Structure and optical properties of amorphous GeSx films prepared by PLD

Amorphous GeSx (x = 2, 4, 6) films were prepared by the pulsed laser deposition (PLD) technique. The optical band gaps (Egopt) and refractive indices of the films were obtained from the optical absorption spectra and transmission spectra, respectively. The short-wave absorption edges of the films were described using the ‘non-direct transition’ model proposed by Tauc. The dispersion of the refractive index was analyzed in terms of the single-oscillator Wemple–Di Domenico model. The structural units of the films were characterized using Raman spectroscopy. In addition to the basic structural units of edge-sharing and corner-sharing [GeS4] tetrahedra, there are S–S homopolar bonds in S-rich GeS4 and GeS6 films while Ge–Ge bonds exist in stoichiometric GeS2 film. The results show that the index of refraction decreases while Egopt increases with the sulphur content in the GeSx films. The changes of Egopt were discussed in relation to the structure of GeSx films, which were confirmed by the Raman spectra analysis.