotechnol. 2018, 17, 1133– 1137,  DOI: 10.1109/tnano.2018.2853798
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    Jin, C.; Rasmussen, F. A.; Thygesen, K. S. Tuning the Schottky Barrier at the Graphene/MoS2 Interface by Electron Doping: Density Functional Theory and Many-Body Calculations. J. Phys. Chem. C 2015, 119, 19928– 19933,  DOI: 10.1021/acs.jpcc.5b05580
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    24
    Tuning the Schottky Barrier at the Graphene/MoS2 Interface by Electron Doping: Density Functional Theory and Many-Body Calculations
    Jin, Chengjun; Rasmussen, Filip A.; Thygesen, Kristian S.
    Journal of Physical Chemistry C (2015), 119 (34), 19928-19933CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Using ab initio calcns. we investigate the energy level alignment at the graphene/MoS2 heterostructure and the use of electron doping as a strategy to lower the Schottky barrier and achieve a low-resistance Ohmic contact. For the neutral heterostructure, d. functional theory (DFT) with a generalized gradient approxn. predicts a Schottky barrier height of 0.18 eV, whereas the G0W0 method increases this value to 0.60 eV. While the DFT band gap of MoS2 does not change when the heterostructure is formed, the G0W0 gap is reduced by 0.30 eV as a result of the enhanced screening by the graphene layer. In contrast to the case of metal substrates, where the band alignment is governed by Pauli repulsion-induced interface dipoles, the graphene/MoS2 heterostructure shows only a negligible interface dipole. As a consequence, the band alignment at the neutral heterostructure is not changed when the two layers are brought into contact. We systematically follow the band alignment as a function of doping concn. and find that the Fermi level of the graphene crosses the MoS2 conduction band at a doping concn. of around 1012 cm-2. The variation of the energy levels with doping concn. is shown to be mainly governed by the electrostatic potential resulting from the doping charge.
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  25. 25
    Grillo, A.; Di Bartolomeo, A.; Urban, F.; Passacantando, M.; Caridad, J. M.; Sun, J.; Camilli, L. Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors. ACS Appl. Mater. Interfaces 2020, 12, 12998– 13004,  DOI: 10.1021/acsami.0c00348
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    Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors
    Grillo Alessandro; Di Bartolomeo Antonio; Urban Francesca; Grillo Alessandro; Di Bartolomeo Antonio; Urban Francesca; Passacantando Maurizio; Caridad Jose M; Sun Jianbo; Camilli Luca
    ACS applied materials & interfaces (2020), 12 (11), 12998-13004 ISSN:.
    We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20-280 K temperature range, revealing that the p-type electrical conductivity and the field-effect mobility are growing functions of temperature. An unexpected peak is observed in the temperature dependence of the carrier density per area at ∼75 K. Such a feature is explained considering that the increased carrier concentration at higher temperatures and the vertical band bending combined with the gate field lead to the formation of a two-dimensional (2D) conducting channel, limited to few interfacial GeAs layers, which dominates the channel conductance. The conductivity follows the variable-range hopping model at low temperatures and becomes the band-type at higher temperatures when the 2D channel is formed. The formation of the 2D channel is validated through a numerical simulation that shows excellent agreement with the experimental data.
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    Schleberger, M.; Kotakoski, J. 2D Material Science: Defect Engineering by Particle Irradiation. Materials 2018, 11, 1885,  DOI: 10.3390/ma11101885
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    2D material science: defect engineering by particle irradiation
    Schleberger, Marika; Kotakoski, Jani
    Materials (2018), 11 (10), 1885/1-1885/29CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
    Two-dimensional (2D) materials are at the heart of many novel devices due to their unique and often superior properties. For simplicity, 2D materials are often assumed to exist in their text-book form, i.e., as an ideal solid with no imperfections. However, defects are ubiquitous in macroscopic samples and play an important - if not imperative - role for the performance of any device. Thus, many independent studies have targeted the artificial introduction of defects into 2D materials by particle irradn. In our view it would be beneficial to develop general defect engineering strategies for 2D materials based on a thorough understanding of the defect creation mechanisms, which may significantly vary from the ones relevant for 3D materials. This paper reviews the state-of-the-art in defect engineering of 2D materials by electron and ion irradn. with a clear focus on defect creation on the at. scale and by individual impacts. Whenever possible we compile reported exptl. data alongside corresponding theor. studies. We show that, on the one hand, defect engineering by particle irradn. covers a wide range of defect types that can be fabricated with great precision in the most commonly investigated 2D materials. On the other hand, gaining a complete understanding still remains a challenge, that can be met by combining advanced theor. methods and improved exptl. set-ups, both of which only now begin to emerge. In conjunction with novel 2D materials, this challenge promises attractive future opportunities for researchers in this field.
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  27. 27
    Giubileo, F.; Iemmo, L.; Passacantando, M.; Urban, F.; Luongo, G.; Sun, L.; Amato, G.; Enrico, E.; Di Bartolomeo, A. Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS2 Field-Effect Transistors. J. Phys. Chem. C 2019, 123, 1454– 1461,  DOI: 10.1021/acs.jpcc.8b09089
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    27
    Effect of electron irradiation on the transport and field emission properties of few-layer MoS2 field-effect transistors
    Giubileo, Filippo; Iemmo, Laura; Passacantando, Maurizio; Urban, Francesca; Luongo, Giuseppe; Sun, Linfeng; Amato, Giampiero; Enrico, Emanuele; Di Bartolomeo, Antonio
    Journal of Physical Chemistry C (2019), 123 (2), 1454-1461CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Elec. characterization of few-layer MoS2-based field-effect transistors with Ti/Au electrodes is performed in the vacuum chamber of a scanning electron microscope in order to study the effects of electron-beam irradn. on the transport properties of the device. A neg. threshold voltage shift and a carrier mobility enhancement are obsd. and explained in terms of pos. charges trapped in the SiO2 gate oxide, during the irradn. The transistor channel current is increased up to 3 orders of magnitudes after the exposure to an irradn. dose of 100 e-/nm2. Finally, a complete field emission characterization of the MoS2 flake, achieving emission stability for several hours and a min. turn-on field of ≈20 V/μm with a field enhancement factor of about 500 at an anode-cathode distance of ∼1.5 μm, demonstrates the suitability of few-layer MoS2 as a 2-dimensional emitting surface for cold-cathode applications.
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  28. 28
    Di Bartolomeo, A.; Urban, F.; Pelella, A.; Grillo, A.; Passacantando, M.; Liu, X.; Giubileo, F. Electron Irradiation of Multilayer PdSe2 Field Effect Transistors. Nanotechnology 2020, 31, 375204,  DOI: 10.1088/1361-6528/ab9472
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    Electron irradiation of multilayer PdSe2 field effect transistors
    Di Bartolomeo, A.; Urban, F.; Pelella, A.; Grillo, A.; Passacantando, M.; Liu, X.; Giubileo, F.
    Nanotechnology (2020), 31 (37), 375204CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Palladium diselenide (PdSe2) is a recently isolated layered material that has attracted a lot of interest for its pentagonal structure, air stability and elec. properties that are largely tunable by the no. of layers. In this work, multilayer PdSe2 is used as the channel of back-gate field-effect transistors, which are studied under repeated electron irradiations. Source-drain Pd electrodes enable contacts with resistance below 350 kΩ μm. The transistors exhibit a prevailing n-type conduction in high vacuum, which reversibly turns into ambipolar elec. transport at atm. pressure. Irradn. by 10 keV electrons suppresses the channel conductance and promptly transforms the device from n-type to p-type. An electron fluence as low as 160 e-/nm2 dramatically changes the transistor behavior, demonstrating a high sensitivity of PdSe2 to electron irradn. The sensitivity is lost after a few exposures, with a satn. condition being reached for fluence higher than ~ 4000 e-/nm2. The damage induced by high electron fluence is irreversible as the device persists in the radiation-modified state for several hours, if kept in vacuum and at room temp. With the support of numerical simulation, we explain such a behavior by electron-induced Se atom vacancy formation and charge trapping in slow trap states at the Si/SiO2 interface.
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  29. 29
    Ochedowski, O.; Marinov, K.; Wilbs, G.; Keller, G.; Scheuschner, N.; Severin, D.; Bender, M.; Maultzsch, J.; Tegude, F. J.; Schleberger, M. Radiation Hardness of Graphene and MoS2 Field Effect Devices against Swift Heavy Ion Irradiation. J. Appl. Phys. 2013, 113, 214306,  DOI: 10.1063/1.4808460
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    Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation
    Ochedowski, O.; Marinov, K.; Wilbs, G.; Keller, G.; Scheuschner, N.; Severin, D.; Bender, M.; Maultzsch, J.; Tegude, F. J.; Schleberger, M.
    Journal of Applied Physics (Melville, NY, United States) (2013), 113 (21), 214306/1-214306/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradn. with swift heavy ions. Devices were prepd. with exfoliated single layers of MoS2 and graphene, resp. They were characterized before and after irradn. with 1.14 GeV U28+ ions using three different fluences. By elec. characterization, at. force microscopy, and Raman spectroscopy, we show that the irradn. leads to significant changes of structural and elec. properties. At the highest fluence of 4 × 1011 ions/cm2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance. (c) 2013 American Institute of Physics.
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  30. 30
    Ernst, P.; Kozubek, R.; Madauß, L.; Sonntag, J.; Lorke, A.; Schleberger, M. Irradiation of Graphene Field Effect Transistors with Highly Charged Ions. Nucl. Instrum. Methods Phys. Res., Sect. B 2016, 382, 71– 75,  DOI: 10.1016/j.nimb.2016.03.043
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    Irradiation of graphene field effect transistors with highly charged ions
    Ernst, P.; Kozubek, R.; Madauss, L.; Sonntag, J.; Lorke, A.; Schleberger, M.
    Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms (2016), 382 (), 71-75CODEN: NIMBEU; ISSN:0168-583X. (Elsevier B.V.)
    In this work, graphene field-effect transistors are used to detect defects due to irradn. with slow, highly charged ions. In order to avoid contamination effects, a dedicated ultra-high vacuum set up has been designed and installed for the in situ cleaning and elec. characterization of graphene field-effect transistors during irradn. To investigate the elec. and structural modifications of irradiated graphene field-effect transistors, their transfer characteristics as well as the corresponding Raman spectra are analyzed as a function of ion fluence for two different charge states. The irradn. expts. show a decreasing mobility with increasing fluences. The mobility redn. scales with the potential energy of the ions. In comparison to Raman spectroscopy, the transport properties of graphene show an extremely high sensitivity with respect to ion irradn.: a significant drop of the mobility is obsd. already at fluences below 15 ions/μm2, which is more than one order of magnitude lower than what is required for Raman spectroscopy.
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  31. 31
    Madauß, L.; Ochedowski, O.; Lebius, H.; Ban-d’Etat, B.; Naylor, C. H.; Johnson, A. T. C.; Kotakoski, J.; Schleberger, M. Defect Engineering of Single- and Few-Layer MoS2 by Swift Heavy Ion Irradiation. 2D Mater. 2016, 4, 015034,  DOI: 10.1088/2053-1583/4/1/015034
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    Kozubek, R.; Tripathi, M.; Ghorbani-Asl, M.; Kretschmer, S.; Madauß, L.; Pollmann, E.; O’Brien, M.; McEvoy, N.; Ludacka, U.; Susi, T.; Duesberg, G. S.; Wilhelm, R. A.; Krasheninnikov, A. V.; Kotakoski, J.; Schleberger, M. Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions. J. Phys. Chem. Lett. 2019, 10, 904– 910,  DOI: 10.1021/acs.jpclett.8b03666
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    Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions
    Kozubek, Roland; Tripathi, Mukesh; Ghorbani-Asl, Mahdi; Kretschmer, Silvan; Madauss, Lukas; Pollmann, Erik; O'Brien, Maria; McEvoy, Niall; Ludacka, Ursula; Susi, Toma; Duesberg, Georg S.; Wilhelm, Richard A.; Krasheninnikov, Arkady V.; Kotakoski, Jani; Schleberger, Marika
    Journal of Physical Chemistry Letters (2019), 10 (5), 904-910CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradn. with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the crit. role of energy deposition from the ion to the material through electronic excitation in the defect creation process and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Anal. of the irradiated samples with at. resoln. scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradn. with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.
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    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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    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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    Shahzad, K.; Jia, K.; Zhao, C.; Wang, D.; Usman, M.; Luo, J. Effects of Different Ion Irradiation on the Contact Resistance of Pd/Graphene Contacts. Materials 2019, 12, 3928,  DOI: 10.3390/ma12233928
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    Effects of different ion irradiation on the contact resistance of Pd/graphene contacts
    Shahzad, Kashif; Jia, Kunpeng; Zhao, Chao; Wang, Dahai; Usman, Muhammad; Luo, Jun
    Materials (2019), 12 (23), 3928CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
    The effect of ion-induced defects on graphene was studied to investigate the contact resistance of 40 nm palladium (Pd) contacting on graphene. The defect development was considered and analyzed by irradiating boron (B), carbon (C), nitrogen (N2), and argon (Ar) ions on as-transferred graphene before metalization. The bombardment energy was set at 1.5 keV and ion dose at 1 x 1014 ions/cm2. The defect yields under different ion irradn. conditions were examd. by Raman spectroscopy. Although, dissoln. process occurs spontaneously upon metal deposition, chem. reaction between metal and graphene is more pronounced at higher temps. The rapid thermal annealing (RTA) treatment was performed to improve the Pd/graphene contact after annealing at 450 °C, 500 °C, 550 °C, and 600 °C. The lowest contact resistance of 95.2 Ω-μm was achieved at 550 °C RTA with Ar ion irradn. We have proved that ion irradn. significantly enhance the Pd/graphene contact instead of pd/pristine graphene contact. Therefore, in view of the contention of results ion induced defects before metalization plus the RTA served an excellent purpose to reduce the contact resistance.
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  35. 35
    Yan, X.; Jia, K.; Su, Y.; Ma, Y.; Luo, J.; Zhu, H.; Wei, Y. Edge-Contact Formed by Oxygen Plasma and Rapid Thermal Annealing to Improve Metal-Graphene Contact Resistance. ECS J. Solid State Sci. Technol. 2018, 7, M11– M15,  DOI: 10.1149/2.0251802jss
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    Edge-Contact Formed by Oxygen Plasma and Rapid Thermal Annealing to Improve Metal-Graphene Contact Resistance
    Yan, Xiangyu; Jia, Kunpeng; Su, Yajuan; Ma, Yuanjun; Luo, Jun; Zhu, Huilong; Wei, Yayi
    ECS Journal of Solid State Science and Technology (2018), 7 (2), M11-M15CODEN: EJSSBG; ISSN:2162-8769. (Electrochemical Society)
    Graphene is the first sepd. 2-dimensional material which has extremely high carrier mobility. The high carrier mobility puts forward stricter requirement on the contact resistance of graphene devices. In this paper, graphene is treated by oxygen plasma with varied time to form different contact type between graphene and metal, esp. edge contact which enhances the graphene/metal bond strength and improves the contact resistance. To investigate the effect of thermal annealing on the contact resistance, the graphene devices with different oxygen plasma treatment were annealed by rapid thermal annealing. With the optimized annealing treatment, the av. edge contact resistance is reduced by 78.6% to 118 Ω•μm, with a min. value of 92 Ω•μm, which is very close to the best results reported so far. For the surface contact devices, thermal annealing resulted in reducing of contact resistance by 48% which is far less than edge contact. Due to the different influence of thermal annealing on various contact type, the edge contact resistance is 37.1% less than surface contact resistance. Since oxygen plasma treatment and rapid thermal annealing both are typical process of regular integrated circuit manuf. flow, the method proposed in the paper could provide an effective and feasible way to optimize the graphene/metal contact, hence promote the performance of future graphene and other two-dimensional material based devices.
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  36. 36
    Choi, B. Y.; Cho, K.; Pak, J.; Kim, T.-Y.; Kim, J.-K.; Shin, J.; Seo, J.; Chung, S.; Lee, T. Effects of Electron Beam Irradiation and Thiol Molecule Treatment on the Properties of MoS2 Field Effect Transistors. J. Korean Phys. Soc. 2018, 72, 1203– 1208,  DOI: 10.3938/jkps.72.1203
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    Effects of Electron Beam Irradiation and Thiol Molecule Treatment on the Properties of Molybdenum disulfide Field Effect Transistors
    Choi, Barbara Yuri; Cho, Kyungjune; Pak, Jinsu; Kim, Tae-Young; Kim, Jae-Keun; Shin, Jiwon; Seo, Junseok; Chung, Seungjun; Lee, Takhee
    Journal of the Korean Physical Society (2018), 72 (10), 1203-1208CODEN: JKPSDV; ISSN:0374-4884. (Korean Physical Society)
    We investigated the effects of the structural defects intentionally created by electron-beam irradn. with an energy of 30 keV on the elec. properties of monolayer MoS2 field effect transistors (FETs). We obsd. that the created defects by electron beam irradn. on the MoS2 surface working as trap sites deteriorated the carrier mobility and carrier concn. with increasing the subthreshold swing value and shifting the threshold voltage in MoS2 FETs. The elec. properties of electron-beam irradiated MoS2 FETs were slightly improved by treating the devices with thiol-terminated mols. which presumably passivated the structural defects of MoS2. The results of this study may enhance the understanding of the elec. properties of MoS2 FETs in terms of creating and passivating defect sites.
