Thin film binary alloys of InxSe1-x (0.05£ X ≤0.30) have been prepared by the thermal evaporation technique. The optical transmission and reflection spectrum of these films were measured in the range 300-1100 nm. Both refractive index, 11 and extinction coefficient k have been determined from transmission and reflection measurements in terms of Murrnanns equations. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. The width of band tail is determined and the optical absorption edge is described using the 'non-direct transition' model proposed by Tauc. Finally, the relationship between the optical gap and chemical composition in InxSe1-x amorphous system is discussed in terms of the average heat of atomization H, and average coordination number Nc. The results of these calculations can be used rationalize the observed optical properties of these materials. Finally, the chemical bond approach has been also applied to interpret the decrease of the glass optical gap with increasing In content.
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