Imaging Current in Si-doped InAs/GaSb Quantum Wells

Quantum spin hall (QSH) insulators are characterized by current-carrying edges in which single-particle elastic backscattering is forbidden, resulting in a theoretical conductance of e2/h per edge. Various theoretical mechanisms have been proposed to explain why, in devices with edges longer than several microns, the measured resistance is greater than expected. We used a scanning superconducting quantum interference device to image 2D current flow in inverted InAs/GaSb composite quantum wells with edges of tens of microns. We compared wells with Si doping at the InAs/GaSb interface (which acts to suppress residual bulk conductivity) to wells without doping. In the Si-doped samples, we observed that the majority of current flowed along the edge of the device when it was tuned into the bulk gap using a front gate. The current at the edges is consistent with an edge resistance that remained unchanged over a wide range of temperature and gate voltage, even in the presence of bulk conduction. These results set strong limits on candidate mechanisms for edge scattering.