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  37. 37
    Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615– 2622,  DOI: 10.1021/nl4007479
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    Intrinsic Structural Defects in Monolayer Molybdenum Disulfide
    Zhou, Wu; Zou, Xiaolong; Najmaei, Sina; Liu, Zheng; Shi, Yumeng; Kong, Jing; Lou, Jun; Ajayan, Pulickel M.; Yakobson, Boris I.; Idrobo, Juan-Carlos
    Nano Letters (2013), 13 (6), 2615-2622CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mech., electronic, optical, and chem. properties that can be used for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphol. of the MoS2 layers. Here the authors provide a systematic study of intrinsic structural defects in chem. vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct at. resoln. imaging, and explore their energy landscape and electronic properties using 1st-principles calcns. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were obsd. in MoS2. One-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The at. scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.
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  38. 38
    Durand, C.; Zhang, X.; Fowlkes, J.; Najmaei, S.; Lou, J.; Li, A.-P. Defect-Mediated Transport and Electronic Irradiation Effect in Individual Domains of CVD-Grown Monolayer MoS2. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2015, 33, 02B110,  DOI: 10.1116/1.4906331
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    Rice, C.; Young, R. J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K. S. Raman-Scattering Measurements and First-Principles Calculations of Strain-Induced Phonon Shifts in Monolayer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 081307,  DOI: 10.1103/physrevb.87.081307
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    39
    Raman-scattering measurements and first-principles calculations of strain-induced phonon shifts in monolayer MoS2
    Rice, C.; Young, R. J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K. S.
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (8), 081307/1-081307/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The effect of strain on the phonon modes of monolayer and few-layer MoS2 has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS2 sandwiched between two layers of optically transparent polymer. The resulting band shifts of the E12g (∼385.3 cm-1) and A1g (∼402.4 cm-1) Raman modes were found to be small but observable. First-principles plane-wave calcns. based on d. functional perturbation theory were used to det. the Gruneisen parameters for the E1g, E12g, A1g, and A2u modes and predict the exptl. obsd. band shifts for the monolayer material. The polymer-MoS2 interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS2 into devices.
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  40. 40
    Chakraborty, B.; Bera, A.; Muthu, D. V. S.; Bhowmick, S.; Waghmare, U. V.; Sood, A. K. Symmetry-Dependent Phonon Renormalization in Monolayer MoS2 Transistor. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 161403,  DOI: 10.1103/physrevb.85.161403
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    40
    Symmetry-dependent phonon renormalization in monolayer MoS2 transistor
    Chakraborty, Biswanath; Bera, Achintya; Muthu, D. V. S.; Bhowmick, Somnath; Waghmare, U. V.; Sood, A. K.
    Physical Review B: Condensed Matter and Materials Physics (2012), 85 (16), 161403/1-161403/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS2 electrochem. top-gated field-effect transistor (FET), we show softening and broadening of the A1g phonon with electron doping, whereas the other Raman-active E12g mode remains essentially inert. Confirming these results with first-principles d. functional theory based calcns., we use group theor. arguments to explain why the A1g mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS2-based FETs, which have a high on-off ratio and are of technol. significance.
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  41. 41
    Scheuschner, N.; Ochedowski, O.; Kaulitz, A.-M.; Gillen, R.; Schleberger, M.; Maultzsch, J. Photoluminescence of Freestanding Single- and Few-Layer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 125406,  DOI: 10.1103/physrevb.89.125406
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    Photoluminescence of freestanding single- and few-layer MoS2
    Scheuschner, Nils; Ochedowski, Oliver; Kaulitz, Anne-Marie; Gillen, Roland; Schleberger, Marika; Maultzsch, Janina
    Physical Review B: Condensed Matter and Materials Physics (2014), 89 (12), 125406/1-125406/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only obsd. in freestanding MoS2. The photoluminescence of supported single-layer MoS2 instead originates from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to ≈80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.
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  42. 42
    Conley, H. J.; Wang, B.; Ziegler, J. I.; Haglund, R. F.; Pantelides, S. T.; Bolotin, K. I. Bandgap Engineering of Strained Monolayer and Bilayer MoS2. Nano Lett. 2013, 13, 3626– 3630,  DOI: 10.1021/nl4014748
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    Bandgap Engineering of Strained Monolayer and Bilayer MoS2
    Conley, Hiram J.; Wang, Bin; Ziegler, Jed I.; Haglund, Richard F.; Pantelides, Sokrates T.; Bolotin, Kirill I.
    Nano Letters (2013), 13 (8), 3626-3630CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The influence of uniaxial tensile mech. strain in the range 0-2.2% on the phonon spectra and band structures of monolayer and bilayer MoS2 2-dimensional crystals are reported. First, Raman spectra were used to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and ext. a Grueneisen parameter of ∼1.06. Second, using luminescence a decrease in the optical band gap of MoS2 was measured that is approx. linear with strain, ∼45 meV/% strain for monolayer MoS2 and ∼120 meV/% strain for bilayer MoS2. Third, a pronounced strain-induced decrease in the luminescence intensity of monolayer MoS2 was obsd. that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ∼1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of 2-dimensional crystals, transition metal dichalcogenides.
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  43. 43
    Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207– 211,  DOI: 10.1038/nmat3505
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    Tightly bound trions in monolayer MoS2
    Mak, Kin Fai; He, Keliang; Lee, Changgu; Lee, Gwan Hyoung; Hone, James; Heinz, Tony F.; Shan, Jie
    Nature Materials (2013), 12 (3), 207-211CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Two-dimensional (2D) at. crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable phys. properties. In contrast to graphene, monolayer MoS2 is a noncentrosym. material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 108 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here the authors report the spectroscopic identification in a monolayer MoS2 field-effect transistor of tightly bound neg. trions, a quasiparticle composed of 2 electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analog in conventional semiconductors. They also possess a large binding energy (∼ 20 meV), rendering them significant even at room temp. Results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2-dimensional at. crystals.
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  44. 44
    Pollmann, E.; Madauß, L.; Schumacher, S.; Kumar, U.; Heuvel, F.; Ende, C. vom.; Yilmaz, S.; Gündörmüs, S.; Schleberger, M. Apparent Differences between Single Layer Molybdenum Disulfide Fabricated via Chemical Vapor Deposition and Exfoliation. 2020, arXiv:2006.05789 [cond-mat].
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    Di Bartolomeo, A.; Grillo, A.; Urban, F.; Iemmo, L.; Giubileo, F.; Luongo, G.; Amato, G.; Croin, L.; Sun, L.; Liang, S.-J.; Ang, L. K. Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors. Adv. Funct. Mater. 2018, 28, 1800657,  DOI: 10.1002/adfm.201800657
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    Di Bartolomeo, A.; Urban, F.; Passacantando, M.; McEvoy, N.; Peters, L.; Iemmo, L.; Luongo, G.; Romeo, F.; Giubileo, F. A WSe2 Vertical Field Emission Transistor. Nanoscale 2019, 11, 1538– 1548,  DOI: 10.1039/c8nr09068h
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    A WSe2 vertical field emission transistor
    Di Bartolomeo, Antonio; Urban, Francesca; Passacantando, Maurizio; McEvoy, Niall; Peters, Lisanne; Iemmo, Laura; Luongo, Giuseppe; Romeo, Francesco; Giubileo, Filippo
    Nanoscale (2019), 11 (4), 1538-1548CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    We report the first observation of a gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chem.-vapor deposition on a SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, resp. The gate-tuned n-type conduction enables field emission, i.e. the extn. of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an elec. field ∼100 V μm-1 and exhibits good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on the WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.
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  47. 47
    Smyth, C. M.; Addou, R.; McDonnell, S.; Hinkle, C. L.; Wallace, R. M. Contact Metal–MoS2 Interfacial Reactions and Potential Implications on MoS2 -Based Device Performance. J. Phys. Chem. C 2016, 120, 14719– 14729,  DOI: 10.1021/acs.jpcc.6b04473
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    47
    Contact Metal-MoS2 Interfacial Reactions and Potential Implications on MoS2-Based Device Performance
    Smyth, Christopher M.; Addou, Rafik; McDonnell, Stephen; Hinkle, Christopher L.; Wallace, Robert M.
    Journal of Physical Chemistry C (2016), 120 (27), 14719-14729CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS2 using electron beam deposition under two different reactor base pressures to det. the contact metal-MoS2 interface chem. and its dependence on the reactor ambient. The high work function metal Au does not react with MoS2 regardless of reactor ambient. In contrast, interfacial reactions between MoS2 and another high work function metal, Ir, are obsd. when it is deposited under both high vacuum (HV, ∼ 1 × 10-6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10-9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS2 when the contact metal is deposited under UHV conditions. Sc is rapidly oxidized on the MoS2 surface, whereas Cr is only partially oxidized when deposited under HV conditions. Deposition chamber ambient can affect the contact metal chem. in addn. to the chem. present at the contact metal-MoS2 interface. These results elucidate the true chem. of some contact metal-MoS2 interfaces and its dependence on the deposition ambient, and highlight the need to consider the chem. states present at the interface and their impact on contact resistance with MoS2.
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  48. 48
    Kwon, H.; Baik, S.; Jang, J.; Jang, J.; Kim, S.; Grigoropoulos, C.; Kwon, H.-J. Ultra-Short Pulsed Laser Annealing Effects on MoS2 Transistors with Asymmetric and Symmetric Contacts. Electronics 2019, 8, 222,  DOI: 10.3390/electronics8020222
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    48
    Ultra-short pulsed laser annealing effects on MoS2 transistors with asymmetric and symmetric contacts
    Kwon, Hyeokjin; Baik, Seunghun; Eun Jang, Jae; Jang, Jaewon; Kim, Sunkook; Grigoropoulos, Costas P.; Kwon, Hyuk-Jun
    Electronics (Basel, Switzerland) (2019), 8 (2), 222CODEN: ELECGJ; ISSN:2079-9292. (MDPI AG)
    The ultra-short pulsed laser annealing process enhances the performance of MoS2 thin film transistors (TFTs) without thermal damage on plastic substrates. However, there has been insufficient investigation into how much improvement can be brought about by the laser process. In this paper, we obsd. how the parameters of TFTs, i.e., mobility, subthreshold swing, Ion/Ioff ratio, and Vth, changed as the TFTs' contacts were (1) not annealed, (2) annealed on one side, or (3) annealed on both sides. The results showed that the linear effective mobility (μeff_lin) increased from 13.14 [cm2/Vs] (not annealed) to 18.84 (one side annealed) to 24.91 (both sides annealed). Also, Ion/Ioff ratio increased from 2.27 × 105 (not annealed) to 3.14 × 105 (one side annealed) to 4.81 × 105 (both sides annealed), with Vth shifting to neg. direction. Analyzing the main reason for the improvement through the Y function method (YFM), we found that both the contact resistance (Rc) and the channel interface resistance (Rch) improves after the pulsed laser annealings under different conditions. Moreover, the Rc enhances more dramatically than the Rch does. In conclusion, our picosecond laser annealing improves the performance of TFTs (esp., the Rc) in direct proportion to the no. of annealings applied. The results will contribute to the investigation about correlations between the laser annealing process and the performance of devices.
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  49. 49
    Freedy, K. M.; Zhang, H.; Litwin, P. M.; Bendersky, L. A.; Davydov, A. V.; McDonnell, S. Thermal Stability of Titanium Contacts to MoS2. ACS Appl. Mater. Interfaces 2019, 11, 35389– 35393,  DOI: 10.1021/acsami.9b08829
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    49
    Thermal stability of titanium contacts to MoS2
    Freedy, Keren M.; Zhang, Huairuo; Litwin, Peter M.; Bendersky, Leonid A.; Davydov, Albert V.; McDonnell, Stephen
    ACS Applied Materials & Interfaces (2019), 11 (38), 35389-35393CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices; however, its effects on interface chem. have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geol. bulk single crystals of MoS2 in ultrahigh vacuum (UHV) is investigated with XPS and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is obsd. resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compds. Annealing Ti/MoS2 sequentially at 100, 300, and 600°C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at a transistor operating temp. of 100°C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for elec. transport properties, highlighting the importance of interface chem. in the metal contact design and fabrication.
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  50. 50
    McDonnell, S.; Smyth, C.; Hinkle, C. L.; Wallace, R. M. MoS2 −Titanium Contact Interface Reactions. ACS Appl. Mater. Interfaces 2016, 8, 8289– 8294,  DOI: 10.1021/acsami.6b00275
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    50
    MoS2-titanium contact interface reactions
    McDonnell, Stephen; Smyth, Christopher; Hinkle, Christopher L.; Wallace, Robert M.
    ACS Applied Materials & Interfaces (2016), 8 (12), 8289-8294CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The formation of the Ti-MoS2 interface, which is heavily utilized in nanoelectronic device research, is studied by XPS. It is found that, if deposition under high vacuum (∼1 × 10-6 mbar) as opposed to ultrahigh vacuum (∼1 × 10-9 mbar) conditions are used, TiO2 forms at the interface rather than Ti. The high vacuum deposition results in an interface free of any detectable reaction between the semiconductor and the deposited contact. In contrast, when metallic titanium is successfully deposited by carrying out depositions in ultrahigh vacuum, the titanium reacts with MoS2 forming TixSy and metallic Mo at the interface. These results have far reaching implications as many prior studies assuming Ti contacts may have actually used TiO2 due to the nature of the deposition tools used.
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  51. 51
    English, C. D.; Shine, G.; Dorgan, V. E.; Saraswat, K. C.; Pop, E. Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition. Nano Lett. 2016, 16, 3824– 3830,  DOI: 10.1021/acs.nanolett.6b01309
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    Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition
    English, Chris D.; Shine, Gautam; Dorgan, Vincent E.; Saraswat, Krishna C.; Pop, Eric
    Nano Letters (2016), 16 (6), 3824-3830CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (RC). Here the authors present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temps. (80 to 500 K), carrier densities (1012 to 1013 cm-2), and contact dimensions (20 to 500 nm). The authors uncover that Au deposited in ultra-high vacuum (∼10-9 Torr) yields three times lower RC than under normal conditions, reaching 740 Ω μm and specific contact resistivity 3 × 10-7 Ω cm2, stable for over four months. Modeling reveals sep. RC contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified exptl. using devices with 20 nm contacts and 70 nm contact pitch (CP), equiv. to the 14 nm technol. node.
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  52. 52
    Wang, Q.; Deng, B.; Shi, X. A New Insight for Ohmic Contacts to MoS2 : By Tuning MoS2 Affinity Energies but Not Metal Work-Functions. Phys. Chem. Chem. Phys. 2017, 19, 26151– 26157,  DOI: 10.1039/c7cp05109c
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    52
    A new insight for ohmic contacts to MoS2: by tuning MoS2 affinity energies but not metal work-functions
    Wang, Qian; Deng, Bei; Shi, Xingqiang
    Physical Chemistry Chemical Physics (2017), 19 (38), 26151-26157CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently attracted tremendous interest for fundamental studies and applications. High contact resistances between the metal electrodes and the 2D TMDCs, usually composed of a tunneling barrier (TB) and a Schottky barrier (SB), are the key bottleneck to the realization of high performance devices based on such systems. Here, from van der Waals d. functional theory calcns., we demonstrate that strain can provide a feasible means to reduce the contact resistances between, for example, 2D semiconductor MoS2 and metal surfaces, in both strong and weak coupling regimes. Both the SB and TB are lowered significantly with the increasing tensile strain in both the coupling regimes. Esp., the SB can reduce to zero in all configurations considered, with tensile strain increasing to ∼4% or above. The mechanism of SB redn. under tensile strain is attributed to the increase of the MoS2 affinity energy since the monolayer MoS2 conduction band min. (CBm) is derived from anti-bonding states. Thus, the SB in other semiconducting TMDCs with an anti-bonding CBm (for n-type contact) could also be reduced to zero by tensile strain. Our discoveries thus shed a new and general light on minimizing the contact resistance of semiconducting TMDCs-metal based contacts and this can also prove applicable to other 2D semiconductors, e.g. phosphorene.
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  53. 53
