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 (E g opt ) 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 E g opt increases with the sulphur content in the GeSx films. The changes of E g opt were discussed in relation to the structure of GeSx films, which were confirmed by the Raman spectra analysis.