Characterizing the Morphology of the Different Grown Homo/Hetero TMD Structures By Controlling Parameters – a Multiscale Computational Approach

Abstract

Transition Metal Dichalcogenides (TMDs) and their homo/heterostructures have demonstrated excellent mechanical, optical, and electrical properties, which make this family of materials promising candidates for various future applications. TMD films are now being produced mainly by Chemical Vapor Deposition(CVD) and Molecular Beam Epitaxy(MBE), where it was only mechanical exfoliation technique in the beginning. A major drawback of mechanical exfoliation technique was that it could not be used for the mass production of the TMD layer similar to the graphene. However, CVD & MBE synthesis techniques for TMDs are still in a very early stage of development, leading to a poor understanding of what impacts the growth mechanism and post-growth products. Furthermore, a fundamental understanding of what controls morphology over a range of parameters (temperature, pressure, metal: chalcogen flux ratio) is still lacking. The primary reason for the presence of these challenges in a higher degree of complexities than graphene or any other single planar film is that TMDs are multi-planar films, which increases the number of variables involved and ultimately decreasing the control over the growth process. To gain fundamental insight into the growth mechanism of TMDs, we performed Grand Canonical Monte Carlo(GCMC) simulation of growth process over varying substrates. Besides GCMC, we developed in-house Kinetic Monte Carlo (KMC) code to obtain further insights at the macro-domain taking into account spatial and temporal scales. Density Functional Theory (DFT) calculations provide input for the KMC modeling. We have mainly considered the Homo/Heterostructures of MoS2/WS2to establish the relation between different controlling parameters (i.e., temperature, pressure, flux ratios) and the resulting morphology along with defect concentration of underlying substrates. Our preliminary Monte Carlo study captures the formation of the MoS2on the perfect MoS2surface starting with the individual Mo and S flux. We find that the temperature plays a crucial role in governing growth mechanisms. Further investigation is on-going on the growth physics of MoS2and other TMDs (e.g., WS2) on pristine and defective substrates. Our modeling approach provides guidelines for the experimentalists for the optimal design of TMD bilayer growth.