    Kim, C.; Moon, I.; Lee, D.; Choi, M. S.; Ahmed, F.; Nam, S.; Cho, Y.; Shin, H.-J.; Park, S.; Yoo, W. J. Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides. ACS Nano 2017, 11, 1588– 1596,  DOI: 10.1021/acsnano.6b07159
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    53
    Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides
    Kim, Changsik; Moon, Inyong; Lee, Daeyeong; Choi, Min Sup; Ahmed, Faisal; Nam, Seunggeol; Cho, Yeonchoo; Shin, Hyeon-Jin; Park, Seongjun; Yoo, Won Jong
    ACS Nano (2017), 11 (2), 1588-1596CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Elec. metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs is reported only theor., although that of bulk TMDCs is reported exptl. Here, the authors report the exptl. study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. The authors also quant. compared the authors' results with the theor. simulation results of the monolayer structure as well as the exptl. results of the bulk structure. The authors measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, resp., suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theor. prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. The authors' results further imply that metal work functions have very little influence on contact properties of 2-dimensional-material-based devices. Also, Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chem. doping into the 2-dimensional materials. These findings provide a practical guideline for depinning Fermi level at the 2-dimensional interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.
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  54. 54
    Guo, Y.; Liu, D.; Robertson, J. 3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides. ACS Appl. Mater. Interfaces 2015, 7, 25709– 25715,  DOI: 10.1021/acsami.5b06897
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    3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides
    Guo, Yuzheng; Liu, Dameng; Robertson, John
    ACS Applied Materials & Interfaces (2015), 7 (46), 25709-25715CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The transition metal dichalcogenides (TMDs) are two-dimensional layered solids with van der Waals bonding between layers. We calc. their Schottky barrier heights (SBHs) using supercell models and d. functional theory. It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S = 0.3, a similar value for both top and edge contact geometries. This arises because there is direct bonding between the contact metal atoms and the TMD chalcogen atoms, for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials. The Schottky barriers largely follow the metal-induced gap state (MIGS) model, like those of three-dimensional semiconductors, despite the bonding in the TMDs being largely constrained within the layers. The pinning energies are found to be lower in the gap for edge contact geometries than for top contact geometries, which might be used to obtain p-type contacts on MoS2.
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    Pan, Y.; Gu, J.; Tang, H.; Zhang, X.; Li, J.; Shi, B.; Yang, J.; Zhang, H.; Yan, J.; Liu, S.; Hu, H.; Wu, M.; Lu, J. Reexamination of the Schottky Barrier Heights in Monolayer MoS2 Field-Effect Transistors. ACS Appl. Nano Mater. 2019, 2, 4717– 4726,  DOI: 10.1021/acsanm.9b00200
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    Reexamination of the Schottky barrier heights in monolayer MoS2 field-effect transistors
    Pan, Yuanyuan; Gu, Jihuan; Tang, Hao; Zhang, Xiuying; Li, Jingzhen; Shi, Bowen; Yang, Jie; Zhang, Han; Yan, Jiahuan; Liu, Shiqi; Hu, Han; Wu, Mingbo; Lu, Jing
    ACS Applied Nano Materials (2019), 2 (8), 4717-4726CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    Owing to its promising electronic application of monolayer (ML) MoS2, ML MoS2-metal contacts have been widely explored. The expts. reveal a very strong Fermi level pinning, and the corresponding pinning factor is about 0.1, but all the existing calcns. give a larger pinning factor of about 0.3. Such an apparent discrepancy is attributed to the defects in samples. In this paper, the Schottky barriers are reexamd. in the pristine ML MoS2 field effect transistors (FETs) with a series of metal electrodes (Au, Pt, Ag, Ti, Cr, Pd, Ni, and ML CCr2) by using ab initio quantum transport simulation. The Schottky barrier heights obtained from our ab initio quantum transport simulation are in better agreement with those obsd. in expts. for Au and Pt electrodes, and the calcd. pinning factor is also improved. Our work highlights the importance of the inclusion of the coupling between the electrode and channel in detg. the pinning behavior. Hence, ab initio quantum transport simulation is an improved method to det. the SBH and the pinning factor in low-dimensional semiconductor FETs.
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    Gong, C.; Colombo, L.; Wallace, R. M.; Cho, K. The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces. Nano Lett. 2014, 14, 1714– 1720,  DOI: 10.1021/nl403465v
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    The Unusual Mechanism of Partial Fermi Level Pinning at Metal-MoS2 Interfaces
    Gong, Cheng; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae
    Nano Letters (2014), 14 (4), 1714-1720CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    D. functional theory calcns. were performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with Pt, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is unique for metal-2D-semiconductor interfaces, remarkably different from the known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: 1st by a metal work function modification by interface dipole formation due to the charge redistribution, and 2nd by the prodn. of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form ohmic contact to MoS2.
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    Zhong, H.; Quhe, R.; Wang, Y.; Ni, Z.; Ye, M.; Song, Z.; Pan, Y.; Yang, J.; Yang, L.; Lei, M.; Shi, J.; Lu, J. Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations. Sci. Rep. 2016, 6, 21786,  DOI: 10.1038/srep21786
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    Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations
    Zhong, Hongxia; Quhe, Ruge; Wang, Yangyang; Ni, Zeyuan; Ye, Meng; Song, Zhigang; Pan, Yuanyuan; Yang, Jinbo; Yang, Li; Lei, Ming; Shi, Junjie; Lu, Jing
    Scientific Reports (2016), 6 (), 21786CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calcd. and obsd. Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calcn. scheme fails to reproduce the obsd. SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the obsd. SBH in 2D MoS2-Sc interface and highlights the importance of a higher level theor. approach beyond the energy band calcn. in the interface study. BL MoS2-metal contacts generally have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.
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    Graphene Schottky diodes: An experimental review of the rectifying graphene/semiconductor heterojunction
    Di Bartolomeo, Antonio
    Physics Reports (2016), 606 (), 1-58CODEN: PRPLCM; ISSN:0370-1573. (Elsevier B.V.)
    In the past decade graphene has been one of the most studied materials for several unique and excellent properties. Due to its two dimensional nature, phys. and chem. properties and ease of manipulation, graphene offers the possibility of integration with the existing semiconductor technol. for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chem. and biol. sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.
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    Anwar, Amro; Nabet, Bahram; Culp, James; Castro, Fransisco
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    The authors discuss mechanisms responsible for the redn. of electron thermionic emission current from a Schottky contact to a modulation doped semiconductor compared to a bulk semiconductor. The effects discussed include metal to semiconductor barrier height enhancement due to proposed electron-electron cloud interaction, confined potential of the reduced dimensional systems, and the reduced dimensional nature of the d. of states in the semiconductor. These effects describe the obsd. lowering of the dark current, and hence noise, of a modulation doped heterojunction based photodetector compared to a conventional bulk device.
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    Di Bartolomeo, A.; Genovese, L.; Foller, T.; Giubileo, F.; Luongo, G.; Croin, L.; Liang, S.-J.; Ang, L. K.; Schleberger, M. Electrical Transport and Persistent Photoconductivity in Monolayer MoS2 Phototransistors. Nanotechnology 2017, 28, 214002,  DOI: 10.1088/1361-6528/aa6d98
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    Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors
    Di Bartolomeo, Antonio; Genovese, Luca; Foller, Tobias; Giubileo, Filippo; Luongo, Giuseppe; Croin, Luca; Liang, Shi-Jun; Ang, L. K.; Schleberger, Marika
    Nanotechnology (2017), 28 (21), 214002/1-214002/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    We study elec. transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photocond. increases logarithmically with the light intensity and can persist with a decay time longer than 104s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 kW μm-1, ON current as high as 1.25 nA μm-1, 105 ON-OFF ratio, mobility of ∼1 cm2 V-1s-1 and photoresponsivity R ≈ 1 A W-1.
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    Zhang, K.; Peng, M.; Yu, A.; Fan, Y.; Zhai, J.; Wang, Z. L. A Substrate-Enhanced MoS2 Photodetector through a Dual-Photogating Effect. Mater. Horiz. 2019, 6, 826– 833,  DOI: 10.1039/c8mh01429a
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    A substrate-enhanced MoS2 photodetector through a dual-photogating effect
    Zhang, Ke; Peng, Mingzeng; Yu, Aifang; Fan, Youjun; Zhai, Junyi; Wang, Zhong Lin
    Materials Horizons (2019), 6 (4), 826-833CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
    The high-performance, miniaturization and flexibility of two-dimensional (2D) material-based electronics and optoelectronics make them promising substitutes for silicon-based devices in the future. However, the complex structures and the assistance strategies needed to compensate for the intrinsic deficiencies of 2D materials are major challenges hindering practical applications. It is hoped to achieve high-performance 2D material-based devices of the simplest construction without external assistance. Therefore, making full use of the basic components (host material, substrate and electrode) of 2D devices is crucial. To date, the role of the substrate has always been ignored. Here, we choose undoped-silicon carbon (SiC) and Kapton (polyimide, PI) film as substrates for MoS2 photodetectors (PDs) for rigid and flexible applications, resp. A new concept, the "dual-photogating effect" induced at the MoS2/substrate interface, is proposed and applied to obtain ultra-high photoresponse performance, such as high photoresponsivity of ∼104 A W-1 in MoS2/SiC PD and an on/off ratio of ∼105 in a MoS2/Kapton PD.
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    Cheng, Y. J.; Yan, L.; Shi, F.; Liu, F.; Li, M.; Shi, H. L.; Hou, Z. P. Monte Carlo Simulation of Electron Scattering in Ion Barrier Film in Generation III Image Intensifier. Key Eng. Mater. 2013, 552, 193– 200,  DOI: 10.4028/www.scientific.net/kem.552.193
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    Monte Carlo simulation of electron scattering in ion barrier film in generation III image intensifier
    Cheng, Yaojin; Yan, Lei; Shi, Feng; Liu, Feng; Li, Min; Shi, Hongli; Hou, Zhipeng
    Key Engineering Materials (2013), 552 (), 193-200, 9 pp.CODEN: KEMAEY; ISSN:1662-9795. (Trans Tech Publications Ltd.)
    In Generation III image intensifier tubes, the input of the microchannel plate is typically coated with a thin Al2O3 film to prevent ions penetrating and damaging the delicate Cs-O activation layer of GaAs cathode. While the application of an ion barrier film to the MCP greatly improves the mean time to failure of the image intensifier tube, it unfortunately produces electron scattering to cause a significant degrdn. in the Modulation Transfer Function and resoln. of the image tubes. To obtain an overview of the role of the ion barrier in Generation III image intensifiers, in this paper, we present the results of a Monte Carlo simulation on scattering electron transport through Al2O3 film with various photocathode voltage, d., thickness, and stoichiometry. Using Monte Carlo simulation on scattering electron ways, we get the MTF curve of film at the accelerating voltage of 400v, a thickness of 10nm, and a d. of 0.3 g/cm3. The simulation results play an important role on optimizing the quality and facture technique of ion barrier film.
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    Movla, H.; Babazadeh, M. Simulation Analysis of the Aluminum Thin Film Thickness Measurement by Using Low Energy Electron Beam. Optik 2014, 125, 71– 74,  DOI: 10.1016/j.ijleo.2013.06.033
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    Simulation analysis of the aluminum thin film thickness measurement by using low energy electron beam
    Movla, Hossein; Babazadeh, Mohammad
    Optik (Munich, Germany) (2014), 125 (1), 71-74CODEN: OTIKAJ; ISSN:0030-4026. (Elsevier GmbH)
    This paper indicates a simulation anal. for estg. the aluminum (Al) thin film thickness measurements by using the low energy electron beam. In order to calc. the Al thickness estn., the energy of the incident electron beams was varied from 10 to 30 keV, while the thickness of the Al film was varied between 6 and 14 μm. From the simulation results it was found that electron transmittance fraction in 14 μm sample is about nine orders of magnitude more than 6 μm sample at the same incident electron beam energy. Simulation results show that max. transmitted electrons vs. Al layer thickness has a parabolic relation and by using the obtained equation, it is possible to est. unknown thickness of the thin film Al layer. All calcns. here were done by CASINO numerical simulation package.
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    Drouin, D.; Couture, A. R.; Joly, D.; Tastet, X.; Aimez, V.; Gauvin, R. CASINO V2.42—A Fast and Easy-to-Use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users. Scanning 2007, 29, 92– 101,  DOI: 10.1002/sca.20000
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    CASINO V2.42-a fast and easy-to-use modeling tool for scanning electron microscopy and microanalysis users
    Drouin, Dominique; Couture, Alexandre Real; Joly, Dany; Tastet, Xavier; Aimez, Vincent; Gauvin, Raynald
    Scanning (2007), 29 (3), 92-101CODEN: SCNNDF; ISSN:0161-0457. (John Wiley & Sons Ltd.)
    Monte Carlo simulations have been widely used by microscopists for the last few decades. In the beginning it was a tedious and slow process, requiring a high level of computer skills from users and long computational times. Recent progress in the microelectronics industry now provides researchers with affordable desktop computers with clock rates greater than 3 GHz. With this type of computing power routinely available, Monte Carlo simulation is no longer an exclusive or long (overnight) process. The aim of this paper is to present a new user-friendly simulation program based on the earlier CASINO Monte Carlo program. The intent of this software is to assist scanning electron microscope users in interpretation of imaging and microanal. and also with more advanced procedures including electron-beam lithog. This version uses a new architecture that provides results twice as quickly. This program is freely available to the scientific community and can be downloaded from the website: www.gel.usherb.ca/casino.
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    Abraham, M.; Mohney, S. E. Annealed Ag Contacts to MoS2 Field-Effect Transistors. J. Appl. Phys. 2017, 122, 115306,  DOI: 10.1063/1.4991961
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    Annealed Ag contacts to MoS2 field-effect transistors
    Abraham, Michael; Mohney, Suzanne E.
    Journal of Applied Physics (Melville, NY, United States) (2017), 122 (11), 115306/1-115306/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Silver contacts to few-layer (5 to 14 layers thick) MoS2 have been studied before and after annealing. Annealing was found to be crit. for reducing the contact resistance but did not degrade the operation of field-effect transistors that are part of the test structure. The contact resistance for the as-deposited samples was in the range of 0.8-3.5 kΩ μm. On the other hand, the contact resistance was reduced to 0.2-0.7 kΩ μm, evaluated at a const. sheet resistance of 32 kΩ/.box., after annealing at 250 or 300 °C. The reduced contact resistance is attributed to diffusion of Ag into the MoS2 and doping, as supported by further elec. characterization of the contacts and devices. (c) 2017 American Institute of Physics.
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    Goyal, N.; Mackenzie, D. M. A.; Panchal, V.; Jawa, H.; Kazakova, O.; Petersen, D. H.; Lodha, S. Enhanced Thermally Aided Memory Performance Using Few-Layer ReS2 Transistors. Appl. Phys. Lett. 2020, 116, 052104,  DOI: 10.1063/1.5126809
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    Enhanced thermally aided memory performance using few-layer ReS2 transistors
    Goyal, Natasha; Mackenzie, David M. A.; Panchal, Vishal; Jawa, Himani; Kazakova, Olga; Petersen, Dirch Hjorth; Lodha, Saurabh
    Applied Physics Letters (2020), 116 (5), 052104CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Thermally varying hysteretic gate operation in few-layer ReS2 and MoS2 back gate field effect transistors (FETs) is studied and compared for memory applications. Clockwise hysteresis at room temp. and anti-clockwise hysteresis at higher temp. (373 K for ReS2 and 400 K for MoS2) are accompanied by step-like jumps in transfer curves for both forward and reverse voltage sweeps. Hence, a step-like conductance (STC) crossover hysteresis between the transfer curves for the two sweeps is obsd. at high temp. Furthermore, memory parameters such as the RESET-to-WRITE window and READ window are defined and compared for clockwise hysteresis at low temp. and STC-type hysteresis at high temp., showing better memory performance for ReS2 FETs as compared to MoS2 FETs. Smaller operating temp. and voltage along with larger READ and RESET-to-WRITE windows make ReS2 FETs a better choice for thermally aided memory applications. Finally, temp. dependent Kelvin probe force microscopy measurements show decreasing (const.) surface potential with increasing temp. for ReS2 (MoS2). This indicates less effective intrinsic trapping at high temp. in ReS2, leading to earlier occurrence of STC-type hysteresis in ReS2 FETs as compared to MoS2 FETs with increasing temp. (c) 2020 American Institute of Physics.
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Cite this: ACS Appl. Mater. Interfaces 2020, 12, 36, 40532–40540
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  • Abstract

