In-situ Fermi level tuning of semiconductor thin films on graphene during scanning tunneling microscopy studies

Doping can often drive the system into interesting quantum phases such as superconductivity or the pseudo-gap phase. Typically, doping is controled during fabrication by adjusting the dopant impurity ratios or tuning the chemical composition of the main elements. For a detailed atomically-resolved study of a material with a complex phase diagram, it is beneficial to implement in-situ Fermi level (E_F) tuning that can be done by gating. Gating is a somewhat robust technique in transport measurements, but combining it with epitaxial film growth and scanning tunneling microscopy (STM) presents many technical challenges. Here, we report the design of a robust back-gating device for a versatile thin film growth and subsequent E_F tuning inside STM. Graphene serves as the platform for epitaxial growth, while a range of materials can be used as an insulating gate depending on the particular experiment. We demonstrate successful E_F tuning in both Bi₂Te₃ and SnTe films in STM. We study the strength of the effect in terms of Schottky barriers between materials, film thickness, and quality of the gate and report a magnetic field study of back-gated thin films of topological insulators.