Abstract
The optical properties of a GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As hetero-n-i-p-i crystal with GaAs quantum wells in the intrinsic region (``type II'') have been investigated by photoreflectance measurements. To understand the complex spectra we have varied several measurement parameters such as ac pump intensity, dc pump intensity, pump frequency, and temperature. The latter one represents a very useful tool to vary the degree of excitation of the sample over a wide range while keeping the pump and probe light intensities at a constant level. The role of temperature is described by a simple model which takes into account the strong temperature dependence of the thermally activated recombination kinetics. The photoreflectance spectra are compared with spectra calculated within the framework of a semiclassical Franz-Keldysh model and a model considering an infinite quantum well in an electric field (quantum-confined Franz-Keldysh model). From this comparison the internal electric field can be deduced. We demonstrate that it is essential to take into account the fact that the dielectric function has a spatial dependence with the period of the superlattice.
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