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    Figure 1

    Figure 1. (a) SEM image of the MoS2 device and contact labels. (b) MoS2 FET layout and schematic of the common source configuration used for electrical characterization. (c) AFM image of the MoS2 flake between the electrical contacts, which appear here in white as the scale has been adjusted to properly image the MoS2 flake. (d) Zoom-in into the upper region of (c), showing that the flake is flat and structurally intact. The rms roughness is 0.221 nm for the SiO2 substrate and 0.237 nm for MoS2. (e) Height distribution taken from image (d), yielding a flake height of ∼1.2 nm.

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    Figure 2

    Figure 2. (a) PL and (b) Raman spectrum of monolayer MoS2 after FET processing. Blue: contacted MoS2 monolayer flake and red: noncontacted monolayer MoS2 flake.

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    Figure 3

    Figure 3. Output (a) and transfer (b) characteristics of the device between C2 and C3 contacts, with C3 used as the drain and C2 as the grounded source.

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    Figure 4

    Figure 4. (a) Output characteristics at Vgs = 0 V of the transistor formed by contacts C2–C3 exposed to two sets of electron irradiations performed first on contact C3 and then on C2. (b) Rectification ratio and (c) maximum forward and reverse current, at Vds = ±5 V, as a function of the irradiation number. (d) Zero-bias Schottky barrier variation at the contacts C2 and C3 as a function of the irradiation number.

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    Figure 5

    Figure 5. Low-bias energy band diagrams (black) and their modification under electron irradiation (red) of C3 (a) and of C2 (b) contacts resulting in barrier lowering (φ̅B).

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    Figure 6

    Figure 6. (a) FET transfer characteristics at Vds = −4 V before and after e-beam irradiations of contacts C3 and C2 and of the channel. (b) Left shift of the threshold voltage extrapolated from the transfer characteristics over the e-beam exposure.

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    Figure 7

    Figure 7. Monte Carlo simulation using CASINO v2 of e-beam irradiation of the device (a) contacts and (b) of the MoS2 channel. (c) Simulated cathodoluminescence intensity through the sample, with the e-beam focused onto the contacts and the flake. (d) Simulation of the electrons' penetration depth through the sample.

