OPTICAL-PROPERTIES OF TERNARY AGSBS2 THIN-FILMS

Structural and optical properties of amorphous and crystalline ternary AgSbS2 thin films as well as the effect of annealing temperature on these properties have been investigated. From the study of x-ray diffraction patterns, it is clear that the as-deposited films (300 K) are amorphous in nature, whilst those heat treated under vacuum at moderate temperatures (373 K) for 1 h have been transformed from the amorphous structure to a crystalline structure and the crystallinity increases with increasing temperature. At 500 K a new crystalline plane, (200), appears, characteristic of the binary alpha-AgS2 besides AgSbS2. The optical constants, absorption coefficient alpha, reflective index n, optical energy gap E(g), valence band density of states g(i) and dielectric constants epsilon' and epsilon'', were computed for amorphous and crystalline AgSbS2 films of thickness 169 nm. The analysis of the absorption coefficient data revealed the existence of two optical transition mechanisms, indirect and direct transitions, depending on the value of wavelength range. These energy gaps were found to have the values (1.38-1.17 eV) and (2-1.77 eV) in the temperature range from 300 to 473 K for indirect and direct energy gaps respectively.