  The two-dimensional topological insulator state in InAs/GaSb quantum wells
manifests itself by topologically protected helical edge channel transport
relying on an insulating bulk. This work investigates a way of suppressing bulk
conductivity by using gallium source materials of different degrees of impurity
concentrations. While highest-purity gallium is accompanied by clear conduction
through the sample bulk, intentional impurity incorporation lead to a bulk
resistance over 1 M\Omega. This resistance was found to be independent of
applied magnetic fields. Ultra high electron mobilities for GaAs/AlGaAs
structures fabricated in a molecular beam epitaxy system used for the growth of
Sb-based samples are reported.
