Grain size dependent optical properties of CdI2 films

The optical properties (refractive index and optical energy gap) were studied as a function of film thickness, substrate temperature and heat-treatment temperature for structurally, compositionally and morphologically well characterized CdI2 films grown by thermal evaporation. The variation of residual stress in the film (determined by X-ray diffraction), grain size (D), refractive index (n) and optical energy gap (E-g) was observed to be different in the above three cases. Combining the results of all three experiments, the grain size dependence of n and E-g was derived. Variation of n with grain size may be attributed to the changing packing density of crystallites with grain size and its distribution consistent with SEM results. E-g was found to decrease with D non-linearly and the data fits well with the equation of the type either E-g(D) - 3.61 - 0.023D - 0.0034D(2) or E-g(D) - 3.59 - 0.075 D-2/(2+D). The variation of E-g with D could be attributed as due to the defect densities in the film.