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Migration-Enhanced Metal–Organic Chemical Vapor Deposition of Wafer-Scale Fully Coalesced WS2 and WSe2 Monolayers
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Crystal Growth & Design

Cite this: Cryst. Growth Des. 2023, 23, 3, 1547–1558
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https://doi.org/10.1021/acs.cgd.2c01134
Published January 24, 2023

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Abstract

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Metal–organic chemical vapor deposition (MOCVD) is widely employed for the wafer-scale synthesis of transition metal dichalcogenide (TMDC) monolayers (MLs). Despite large efforts devoted to understanding the intricate nucleation and lateral growth mechanisms of TMDCs, little attention has been paid to the migration of adatoms on the top of an ML and its influence on parasitic/premature bilayer (BL) nucleation. In this work, using a commercial multi-wafer MOCVD platform, a novel two-stage migration-enhanced MOCVD process is introduced to realize the deposition of wafer-scale fully coalesced tungsten disulfide (WS2) and tungsten diselenide (WSe2) MLs with only sparse BL nucleation in a reasonable deposition time. With the WS2 ML coverage exceeding 99% on 2 in. sapphire substrates within 3 h, BL coverage is suppressed to ∼15%. Following the same migration enhancement approach, WSe2 MLs are synthesized in 90 min with <20% BL coverage. The migration of W adatoms on the already formed stable WS2 (or WSe2) ML domains is promoted by ramping down the delivery of the tungsten precursor. From the qualitative analysis of the nanomorphology, the migration length of W adatoms is estimated to be ≤100 nm. This approach can be seen as a reliable and solid basis for the development of future large-scale TMDC ML deposition techniques.

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Synopsis

The upper half describes two possible cases during growth of individual domains. The triangle outlined in blue highlights lateral monolayer growth when the domain size is small compared to the migration length of W adatoms. The yellow one depicts bilayer nucleation caused by W adatoms confined on a monolayer domain. The lower half shows the mechanism of WCO ramp-down.

Introduction

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In the last decade, transition metal dichalcogenides (TMDCs) as an emerging class of 2D (semi)conducting materials have received great interest in various application fields such as (opto-)electronics, (1−3) sensing, (4,5) and catalysis. (6,7) High-quality single crystals, fabricated by either exfoliation or solid-source chemical vapor deposition, have been the subject of overwhelming activities to develop TMDC-based device technology, highlighting the great potential of this material system. However, all future applications require wafer-scale deposition techniques that fulfill the demands of industrialization and reproducible large-scale device fabrication.
Among a couple of methods (e.g., exfoliation, chemical vapor deposition, and atomic layer deposition) reported to prepare TMDC thin films, metal–organic chemical vapor deposition (MOCVD) stands out due to its excellent scalability, uniformity, and controllability. (8,9) Until now, various types of TMDC monolayers (MLs) have been successfully synthesized via MOCVD. Kang et al. (10) reported on the deposition of MoS2 and WS2 MLs on 4 in. fused silica and SiO2 substrates at a low growth temperature (550 °C), in which the layer-by-layer growth mode is achieved by low flow rates of precursors. Similarly, Grundmann et al. (11) employed a commercial MOCVD hot-wall reactor and deposited fully coalesced WS2 MLs with only sparse bilayer (BL) nucleation (∼10%) on 2 in. sapphire (0001) substrates at an elevated temperature (820 °C). However, a common drawback of these approaches is the long growth time (≥20 h). Boosting the lateral growth rate (after nucleation) by simply increasing the injection flux of precursors and thus gas-phase supersaturation will typically lead to the formation of a high density of premature BL nuclei. This process delays the coalescence of the first ML due to excessive consumption of adatoms for the new nuclei formed on the top of the ML. As growth proceeds, multilayers and even vertically standing nanostructures (12) are obtained.
Another technical challenge is to avoid unintentional carbon co-deposition in MOCVD. Based on vapor-phase chemistry, such carbon impurities are presumed to be originating from parasitic thermal decomposition, especially of side groups of organic chalcogen precursors. (13) As disclosed by many researchers, co-deposited carbon not only suppresses the lateral growth of ML but also deteriorates the properties of TMDCs. (13−16) Several countermeasures have been developed to prevent carbon co-deposition. For example, reducing the total pressure in the reactor chamber or adding H2 into the carrier gas has shown promising results. (13,15,17)
Recent studies have suggested using domain ripening and varied precursor fluxes to suppress BL nucleation and accelerate deposition. (18−20) The purpose of ripening is to dissociate unstable non-stoichiometric or sub-critically small clusters (on either the substrate (18) or the existing ML (20)) and to have the dissociated adatoms reassemble at the energetically preferential edge of ML domains, which results in the lateral growth of the existing ML domains. Practically, ripening is performed by stopping the supply of the growth-limiting species. Zhang et al. (18) demonstrated a diffusion-controlled multi-step MOCVD process in which nucleation, ripening, and lateral growth were designed with different precursor flow settings. A low flow rate in the later growth stage was chosen to reduce the gas-phase supersaturation. Finally, a closed WSe2 ML with 36% BL coverage was obtained on a 1 cm × 1 cm sapphire substrate. Such a multi-step MOCVD process was later applied to prepare crystallographically unidirectional WS2 ML with minimal BL coverage (<1%) on 2 in. sapphire. (19) As a step further, Seol et al. (20) introduced vertical Ostwald ripening in a pulsed MOCVD process, in which the injection of precursors is periodically interrupted. The authors claimed that the adatoms and sub-critical clusters are able to diffuse from the top of the MLs to their edges (i.e., vertical motion) during growth interruptions. Using this strategy, MoS2 and WS2 ML can be deposited on 6 in. SiO2 substrates in only 12 min. Recently, Choi et al. (21) employed a similar pulsed MOCVD technique and successfully synthesized wafer-scale MoS2 thin films at a relatively low temperature. By introducing water during idle times, Cohen et al. (22) proposed a growth–etch MOCVD process and realized the deposition of WS2 MLs on 2 in. sapphire with a large domain size and a low degree of carbon contamination.
Most of these studies explain the working principle of ripening/pulsing by the diffusion mechanism of adatoms on the substrate. It has been comprehensively elucidated that the diffusive motion of adatoms on the substrates, as an essential step in the lateral growth of 2D TMDCs, can influence growth kinetics (23−26) and the evolution of the domain morphology. (18,24,27) Adsorption and diffusive motion of adatoms on the surface of ML domains (which will be termed “migration” in this work) as well as their vertical climb-down motion across step edges of individual ML domains are commonly ignored to simplify the above model and discussion. The main justification for such simplification is that adsorption and sticking of adatoms are weaker on the ML surface due to the lower density of dangling bonds than that on the sapphire surface. However, as the ML coverage increases and approaches coalescence, the available free area of the sapphire surface will reduce and the number of adatoms arriving at the ML surface will increase. As a result, the adsorption and migration of adatoms on the ML surface start to play a vital role in premature parasitic BL nucleation on ML domains. Therefore, the migration behavior of adatoms in MOCVD, especially near full coalescence of MLs, requires dedicated experimental studies and discussion.
Previously, wafer-scale fully coalesced MoS2 MLs with ∼15% BL coverage have been successfully synthesized in our group without specifically considering the migration behavior of Mo adatoms. (26) However, for the case of WS2 discussed here, it is found that the BL coverage can easily exceed 20%. W-containing TMDCs appear to be more challenging for ML coalescence and BL suppression. We attribute this to a generally lower adatom mobility of W than that of Mo. (28) It means that W adatoms are prone to be confined on the top of ML domains and form BLs (compared to Mo adatoms), so the migration of W adatoms needs to be promoted.
In this work, wafer-scale fully coalesced WS2 and WSe2 MLs with largely suppressed BL formation are synthesized via two-stage migration-enhanced MOCVD processes. Similar to previous studies, (18,19,26) the nucleation (stage I) and lateral growth (stage II) are designed to take place at two distinct temperatures. Furthermore, the flux of tungsten hexacarbonyl (WCO) is gradually reduced by 75% during lateral growth stage II. Such ramp-down processes can be precisely controlled using mass flow controllers and monitored via in situ techniques in real time.
By studying the nanoscale morphology and structural properties of as-deposited WS2 thin films, the role of adatom migration is discussed, and the impact of WCO ramp-down is revealed. The influence of premature BL nucleation and carbon co-deposition on optical properties is unveiled experimentally. Additionally, the synergy between migration length of adatoms and growth conditions is discussed toward the realization of high-quality fully coalesced WS2 MLs. Such WS2 processes can be directly translated for the successful growth of WSe2 MLs, again evidencing the central role of W adatoms in ML formation, as well as showing the influence of different chalcogen species.

Methods

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A commercial AIXTRON cold-wall close-coupled showerhead reactor in 7 × 2″ geometry was employed, which was equipped with LayTec’s EpiTT in situ true temperature measurement and spectroscopic reflectance monitoring. EpiTT employs a high-brightness light-emitting diode as the light source and measures the reflectance of the sample surface at 950, 633, and 405 nm during MOCVD processes. A built-in ARGUS pyrometric profiling system controls the temperature distribution across the susceptor. Unless stated otherwise, the temperatures mentioned in this work refer to the emissivity-corrected true temperature on the wafer surface.
Tungsten hexacarbonyl [W(CO)6 or WCO, 99.99% purity], di-tert-butyl sulfide (DTBS, 99.999% purity), and di-iso-propyl selenide (DiPSe, 99.999% purity), all from Dockweiler Chemicals, were chosen as precursors. H2 was used as the carrier gas and reactor atmosphere. All processes were performed on (0001) oriented 2 in. sapphire substrates with a 0.2° offcut toward the m-plane and started with a H2 substrate desorption procedure for 30 min under 150 hPa at 1050 °C (which is adapted from our previous work (15)). For the deposition, a low reactor pressure of 20 hPa was chosen due to the suppression of Raman-detectable carbon incorporation in the TMDC films (see Figure S1 for details). The susceptor rotation was set to 25 rpm during deposition. After desorption, the measured reflectance values are normalized to the standard reflectance spectrum of sapphire provided by LayTec.
For WS2, the two-stage migration-enhanced MOCVD process consists of (i) nucleation stage I at relatively low temperatures (i.e., 700 and 750 °C) for only 15 min and (ii) lateral growth stage II at a relatively high temperature (820 °C) for 2–3 h. A temperature ramp at the beginning of stage II is introduced at a rate of 10 K/min without growth interruption (Figure 1a). In stage II, the molar flow of WCO is linearly ramped down by −75%, from 28 to 7 nmol/min (in, for example, 144 min, as shown in Figure 1b). With a constant DTBS flow (180 μmol/min), the S/W ratio increases from initially approx. 6,400 to 25,600 at the end of the process.

Figure 1

Figure 1. (a) Temperature profile with two different nucleation temperatures and (b) variation of molar flows and S/W ratio in the two-stage migration-enhanced MOCVD process (shown for sample S_R144, see below).

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To evaluate the effect of WCO ramp-down (R), several “constant-flow” processes (C) were also conducted for comparison, in which the molar flow of WCO is kept constant in both stages. The temperature profile in these C processes is the same as that in R processes. Detailed growth parameters for all WS2 samples discussed in this work are summarized in Table 1. Samples S_N700 and S_N750 are reference samples to study nucleation at different temperatures, grown only for the usual duration time of 15 min. The three digits after R/C represent the lateral growth time in minutes. “HT” stands for high nucleation temperature (i.e., 750 °C). Samples S_R144 and S_R160.HT are compared to reveal the influence of nucleation density on BL formation. Samples S_R128, S_R144, S_C090, and S_C100 are used to discuss the impact of WCO ramp-down in detail. Coverage values are also given for the ML and BL (if applicable) and show the sample evolution toward a closed ML film.
Table 1. Detailed Summary of Growth Conditions of WS2 Samples (S_xxxx)a
a

Yellow- and gray-shaded samples are grown with low and high nucleation temperatures, respectively. Red- and green-shaded samples are deposited via the ramped (R) WCO flux and constant (C) WCO flux, respectively. The saturation of color reflects the coverage of the corresponding sample.

The development and details of migration-enhanced MOCVD processes is elucidated in the following using the WS2 samples listed in Table 1. WSe2 deposition shares most of the characteristics of WS2 growth. Therefore, WSe2 results will be presented in a compact form, introducing five samples (Se_N28, Se_N56, Se_R45, Se_R60, and Se_R75 as listed in Table 2) of different coalescence levels. The two digits after Se_N (nucleation-only samples) represent the WCO flux in nanomoles per minute during nucleation. As before, the digits after R represent the lateral growth time in minutes.
Table 2. Detailed Summary of Growth Conditions of WSe2 Samples (Se_xxx)a
a

Gray- and yellow-shaded samples are grown with low WCO flux and high WCO flux, respectively. The saturation of green color reflects the coverage of the corresponding sample.

Scanning electron microscopy (SEM; Zeiss Sigma), Raman spectroscopy (Renishaw inVia), photoluminescence (PL) spectroscopy (WiTec), and X-ray photoelectron spectroscopy (XPS; Physical Electronics) were employed to study the structural and optical properties of the as-deposited thin films. SEM images taken from different positions on the samples were segmented via ImageJ (29) to determine the quantities of nuclei as well as the coverage of the ML and BL. In this work, if ML coverage reaches 99%, the thin films will be considered as “fully coalesced”. For both Raman and PL measurements, the excitation wavelength is 532 nm and the laser power is ≤0.7 mW (to avoid damaging samples). The grating used for Raman measurements has 1800 grooves/mm and for PL measurements has 300 grooves/mm. Lorentzian fitting was applied to process all measured spectra. The source of XPS is the monochromatic Al Kα line at 1486.6 eV. An electron neutralizer with the emission of 20 μA current and a bias of 1.37 eV was used to avoid charging of samples during measurements. High-resolution scans were performed with a pass energy at 10 eV and a step width of 0.1 eV. All XPS spectra were deconvoluted using the Voigt function.

Results and Discussion

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Migration and Migration Enhancement

Two fundamental diffusive processes are relevant for W adatoms arriving from the gas phase. One takes place on the bare substrate surface and the other takes place on the surface of the already formed ML. In order to distinguish these two, in the following, the former is referred to as “diffusion”, while the latter is termed “migration”. “Migration” is used in analogy with “migration-enhanced MOCVD” (MEMOCVD) in III-nitride epitaxy, (30) which relates to the diffusive motion of precursor species on the surface of the topmost deposited layer. Similar to the diffusion length on the substrate, the migration length of adatoms on the ML λM is proportional to the product of the square roots of the diffusion coefficient D and mean mobile lifetime τ of adatoms. (18,25)
The contribution of diffusion and migration to the evolution of the WS2 morphology is illustrated in Figure 2. After 15 min of nucleation at 700 °C (sample S_N700), the shape of WS2 domains appears to be similar to a three-point star, as seen in Figure 2a. This curved triangular morphology was reported to be the result of a huge difference in the growth rate between W- and S-zigzag terminations under the S-rich condition. (31,32) Nevertheless, a more straightforward geometric approach to understand this shape evolution is illustrated in Figure 2b. Because growth is determined by the limited diffusion of adatoms from the surrounding free sapphire surface and assuming insignificant diffusion along nuclei edges, the corners of triangular domains will bind more adatoms and stretch out. Therefore, the initially tiny triangles will gradually turn into small three-point stars.

Figure 2

Figure 2. (a) Morphology of WS2 after 15 min nucleation at 700 °C (sample S_N700). (b) Schematic of the WS2 domain deviation from the triangular shape. (c) Morphology of WS2 after 128 min lateral growth (sample S_R128). (d) Schematic of WS2 domain shape recovery from three-point stars to triangles. (e) Schematic explaining parasitic/premature BL nucleation. λM stands for the migration length of adatoms.

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However, as growth proceeds toward ML coalescence, the triangular shape is recovered. From the thermodynamics perspective, it can be explained by the fact that the curved concave edges are composed of kinks and are less stable than the straight S-zigzag edges, which results in the transformation from three-point stars to equilateral triangles during growth. (33) In this work, from the perspective of adatom migration, the authors additionally highlight the increasing contribution of adatoms from the surface of the ML domain itself to explain the recovery of the triangular shape. These adatoms can migrate to the nearest edge, overcome the Ehrlich–Schwoebel (E–S) barrier, (34,35) and attach at the concave frontiers of ML domains. As seen in Figure 2c, near the coalescence of MLs in sample S_R128, only equilateral triangles can be recognized.
In addition to the shape evolution of the WS2 domains, adatom migration can also determine parasitic/premature BL nucleation on the ML. As the total ML coverage increases by laterally growing ML domains, an increasing fraction of adatoms will arrive and migrate on the top of the ML. When a certain threshold value related to the ML domain size L exceeds the migration length λM, adatoms will be confined to the ML domains, forming BL clusters (Figure 2e). Once the critical nucleus size is reached, premature BL growth will be triggered. In the typical triangular geometry, this threshold value is equal to the distance from the centroid to the edge of equilateral triangles (i.e., 36L). Due to the existence of the E–S barrier at step edges of ML domains, adatom residence time (or effective adatom density) on ML domains will be even longer (or larger). Furthermore, BL nucleation can be seeded by intrinsic defects on the exposed surface of the ML (24) (which is a dangling-bond-free 2D surface). As a result, the effective migration length of adatoms λM,eff observed in experiments becomes much shorter. The formation of BL nuclei is a competing parasitic process, which can be attributed to insufficient λM (in relation to the ML domain size), the E–S barrier, and intrinsic defects within ML domains.
In order to close an ML while suppressing BL nucleation, the migration of adatoms needs to be enhanced. An intuitive approach is to increase the substrate temperature to improve the mobility of adatoms and to reduce the impact of the E–S barrier. However, this approach has practical limits because it increases the probability of gas-phase pre-reactions, (20) parasitic carbon co-deposition, (13,16) and adatom desorption. (24)
In this work, we propose to gradually ramp down the WCO flux with growing ML domain sizes, which reduces the partial pressure of W species in the gas phase as well as the arrival rate of W adatoms on the surface. This increases the critical nucleus size, thereby hindering the formation of stable/super-critical clusters. Additionally, it extends the adatom migration length, hence promoting lateral growth and coalescence of the ML. The mechanism of ramp-down is similar to the principle of vertical Ostwald ripening used by Seol et al. (20) (interrupted precursor injection).
In addition to migration enhancement via increasing temperature and ramping down the WCO supply, another method to prevent BL nucleation is to increase the initial nucleation density so that the required migration length just before coalescence is reduced. Of course, this comes at the cost of an increased density of grain boundaries. The influence of nucleation density on the quality of the final WS2 ML is discussed in the coming section. As stated above, these considerations apply to the growth of WSe2 as well.

Role of WS2 Nucleation Density in ML Coalescence

Controlling the nucleation density can be easily realized by varying the nucleation temperature. (11) WS2 nuclei after 15 min at 750 and 700 °C are shown in Figure 3a,b, respectively. When the temperature is reduced from 750 to 700 °C, the nucleation density increases from ∼140 to ∼210 nuclei/μm2. It can be estimated that the average distance between nuclei grown at 750 °C in Figure 3a is ∼85 nm, while this value drops to ∼70 nm at 700 °C in Figure 3b. The typical size of triangles at 750 °C is ∼60 nm and at 700 °C is ∼ 50 nm. Although these differences are comparably small, the resulting BL coverage after ML coalescence is clearly different for the two nucleation temperatures. Comparing Figure 3c with Figure 3d, we note that BL coverage decreases from ∼26% for growth after 750 °C nucleation (sample S_R160.HT) to ∼15% for growth after 700 °C nucleation (sample S_R144). It should be noted that the growth time of S_R160.HT needed to be extended to 160 min (Table 1) to reach coalescence. Hence, increasing the initial nucleation density can assist the synthesis of the WS2 ML with minimal BL formation.

