The Dependence of Optical Constants on Selenium and Sulphur-Doping in a-Si:H

Abstract

Hydrogenated amorphous silicon–selenium and silicon–sulphur thin films were prepared by the decomposition of SiH4 and H2Se/H2S gas mixtures in an rf plasma glow discharge at a substrate temperature of 230 °C. Optical and electrical measurements show that increasing the selenium content above the gas ratio, H2Se/SiH4 = 10—3, results in an increase in the size of the optical (Tauc) gap and a decrease in the dark conductivity. The band gap of Se-doped a-Si:H was found to be between 1.79 and 1.89 eV as the gas ratio (H2Se/SiH4) increased from 10—4 to 10—1 and for S-doped a-Si:H lied between 1.80 and 1.96 eV as the gas ratio (H2S/SiH4) increased from 6.8 × 10—7 to 10—4. The refractive index n was calculated from transmission spectra and the value of n was found to decrease with increasing wavelength as well as doping concentration in the wavelength range of 650 to 950 nm. The Urbach energy and defect density were obtained from subgap absorption measurements.