Formation of a Stable Surface Oxide in MnBi₂Te₄ Thin Films

Understanding the air stability of MnBi₂Te₄ thin films is crucial for the development and long-term operation of electronic devices based on magnetic topological insulators. In the present work, we study MnBi₂Te₄ thin films upon exposure to the atmosphere using a combination of synchrotron-based photoelectron spectroscopy, room-temperature electrical transport, and atomic force microscopy to determine the oxidation process. After 2 days of air exposure, a 2 nm thick oxide passivates the surface, corresponding to the oxidation of only the top two surface layers, with the underlying layers preserved. This protective oxide layer results in samples that still exhibit metallic conduction even after several days of air exposure. Furthermore, the work function decreases from 4.4 eV for pristine MnBi₂Te₄ to 4.0 eV after the formation of the oxide, along with only a small shift in the core levels, indicating minimal doping as a result of air exposure. With the oxide confined to the top surface layers, and the underlying layers preserved, it may be possible to explore new avenues in how to handle, prepare, and passivate future MnBi₂Te₄ devices.