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  • References

    This article references 67 other publications.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXitVCks77I&md5=85a7795e1dd2949131e96f043625be9e
    9. 9
      Madauß, L.; Zegkinoglou, I.; Vázquez Muiños, H.; Choi, Y.-W.; Kunze, S.; Zhao, M.-Q.; Naylor, C. H.; Ernst, P.; Pollmann, E.; Ochedowski, O.; Lebius, H.; Benyagoub, A.; Ban-d’Etat, B.; Johnson, A. T. C.; Djurabekova, F.; Roldan Cuenya, B.; Schleberger, M. Highly Active Single-Layer MoS2 Catalysts Synthesized by Swift Heavy Ion Irradiation. Nanoscale 2018, 10, 22908– 22916,  DOI: 10.1039/c8nr04696d
      9
      Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation
      Madauss, Lukas; Zegkinoglou, Ioannis; Vazquez Muinos, Henrique; Choi, Yong-Wook; Kunze, Sebastian; Zhao, Meng-Qiang; Naylor, Carl H.; Ernst, Philipp; Pollmann, Erik; Ochedowski, Oliver; Lebius, Henning; Benyagoub, Abdenacer; Ban-d'Etat, Brigitte; Johnson, A. T. Charlie; Djurabekova, Flyura; Roldan Cuenya, Beatriz; Schleberger, Marika
      Nanoscale (2018), 10 (48), 22908-22916CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compd.'s basal planes can be overcome by either increasing the no.of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradn. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the no.of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochem.c.d.by over 160% as compared to an identical but non-irradiated MoS2 surface.
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    10. 10
      Urban, F.; Lupina, G.; Grillo, A.; Martucciello, N.; Di Bartolomeo, A. Contact Resistance and Mobility in Back-Gate Graphene Transistors. Nano Express 2020, 1, 010001,  DOI: 10.1088/2632-959x/ab7055
      There is no corresponding record for this reference.
    11. 11
      Bolotin, K. I. Electronic Transport in Graphene: Towards High Mobility. Graphene; Elsevier, 2014; pp 199– 227.
      There is no corresponding record for this reference.
    12. 12
      Di Bartolomeo, A.; Santandrea, S.; Giubileo, F.; Romeo, F.; Petrosino, M.; Citro, R.; Barbara, P.; Lupina, G.; Schroeder, T.; Rubino, A. Effect of Back-Gate on Contact Resistance and on Channel Conductance in Graphene-Based Field-Effect Transistors. Diamond Relat. Mater. 2013, 38, 19– 23,  DOI: 10.1016/j.diamond.2013.06.002
      12
      Effect of back-gate on contact resistance and on channel conductance in graphene-based field-effect transistors
      Di Bartolomeo, A.; Santandrea, S.; Giubileo, F.; Romeo, F.; Petrosino, M.; Citro, R.; Barbara, P.; Lupina, G.; Schroeder, T.; Rubino, A.
      Diamond and Related Materials (2013), 38 (), 19-23CODEN: DRMTE3; ISSN:0925-9635. (Elsevier B.V.)
      We study the contact resistance and the transfer characteristics of back-gated field effect transistors of mono- and bi-layer graphene. We measure specific contact resistivity of ∼ 7 k Ω μm2 and ∼ 30k Ω μm2 for Ni and Ti, resp. We show that the contact resistance is a significant contributor to the total source-to-drain resistance and it is modulated by the back-gate voltage. We measure transfer characteristics showing a double dip feature that we explain as the effect of doping due to charge transfer from the contacts causing min. d. of states for graphene under the contacts and in the channel at different gate voltage.
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    13. 13
      Wilmart, Q.; Boukhicha, M.; Graef, H.; Mele, D.; Palomo, J.; Rosticher, M.; Taniguchi, T.; Watanabe, K.; Bouchiat, V.; Baudin, E.; Berroir, J.-M.; Bocquillon, E.; Fève, G.; Pallecchi, E.; Plaçais, B. High-Frequency Limits of Graphene Field-Effect Transistors with Velocity Saturation. Appl. Sci. 2020, 10, 446,  DOI: 10.3390/app10020446
      13
      High-frequency limits of graphene field-effect transistors with velocity saturation
      Wilmart, Quentin; Boukhicha, Mohamed; Graef, Holger; Mele, David; Palomo, Jose; Rosticher, Michael; Taniguchi, Takashi; Watanabe, Kenji; Bouchiat, Vincent; Baudin, Emmanuel; Berroir, Jean-Marc; Bocquillon, Erwann; Feve, Gwendal; Pallecchi, Emiliano; Placais, Bernard
      Applied Sciences (2020), 10 (2), 446CODEN: ASPCC7; ISSN:2076-3417. (MDPI AG)
      The current understanding of phys. principles governing electronic transport in graphene field effect transistors (GFETs) has reached a level where we can model quite accurately device operation and predict intrinsic frequency limits of performance. In this work, we use this knowledge to analyze DC and RF transport properties of bottom-gated graphene on boron nitride field effect transistors exhibiting pronounced velocity satn. by substrate hyperbolic phonon polariton scattering, including Dirac pinch-off effect. We predict and demonstrate a max. oscillation frequency exceeding 20 GHz. We discuss the intrinsic 0.1 THz limit of GFETs and envision plasma resonance transistors as an alternative for sub-THz narrow-band detection.
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    14. 14
      Piccinini, E.; Alberti, S.; Longo, G. S.; Berninger, T.; Breu, J.; Dostalek, J.; Azzaroni, O.; Knoll, W. Pushing the Boundaries of Interfacial Sensitivity in Graphene FET Sensors: Polyelectrolyte Multilayers Strongly Increase the Debye Screening Length. J. Phys. Chem. C 2018, 122, 10181– 10188,  DOI: 10.1021/acs.jpcc.7b11128
      14
      Pushing the Boundaries of Interfacial Sensitivity in Graphene FET Sensors: Polyelectrolyte Multilayers Strongly Increase the Debye Screening Length
      Piccinini, Esteban; Alberti, Sebastian; Longo, Gabriel S.; Berninger, Teresa; Breu, Josef; Dostalek, Jakub; Azzaroni, Omar; Knoll, Wolfgang
      Journal of Physical Chemistry C (2018), 122 (18), 10181-10188CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Nanomaterial-based FET sensors represent an attractive platform for ultrasensitive, real-time, and label-free detection of chem. and biol. species. Nevertheless, because their response is screened by mobile ions, it remains a challenge to use them to sense in physiol. ionic strength solns. It is demonstrated, both exptl. and theor., that polyelectrolyte multilayers are capable of increasing the sensing range of graphene-based FETs. Potential shifts at graphene surfaces and film thickness are recorded upon the construction of PDADMAC/PSS polyelectrolyte multilayer (PEM) films. By correlation of the potential shift with the film thickness, the electrostatic screening length and the concn. of mobile ion inside the films were deduced. Across the polymer interface the Debye length is increased >1 order of magnitude. The fundamentals of this strategy are described by a conceptually simple thermodn. model, which accounts for the entropy loss of ion confinement and incorporates the effect of ions finite vol. The electrostatic screening inside the film strongly depends on the polymer d. and the ionic strength of the soln. Of particular interest in physiol. condition sensing, the PEM interfaces can extend the Debye length from 0.8 to 10 nm.
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    15. 15
      Di Bartolomeo, A.; Giubileo, F.; Iemmo, L.; Romeo, F.; Russo, S.; Unal, S.; Passacantando, M.; Grossi, V.; Cucolo, A. M. Leakage and Field Emission in Side-Gate Graphene Field Effect Transistors. Appl. Phys. Lett. 2016, 109, 023510,  DOI: 10.1063/1.4958618
      15
      Leakage and field emission in side-gate graphene field effect transistors
      Di Bartolomeo, A.; Giubileo, F.; Iemmo, L.; Romeo, F.; Russo, S.; Unal, S.; Passacantando, M.; Grossi, V.; Cucolo, A. M.
      Applied Physics Letters (2016), 109 (2), 023510/1-023510/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We fabricate planar graphene field-effect transistors with self-aligned side-gate at 100 nm from the 500 nm wide graphene conductive channel, using a single lithog. step. We demonstrate side-gating below 1 V with conductance modulation of 35% and transconductance up to 0.5 mS/mm at 10 mV drain bias. We measure the planar leakage along the SiO2/vacuum gate dielec. over a wide voltage range, reporting rapidly growing current above 15 V. We unveil the microscopic mechanisms driving the leakage, as Frenkel-Poole transport through SiO2 up to the activation of Fowler-Nordheim tunneling in vacuum, which becomes dominant at higher voltages. We report a field-emission c.d. as high as 1 μA/μm between graphene flakes. These findings are important for the miniaturization of atomically thin devices. (c) 2016 American Institute of Physics.
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    16. 16
      Bartolomeo, A. D.; Giubileo, F.; Romeo, F.; Sabatino, P.; Carapella, G.; Iemmo, L.; Schroeder, T.; Lupina, G. Graphene Field Effect Transistors with Niobium Contacts and Asymmetric Transfer Characteristics. Nanotechnology 2015, 26, 475202,  DOI: 10.1088/0957-4484/26/47/475202
      16
      Graphene field effect transistors with niobium contacts and asymmetric transfer characteristics
      Bartolomeo Antonio Di; Giubileo Filippo; Romeo Francesco; Sabatino Paolo; Carapella Giovanni; Iemmo Laura; Schroeder Thomas; Lupina Grzegorz
      Nanotechnology (2015), 26 (47), 475202 ISSN:.
      We fabricate back-gated field effect transistors using niobium electrodes on mechanically exfoliated monolayer graphene and perform electrical characterization in the pressure range from atmospheric down to 10(-4) mbar. We study the effect of room temperature vacuum degassing and report asymmetric transfer characteristics with a resistance plateau in the n-branch. We show that weakly chemisorbed Nb acts as p-dopant on graphene and explain the transistor characteristics by Nb/graphene interaction with unpinned Fermi level at the interface.
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    17. 17
      Li, F.; Gao, F.; Xu, M.; Liu, X.; Zhang, X.; Wu, H.; Qi, J. Tuning Transport and Photoelectric Performance of Monolayer MoS2 Device by E-Beam Irradiation. Adv. Mater. Interfaces 2018, 5, 1800348,  DOI: 10.1002/admi.201800348
      There is no corresponding record for this reference.
    18. 18
      Wang, J.; Yao, Q.; Huang, C.-W.; Zou, X.; Liao, L.; Chen, S.; Fan, Z.; Zhang, K.; Wu, W.; Xiao, X.; Jiang, C.; Wu, W.-W. High Mobility MoS2 Transistor with Low Schottky Barrier Contact by Using Atomic Thick h-BN as a Tunneling Layer. Adv. Mater. 2016, 28, 8302– 8308,  DOI: 10.1002/adma.201602757
      18
      High Mobility MoS2 Transistor with Low Schottky Barrier Contact by Using Atomic Thick h-BN as a Tunneling Layer
      Wang, Jingli; Yao, Qian; Huang, Chun-Wei; Zou, Xuming; Liao, Lei; Chen, Shanshan; Fan, Zhiyong; Zhang, Kai; Wu, Wei; Xiao, Xiangheng; Jiang, Changzhong; Wu, Wen-Wei
      Advanced Materials (Weinheim, Germany) (2016), 28 (37), 8302-8308CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this article, CVD hexagonal boron nitride (h-BN) tunneling layer was used to reduce the Schottky barrier height and improve the contact between metal and MoS2. The at. thickness of 1-2 layers h-BN, the Schottky barrier can be greatly reduced with small tunneling resistance.
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    19. 19
      Fiori, G.; Bonaccorso, F.; Iannaccone, G.; Palacios, T.; Neumaier, D.; Seabaugh, A.; Banerjee, S. K.; Colombo, L. Electronics Based on Two-Dimensional Materials. Nat. Nanotechnol. 2014, 9, 768– 779,  DOI: 10.1038/nnano.2014.207
      19
      Electronics based on two-dimensional materials
      Fiori, Gianluca; Bonaccorso, Francesco; Iannaccone, Giuseppe; Palacios, Tomas; Neumaier, Daniel; Seabaugh, Alan; Banerjee, Sanjay K.; Colombo, Luigi
      Nature Nanotechnology (2014), 9 (10), 768-779CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      A review. The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Provided is a review of electronic devices based on 2D materials, outlining their potential as a technol. option beyond scaled complementary metal-oxide-semiconductor switches. It is focused on the performance limits and advantages of these materials and assocd. technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. Also discussed is the use of 2D materials as an enabling factor for flexible electronics and provide perspectives on future developments.
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    20. 20
      Kim, M. J.; Choi, Y.; Seok, J.; Lee, S.; Kim, Y. J.; Lee, J. Y.; Cho, J. H. Defect-Free Copolymer Gate Dielectrics for Gating MoS2 Transistors. J. Phys. Chem. C 2018, 122, 12193– 12199,  DOI: 10.1021/acs.jpcc.8b03092
      20
      Defect-free copolymer gate dielectrics for gating MoS2 transistors
      Kim, Min Je; Choi, Yongsuk; Seok, Jihoo; Lee, Sungjoo; Kim, Young Jun; Lee, Jun Young; Cho, Jeong Ho
      Journal of Physical Chemistry C (2018), 122 (23), 12193-12199CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      In this study, the poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane-co-cyclohexyl methacrylate) [p(V4D4-co-CHMA)] copolymer was developed for use as a gate dielec. in molybdenum disulfide (MoS2) field-effect transistors (FETs). The p(V4D4-co-CHMA) copolymer was synthesized via the initiated chem. vapor deposition (iCVD) of two types of monomers: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4) and cyclohexyl methacrylate (CHMA). Four vinyl groups of V4D4 monomers and cyclohexyl groups of CHMA monomers were introduced to enhance the elec. strength of gate dielecs. through the formation of a highly crosslinked network and to reduce the charge trap densities at the MoS2-dielec. interface, resp. The iCVD-grown p(V4D4-co-CHMA) copolymer films yielded a dielec. const. of 2.3 and a leakage current of 3.8 × 10-11 A/cm2 at 1 MV/cm. The resulting MoS2 FETs with p(V4D4-co-CHMA) gate dielecs. exhibited excellent elec. properties, including an electron mobility of 35.1 cm2/V s, a subthreshold swing of 0.2 V/dec, and an on-off current ratio of 2.6 × 106. In addn., the environmental and operational stabilities of MoS2 FETs with p(V4D4-co-CHMA) top-gate dielecs. were superior to those of devices with SiO2 back-gate dielecs. The use of iCVD-grown copolymer gate dielecs. as demonstrated in this study provides a novel approach to realizing next-generation 2-dimensional electronics.
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    21. 21
      Rasmussen, F. A.; Thygesen, K. S. Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides. J. Phys. Chem. C 2015, 119, 13169– 13183,  DOI: 10.1021/acs.jpcc.5b02950
      21
      Computational 2D Materials Database: Electronic Structure of Transition-Metal Dichalcogenides and Oxides
      Rasmussen, Filip A.; Thygesen, Kristian S.
      Journal of Physical Chemistry C (2015), 119 (23), 13169-13183CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We present a comprehensive first-principles study of the electronic structure of 51 semiconducting monolayer transition-metal dichalcogenides and -oxides in the 2H and 1T hexagonal phases. The quasiparticle (QP) band structures with spin-orbit coupling are calcd. in the G0W0 approxn., and comparison is made with different d. functional theory descriptions. Pitfalls related to the convergence of GW calcns. for two-dimensional (2D) materials are discussed together with possible solns. The monolayer band edge positions relative to vacuum are used to est. the band alignment at various heterostructure interfaces. The sensitivity of the band structures to the in-plane lattice const. is analyzed and rationalized in terms of the electronic structure. Finally, the q-dependent dielec. functions and effective electron and hole masses are obtained from the QP band structure and used as input to a 2D hydrogenic model to est. exciton binding energies. Throughout the paper we focus on trends and correlations in the electronic structure rather than detailed anal. of specific materials. All the computed data is available in an open database.
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    22. 22
      Di Bartolomeo, A.; Pelella, A.; Liu, X.; Miao, F.; Passacantando, M.; Giubileo, F.; Grillo, A.; Iemmo, L.; Urban, F.; Liang, S. J. Pressure-Tunable Ambipolar Conduction and Hysteresis in Thin Palladium Diselenide Field Effect Transistors. Adv. Funct. Mater. 2019, 29, 1902483,  DOI: 10.1002/adfm.201902483
      There is no corresponding record for this reference.
    23. 23
      Di Bartolomeo, A.; Luongo, G.; Iemmo, L.; Urban, F.; Giubileo, F. Graphene–Silicon Schottky Diodes for Photodetection. IEEE Trans. Nanotechnol. 2018, 17, 1133– 1137,  DOI: 10.1109/tnano.2018.2853798
      There is no corresponding record for this reference.
    24. 24
      Jin, C.; Rasmussen, F. A.; Thygesen, K. S. Tuning the Schottky Barrier at the Graphene/MoS2 Interface by Electron Doping: Density Functional Theory and Many-Body Calculations. J. Phys. Chem. C 2015, 119, 19928– 19933,  DOI: 10.1021/acs.jpcc.5b05580
      24
      Tuning the Schottky Barrier at the Graphene/MoS2 Interface by Electron Doping: Density Functional Theory and Many-Body Calculations
      Jin, Chengjun; Rasmussen, Filip A.; Thygesen, Kristian S.
      Journal of Physical Chemistry C (2015), 119 (34), 19928-19933CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Using ab initio calcns. we investigate the energy level alignment at the graphene/MoS2 heterostructure and the use of electron doping as a strategy to lower the Schottky barrier and achieve a low-resistance Ohmic contact. For the neutral heterostructure, d. functional theory (DFT) with a generalized gradient approxn. predicts a Schottky barrier height of 0.18 eV, whereas the G0W0 method increases this value to 0.60 eV. While the DFT band gap of MoS2 does not change when the heterostructure is formed, the G0W0 gap is reduced by 0.30 eV as a result of the enhanced screening by the graphene layer. In contrast to the case of metal substrates, where the band alignment is governed by Pauli repulsion-induced interface dipoles, the graphene/MoS2 heterostructure shows only a negligible interface dipole. As a consequence, the band alignment at the neutral heterostructure is not changed when the two layers are brought into contact. We systematically follow the band alignment as a function of doping concn. and find that the Fermi level of the graphene crosses the MoS2 conduction band at a doping concn. of around 1012 cm-2. The variation of the energy levels with doping concn. is shown to be mainly governed by the electrostatic potential resulting from the doping charge.
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    25. 25
      Grillo, A.; Di Bartolomeo, A.; Urban, F.; Passacantando, M.; Caridad, J. M.; Sun, J.; Camilli, L. Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors. ACS Appl. Mater. Interfaces 2020, 12, 12998– 13004,  DOI: 10.1021/acsami.0c00348
      25
      Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors
      Grillo Alessandro; Di Bartolomeo Antonio; Urban Francesca; Grillo Alessandro; Di Bartolomeo Antonio; Urban Francesca; Passacantando Maurizio; Caridad Jose M; Sun Jianbo; Camilli Luca
      ACS applied materials & interfaces (2020), 12 (11), 12998-13004 ISSN:.
      We report the fabrication and electrical characterization of germanium arsenide (GeAs) field-effect transistors with ultrathin channels. The electrical transport is investigated in the 20-280 K temperature range, revealing that the p-type electrical conductivity and the field-effect mobility are growing functions of temperature. An unexpected peak is observed in the temperature dependence of the carrier density per area at ∼75 K. Such a feature is explained considering that the increased carrier concentration at higher temperatures and the vertical band bending combined with the gate field lead to the formation of a two-dimensional (2D) conducting channel, limited to few interfacial GeAs layers, which dominates the channel conductance. The conductivity follows the variable-range hopping model at low temperatures and becomes the band-type at higher temperatures when the 2D channel is formed. The formation of the 2D channel is validated through a numerical simulation that shows excellent agreement with the experimental data.
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    26. 26
      Schleberger, M.; Kotakoski, J. 2D Material Science: Defect Engineering by Particle Irradiation. Materials 2018, 11, 1885,  DOI: 10.3390/ma11101885
      26
      2D material science: defect engineering by particle irradiation
      Schleberger, Marika; Kotakoski, Jani
      Materials (2018), 11 (10), 1885/1-1885/29CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      Two-dimensional (2D) materials are at the heart of many novel devices due to their unique and often superior properties. For simplicity, 2D materials are often assumed to exist in their text-book form, i.e., as an ideal solid with no imperfections. However, defects are ubiquitous in macroscopic samples and play an important - if not imperative - role for the performance of any device. Thus, many independent studies have targeted the artificial introduction of defects into 2D materials by particle irradn. In our view it would be beneficial to develop general defect engineering strategies for 2D materials based on a thorough understanding of the defect creation mechanisms, which may significantly vary from the ones relevant for 3D materials. This paper reviews the state-of-the-art in defect engineering of 2D materials by electron and ion irradn. with a clear focus on defect creation on the at. scale and by individual impacts. Whenever possible we compile reported exptl. data alongside corresponding theor. studies. We show that, on the one hand, defect engineering by particle irradn. covers a wide range of defect types that can be fabricated with great precision in the most commonly investigated 2D materials. On the other hand, gaining a complete understanding still remains a challenge, that can be met by combining advanced theor. methods and improved exptl. set-ups, both of which only now begin to emerge. In conjunction with novel 2D materials, this challenge promises attractive future opportunities for researchers in this field.
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    27. 27
      Giubileo, F.; Iemmo, L.; Passacantando, M.; Urban, F.; Luongo, G.; Sun, L.; Amato, G.; Enrico, E.; Di Bartolomeo, A. Effect of Electron Irradiation on the Transport and Field Emission Properties of Few-Layer MoS2 Field-Effect Transistors. J. Phys. Chem. C 2019, 123, 1454– 1461,  DOI: 10.1021/acs.jpcc.8b09089
      27
      Effect of electron irradiation on the transport and field emission properties of few-layer MoS2 field-effect transistors
      Giubileo, Filippo; Iemmo, Laura; Passacantando, Maurizio; Urban, Francesca; Luongo, Giuseppe; Sun, Linfeng; Amato, Giampiero; Enrico, Emanuele; Di Bartolomeo, Antonio
      Journal of Physical Chemistry C (2019), 123 (2), 1454-1461CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Elec. characterization of few-layer MoS2-based field-effect transistors with Ti/Au electrodes is performed in the vacuum chamber of a scanning electron microscope in order to study the effects of electron-beam irradn. on the transport properties of the device. A neg. threshold voltage shift and a carrier mobility enhancement are obsd. and explained in terms of pos. charges trapped in the SiO2 gate oxide, during the irradn. The transistor channel current is increased up to 3 orders of magnitudes after the exposure to an irradn. dose of 100 e-/nm2. Finally, a complete field emission characterization of the MoS2 flake, achieving emission stability for several hours and a min. turn-on field of ≈20 V/μm with a field enhancement factor of about 500 at an anode-cathode distance of ∼1.5 μm, demonstrates the suitability of few-layer MoS2 as a 2-dimensional emitting surface for cold-cathode applications.
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    28. 28
      Di Bartolomeo, A.; Urban, F.; Pelella, A.; Grillo, A.; Passacantando, M.; Liu, X.; Giubileo, F. Electron Irradiation of Multilayer PdSe2 Field Effect Transistors. Nanotechnology 2020, 31, 375204,  DOI: 10.1088/1361-6528/ab9472
      28
      Electron irradiation of multilayer PdSe2 field effect transistors
      Di Bartolomeo, A.; Urban, F.; Pelella, A.; Grillo, A.; Passacantando, M.; Liu, X.; Giubileo, F.
      Nanotechnology (2020), 31 (37), 375204CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      Palladium diselenide (PdSe2) is a recently isolated layered material that has attracted a lot of interest for its pentagonal structure, air stability and elec. properties that are largely tunable by the no. of layers. In this work, multilayer PdSe2 is used as the channel of back-gate field-effect transistors, which are studied under repeated electron irradiations. Source-drain Pd electrodes enable contacts with resistance below 350 kΩ μm. The transistors exhibit a prevailing n-type conduction in high vacuum, which reversibly turns into ambipolar elec. transport at atm. pressure. Irradn. by 10 keV electrons suppresses the channel conductance and promptly transforms the device from n-type to p-type. An electron fluence as low as 160 e-/nm2 dramatically changes the transistor behavior, demonstrating a high sensitivity of PdSe2 to electron irradn. The sensitivity is lost after a few exposures, with a satn. condition being reached for fluence higher than ~ 4000 e-/nm2. The damage induced by high electron fluence is irreversible as the device persists in the radiation-modified state for several hours, if kept in vacuum and at room temp. With the support of numerical simulation, we explain such a behavior by electron-induced Se atom vacancy formation and charge trapping in slow trap states at the Si/SiO2 interface.
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    29. 29
      Ochedowski, O.; Marinov, K.; Wilbs, G.; Keller, G.; Scheuschner, N.; Severin, D.; Bender, M.; Maultzsch, J.; Tegude, F. J.; Schleberger, M. Radiation Hardness of Graphene and MoS2 Field Effect Devices against Swift Heavy Ion Irradiation. J. Appl. Phys. 2013, 113, 214306,  DOI: 10.1063/1.4808460
      29
      Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation
      Ochedowski, O.; Marinov, K.; Wilbs, G.; Keller, G.; Scheuschner, N.; Severin, D.; Bender, M.; Maultzsch, J.; Tegude, F. J.; Schleberger, M.
      Journal of Applied Physics (Melville, NY, United States) (2013), 113 (21), 214306/1-214306/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradn. with swift heavy ions. Devices were prepd. with exfoliated single layers of MoS2 and graphene, resp. They were characterized before and after irradn. with 1.14 GeV U28+ ions using three different fluences. By elec. characterization, at. force microscopy, and Raman spectroscopy, we show that the irradn. leads to significant changes of structural and elec. properties. At the highest fluence of 4 × 1011 ions/cm2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance. (c) 2013 American Institute of Physics.
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    30. 30
      Ernst, P.; Kozubek, R.; Madauß, L.; Sonntag, J.; Lorke, A.; Schleberger, M. Irradiation of Graphene Field Effect Transistors with Highly Charged Ions. Nucl. Instrum. Methods Phys. Res., Sect. B 2016, 382, 71– 75,  DOI: 10.1016/j.nimb.2016.03.043
      30
      Irradiation of graphene field effect transistors with highly charged ions
      Ernst, P.; Kozubek, R.; Madauss, L.; Sonntag, J.; Lorke, A.; Schleberger, M.
      Nuclear Instruments & Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms (2016), 382 (), 71-75CODEN: NIMBEU; ISSN:0168-583X. (Elsevier B.V.)
      In this work, graphene field-effect transistors are used to detect defects due to irradn. with slow, highly charged ions. In order to avoid contamination effects, a dedicated ultra-high vacuum set up has been designed and installed for the in situ cleaning and elec. characterization of graphene field-effect transistors during irradn. To investigate the elec. and structural modifications of irradiated graphene field-effect transistors, their transfer characteristics as well as the corresponding Raman spectra are analyzed as a function of ion fluence for two different charge states. The irradn. expts. show a decreasing mobility with increasing fluences. The mobility redn. scales with the potential energy of the ions. In comparison to Raman spectroscopy, the transport properties of graphene show an extremely high sensitivity with respect to ion irradn.: a significant drop of the mobility is obsd. already at fluences below 15 ions/μm2, which is more than one order of magnitude lower than what is required for Raman spectroscopy.
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    31. 31
      Madauß, L.; Ochedowski, O.; Lebius, H.; Ban-d’Etat, B.; Naylor, C. H.; Johnson, A. T. C.; Kotakoski, J.; Schleberger, M. Defect Engineering of Single- and Few-Layer MoS2 by Swift Heavy Ion Irradiation. 2D Mater. 2016, 4, 015034,  DOI: 10.1088/2053-1583/4/1/015034
      There is no corresponding record for this reference.
    32. 32
      Kozubek, R.; Tripathi, M.; Ghorbani-Asl, M.; Kretschmer, S.; Madauß, L.; Pollmann, E.; O’Brien, M.; McEvoy, N.; Ludacka, U.; Susi, T.; Duesberg, G. S.; Wilhelm, R. A.; Krasheninnikov, A. V.; Kotakoski, J.; Schleberger, M. Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions. J. Phys. Chem. Lett. 2019, 10, 904– 910,  DOI: 10.1021/acs.jpclett.8b03666
      32
      Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions
      Kozubek, Roland; Tripathi, Mukesh; Ghorbani-Asl, Mahdi; Kretschmer, Silvan; Madauss, Lukas; Pollmann, Erik; O'Brien, Maria; McEvoy, Niall; Ludacka, Ursula; Susi, Toma; Duesberg, Georg S.; Wilhelm, Richard A.; Krasheninnikov, Arkady V.; Kotakoski, Jani; Schleberger, Marika
      Journal of Physical Chemistry Letters (2019), 10 (5), 904-910CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Porous single-layer molybdenum disulfide (MoS2) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradn. with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS2. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies. Comparison to atomistic simulations reveals the crit. role of energy deposition from the ion to the material through electronic excitation in the defect creation process and suggests an enrichment in molybdenum in the vicinity of the pore edges at least for ions with low potential energies. Anal. of the irradiated samples with at. resoln. scanning transmission electron microscopy reveals a clear dependence of the pore size on the potential energy of the projectiles, establishing irradn. with highly charged ions as an effective method to create pores with narrow size distributions and radii between ca. 0.3 and 3 nm.
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    33. 33
      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
      33
      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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    34. 34
      Shahzad, K.; Jia, K.; Zhao, C.; Wang, D.; Usman, M.; Luo, J. Effects of Different Ion Irradiation on the Contact Resistance of Pd/Graphene Contacts. Materials 2019, 12, 3928,  DOI: 10.3390/ma12233928
      34
      Effects of different ion irradiation on the contact resistance of Pd/graphene contacts
      Shahzad, Kashif; Jia, Kunpeng; Zhao, Chao; Wang, Dahai; Usman, Muhammad; Luo, Jun
      Materials (2019), 12 (23), 3928CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      The effect of ion-induced defects on graphene was studied to investigate the contact resistance of 40 nm palladium (Pd) contacting on graphene. The defect development was considered and analyzed by irradiating boron (B), carbon (C), nitrogen (N2), and argon (Ar) ions on as-transferred graphene before metalization. The bombardment energy was set at 1.5 keV and ion dose at 1 x 1014 ions/cm2. The defect yields under different ion irradn. conditions were examd. by Raman spectroscopy. Although, dissoln. process occurs spontaneously upon metal deposition, chem. reaction between metal and graphene is more pronounced at higher temps. The rapid thermal annealing (RTA) treatment was performed to improve the Pd/graphene contact after annealing at 450 °C, 500 °C, 550 °C, and 600 °C. The lowest contact resistance of 95.2 Ω-μm was achieved at 550 °C RTA with Ar ion irradn. We have proved that ion irradn. significantly enhance the Pd/graphene contact instead of pd/pristine graphene contact. Therefore, in view of the contention of results ion induced defects before metalization plus the RTA served an excellent purpose to reduce the contact resistance.
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    35. 35
      Yan, X.; Jia, K.; Su, Y.; Ma, Y.; Luo, J.; Zhu, H.; Wei, Y. Edge-Contact Formed by Oxygen Plasma and Rapid Thermal Annealing to Improve Metal-Graphene Contact Resistance. ECS J. Solid State Sci. Technol. 2018, 7, M11– M15,  DOI: 10.1149/2.0251802jss
      35
      Edge-Contact Formed by Oxygen Plasma and Rapid Thermal Annealing to Improve Metal-Graphene Contact Resistance
      Yan, Xiangyu; Jia, Kunpeng; Su, Yajuan; Ma, Yuanjun; Luo, Jun; Zhu, Huilong; Wei, Yayi
      ECS Journal of Solid State Science and Technology (2018), 7 (2), M11-M15CODEN: EJSSBG; ISSN:2162-8769. (Electrochemical Society)
      Graphene is the first sepd. 2-dimensional material which has extremely high carrier mobility. The high carrier mobility puts forward stricter requirement on the contact resistance of graphene devices. In this paper, graphene is treated by oxygen plasma with varied time to form different contact type between graphene and metal, esp. edge contact which enhances the graphene/metal bond strength and improves the contact resistance. To investigate the effect of thermal annealing on the contact resistance, the graphene devices with different oxygen plasma treatment were annealed by rapid thermal annealing. With the optimized annealing treatment, the av. edge contact resistance is reduced by 78.6% to 118 Ω•μm, with a min. value of 92 Ω•μm, which is very close to the best results reported so far. For the surface contact devices, thermal annealing resulted in reducing of contact resistance by 48% which is far less than edge contact. Due to the different influence of thermal annealing on various contact type, the edge contact resistance is 37.1% less than surface contact resistance. Since oxygen plasma treatment and rapid thermal annealing both are typical process of regular integrated circuit manuf. flow, the method proposed in the paper could provide an effective and feasible way to optimize the graphene/metal contact, hence promote the performance of future graphene and other two-dimensional material based devices.
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    36. 36
      Choi, B. Y.; Cho, K.; Pak, J.; Kim, T.-Y.; Kim, J.-K.; Shin, J.; Seo, J.; Chung, S.; Lee, T. Effects of Electron Beam Irradiation and Thiol Molecule Treatment on the Properties of MoS2 Field Effect Transistors. J. Korean Phys. Soc. 2018, 72, 1203– 1208,  DOI: 10.3938/jkps.72.1203
      36
      Effects of Electron Beam Irradiation and Thiol Molecule Treatment on the Properties of Molybdenum disulfide Field Effect Transistors
      Choi, Barbara Yuri; Cho, Kyungjune; Pak, Jinsu; Kim, Tae-Young; Kim, Jae-Keun; Shin, Jiwon; Seo, Junseok; Chung, Seungjun; Lee, Takhee
      Journal of the Korean Physical Society (2018), 72 (10), 1203-1208CODEN: JKPSDV; ISSN:0374-4884. (Korean Physical Society)
      We investigated the effects of the structural defects intentionally created by electron-beam irradn. with an energy of 30 keV on the elec. properties of monolayer MoS2 field effect transistors (FETs). We obsd. that the created defects by electron beam irradn. on the MoS2 surface working as trap sites deteriorated the carrier mobility and carrier concn. with increasing the subthreshold swing value and shifting the threshold voltage in MoS2 FETs. The elec. properties of electron-beam irradiated MoS2 FETs were slightly improved by treating the devices with thiol-terminated mols. which presumably passivated the structural defects of MoS2. The results of this study may enhance the understanding of the elec. properties of MoS2 FETs in terms of creating and passivating defect sites.
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    37. 37
      Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.; Yakobson, B. I.; Idrobo, J.-C. Intrinsic Structural Defects in Monolayer Molybdenum Disulfide. Nano Lett. 2013, 13, 2615– 2622,  DOI: 10.1021/nl4007479
      37
      Intrinsic Structural Defects in Monolayer Molybdenum Disulfide
      Zhou, Wu; Zou, Xiaolong; Najmaei, Sina; Liu, Zheng; Shi, Yumeng; Kong, Jing; Lou, Jun; Ajayan, Pulickel M.; Yakobson, Boris I.; Idrobo, Juan-Carlos
      Nano Letters (2013), 13 (6), 2615-2622CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mech., electronic, optical, and chem. properties that can be used for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphol. of the MoS2 layers. Here the authors provide a systematic study of intrinsic structural defects in chem. vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct at. resoln. imaging, and explore their energy landscape and electronic properties using 1st-principles calcns. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were obsd. in MoS2. One-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The at. scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.
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    38. 38