Figure 3

Figure 3. (a,b) SEM images of WS2 after 15 min nucleation at 750 °C (S_N750) and 700 °C (S_N700). (c,d) SEM images of samples S_R160.HT and S_R144, where the ML is coalesced and BL nuclei are visible. Blue, green, and red arrows point at sapphire, ML, and BL regions, respectively. (e) Comparison of RT PL intensities of samples S_R160.HT and S_R144. (f) Deconvolution of PL spectra in different excitonic peaks. XD stands for bound excitons; XT stands for trions; and A stands for A excitons.

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However, a high initial nucleation density is usually seen critically because it leads to a large density of grain boundaries in the coalesced film. To study their impact, room-temperature (RT) PL spectra were measured on the center of samples S_R160.HT and S_R144 and are shown in Figure 3e. Lower PL intensity is recorded from the WS2 ML with higher BL coverage (S_R160.HT), which is explained by the transition from the direct band gap to indirect band gap. (36,37) On the other hand, the peak position and the full width at half maximum (FWHM) of A-exciton emissions can be used as indicators for the crystal quality of WS2. (38) Interestingly, the deconvoluted A-exciton peaks of both samples are found to be almost identical. The FWHM of S_R160.HT (∼71 meV) is even slightly larger than that of S_R144 (∼66 meV). This observation suggests that the BL nuclei and their edge states might serve as defects in excitonic processes.
To sum up, no significant degradation of PL properties could be found as a result of an increased nucleation/grain boundary density, at least when it is accompanied by a reduction of BL coverage. Recently, some reports claimed that homogeneous and even unidirectional nucleation domains can be achieved by properly treating the sapphire substrates. (19,39) In the future, similar treatments can also be applied to this migration-enhanced MOCVD process in order to further optimize the crystal quality and properties of the WS2 ML. In the following sections, all WS2 MLs are grown using the nucleation process shown for sample S_N700.

Impact of Ramping Down the WCO Flux on WS2 Growth

To study the impact of WCO ramp-down (100% → 25%) on the lateral growth of the WS2 ML, the total amount of the WCO precursor injected was kept constant for samples S_R144 and S_C090. According to Figure 1 and Table 1, the total molar amount of WCO used for sample S_R144 can be calculated as 15 min × 28 nmol/min + 144 min × (28 + 7)/2 nmol/min = 2940 nmol. The same amount was used for sample S_C090: 15 min × 28 nmol/min + 90 min × 28 nmol/min = 2940 nmol.
During deposition, the impact of WCO ramp-down can be clearly recognized via the in situ reflectance transients in Figure 4a, in which the slope of reflectance at 405 nm gradually decreases. After linear fitting (the slope between 15 and 27 min is not considered because it is directly related to the temperature ramp-up), the slope for S_C090 between 27 and 105 min is about 8.6‰ per min, while that for S_R144 between 27 and 51 min is ∼6.4‰ per min and drops to ∼1.5‰ per min between 135 and 159 min (end of deposition). Based on our previous work, (26) there is an approximately linear dependence between the transient reflectance slope and the deposition rate during the MOCVD of the TMDC ML. By analogy, the in situ data comparison here indicates a decelerated growth for sample S_R144 near ML coalescence, which is explained by the WCO ramp-down.

Figure 4

Figure 4. (a) In situ reflectance transients of WS2 samples S_R144 and S_C090. The temperature ramp-up (between 15 and 27 min, marked by the red box) increases the slope. (Unit of slope: ‰ per min.) (b,c) SEM images of samples S_R144 and S_C090, respectively. (d) Raman spectra of both samples. (e) Variation of ML and BL coverages according to the type and duration of processes. The red and green arrows are indicators for increasing tendency of coverages.

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The morphologies of samples S_R144 and S_C090 are presented in Figure 4b,c, respectively. After segmenting the SEM images taken from multiple positions on the 2 in. wafer, the average ML and BL coverages can be determined to be 99.0% ML and 14.5% BL for S_R144, and 96.9% ML and 11.5% BL for S_C090, respectively. Both the ML coverage and the BL coverage of WS2 after the ramp-down process (S_R144) are slightly (∼2%) higher than those on sample S_C090. This implies that given a certain molar amount of WCO, the ramp-down process features a higher incorporation efficiency of W, which could be the result of a higher mean S/W ratio (from constant DTBS injection) stabilizing the WS2 growth front during ramp-down.
Concerning BL formation, quantitative analyses of BL nuclei are carried out and listed in Table 3. Two facts are noticed from the comparison: (i) the BL nucleation density on sample S_R144 is slightly lower than that on sample S_C090 and (ii) the number of small clusters (with an area no larger than 100 nm2, probably formed during later stages of growth) on sample S_R144 is only about half of that on S_C090. Hence, it can be concluded that BL nucleation in the ramp-down process is suppressed in comparison with that in the constant process.
Table 3. Morphological Comparison of WS2 Samples S_R144 and S_C090
ProcessML coverage [%]BL coverage [%]BL nucleation density [/μm2]Counts of BL nuclei ≤ 100 nm2BL/ML ratio (coverage)I2LA/IA1g ratio (Raman)
S_R14499.0 ± 0.214.5 ± 0.7111 ± 531 in Figure 4b0.154.0 ± 0.4
S_C09096.9 ± 0.511.5 ± 0.4125 ± 763 in Figure 4c0.124.5 ± 0.2
Raman spectra in Figure 4d are used to identify the structural properties. The dominant ML nature of both samples can be confirmed by the high intensity ratio between deconvoluted 2LA(M) and A1g(Γ) modes. (40) Comparing both spectra, although the positions of individual Raman peaks barely differ, the intensity ratio I2LA/IA1g of S_R144 (∼4.0) is found to be lower than that of S_C090 (∼4.5). Details are listed in Table 3. The decrease of the intensity ratio I2LA/IA1g is often linked to a transition from the ML to BL (or multilayer). (22,40) The lower intensity ratio of the ramp-down sample here could therefore be conclusively related to its higher BL to ML coverage ratio. A similar relationship can be observed when comparing the intensity ratio I2LA/IE2g1 with the BL/ML coverage ratio. A summary about the intensity ratios of all WS2 ML samples can be found in the Supporting Information (Figure S2), which further confirms their dependence on the BL/ML ratio. On the other hand, it is important to notice that this intensity ratio is highly sensitive to the small variation of BL/ML coverage.
To emphasize the impact of WCO ramp-down on WS2 growth, the evolution of ML and BL coverages from different processes described in Table 1 is depicted in Figure 4e. It is clearly visible that in the final stage of coalescence (from 90 to 100%), the lateral growth of the ML slows down. In constant processes (samples S_C090 and S_C100), with the ML coverage increasing from 96.9 to 99.3% (ΔML = 2.4%), the BL coverage increases from 11.5 to 19.9% (ΔBL = 8.4%). Such an “obstacle” is less evident in ramp-down processes (samples S_R128 and S_R144): when the ML coverage grows from 94.3 to 99.0% (ΔML = 4.7%), the BL coverage only increases from 9.5 to 14.5% (ΔBL = 5.0%). This can be regarded as additional evidence for the improved lateral growth of the ML via ramping down WCO.
To study the impact of ramp-down on the optical properties of fully coalesced WS2 MLs, PL spectra of samples S_C100 and S_R144 are compared and correlated with the sample morphology. As seen in Figure 5a, the morphology of S_R144 is quite similar to that of S_C100, except for a 5.4% smaller BL coverage for S_R144. This offset of BL coverage leads to a distinct difference in the PL intensities in Figure 5b. Although PL emission of WS2 is expected to strongly drop from the ML to BL due to the direct-to-indirect band gap transition, a higher PL intensity as well as a slightly narrower FWHM of the deconvoluted A-exciton peak are found for S_C100 despite its higher BL coverage. Since the two processes were based on the same nucleation procedure, the influence of the density of grain boundaries can be excluded. The inferior PL properties of S_R144 are speculated to be related to defects within WS2 MLs themselves. The most common type of defect is sulfur vacancies, which can lead to the quenching of PL in WS2. (41−43) Other studies also point out that carbon co-deposition can reduce PL efficiency of TMDCs due to a charge transfer between TMDCs and graphitic carbon. (13,16) Furthermore, an impact of oxidation due to exposure to the atmosphere cannot be excluded.

Figure 5

Figure 5. (a) SEM image of WS2 with an almost identical ML coverage and an approx. 5% difference in BL coverage (top: S_C100 and bottom: S_R144). (b) PL comparison of two MLs, where the red-colored region stands for the assembly of PL spectra measured from different positions on sample S_R144. Green stands for those from sample S_C100. (c) XPS spectra of C 1s, S 2p, and W 4f core levels. Both axes are normalized to the same scale.

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In order to figure out the exact reason for the low PL intensity of S_R144, the XPS spectra of C 1s, S 2p and W 4f core levels for samples S_C100 and S_R144 have been analyzed (Figure 5c). The peak positions of the W 4f doublet (33.2 and 35.4 eV) and S 2p doublet (162.8 and 164.0 eV) are in good agreement with reference values (16,20,41) as well as with XPS data of an exfoliated WS2 single-crystal flake. The additional minor doublet at 36.6 and 38.8 eV can be ascribed to the oxidized W species. (16,43) The S/W compositional ratio of WS2 grown from the two processes is calculated based on area fractions and relative sensitivity factors. After normalization with respect to the WS2 single-crystal flake, the stoichiometry of WS2 grown from both the constant (S_C100) and ramp-down (S_R144) processes is approximately WS2.0 (error margin approx. relative 5%). This implies that the stoichiometry of both MOCVD samples matches with that of the single crystal, which proves a high quality. On the other hand, based on the comparison of C 1s core levels, a stronger trace of carbon can be found in sample S_R144 (the atomic concentration is ∼14%, while that of S_C100 is ∼ 10%). This can be correlated to the higher total amount of DTBS resulting from the longer growth time and the larger S/W precursor ratio. Therefore, the reduction of PL intensity is likely caused by a higher degree of carbon incorporation within the ML. One possible approach to improvement may be to simultaneously ramp down the fluxes of both WCO and DTBS so that the potential source of carbon contamination can be minimized.

Estimate of Migration Length for WS2 Growth

Based on the nanoscale morphology of WS2 thin films, the migration length of W adatoms can be roughly estimated by various methods. The first is by identifying circular areas without the BL nucleus in the contour of merged ML domains. The largest radius of such areas is defined as the nominal migration length of adatoms. Larger coherent ML areas cannot be found on the sample because adatoms would have accumulated in these oversized ML regions, forming small BL clusters. Such an estimate is carried out on the samples with relatively few BL nuclei (S_R128 and S_C090) as the adsorption of adatoms to the edges of existing BL domains could shorten the mean travel distance of adatoms and underrate the extracted value of the migration length. As shown in Figure 6a, the nominal migration length λM measured in a ramp-down process (∼100 nm) is generally larger than that in a constant process (∼75 nm).

Figure 6

Figure 6. (a) Morphology of WS2 samples S_R128 (left) and S_C090 (right), on which an estimate of nominal migration length is extracted by red inscribed circles inside the ML contour. The corresponding BL nucleation density and effective migration length after calculations are noted on the top right of SEM images. (b) Illustration for calculating Lav from the density of BL nuclei, where the ML region is colored in light green and BL nuclei is colored in dark green.

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As a second approach, the effective migration length can be approximated as the half of the average distance between BL nuclei Lav, which can be derived from the density of BL nuclei N via λM,eff=12×Lav=12×1/N. A similar method has been reported to calculate the average separation of WSe2 clusters on sapphire during ripening. (18) However, this estimate is based on the essential assumption that the size of individual clusters is negligible. In our case, we simply accept the same assumption because the number of small circular clusters is still dominant in comparison with the number of large domains. The results indicate that the effective migration length λM,eff in a ramp-down process (51 ± 2 nm) is also larger than that in a constant process (44 ± 1 nm).
Due to the above simplifications, the absolute values of the migration lengths reported here need to be treated with care. For instance, grain boundaries (or other morphological defects) in MLs could serve as favorable nucleation sites for BL nuclei, which leads to a significant underestimation of the migration length. Nevertheless, the results imply that the migration of adatoms on the top of the ML is rather restricted, with λM,eff in the range of only tens of nanometers up to 100 nm. Besides, by comparing the estimated migration lengths of different processes, it is confirmed that the WCO ramp-down can enlarge the migration length of W adatoms.

Migration-Enhanced MOCVD of WSe2 MLs

The above growth strategy described for WS2 can be directly applied to synthesize WSe2 MLs on 2 in. sapphire. The development of WSe2 processes also starts with two distinct nucleation tests. As shown in Figure 7a, the nucleation density of sample Se_N28 is only ∼20 nuclei/μm2. Based on the same reasoning regarding the role of nucleation density in ML coalescence for WS2, the nucleation density of WSe2 is increased up to ∼50 nuclei/μm2 (in order to fully close the ML) by increasing the WCO flux as shown for sample Se_N56 (in Figure 7b). This can be attributed to the smaller critical nucleus size (under higher partial pressure of W adatoms in the gas phase in the case of Se_N56) and the resulting higher nucleation rate.

Figure 7

Figure 7. (a,b) SEM image of WSe2 nucleation in samples Se_N28 and Se_N56, respectively. (c) Schematic of temperature and WCO flux variations during three MOCVD processes. The resulting morphologies of samples Se_R45, Se_R60, and Se_R75 are shown in figures (d–f). Blue, green, and red arrows in SEM images point at sapphire, ML, and BL regions, respectively. Trilayers are clearly visible in (f) marked by the red arrows.

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Additional attention needs to be paid to the difference in the nucleation behavior between WS2 and WSe2, which is clearly discernible when comparing Figures 3a and 7a. The nucleation density of WSe2 in sample Se_N28 is much lower than that of WS2 in S_N750, even though both samples are grown under identical conditions (i.e., temperature, precursor fluxes, nucleation time, and so on). The average size of individual nuclei of WSe2 is also drastically larger than that of WS2, while the coverage of Se_N28 is lower than that of S_N750. The possible causes include the different critical nucleus sizes (WSe2 > WS2) and the stabilizing effect of different chalcogen precursors (DiPSe < DTBS). Thinking from the atomic perspective, it is indicated that S and Se adatoms interact with W adatoms in different kinetics. Further experiments need to be carried out to unveil the nature of this difference between S and Se adatoms and to discuss the detailed influence of chalcogen species in MOCVD processes.
Based on the nucleation, as shown for sample Se_N56, WCO ramp-down (from 56 to 14 nmol/min, −75%) is performed over different periods of lateral growth time, as seen in Figure 7c. From the morphology evolution from Figure 7d–f, the WSe2 ML is found to be coalesced after 75 min of lateral growth (sample Se_R75). Evaluating the variation of ML and BL coverages, similar to the previous analysis of the WS2 samples, we conclude that near ML coalescence, the growth of the BL is dominating (e.g., from Se_R60 to Se_R75: ΔBL = 8.4% while ΔML = 3.1%). With the help of WCO ramp-down, the BL coverage on the coalesced ML can be controlled to be below 20%. Furthermore, it is estimated that when the ML coverage of WSe2 just hits 99.0%, its BL coverage is around 15%, which coincides with the morphology of S_R144. This indicates that the W species is the decisive element for BL coverage in the MOCVD of both WS2 and WSe2, and the WCO ramp-down has similar impacts in both cases.
Nevertheless, the BL nucleation behavior in WSe2 samples is quite different from that in the WS2 one: BL domains of WSe2 as large as ∼500 nm (Figure 7f) are typically bigger than WS2 BL domains, which do not exceed 100 nm (Figure 4b). Some trilayer and even multilayer growth features can be recognized (marked by the red arrows in Figure 7f), which can also be explained by the strong confinement of adatoms on the top of large BL domains. Similar to the above comparison of ML nucleation behavior on a sapphire substrate, the difference in BL nucleation can also be attributed to the different reactivity of chalcogen species. Additionally, since the ML nucleation density in the case of WSe2 is much lower than that of WS2, it will lead to a lower density of grain boundaries throughout the WSe2 ML. Such grain boundaries can be seen as defects that favor the nucleation on top of them. Therefore, the BL nucleation in the case of WSe2 is less favored due to the suppression of defect-related nucleation sites on the surface of the WSe2 ML.

Conclusions

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In this work, a novel two-stage migration-enhanced MOCVD process is designed to prepare wafer-scale fully coalesced WS2 and WSe2 MLs with a BL coverage of <20%. Continuously ramping down the W precursor in the lateral growth stage is a commensurate and suitable strategy to enhance migration while the coverage of ML approaches 100%. The lateral growth of the ML is promoted and the formation of BL nuclei is suppressed. The estimated migration length of adatoms is also found to be increased by the WCO ramp-down. Furthermore, the influence of additional BL formation on the properties of the WS2 MLs is discussed. The PL emission is affected by not only the BL coverage but also by carbon co-deposition during synthesis. Although some growth parameters need further optimization, a generally high quality of the WS2 ML produced by ramp-down processes can be concluded as shown by Raman spectroscopy, PL spectroscopy, and XPS analyses. The extensive discussion about WSe2 allows us to extract information on the impact of the chalcogen species on nucleation and growth. This demonstrates that our model of TMDC ML formation in MOCVD should be expanded with the impact of the group VI species in future studies.
The migration-enhanced MOCVD process itself can be seen as a practical and useful technique supporting the large-scale fabrication of 2D TMDCs. This work can also be treated as a pioneer path to the direct growth of vertical 2D–2D all-planar heterostructures. The discussion about migration of adatoms sheds lights on the whole picture of the growth mechanism of 2D TMDCs.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.cgd.2c01134.

  • Raman spectra of WS2 and WSe2 MLs, XPS spectra of MOCVD-grown and exfoliated WS2, reproducibility of WS2 ML growth, and a summary about the Raman intensity ratios and BL/ML coverage ratio (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Annika Grundmann - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany
    • Hleb Fiadziushkin - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany;  Orcidhttps://orcid.org/0000-0002-0456-7480
    • Zhaodong Wang - PGI-7&10 and JARA-FIT, Forschungszentrum Jülich GmbH, Wilhelm Johnen Straße, 52428Jülich, Germany
    • Susanne Hoffmann-Eifert - PGI-7&10 and JARA-FIT, Forschungszentrum Jülich GmbH, Wilhelm Johnen Straße, 52428Jülich, Germany;  Orcidhttps://orcid.org/0000-0003-1682-826X
    • Amir Ghiami - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany;  Orcidhttps://orcid.org/0000-0002-2451-1388
    • Arne Debald - AIXTRON SE, Dornkaulstr. 2, 52134Herzogenrath, Germany
    • Michael Heuken - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany;  AIXTRON SE, Dornkaulstr. 2, 52134Herzogenrath, Germany
    • Andrei Vescan - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany;  Orcidhttps://orcid.org/0000-0001-9465-2621
    • Holger Kalisch - Compound Semiconductor Technology, RWTH Aachen University, Sommerfeldstr. 18, 52074Aachen, Germany
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was in part funded by the Federal Ministry of Education and Research (BMBF, Germany) in the projects NEUROTEC (16ME0399, 16ME0398K, and 16ME0403) and ForLab (16ES0940).

References

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This article references 43 other publications.

