Study of optical properties of some sol–gel derived films of ZnO

The optical transmission and reflection data for aluminum doped zinc oxide films have been analyzed. The optical absorption coefficient (α) and hence the refractive index (n), extinction coefficient (k) and the band gap (Eg) have been determined for these films using different methods. The films are prepared by sol–gel technique and are optically transparent. It is observed that the band gap increases with aluminum doping from 3.19 to 3.24 eV, but is less than the bulk ZnO crystal. A qualitative explanation has been put forward for the band gap widening with doping. The width of the band tail states which is connected to localized states in the band gap is least for 1 at% doped aluminum film. The porosity of the films as deduced from the refractive index seems to be of the same order in all the cases, but relatively higher for 1 at% aluminum doped films.