Optical band gap, refractive index dispersion and single-oscillator parameters of amorphous Se₇₀S_30−xSb_x semiconductor thin films

Abstract

The optical band gap, refractive index and oscillator parameters of amorphous semiconductor Se₇₀S_{30− x} Sb _x thin films with x = 0, 18, 12 and 30 at. % deposited by electron-beam evaporation were investigated by optical spectrophotometry. The energy gap decreases from 2.25 to 1.08 eV with increasing Sb content introduced at the expense of S, which is explained in terms of chemical bond and cohesive energy approaches. The method proposed by Swanepoel is applied to extract the refractive index n. It was found that the refractive index is strongly related to the change in the concentration of Sb, which is related to the increase density of the film material with increasing Sb content, and is interpreted in terms of polarisability and the mean coordination number. The ratio of the refractive indices of (Se₇₀Sb₃₀)/(Se₇₀S₃₀) is ≈ 1.32 at a wavelength λ ≈ 1600 nm and increases to 1.48 at λ ≈ 950 nm. Such a refractive index contrast could be used to fabricate a distributed Bragg reflector (DBR) stack with limited number of layers. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-oscillator model. The oscillator parameters, i.e. the single-oscillator energy E_o , the dispersion energy E_d , the static refractive index n ₀, the average interband oscillator wavelength λ ₀, and the average oscillator strength S _o, and the optical conductivity have been estimated.

Keywords:

• chalcogenide glass
• amorphous semiconductor
• optical properties
• thin film
• optical constant

Acknowledgement

This work was supported by the Deanship of the Scientific Research at Qassim University, Kingdom of Saudi Arabia.