Two-dimensional magnetotransport in AlAs quantum wells

We have made magnetotransport measurements on two-dimensional electronic systems confined to AlAs by ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As. We have determined the effective mass ($m_{111}$=0.49$m_0$), the effective g factor (3.1), and the relaxation time. Even though the lattice constants of GaAs and AlAs are nearly equal, the stress arising from this small mismatch strongly influences the electronic properties of AlAs. For [111]-oriented samples the valley degeneracy is one, and there is an anomalous phase shift in the Shubnikov–de Haas oscillations.