      Durand, C.; Zhang, X.; Fowlkes, J.; Najmaei, S.; Lou, J.; Li, A.-P. Defect-Mediated Transport and Electronic Irradiation Effect in Individual Domains of CVD-Grown Monolayer MoS2. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2015, 33, 02B110,  DOI: 10.1116/1.4906331
      There is no corresponding record for this reference.
    39. 39
      Rice, C.; Young, R. J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K. S. Raman-Scattering Measurements and First-Principles Calculations of Strain-Induced Phonon Shifts in Monolayer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 081307,  DOI: 10.1103/physrevb.87.081307
      39
      Raman-scattering measurements and first-principles calculations of strain-induced phonon shifts in monolayer MoS2
      Rice, C.; Young, R. J.; Zan, R.; Bangert, U.; Wolverson, D.; Georgiou, T.; Jalil, R.; Novoselov, K. S.
      Physical Review B: Condensed Matter and Materials Physics (2013), 87 (8), 081307/1-081307/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The effect of strain on the phonon modes of monolayer and few-layer MoS2 has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS2 sandwiched between two layers of optically transparent polymer. The resulting band shifts of the E12g (∼385.3 cm-1) and A1g (∼402.4 cm-1) Raman modes were found to be small but observable. First-principles plane-wave calcns. based on d. functional perturbation theory were used to det. the Gruneisen parameters for the E1g, E12g, A1g, and A2u modes and predict the exptl. obsd. band shifts for the monolayer material. The polymer-MoS2 interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS2 into devices.
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    40. 40
      Chakraborty, B.; Bera, A.; Muthu, D. V. S.; Bhowmick, S.; Waghmare, U. V.; Sood, A. K. Symmetry-Dependent Phonon Renormalization in Monolayer MoS2 Transistor. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 161403,  DOI: 10.1103/physrevb.85.161403
      40
      Symmetry-dependent phonon renormalization in monolayer MoS2 transistor
      Chakraborty, Biswanath; Bera, Achintya; Muthu, D. V. S.; Bhowmick, Somnath; Waghmare, U. V.; Sood, A. K.
      Physical Review B: Condensed Matter and Materials Physics (2012), 85 (16), 161403/1-161403/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS2 electrochem. top-gated field-effect transistor (FET), we show softening and broadening of the A1g phonon with electron doping, whereas the other Raman-active E12g mode remains essentially inert. Confirming these results with first-principles d. functional theory based calcns., we use group theor. arguments to explain why the A1g mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS2-based FETs, which have a high on-off ratio and are of technol. significance.
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    41. 41
      Scheuschner, N.; Ochedowski, O.; Kaulitz, A.-M.; Gillen, R.; Schleberger, M.; Maultzsch, J. Photoluminescence of Freestanding Single- and Few-Layer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 125406,  DOI: 10.1103/physrevb.89.125406
      41
      Photoluminescence of freestanding single- and few-layer MoS2
      Scheuschner, Nils; Ochedowski, Oliver; Kaulitz, Anne-Marie; Gillen, Roland; Schleberger, Marika; Maultzsch, Janina
      Physical Review B: Condensed Matter and Materials Physics (2014), 89 (12), 125406/1-125406/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a photoluminescence study of freestanding and Si/SiO2 supported single- and few-layer MoS2. The single-layer exciton peak (A) is only obsd. in freestanding MoS2. The photoluminescence of supported single-layer MoS2 instead originates from the A- (trion) peak as the MoS2 is n-type doped from the substrate. In bilayer MoS2, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to ≈80 meV. Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced by interference effects.
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    42. 42
      Conley, H. J.; Wang, B.; Ziegler, J. I.; Haglund, R. F.; Pantelides, S. T.; Bolotin, K. I. Bandgap Engineering of Strained Monolayer and Bilayer MoS2. Nano Lett. 2013, 13, 3626– 3630,  DOI: 10.1021/nl4014748
      42
      Bandgap Engineering of Strained Monolayer and Bilayer MoS2
      Conley, Hiram J.; Wang, Bin; Ziegler, Jed I.; Haglund, Richard F.; Pantelides, Sokrates T.; Bolotin, Kirill I.
      Nano Letters (2013), 13 (8), 3626-3630CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The influence of uniaxial tensile mech. strain in the range 0-2.2% on the phonon spectra and band structures of monolayer and bilayer MoS2 2-dimensional crystals are reported. First, Raman spectra were used to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and ext. a Grueneisen parameter of ∼1.06. Second, using luminescence a decrease in the optical band gap of MoS2 was measured that is approx. linear with strain, ∼45 meV/% strain for monolayer MoS2 and ∼120 meV/% strain for bilayer MoS2. Third, a pronounced strain-induced decrease in the luminescence intensity of monolayer MoS2 was obsd. that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ∼1%. These observations constitute a demonstration of strain engineering the band structure in the emergent class of 2-dimensional crystals, transition metal dichalcogenides.
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    43. 43
      Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207– 211,  DOI: 10.1038/nmat3505
      43
      Tightly bound trions in monolayer MoS2
      Mak, Kin Fai; He, Keliang; Lee, Changgu; Lee, Gwan Hyoung; Hone, James; Heinz, Tony F.; Shan, Jie
      Nature Materials (2013), 12 (3), 207-211CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Two-dimensional (2D) at. crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable phys. properties. In contrast to graphene, monolayer MoS2 is a noncentrosym. material with a direct energy gap. Strong photoluminescence, a current on/off ratio exceeding 108 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here the authors report the spectroscopic identification in a monolayer MoS2 field-effect transistor of tightly bound neg. trions, a quasiparticle composed of 2 electrons and a hole. These quasiparticles, which can be optically created with valley and spin polarized holes, have no analog in conventional semiconductors. They also possess a large binding energy (∼ 20 meV), rendering them significant even at room temp. Results open up possibilities both for fundamental studies of many-body interactions and for optoelectronic and valleytronic applications in 2-dimensional at. crystals.
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    44. 44
      Pollmann, E.; Madauß, L.; Schumacher, S.; Kumar, U.; Heuvel, F.; Ende, C. vom.; Yilmaz, S.; Gündörmüs, S.; Schleberger, M. Apparent Differences between Single Layer Molybdenum Disulfide Fabricated via Chemical Vapor Deposition and Exfoliation. 2020, arXiv:2006.05789 [cond-mat].
      There is no corresponding record for this reference.
    45. 45
      Di Bartolomeo, A.; Grillo, A.; Urban, F.; Iemmo, L.; Giubileo, F.; Luongo, G.; Amato, G.; Croin, L.; Sun, L.; Liang, S.-J.; Ang, L. K. Asymmetric Schottky Contacts in Bilayer MoS2 Field Effect Transistors. Adv. Funct. Mater. 2018, 28, 1800657,  DOI: 10.1002/adfm.201800657
      There is no corresponding record for this reference.
    46. 46
      Di Bartolomeo, A.; Urban, F.; Passacantando, M.; McEvoy, N.; Peters, L.; Iemmo, L.; Luongo, G.; Romeo, F.; Giubileo, F. A WSe2 Vertical Field Emission Transistor. Nanoscale 2019, 11, 1538– 1548,  DOI: 10.1039/c8nr09068h
      46
      A WSe2 vertical field emission transistor
      Di Bartolomeo, Antonio; Urban, Francesca; Passacantando, Maurizio; McEvoy, Niall; Peters, Lisanne; Iemmo, Laura; Luongo, Giuseppe; Romeo, Francesco; Giubileo, Filippo
      Nanoscale (2019), 11 (4), 1538-1548CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      We report the first observation of a gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chem.-vapor deposition on a SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, resp. The gate-tuned n-type conduction enables field emission, i.e. the extn. of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an elec. field ∼100 V μm-1 and exhibits good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on the WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.
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    47. 47
      Smyth, C. M.; Addou, R.; McDonnell, S.; Hinkle, C. L.; Wallace, R. M. Contact Metal–MoS2 Interfacial Reactions and Potential Implications on MoS2 -Based Device Performance. J. Phys. Chem. C 2016, 120, 14719– 14729,  DOI: 10.1021/acs.jpcc.6b04473
      47
      Contact Metal-MoS2 Interfacial Reactions and Potential Implications on MoS2-Based Device Performance
      Smyth, Christopher M.; Addou, Rafik; McDonnell, Stephen; Hinkle, Christopher L.; Wallace, Robert M.
      Journal of Physical Chemistry C (2016), 120 (27), 14719-14729CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS2 using electron beam deposition under two different reactor base pressures to det. the contact metal-MoS2 interface chem. and its dependence on the reactor ambient. The high work function metal Au does not react with MoS2 regardless of reactor ambient. In contrast, interfacial reactions between MoS2 and another high work function metal, Ir, are obsd. when it is deposited under both high vacuum (HV, ∼ 1 × 10-6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10-9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS2 when the contact metal is deposited under UHV conditions. Sc is rapidly oxidized on the MoS2 surface, whereas Cr is only partially oxidized when deposited under HV conditions. Deposition chamber ambient can affect the contact metal chem. in addn. to the chem. present at the contact metal-MoS2 interface. These results elucidate the true chem. of some contact metal-MoS2 interfaces and its dependence on the deposition ambient, and highlight the need to consider the chem. states present at the interface and their impact on contact resistance with MoS2.
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    48. 48
      Kwon, H.; Baik, S.; Jang, J.; Jang, J.; Kim, S.; Grigoropoulos, C.; Kwon, H.-J. Ultra-Short Pulsed Laser Annealing Effects on MoS2 Transistors with Asymmetric and Symmetric Contacts. Electronics 2019, 8, 222,  DOI: 10.3390/electronics8020222
      48
      Ultra-short pulsed laser annealing effects on MoS2 transistors with asymmetric and symmetric contacts
      Kwon, Hyeokjin; Baik, Seunghun; Eun Jang, Jae; Jang, Jaewon; Kim, Sunkook; Grigoropoulos, Costas P.; Kwon, Hyuk-Jun
      Electronics (Basel, Switzerland) (2019), 8 (2), 222CODEN: ELECGJ; ISSN:2079-9292. (MDPI AG)
      The ultra-short pulsed laser annealing process enhances the performance of MoS2 thin film transistors (TFTs) without thermal damage on plastic substrates. However, there has been insufficient investigation into how much improvement can be brought about by the laser process. In this paper, we obsd. how the parameters of TFTs, i.e., mobility, subthreshold swing, Ion/Ioff ratio, and Vth, changed as the TFTs' contacts were (1) not annealed, (2) annealed on one side, or (3) annealed on both sides. The results showed that the linear effective mobility (μeff_lin) increased from 13.14 [cm2/Vs] (not annealed) to 18.84 (one side annealed) to 24.91 (both sides annealed). Also, Ion/Ioff ratio increased from 2.27 × 105 (not annealed) to 3.14 × 105 (one side annealed) to 4.81 × 105 (both sides annealed), with Vth shifting to neg. direction. Analyzing the main reason for the improvement through the Y function method (YFM), we found that both the contact resistance (Rc) and the channel interface resistance (Rch) improves after the pulsed laser annealings under different conditions. Moreover, the Rc enhances more dramatically than the Rch does. In conclusion, our picosecond laser annealing improves the performance of TFTs (esp., the Rc) in direct proportion to the no. of annealings applied. The results will contribute to the investigation about correlations between the laser annealing process and the performance of devices.
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    49. 49
      Freedy, K. M.; Zhang, H.; Litwin, P. M.; Bendersky, L. A.; Davydov, A. V.; McDonnell, S. Thermal Stability of Titanium Contacts to MoS2. ACS Appl. Mater. Interfaces 2019, 11, 35389– 35393,  DOI: 10.1021/acsami.9b08829
      49
      Thermal stability of titanium contacts to MoS2
      Freedy, Keren M.; Zhang, Huairuo; Litwin, Peter M.; Bendersky, Leonid A.; Davydov, Albert V.; McDonnell, Stephen
      ACS Applied Materials & Interfaces (2019), 11 (38), 35389-35393CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices; however, its effects on interface chem. have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geol. bulk single crystals of MoS2 in ultrahigh vacuum (UHV) is investigated with XPS and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is obsd. resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compds. Annealing Ti/MoS2 sequentially at 100, 300, and 600°C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at a transistor operating temp. of 100°C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for elec. transport properties, highlighting the importance of interface chem. in the metal contact design and fabrication.
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    50. 50
      McDonnell, S.; Smyth, C.; Hinkle, C. L.; Wallace, R. M. MoS2 −Titanium Contact Interface Reactions. ACS Appl. Mater. Interfaces 2016, 8, 8289– 8294,  DOI: 10.1021/acsami.6b00275
      50
      MoS2-titanium contact interface reactions
      McDonnell, Stephen; Smyth, Christopher; Hinkle, Christopher L.; Wallace, Robert M.
      ACS Applied Materials & Interfaces (2016), 8 (12), 8289-8294CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The formation of the Ti-MoS2 interface, which is heavily utilized in nanoelectronic device research, is studied by XPS. It is found that, if deposition under high vacuum (∼1 × 10-6 mbar) as opposed to ultrahigh vacuum (∼1 × 10-9 mbar) conditions are used, TiO2 forms at the interface rather than Ti. The high vacuum deposition results in an interface free of any detectable reaction between the semiconductor and the deposited contact. In contrast, when metallic titanium is successfully deposited by carrying out depositions in ultrahigh vacuum, the titanium reacts with MoS2 forming TixSy and metallic Mo at the interface. These results have far reaching implications as many prior studies assuming Ti contacts may have actually used TiO2 due to the nature of the deposition tools used.
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    51. 51
      English, C. D.; Shine, G.; Dorgan, V. E.; Saraswat, K. C.; Pop, E. Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition. Nano Lett. 2016, 16, 3824– 3830,  DOI: 10.1021/acs.nanolett.6b01309
      51
      Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition
      English, Chris D.; Shine, Gautam; Dorgan, Vincent E.; Saraswat, Krishna C.; Pop, Eric
      Nano Letters (2016), 16 (6), 3824-3830CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (RC). Here the authors present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temps. (80 to 500 K), carrier densities (1012 to 1013 cm-2), and contact dimensions (20 to 500 nm). The authors uncover that Au deposited in ultra-high vacuum (∼10-9 Torr) yields three times lower RC than under normal conditions, reaching 740 Ω μm and specific contact resistivity 3 × 10-7 Ω cm2, stable for over four months. Modeling reveals sep. RC contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified exptl. using devices with 20 nm contacts and 70 nm contact pitch (CP), equiv. to the 14 nm technol. node.
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    52. 52
      Wang, Q.; Deng, B.; Shi, X. A New Insight for Ohmic Contacts to MoS2 : By Tuning MoS2 Affinity Energies but Not Metal Work-Functions. Phys. Chem. Chem. Phys. 2017, 19, 26151– 26157,  DOI: 10.1039/c7cp05109c
      52
      A new insight for ohmic contacts to MoS2: by tuning MoS2 affinity energies but not metal work-functions
      Wang, Qian; Deng, Bei; Shi, Xingqiang
      Physical Chemistry Chemical Physics (2017), 19 (38), 26151-26157CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently attracted tremendous interest for fundamental studies and applications. High contact resistances between the metal electrodes and the 2D TMDCs, usually composed of a tunneling barrier (TB) and a Schottky barrier (SB), are the key bottleneck to the realization of high performance devices based on such systems. Here, from van der Waals d. functional theory calcns., we demonstrate that strain can provide a feasible means to reduce the contact resistances between, for example, 2D semiconductor MoS2 and metal surfaces, in both strong and weak coupling regimes. Both the SB and TB are lowered significantly with the increasing tensile strain in both the coupling regimes. Esp., the SB can reduce to zero in all configurations considered, with tensile strain increasing to ∼4% or above. The mechanism of SB redn. under tensile strain is attributed to the increase of the MoS2 affinity energy since the monolayer MoS2 conduction band min. (CBm) is derived from anti-bonding states. Thus, the SB in other semiconducting TMDCs with an anti-bonding CBm (for n-type contact) could also be reduced to zero by tensile strain. Our discoveries thus shed a new and general light on minimizing the contact resistance of semiconducting TMDCs-metal based contacts and this can also prove applicable to other 2D semiconductors, e.g. phosphorene.
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    53. 53
      Kim, C.; Moon, I.; Lee, D.; Choi, M. S.; Ahmed, F.; Nam, S.; Cho, Y.; Shin, H.-J.; Park, S.; Yoo, W. J. Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides. ACS Nano 2017, 11, 1588– 1596,  DOI: 10.1021/acsnano.6b07159
      53
      Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides
      Kim, Changsik; Moon, Inyong; Lee, Daeyeong; Choi, Min Sup; Ahmed, Faisal; Nam, Seunggeol; Cho, Yeonchoo; Shin, Hyeon-Jin; Park, Seongjun; Yoo, Won Jong
      ACS Nano (2017), 11 (2), 1588-1596CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Elec. metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs is reported only theor., although that of bulk TMDCs is reported exptl. Here, the authors report the exptl. study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. The authors also quant. compared the authors' results with the theor. simulation results of the monolayer structure as well as the exptl. results of the bulk structure. The authors measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, resp., suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theor. prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. The authors' results further imply that metal work functions have very little influence on contact properties of 2-dimensional-material-based devices. Also, Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chem. doping into the 2-dimensional materials. These findings provide a practical guideline for depinning Fermi level at the 2-dimensional interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.