  1. 1
    Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147– 150,  DOI: 10.1038/nnano.2010.279
    Google Scholar
    1
    Single-layer MoS2 transistors
    Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.
    Nature Nanotechnology (2011), 6 (3), 147-150CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to 1D materials, it is relatively easy to fabricate complex structures from them. The most widely studied 2D material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained Si films or requires high voltages. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV, previously reported mobilities in the 0.5-3 cm2 V-1 s-1 range are too low for practical devices. Here, we use a HfO2 gate dielec. to demonstrate a room-temp. single-layer MoS2 mobility of at least 200 cm2 V-1 s-1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temp. current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
  2. 2
    Akama, T.; Okita, W.; Nagai, R.; Li, C.; Kaneko, T.; Kato, T. Schottky Solar Cell Using Few-Layered Transition Metal Dichalcogenides toward Large-Scale Fabrication of Semitransparent and Flexible Power Generator. Sci. Rep. 2017, 7, 11967,  DOI: 10.1038/s41598-017-12287-6
    Google Scholar
    2
    Schottky solar cell using few-layered transition metal dichalcogenides toward large-scale fabrication of semitransparent and flexible power generator
    Akama Toshiki; Okita Wakana; Nagai Reito; Li Chao; Kaneko Toshiro; Kato Toshiaki
    Scientific reports (2017), 7 (1), 11967 ISSN:.
    Few-layered transition metal dichalcogenides (TMDs) are known as true two-dimensional materials, with excellent semiconducting properties and strong light-matter interaction. Thus, TMDs are attractive materials for semitransparent and flexible solar cells for use in various applications. Hoewver, despite the recent progress, the development of a scalable method to fabricate semitransparent and flexible solar cells with mono- or few-layered TMDs remains a crucial challenge. Here, we show easy and scalable fabrication of a few-layered TMD solar cell using a Schottky-type configuration to obtain a power conversion efficiency (PCE) of approximately 0.7%, which is the highest value reported with few-layered TMDs. Clear power generation was also observed for a device fabricated on a large SiO2 and flexible substrate, demonstrating that our method has high potential for scalable production. In addition, systematic investigation revealed that the PCE and external quantum efficiency (EQE) strongly depended on the type of photogenerated excitons (A, B, and C) because of different carrier dynamics. Because high solar cell performance along with excellent scalability can be achieved through the proposed process, our fabrication method will contribute to accelerating the industrial use of TMDs as semitransparent and flexible solar cells.
  3. 3
    Andrzejewski, D.; Myja, H.; Heuken, M.; Grundmann, A.; Kalisch, H.; Vescan, A.; Kümmell, T.; Bacher, G. Scalable Large-Area p–i–n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD. ACS Photonics 2019, 6, 1832– 1839,  DOI: 10.1021/acsphotonics.9b00311
    Google Scholar
    3
    Scalable Large-Area p-i-n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD
    Andrzejewski, Dominik; Myja, Henrik; Heuken, Michael; Grundmann, Annika; Kalisch, Holger; Vescan, Andrei; Kuemmell, Tilmar; Bacher, Gerd
    ACS Photonics (2019), 6 (8), 1832-1839CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chem. vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal-org. (MO-)CVD WS2 monolayers into a vertical p-i-n device architecture using org. and inorg. injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V.
  4. 4
    Late, D. J.; Doneux, T.; Bougouma, M. Single-Layer MoSe2 Based NH3 Gas Sensor. Appl. Phys. Lett. 2014, 105, 233103,  DOI: 10.1063/1.4903358
    Google Scholar
    4
    Single-layer MoSe2 based NH3 gas sensor
    Late, Dattatray J.; Doneux, Thomas; Bougouma, Moussa
    Applied Physics Letters (2014), 105 (23), 233103/1-233103/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Describe is the utilization of single-layer MoSe2 as high-performance room temp. NH3 gas sensors. The single-layer MoSe2-based gas sensor device shows comprehensible detection of NH3 gas down to 50 ppm. Also confirmed was a gas sensing measurement by recording the Raman spectra before and after exposing the device to NH3 gas, which subsequently shows the shift due to charger transfer and analyte gas mol. adsorption on surface of single-layer MoSe2 nanosheet. The investigations show the potential use of single-layer and few layer thick MoSe2 and other TMDC as high-performance gas sensors. (c) 2014 American Institute of Physics.
  5. 5
    Fathi-Hafshejani, P.; Azam, N.; Wang, L.; Kuroda, M. A.; Hamilton, M. C.; Hasim, S.; Mahjouri-Samani, M. Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2). ACS Nano 2021, 15, 11461– 11469,  DOI: 10.1021/acsnano.1c01188
    Google Scholar
    5
    Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2)
    Fathi-Hafshejani, Parvin; Azam, Nurul; Wang, Lu; Kuroda, Marcelo A.; Hamilton, Michael C.; Hasim, Sahar; Mahjouri-Samani, Masoud
    ACS Nano (2021), 15 (7), 11461-11469CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The emergence of rapidly expanding infectious diseases such as coronavirus (COVID-19) demands effective biosensors that can promptly detect and recognize the pathogens. Field-effect transistors based on semiconducting two-dimensional (2D) materials (2D-FETs) have been identified as potential candidates for rapid and label-free sensing applications. This is because any perturbation of such atomically thin 2D channels can significantly impact their electronic transport properties. Here, we report the use of FET based on semiconducting transition metal dichalcogenide (TMDC) WSe2 as a promising biosensor for the rapid and sensitive detection of SARS-CoV-2 in vitro. The sensor is created by functionalizing the WSe2 monolayers with a monoclonal antibody against the SARS-CoV-2 spike protein and exhibits a detection limit of down to 25 fg/μL in 0.01X phosphate-buffered saline (PBS). Comprehensive theor. and exptl. studies, including d. functional theory, at. force microscopy, Raman and photoluminescence spectroscopies, and electronic transport properties, were performed to characterize and explain the device performance. The results demonstrate that TMDC-based 2D-FETs can potentially serve as sensitive and selective biosensors for the rapid detection of infectious diseases.
  6. 6
    Kang, S.; Koo, J.-J.; Seo, H.; Truong, Q. T.; Park, J. B.; Park, S. C.; Jung, Y.; Cho, S.-P.; Nam, K. T.; Kim, Z. H.; Hong, B. H. Defect-Engineered MoS2 with Extended Photoluminescence Lifetime for High-Performance Hydrogen Evolution. J. Mater. Chem. C 2019, 7, 10173– 10178,  DOI: 10.1039/c9tc02256b
    Google Scholar
    6
    Defect-engineered MoS2 with extended photoluminescence lifetime for high-performance hydrogen evolution
    Kang, Sangmin; Koo, Ja-Jung; Seo, Hongmin; Truong, Quang Trung; Park, Jong Bo; Park, Seong Chae; Jung, Youngjin; Cho, Sung-Pyo; Nam, Ki Tae; Kim, Zee Hwan; Hong, Byung Hee
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (33), 10173-10178CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    It has been reported that defects in molybdenum disulfide (MoS2) enable the hydrogen evolution reaction (HER). The most widely employed method of argon-plasma treatment for defect generation suffers from poor material stability and loss of cond. Here, we report a new method to synthesize highly polycryst. molybdenum disulfide MoS2 bilayers with enhanced HER performance and material stability. This new method is based on metal org. chem. vapor deposition (MOCVD) followed by UV/ozone treatment to generate defects. The defect densities on MoS2 were identified by the increase in lifetime (∼76%) and intensity (∼15%) in photoluminescence (PL) as compared to those of pristine MoS2. Our fabrication and characterization methods can be further applied to optimize defect densities for catalytic effects in various transition metal dichalcogenide (TMDC) materials.
  7. 7
    Ju, L.; Bie, M.; Shang, J.; Tang, X.; Kou, L. Janus Transition Metal Dichalcogenides: A Superior Platform for Photocatalytic Water Splitting. JPhys Mater. 2020, 3, 022004,  DOI: 10.1088/2515-7639/ab7c57
    Google Scholar
    7
    Janus transition metal dichalcogenides: a superior platform for photocatalytic water splitting
    Ju, Lin; Bie, Mei; Shang, Jing; Tang, Xiao; Kou, Liangzhi
    JPhys Materials (2020), 3 (2), 022004CODEN: JPMOC4; ISSN:2515-7639. (IOP Publishing Ltd.)
    Janus two-dimensional (2D) materials, referring to the layers with different surfaces, have attracted intensive research interest due to the unique properties induced by symmetry breaking, and promising applications in energy conversion. Based on the successful exptl. synthesis of Janus transition metal dichalcogenides (TMDC), here we present a review on their potential application in photocatalytic overall water splitting, from the perspectives of the latest theor. and exptl. progress. Four aspects which are related to photocatalytic reaction, including the adsorption of water mols., utilization of sunlight, charge sepn. and transport, and surface chem. reactions have been discussed, and it is concluded that the Janus structures have better performances than sym. TMDCs. At the end of this review, we raise further challenges and possible future research directions for Janus 2D materials as water-splitting photocatalysts.
  8. 8
    Liu, Y.; Gu, F. A Wafer-Scale Synthesis of Monolayer MoS2 and Their Field-Effect Transistors toward Practical Applications. Nanoscale Adv. 2021, 3, 2117– 2138,  DOI: 10.1039/d0na01043j
    Google Scholar
    8
    A wafer-scale synthesis of monolayer MoS2 and their field-effect transistors toward practical applications
    Liu, Yuchun; Gu, Fuxing
    Nanoscale Advances (2021), 3 (8), 2117-2138CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
    A review. Molybdenum disulfide (MoS2) has attracted considerable research interest as a promising candidate for downscaling integrated electronics due to the special two-dimensional structure and unique physicochem. properties. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and elec. properties to fulfill the requirement for practical applications. Recently, a variety of investigations have focused on wafer-scale monolayer MoS2 synthesis with high-quality. The 2D MoS2 field-effect transistor (MoS2-FET) array with different configurations utilizes the high-quality MoS2 film as channels and exhibits favorable performance. In this review, we illustrated the latest research advances in wafer-scale monolayer MoS2 synthesis by different methods, including Au-assisted exfoliation, CVD, thin film sulfurization, MOCVD, ALD, VLS method, and the thermolysis of thiosalts. Then, an overview of MoS2-FET developments was provided based on large-area MoS2 film with different device configurations and performances. The different applications of MoS2-FET in logic circuits, basic memory devices, and integrated photodetectors were also summarized. Lastly, we considered the perspective and challenges based on wafer-scale monolayer MoS2 synthesis and MoS2-FET for developing practical applications in next-generation integrated electronics and flexible optoelectronics.
  9. 9
    Lee, D. H.; Sim, Y.; Wang, J.; Kwon, S.-Y. Metal–Organic Chemical Vapor Deposition of 2D van Der Waals Materials─The Challenges and the Extensive Future Opportunities. APL Mater. 2020, 8, 030901,  DOI: 10.1063/1.5142601
    Google Scholar
    9
    Metal-organic chemical vapor deposition of 2D van der Waals materials-The challenges and the extensive future opportunities
    Lee, Do Hee; Sim, Yeoseon; Wang, Jaewon; Kwon, Soon-Yong
    APL Materials (2020), 8 (3), 030901CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    A review. The last decade has witnessed significant progress in two-dimensional van der Waals (2D vdW) materials research; however, a no. of challenges remain for their practical applications. The most significant challenge for 2D vdW materials is the control of the early stages of nucleation and growth of the material on preferred surfaces to eventually create large grains with digital thickness controllability, which will enable their incorporation into high-performance electronic and optoelectronic devices. This Perspective discusses the tech. challenges to be overcome in the metal-org. chem. vapor deposition (MOCVD) growth of 2D group 6 transition metal dichalcogenide (TMD) at. crystals and their heterostructures, as well as future research aspects in vdW epitaxy for 2D TMDs via MOCVD. In addn., we encourage the traditional MOCVD community to apply their expertise in the field of "2D vdW materials," which will continue to grow at an exponential rate. (c) 2020 American Institute of Physics.
  10. 10
    Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656– 660,  DOI: 10.1038/nature14417
    Google Scholar
    10
    High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity
    Kang, Kibum; Xie, Saien; Huang, Lujie; Han, Yimo; Huang, Pinshane Y.; Mak, Kin Fai; Kim, Cheol-Joo; Muller, David; Park, Jiwoong
    Nature (London, United Kingdom) (2015), 520 (7549), 656-660CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the at., sub-nanometer length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technol. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high elec. carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technol. relevant scale without film transfer. In addn., their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high elec. performance remains an unsolved challenge. Here we report the prepn. of high-mobility 4-in. wafer-scale films of monolayer molybdenum disulfide (MoS2) and tungsten disulfide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal-org. chem. vapor deposition technique, and show high elec. performance, including an electron mobility of 30 cm2 V-1 s-1 at room temp. and 114 cm2 V-1 s-1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.
  11. 11
    Grundmann, A.; Andrzejewski, D.; Kümmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A. H2S-Free Metal-Organic Vapor Phase Epitaxy of Coalesced 2D WS2 Layers on Sapphire─ERRATUM. MRS Adv. 2019, 4, e1  DOI: 10.1557/adv.2019.229
    Google Scholar
    There is no corresponding record for this reference.
  12. 12
    Li, H.; Wu, H.; Yuan, S.; Qian, H. Synthesis and Characterization of Vertically Standing MoS2 Nanosheets. Sci. Rep. 2016, 6, 21171,  DOI: 10.1038/srep21171
    Google Scholar
    12
    Synthesis and characterization of vertically standing MoS2 nanosheets
    Li, Han; Wu, Huaqiang; Yuan, Shuoguo; Qian, He
    Scientific Reports (2016), 6 (), 21171CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    Molybdenum disulfide (MoS2) has been attracting much attentions due to its excellent elec. and optical properties. We report here the synthesis of large-scale and uniform MoS2 nanosheets with vertically standing morphol. using chem. vapor deposition method. TEM observations clearly reveal the growth mechanism of these vertical structures. It is suggested that the vertical structures are caused by the compression and extrusion between MoS2 islands. More importantly, the vertical morphol. of two dimensional (2D) materials hold many promising potential applications. We demonstrate here the as-synthesized vertically standing MoS2 nanosheets could be used for hydrogen evolution reaction, where the exchange c.d. is about 70 times of bulk MoS2. The field emission performance of vertically standing MoS2 were also improved due to the abundantly exposed edges.
  13. 13
    Schaefer, C. M.; Caicedo Roque, J. M.; Sauthier, G.; Bousquet, J.; Hébert, C.; Sperling, J. R.; Pérez-Tomás, A.; Santiso, J.; del Corro, E.; Garrido, J. A. Carbon Incorporation in MOCVD of MoS2 Thin Films Grown from an Organosulfide Precursor. Chem. Mater. 2021, 33, 4474– 4487,  DOI: 10.1021/acs.chemmater.1c00646
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    Carbon Incorporation in MOCVD of MoS2 Thin Films Grown from an Organosulfide Precursor
    Schaefer, Christian M.; Caicedo Roque, Jose M.; Sauthier, Guillaume; Bousquet, Jessica; Hebert, Clement; Sperling, Justin R.; Perez-Tomas, Amador; Santiso, Jose; del Corro, Elena; Garrido, Jose A.
    Chemistry of Materials (2021), 33 (12), 4474-4487CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in com. (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-org. chem. vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-esp. in instances where org. chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-contg. side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and di-Et sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temp. and Mo(CO)6/DES/H2 gas mixt. ratios on film morphol., chem. compn., and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temps. and high DES/Mo(CO)6 precursor ratios, we obsd. that temps. above DES pyrolysis onset (⪆600°C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-to-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices.
  14. 14
    Zhang, X.; Al Balushi, Z. Y.; Zhang, F.; Choudhury, T. H.; Eichfeld, S. M.; Alem, N.; Jackson, T. N.; Robinson, J. A.; Redwing, J. M. Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films. J. Electron. Mater. 2016, 45, 6273– 6279,  DOI: 10.1007/s11664-016-5033-0
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    14
    Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films
    Zhang, Xiaotian; Al Balushi, Zakaria Y.; Zhang, Fu; Choudhury, Tanushree H.; Eichfeld, Sarah M.; Alem, Nasim; Jackson, Thomas N.; Robinson, Joshua A.; Redwing, Joan M.
    Journal of Electronic Materials (2016), 45 (12), 6273-6279CODEN: JECMA5; ISSN:0361-5235. (Springer)
    Metalorg. chem. vapor deposition (MOCVD) is a promising technique to form large-area, uniform films of monolayer or few-layer transition metal dichalcogenide (TMD) thin films; however, unintentional carbon incorporation is a concern. In this work, we report the presence of a defective graphene layer that forms simultaneously during MOCVD growth of tungsten diselenide (WSe2) on sapphire at high growth temp. and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and di-Me selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. The use of hydrogen selenide (H2Se) instead of DMSe eliminates the defective graphene layer enabling coalesced monolayer and few-layer WSe2 films.
  15. 15
    Marx, M.; Grundmann, A.; Lin, Y.-R.; Andrzejewski, D.; Kümmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A. Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2. J. Electron. Mater. 2018, 47, 910– 916,  DOI: 10.1007/s11664-017-5937-3
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    15
    Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
    Marx, M.; Grundmann, A.; Lin, Y.-R.; Andrzejewski, D.; Kuemmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A.
    Journal of Electronic Materials (2018), 47 (2), 910-916CODEN: JECMA5; ISSN:0361-5235. (Springer)
    The influence of the main growth parameters on the growth mechanism and film formation processes during metalorg. vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-Bu sulfide as metalorg. precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by at. force microscopy, SEM, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation d. of the polycryst. film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Addnl., the influence of growth parameters such as reactor pressure and surface temp. is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temp. above 900°C.
  16. 16
    Choudhury, T. H.; Simchi, H.; Boichot, R.; Chubarov, M.; Mohney, S. E.; Redwing, J. M. Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor Deposition Growth of WS2. Cryst. Growth Des. 2018, 18, 4357– 4364,  DOI: 10.1021/acs.cgd.8b00306
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    16
    Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor Deposition Growth of WS2
    Choudhury, Tanushree H.; Simchi, Hamed; Boichot, Raphael; Chubarov, Mikhail; Mohney, Suzanne E.; Redwing, Joan M.
    Crystal Growth & Design (2018), 18 (8), 4357-4364CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
    Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas-source chem. vapor deposition system to ascertain the effect of the chalcogen precursor on the film growth and properties. Tungsten hexacarbonyl (W(CO)6) was used as the tungsten source, and hydrogen sulfide (H2S) and di-Et sulfide (DES-(C2H5)2S) were the chalcogen sources. The film deposition was studied at different temps. and chalcogen-to-metal ratios to understand the effect of each chalcogen precursor on the film growth rate, thickness, coverage, photoluminescence, and stoichiometry. Larger lateral growth was obsd. in films grown with H2S than DES. The reduced lateral growth with DES can be attributed to carbon contamination, which also quenches the photoluminescence. Thermodn. calcns. agreed well with the exptl. observations, suggesting formation of WS2 with both sulfur precursors and addnl. formation of carbon when deposition is done using DES.
  17. 17
    Eichfeld, S. M.; Hossain, L.; Lin, Y.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kim, M. J.; Mayer, T. S.; Redwing, J. M.; Robinson, J. A. Highly Scalable, Atomically Thin WSe2 Grown via Metal–Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080– 2087,  DOI: 10.1021/nn5073286
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    17
    Highly Scalable, Atomically Thin WSe2 Grown via Metal-Organic Chemical Vapor Deposition
    Eichfeld, Sarah M.; Hossain, Lorraine; Lin, Yu-Chuan; Piasecki, Aleksander F.; Kupp, Benjamin; Birdwell, A. Glen; Burke, Robert A.; Lu, Ning; Peng, Xin; Li, Jie; Azcatl, Angelica; McDonnell, Stephen; Wallace, Robert M.; Kim, Moon J.; Mayer, Theresa S.; Redwing, Joan M.; Robinson, Joshua A.
    ACS Nano (2015), 9 (2), 2080-2087CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2-dimensional material cannot be realized without a scalable synthesis process. Here, the authors demonstrate the 1st scalable synthesis of large-area, mono and few-layer WSe2via metal-org. CVD using tungsten hexacarbonyl (W(CO)6) and dimethylselenium (Me2Se). In addn. to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chem., which is unobtainable using more traditional oxide vaporization routes. Temp., pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing at. layer structure, with optimized conditions yielding >8 μm size domains. Raman spectroscopy, at. force microscopy (AFM), and cross-sectional TEM confirm cryst. monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.
  18. 18
    Zhang, X.; Choudhury, T. H.; Chubarov, M.; Xiang, Y.; Jariwala, B.; Zhang, F.; Alem, N.; Wang, G.-C.; Robinson, J. A.; Redwing, J. M. Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire. Nano Lett. 2018, 18, 1049– 1056,  DOI: 10.1021/acs.nanolett.7b04521
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    18
    Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire
    Zhang, Xiaotian; Choudhury, Tanushree H.; Chubarov, Mikhail; Xiang, Yu; Jariwala, Bhakti; Zhang, Fu; Alem, Nasim; Wang, Gwo-Ching; Robinson, Joshua A.; Redwing, Joan M.
    Nano Letters (2018), 18 (2), 1049-1056CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    A multistep diffusion-mediated process was developed to control the nucleation d., size, and lateral growth rate of WSe2 domains on c-plane Al2O3 for the epitaxial growth of large area monolayer films by gas source CVD. The process consists of an initial nucleation step followed by an annealing period in H2Se to promote surface diffusion of W-contg. species to form oriented WSe2 islands with uniform size and controlled d. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in <1 h. Postgrowth structural characterization demonstrates that the WSe2 monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe2 domains. The process also provides fundamental insights into the 2-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster d. with annealing time follows a 2D ripening process, enabling an est. of the W-species surface diffusivity. The lateral growth rate of domains is relatively independent of substrate temp. at 700-900° suggesting a mass transport limited process, however, the domain shape (triangular vs. truncated triangular) varied with temp. over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward at. level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication.
  19. 19
    Chubarov, M.; Choudhury, T. H.; Hickey, D. R.; Bachu, S.; Zhang, T.; Sebastian, A.; Bansal, A.; Zhu, H.; Trainor, N.; Das, S.; Terrones, M.; Alem, N.; Redwing, J. M. Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire. ACS Nano 2021, 15, 2532– 2541,  DOI: 10.1021/acsnano.0c06750
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    19
    Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire
    Chubarov, Mikhail; Choudhury, Tanushree H.; Hickey, Danielle Reifsnyder; Bachu, Saiphaneendra; Zhang, Tianyi; Sebastian, Amritanand; Bansal, Anushka; Zhu, Haoyue; Trainor, Nicholas; Das, Saptarshi; Terrones, Mauricio; Alem, Nasim; Redwing, Joan M.
    ACS Nano (2021), 15 (2), 2532-2541CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallog. direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diam. c-plane sapphire by metalorg. chem. vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy anal. reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the d. of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ~ 107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.
  20. 20
    Seol, M.; Lee, M.; Kim, H.; Shin, K. W.; Cho, Y.; Jeon, I.; Jeong, M.; Lee, H.; Park, J.; Shin, H. High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening. Adv. Mater. 2020, 32, 2003542,  DOI: 10.1002/adma.202003542
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    High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening
    Seol, Minsu; Lee, Min-Hyun; Kim, Haeryong; Shin, Keun Wook; Cho, Yeonchoo; Jeon, Insu; Jeong, Myoungho; Lee, Hyung-Ik; Park, Jiwoong; Shin, Hyeon-Jin
    Advanced Materials (Weinheim, Germany) (2020), 32 (42), 2003542CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high-throughput, large-scale prodn. of high-quality, spatially uniform materials, and reliable integration into devices. Here, high-throughput growth, completed in 12 min, of 6-in. wafer-scale monolayer MoS2 and WS2 is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal-org. chem. vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concns. The as-grown TMD films show excellent spatial homogeneity and well-stitched grain boundaries, enabling facile transfer to various target substrates without degrdn. Using these films, batch fabrication of high-performance field-effect transistor (FET) arrays in wafer-scale is demonstrated, and the FETs show remarkable uniformity. The high-throughput prodn. and wafer-scale automatable transfer will facilitate the integration of TMDs into Si-complementary metal-oxide-semiconductor platforms.
  21. 21
    Choi, J.; Ha, M.; Park, J. C.; Park, T. J.; Kim, W.; Lee, M.; Ahn, J. A Strategy for Wafer-Scale Crystalline MoS2 Thin Films with Controlled Morphology Using Pulsed Metal–Organic Chemical Vapor Deposition at Low Temperature. Adv. Mater. Interfaces 2022, 9, 2101785,  DOI: 10.1002/admi.202101785
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    Strategy for wafer-scale crystalline molybdenum sulfide thin films with controlled morphology using pulsed metal-organic chemical vapor deposition at low temperature
    Choi, Jeong-Hun; Ha, Min-Ji; Park, Jae Chan; Park, Tae Joo; Kim, Woo-Hee; Lee, Myoung-Jae; Ahn, Ji-Hoon
    Advanced Materials Interfaces (2022), 9 (4), 2101785CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
    2D semiconductor materials with layered crystal structures have attracted great interest as promising candidates for electronic, optoelectronic, and sensor applications due to their unique and superior characteristics. However, a large-area synthesis process for various applications and practical mass prodn. is still lacking. In particular, there is a limitation in that a high process temp. and a very long process time are required to deposit a crystd. 2D material on a large area. Herein, pulsed metal-org. chem. vapor deposition (p-MOCVD) is proposed for the growth of wafer-scale cryst. MoS2 thin films to overcome the existing limitations. In the p-MOCVD process, precursors are repeatedly injected at regular intervals to enhance the migration of precursors on the surface. As a result, cryst. MoS2 is successfully synthesized at the lowest temp. (350°C) reported so far in a very short process time of 550 s. In addn., it is found that the horizontal and vertical growth modes of MoS2 can be effectively controlled by adjusting key process parameters. Finally, various applications are presented by demonstrating the photodetector (detectivity = 18.1 x 106 at light power of 1 mW) and chem. sensor (response = 38% at 100 ppm of NO2 gas) devices.
  22. 22
    Cohen, A.; Patsha, A.; Mohapatra, P. K.; Kazes, M.; Ranganathan, K.; Houben, L.; Oron, D.; Ismach, A. Growth-Etch Metal–Organic Chemical Vapor Deposition Approach of WS2 Atomic Layers. ACS Nano 2021, 15, 526– 538,  DOI: 10.1021/acsnano.0c05394
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    22
    Growth-etch metal-organic chemical vapor deposition approach of WS2 atomic layers
    Cohen, Assael; Patsha, Avinash; Mohapatra, Pranab K.; Kazes, Miri; Ranganathan, Kamalakannan; Houben, Lothar; Oron, Dan; Ismach, Ariel
