Suppression of the Virtual Anderson Transition in the Impurity Band of Doped Quantum Well Structures

We have previously observed activation-type conductivity with low activation energies of heavily doped p-GaAs/AlGaAs quantum well structures at low temperatures. It has been attributed to the delocalization of the electron states near the maximum of a narrow impurity band in the sense of the Anderson transition. The possibility of this delocalization at a relatively low impurity concentration is associated with the narrowness of the impurity band in the presence of weak disorder. In this case, charge carriers were activated from the tail of the band and their presence was due to the background (weak) compensation. In this work, we study the dependence of the above virtual Anderson transition on the external compensation and impurity concentration. It has been found that an increase in the compensation does not initially affect the Anderson transition; however, at a higher compensation, it leads to the suppression of the transition owing to the growing disorder. An increase in the impurity concentration also initially leads to the suppression of the Anderson transition due to the disorder associated with the partial overlap of the Hubbard bands. However, the conductivity becomes metallic at a fairly high concentration due to the Mott transition.