DEPENDENCE OF OPTICAL BAND-GAP ON THE COMPOSITIONS OF SE(1-X)TEX THIN-FILMS

Thin films of binary system Se(1-x)Tex (x = 0.2, 0.4, 0.5, 0.8) have been prepared by thermal evaporation under vacuum of 10(-5) Torr. The thickness of the films were about 1000 angstrom. The optical gap is determined as a function of composition. It is observed that the width of optical gap lies between 1.8 and 1.06 eV. The optical gap decreases with increasing Te content. The structure of compositions is transformed from a chain-like structure to a trigonal one depending on Te contents. The validity of the Urbach relation has been proved for compositions of 0.2 < x < 0.5. X-ray diffraction patterns show that polycrystalline films start to appear for x = 0.5 and 0.8. The optical constants (n, k) were determined ellipsometrically in the wave length range 4000 - 5400 angstrom.