(Invited) Wafer-Scale Epitaxy of Transition Metal Dichalcogenides By MOCVD

Abstract

Monolayer semiconducting transition metal dichalcogenides (MX2, M=W, Mo and X=S, Se) exhibit novel electronic and optical properties. Availability of epitaxial single-crystal monolayers over a wafer scale is a major hurdle in integrating these materials with existing device fabrication processes. Our approach focusses on synthesis of these monolayers using gas source precursors, like those used in traditional III-V semiconductor industry. The precursors are kept in temperature and pressure-controlled bubblers outside the growth chamber and can be metered independently into the growth zone with precision. Uniform epitaxial binary TMD monolayers including MoS2, WS2, WSe2 and MoSe2 have been deposited on 2” sapphire wafers in a cold-wall CVD reactor using metal hexacarbonyl and hydride chalcogen precursors. A multi-step growth process comprising of precursor modulation was developed for WSe2 to independently control nucleation density and the lateral growth rate of monolayer domains on the sapphire substrate [1]. For the sulfides, additional temperature modulation was introduced to increase crystalline quality. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to sapphire with narrow X-ray full-width-at-half-maximum indicating minimal rotational misorientation of domains within the basal plane [2]. Deposition of these materials in the same reactor provides insight into the factors controlling their growth, which in essential for the growth of heterostructures and/or alloys. Transmission electron microscopy analysis of these materials revealed additional information about the coalesced film structure. For instance, ~95 % single oriented WS2 domains were observed when the film growth rate was modulated. For WS2, it was also confirmed that despite translational line defects present in the film, mirror boundaries were predominantly absent. WS2 also showed no defect related photoluminescence peak at 80 K, indicating its high quality.In this talk the factors affecting the growth of TMDs (MX2, M=W, Mo and X=S, Se) will be presented. The role of the substrate surface in the nucleation and growth of these TMDs will be discussed. In addition, the structural and optoelectronic characteristics for these films will also be presented.The authors acknowledge financial support of the U.S. National Science Foundation through the Penn State 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and EFRI 2-DARE Grant EFRI-1433378.[1] Zhang X, Choudhury TH, Chubarov M et al., 2018. Nano Lett. 18(2):1049–56[2] Chubarov M, Choudhury TH, Zhang X et al., 2018. Nanotechnology. 29(5):55706