PHOTOREFLECTANCE EVALUATION OF INTERNAL ELECTRIC-FIELDS IN GAAS SI/GAAS AND ALAS/SI/ALAS HETEROSTRUCTURES/

The possibility of controlling the band offset between layers of III-V compounds by inserting a few monolayers of silicon have given to thes e systems an increasing theoretical and experimental interest. Photore flectance is a technique very sensitive to interface effects in hetero structures and particularly sensitive to effects directly related to b and offsets, such as interfacial electric fields. A signature of the p resence of internal electric fields is the observation of Franz-Keldys h oscillations in the photoreflectance spectra. We carried out a detai led study of Franz-Keldysh oscillations observed in the room temperatu re photoreflectance spectra of GaAs/Si/GaAs and AlAs/Si/AlAs, heterost ructures grown by molecular beam epitaxy where the Si nominal thicknes s was two monolayers. The data were analyzed employing the asymptotic Franz-Keldysh theory which allowed us to calculate the magnitudes of t he internal electric fields. The origin of the internal electric field s is discussed taken into account the silicon distribution as determin ed by secondary ion mass spectroscopy and the contribution from the in ner interfaces. It is found that different contributions from degenera te heavy and light hole bands, to transitions around the GAMMA point o f the Brillouin zone, must be expected for different heterostructures depending upon the particular characteristics of the electric fields p resent in each sample.