Materials considerations for forming the topological insulator phase in InAs/GaSb heterostructures

In an ideal InAs/GaSb bilayer of appropriate dimension in-plane electron and hole bands overlap and hybridize, and a topologically non-trivial, or quantum spin Hall (QSH) insulator, phase is predicted to exist. The in-plane dispersion's potential landscape, however, is subject to microscopic perturbations originating from material imperfections. In this work, the effect of disorder on the electronic structure of InAs/GaSb bilayers was studied by the temperature and magnetic field dependence of the resistance of a dual-gated heterostructures gate-tuned through the inverted to normal gap regimes. Conduction in the inverted (predicted topological) regime was qualitatively similar to behavior in a disordered two-dimensional system. The impact of charged impurities and interface roughness on the formation of topologically protected edge states and an insulating bulk was estimated. The experimental evidence and estimates of disorder in the potential landscape indicated the potential fluctuations in state-of-the-art films are sufficiently strong such that conduction in the predicted topological insulator (TI) regime was dominated by a symplectic metal phase rather than a TI phase. The implications are that future efforts must address disorder in this system and focus must be placed on the reduction of defects and disorder in these heterostructures if a TI regime is to be achieved.