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Three models for the dielectric function εx(hν) of AlxGa1−xAs are reviewed. All are based on measured optical constants at discrete compositions. The validity of each model near critical point energies, and otherwise, is evaluated. Only the energy-shift model is appropriate over the entire available spectrum (1.5–6.0 eV), including the band-gap (E0) region.

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The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs-AlxGa1−xAs-GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength-dependent principal angle. These results are verified by experimental measurements on two molecular-beam epitaxy grown samples. New spectral features, not found in previous ellipsometry studies of similar structures, are also reported.

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