Anisotropic transport in modulation-doped quantum-well structures

Abstract

Anisotropic electron transport has been observed in GaAs modulation-doped quantum wells grown by molecular-beam epitaxy on a thick (001) Al0.3Ga0.7As buffer grown at 620 °C. The low-field electron mobility at 77 K in the [110] direction is a factor of 2 larger than the mobility in the [1̄10] direction for a 90-Å quantum well. Thicker quantum wells (150, 200, and 300 Å) show progressively less anisotropy, which vanishes for a 300-Å quantum well. The degree of anisotropy is also reduced or eliminated by suspending growth of the Al0.3Ga0.7As for a period of 300 s prior to growing the GaAs quantum well. Growing the Al0.3Ga0.7As buffer at higher temperatures (680 °C) also reduces the degree of anisotropy. Higher two-dimensional electron gas sheet densities result in less anisotropy. The anisotropy is eliminated by replacing the thick Al0.3Ga0.7As buffer with a periodic multilayer structure comprising 15 Å of GaAs and 200 Å of Al0.3Ga0.7As. The degree of anisotropy is related to the thickness and growth parameters of the Al0.3Ga0.7As layer grown just prior to the growth of the GaAs quantum well.