Optical band gap of Pb modified Ge–Se–Te glasses investigated by photoacoustic technique

Optical band gap (Eopt) for Pb modified Ge–Se–Te bulk glasses has been measured using the photoacoustic (PA) technique. The advantage of this technique over the absorption spectrophotometry technique is that this method is independent of sample thickness and provides optical band gap energy (Eopt) values corresponding to a high (∼104 cm−1) absorption coefficient. Hence this technique is especially suited for highly absorbing and brittle bulk semiconductors. The composition dependence of the optical energy Eopt of two series of glasses, namely, PbxGe42−xSe48Te10 (3≤x≤13) and Pb20GexSe70Te10 (17≤x≤24) has been measured by recording the variation of the PA signal as a function of wavelength in the range 400–1200 nm. Eopt has a minimum value for the composition with 9 at. wt.% Pb in the PbxGe42−xSe48Te10 series of glasses. Eopt exhibits a maximum for the 21 at. wt.% Ge composition in the Pb20GexSe70Te10 series of glasses. The above results have been interpreted in terms of the variation of the average bond energy of these glasses with change in composition. It is a matter of interest that Eopt shows anomalous behaviour at compositions at which the majority charge carrier reversal is observed in these glasses.