The destruction of the virtual Anderson transition in a narrow impurity band of doped quantum well structures

Earlier we reported an observation at low temperatures of activation conductivity with small activation energies in strongly doped uncompensated layers of p-GaAs/AlGaAs quantum wells. We attributed it to Anderson delocalization of electronic states in the vicinity of the maximum of the narrow impurity band due to the small width of the impurity band characterized by weak disorder. In this case the carriers were activated from the 'band tail' while its presence was related to weak background compensation. Here we study an effect of the extrinsic compensation and of the impurity concentration on this 'virtual' Anderson transition. It was shown that an increase of compensation initially does not affect the Anderson transition, however at strong compensations the transition is suppressed due to increase of disorder. However at larger impurity concentration the conductivity becomes to be metallic due to Mott transition.