UTILIZING COMPLEX OXIDE SUBSTRATES TO CONTROL CARRIER CONCENTRATION IN LARGE-AREA MONOLAYER MOS2 FILMS

Abstract

Band-gap engineering is central to the design of heterojunction devices. It is a powerful technique to overcome the constrain of natural material properties and realize novel functionalities of materials. Monolayer transitional metal dichalcogenides (TMDs) provides unprecedented opportunities for band-gap engineering with their atomically-thin thickness, direct bandgap in the near-infrared to the visible region, large excitonic effect, and strong spin-orbit coupling.For heterojunctions involving monolayer TMDs, the electronic properties of the atomically thin films are widely tunable by external perturbations. For instance, the carrier concentration of monolayer MoS2 can vary significantly depending on the amount of charge transfer between the MoS2 and the substrate. This makes substrates with a range of charge neutrality levels—as is the case for complex oxide substrates—a powerful addition to electrostatic gating or chemical doping to control the doping of overlying MoS2 layers. In this thesis, I demonstrate this approach (charge transfer doping) by growing monolayer MoS2 on perovskite (SrTiO3 and LaAlO3), spinel (MgAl2O4), and SiO2 substrates with multi-inch uniformity. The as-grown MoS2 films on these substrates exhibit a controlled, reproducible, and uniform carrier concentration ranging from (1-4) ×1013 cm-2, depending on the oxide substrate employed. The observed carrier concentrations are further confirmed by our density-functional theory calculations based on ab initio mismatched interface theory (MINT). This approach is relevant to large-scale heterostructures involving monolayer-thick materials in which it is desired to precisely control carrier concentrations for applications.