Composition and temperature dependence of the low-frequency Raman scattering in Ge–As–S glasses

The Raman spectra of ‘stoichiometric’ (GeS2)x(As2S3)1−x glasses are presented for various compositions (x = 0.40, 0.60, 0.80, 0.83, 0.90) and compared to that of the ‘non-stoichiometric’ Ge0.30As0.10S0.60 glass (in which there is excess of Ge with regard to the stoichiometric system). The study is focused on the variation of the low-frequency Boson peak with glass composition and temperature. The ratio of intensity of the as-recorded Boson peak to that of the high frequency main molecular band remains almost composition-independent for all glasses, except for the non-stoichiometric one. This ratio is considerably larger in the latter glass, implying a higher degree of disorder which is attributed to an increased number of defects. In the (GeS2)x(As2S3)1−x glasses, there is evidence of a shift of the Boson peak towards the lower frequencies with increasing Ge-content, indicating an increase of the correlation length of the medium range structure. On the other hand, the non-stoichiometric glass displays a lower Boson peak frequency than any (GeS2)x(As2S3)1−x glass, implying, again, a higher correlation length of the medium range structure. Both these results, i.e., the shifts towards the lower frequencies, are interpreted in terms of the higher free volume displayed by the Ge-rich glasses. In the case of the stoichiometric (GeS2)0.83(As2S3)0.17 (or Ge0.25As0.10S0.65) glass and the non-stoichiometric Ge0.30As0.10S0.60 one, Raman spectra have also been measured over the temperature range 20–295 K and shown significantly different dependence. Comparison of the intensity variation of the BP with temperature further confirms the higher degree of disorder of the non-stoichiometric glass. Finally, the reduced BP intensities of the glasses show almost the same spectral profiles over the entire temperature range, thus displaying universal lineshape behavior with temperature.