Effects of impurity transitions on electroreflectance in thin epitaxial GaAs and Ga1−xAlxAs/GaAs layers

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Abstract
A detailed electroreflectance (ER) and photoluminescence study has been performed of impurity‐related, below‐gap electroabsorption (EA) features in bulk Sn‐ and Si‐doped GaAs as well as in thin epitaxial layers of GaAs and Ga1−xAlxAs/GaAs, with and without a known carbon impurity. A correlation is observed between the below‐gap electroreflectance structure and impurity related emission from the conduction band and donor levels (when present) to acceptor states. In all cases in which impurities are present, the dc bias dependence of the EA below‐gap structure and the current through the sample are found to be similar suggesting impact ionization as the mechanism which allows observation of the impurity transitions in ER. It is also shown that regardless of the carrier type of the material, upward band bending at the sample surface is required to prevent rapid recombination and maintain empty impurity levels whose population can be modulated by impact ionization. The photoresponse of the samples verifies these observations. For thin epitaxial films, criteria are presented for reducing the effects of below‐gap structure arising from the substrate material.