Transport in gated InAs/GaSb coupled quantum wells: exploring carrier regime crossover and integrating ferromagnetic contacts

This thesis presents the study of carrier dynamics at the boundary of the charge neutral transport regime in InAs/GaSb quantum wells (QWs) and the implementation of integrated ferromagnetic contacts onto InAs/GaSb QW devices. As promising candidate materials for hosting a quantum spin Hall insulator state, InAs/GaSb QWs have been the subject of intensive study for over fifteen years. The conducting helical edge states characteristic of this phase are of particular interest, as the associated spin-momentum locked transport has yet to be measured directly. In this QW structure, the band inversion required to host a quantum spin Hall state is promoted by the broken-gap alignment of the InAs and GaSb. The band alignment can also be tuned by gate voltage application.
The first focus of this thesis is the gate tuning of InAs/GaSb QW Hall bar devices to the boundary of the charge neutral regime, where the Lifshitz-Kosevich model is applied at varying carrier densities in a scan-like approach to determine a breakdown region of (4.3 - 6.8) × 10^(11) cm^(-2), in which its associated assumptions no longer apply. The contested existence of a non-trivial Berry phase is also explored, determining that no such phase is present.
The remainder of this thesis is devoted to the fabrication and optimisation of novel micron-scale and nano-scale InAs/GaSb devices with integrated ferromagnetic contacts. First, micron-scale transmission line devices were fabricated, which enabled the refinement of the ferromagnetic contact-(InAs/GaSb) interface to produce a reliably ohmic junction, along with the determination of contact resistance and other characteristics. This established a foundation for the fabrication of transmission line devices with low micron- to nano-scale contact separations. With the design and fabrication adjustments outlined in this thesis, this work provides a platform for future pioneering experiments detecting spin-dependent transport in InAs/GaSb QWs.