Abstract
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.
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