The relationship between optical gap and chemical composition in SbxSe1-x system

The optical constants of vacuum deposited SbxS1-x amorphous thin films (x = 0.075, 0.15 and 0.20 at%) of different thicknesses (90-150 nm) were studied in the wavelength range 550-800 nm. It was found that both the refractive index n and the absorption index k are independent of the film thickness, whilst there is a distinct dependence of n and k on the Sb content. The allowed optical transition was found to be non-direct, the corresponding optical gaps E(g)(opt) are determined. The width of the band tail E(e) is also determined. As a result, E(g)(opt) decreases while E(e) increases with increasing Sb contents. The variation of E(g)(opt) with x for SbxSe1-x agree with the optical gap E(o) predicted theoretically in terms of random bond network proposed by Shimakawa. The relationship between the optical gap and chemical composition in SbxSe1-x amorphous system is discussed in terms of the average heat of atomization H-s and the average coordination number m. The results of these calculations can be used to rationalize the observed optical properties of these materials.