Control over Epitaxy and the Role of the InAs/Al Interface in Hybrid Two-Dimensional Electron Gas Systems

In situ synthesized semiconductor/superconductor hybrid structures became an important material platform in condensed matter physics. Their development enabled a plethora of novel quantum transport experiments with focus on Andreev and Majorana physics. The combination of InAs and Al has become the workhorse material and has been successfully implemented in the form of one-dimensional structures and two-dimensional electron gases. In contrast to the well-developed semiconductor parts of the hybrid materials, the direct effect of the crystal nanotexture of Al films on the electron transport still remains unclear. This is mainly due to the complex epitaxial relation between Al and the semiconductor. Here, we present characterization of Al thin films grown on shallow InAs two-dimensional electron gas systems by molecular beam epitaxy. Using a growth approach based on an intentional roughening of the epitaxial interface, we demonstrate growth of grain-boundary-free Al. We show that the implemented roughening does not negatively impact either the electron mobility of the two-dimensional electron gas or the basic superconducting properties of the proximitized system. This is an important step in understanding the role of properties of the InAs/Al interface in hybrid devices. Ultimately, our results provide a growth approach to achieve a high-degree of epitaxy in lattice-mismatched materials.