Observation of strong localization effects in (AlGa)As–GaAs two-dimensional electron gas structures at low magnetic fields

Abstract

Electrical measurements have been made on films of lightly Si-doped GaAs and on (AlGa)As/GaAs two-dimensional electron gas (2DEG) structures grown by MBE. These have shown residual donor and acceptor concentrations of ∼2×1014 cm−3 in the GaAs, and mobilities for 2DEG layers which varied systematically with carrier density ns and undoped spacer layer thickness d; the highest mobility obtained was 2.2×106 cm2 V−1 s−1 for a sample with ns=2×1011 cm−2 and d=800 Å. Analysis of the data shows that scattering by remote ionized impurities is the dominant mechanism in most of our 2DEG samples. In such layers, the mobility μ at 4 K varied with photoexcited carrier density ns approximately as ns1.5. This behavior is expected for photoexcitation of carriers from DX centers in the doped (AlGa)As, and results from increased screening in the 2DEG and increased scattering due to the greater number of charged impurities in the (AlGa)As. However, some samples showed a much steeper variation of μ with ns, and also different temperature dependences of μ for high and low ns values. Comparison with previously reported data on Si–SiO2 2DEG systems suggests that this effect is due to strong localization effects occurring at the (AlGa)As–GaAs interface.