Photoreflectance spectra from a surface and an interface of n-type GaAs epitaxial layers and their modulation frequency dependence

We have performed photoreflectance (PR) measurements for n-type GaAs epitaxial layers grown on n+ and S.I. substrates and have observed that the PR spectrum is a superposition of two signals which originate from the surface and from the interface between a grown layer and a substrate. From the modulation beam power dependence, we reveal that the photo-carriers, which cause the PR signal from the interface, are generated at the vicinity of the interface. We have also demonstrated that the PR signal from the surface can be separated from the other contributions at a high modulation frequency. Through measuring the modulation frequency dependence, we have confirmed that this results from the difference of the lifetimes of photo-carriers at the surface and the interface.