Effect of deposition parameters on the optical absorption in thermally evaporated cadmium telluride thin films

The dependence of the absorption coefficient, alpha, on the photon energy, h nu, has been determined for the CdTe films thermally deposited onto amorphous substrates. Analysis of the results showed that, for CdTe films of different thicknesses (126-1115 nm) deposited at 3 rate of 0.5 nm s(-1) at a substrate temperature of 250 degrees C, direct and indirect transitions occur with energies 1.54+/-0.01 eV and 1.38+/-0.001 eV, respectively. The influence of film thickness, substrate temperature and deposition rate on the determined energy gaps have been studied separately. The results showed thickness-independent values and a negligible effect of the rate of evaporation. The substrate temperature has a significant influence and it was found that, as the growth temperature increases from 30 to 250 degrees C, the direct energy gap increases from 1.51 to 1.56 eV and the indirect gap from 1.30 to 1.44 eV, respectively. This shift is explained in terms of a change in him stoichiometry as revealed by X-ray study. It is also shown that films which are more strained and have crystallites of small sizes and are weakly oriented are more absorbing. The results are also compared with previous findings.