Optical constants in the subgap region and vibrational behaviour by far-infrared spectroscopy of wedge-shaped obliquely-deposited amorphous GeS2 films

Optical transmission spectra are very sensitive to inhomogeneities of thin films. In particular, non-uniform him thickness leads to shrinking of the t ransmission spectrum at normal incidence. This non-uniformity has to be tak en into account, because its ignorance may lead to serious errors in the ca lculated 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 cal culating the refractive index, the average thickness, as well as a paramete r indicating the degree of film thickness uniformity. The method is success fully applied to wedge-shaped obliquely-deposited chalcogenide glass thin f ilms of chemical composition GeS2. The dispersion of the refractive index i n the subgap region is discussed in terms of the Wemple-DiDomenico single-o scillator model. The optical-absorption edge is described using the 'non-di rect transition' model suggested by Tauc, and the optical gap is calculated by Tauc's extrapolation. Far-infrared spectroscopy has been used to clearl y complement the results derived from the present optical measurements.