Calculation of optical constant of amorphous germanium arsenoselenide wedge-shaped thin films from their shrunk transmittance and reflectance spectra

Abstract

The optical transmittance spectrum is influenced by inhomogeneities in germanium arsenoselenide thin films. The non-uniformity of thickness, found under the present deposition conditions, gives rise to a clear shrinking of the interference fringes of the transmittance spectrum at normal incidence. Inaccuracies and even serious errors occur if the refractive index and film thickness are calculated from such a shrunken transmittance spectrum, under the unrealistic assumption that the film is uniform. The analytical expressions proposed by Swanepoel [J. Phys. E. Sci. Instrum. 17 (1984) 896] enabled derivation of the refractive index and film thickness of a wedge-shaped thin film from its shrunk transmittance spectrum. This method was applied in this study making it possible to derive the refractive index and average thickness to an accuracy better than 1%. Dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model [Phys. Rev. B 3 (1971) 1338]. The absorption coefficient and, thus the extinction coefficient, can be calculated from transmittance and reflectance spectra in the strong absorption region. The optical energy gap is derived from Tauc's extrapolation [Amorphous and Liquid Semiconductor (Plenum Press, New York, 1974)]. The relationship between the optical gap and chemical composition in the Ge x As30– x Se70 (with 0 ≤ x ≤ 30) amorphous system is discussed in terms of the chemical bond approach and cohesive energy.