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 (\(\le\) 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.