Photoreflectance Characterization of InGaAs/GaAs Superlattices Grown on [111]-Oriented Substrates

We report the results of a photoreflectance (PR) study of InGaAs/GaAs strained-layer quantum wells and superlattices (SLSs) grown by MBE on [111]B GaAs substrates. Under our measurement conditions, the PR spectra display features we can relate to the bandgaps of both materials and to optical transitions in the quantum structures. Using the photovoltaic effect to vary the surface electric field of our i-n+ and p+-i-n+ samples in a strictly contactless manner, we find optical transitions red-shifting with increasing intensity of illumination from a CW HeNe laser in [111]-grown structures, a well known effect which can be attributed to the straingenerated electric field (SGEF) present in these structures. We also find experimental support for the predicted effectiveness of free-carriers in screening the SGEF and thereby originating highly non-linear absorption.