Compositional dependence of the optical properties of amorphous semiconducting glass Ge 10As xSe (90−x) thin films

Abstract

Optical properties of ternary chalcognide amorphous Ge 10As x Se (90− x ) (with 10⩽ x⩽25 at%) thin films prepared by thermal evaporation have been measured in visible and near-infrared spectral region. The straightforward analysis proposed by Swanepoel has been successfully employed, and it has allowed us to determine the average thickness d ¯ , and the refractive index, n, of the films, with high accuracy. The refractive index, n and the average thickness d ¯ has been determined from the upper and lower envelopes of the transmission spectra measured at normal incidence, in the spectral range 400–2500 nm. The absorption coefficient α, and therefore extinction coefficient k, have been determined from the transmission spectra in the strong-absorption region. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–DiDomenico model, and the optical absorption edge is described using the ‘nondirect transition’ model proposed by Tauc. Likewise, the optical energy gap is derived from Tauc's extrapolation. The relationship between the optical gap and chemical composition in Ge 10As x Se (90− x ) amorphous system is discussed in terms of the average heat of atomization H s and average coordination number N c. Finally, the chemical bond approach has been also applied successfully to interpret the decrease of the glass optical gap with increasing As content.