Control over epitaxy and the role of the InAs/Al interface in hybrid two-dimensional electron gas systems

Abstract

In-situ synthesised 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. We present a study of Al films on shallow InAs two-dimensional electron gas systems grown by molecular beam epitaxy, with focus on control of the Al crystal structure. We identify the dominant grain types present in our Al films and show that the formation of grain boundaries can be significantly reduced by controlled roughening of the epitaxial interface. Finally, we demonstrate 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.

Measurements of Phase Dynamics in Planar Josephson Junctions and SQUIDs

“…The 8 nm InAs layer hosted a two-dimensional electron gas (2DEG), buried 13.4 nm below the semiconductor surface, as measured by transmission electron microscopy. 51 A 15 nm layer of Al was deposited onto the semiconductor surface, in situ without breaking vacuum in the growth chamber. Measurements of a gated Hall bar in this material showed a peak mobility of 18000 cm 2 V −1 s −1 at an electron sheet density of 8 • 10 11 cm −2 .…”

“…Figure 1(a) shows a false-colored scanning electron micrograph of Device 1, the principal device under study, which consisted of a planar SQUID fabricated in a heterostructure of InAs (pink) and epitaxial Al (blue). 50,51 The device was covered by a HfO 2 dielectric layer, onto which Au gate electrodes (yellow) were deposited.…”

“…Experiments were performed on four devices (Devices 1 to 4) defined in the same epitaxial heterostructure of InAs and Al, 3,49 measured in a dilution refrigerator with a base temperature below 10 mK. Figure 1 The measurement setup used to measure switching currents is schematically depicted in Fig.…”