  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.
