Structural, optical and photocurrent properties of undoped and Al-doped ZnO thin films deposited by sol-gel spin coating technique

Undoped and Al-doped ZnO thin films have been deposited onto glass substrates by the sol-gel spin coating method and their structural and optical properties have been investigated. The XRD results showed that all films were crystallized under hexagonal wurtzite structure and presented a preferential orientation along the c-axis where the maximum crystallite size was found to be 25 nm for undoped film. The average transmittance of all films is over 95% in the visible region and the band gap energy increases from 3.25 to 3.29 eV with the increase of Al concentration. In addition to the vibrational modes from undoped ZnO, longitudinal optical (LO) mode in the Raman spectra was enhanced by the Al doping. Photoluminescence of the films showed an ultraviolet (UV) and defect related visible emissions like violet, blue and green. 3% Al doped ZnO film shows the best photocurrent properties.