Formation of a stable surface oxide in MnBi$_2$Te$_4$ thin films

TL;DR

This study reveals that MnBi$_2$Te$_4$ thin films form a stable, protective surface oxide after air exposure, preserving metallic conduction and minimal doping, which is promising for device stability and handling.

Contribution

The paper demonstrates that MnBi$_2$Te$_4$ thin films develop a self-limiting oxide layer that maintains their electronic properties, offering insights into air-stability and passivation strategies.

Findings

A 2 nm oxide passivates the surface after 2 days in air.

The oxide layer preserves the underlying layers and metallic conduction.

Work function decreases from 4.4 eV to 4.0 eV after oxidation.

Abstract

Understanding the air-stability of MnBi$_2$Te$_4$ thin films is crucial for the development and long-term operation of electronic devices based around magnetic topological insulators. In the present work, we study MnBi$_2$Te$_4$ thin films upon exposure to 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 air exposure a 2 nm thick oxide passivates the surface, corresponding to 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 air exposure. Furthermore, the work function decreases from 4.4 eV for pristine MnBi$_2$Te$_4$ 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 MnBi$_2$Te$_4$ devices.