    ACS Nano (2021), 15 (1), 526-538CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Metal-org. chem. vapor deposition (MOCVD) is one of the main methodologies used for thin-film fabrication in the semiconductor industry today and is considered one of the most promising routes to achieve large-scale and high-quality 2D transition metal dichalcogenides (TMDCs). However, if special measures are not taken, MOCVD suffers from some serious drawbacks, such as small domain size and carbon contamination, resulting in poor optical and crystal quality, which may inhibit its implementation for the large-scale fabrication of at.-thin semiconductors. Here we present a growth-etch MOCVD (GE-MOCVD) methodol., in which a small amt. of water vapor is introduced during the growth, while the precursors are delivered in pulses. The evolution of the growth as a function of the amt. of water vapor, the no. and type of cycles, and the gas compn. is described. We show a significant domain size increase is achieved relative to our conventional process. The improved crystal quality of WS2 (and WSe2) domains is demonstrated by means of Raman spectroscopy, photoluminescence (PL) spectroscopy, and HRTEM studies. Moreover, time-resolved PL studies show very long exciton lifetimes, comparable to those obsd. in mech. exfoliated flakes. Thus, the GE-MOCVD approach presented here may facilitate their integration into a wide range of applications.
  23. 23
    Shang, S.-L.; Lindwall, G.; Wang, Y.; Redwing, J. M.; Anderson, T.; Liu, Z.-K. Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2. Nano Lett. 2016, 16, 5742– 5750,  DOI: 10.1021/acs.nanolett.6b02443
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    23
    Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2
    Shang, Shun-Li; Lindwall, Greta; Wang, Yi; Redwing, Joan M.; Anderson, Tim; Liu, Zi-Kui
    Nano Letters (2016), 16 (9), 5742-5750CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodn. and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated d. functional theory (DFT) and calcn. of phase diagram (CALPHAD) modeling approach is employed to provide thermodn. insight into lateral vs. vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral vs. vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The crit. 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temp.-compn. (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calcd. MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available expts., and the present data provide quant. insight into the controlled growth of 2D layered MoS2.
  24. 24
    Nie, Y.; Liang, C.; Cha, P.-R.; Colombo, L.; Wallace, R. M.; Cho, K. A Kinetic Monte Carlo Simulation Method of van Der Waals Epitaxy for Atomistic Nucleation-Growth Processes of Transition Metal Dichalcogenides. Sci. Rep. 2017, 7, 2977,  DOI: 10.1038/s41598-017-02919-2
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    A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides
    Nie Yifan; Liang Chaoping; Wallace Robert M; Cho Kyeongjae; Cha Pil-Ryung; Colombo Luigi
    Scientific reports (2017), 7 (1), 2977 ISSN:.
    Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.
  25. 25
    Ye, H.; Zhou, J.; Er, D.; Price, C. C.; Yu, Z.; Liu, Y.; Lowengrub, J.; Lou, J.; Liu, Z.; Shenoy, V. B. Toward a Mechanistic Understanding of Vertical Growth of van Der Waals Stacked 2D Materials: A Multiscale Model and Experiments. ACS Nano 2017, 11, 12780– 12788,  DOI: 10.1021/acsnano.7b07604
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    25
    Toward a Mechanistic Understanding of Vertical Growth of van der Waals Stacked 2D Materials: A Multiscale Model and Experiments
    Ye, Han; Zhou, Jiadong; Er, Dequan; Price, Christopher C.; Yu, Zhongyuan; Liu, Yumin; Lowengrub, John; Lou, Jun; Liu, Zheng; Shenoy, Vivek B.
    ACS Nano (2017), 11 (12), 12780-12788CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering phys. properties of two-dimensional (2D) materials and designing atomically thin devices. However, due to the lack of mechanistic understanding, challenges remain in the controlled fabrication of these structures via scalable methods such as chem. vapor deposition (CVD) onto substrates. In this paper, we develop a general multiscale model to describe the size evolution of 2D layers and predict the necessary growth conditions for vertical (initial + subsequent layers) vs. in-plane lateral (monolayer) growth. An analytic thermodn. criterion is established for subsequent layer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. Considering the time-dependent growth process, we find that temp. and adatom flux from vapor are the primary criteria affecting the self-assembled growth. The proposed model clearly demonstrates the distinct roles of thermodn. and kinetic mechanisms governing the final structure. This model agrees with exptl. observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD and provides a predictive framework to guide the fabrication of vertically stacked 2D materials.
  26. 26
    Tang, S.; Grundmann, A.; Fiadziushkin, H.; Ghiami, A.; Heuken, M.; Vescan, A.; Kalisch, H. Detailed Study on MOCVD of Wafer-Scale MoS2 Monolayers: From Nucleation to Coalescence. MRS Adv. 2022, 7, 751– 756,  DOI: 10.1557/s43580-022-00312-4
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    26
    Detailed study on MOCVD of wafer-scale MoS2 monolayers: From nucleation to coalescence
    Tang, Songyao; Grundmann, Annika; Fiadziushkin, Hleb; Ghiami, Amir; Heuken, Michael; Vescan, Andrei; Kalisch, Holger
    MRS Advances (2022), 7 (30), 751-756CODEN: MARDCQ; ISSN:2059-8521. (Springer International Publishing AG)
    Metal-org. chem. vapor deposition (MOCVD) has become one of the most promising techniques for the large-scale fabrication of 2D transition metal dichalcogenide (TMDC) materials. Despite efforts devoted to the development of MOCVD for TMDC monolayers, the whole picture of the growth process has not been fully unveiled yet. In this work, we employ a com. AIXTRON CCS MOCVD tool for the deposition of MoS2 on sapphire using std. precursors and H2 as carrier gas. Adsorption and diffusion of Mo adatoms on the substrate are found to be decisive for nucleation. By lowering temp. from 650 to 450°C, a uniform distribution of nuclei on sapphire terraces is achieved. Full coalescence of MoS2 monolayers with limited bilayer formation (∼ 15%) is then realized at 700°C. This study highlights the importance of understanding the details of film formation mechanisms and developing multi-stage MOCVD processes for 2D TMDC films.
  27. 27
    Xu, J.; Srolovitz, D. J.; Ho, D. The Adatom Concentration Profile: A Paradigm for Understanding Two-Dimensional MoS2 Morphological Evolution in Chemical Vapor Deposition Growth. ACS Nano 2021, 15, 6839– 6848,  DOI: 10.1021/acsnano.0c10474
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    27
    The Adatom Concentration Profile: A Paradigm for Understanding Two-Dimensional MoS2 Morphological Evolution in Chemical Vapor Deposition Growth
    Xu, Jiangang; Srolovitz, David J.; Ho, Derek
    ACS Nano (2021), 15 (4), 6839-6848CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The two-dimensional (2D) transition metal dichalcogenide (TMD) MoS2 possesses many intriguing electronic and optical properties. Potential technol. applications have focused much attention on tuning MoS2 properties through control of its morphologies during growth. In this paper, we present a unified spatial-temporal model for the growth of MoS2 crystals with a full spectrum of shapes from triangles, concave triangles, three-point stars, to dendrites through the concept of the adatom concn. profile (ACP). We perform a series of chem. vapor deposition (CVD) expts. controlling adatom concn. on the substrate and growth temp. and present a method for exptl. measuring the ACP in the vicinity of growing islands. We apply a phase-field model of growth that explicitly considers similar variables (adatom concn., adatom diffusion, and noise effects) and cross-validate the simulations and expts. through the ACP and island morphologies as a function of phys. controllable variables. Our calcns. reproduce the exptl. observations with high fidelity. The ACP is an alternative paradigm to conceptualize the growth of crystals through time, which is expected to be instrumental in guiding the rational shape engineering of MoS2 crystals.
  28. 28
    Fruchart, O.; Jaren, S.; Rothman, J. Growth Modes of W and Mo Thin Epitaxial (110) Films on (112̅0) Sapphire. Appl. Surf. Sci. 1998, 135, 218– 232,  DOI: 10.1016/s0169-4332(98)00261-x
    Google Scholar
    28
    Growth modes of W and Mo thin epitaxial (110) films on (11‾20) sapphire
    Fruchart, O.; Jaren, S.; Rothman, J.
    Applied Surface Science (1998), 135 (1-4), 218-232CODEN: ASUSEE; ISSN:0169-4332. (Elsevier Science B.V.)
    Growth modes of W(110) and Mo(110) epitaxial thin films grown with pulsed laser deposition (PLD) on (11‾20) sapphire were studied. The films properties were studied both in reciprocal space using RHEED and X-ray diffraction and in real space using at. force microscope (AFM). The conventionally used high temp. growth process, usually considered to be the best process - according to reciprocal space characterization - to minimize in-plane crystallites twinning and mosaicity, yields three-dimensional growth and rough surfaces. A sample continuously wedged up to a thickness of 600 Å was studied and only partial islands coalescence was obsd. An alternative optimized growth process is proposed, based on 200°C deposition, subsequent annealing, and further high temp. deposition, that enables the elimination of in-plane twinned-crystallites while yielding to nearly perfect W(110) or Mo(110) surfaces, from both crystallog. and geometrical points of view. The comparison with MBE grown films suggests that the reported behavior is not PLD-specific. A R(15 × 6) Mo surface reconstruction is reported, which had never been obsd. before.
  29. 29
    Kapur, J. N.; Sahoo, P. K.; Wong, A. K. C. A New Method for Gray-Level Picture Thresholding Using the Entropy of the Histogram. Comput. Vis. Graph Image Process 1985, 29, 273– 285,  DOI: 10.1016/0734-189x(85)90125-2
    Google Scholar
    There is no corresponding record for this reference.
  30. 30
    Fareed, R. S. Q.; Jain, R.; Gaska, R.; Shur, M. S.; Wu, J.; Walukiewicz, W.; Khan, M. A. High Quality InN/GaN Heterostructures Grown by Migration Enhanced Metalorganic Chemical Vapor Deposition. Appl. Phys. Lett. 2004, 84, 1892– 1894,  DOI: 10.1063/1.1686889
    Google Scholar
    30
    High-quality InN/GaN heterostructures grown by migration-enhanced metalorganic chemical vapor deposition
    Fareed, R. S. Qhalid; Jain, R.; Gaska, R.; Shur, M. S.; Wu, J.; Walukiewicz, W.; Khan, M. Asif
    Applied Physics Letters (2004), 84 (11), 1892-1894CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors report on the structural properties and optical and elec. characteristics of InN epitaxial layers grown on highly resistive GaN templates using migration-enhanced metalorg. chem. vapor deposition (MEMOCVD). The material quality of the InN improved significantly for layer thickness larger than 150 nm. The highest extd. value of the room-temp. electron mobility was close to 850 cm2/V-s for samples with electron carrier concn. of ∼4 × 1018 cm-3. The measured dependence of the electron mobility on electron concn. is interpreted using a model accounting for ionized impurity scattering, polar optical scattering, and compensation. The MEMOCVD-grown material exhibited stronger photoluminescence (PL) compared to InN deposited using conventional metalorg. chem. vapor deposition. Room-temp. PL spectra were similar to InN grown using mol.-beam epitaxy (MBE) with peak emission at 0.8 eV. The obtained results demonstrate the potential of the MEMOCVD technique for the deposition of high-quality InN epitaxial layers at reduced temps. comparable to those used in MBE growth.
  31. 31
    Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371– 6379,  DOI: 10.1021/cm5025662
    Google Scholar
    31
    Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition
    Wang, Shanshan; Rong, Youmin; Fan, Ye; Pacios, Merce; Bhaskaran, Harish; He, Kuang; Warner, Jamie H.
    Chemistry of Materials (2014), 26 (22), 6371-6379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Atm.-pressure CVD was used to grow monolayer MoS2 two-dimensional crystals at elevated temps. on silicon substrates with a 300. nm oxide layer. The authors' CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace sep. from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. The authors intentionally establish a sharp gradient of MoO3 precursor concn. on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temp. range. The shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve the authors' understanding of the factors that influence the growth of MoS2 domains and their shape evolution.
  32. 32
    Shinde, N. B.; Francis, B.; Ramachandra Rao, M. S.; Ryu, B. D.; Chandramohan, S.; Eswaran, S. K. Rapid Wafer-Scale Fabrication with Layer-by-Layer Thickness Control of Atomically Thin MoS2 Films Using Gas-Phase Chemical Vapor Deposition. APL Mater. 2019, 7, 081113,  DOI: 10.1063/1.5095451
    Google Scholar
    32
    Rapid wafer-scale fabrication with layer-by-layer thickness control of atomically thin MoS2 films using gas-phase chemical vapor deposition
    Shinde, Nitin Babu; Francis, Bellarmine; Ramachandra Rao, M. S.; Ryu, Beo Deul; Chandramohan, S.; Eswaran, Senthil Kumar
    APL Materials (2019), 7 (8), 081113/1-081113/8CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Design and development of the growth-process for the prodn. of wafer-scale spatially homogeneous thickness controlled atomically thin transition metal dichalcogenides (TMDs) is one of the key challenges to realize modern electronic devices. Here, we demonstrate rapid and scalable synthesis of MoS2 films with precise thickness control via gas-phase chem. vapor deposition approach. We show that a monolayer MoS2 can be synthesized over a 2-in. sapphire wafer in a growth time as low as 4 min. With a linear growth rate of 1-layer per 4 min, MoS2 films with thicknesses varying from 1- to 5-layers with monolayer precision are produced. We propose that, in addn. to Raman spectroscopy, the energy splitting of exciton bands in optical-absorbance spectra may be another choice for layer thickness identification. With suitable precursor selection, our approach can facilitate the rapid synthesis of spatially homogeneous atomically thin TMDs on a large scale. (c) 2019 American Institute of Physics.
  33. 33
    Reifsnyder Hickey, D.; Nayir, N.; Chubarov, M.; Choudhury, T. H.; Bachu, S.; Miao, L.; Wang, Y.; Qian, C.; Crespi, V. H.; Redwing, J. M.; van Duin, A. C. T.; Alem, N. Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films. Nano Lett. 2021, 21, 6487– 6495,  DOI: 10.1021/acs.nanolett.1c01517
    Google Scholar
    33
    Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films
    Reifsnyder Hickey, Danielle; Nayir, Nadire; Chubarov, Mikhail; Choudhury, Tanushree H.; Bachu, Saiphaneendra; Miao, Leixin; Wang, Yuanxi; Qian, Chenhao; Crespi, Vincent H.; Redwing, Joan M.; van Duin, Adri C. T.; Alem, Nasim
    Nano Letters (2021), 21 (15), 6487-6495CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Engineering at.-scale defects is crucial for realizing wafer-scale, single-cryst. transition metal dichalcogenide monolayers for electronic devices. However, connecting at.-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and atomistic simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that at.-resoln. STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resoln. imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF reactive force field-based mol. dynamics simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical anal. of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.
  34. 34
    Liu, S. J.; Huang, H.; Woo, C. H. Schwoebel-Ehrlich Barrier: From Two to Three Dimensions. Appl. Phys. Lett. 2002, 80, 3295– 3297,  DOI: 10.1063/1.1475774
    Google Scholar
    34
    Schwoebel-Ehrlich barrier: from two to three dimensions
    Liu, S. J.; Huang, Hanchen; Woo, C. H.
    Applied Physics Letters (2002), 80 (18), 3295-3297CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The Schwoebel-Ehrlich barrier, the addnl. barrier for an adatom to diffuse down a surface step, dictates the growth modes of thin films. The conventional concept of this barrier is two dimensional (2D), with the surface step being one monolayer. We propose the concept of a three-dimensional (3D) Schwoebel-Ehrlich barrier, and identify the 2D to 3D transition, taking aluminum as a prototype and using the mol. statics method. Our results show that: (1) substantial differences exist between the 2D and 3D barriers; (2) the transition completes in four monolayers; and (3) there is a major disparity in the 3D barriers between two facets; further, alteration of this disparity using surfactants can lead to the dominance of surface facet against thermodn.
  35. 35
    Rao, D.; Biswas, B.; Acharya, S.; Bhatia, V.; Pillai, A. I. K.; Garbrecht, M.; Saha, B. Effects of Adatom Mobility and Ehrlich–Schwoebel Barrier on Heteroepitaxial Growth of Scandium Nitride (ScN) Thin Films. Appl. Phys. Lett. 2020, 117, 212101,  DOI: 10.1063/5.0027091
    Google Scholar
    35
    Effects of adatom mobility and Ehrlich-Schwoebel barrier on heteroepitaxial growth of scandium nitride (ScN) thin films
    Rao, Dheemahi; Biswas, Bidesh; Acharya, Shashidhara; Bhatia, Vijay; Pillai, Ashalatha Indiradevi Kamalasanan; Garbrecht, Magnus; Saha, Bivas
    Applied Physics Letters (2020), 117 (21), 212101CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Scandium nitride (ScN) is an emerging rock salt indirect bandgap semiconductor and has attracted significant interest in recent years for thermoelec. energy conversion, as a substrate for defect-free GaN growth, as a semiconducting component in single-cryst. metal/semiconductor superlattices for thermionic energy conversion, as well as for Al1-xScxN-based bulk and surface acoustic devices for 5G technologies. Most ScN film growth traditionally utilizes phys. vapor deposition techniques such as magnetron sputtering and mol. beam epitaxy, which results in stoichiometric films but with varying crystal quality, orientations, microstructures, and phys. properties. As epitaxial single-cryst. ScN films with smooth surfaces are essential for device applications, it is important to understand the ScN growth modes and parameters that impact and control their microstructure. In this Letter, we demonstrate that large adatom mobility is essential to overcome the Ehrlich-Schwoebel (E-S) and grain boundary migration barriers and achieve defect (voids, dislocations, stacking faults, etc.)-free single-cryst. ScN films. Using the substrate temp. to tune adatom mobility, we show that nominally single-cryst. ScN films are achieved when the homologous temp. is higher than ∼0.3. For homologous temps. ranging from 0.23 to 0.30, ScN films are found to exhibit significant structural voids in between pyramidal growth regions with multiple in-plane orientations resulting from addnl. lateral growth off the facets of the pyramids and broken epitaxy after ∼80 nm of growth. The in-depth discussion of the growth modes of ScN presented here explains its varying elec. and optical properties and will help achieve high-quality ScN for device applications. (c) 2020 American Institute of Physics.
  36. 36
    Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.-H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2. ACS Nano 2013, 7, 791– 797,  DOI: 10.1021/nn305275h
    Google Scholar
    36
    Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2
    Zhao, Weijie; Ghorannevis, Zohreh; Chu, Leiqiang; Toh, Minglin; Kloc, Christian; Tan, Ping-Heng; Eda, Goki
    ACS Nano (2013), 7 (1), 791-797CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS2) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS2 opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS2 and WSe2, resp. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepd. by a chem. vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.
  37. 37
    Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447– 3454,  DOI: 10.1021/nl3026357
    Google Scholar
    37
    Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers
    Gutierrez, Humberto R.; Perea-Lopez, Nestor; Elias, Ana Laura; Berkdemir, Ayse; Wang, Bei; Lv, Ruitao; Lopez-Urias, Florentino; Crespi, Vincent H.; Terrones, Humberto; Terrones, Mauricio
    Nano Letters (2013), 13 (8), 3447-3454CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Individual monolayers of metal dichalcogenides are atomically thin 2-dimensional crystals with attractive phys. properties different from those of their bulk counterparts. Here the authors describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temp. photoluminescence (PL). The Raman response as well as the luminescence as a function of the no. of S-W-S layers is also reported. The PL weakens with increasing no. of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet's center. The structure and chem. compn. of the platelet edges appear to be crit. for PL enhancement.
  38. 38
    McCreary, A.; Berkdemir, A.; Wang, J.; Nguyen, M. A.; Elías, A. L.; Perea-López, N.; Fujisawa, K.; Kabius, B.; Carozo, V.; Cullen, D. A.; Mallouk, T. E.; Zhu, J.; Terrones, M. Distinct Photoluminescence and Raman Spectroscopy Signatures for Identifying Highly Crystalline WS2 Monolayers Produced by Different Growth Methods. J. Mater. Res. 2016, 31, 931– 944,  DOI: 10.1557/jmr.2016.47
    Google Scholar
    38
    Distinct photoluminescence and Raman spectroscopy signatures for identifying highly crystalline WS2 monolayers produced by different growth methods
    McCreary, Amber; Berkdemir, Ayse; Wang, Junjie; Nguyen, Minh An; Elias, Ana Laura; Perea-Lopez, Nestor; Fujisawa, Kazunori; Kabius, Bernd; Carozo, Victor; Cullen, David A.; Mallouk, Thomas E.; Zhu, J.; Terrones, Mauricio
    Journal of Materials Research (2016), 31 (7), 931-944CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
    Transition metal dichalcogenides such as WS2 show exciting promise in electronic and optoelectronic applications. Significant variations in the transport, Raman, and photoluminescence (PL) can be found in the literature, yet it is rarely addressed why this is. In this report, Raman and PL of monolayered WS2 produced via different methods are studied and distinct features that indicate the degree of crystallinity of the material are obsd. While the intensity of the LA(M) Raman mode is found to be a useful indicator to assess the crystallinity, PL is drastically more sensitive to the quality of the material than Raman spectroscopy. We also show that even exfoliated crystals, which are usually regarded as the most pristine material, can contain large amts. of defects that would not be apparent without Raman and PL measurements. These findings can be applied to the understanding of other two-dimensional heterostructured systems.
  39. 39
    Shi, Y.; Groven, B.; Serron, J.; Wu, X.; Nalin Mehta, A.; Minj, A.; Sergeant, S.; Han, H.; Asselberghs, I.; Lin, D.; Brems, S.; Huyghebaert, C.; Morin, P.; Radu, I.; Caymax, M. Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics. ACS Nano 2021, 15, 9482– 9494,  DOI: 10.1021/acsnano.0c07761
    Google Scholar
    39
    Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics
    Shi, Yuanyuan; Groven, Benjamin; Serron, Jill; Wu, Xiangyu; Nalin Mehta, Ankit; Minj, Albert; Sergeant, Stefanie; Han, Han; Asselberghs, Inge; Lin, Dennis; Brems, Steven; Huyghebaert, Cedric; Morin, Pierre; Radu, Iuliana; Caymax, Matty
    ACS Nano (2021), 15 (6), 9482-9494CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    In view of its epitaxial seeding capability, c-plane single cryst. sapphire represents one of the most enticing, industry-compatible templates to realize manufacturable deposition of single cryst. two-dimensional transition metal dichalcogenides (MX2) for functional, ultrascaled, nanoelectronic devices beyond silicon. Despite sapphire being atomically flat, the surface topog., structure, and chem. termination vary between sapphire terraces during the fabrication process. To date, it remains poorly understood how these sapphire surface anomalies affect the local epitaxial registry and the intrinsic elec. properties of the deposited MX2 monolayer. Therefore, molybdenum disulfide (MoS2) is deposited by metal-org. chem. vapor deposition (MOCVD) in an industry-std. epitaxial reactor on two types of c-plane sapphire with distinctly different terrace and step dimensions. Complementary scanning probe microscopy techniques reveal an inhomogeneous cond. profile in the first epitaxial MoS2 monolayer on both sapphire templates. MoS2 regions with poor cond. correspond to sapphire terraces with uncontrolled topog. and surface structure. By intentionally applying a substantial off-axis cut angle (1° in this work), the sapphire terrace width and step height-and thus also surface structure-become more uniform across the substrate and MoS2 conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS2 device performance: the field-effect transistor variability reduces both within and across wafers at higher median electron mobility. Carefully controlling the sapphire surface topog. and structure proves an essential prerequisite to systematically study and control the MX2 growth behavior and capture the influence on its structural and elec. properties.
  40. 40
    Berkdemir, A.; Gutiérrez, H. R.; Botello-Méndez, A. R.; Perea-López, N.; Elías, A. L.; Chia, C.-I.; Wang, B.; Crespi, V. H.; López-Urías, F.; Charlier, J.-C.; Terrones, H.; Terrones, M. Identification of Individual and Few Layers of WS2 Using Raman Spectroscopy. Sci. Rep. 2013, 3, 1755,  DOI: 10.1038/srep01755
    Google Scholar
    40
    Identification of individual and few layers of WS2 using Raman spectroscopy
    Berkdemir, Ayse; Gutierrez, Humberto R.; Botello-Mendez, Andres R.; Perea-Lopez, Nestor; Elias, Ana Laura; Chia, Chen-Ing; Wang, Bei; Crespi, Vincent H.; Lopez-Urias, Florentino; Charlier, Jean-Christophe; Terrones, Humberto; Terrones, Mauricio
    Scientific Reports (2013), 3 (), 1755, 8 pp.CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    The Raman scattering of single- and few-layered WS2 is studied as a function of the no. of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the no. of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calcns. were used to det. the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.
  41. 41
    Rosenberger, M. R.; Chuang, H.-J.; McCreary, K. M.; Li, C. H.; Jonker, B. T. Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2. ACS Nano 2018, 12, 1793– 1800,  DOI: 10.1021/acsnano.7b08566
    Google Scholar
    41
    Electrical characterization of discrete defects and impact of defect density on photoluminescence in monolayer WS2
    Rosenberger, Matthew R.; Chuang, Hsun-Jen; McCreary, Kathleen M.; Li, Connie H.; Jonker, Berend T.
    ACS Nano (2018), 12 (2), 1793-1800CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quant. nanometer-scale characterization of defects over large areas in order to understand the relation between defects and obsd. properties, such as photoluminescence (PL) and elec. cond. In this work, the authors present conductive at. force microscopy measurements which reveal nanometer-scale electronically active defects in chem. vapor deposition-grown WS2 monolayers with defect d. varying from 2.3 × 1010 cm-2 to 4.5 × 1011 cm-2. Comparing these defect d. measurements with PL measurements across large areas (>20 μm distances) reveals a strong inverse relationship between WS2 PL intensity and defect d. They propose a model in which the obsd. electronically active defects serve as nonradiative recombination centers and obtain good agreement between the expts. and model.
  42. 42
    Li, J.; Su, W.; Chen, F.; Fu, L.; Ding, S.; Song, K.; Huang, X.; Zhang, L. Atypical Defect-Mediated Photoluminescence and Resonance Raman Spectroscopy of Monolayer WS2. J. Phys. Chem. C 2019, 123, 3900– 3907,  DOI: 10.1021/acs.jpcc.8b11647
    Google Scholar
    42
    Atypical Defect-Mediated Photoluminescence and Resonance Raman Spectroscopy of Monolayer WS2
    Li, Jiake; Su, Weitao; Chen, Fei; Fu, Li; Ding, Su; Song, Kaixin; Huang, Xiwei; Zhang, Lijie
    Journal of Physical Chemistry C (2019), 123 (6), 3900-3907CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Defects play an indispensable role in tuning the optical properties of 2-dimensional materials. The influence of defects on the luminescence and resonance Raman spectra of as-grown monolayer (1L) WS2 was studied. Increasing the d. of defects significantly lowers the excitonic binding energy by ≤110 meV. These defect-modified excitonic binding energies in 1L-WS2 strongly mediate the Raman resonance condition, resulting in unexpected Raman intensity variations in the LA(M), 2LA(M), and A'1(Γ) phonon modes. The sample with the highest d. of defects exhibits an almost temp.-independent resonance in different Raman modes at low temp., whereas the samples with low densities of defects exhibit a clear resonance with decreasing temp. This study will further increase the understanding of the role of defects in resonance Raman spectroscopy and of the phonon-exciton interaction in 1L-WS2.
  43. 43
    Marinov, D.; de Marneffe, J.-F.; Smets, Q.; Arutchelvan, G.; Bal, K. M.; Voronina, E.; Rakhimova, T.; Mankelevich, Y.; El Kazzi, S.; Nalin Mehta, A.; Wyndaele, P.-J.; Heyne, M. H.; Zhang, J.; With, P. C.; Banerjee, S.; Neyts, E. C.; Asselberghs, I.; Lin, D.; De Gendt, S. Reactive Plasma Cleaning and Restoration of Transition Metal Dichalcogenide Monolayers. npj 2D Mater. Appl. 2021, 5, 17,  DOI: 10.1038/s41699-020-00197-7
    Google Scholar
    43
    Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers
    Marinov, Daniil; de Marneffe, Jean-Francois; Smets, Quentin; Arutchelvan, Goutham; Bal, Kristof M.; Voronina, Ekaterina; Rakhimova, Tatyana; Mankelevich, Yuri; El Kazzi, Salim; Nalin Mehta, Ankit; Wyndaele, Pieter-Jan; Heyne, Markus Hartmut; Zhang, Jianran; With, Patrick C.; Banerjee, Sreetama; Neyts, Erik C.; Asselberghs, Inge; Lin, Dennis; De Gendt, Stefan
    npj 2D Materials and Applications (2021), 5 (1), 17CODEN: DMAAAH; ISSN:2397-7132. (Nature Research)
    The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique phys., optical, and chem. properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of "monolayer" TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temp. processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temp. in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temp. H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for std. semiconductor pilot prodn. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technol.

