Phase and orientation evolution of Bismuth during heteroepitaxy on ML-MoS₂/Sapphire (0001) surfaces

Abstract

Due to the semimetal properties of Bismuth (Bi), this material is considered as one of the candidates for forming ohmic contacts with molybdenum disulfide (MoS2) in advanced 2D-based MOS transistors. Low contact resistances have already been demonstrated using Bismuth contacts on monolayer MoS2 (ML-MoS2) transistors. However, the precise crystalline structure as well as the heteroepitaxy of Bi is generally overlooked and the insight of research on Bi thin film crystal phase control and orientation on ML-MoS2 lacks. In this framework, we conducted an in-depth study on the control of the crystalline phase and orientation of Bi thin film on ML-MoS2 by molecular beam epitaxy (MBE) technique. By careful control of the thickness and growth temperature, we highlighted different phase and orientation transitions during the heteroepitaxy process of Bi on ML-MoS2. Ultrathin layer and low temperature ( 4 nm & < 110 °C) will stabilize the α-phase (metal) of Bi thin film on ML-MoS2 while for thicker layers the β-phase (semimetal) Bi is obtained which is beneficial for the 2D material Ohmic contact, but an orientation growth front transition is happening at 110 °C between the (111) and (110) planes.