Optical Constants in the Subgap Region and Vibrational Behaviour by Far-Infrared Spectroscopy of Wedge-Shaped Obliquely-Deposited Amorphous GeS2 Films

Abstract

Optical transmission spectra are very sensitive to inhomogeneities of thin films. In particular, non-uniform film thickness leads to shrinking of the transmission spectrum at normal incidence. This non-uniformity has to be taken into account, because its ignorance may lead to serious errors in the calculated values of the refractive index and the film thickness. This paper presents a method, which transforms the optical transmission spectrum of a wedge-shaped thin film into a spectrum of a uniform film whose thickness is equal to the average thickness of the non-uniform film. This allows to calculating the refractive index, the average thickness, as well as a parameter indicating the degree of film thickness uniformity. The method is successfully applied to wedge-shaped obliquely-deposited chalcogenide glass thin films of chemical composition GeS2. The dispersion of the refractive index in the subgap region is discussed in terms of the Wemple-DiDomenico single-oscillator model. The optical -absorption edge is described using the 'non-direct transition' model suggested by Tauc, and the optical gap is calculated by Tauc's extrapolation. Far-infrared spectroscopy has been used to clearly complement the results derived from the present optical measurements.