Electronic Transport Measurements of Two-Dimensional Semiconductor Homostructures

Abstract

The ever-increasing demand for emerging technologies requires advancements in semiconductor devices beyond what is hitherto envisaged. Two-dimensional (2D) semiconductors have recently gained considerable attention for field-effect transistor technologies. However, there are various challenges regarding the growth of 2D semiconductors and the optimisation of their channel properties. Specifically, the growth of 2D semiconductors by chemical vapour deposition (CVD) can result in bilayer patches and twisted layered structures, leading to the formation of homojunctions. These homojunctions may exhibit non-uniform charge transport due to various growth-related defects. Therefore, it is imperative to investigate charge transport across such homojunctions in 2D semiconductor field-effect transistor devices. In this thesis, charge transport across monolayer-bilayer homojunctions in twisted 2D WS2-homostructures is demonstrated. Devices were fabricated from WS2 flakes containing monolayer-bilayer junctions with twist angles of 0◦ and 60◦. Transport measurements conducted at room temperature revealed rectifying behaviour across monolayer-bilayer WS2-homojunctions. This rectifying behaviour is attributed to the different band gaps and work functions of monolayer WS2 and bilayer WS2. Additionally, it is likely that intrinsic defects around the nucleation site, originating from the CVD growth, increased the doping concentration in the bilayer region. Together, these factors affect the charge transport across the homojunctions and result in the observed diodic behaviour. These findings contribute to a deeper understanding of the physics of 2D semiconductor devices, thereby highlighting the pivotal role of these devices in shaping future technological developments.