The effect of composition on the optical constants of Se 100xTex (x = 30, 50, 70) chalcogenide thin films of
thickn ess 150 nm are studied. The film is deposited on glass substrate using thermal evaporation technique
under vacuum. X-ray diffraction reveals that the poorly Te content Se 70Te30 film has amorphous
structure while the other films have crystalline phases. Both the transmittance and reflectance are
strongl y dependent on the film composition. The analyses of optical measurements show that the Se 30-
Te70 film has two absorption mechanisms which are direct and indir ect allowed transitions giving raise
two optical energy gaps. Increasing of Se content changes the direct gap to an indirect one. The increase of
Se contents (from x = 30 to 70) leads to increase in the indirect gap from 1.33 to 1.85 eV. The absorption
coefficient, refractive index, extinction coefficient and dispersion energy of the films are found to be
depende d on the film composition.