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Cite this: Cryst. Growth Des. 2023, 23, 3, 1547–1558
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Published January 24, 2023

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  • Abstract

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    Figure 1

    Figure 1. (a) Temperature profile with two different nucleation temperatures and (b) variation of molar flows and S/W ratio in the two-stage migration-enhanced MOCVD process (shown for sample S_R144, see below).

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    Figure 2

    Figure 2. (a) Morphology of WS2 after 15 min nucleation at 700 °C (sample S_N700). (b) Schematic of the WS2 domain deviation from the triangular shape. (c) Morphology of WS2 after 128 min lateral growth (sample S_R128). (d) Schematic of WS2 domain shape recovery from three-point stars to triangles. (e) Schematic explaining parasitic/premature BL nucleation. λM stands for the migration length of adatoms.

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    Figure 3

    Figure 3. (a,b) SEM images of WS2 after 15 min nucleation at 750 °C (S_N750) and 700 °C (S_N700). (c,d) SEM images of samples S_R160.HT and S_R144, where the ML is coalesced and BL nuclei are visible. Blue, green, and red arrows point at sapphire, ML, and BL regions, respectively. (e) Comparison of RT PL intensities of samples S_R160.HT and S_R144. (f) Deconvolution of PL spectra in different excitonic peaks. XD stands for bound excitons; XT stands for trions; and A stands for A excitons.

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    Figure 4

    Figure 4. (a) In situ reflectance transients of WS2 samples S_R144 and S_C090. The temperature ramp-up (between 15 and 27 min, marked by the red box) increases the slope. (Unit of slope: ‰ per min.) (b,c) SEM images of samples S_R144 and S_C090, respectively. (d) Raman spectra of both samples. (e) Variation of ML and BL coverages according to the type and duration of processes. The red and green arrows are indicators for increasing tendency of coverages.

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    Figure 5

    Figure 5. (a) SEM image of WS2 with an almost identical ML coverage and an approx. 5% difference in BL coverage (top: S_C100 and bottom: S_R144). (b) PL comparison of two MLs, where the red-colored region stands for the assembly of PL spectra measured from different positions on sample S_R144. Green stands for those from sample S_C100. (c) XPS spectra of C 1s, S 2p, and W 4f core levels. Both axes are normalized to the same scale.

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    Figure 6

    Figure 6. (a) Morphology of WS2 samples S_R128 (left) and S_C090 (right), on which an estimate of nominal migration length is extracted by red inscribed circles inside the ML contour. The corresponding BL nucleation density and effective migration length after calculations are noted on the top right of SEM images. (b) Illustration for calculating Lav from the density of BL nuclei, where the ML region is colored in light green and BL nuclei is colored in dark green.

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    Figure 7

    Figure 7. (a,b) SEM image of WSe2 nucleation in samples Se_N28 and Se_N56, respectively. (c) Schematic of temperature and WCO flux variations during three MOCVD processes. The resulting morphologies of samples Se_R45, Se_R60, and Se_R75 are shown in figures (d–f). Blue, green, and red arrows in SEM images point at sapphire, ML, and BL regions, respectively. Trilayers are clearly visible in (f) marked by the red arrows.

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  • References

    This article references 43 other publications.

