We report the realization of a modulation-doped quasi-three-dimensional electron system. The structure consists of a 2000-Å-wide undoped AlxGa1−xAs well bounded by undoped (spacer) and doped layers of AlyGa1−yAs (y>x) on both sides. The alloy composition in the well (x) is varied quadratically so that the combined potentials due to the AlxGa1−xAs and the electric charge in the well produce a square potential well with a nearly uniform carrier density. Magnetotransport data reveal that the system contains ≂2.5×1011 cm−2 electrons, which occupy four electric subbands and have a low-temperature mobility in excess of 1×105 cm2/V s indicating the high quality of the structure.

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Segregation of Ge in the ‘‘leading edge’’ of Si1−xGex alloys grown by molecular beam epitaxy is investigated using x-ray photoelectron spectroscopy (XPS). Alloys of 5%, 10%, 20%, and 40% Ge were grown in varying thickness (0–20 nm) at 500 °C to observe segregation during the initial stages of alloy growth. The length of the leading edge was found to decrease with increasing Ge concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). The amount of segregated Ge was found to increase with Ge concentration. A complete monolayer of Ge was found on the surface for all Ge concentrations and an increasing amount of Ge (20%, 55%, 80%, and 95%, respectively) was found in the second atomic layer.