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 b een prepared by thermal evaporation under vacuum of 10(-5) Torr. The t hickness of the films were about 1000 angstrom. The optical gap is det ermined 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 w ith increasing Te content. The structure of compositions is transforme d from a chain-like structure to a trigonal one depending on Te conten ts. The validity of the Urbach relation has been proved for compositio ns of 0.2 < x < 0.5. X-ray diffraction patterns show that polycrystall ine 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.