Photoreflectance of AlxGa1−xAs and AlxGa1−xAs/GaAs interfaces and high-electron-mobility transistors

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Abstract
Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si-doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation-doped heterostructures is apparently associated with samples that show the presence of two-dimensional electron gas.