Optical-constant calculation of non-uniform thickness thin films of the Ge10As15Se75 chalcogenide glassy alloy in the sub-band-gap region (0.1-1.8 eV)

Abstract

Optical-transmission spectra are very sensitive to inhomogeneities in thin films. 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 film thickness. In this paper, a method first applied by Swanepoel for enabling the transformation of an optical-transmission spectrum of a thin film of wedge-shaped thickness into the spectrum of a uniform film, 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 film-thickness uniformity. This optical procedure is applied to the particular case of freshly-prepared films of the Ge10As15Se75 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 a-Ge10As15Se75 layers are also studied. (C) 1999 Elsevier Science S.A. All rights reserved.