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1. Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire

   https://doi.org/https://doi.org/10.1021/acsnano.0c06750  

   Chubarov, Mikhail ; Choudhury, Tanushree H. ; Hickey, Danielle Reifsnyder ; Bachu, Saiphaneendra ; Zhang, Tianyi ; Sebastian, Amritanand ; Bansal, Anushka ; Zhu, Haoyue ; Trainor, Nicholas ; Das, Saptarshi ; et al ( January 2021 , ACS nano)

   null (Ed.)

   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 crystallographic 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. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis 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 density 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. 

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2. Epitaxial growth of wafer-scale transition metal dichalcogenide monolayers by metalorganic chemical vapor deposition

   https://doi.org/10.1109/EDTM53872.2022.9797981  

   Trainor, Nicholas ; Chen, Chen ; Zhu, Haoyue ; Mc Knight, Thomas V. ; Choudhury, Tanushree H. ; Redwing, Joan M. ( March 2022 , 2022 6th IEEE Electron Devices Technology & Manufacturing Conference (EDTM))

   The epitaxial growth of wafer-scale semiconducting TMDs monolayers (MoS 2 , WS 2 , WSe 2 ) on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD) is demonstrated and the resulting structural and optical properties of the films are compared to elucidate trends based on metal and chalcogen species. The sulfur based TMDs exhibit improved epitaxy, fewer defects and increased photoluminescence intensity on sapphire compared to WSe 2 which is attributed to a smaller effective lattice mismatch and improved stability. 

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3. The role of carbon and C-H neutralization in MOCVD β-Ga2O3 using TMGa as precursor

   https://doi.org/10.1063/5.0153626  

   Meng, Lingyu ; Bhuiyan, A F ; Zhao, Hongping ( June 2023 , Applied Physics Letters)

   In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications. 

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4. Benchmarking monolayer MoS2 and WS2 field-effect transistors

   https://doi.org/10.1038/s41467-020-20732-w  

   Sebastian, Amritanand ; Pendurthi, Rahul ; Choudhury, Tanushree H. ; Redwing, Joan M. ; Das, Saptarshi ( January 2021 , Nature Communications)

   Here we benchmark device-to-device variation in field-effect transistors (FETs) based on monolayer MoS₂and WS₂films grown using metal-organic chemical vapor deposition process. Our study involves 230 MoS₂FETs and 160 WS₂FETs with channel lengths ranging from 5 μm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of 2D FETs across 1 × 1 cm²chips owing to high quality and uniform growth of these TMDs followed by clean transfer onto device substrates. We are able to demonstrate record high carrier mobility of 33 cm² V⁻¹ s⁻¹in WS₂FETs, which is a 1.5X improvement compared to the best reported in the literature. Our experimental demonstrations confirm the technological viability of 2D FETs in future integrated circuits.

    

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5. Role of tilt grain boundaries on the structural integrity of WSe 2 monolayers

   https://doi.org/10.1039/D2CP03492A  

   Sakib, Nuruzzaman ; Paul, Shiddartha ; Nayir, Nadire ; van Duin, Adri C. ; Neshani, Sara ; Momeni, Kasra ( November 2022 , Physical Chemistry Chemical Physics)

   Transition metal dichalcogenides (TMDCs) are potential materials for future optoelectronic devices. Grain boundaries (GBs) can significantly influence the optoelectronic properties of TMDC materials. Here, we have investigated the mechanical characteristics of tungsten diselenide (WSe 2 ) monolayers and failure process with symmetric tilt GBs using ReaxFF molecular dynamics simulations. In particular, the effects of topological defects, loading rates, and temperatures are investigated. We considered nine different grain boundary structures of monolayer WSe 2 , of which six are armchair (AC) tilt structures, and the remaining three are zigzag (ZZ) tilt structures. Our results indicate that both tensile strength and fracture strain of WSe 2 with symmetric tilt GBs decrease as the temperature increases. We revealed an interfacial phase transition for high-angle GBs reduces the elastic strain energy within the interface at finite temperatures. Furthermore, brittle cracking is the dominant failure mode in the WSe 2 monolayer with tilted GBs. WSe 2 GB structures showed more strain rate sensitivity at high temperatures than at low temperatures. 

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