arXiv subjects

Mikhail Chubarov

Publications and source records attributed to Mikhail Chubarov.

4 recordsLinked to original sources

arXiv2020

Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-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 the microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Imaging reveals the films to have nearly a single orientation with imperfectly stitched domains. Through atomic-resolution imaging and ReaxFF reactive force field-based molecular dynamics simulations, we observe two types of translational mismatch and discuss their atomic structures and origin. Our results indicate that the mismatch results from relatively fast growth rates. Through statistical analysis of >1300 facets, we demonstrate that the macrostructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multi-micrometer length scales.

cond-mat.mtrl-sci↗

arXiv2020

Wafer-scale epitaxial growth of single orientation WS2 monolayers on sapphire

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as tungsten sulfide 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 avoid the formation of metallic inversion domain boundaries (IDBs) which are a common feature of layered chalcogenides. Here we demonstrate fully-coalesced single orientation tungsten sulfide monolayers on 2-inch diameter c-plane sapphire by metalorganic chemical vapor deposition using a multi-step growth process. High growth temperatures and sulfur/metal ratios were required to reduce domain misorientation and achieve epitaxial tungsten sulfide monolayers with low in-plane rotational twist (0.09 deg). Transmission electron microscopy analysis reveals that the tungsten sulfide monolayers lack IDBs but instead have translational boundaries that arise when tungsten sulfide domains with slightly off-set lattices merge together. By adjusting the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The preferred orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions promote surface diffusion and oriented attachment. The transferred tungsten sulfide monolayers show neutral and charged exciton emission at 80K with negligible defect-related luminescence. Back-gated tungsten sulfide field effect transistors exhibited mobility of 16 cm2/Vs. The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching that of exfoliated flakes.

cond-mat.mtrl-sci↗

arXiv2019

Epitaxial Growth of 2D Layered Transition Metal Dichalcogenides

Transition metal dichalcogenide (TMD) monolayers and heterostructures have emerged as a compelling class of materials with transformative new science that may be harnessed for novel device technologies. These materials are commonly fabricated by exfoliation of flakes from bulk crystals, but wafer-scale epitaxy of single crystal films is required to advance the field. This article reviews the fundamental aspects of epitaxial growth of van der Waals bonded crystals specific to TMD films. The structural and electronic properties of TMD crystals are initially described along with sources and methods used for vapor phase deposition. Issues specific to TMD epitaxy are critically reviewed including substrate properties and film-substrate orientation and bonding. The current status of TMD epitaxy on different substrate types is discussed along with characterization techniques for large area epitaxial films. Future directions are proposed including developments in substrates, in situ and full wafer characterization techniques and heterostructure growth.

cond-mat.mtrl-sci↗

arXiv2018

Considerations for utilizing sodium chloride in epitaxial molybdenum disulfide

The utilization of alkali salts, such as NaCl and KI, have enabled the successful growth of large single domain and fully coalesced polycrystalline two-dimensional (2D) transition metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties are not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkali-assisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in as-grown large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and a high density of nanoscale MoS2 particles (4/μm2). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ~20x and degrades transistor performance.

cond-mat.mtrl-sci↗