Study and improvement of aluminium doped ZnO thin films: Limits and advantages

ZnO:Al films were deposited at 70°C at a fixed −1.1V potential onto ITO substrates from a 0.01M Zn(NO3)2+x Al(NO3)3·9H2O electrochemical bath, with Al3+ concentrations between 0 and 2mM. Electrodeposition conditions were optimized to remove bubbles, increase grain size homogeneity and ensure adherence. Films were characterized by field emission scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis transmittance, electrochemical impedance spectroscopy and photocurrent spectroscopy. Films were crystalline with the wurtzite structure and present a morphology made of hexagonal nano-pillars. It was found that Al incorporation increases gradually up to ∼11at% for samples prepared within the concentration range 0.0–0.3mM Al3+ in the bath. For higher Al3+ contents (>0.4mM) an amorphous Al2O3-like compound develops on top of the films. In the grown films with Al contents up to 11at%, changes in the optical band gap from 2.88eV to 3.45eV and in the carrier densities from 1019 to 1020cm−3 were observed. The blue shift in the band gap energy was attributed to the Burstein-Moss effect. Changes in the photocurrent response and the electronic disorder were also discussed in the light of Al doping. Optical transmittances up to 60% at 550nm were obtained, thus making these films suitable as transparent and conductive oxide films.