    1. 1
      Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147– 150,  DOI: 10.1038/nnano.2010.279
      1
      Single-layer MoS2 transistors
      Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.
      Nature Nanotechnology (2011), 6 (3), 147-150CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to 1D materials, it is relatively easy to fabricate complex structures from them. The most widely studied 2D material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained Si films or requires high voltages. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV, previously reported mobilities in the 0.5-3 cm2 V-1 s-1 range are too low for practical devices. Here, we use a HfO2 gate dielec. to demonstrate a room-temp. single-layer MoS2 mobility of at least 200 cm2 V-1 s-1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temp. current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
    2. 2
      Akama, T.; Okita, W.; Nagai, R.; Li, C.; Kaneko, T.; Kato, T. Schottky Solar Cell Using Few-Layered Transition Metal Dichalcogenides toward Large-Scale Fabrication of Semitransparent and Flexible Power Generator. Sci. Rep. 2017, 7, 11967,  DOI: 10.1038/s41598-017-12287-6
      2
      Schottky solar cell using few-layered transition metal dichalcogenides toward large-scale fabrication of semitransparent and flexible power generator
      Akama Toshiki; Okita Wakana; Nagai Reito; Li Chao; Kaneko Toshiro; Kato Toshiaki
      Scientific reports (2017), 7 (1), 11967 ISSN:.
      Few-layered transition metal dichalcogenides (TMDs) are known as true two-dimensional materials, with excellent semiconducting properties and strong light-matter interaction. Thus, TMDs are attractive materials for semitransparent and flexible solar cells for use in various applications. Hoewver, despite the recent progress, the development of a scalable method to fabricate semitransparent and flexible solar cells with mono- or few-layered TMDs remains a crucial challenge. Here, we show easy and scalable fabrication of a few-layered TMD solar cell using a Schottky-type configuration to obtain a power conversion efficiency (PCE) of approximately 0.7%, which is the highest value reported with few-layered TMDs. Clear power generation was also observed for a device fabricated on a large SiO2 and flexible substrate, demonstrating that our method has high potential for scalable production. In addition, systematic investigation revealed that the PCE and external quantum efficiency (EQE) strongly depended on the type of photogenerated excitons (A, B, and C) because of different carrier dynamics. Because high solar cell performance along with excellent scalability can be achieved through the proposed process, our fabrication method will contribute to accelerating the industrial use of TMDs as semitransparent and flexible solar cells.
    3. 3
      Andrzejewski, D.; Myja, H.; Heuken, M.; Grundmann, A.; Kalisch, H.; Vescan, A.; Kümmell, T.; Bacher, G. Scalable Large-Area p–i–n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD. ACS Photonics 2019, 6, 1832– 1839,  DOI: 10.1021/acsphotonics.9b00311
      3
      Scalable Large-Area p-i-n Light-Emitting Diodes Based on WS2 Monolayers Grown via MOCVD
      Andrzejewski, Dominik; Myja, Henrik; Heuken, Michael; Grundmann, Annika; Kalisch, Holger; Vescan, Andrei; Kuemmell, Tilmar; Bacher, Gerd
      ACS Photonics (2019), 6 (8), 1832-1839CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chem. vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal-org. (MO-)CVD WS2 monolayers into a vertical p-i-n device architecture using org. and inorg. injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V.
    4. 4
      Late, D. J.; Doneux, T.; Bougouma, M. Single-Layer MoSe2 Based NH3 Gas Sensor. Appl. Phys. Lett. 2014, 105, 233103,  DOI: 10.1063/1.4903358
      4
      Single-layer MoSe2 based NH3 gas sensor
      Late, Dattatray J.; Doneux, Thomas; Bougouma, Moussa
      Applied Physics Letters (2014), 105 (23), 233103/1-233103/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Describe is the utilization of single-layer MoSe2 as high-performance room temp. NH3 gas sensors. The single-layer MoSe2-based gas sensor device shows comprehensible detection of NH3 gas down to 50 ppm. Also confirmed was a gas sensing measurement by recording the Raman spectra before and after exposing the device to NH3 gas, which subsequently shows the shift due to charger transfer and analyte gas mol. adsorption on surface of single-layer MoSe2 nanosheet. The investigations show the potential use of single-layer and few layer thick MoSe2 and other TMDC as high-performance gas sensors. (c) 2014 American Institute of Physics.
    5. 5
      Fathi-Hafshejani, P.; Azam, N.; Wang, L.; Kuroda, M. A.; Hamilton, M. C.; Hasim, S.; Mahjouri-Samani, M. Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2). ACS Nano 2021, 15, 11461– 11469,  DOI: 10.1021/acsnano.1c01188
      5
      Two-Dimensional-Material-Based Field-Effect Transistor Biosensor for Detecting COVID-19 Virus (SARS-CoV-2)
      Fathi-Hafshejani, Parvin; Azam, Nurul; Wang, Lu; Kuroda, Marcelo A.; Hamilton, Michael C.; Hasim, Sahar; Mahjouri-Samani, Masoud
      ACS Nano (2021), 15 (7), 11461-11469CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The emergence of rapidly expanding infectious diseases such as coronavirus (COVID-19) demands effective biosensors that can promptly detect and recognize the pathogens. Field-effect transistors based on semiconducting two-dimensional (2D) materials (2D-FETs) have been identified as potential candidates for rapid and label-free sensing applications. This is because any perturbation of such atomically thin 2D channels can significantly impact their electronic transport properties. Here, we report the use of FET based on semiconducting transition metal dichalcogenide (TMDC) WSe2 as a promising biosensor for the rapid and sensitive detection of SARS-CoV-2 in vitro. The sensor is created by functionalizing the WSe2 monolayers with a monoclonal antibody against the SARS-CoV-2 spike protein and exhibits a detection limit of down to 25 fg/μL in 0.01X phosphate-buffered saline (PBS). Comprehensive theor. and exptl. studies, including d. functional theory, at. force microscopy, Raman and photoluminescence spectroscopies, and electronic transport properties, were performed to characterize and explain the device performance. The results demonstrate that TMDC-based 2D-FETs can potentially serve as sensitive and selective biosensors for the rapid detection of infectious diseases.
    6. 6
      Kang, S.; Koo, J.-J.; Seo, H.; Truong, Q. T.; Park, J. B.; Park, S. C.; Jung, Y.; Cho, S.-P.; Nam, K. T.; Kim, Z. H.; Hong, B. H. Defect-Engineered MoS2 with Extended Photoluminescence Lifetime for High-Performance Hydrogen Evolution. J. Mater. Chem. C 2019, 7, 10173– 10178,  DOI: 10.1039/c9tc02256b
      6
      Defect-engineered MoS2 with extended photoluminescence lifetime for high-performance hydrogen evolution
      Kang, Sangmin; Koo, Ja-Jung; Seo, Hongmin; Truong, Quang Trung; Park, Jong Bo; Park, Seong Chae; Jung, Youngjin; Cho, Sung-Pyo; Nam, Ki Tae; Kim, Zee Hwan; Hong, Byung Hee
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (33), 10173-10178CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      It has been reported that defects in molybdenum disulfide (MoS2) enable the hydrogen evolution reaction (HER). The most widely employed method of argon-plasma treatment for defect generation suffers from poor material stability and loss of cond. Here, we report a new method to synthesize highly polycryst. molybdenum disulfide MoS2 bilayers with enhanced HER performance and material stability. This new method is based on metal org. chem. vapor deposition (MOCVD) followed by UV/ozone treatment to generate defects. The defect densities on MoS2 were identified by the increase in lifetime (∼76%) and intensity (∼15%) in photoluminescence (PL) as compared to those of pristine MoS2. Our fabrication and characterization methods can be further applied to optimize defect densities for catalytic effects in various transition metal dichalcogenide (TMDC) materials.
    7. 7
      Ju, L.; Bie, M.; Shang, J.; Tang, X.; Kou, L. Janus Transition Metal Dichalcogenides: A Superior Platform for Photocatalytic Water Splitting. JPhys Mater. 2020, 3, 022004,  DOI: 10.1088/2515-7639/ab7c57
      7
      Janus transition metal dichalcogenides: a superior platform for photocatalytic water splitting
      Ju, Lin; Bie, Mei; Shang, Jing; Tang, Xiao; Kou, Liangzhi
      JPhys Materials (2020), 3 (2), 022004CODEN: JPMOC4; ISSN:2515-7639. (IOP Publishing Ltd.)
      Janus two-dimensional (2D) materials, referring to the layers with different surfaces, have attracted intensive research interest due to the unique properties induced by symmetry breaking, and promising applications in energy conversion. Based on the successful exptl. synthesis of Janus transition metal dichalcogenides (TMDC), here we present a review on their potential application in photocatalytic overall water splitting, from the perspectives of the latest theor. and exptl. progress. Four aspects which are related to photocatalytic reaction, including the adsorption of water mols., utilization of sunlight, charge sepn. and transport, and surface chem. reactions have been discussed, and it is concluded that the Janus structures have better performances than sym. TMDCs. At the end of this review, we raise further challenges and possible future research directions for Janus 2D materials as water-splitting photocatalysts.
    8. 8
      Liu, Y.; Gu, F. A Wafer-Scale Synthesis of Monolayer MoS2 and Their Field-Effect Transistors toward Practical Applications. Nanoscale Adv. 2021, 3, 2117– 2138,  DOI: 10.1039/d0na01043j
      8
      A wafer-scale synthesis of monolayer MoS2 and their field-effect transistors toward practical applications
      Liu, Yuchun; Gu, Fuxing
      Nanoscale Advances (2021), 3 (8), 2117-2138CODEN: NAADAI; ISSN:2516-0230. (Royal Society of Chemistry)
      A review. Molybdenum disulfide (MoS2) has attracted considerable research interest as a promising candidate for downscaling integrated electronics due to the special two-dimensional structure and unique physicochem. properties. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and elec. properties to fulfill the requirement for practical applications. Recently, a variety of investigations have focused on wafer-scale monolayer MoS2 synthesis with high-quality. The 2D MoS2 field-effect transistor (MoS2-FET) array with different configurations utilizes the high-quality MoS2 film as channels and exhibits favorable performance. In this review, we illustrated the latest research advances in wafer-scale monolayer MoS2 synthesis by different methods, including Au-assisted exfoliation, CVD, thin film sulfurization, MOCVD, ALD, VLS method, and the thermolysis of thiosalts. Then, an overview of MoS2-FET developments was provided based on large-area MoS2 film with different device configurations and performances. The different applications of MoS2-FET in logic circuits, basic memory devices, and integrated photodetectors were also summarized. Lastly, we considered the perspective and challenges based on wafer-scale monolayer MoS2 synthesis and MoS2-FET for developing practical applications in next-generation integrated electronics and flexible optoelectronics.
    9. 9
      Lee, D. H.; Sim, Y.; Wang, J.; Kwon, S.-Y. Metal–Organic Chemical Vapor Deposition of 2D van Der Waals Materials─The Challenges and the Extensive Future Opportunities. APL Mater. 2020, 8, 030901,  DOI: 10.1063/1.5142601
      9
      Metal-organic chemical vapor deposition of 2D van der Waals materials-The challenges and the extensive future opportunities
      Lee, Do Hee; Sim, Yeoseon; Wang, Jaewon; Kwon, Soon-Yong
      APL Materials (2020), 8 (3), 030901CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      A review. The last decade has witnessed significant progress in two-dimensional van der Waals (2D vdW) materials research; however, a no. of challenges remain for their practical applications. The most significant challenge for 2D vdW materials is the control of the early stages of nucleation and growth of the material on preferred surfaces to eventually create large grains with digital thickness controllability, which will enable their incorporation into high-performance electronic and optoelectronic devices. This Perspective discusses the tech. challenges to be overcome in the metal-org. chem. vapor deposition (MOCVD) growth of 2D group 6 transition metal dichalcogenide (TMD) at. crystals and their heterostructures, as well as future research aspects in vdW epitaxy for 2D TMDs via MOCVD. In addn., we encourage the traditional MOCVD community to apply their expertise in the field of "2D vdW materials," which will continue to grow at an exponential rate. (c) 2020 American Institute of Physics.
    10. 10
      Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656– 660,  DOI: 10.1038/nature14417
      10
      High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity
      Kang, Kibum; Xie, Saien; Huang, Lujie; Han, Yimo; Huang, Pinshane Y.; Mak, Kin Fai; Kim, Cheol-Joo; Muller, David; Park, Jiwoong
      Nature (London, United Kingdom) (2015), 520 (7549), 656-660CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The large-scale growth of semiconducting thin films forms the basis of modern electronics and optoelectronics. A decrease in film thickness to the ultimate limit of the at., sub-nanometer length scale, a difficult limit for traditional semiconductors (such as Si and GaAs), would bring wide benefits for applications in ultrathin and flexible electronics, photovoltaics and display technol. For this, transition-metal dichalcogenides (TMDs), which can form stable three-atom-thick monolayers, provide ideal semiconducting materials with high elec. carrier mobility, and their large-scale growth on insulating substrates would enable the batch fabrication of atomically thin high-performance transistors and photodetectors on a technol. relevant scale without film transfer. In addn., their unique electronic band structures provide novel ways of enhancing the functionalities of such devices, including the large excitonic effect, bandgap modulation, indirect-to-direct bandgap transition, piezoelectricity and valleytronics. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high elec. performance remains an unsolved challenge. Here we report the prepn. of high-mobility 4-in. wafer-scale films of monolayer molybdenum disulfide (MoS2) and tungsten disulfide, grown directly on insulating SiO2 substrates, with excellent spatial homogeneity over the entire films. They are grown with a newly developed, metal-org. chem. vapor deposition technique, and show high elec. performance, including an electron mobility of 30 cm2 V-1 s-1 at room temp. and 114 cm2 V-1 s-1 at 90 K for MoS2, with little dependence on position or channel length. With the use of these films we successfully demonstrate the wafer-scale batch fabrication of high-performance monolayer MoS2 field-effect transistors with a 99% device yield and the multi-level fabrication of vertically stacked transistor devices for three-dimensional circuitry. Our work is a step towards the realization of atomically thin integrated circuitry.
    11. 11
      Grundmann, A.; Andrzejewski, D.; Kümmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A. H2S-Free Metal-Organic Vapor Phase Epitaxy of Coalesced 2D WS2 Layers on Sapphire─ERRATUM. MRS Adv. 2019, 4, e1  DOI: 10.1557/adv.2019.229
      There is no corresponding record for this reference.
    12. 12
      Li, H.; Wu, H.; Yuan, S.; Qian, H. Synthesis and Characterization of Vertically Standing MoS2 Nanosheets. Sci. Rep. 2016, 6, 21171,  DOI: 10.1038/srep21171
      12
      Synthesis and characterization of vertically standing MoS2 nanosheets
      Li, Han; Wu, Huaqiang; Yuan, Shuoguo; Qian, He
      Scientific Reports (2016), 6 (), 21171CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Molybdenum disulfide (MoS2) has been attracting much attentions due to its excellent elec. and optical properties. We report here the synthesis of large-scale and uniform MoS2 nanosheets with vertically standing morphol. using chem. vapor deposition method. TEM observations clearly reveal the growth mechanism of these vertical structures. It is suggested that the vertical structures are caused by the compression and extrusion between MoS2 islands. More importantly, the vertical morphol. of two dimensional (2D) materials hold many promising potential applications. We demonstrate here the as-synthesized vertically standing MoS2 nanosheets could be used for hydrogen evolution reaction, where the exchange c.d. is about 70 times of bulk MoS2. The field emission performance of vertically standing MoS2 were also improved due to the abundantly exposed edges.
    13. 13
      Schaefer, C. M.; Caicedo Roque, J. M.; Sauthier, G.; Bousquet, J.; Hébert, C.; Sperling, J. R.; Pérez-Tomás, A.; Santiso, J.; del Corro, E.; Garrido, J. A. Carbon Incorporation in MOCVD of MoS2 Thin Films Grown from an Organosulfide Precursor. Chem. Mater. 2021, 33, 4474– 4487,  DOI: 10.1021/acs.chemmater.1c00646
      13
      Carbon Incorporation in MOCVD of MoS2 Thin Films Grown from an Organosulfide Precursor
      Schaefer, Christian M.; Caicedo Roque, Jose M.; Sauthier, Guillaume; Bousquet, Jessica; Hebert, Clement; Sperling, Justin R.; Perez-Tomas, Amador; Santiso, Jose; del Corro, Elena; Garrido, Jose A.
      Chemistry of Materials (2021), 33 (12), 4474-4487CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in com. (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-org. chem. vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-esp. in instances where org. chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-contg. side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and di-Et sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temp. and Mo(CO)6/DES/H2 gas mixt. ratios on film morphol., chem. compn., and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temps. and high DES/Mo(CO)6 precursor ratios, we obsd. that temps. above DES pyrolysis onset (⪆600°C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-to-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices.
    14. 14
      Zhang, X.; Al Balushi, Z. Y.; Zhang, F.; Choudhury, T. H.; Eichfeld, S. M.; Alem, N.; Jackson, T. N.; Robinson, J. A.; Redwing, J. M. Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films. J. Electron. Mater. 2016, 45, 6273– 6279,  DOI: 10.1007/s11664-016-5033-0
      14
      Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films
      Zhang, Xiaotian; Al Balushi, Zakaria Y.; Zhang, Fu; Choudhury, Tanushree H.; Eichfeld, Sarah M.; Alem, Nasim; Jackson, Thomas N.; Robinson, Joshua A.; Redwing, Joan M.
      Journal of Electronic Materials (2016), 45 (12), 6273-6279CODEN: JECMA5; ISSN:0361-5235. (Springer)
      Metalorg. chem. vapor deposition (MOCVD) is a promising technique to form large-area, uniform films of monolayer or few-layer transition metal dichalcogenide (TMD) thin films; however, unintentional carbon incorporation is a concern. In this work, we report the presence of a defective graphene layer that forms simultaneously during MOCVD growth of tungsten diselenide (WSe2) on sapphire at high growth temp. and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and di-Me selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. The use of hydrogen selenide (H2Se) instead of DMSe eliminates the defective graphene layer enabling coalesced monolayer and few-layer WSe2 films.
    15. 15
      Marx, M.; Grundmann, A.; Lin, Y.-R.; Andrzejewski, D.; Kümmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A. Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2. J. Electron. Mater. 2018, 47, 910– 916,  DOI: 10.1007/s11664-017-5937-3
      15
      Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
      Marx, M.; Grundmann, A.; Lin, Y.-R.; Andrzejewski, D.; Kuemmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A.
      Journal of Electronic Materials (2018), 47 (2), 910-916CODEN: JECMA5; ISSN:0361-5235. (Springer)
      The influence of the main growth parameters on the growth mechanism and film formation processes during metalorg. vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-Bu sulfide as metalorg. precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by at. force microscopy, SEM, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation d. of the polycryst. film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Addnl., the influence of growth parameters such as reactor pressure and surface temp. is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temp. above 900°C.
    16. 16
      Choudhury, T. H.; Simchi, H.; Boichot, R.; Chubarov, M.; Mohney, S. E.; Redwing, J. M. Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor Deposition Growth of WS2. Cryst. Growth Des. 2018, 18, 4357– 4364,  DOI: 10.1021/acs.cgd.8b00306
      16
      Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor Deposition Growth of WS2
      Choudhury, Tanushree H.; Simchi, Hamed; Boichot, Raphael; Chubarov, Mikhail; Mohney, Suzanne E.; Redwing, Joan M.
