Extrinsic contributions to photoreflectance of ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs quantum wells: An investigation of the ‘‘donor-related’’ feature

Above 6 K, excitonic features in reflectance spectra of a Si-doped 175-Å ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs single quantum well shift negligibly in energy with the application of a pump laser. Thus the built-in electric field changes insignificantly in the presence of excess photoexcited carriers. These spectra are successfully described by assuming that linewidth broadening dominates the observed change in reflectance line shapes. Such measurements are shown to be equivalent to photoreflectance (PR) and provide insight into the PR modulation mechanisms involved. An additional feature is resolved below 6 K which correlates to excitons bound to neutral donors ($D^0$X) observed in photoluminescence measurements acquired simultaneously. The broadening at higher temperatures is speculated to arise from an unresolved combination of a quantum-confined Stark-effect blue shift and a light activation of $D^0$X which produces a red component. Therefore, it is demonstrated that PR differs from electroreflectance in doped quantum wells in that PR is not solely due to electromodulation but may contain extrinsic contributions such as $D^0$X.