The effects of a change in the chemical environment of an atom in a surface region are manifested in several subtle ways on the measured Auger electron spectra. The first, a shift in the energies of the Auger electrons, is the result of charge transfer and is a measure of the valence state of an atom. Examples of this effect are the oxidation results on refractory metals presented here. It is found that measurable changes are found even for submonolayer coverages of oxygen on an otherwise clean surface. The second main effect is a change in the shape of a complex spectrum, which occurs when some of the electrons involved in this Auger process are valence electrons. The best example of this behavior is that of carbon, and it is found that these complex spectra serve as a fingerprint for the identification of the form of the carbon at a surface. Problems and opportunities for the exploitation of both these effects are discussed, the data included here being illustrative of the usefulness of this type of measurement.
Photorefiectance is used to measure AlxGa 1 _ xAs composition, and to determine carrier concentrations in Si-doped AIGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AIGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AiGaAs band edges precisely. A similar broad signal from modulation~deped heterostructures is apparently associated with samples that show the presence of two-dimensional electron gas.
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