Chemical vapor deposition of two-dimensional transition metal dichalcogenides and their applications in metal-oxide-semiconductor field-effect transistors

Abstract

Atomically thin two-dimensional (2D) transitional metal dichalcogenides (TMDCs) offer a potential ultimate-scaling solution for metal-oxide-semiconductor field-effect transistors (MOSFETs). However, the Schottky barrier height (SBH) at the metal/TMDC interface leads to high contact resistance, which remains a challenging issue that hinders the performance of these devices.To utilize TMDCs for practical applications, massive production of high-quality wafer-scale TMDCs at low cost are necessary. Here, I present the chemical vapor deposition (CVD) growth of various TMDCs, including MoTe2, MoS2, WS2, and NbS2 as well as other low dimensional materials such as one-dimensional (1D) Mo6Te6 and 2D black phosphorus. Among them, MoS2 and WS2 can be synthesized in monolayer (sub-1-nm) thickness, which is a critical requirement for ultra-scaled MOSFETs. Furthermore, wafer-scale monolayer WS2 can be synthesized on 2-inch c-plane sapphire wafers with good reproducibility. With the capability of growing these 2D materials, 2H/1T’ MoTe2 homojunctions, 2H-MoS2/1T’-MoTe2, 2H-WS2/1T’-MoTe2, 2H-WS2/1T’-WTe2, and 2H-WS2/2H-NbS2 heterostructures are realized. The second part of my work focuses on the application of semiconducting 2D TMDCs in MOSFETs, particularly addressing the issue of high contact resistance, which has been a bottleneck of their electrical performance. Three different strategies are demonstrated to lower the contact resistance, including phase engineering of semiconducting 2H- and semi-metallic 1T’-MoTe2 homojunctions, suppression of metal-induced gap states (MIGSs) in monolayer WS2 by using low work function semi-metallic Bi contacts, and Fermi leveling depinning of monolayer WS2 through the insertion of ultra-thin 2D buffer layer at metal/WS2 interfaces. With the substantially reduced contact resistance, devices benchmark the electrical performance of CVD grown monolayer WS2 MOSFETs and are promising for future applications for high-performance transistors.