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    54. 54
      Guo, Y.; Liu, D.; Robertson, J. 3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides. ACS Appl. Mater. Interfaces 2015, 7, 25709– 25715,  DOI: 10.1021/acsami.5b06897
      54
      3D Behavior of Schottky Barriers of 2D Transition-Metal Dichalcogenides
      Guo, Yuzheng; Liu, Dameng; Robertson, John
      ACS Applied Materials & Interfaces (2015), 7 (46), 25709-25715CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The transition metal dichalcogenides (TMDs) are two-dimensional layered solids with van der Waals bonding between layers. We calc. their Schottky barrier heights (SBHs) using supercell models and d. functional theory. It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S = 0.3, a similar value for both top and edge contact geometries. This arises because there is direct bonding between the contact metal atoms and the TMD chalcogen atoms, for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials. The Schottky barriers largely follow the metal-induced gap state (MIGS) model, like those of three-dimensional semiconductors, despite the bonding in the TMDs being largely constrained within the layers. The pinning energies are found to be lower in the gap for edge contact geometries than for top contact geometries, which might be used to obtain p-type contacts on MoS2.
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    55. 55
      Pan, Y.; Gu, J.; Tang, H.; Zhang, X.; Li, J.; Shi, B.; Yang, J.; Zhang, H.; Yan, J.; Liu, S.; Hu, H.; Wu, M.; Lu, J. Reexamination of the Schottky Barrier Heights in Monolayer MoS2 Field-Effect Transistors. ACS Appl. Nano Mater. 2019, 2, 4717– 4726,  DOI: 10.1021/acsanm.9b00200
      55
      Reexamination of the Schottky barrier heights in monolayer MoS2 field-effect transistors
      Pan, Yuanyuan; Gu, Jihuan; Tang, Hao; Zhang, Xiuying; Li, Jingzhen; Shi, Bowen; Yang, Jie; Zhang, Han; Yan, Jiahuan; Liu, Shiqi; Hu, Han; Wu, Mingbo; Lu, Jing
      ACS Applied Nano Materials (2019), 2 (8), 4717-4726CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Owing to its promising electronic application of monolayer (ML) MoS2, ML MoS2-metal contacts have been widely explored. The expts. reveal a very strong Fermi level pinning, and the corresponding pinning factor is about 0.1, but all the existing calcns. give a larger pinning factor of about 0.3. Such an apparent discrepancy is attributed to the defects in samples. In this paper, the Schottky barriers are reexamd. in the pristine ML MoS2 field effect transistors (FETs) with a series of metal electrodes (Au, Pt, Ag, Ti, Cr, Pd, Ni, and ML CCr2) by using ab initio quantum transport simulation. The Schottky barrier heights obtained from our ab initio quantum transport simulation are in better agreement with those obsd. in expts. for Au and Pt electrodes, and the calcd. pinning factor is also improved. Our work highlights the importance of the inclusion of the coupling between the electrode and channel in detg. the pinning behavior. Hence, ab initio quantum transport simulation is an improved method to det. the SBH and the pinning factor in low-dimensional semiconductor FETs.
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    56. 56
      Gong, C.; Colombo, L.; Wallace, R. M.; Cho, K. The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces. Nano Lett. 2014, 14, 1714– 1720,  DOI: 10.1021/nl403465v
      56
      The Unusual Mechanism of Partial Fermi Level Pinning at Metal-MoS2 Interfaces
      Gong, Cheng; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae
      Nano Letters (2014), 14 (4), 1714-1720CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      D. functional theory calcns. were performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are present for a variety of metals with the work functions spanning over 4.2-6.1 eV. Except for the p-type Schottky contact with Pt, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal-MoS2 contact is unique for metal-2D-semiconductor interfaces, remarkably different from the known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: 1st by a metal work function modification by interface dipole formation due to the charge redistribution, and 2nd by the prodn. of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form ohmic contact to MoS2.
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    57. 57
      Zhong, H.; Quhe, R.; Wang, Y.; Ni, Z.; Ye, M.; Song, Z.; Pan, Y.; Yang, J.; Yang, L.; Lei, M.; Shi, J.; Lu, J. Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations. Sci. Rep. 2016, 6, 21786,  DOI: 10.1038/srep21786
      57
      Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations
      Zhong, Hongxia; Quhe, Ruge; Wang, Yangyang; Ni, Zeyuan; Ye, Meng; Song, Zhigang; Pan, Yuanyuan; Yang, Jinbo; Yang, Li; Lei, Ming; Shi, Junjie; Lu, Jing
      Scientific Reports (2016), 6 (), 21786CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calcd. and obsd. Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calcn. scheme fails to reproduce the obsd. SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the obsd. SBH in 2D MoS2-Sc interface and highlights the importance of a higher level theor. approach beyond the energy band calcn. in the interface study. BL MoS2-metal contacts generally have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.
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    58. 58
      Di Bartolomeo, A. Graphene Schottky Diodes: An Experimental Review of the Rectifying Graphene/Semiconductor Heterojunction. Phys. Rep. 2016, 606, 1– 58,  DOI: 10.1016/j.physrep.2015.10.003
      58
      Graphene Schottky diodes: An experimental review of the rectifying graphene/semiconductor heterojunction
      Di Bartolomeo, Antonio
      Physics Reports (2016), 606 (), 1-58CODEN: PRPLCM; ISSN:0370-1573. (Elsevier B.V.)
      In the past decade graphene has been one of the most studied materials for several unique and excellent properties. Due to its two dimensional nature, phys. and chem. properties and ease of manipulation, graphene offers the possibility of integration with the existing semiconductor technol. for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chem. and biol. sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.
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    59. 59
      Anwar, A.; Nabet, B.; Culp, J.; Castro, F. Effects of Electron Confinement on Thermionic Emission Current in a Modulation Doped Heterostructure. J. Appl. Phys. 1999, 85, 2663– 2666,  DOI: 10.1063/1.369627
      59
      Effects of electron confinement on thermionic emission current in a modulation doped heterostructure
      Anwar, Amro; Nabet, Bahram; Culp, James; Castro, Fransisco
      Journal of Applied Physics (1999), 85 (5), 2663-2666CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The authors discuss mechanisms responsible for the redn. of electron thermionic emission current from a Schottky contact to a modulation doped semiconductor compared to a bulk semiconductor. The effects discussed include metal to semiconductor barrier height enhancement due to proposed electron-electron cloud interaction, confined potential of the reduced dimensional systems, and the reduced dimensional nature of the d. of states in the semiconductor. These effects describe the obsd. lowering of the dark current, and hence noise, of a modulation doped heterojunction based photodetector compared to a conventional bulk device.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1MXhtF2gs70%253D&md5=541e8db98f90f5241da7c6c70f13b68e
    60. 60
      Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2006.
      There is no corresponding record for this reference.
    61. 61
      Di Bartolomeo, A.; Genovese, L.; Foller, T.; Giubileo, F.; Luongo, G.; Croin, L.; Liang, S.-J.; Ang, L. K.; Schleberger, M. Electrical Transport and Persistent Photoconductivity in Monolayer MoS2 Phototransistors. Nanotechnology 2017, 28, 214002,  DOI: 10.1088/1361-6528/aa6d98
      61
      Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors
      Di Bartolomeo, Antonio; Genovese, Luca; Foller, Tobias; Giubileo, Filippo; Luongo, Giuseppe; Croin, Luca; Liang, Shi-Jun; Ang, L. K.; Schleberger, Marika
      Nanotechnology (2017), 28 (21), 214002/1-214002/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      We study elec. transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photocond. increases logarithmically with the light intensity and can persist with a decay time longer than 104s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 kW μm-1, ON current as high as 1.25 nA μm-1, 105 ON-OFF ratio, mobility of ∼1 cm2 V-1s-1 and photoresponsivity R ≈ 1 A W-1.
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    62. 62
      Zhang, K.; Peng, M.; Yu, A.; Fan, Y.; Zhai, J.; Wang, Z. L. A Substrate-Enhanced MoS2 Photodetector through a Dual-Photogating Effect. Mater. Horiz. 2019, 6, 826– 833,  DOI: 10.1039/c8mh01429a
      62
      A substrate-enhanced MoS2 photodetector through a dual-photogating effect
      Zhang, Ke; Peng, Mingzeng; Yu, Aifang; Fan, Youjun; Zhai, Junyi; Wang, Zhong Lin
      Materials Horizons (2019), 6 (4), 826-833CODEN: MHAOBM; ISSN:2051-6355. (Royal Society of Chemistry)
      The high-performance, miniaturization and flexibility of two-dimensional (2D) material-based electronics and optoelectronics make them promising substitutes for silicon-based devices in the future. However, the complex structures and the assistance strategies needed to compensate for the intrinsic deficiencies of 2D materials are major challenges hindering practical applications. It is hoped to achieve high-performance 2D material-based devices of the simplest construction without external assistance. Therefore, making full use of the basic components (host material, substrate and electrode) of 2D devices is crucial. To date, the role of the substrate has always been ignored. Here, we choose undoped-silicon carbon (SiC) and Kapton (polyimide, PI) film as substrates for MoS2 photodetectors (PDs) for rigid and flexible applications, resp. A new concept, the "dual-photogating effect" induced at the MoS2/substrate interface, is proposed and applied to obtain ultra-high photoresponse performance, such as high photoresponsivity of ∼104 A W-1 in MoS2/SiC PD and an on/off ratio of ∼105 in a MoS2/Kapton PD.
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    63. 63
      Cheng, Y. J.; Yan, L.; Shi, F.; Liu, F.; Li, M.; Shi, H. L.; Hou, Z. P. Monte Carlo Simulation of Electron Scattering in Ion Barrier Film in Generation III Image Intensifier. Key Eng. Mater. 2013, 552, 193– 200,  DOI: 10.4028/www.scientific.net/kem.552.193
      63
      Monte Carlo simulation of electron scattering in ion barrier film in generation III image intensifier
      Cheng, Yaojin; Yan, Lei; Shi, Feng; Liu, Feng; Li, Min; Shi, Hongli; Hou, Zhipeng
      Key Engineering Materials (2013), 552 (), 193-200, 9 pp.CODEN: KEMAEY; ISSN:1662-9795. (Trans Tech Publications Ltd.)
      In Generation III image intensifier tubes, the input of the microchannel plate is typically coated with a thin Al2O3 film to prevent ions penetrating and damaging the delicate Cs-O activation layer of GaAs cathode. While the application of an ion barrier film to the MCP greatly improves the mean time to failure of the image intensifier tube, it unfortunately produces electron scattering to cause a significant degrdn. in the Modulation Transfer Function and resoln. of the image tubes. To obtain an overview of the role of the ion barrier in Generation III image intensifiers, in this paper, we present the results of a Monte Carlo simulation on scattering electron transport through Al2O3 film with various photocathode voltage, d., thickness, and stoichiometry. Using Monte Carlo simulation on scattering electron ways, we get the MTF curve of film at the accelerating voltage of 400v, a thickness of 10nm, and a d. of 0.3 g/cm3. The simulation results play an important role on optimizing the quality and facture technique of ion barrier film.
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    64. 64
      Movla, H.; Babazadeh, M. Simulation Analysis of the Aluminum Thin Film Thickness Measurement by Using Low Energy Electron Beam. Optik 2014, 125, 71– 74,  DOI: 10.1016/j.ijleo.2013.06.033
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      Simulation analysis of the aluminum thin film thickness measurement by using low energy electron beam
      Movla, Hossein; Babazadeh, Mohammad
      Optik (Munich, Germany) (2014), 125 (1), 71-74CODEN: OTIKAJ; ISSN:0030-4026. (Elsevier GmbH)
      This paper indicates a simulation anal. for estg. the aluminum (Al) thin film thickness measurements by using the low energy electron beam. In order to calc. the Al thickness estn., the energy of the incident electron beams was varied from 10 to 30 keV, while the thickness of the Al film was varied between 6 and 14 μm. From the simulation results it was found that electron transmittance fraction in 14 μm sample is about nine orders of magnitude more than 6 μm sample at the same incident electron beam energy. Simulation results show that max. transmitted electrons vs. Al layer thickness has a parabolic relation and by using the obtained equation, it is possible to est. unknown thickness of the thin film Al layer. All calcns. here were done by CASINO numerical simulation package.
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    65. 65
      Drouin, D.; Couture, A. R.; Joly, D.; Tastet, X.; Aimez, V.; Gauvin, R. CASINO V2.42—A Fast and Easy-to-Use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users. Scanning 2007, 29, 92– 101,  DOI: 10.1002/sca.20000
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      CASINO V2.42-a fast and easy-to-use modeling tool for scanning electron microscopy and microanalysis users
      Drouin, Dominique; Couture, Alexandre Real; Joly, Dany; Tastet, Xavier; Aimez, Vincent; Gauvin, Raynald
      Scanning (2007), 29 (3), 92-101CODEN: SCNNDF; ISSN:0161-0457. (John Wiley & Sons Ltd.)
      Monte Carlo simulations have been widely used by microscopists for the last few decades. In the beginning it was a tedious and slow process, requiring a high level of computer skills from users and long computational times. Recent progress in the microelectronics industry now provides researchers with affordable desktop computers with clock rates greater than 3 GHz. With this type of computing power routinely available, Monte Carlo simulation is no longer an exclusive or long (overnight) process. The aim of this paper is to present a new user-friendly simulation program based on the earlier CASINO Monte Carlo program. The intent of this software is to assist scanning electron microscope users in interpretation of imaging and microanal. and also with more advanced procedures including electron-beam lithog. This version uses a new architecture that provides results twice as quickly. This program is freely available to the scientific community and can be downloaded from the website: www.gel.usherb.ca/casino.
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    66. 66
      Abraham, M.; Mohney, S. E. Annealed Ag Contacts to MoS2 Field-Effect Transistors. J. Appl. Phys. 2017, 122, 115306,  DOI: 10.1063/1.4991961
      66
      Annealed Ag contacts to MoS2 field-effect transistors
      Abraham, Michael; Mohney, Suzanne E.
      Journal of Applied Physics (Melville, NY, United States) (2017), 122 (11), 115306/1-115306/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Silver contacts to few-layer (5 to 14 layers thick) MoS2 have been studied before and after annealing. Annealing was found to be crit. for reducing the contact resistance but did not degrade the operation of field-effect transistors that are part of the test structure. The contact resistance for the as-deposited samples was in the range of 0.8-3.5 kΩ μm. On the other hand, the contact resistance was reduced to 0.2-0.7 kΩ μm, evaluated at a const. sheet resistance of 32 kΩ/.box., after annealing at 250 or 300 °C. The reduced contact resistance is attributed to diffusion of Ag into the MoS2 and doping, as supported by further elec. characterization of the contacts and devices. (c) 2017 American Institute of Physics.
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    67. 67
      Goyal, N.; Mackenzie, D. M. A.; Panchal, V.; Jawa, H.; Kazakova, O.; Petersen, D. H.; Lodha, S. Enhanced Thermally Aided Memory Performance Using Few-Layer ReS2 Transistors. Appl. Phys. Lett. 2020, 116, 052104,  DOI: 10.1063/1.5126809
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      Enhanced thermally aided memory performance using few-layer ReS2 transistors
      Goyal, Natasha; Mackenzie, David M. A.; Panchal, Vishal; Jawa, Himani; Kazakova, Olga; Petersen, Dirch Hjorth; Lodha, Saurabh
      Applied Physics Letters (2020), 116 (5), 052104CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Thermally varying hysteretic gate operation in few-layer ReS2 and MoS2 back gate field effect transistors (FETs) is studied and compared for memory applications. Clockwise hysteresis at room temp. and anti-clockwise hysteresis at higher temp. (373 K for ReS2 and 400 K for MoS2) are accompanied by step-like jumps in transfer curves for both forward and reverse voltage sweeps. Hence, a step-like conductance (STC) crossover hysteresis between the transfer curves for the two sweeps is obsd. at high temp. Furthermore, memory parameters such as the RESET-to-WRITE window and READ window are defined and compared for clockwise hysteresis at low temp. and STC-type hysteresis at high temp., showing better memory performance for ReS2 FETs as compared to MoS2 FETs. Smaller operating temp. and voltage along with larger READ and RESET-to-WRITE windows make ReS2 FETs a better choice for thermally aided memory applications. Finally, temp. dependent Kelvin probe force microscopy measurements show decreasing (const.) surface potential with increasing temp. for ReS2 (MoS2). This indicates less effective intrinsic trapping at high temp. in ReS2, leading to earlier occurrence of STC-type hysteresis in ReS2 FETs as compared to MoS2 FETs with increasing temp. (c) 2020 American Institute of Physics.
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