Photoconductivity of gallium arsenide epitaxial layers

Abstract

We have adapted a model first proposed by DeVore, to fit photoconductivity (PC) spectra of lightly doped GaAs epilayers of different thicknesses, taken at T=20 K. The model involves only the excitonic and interband transitions and the carrier lifetime is assumed to be shortened by recombinations at the surface of the sample. The modeling and the comparison with photoluminescence spectra have shown that the free exciton transition appears as a dip in our PC spectra. The position of the main peak does not correspond to a resonance; it is caused by an enhancement of the carrier recombinations, due to an increase of the absorption coefficient. By calculating PC line shapes with the model, we found that there are two thickness ranges for the signature of the free exciton transition: it should appear as a peak in GaAs layers thinner than about 2 μm and as a dip in thicker ones. The data published in previous papers can also be modeled with the expression used in the present article. We infer that the signature of free exciton transitions is also a dip in most of them.