Device Physics of Two-Dimensional Crystalline Materials

Abstract

The study of two-dimensional (2D) electrons in condensed matter physics has two milestones: quantum Hall effects in semiconductor heterostructures and superconductivity in cuprates. In both, the 2D electrons are embedded in complex material structures. The discovery of graphene allows for the isolation of 2D electrons in their simplest form, existing merely in a single crystalline atomic layer and exposing the states to external controls. Triggered by graphene, the interest in a variety of other 2D crystals, such as atomically thin transition metal dichalcogenides (TMDs), has recently exploded, revising the physics of 2D electrons in the ultimate confinement limit. At the same time, constructing heterostructures based on 2D crystals, either by placing the atomic layer on a special substrate or by combining different 2D crystals together, promises a bright future for observing novel electronic phenomena and engineering new device functionalities. In this thesis, the device physics of atomically thin semiconducting TMDs, such as MoS2, MoSe2 and WSe2, and semi-metallic graphene will be explored, in the form of both individual crystals and heterostructures. We show how these 2D crystalline systems can give rise to unique electronic behaviors and also application potentials by demonstrating the possibility of 2D-crystal based valleytronics, nanoscale lasing, and multiple photo-excited carrier collection. Subsequently, we demonstrate a scalable way to synthesize TMD monolayers, as well as a method to achieve epitaxial growth within an atomic plane, leading to the formation of lateral semiconductor heterostructures in 2D.