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

ZnO:Al films were deposited at 70 °C at a fixed −1.1 V potential onto ITO substrates from a 0.01 M Zn(NO3)2 + x Al(NO3)3·9H2O electrochemical bath, with Al3+ concentrations between 0 and 2 mM. 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 ∼11 at% for samples prepared within the concentration range 0.0-0.3 mM Al3+ in the bath. For higher Al3+ contents (>0.4 mM) an amorphous Al2O3-like compound develops on top of the films. In the grown films with Al contents up to 11 at%, changes in the optical band gap from 2.88 eV to 3.45 eV and in the carrier densities from 1019 to 1020 cm−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 550 nm were obtained, thus making these films suitable as transparent and conductive oxide films.