The role played by a glass substrate on the accurate determination of the optical constants and the thickness of a thin dielectric film deposited on it, when well-known envelope methods are used, is discussed. Analytical expressions for the two envelopes of the optical transmission spectra corresponding to film. with both uniform and nonuniform thicknesses are derived, assuming the substrate to be a weakly absorbing layer. It is shown that accurate determination of the refractive index and the film thickness is notably improved when the absorption of the substrate is considered. The analytical expressions for the upper and lower envelope, are used to characterize optically and geometrically both uniform and nonuniform amorphous chalcogenide films. The results obtained are compared with those derived by use of expressions for the envelopes that neglect the substrate absorption. The comparison shows that overestimated refractive indexes and underestimated thicknesses are obtained when the conventional approach, in which the substrate absorption is neglected, is used.

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Optical-transmission spectra are very sensitive to inhomogeneities in thin ®lms. In particular, a non-uniform thickness produces a clear shrinking in the transmission spectrum at normal incidence. If this deformation is not taken into account, it may lead to serious errors in the calculated values of the refractive index and ®lm thickness. In this paper, a method ®rst applied by Swanepoel for enabling the transformation of an optical-transmission spectrum of a thin ®lm of wedge-shaped thickness into the spectrum of a uniform ®lm, whose thickness is equal to the average thickness of the non-uniform layer, has been employed. This leads subsequently to the accurate derivation of the refractive index in the subgap region (0.1±1.8 eV), the average thickness, as well as a parameter indicating the degree of ®lmthickness uniformity. This optical procedure is applied to the particular case of freshly-prepared ®lms of the Ge 10 As 15 Se 75 ternary chalcogenide glassy alloy. The dispersion of the refractive index is discussed in terms of the Wemple±DiDomenico single-oscillator model. The optical-absorption edge is described using the`non-direct transition' model proposed by Tauc, and the optical energy gap is calculated by Tauc's extrapolation. Finally, the photo-induced and thermally induced changes in the optical properties of the aGe 10 As 15 Se 75 layers are also studied.

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