Proximity Superconductivity in Indium Arsenide Two-Dimensional Electron Gas Devices

Abstract

Of the many theoretical proposals for quantum computers, topological quantum computing is unique in its resistance to decoherence and the reliability of its gate operations. One proposed method for achieving these topological qubits is to harness the unusual non-Abelian exchange statistics of quasiparticle excitations known as Majorana bound states. Historically, research devoted to realizing these states has primarily been in nanowires, but purely one-dimensional devices are limited in their applications. Two-dimensional electron gas devices are an alternative with the benefit of future scalability and increased options for device geometries. To this end, we developed InAs/AlGaSb surface quantum well devices compatible with the proximity-induced superconductivity required to realize a Majorana device.

Magnetotransport measurements investigating mobility-density relationships, I-V characteristics, the Shubnikov de Haas effect, and the quantum Hall effect confirm the very high quality of our dielectric deposition method and growths. Even with quantum wells so near the surface of the device, we achieve high mobilities and stable, reproducible gating characteristics. These devices have high spin-orbit coupling coefficients, confirming that we can simultaneously benefit from the inherent bulk properties of InAs and properties imparted by the rest of the growth and lithography steps. Analytical comparisons of devices with different quantum well widths, interface characters, and dielectric deposition methods reinforce the need for the rigorous optimization of numerous factors. From this analysis, we conclude that devices with smooth surface morphologies, SiO2 dielectric deposited by atomic layer deposition, and InSb-like interfaces provide the ingredients necessary to achieve near-record mobilities and consistent gating properties.

On these same excellent wafers, we fabricated superconductor-normal-superconductor (SNS)-type devices of three different normal region dimensions with ex-situ deposited niobium as the proximitizing superconductor. The universally high quality of these devices challenges the long-held norm that epitaxial aluminum is the best choice for the superconductor in these types of devices. Specifically, we achieved figures of merit much higher than those previously reported in Nb-InAs-Nb devices and on par with those using epitaxial aluminum. Using two separate mathematical models, we found that our devices have very high transparencies, indicating high-quality interfaces. Detailed plans for future devices are also discussed in this thesis, including gated SNS devices, quantum point contacts, and an attempt at observing Majorana signatures.