Abstract
In modern GaAs/Al_xGa_1-xAs heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor δ-layers placed on both sides of the well. Each δ-layer is located within a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. Nevertheless, excess electrons screen both the remote donors and background impurities. When the fraction of remote donors filled by excess electrons f is small, the remote donor limited quantum mobility grows as f³ and becomes larger than the background impurity limited one at a characteristic value f_c. We also calculate both the mobility and the quantum mobility limited by the screened background impurities with concentrations N₁ in Al_xGa_1-xAs and N₂ in GaAs, which allows one to estimate N₁ and N₂ from the measured mobilities. Taken together, our findings should help to identify avenues for further improvement of modern heterostructures.