Band Structure of InAs/GaSb coupled quantum wells studied by magnetotransport

This thesis describes the characterisation of InAs/GaSb coupled quantum wells in the electron-dominated regime. The “inverted” band gap, created by electron-hole coupling within this material is predicted to give rise to a quantum spin Hall phase, where carrier dynamics are dominated by ballistic, spin-polarised, edge states. If it was possible to inject spin into these edge-states, this material could become the foundation for future spintronic devices. However, the effect of this electron-hole coupling on the electron dominated transport is relatively unexplored. Therefore, this thesis studies the electron dominated regime in a variety of quantum wells, and highlights the differences between this coupled system and the single 2DEG transport, seen in previous studies of single InAs wells. The effect the coupling has on the transport in the absence of a gate bias is probed by comparing the transport in a simple InAs/GaSb well to a similar well with an inter-layer AlSb barrier. Once it was determined that two carrier, electron-hole, transport exists in the InAs/AlSb/GaSb well, but not in the simple InAs/GaSb well, the effect of gate biases on the magnetotransport through the simple coupled well was also probed. In that case, the effects of higher order electron-hole coupling on the band-structure within this interesting coupled quantum well system was observed. Finally, the spin-orbit coupling within this strongly coupled quantum well system is investigated. The Dresselhaus and Rashba spin-orbit coupling terms are explicitly calculated and their dependences on a small gate bias are noted. In particular, the Rashba spin orbit coupling is not only much larger than that observed in single InAs quantum wells, but it shows a remarkable dependence on the quantum well growth conditions. This strong spin-orbit coupling could have a profound impact on the quantum spin Hall effect predicted to arise in this material system, perturbing the polarisation axis of the quantum spin Hall state