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.