Influence of the deposition technique on the structural and optical properties of amorphous As–S films

Amorphous chalcogenide films of stoichiometric composition As40S60 have been prepared by three different deposition techniques, namely, vacuum thermal evaporation, plasma-enhanced chemical vapour deposition (PECVD) and spin coating. Indications of film-thickness inhomogeneities were found in all samples. Thermally evaporated and chemically deposited samples showed wedge-shaped surface profiles, while significant surface roughness was evidenced in the spin-coated ones. Refractive-index values of the film samples were obtained, with accuracy better than 1%, by using the envelope method most suitable for each particular film surface profile. Structural information of the samples has been gained from X-ray diffraction experiments, and also inferred from the analysis of the dispersion of the refractive index, on the basis of a single-oscillator model. Analysis of the optical absorption spectra allowed both calculating the optical band gaps and estimating the localisedstate tail width of these semiconducting films. In addition, information about the degree of structural randomness of these thinfilm amorphous alloys was also obtained from this analysis, which is in good agreement with the conclusions derived from the Xray diffraction results