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1. Effect of growth temperature on the microstructure and properties of epitaxial MoS2 monolayers grown by metalorganic chemical vapor deposition

   https://doi.org/10.1116/6.0003296  

   Chen, Chen; Trainor, Nicholas; Kumari, Shalini; Myja, Henrik; Kümmell, Tilmar; Zhang, Zhiyu; Zhang, Yuxi; Bisht, Anuj; Sadaf, Muhtasim Ul; Sakib, Najam U.; et al (March 2024, Journal of Vacuum Science & Technology A)

   Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS2 monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS2 on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS2 monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900–1000 °C). The density of low-angle grain boundaries in the MoS2 monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A1g peak, which are typically indicative of a general reduction in defects in MoS2. However, the best transport properties (e.g., mean field-effect mobility mFE = 17.3 cm2/V s) were obtained in MoS2 monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS2 2D-FETs. 

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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. Formation of metal vacancy arrays in coalesced WS2 monolayer films

   Hickey, Danielle Reifsnyder; Yilmaz, Dundar E; Chubarov, Mikhail; Bachu, Saiphaneendra; Choudhury, Tanushree H; Miao, Leixin; Qian, Chenhao; Redwing, Joan M; van Duin, Adri C; Alem, Nasim (November 2020, 2D materials)

   null (Ed.)

   Defects have a profound impact on the electronic and physical properties of crystals. For two-dimensional (2D) materials, many intrinsic point defects have been reported, but much remains to be understood about their origin. Using scanning transmission electron microscopy imaging, this study discovers various linear arrays of W-vacancy defects that are explained in the context of the crystal growth of coalesced, monolayer WS2. Atomistic-scale simulations show that vacancy arrays can result from steric hindrance of bulky gas-phase precursors at narrowly separated growth edges, and that increasing the edge separation leads to various intact and defective growth modes, which are driven by competition between the catalytic effects of the sapphire substrate and neighboring growth edge. Therefore, we hypothesize that the arrays result from combined growth modes, which directly result from film coalescence. The connections drawn here will guide future synthetic and processing strategies to harness the engineering potential of defects in 2D monolayers. 

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4. Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization

   https://doi.org/10.1002/adma.202201387  

   Wang, Ping; Lee, Woncheol; Corbett, Joseph_P; Koll, William_H; Vu, Nguyen_M; Laleyan, David_Arto; Wen, Qiannan; Wu, Yuanpeng; Pandey, Ayush; Gim, Jiseok; et al (April 2022, Advanced Materials)

   Abstract Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface‐mediated growth process for the controlled synthesis of high‐quality monolayer hBN is proposed and further demonstrated. It is discovered that the in‐plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single‐domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep‐ultraviolet emission at 6.12 eV stems from the 1s‐exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer‐scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices. 

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5. A First-Principles Study on the Electronic, Thermodynamic and Dielectric Properties of Monolayer Ca(OH)2 and Mg(OH)2

   https://doi.org/10.3390/nano12101774  

   Rostami Osanloo, Mehrdad; Oyekan, Kolade A.; Vandenberghe, William G. (May 2022, Nanomaterials)

   We perform first-principles calculations to explore the electronic, thermodynamic and dielectric properties of two-dimensional (2D) layered, alkaline-earth hydroxides Ca(OH)2 and Mg(OH)2. We calculate the lattice parameters, exfoliation energies and phonon spectra of monolayers and also investigate the thermal properties of these monolayers, such as the Helmholtz free energy, heat capacity at constant volume and entropy as a function of temperature. We employ Density Functional Perturbation Theory (DFPT) to calculate the in-plane and out-of-plane static dielectric constant of the bulk and monolayer samples. We compute the bandgap and electron affinity values using the HSE06 functional and estimate the leakage current density of transistors with monolayer Ca(OH)2 and Mg(OH)2 as dielectrics when combined with HfS2 and WS2, respectively. Our results show that bilayer Mg(OH)2 (EOT∼0.60 nm) with a lower solubility in water offers higher out-of-plane dielectric constants and lower leakage currents than does bilayer Ca(OH)2 (EOT∼0.56 nm). Additionally, the out-of-plane dielectric constant, leakage current and EOT of Mg(OH)2 outperform bilayer h-BN. We verify the applicability of Anderson’s rule and conclude that bilayers of Ca(OH)2 and Mg(OH)2, respectively, paired with lattice-matched monolayer HfS2 and WS2, are effective structural combinations that could lead to the development of innovative multi-functional Field Effect Transistors (FETs). 

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