Temperature and field dependence of the optical absorption edge in amorphous As2S3

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Abstract
The optical absorption coefficients and the temperature dependence of the edge in amorphous As2S3have been measured at temperatures down to 10 K. The temperature coefficients of the gap at a fixed absorption level ( delta E/ delta T)alpha, can be separated into two components. The first is attributed to electron-phonon interactions and the shift is found to be proportional to the internal energy on a Debye approximation, as predicted from the theory of zero-phonon transitions in colour centres. The second is the temperature broadening of the Urbach tail. The slope of the tail is interpreted in terms of random electric fields arising from disorder and phonons; the former is estimated to be three times larger than the contribution from zero-point motion. The electro-modulation spectrum has been measured in the region of the absorption edge and is discussed in terms of exciton and other models.