Effect of composition on the optical constants of Se100−xTex thin films

• The Se100−xTex thin films are deposited using thermal evaporation technique.
• Only highly Te content Se30Te70 film has amorphous structure.
• Increasing of Se content changes the direct optical gap to an indirect one.
• The optical constant of the films depend strongly on the film composition.

The effect of composition on the optical constants of Se100−xTex (x = 30, 50, 70) chalcogenide thin films of thickness 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 Se70Te30 film has amorphous structure while the other films have crystalline phases. Both the transmittance and reflectance are strongly dependent on the film composition. The analyses of optical measurements show that the Se30Te70 film has two absorption mechanisms which are direct and indirect 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 depended on the film composition.