      Crystal Growth & Design (2018), 18 (8), 4357-4364CODEN: CGDEFU; ISSN:1528-7483. (American Chemical Society)
      Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas-source chem. vapor deposition system to ascertain the effect of the chalcogen precursor on the film growth and properties. Tungsten hexacarbonyl (W(CO)6) was used as the tungsten source, and hydrogen sulfide (H2S) and di-Et sulfide (DES-(C2H5)2S) were the chalcogen sources. The film deposition was studied at different temps. and chalcogen-to-metal ratios to understand the effect of each chalcogen precursor on the film growth rate, thickness, coverage, photoluminescence, and stoichiometry. Larger lateral growth was obsd. in films grown with H2S than DES. The reduced lateral growth with DES can be attributed to carbon contamination, which also quenches the photoluminescence. Thermodn. calcns. agreed well with the exptl. observations, suggesting formation of WS2 with both sulfur precursors and addnl. formation of carbon when deposition is done using DES.
    17. 17
      Eichfeld, S. M.; Hossain, L.; Lin, Y.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kim, M. J.; Mayer, T. S.; Redwing, J. M.; Robinson, J. A. Highly Scalable, Atomically Thin WSe2 Grown via Metal–Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080– 2087,  DOI: 10.1021/nn5073286
      17
      Highly Scalable, Atomically Thin WSe2 Grown via Metal-Organic Chemical Vapor Deposition
      Eichfeld, Sarah M.; Hossain, Lorraine; Lin, Yu-Chuan; Piasecki, Aleksander F.; Kupp, Benjamin; Birdwell, A. Glen; Burke, Robert A.; Lu, Ning; Peng, Xin; Li, Jie; Azcatl, Angelica; McDonnell, Stephen; Wallace, Robert M.; Kim, Moon J.; Mayer, Theresa S.; Redwing, Joan M.; Robinson, Joshua A.
      ACS Nano (2015), 9 (2), 2080-2087CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2-dimensional material cannot be realized without a scalable synthesis process. Here, the authors demonstrate the 1st scalable synthesis of large-area, mono and few-layer WSe2via metal-org. CVD using tungsten hexacarbonyl (W(CO)6) and dimethylselenium (Me2Se). In addn. to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chem., which is unobtainable using more traditional oxide vaporization routes. Temp., pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing at. layer structure, with optimized conditions yielding >8 μm size domains. Raman spectroscopy, at. force microscopy (AFM), and cross-sectional TEM confirm cryst. monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.
    18. 18
      Zhang, X.; Choudhury, T. H.; Chubarov, M.; Xiang, Y.; Jariwala, B.; Zhang, F.; Alem, N.; Wang, G.-C.; Robinson, J. A.; Redwing, J. M. Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire. Nano Lett. 2018, 18, 1049– 1056,  DOI: 10.1021/acs.nanolett.7b04521
      18
      Diffusion-Controlled Epitaxy of Large Area Coalesced WSe2 Monolayers on Sapphire
      Zhang, Xiaotian; Choudhury, Tanushree H.; Chubarov, Mikhail; Xiang, Yu; Jariwala, Bhakti; Zhang, Fu; Alem, Nasim; Wang, Gwo-Ching; Robinson, Joshua A.; Redwing, Joan M.
      Nano Letters (2018), 18 (2), 1049-1056CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      A multistep diffusion-mediated process was developed to control the nucleation d., size, and lateral growth rate of WSe2 domains on c-plane Al2O3 for the epitaxial growth of large area monolayer films by gas source CVD. The process consists of an initial nucleation step followed by an annealing period in H2Se to promote surface diffusion of W-contg. species to form oriented WSe2 islands with uniform size and controlled d. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe2 islands to form a fully coalesced monolayer film in <1 h. Postgrowth structural characterization demonstrates that the WSe2 monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe2 domains. The process also provides fundamental insights into the 2-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster d. with annealing time follows a 2D ripening process, enabling an est. of the W-species surface diffusivity. The lateral growth rate of domains is relatively independent of substrate temp. at 700-900° suggesting a mass transport limited process, however, the domain shape (triangular vs. truncated triangular) varied with temp. over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward at. level control of the epitaxial growth of WSe2 monolayers in a scalable process that is suitable for large area device fabrication.
    19. 19
      Chubarov, M.; Choudhury, T. H.; Hickey, D. R.; Bachu, S.; Zhang, T.; Sebastian, A.; Bansal, A.; Zhu, H.; Trainor, N.; Das, S.; Terrones, M.; Alem, N.; Redwing, J. M. Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire. ACS Nano 2021, 15, 2532– 2541,  DOI: 10.1021/acsnano.0c06750
      19
      Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire
      Chubarov, Mikhail; Choudhury, Tanushree H.; Hickey, Danielle Reifsnyder; Bachu, Saiphaneendra; Zhang, Tianyi; Sebastian, Amritanand; Bansal, Anushka; Zhu, Haoyue; Trainor, Nicholas; Das, Saptarshi; Terrones, Mauricio; Alem, Nasim; Redwing, Joan M.
      ACS Nano (2021), 15 (2), 2532-2541CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallog. direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diam. c-plane sapphire by metalorg. chem. vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy anal. reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the d. of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ~ 107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.
    20. 20
      Seol, M.; Lee, M.; Kim, H.; Shin, K. W.; Cho, Y.; Jeon, I.; Jeong, M.; Lee, H.; Park, J.; Shin, H. High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening. Adv. Mater. 2020, 32, 2003542,  DOI: 10.1002/adma.202003542
      20
      High-Throughput Growth of Wafer-Scale Monolayer Transition Metal Dichalcogenide via Vertical Ostwald Ripening
      Seol, Minsu; Lee, Min-Hyun; Kim, Haeryong; Shin, Keun Wook; Cho, Yeonchoo; Jeon, Insu; Jeong, Myoungho; Lee, Hyung-Ik; Park, Jiwoong; Shin, Hyeon-Jin
      Advanced Materials (Weinheim, Germany) (2020), 32 (42), 2003542CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high-throughput, large-scale prodn. of high-quality, spatially uniform materials, and reliable integration into devices. Here, high-throughput growth, completed in 12 min, of 6-in. wafer-scale monolayer MoS2 and WS2 is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal-org. chem. vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concns. The as-grown TMD films show excellent spatial homogeneity and well-stitched grain boundaries, enabling facile transfer to various target substrates without degrdn. Using these films, batch fabrication of high-performance field-effect transistor (FET) arrays in wafer-scale is demonstrated, and the FETs show remarkable uniformity. The high-throughput prodn. and wafer-scale automatable transfer will facilitate the integration of TMDs into Si-complementary metal-oxide-semiconductor platforms.
    21. 21
      Choi, J.; Ha, M.; Park, J. C.; Park, T. J.; Kim, W.; Lee, M.; Ahn, J. A Strategy for Wafer-Scale Crystalline MoS2 Thin Films with Controlled Morphology Using Pulsed Metal–Organic Chemical Vapor Deposition at Low Temperature. Adv. Mater. Interfaces 2022, 9, 2101785,  DOI: 10.1002/admi.202101785
      21
      Strategy for wafer-scale crystalline molybdenum sulfide thin films with controlled morphology using pulsed metal-organic chemical vapor deposition at low temperature
      Choi, Jeong-Hun; Ha, Min-Ji; Park, Jae Chan; Park, Tae Joo; Kim, Woo-Hee; Lee, Myoung-Jae; Ahn, Ji-Hoon
      Advanced Materials Interfaces (2022), 9 (4), 2101785CODEN: AMIDD2; ISSN:2196-7350. (Wiley-VCH Verlag GmbH & Co. KGaA)
      2D semiconductor materials with layered crystal structures have attracted great interest as promising candidates for electronic, optoelectronic, and sensor applications due to their unique and superior characteristics. However, a large-area synthesis process for various applications and practical mass prodn. is still lacking. In particular, there is a limitation in that a high process temp. and a very long process time are required to deposit a crystd. 2D material on a large area. Herein, pulsed metal-org. chem. vapor deposition (p-MOCVD) is proposed for the growth of wafer-scale cryst. MoS2 thin films to overcome the existing limitations. In the p-MOCVD process, precursors are repeatedly injected at regular intervals to enhance the migration of precursors on the surface. As a result, cryst. MoS2 is successfully synthesized at the lowest temp. (350°C) reported so far in a very short process time of 550 s. In addn., it is found that the horizontal and vertical growth modes of MoS2 can be effectively controlled by adjusting key process parameters. Finally, various applications are presented by demonstrating the photodetector (detectivity = 18.1 x 106 at light power of 1 mW) and chem. sensor (response = 38% at 100 ppm of NO2 gas) devices.
    22. 22
      Cohen, A.; Patsha, A.; Mohapatra, P. K.; Kazes, M.; Ranganathan, K.; Houben, L.; Oron, D.; Ismach, A. Growth-Etch Metal–Organic Chemical Vapor Deposition Approach of WS2 Atomic Layers. ACS Nano 2021, 15, 526– 538,  DOI: 10.1021/acsnano.0c05394
      22
      Growth-etch metal-organic chemical vapor deposition approach of WS2 atomic layers
      Cohen, Assael; Patsha, Avinash; Mohapatra, Pranab K.; Kazes, Miri; Ranganathan, Kamalakannan; Houben, Lothar; Oron, Dan; Ismach, Ariel
      ACS Nano (2021), 15 (1), 526-538CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Metal-org. chem. vapor deposition (MOCVD) is one of the main methodologies used for thin-film fabrication in the semiconductor industry today and is considered one of the most promising routes to achieve large-scale and high-quality 2D transition metal dichalcogenides (TMDCs). However, if special measures are not taken, MOCVD suffers from some serious drawbacks, such as small domain size and carbon contamination, resulting in poor optical and crystal quality, which may inhibit its implementation for the large-scale fabrication of at.-thin semiconductors. Here we present a growth-etch MOCVD (GE-MOCVD) methodol., in which a small amt. of water vapor is introduced during the growth, while the precursors are delivered in pulses. The evolution of the growth as a function of the amt. of water vapor, the no. and type of cycles, and the gas compn. is described. We show a significant domain size increase is achieved relative to our conventional process. The improved crystal quality of WS2 (and WSe2) domains is demonstrated by means of Raman spectroscopy, photoluminescence (PL) spectroscopy, and HRTEM studies. Moreover, time-resolved PL studies show very long exciton lifetimes, comparable to those obsd. in mech. exfoliated flakes. Thus, the GE-MOCVD approach presented here may facilitate their integration into a wide range of applications.
    23. 23
      Shang, S.-L.; Lindwall, G.; Wang, Y.; Redwing, J. M.; Anderson, T.; Liu, Z.-K. Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2. Nano Lett. 2016, 16, 5742– 5750,  DOI: 10.1021/acs.nanolett.6b02443
      23
      Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2
      Shang, Shun-Li; Lindwall, Greta; Wang, Yi; Redwing, Joan M.; Anderson, Tim; Liu, Zi-Kui
      Nano Letters (2016), 16 (9), 5742-5750CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodn. and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated d. functional theory (DFT) and calcn. of phase diagram (CALPHAD) modeling approach is employed to provide thermodn. insight into lateral vs. vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral vs. vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The crit. 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temp.-compn. (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calcd. MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available expts., and the present data provide quant. insight into the controlled growth of 2D layered MoS2.
    24. 24
      Nie, Y.; Liang, C.; Cha, P.-R.; Colombo, L.; Wallace, R. M.; Cho, K. A Kinetic Monte Carlo Simulation Method of van Der Waals Epitaxy for Atomistic Nucleation-Growth Processes of Transition Metal Dichalcogenides. Sci. Rep. 2017, 7, 2977,  DOI: 10.1038/s41598-017-02919-2
      24
      A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides
      Nie Yifan; Liang Chaoping; Wallace Robert M; Cho Kyeongjae; Cha Pil-Ryung; Colombo Luigi
      Scientific reports (2017), 7 (1), 2977 ISSN:.
      Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.
    25. 25
      Ye, H.; Zhou, J.; Er, D.; Price, C. C.; Yu, Z.; Liu, Y.; Lowengrub, J.; Lou, J.; Liu, Z.; Shenoy, V. B. Toward a Mechanistic Understanding of Vertical Growth of van Der Waals Stacked 2D Materials: A Multiscale Model and Experiments. ACS Nano 2017, 11, 12780– 12788,  DOI: 10.1021/acsnano.7b07604
      25
      Toward a Mechanistic Understanding of Vertical Growth of van der Waals Stacked 2D Materials: A Multiscale Model and Experiments
      Ye, Han; Zhou, Jiadong; Er, Dequan; Price, Christopher C.; Yu, Zhongyuan; Liu, Yumin; Lowengrub, John; Lou, Jun; Liu, Zheng; Shenoy, Vivek B.
      ACS Nano (2017), 11 (12), 12780-12788CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Vertical stacking of monolayers via van der Waals (vdW) interaction opens promising routes toward engineering phys. properties of two-dimensional (2D) materials and designing atomically thin devices. However, due to the lack of mechanistic understanding, challenges remain in the controlled fabrication of these structures via scalable methods such as chem. vapor deposition (CVD) onto substrates. In this paper, we develop a general multiscale model to describe the size evolution of 2D layers and predict the necessary growth conditions for vertical (initial + subsequent layers) vs. in-plane lateral (monolayer) growth. An analytic thermodn. criterion is established for subsequent layer growth that depends on the sizes of both layers, the vdW interaction energies, and the edge energy of 2D layers. Considering the time-dependent growth process, we find that temp. and adatom flux from vapor are the primary criteria affecting the self-assembled growth. The proposed model clearly demonstrates the distinct roles of thermodn. and kinetic mechanisms governing the final structure. This model agrees with exptl. observations of various monolayer and bilayer transition metal dichalcogenides grown by CVD and provides a predictive framework to guide the fabrication of vertically stacked 2D materials.
    26. 26
      Tang, S.; Grundmann, A.; Fiadziushkin, H.; Ghiami, A.; Heuken, M.; Vescan, A.; Kalisch, H. Detailed Study on MOCVD of Wafer-Scale MoS2 Monolayers: From Nucleation to Coalescence. MRS Adv. 2022, 7, 751– 756,  DOI: 10.1557/s43580-022-00312-4
      26
      Detailed study on MOCVD of wafer-scale MoS2 monolayers: From nucleation to coalescence
      Tang, Songyao; Grundmann, Annika; Fiadziushkin, Hleb; Ghiami, Amir; Heuken, Michael; Vescan, Andrei; Kalisch, Holger
      MRS Advances (2022), 7 (30), 751-756CODEN: MARDCQ; ISSN:2059-8521. (Springer International Publishing AG)
      Metal-org. chem. vapor deposition (MOCVD) has become one of the most promising techniques for the large-scale fabrication of 2D transition metal dichalcogenide (TMDC) materials. Despite efforts devoted to the development of MOCVD for TMDC monolayers, the whole picture of the growth process has not been fully unveiled yet. In this work, we employ a com. AIXTRON CCS MOCVD tool for the deposition of MoS2 on sapphire using std. precursors and H2 as carrier gas. Adsorption and diffusion of Mo adatoms on the substrate are found to be decisive for nucleation. By lowering temp. from 650 to 450°C, a uniform distribution of nuclei on sapphire terraces is achieved. Full coalescence of MoS2 monolayers with limited bilayer formation (∼ 15%) is then realized at 700°C. This study highlights the importance of understanding the details of film formation mechanisms and developing multi-stage MOCVD processes for 2D TMDC films.
    27. 27
      Xu, J.; Srolovitz, D. J.; Ho, D. The Adatom Concentration Profile: A Paradigm for Understanding Two-Dimensional MoS2 Morphological Evolution in Chemical Vapor Deposition Growth. ACS Nano 2021, 15, 6839– 6848,  DOI: 10.1021/acsnano.0c10474
      27
      The Adatom Concentration Profile: A Paradigm for Understanding Two-Dimensional MoS2 Morphological Evolution in Chemical Vapor Deposition Growth
      Xu, Jiangang; Srolovitz, David J.; Ho, Derek
      ACS Nano (2021), 15 (4), 6839-6848CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The two-dimensional (2D) transition metal dichalcogenide (TMD) MoS2 possesses many intriguing electronic and optical properties. Potential technol. applications have focused much attention on tuning MoS2 properties through control of its morphologies during growth. In this paper, we present a unified spatial-temporal model for the growth of MoS2 crystals with a full spectrum of shapes from triangles, concave triangles, three-point stars, to dendrites through the concept of the adatom concn. profile (ACP). We perform a series of chem. vapor deposition (CVD) expts. controlling adatom concn. on the substrate and growth temp. and present a method for exptl. measuring the ACP in the vicinity of growing islands. We apply a phase-field model of growth that explicitly considers similar variables (adatom concn., adatom diffusion, and noise effects) and cross-validate the simulations and expts. through the ACP and island morphologies as a function of phys. controllable variables. Our calcns. reproduce the exptl. observations with high fidelity. The ACP is an alternative paradigm to conceptualize the growth of crystals through time, which is expected to be instrumental in guiding the rational shape engineering of MoS2 crystals.
    28. 28
      Fruchart, O.; Jaren, S.; Rothman, J. Growth Modes of W and Mo Thin Epitaxial (110) Films on (112̅0) Sapphire. Appl. Surf. Sci. 1998, 135, 218– 232,  DOI: 10.1016/s0169-4332(98)00261-x
      28
      Growth modes of W and Mo thin epitaxial (110) films on (11‾20) sapphire
      Fruchart, O.; Jaren, S.; Rothman, J.
      Applied Surface Science (1998), 135 (1-4), 218-232CODEN: ASUSEE; ISSN:0169-4332. (Elsevier Science B.V.)
      Growth modes of W(110) and Mo(110) epitaxial thin films grown with pulsed laser deposition (PLD) on (11‾20) sapphire were studied. The films properties were studied both in reciprocal space using RHEED and X-ray diffraction and in real space using at. force microscope (AFM). The conventionally used high temp. growth process, usually considered to be the best process - according to reciprocal space characterization - to minimize in-plane crystallites twinning and mosaicity, yields three-dimensional growth and rough surfaces. A sample continuously wedged up to a thickness of 600 Å was studied and only partial islands coalescence was obsd. An alternative optimized growth process is proposed, based on 200°C deposition, subsequent annealing, and further high temp. deposition, that enables the elimination of in-plane twinned-crystallites while yielding to nearly perfect W(110) or Mo(110) surfaces, from both crystallog. and geometrical points of view. The comparison with MBE grown films suggests that the reported behavior is not PLD-specific. A R(15 × 6) Mo surface reconstruction is reported, which had never been obsd. before.
    29. 29
      Kapur, J. N.; Sahoo, P. K.; Wong, A. K. C. A New Method for Gray-Level Picture Thresholding Using the Entropy of the Histogram. Comput. Vis. Graph Image Process 1985, 29, 273– 285,  DOI: 10.1016/0734-189x(85)90125-2
      There is no corresponding record for this reference.
    30. 30
      Fareed, R. S. Q.; Jain, R.; Gaska, R.; Shur, M. S.; Wu, J.; Walukiewicz, W.; Khan, M. A. High Quality InN/GaN Heterostructures Grown by Migration Enhanced Metalorganic Chemical Vapor Deposition. Appl. Phys. Lett. 2004, 84, 1892– 1894,  DOI: 10.1063/1.1686889
      30
      High-quality InN/GaN heterostructures grown by migration-enhanced metalorganic chemical vapor deposition
      Fareed, R. S. Qhalid; Jain, R.; Gaska, R.; Shur, M. S.; Wu, J.; Walukiewicz, W.; Khan, M. Asif
      Applied Physics Letters (2004), 84 (11), 1892-1894CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The authors report on the structural properties and optical and elec. characteristics of InN epitaxial layers grown on highly resistive GaN templates using migration-enhanced metalorg. chem. vapor deposition (MEMOCVD). The material quality of the InN improved significantly for layer thickness larger than 150 nm. The highest extd. value of the room-temp. electron mobility was close to 850 cm2/V-s for samples with electron carrier concn. of ∼4 × 1018 cm-3. The measured dependence of the electron mobility on electron concn. is interpreted using a model accounting for ionized impurity scattering, polar optical scattering, and compensation. The MEMOCVD-grown material exhibited stronger photoluminescence (PL) compared to InN deposited using conventional metalorg. chem. vapor deposition. Room-temp. PL spectra were similar to InN grown using mol.-beam epitaxy (MBE) with peak emission at 0.8 eV. The obtained results demonstrate the potential of the MEMOCVD technique for the deposition of high-quality InN epitaxial layers at reduced temps. comparable to those used in MBE growth.
