Manifestation of coulomb gap in two-dimensional $p$-GaAs-AlGaAs structures with filled upper Hubbard band

The transport properties of multilayer GaAs/AlGaAs structures doped modulationally with Be so as to fill, in equilibrium, the states of upper Hubbard band (A$^+$-centers) with holes were studied. For the concentration of dopants on the order of $5\cdot10^{11}\,$cm$^{-2}$, the hopping conduction over the states in the Coulomb gap was observed in the temperature range 0.4–4 K. The characteristic temperature ($T_1$) was determined from the temperature dependence of conductance and found to be appreciably lower (by 30 times) than its theoretically predicted value. This discrepancy is assumed to be due to the correlated hopping effect. In the temperature dependence of magnetoresistance, the suppression of negative magnetoresistance was observed with lowering temperature. This is explained by the weakness of underbarrier scattering in the transport via the upper Hubbard band.