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We present the results of a detailed experimental study of the electric current and voltage induced in the 2DEG of a GaAs-AlGaAs heterostructure by a surface acoustic wave (SAW). New results are obtained for these acoustoelectric effects at zero and low (<0.1 T) magnetic fields. At zero magnetic field the acoustoelectric current (voltage) was found to show a non-monotonic temperature dependence with a maximum at 40-50 K. Measurements on high-mobility 2DEGs where the electron mean free path is comparable with the SAW wavelength reveal geometric resonances of the cyclotron orbit with the SAW wavelength. With increasing magnetic field the acoustoelectric effects increase significantly and display rich oscillatory structure. We compare our data for the high-magnetic-field regime with that published in the literature.

Low-temperature mobilities for two-dimensional electron gases (2DEGs) formed in tensile-strained Si/SiGe heterostructures are reported, with values up to 2.7 × 10 5 cm 2 V −1 s −1 for a density of 4.6 × 10 11 cm −2 electrons. The strained layers were grown at 600 • C in a ultra-high-vacuum chemical vapour deposition system using SiH 4 and GeH 4 operating at around 20 Pa. The surface morphology of the layers is also discussed and both the mobility and morphology are linked to the quality of the virtual substrates. The virtual substrate consists of strain-relaxed SiGe alloys grown on Si(001) substrates; we show that it is preferable to grow these substrates at higher temperatures and higher growth rates. For low growth rates and temperatures the 2DEG mobility as a function of sheet carrier density was found to be degraded.