    31. 31
      Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371– 6379,  DOI: 10.1021/cm5025662
      31
      Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition
      Wang, Shanshan; Rong, Youmin; Fan, Ye; Pacios, Merce; Bhaskaran, Harish; He, Kuang; Warner, Jamie H.
      Chemistry of Materials (2014), 26 (22), 6371-6379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Atm.-pressure CVD was used to grow monolayer MoS2 two-dimensional crystals at elevated temps. on silicon substrates with a 300. nm oxide layer. The authors' CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace sep. from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. The authors intentionally establish a sharp gradient of MoO3 precursor concn. on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temp. range. The shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve the authors' understanding of the factors that influence the growth of MoS2 domains and their shape evolution.
    32. 32
      Shinde, N. B.; Francis, B.; Ramachandra Rao, M. S.; Ryu, B. D.; Chandramohan, S.; Eswaran, S. K. Rapid Wafer-Scale Fabrication with Layer-by-Layer Thickness Control of Atomically Thin MoS2 Films Using Gas-Phase Chemical Vapor Deposition. APL Mater. 2019, 7, 081113,  DOI: 10.1063/1.5095451
      32
      Rapid wafer-scale fabrication with layer-by-layer thickness control of atomically thin MoS2 films using gas-phase chemical vapor deposition
      Shinde, Nitin Babu; Francis, Bellarmine; Ramachandra Rao, M. S.; Ryu, Beo Deul; Chandramohan, S.; Eswaran, Senthil Kumar
      APL Materials (2019), 7 (8), 081113/1-081113/8CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      Design and development of the growth-process for the prodn. of wafer-scale spatially homogeneous thickness controlled atomically thin transition metal dichalcogenides (TMDs) is one of the key challenges to realize modern electronic devices. Here, we demonstrate rapid and scalable synthesis of MoS2 films with precise thickness control via gas-phase chem. vapor deposition approach. We show that a monolayer MoS2 can be synthesized over a 2-in. sapphire wafer in a growth time as low as 4 min. With a linear growth rate of 1-layer per 4 min, MoS2 films with thicknesses varying from 1- to 5-layers with monolayer precision are produced. We propose that, in addn. to Raman spectroscopy, the energy splitting of exciton bands in optical-absorbance spectra may be another choice for layer thickness identification. With suitable precursor selection, our approach can facilitate the rapid synthesis of spatially homogeneous atomically thin TMDs on a large scale. (c) 2019 American Institute of Physics.
    33. 33
      Reifsnyder Hickey, D.; Nayir, N.; Chubarov, M.; Choudhury, T. H.; Bachu, S.; Miao, L.; Wang, Y.; Qian, C.; Crespi, V. H.; Redwing, J. M.; van Duin, A. C. T.; Alem, N. Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films. Nano Lett. 2021, 21, 6487– 6495,  DOI: 10.1021/acs.nanolett.1c01517
      33
      Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films
      Reifsnyder Hickey, Danielle; Nayir, Nadire; Chubarov, Mikhail; Choudhury, Tanushree H.; Bachu, Saiphaneendra; Miao, Leixin; Wang, Yuanxi; Qian, Chenhao; Crespi, Vincent H.; Redwing, Joan M.; van Duin, Adri C. T.; Alem, Nasim
      Nano Letters (2021), 21 (15), 6487-6495CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Engineering at.-scale defects is crucial for realizing wafer-scale, single-cryst. transition metal dichalcogenide monolayers for electronic devices. However, connecting at.-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and atomistic simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that at.-resoln. STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resoln. imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF reactive force field-based mol. dynamics simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical anal. of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.
    34. 34
      Liu, S. J.; Huang, H.; Woo, C. H. Schwoebel-Ehrlich Barrier: From Two to Three Dimensions. Appl. Phys. Lett. 2002, 80, 3295– 3297,  DOI: 10.1063/1.1475774
      34
      Schwoebel-Ehrlich barrier: from two to three dimensions
      Liu, S. J.; Huang, Hanchen; Woo, C. H.
      Applied Physics Letters (2002), 80 (18), 3295-3297CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The Schwoebel-Ehrlich barrier, the addnl. barrier for an adatom to diffuse down a surface step, dictates the growth modes of thin films. The conventional concept of this barrier is two dimensional (2D), with the surface step being one monolayer. We propose the concept of a three-dimensional (3D) Schwoebel-Ehrlich barrier, and identify the 2D to 3D transition, taking aluminum as a prototype and using the mol. statics method. Our results show that: (1) substantial differences exist between the 2D and 3D barriers; (2) the transition completes in four monolayers; and (3) there is a major disparity in the 3D barriers between two facets; further, alteration of this disparity using surfactants can lead to the dominance of surface facet against thermodn.
    35. 35
      Rao, D.; Biswas, B.; Acharya, S.; Bhatia, V.; Pillai, A. I. K.; Garbrecht, M.; Saha, B. Effects of Adatom Mobility and Ehrlich–Schwoebel Barrier on Heteroepitaxial Growth of Scandium Nitride (ScN) Thin Films. Appl. Phys. Lett. 2020, 117, 212101,  DOI: 10.1063/5.0027091
      35
      Effects of adatom mobility and Ehrlich-Schwoebel barrier on heteroepitaxial growth of scandium nitride (ScN) thin films
      Rao, Dheemahi; Biswas, Bidesh; Acharya, Shashidhara; Bhatia, Vijay; Pillai, Ashalatha Indiradevi Kamalasanan; Garbrecht, Magnus; Saha, Bivas
      Applied Physics Letters (2020), 117 (21), 212101CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Scandium nitride (ScN) is an emerging rock salt indirect bandgap semiconductor and has attracted significant interest in recent years for thermoelec. energy conversion, as a substrate for defect-free GaN growth, as a semiconducting component in single-cryst. metal/semiconductor superlattices for thermionic energy conversion, as well as for Al1-xScxN-based bulk and surface acoustic devices for 5G technologies. Most ScN film growth traditionally utilizes phys. vapor deposition techniques such as magnetron sputtering and mol. beam epitaxy, which results in stoichiometric films but with varying crystal quality, orientations, microstructures, and phys. properties. As epitaxial single-cryst. ScN films with smooth surfaces are essential for device applications, it is important to understand the ScN growth modes and parameters that impact and control their microstructure. In this Letter, we demonstrate that large adatom mobility is essential to overcome the Ehrlich-Schwoebel (E-S) and grain boundary migration barriers and achieve defect (voids, dislocations, stacking faults, etc.)-free single-cryst. ScN films. Using the substrate temp. to tune adatom mobility, we show that nominally single-cryst. ScN films are achieved when the homologous temp. is higher than ∼0.3. For homologous temps. ranging from 0.23 to 0.30, ScN films are found to exhibit significant structural voids in between pyramidal growth regions with multiple in-plane orientations resulting from addnl. lateral growth off the facets of the pyramids and broken epitaxy after ∼80 nm of growth. The in-depth discussion of the growth modes of ScN presented here explains its varying elec. and optical properties and will help achieve high-quality ScN for device applications. (c) 2020 American Institute of Physics.
    36. 36
      Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.-H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2. ACS Nano 2013, 7, 791– 797,  DOI: 10.1021/nn305275h
      36
      Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2
      Zhao, Weijie; Ghorannevis, Zohreh; Chu, Leiqiang; Toh, Minglin; Kloc, Christian; Tan, Ping-Heng; Eda, Goki
      ACS Nano (2013), 7 (1), 791-797CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Geometrical confinement effect in exfoliated sheets of layered materials leads to significant evolution of energy dispersion in mono- to few-layer thickness regime. Molybdenum disulfide (MoS2) was recently found to exhibit indirect-to-direct gap transition when the thickness is reduced to a single monolayer. Emerging photoluminescence (PL) from monolayer MoS2 opens up opportunities for a range of novel optoelectronic applications of the material. Here we report differential reflectance and PL spectra of mono- to few-layer WS2 and WSe2 that indicate that the band structure of these materials undergoes similar indirect-to-direct gap transition when thinned to a single monolayer. The transition is evidenced by distinctly enhanced PL peak centered at 630 and 750 nm in monolayer WS2 and WSe2, resp. Few-layer flakes are found to exhibit comparatively strong indirect gap emission along with direct gap hot electron emission, suggesting high quality of synthetic crystals prepd. by a chem. vapor transport method. Fine absorption and emission features and their thickness dependence suggest a strong effect of Se p-orbitals on the d electron band structure as well as interlayer coupling in WSe2.
    37. 37
      Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447– 3454,  DOI: 10.1021/nl3026357
      37
      Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers
      Gutierrez, Humberto R.; Perea-Lopez, Nestor; Elias, Ana Laura; Berkdemir, Ayse; Wang, Bei; Lv, Ruitao; Lopez-Urias, Florentino; Crespi, Vincent H.; Terrones, Humberto; Terrones, Mauricio
      Nano Letters (2013), 13 (8), 3447-3454CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Individual monolayers of metal dichalcogenides are atomically thin 2-dimensional crystals with attractive phys. properties different from those of their bulk counterparts. Here the authors describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temp. photoluminescence (PL). The Raman response as well as the luminescence as a function of the no. of S-W-S layers is also reported. The PL weakens with increasing no. of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet's center. The structure and chem. compn. of the platelet edges appear to be crit. for PL enhancement.
    38. 38
      McCreary, A.; Berkdemir, A.; Wang, J.; Nguyen, M. A.; Elías, A. L.; Perea-López, N.; Fujisawa, K.; Kabius, B.; Carozo, V.; Cullen, D. A.; Mallouk, T. E.; Zhu, J.; Terrones, M. Distinct Photoluminescence and Raman Spectroscopy Signatures for Identifying Highly Crystalline WS2 Monolayers Produced by Different Growth Methods. J. Mater. Res. 2016, 31, 931– 944,  DOI: 10.1557/jmr.2016.47
      38
      Distinct photoluminescence and Raman spectroscopy signatures for identifying highly crystalline WS2 monolayers produced by different growth methods
      McCreary, Amber; Berkdemir, Ayse; Wang, Junjie; Nguyen, Minh An; Elias, Ana Laura; Perea-Lopez, Nestor; Fujisawa, Kazunori; Kabius, Bernd; Carozo, Victor; Cullen, David A.; Mallouk, Thomas E.; Zhu, J.; Terrones, Mauricio
      Journal of Materials Research (2016), 31 (7), 931-944CODEN: JMREEE; ISSN:2044-5326. (Cambridge University Press)
      Transition metal dichalcogenides such as WS2 show exciting promise in electronic and optoelectronic applications. Significant variations in the transport, Raman, and photoluminescence (PL) can be found in the literature, yet it is rarely addressed why this is. In this report, Raman and PL of monolayered WS2 produced via different methods are studied and distinct features that indicate the degree of crystallinity of the material are obsd. While the intensity of the LA(M) Raman mode is found to be a useful indicator to assess the crystallinity, PL is drastically more sensitive to the quality of the material than Raman spectroscopy. We also show that even exfoliated crystals, which are usually regarded as the most pristine material, can contain large amts. of defects that would not be apparent without Raman and PL measurements. These findings can be applied to the understanding of other two-dimensional heterostructured systems.
    39. 39
      Shi, Y.; Groven, B.; Serron, J.; Wu, X.; Nalin Mehta, A.; Minj, A.; Sergeant, S.; Han, H.; Asselberghs, I.; Lin, D.; Brems, S.; Huyghebaert, C.; Morin, P.; Radu, I.; Caymax, M. Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics. ACS Nano 2021, 15, 9482– 9494,  DOI: 10.1021/acsnano.0c07761
      39
      Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High-Performance Nanoelectronics
      Shi, Yuanyuan; Groven, Benjamin; Serron, Jill; Wu, Xiangyu; Nalin Mehta, Ankit; Minj, Albert; Sergeant, Stefanie; Han, Han; Asselberghs, Inge; Lin, Dennis; Brems, Steven; Huyghebaert, Cedric; Morin, Pierre; Radu, Iuliana; Caymax, Matty
      ACS Nano (2021), 15 (6), 9482-9494CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      In view of its epitaxial seeding capability, c-plane single cryst. sapphire represents one of the most enticing, industry-compatible templates to realize manufacturable deposition of single cryst. two-dimensional transition metal dichalcogenides (MX2) for functional, ultrascaled, nanoelectronic devices beyond silicon. Despite sapphire being atomically flat, the surface topog., structure, and chem. termination vary between sapphire terraces during the fabrication process. To date, it remains poorly understood how these sapphire surface anomalies affect the local epitaxial registry and the intrinsic elec. properties of the deposited MX2 monolayer. Therefore, molybdenum disulfide (MoS2) is deposited by metal-org. chem. vapor deposition (MOCVD) in an industry-std. epitaxial reactor on two types of c-plane sapphire with distinctly different terrace and step dimensions. Complementary scanning probe microscopy techniques reveal an inhomogeneous cond. profile in the first epitaxial MoS2 monolayer on both sapphire templates. MoS2 regions with poor cond. correspond to sapphire terraces with uncontrolled topog. and surface structure. By intentionally applying a substantial off-axis cut angle (1° in this work), the sapphire terrace width and step height-and thus also surface structure-become more uniform across the substrate and MoS2 conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS2 device performance: the field-effect transistor variability reduces both within and across wafers at higher median electron mobility. Carefully controlling the sapphire surface topog. and structure proves an essential prerequisite to systematically study and control the MX2 growth behavior and capture the influence on its structural and elec. properties.
    40. 40
      Berkdemir, A.; Gutiérrez, H. R.; Botello-Méndez, A. R.; Perea-López, N.; Elías, A. L.; Chia, C.-I.; Wang, B.; Crespi, V. H.; López-Urías, F.; Charlier, J.-C.; Terrones, H.; Terrones, M. Identification of Individual and Few Layers of WS2 Using Raman Spectroscopy. Sci. Rep. 2013, 3, 1755,  DOI: 10.1038/srep01755
      40
      Identification of individual and few layers of WS2 using Raman spectroscopy
      Berkdemir, Ayse; Gutierrez, Humberto R.; Botello-Mendez, Andres R.; Perea-Lopez, Nestor; Elias, Ana Laura; Chia, Chen-Ing; Wang, Bei; Crespi, Vincent H.; Lopez-Urias, Florentino; Charlier, Jean-Christophe; Terrones, Humberto; Terrones, Mauricio
      Scientific Reports (2013), 3 (), 1755, 8 pp.CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The Raman scattering of single- and few-layered WS2 is studied as a function of the no. of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the no. of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calcns. were used to det. the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.
    41. 41
      Rosenberger, M. R.; Chuang, H.-J.; McCreary, K. M.; Li, C. H.; Jonker, B. T. Electrical Characterization of Discrete Defects and Impact of Defect Density on Photoluminescence in Monolayer WS2. ACS Nano 2018, 12, 1793– 1800,  DOI: 10.1021/acsnano.7b08566
      41
      Electrical characterization of discrete defects and impact of defect density on photoluminescence in monolayer WS2
      Rosenberger, Matthew R.; Chuang, Hsun-Jen; McCreary, Kathleen M.; Li, Connie H.; Jonker, Berend T.
      ACS Nano (2018), 12 (2), 1793-1800CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Transition-metal dichalcogenides (TMDs) are an exciting class of 2D materials that exhibit many promising electronic and optoelectronic properties with potential for future device applications. The properties of TMDs are expected to be strongly influenced by a variety of defects which result from growth procedures and/or fabrication. Despite the importance of understanding defect-related phenomena, there remains a need for quant. nanometer-scale characterization of defects over large areas in order to understand the relation between defects and obsd. properties, such as photoluminescence (PL) and elec. cond. In this work, the authors present conductive at. force microscopy measurements which reveal nanometer-scale electronically active defects in chem. vapor deposition-grown WS2 monolayers with defect d. varying from 2.3 × 1010 cm-2 to 4.5 × 1011 cm-2. Comparing these defect d. measurements with PL measurements across large areas (>20 μm distances) reveals a strong inverse relationship between WS2 PL intensity and defect d. They propose a model in which the obsd. electronically active defects serve as nonradiative recombination centers and obtain good agreement between the expts. and model.
    42. 42
      Li, J.; Su, W.; Chen, F.; Fu, L.; Ding, S.; Song, K.; Huang, X.; Zhang, L. Atypical Defect-Mediated Photoluminescence and Resonance Raman Spectroscopy of Monolayer WS2. J. Phys. Chem. C 2019, 123, 3900– 3907,  DOI: 10.1021/acs.jpcc.8b11647
      42
      Atypical Defect-Mediated Photoluminescence and Resonance Raman Spectroscopy of Monolayer WS2
      Li, Jiake; Su, Weitao; Chen, Fei; Fu, Li; Ding, Su; Song, Kaixin; Huang, Xiwei; Zhang, Lijie
      Journal of Physical Chemistry C (2019), 123 (6), 3900-3907CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Defects play an indispensable role in tuning the optical properties of 2-dimensional materials. The influence of defects on the luminescence and resonance Raman spectra of as-grown monolayer (1L) WS2 was studied. Increasing the d. of defects significantly lowers the excitonic binding energy by ≤110 meV. These defect-modified excitonic binding energies in 1L-WS2 strongly mediate the Raman resonance condition, resulting in unexpected Raman intensity variations in the LA(M), 2LA(M), and A'1(Γ) phonon modes. The sample with the highest d. of defects exhibits an almost temp.-independent resonance in different Raman modes at low temp., whereas the samples with low densities of defects exhibit a clear resonance with decreasing temp. This study will further increase the understanding of the role of defects in resonance Raman spectroscopy and of the phonon-exciton interaction in 1L-WS2.
    43. 43
      Marinov, D.; de Marneffe, J.-F.; Smets, Q.; Arutchelvan, G.; Bal, K. M.; Voronina, E.; Rakhimova, T.; Mankelevich, Y.; El Kazzi, S.; Nalin Mehta, A.; Wyndaele, P.-J.; Heyne, M. H.; Zhang, J.; With, P. C.; Banerjee, S.; Neyts, E. C.; Asselberghs, I.; Lin, D.; De Gendt, S. Reactive Plasma Cleaning and Restoration of Transition Metal Dichalcogenide Monolayers. npj 2D Mater. Appl. 2021, 5, 17,  DOI: 10.1038/s41699-020-00197-7
      43
      Reactive plasma cleaning and restoration of transition metal dichalcogenide monolayers
      Marinov, Daniil; de Marneffe, Jean-Francois; Smets, Quentin; Arutchelvan, Goutham; Bal, Kristof M.; Voronina, Ekaterina; Rakhimova, Tatyana; Mankelevich, Yuri; El Kazzi, Salim; Nalin Mehta, Ankit; Wyndaele, Pieter-Jan; Heyne, Markus Hartmut; Zhang, Jianran; With, Patrick C.; Banerjee, Sreetama; Neyts, Erik C.; Asselberghs, Inge; Lin, Dennis; De Gendt, Stefan
      npj 2D Materials and Applications (2021), 5 (1), 17CODEN: DMAAAH; ISSN:2397-7132. (Nature Research)
      The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique phys., optical, and chem. properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of "monolayer" TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temp. processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temp. in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temp. H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of H2 plasma cleaning and OCS restoration. The low-damage plasma cleaning based on H2 and OCS is very reproducible, fast (completed in a few minutes) and uses a 300 mm industrial plasma etch system qualified for std. semiconductor pilot prodn. This process is, therefore, expected to enable the industrial scale-up of 2D-based devices, co-integrated with silicon technol.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.cgd.2c01134.

    • Raman spectra of WS2 and WSe2 MLs, XPS spectra of MOCVD-grown and exfoliated WS2, reproducibility of WS2 ML growth, and a summary about the Raman intensity ratios and BL/ML coverage ratio (PDF)


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