Complexity in Growth Mechanism of MnBi2Te4 using Molecular Beam Epitaxy.

The successful growth of bulk single crystalline intrinsic magnetic topological insulators (MTI) such as MnBi2Te4 has provided a platform to understand exotic quantum phenomena of Quantum Anomalous Hall effect and Axion insulator state. There is a strong driving force behind using molecular beam epitaxy (MBE) to control layer-by-layer growth of MnBi2Te4 as well as the MnBi2Te4/Bi2Te3 superlattices to exploit the rich topological quantum phase diagram. To achieve this goal, it is critical to gain an understanding of the growth kinetics which is a very non-trivial task due to the participation of many chemical species. By combining MBE growth with in-situ scanning tunneling microscopy and ex-situ atomic force microscopy, we report the observation of rich combinations of step heights which represent intermediate phases of MBT in the process of forming a septuple layer. STM study reveals ubiquitous Mn_Bi anti-site defects (Mn in the Bi-layer) and possibly Bi_Mn anti-site defects, suggesting an entropically driven intermixing of Mn and Bi in the septuple layer. We argue that the observed rich step structure on the surface is linked to this entropically driven intermixing in the septuple layer.