Polarized electroabsorption spectra of amorphous semiconductors

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Abstract
Transverse electroabsorption spectra of amorphous Se and hydrogenated amorphous Si (a-Si: H) are presented. These spectra are anisotropic, with fields F parallel to the polarization E of light, causing much larger signals than F perpendicular to E. At temperatures below 350 K the signal depends little on temperature. At higher temperatures the field-induced change δα in the absorption increases in a-Si: H. In Se the anisotropy of δαdepends on photon energy. At low energies the spectrum becomes isotropic and may arise from a quadratic Stark effect of tail states with rather large polarizabilities. The electroabsorption spectra cannot be explained by the Franz-Keldysh effect or by an exciton model. It is proposed that they arise from field mixing of tail states which increases their dynamic polarizability. The mixing is most effective for states near the mobility edges.
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