The signals from buried layer in Si-delta-doped GaAs of different undoped cap thickness have been studied by differential photoreflectance. The first-derivative-like line shape of differential photoreflectance is attributed to the energy transitions related to the modulations of two-dimensional electron gas density. We have observed a change of line shape at low temperatures. This change of line shape is probably due to the change of potential distribution in the conduction band, which is caused by the surface Fermi-level pinning and surface photovoltaic effect.
