Modulation of excitonic reflectance at GaAs/GaAs interfaces

We have studied photoreflectance (PR) and reflectance spectra of undoped molecular‐beam‐epitaxy GaAs films on semi‐insulating GaAs substrates at low temperature. In the PR spectra a sharp structure near the band‐gap energy was observed which is sensitive to pump and probe light intensity. The origin of this structure was investigated by combining time‐resolved and depth‐resolved measurements. It is shown that the sharp PR structure arises from the superposition of a fast and a slow component of the modulated spectra. The slow PR signal originates from the epilayer–substrate interface. Both components are attributed to the modulation of exitonic transitions centered at slightly different energies.