Abstract
By selective doping of the well and barrier regions of p-GaAs/AlGaAs structures we have realized the situation where the upper Hubbard band (A+ centers) has been occupied by holes in equilibrium. Hall effect and conductivity measurements have shown that the binding energy of A+ centers increases significantly (five times) with respect to the bulk case for well width of 15 nm; it is related to the comparability between the well width and the radius of the A+ state. In the low temperature region the conductivity has the character of 2D Mott-type variable range hopping (VRH). Basing on the analysis of quadratic positive magnetoresistance and non-ohmic VRH we have obtained estimates of the localization length. The value of 11 nm obtained is by a factor of five larger than the Bohr radius of the single hole state. The results unambiguously evidence that the hopping transport in our system is over states of the upper Hubbard band.
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