Advances in MOCVD for Wafer-Scale Synthesis of TMD Monolayers

Metalorganic chemical vapor deposition (MOCVD) has emerged as a promising technique for wafer-scale synthesis of transition metal dichalcogenides (TMDs) for device applications. Two general approaches have been pursued: direct growth on oxide-covered substrates at BEOL-compatible temperatures or high temperature epitaxy on single crystal substrates followed by layer transfer. Our work has focused on the later approach with the goal of achieving wafer-scale single crystal TMD films that can be transferred and integrated at BEOL conditions.<br/>It is challenging to reproducibly deposit TMD monolayer films over wafer-scale substrates without additional bilayer islands. Defects in the monolayer serve as nucleation sites for bilayer formation. Once bilayers nucleate, they grow rapidly aided by enhanced adatom diffusion on the monolayer surface. Our work has therefore focused on minimizing defects in the epitaxial TMD monolayers and developing in situ techniques to track and control the process for true layer-by-layer growth. <br/>These efforts are illustrated for epitaxial growth of MoS₂ and WSe₂ on 50 mm diameter c-plane sapphire using metal hexacarbonyls and hydride chalcogen sources in a H₂ carrier gas. The epitaxial orientation of the TMD is found to be strongly dependent on the pre-growth annealing ambient (H₂ vs H₂S/H₂Se) and the growth temperature which can be tuned to minimize inversion domains and high angle grain boundaries which negatively impact field-effect mobility. Spectroscopic ellipsometry is demonstrated as a promising in situ monitoring tool for TMD growth, enabling real time measurements of monolayer and bilayer surface coverage enabling improved control of layer number as well as insights into the epitaxial growth process.