Optical band gap and optical constants of a-Se100-xSnx (x=2, 4, 6 and 8) thin films

The optical band gap and optical constants of amorphous Se100-xSnx (x = 2, 4, 6 and 8) thin films, prepared on glass substrates by evaporating in vacuum Se-Sn alloy previously prepared by quenching method, have been studied in the wavelength range 540-900nm. From the measurement of optical transmittance of the films in the mentioned wave length range, the optical band gap (vary from 1.03 to 1.4eV), refractive index (vary from 4.4 to 8), as well as the real and imaginary parts of the dielectric constants were determined. Results indicate that when higher is the Sn-content in the film, lower is the band gap, E-g = 1.03eV, and also lower is the refractive index of 4.4 at 900nm. Meanwhile, the real epsilon'(r) and imaginary part epsilon ''(r) of the dielectric constant were found to decrease with Sn concentration and then increase at 8 at. % of Sn. The observed decrease in the optical band gap is attributed to the corresponding decrease in the average bond energy of a-Se100-xSnx films. This result was also confirmed and discussed in terms of the heat of atomization H-s and the average co-ordination number Z.