Mobility and quantum mobility of modern GaAs/AlGaAs heterostructures

In modern GaAs/AlxGa1-xAs heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor delta layers placed on both sides of the well. Each delta 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 f(3) 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 N-1 in AlxGa1-xAs and N-2 in GaAs, which allows one to estimate N-1 and N-2 from the measured mobilities. Taken together, our findings should help to identify avenues for further improvement of modern heterostructures.