  We report experimental studies of conductance and magnetoconductance of
GaAs/AlGaAs quantum well structures where both wells and barriers are doped by
acceptor impurity Be. Temperature dependence of conductance demonstrate a
non-monotonic behavior at temperatures around 100 K. At small temperatures
(less than 10 K) we observed strong negative magnetoresistance at moderate
magnetic field which crossed over to positive magnetoresistance at very strong
magnetic fields and was completely suppressed with an increase of temperature.
We ascribe these unusual features to effects of temperature and magnetic field
on a degree of disorder. The temperature dependent disorder is related to
charge redistribution between different localized states with an increase of
temperature. The magnetic field dependent disorder is also related by charge
redistribution between different centers, however in this case an important
role is played by the doubly occupied states of the upper Hubbard band, their
occupation being sensitive to magnetic field due to on-site spin correlations.
The detailed theoretical model is present.
