MODULATION OF EXCITONIC REFLECTANCE AT GAAS GAAS INTERFACES/

We have studied photoreflectance (PR) and reflectance spectra of undop ed molecular-beam-epitaxy GaAs films on semi-insulating GaAs substrate s at low temperature. In the PR spectra a sharp structure near the ban d-gap energy was observed which is sensitive to pump and probe light i ntensity. The origin of this structure was investigated by combining t ime-resolved and depth-resolved measurements. It is shown that the sha rp PR structure arises from the superposition of a fast and a slow com ponent of the modulated spectra. The slow PR signal originates from th e epilayer-substrate interface. Both components are attributed to the modulation of exitonic transitions centered at slightly different ener gies. (C) 1995 American Institute of Physics.