Enhanced redshift of the optical band gap in Sn-doped ZnO free standing films using the sol-gel method

The optical band gap in free standing transparent zinc oxide (ZnO) films using the sol-gel method was studied. The effect of Sn doping on grain size, vibrational structure and on the optical properties of ZnO films was investigated. Contrary to the common observation, the optical band gap of Sn-doped ZnO is red-shifted from 3.38 to 3.18 eV as the doping weight percentage is increased to 3%. The redshift of the optical band gap is due to the deep states in the band gap, and a change of ∼0.2 eV can only be observed when a substrate is not used. This study illustrates that removal of interaction between film boundaries and substrate is essential for developing effective band gap-tunable ZnO thin films.