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 MZn(NO₃)₂+ x Al(NO ₃)₃ ·9H₂O electrochemical bath, with Al³⁺ concentrations between 0 and 2 mM. Elec-trodeposition conditions were optimized to remove bubbles, increase grain size homogeneity and ensureadherence. Films were characterized by field emission scanning electron microscopy, X-ray diffraction,X-ray photoelectron spectroscopy, UV.vis transmittance, electrochemical impedance spectroscopy andphotocurrent spectroscopy. Films were crystalline with the wurtzite structure and present a morphologymade of hexagonal nano-pillars. It was found that Al incorporation increases gradually up to ∼11 at% forsamples prepared within the concentration range 0.0-0.3 mM Al³⁺ in the bath. For higher Al ³⁺ contents(0.4 mM) an amorphous Al₂O₃-like compound develops on top of the films. In the grown films with Alcontents up to 11 at%, changes in the optical band gap from 2.88 eV to 3.45 eV and in the carrier densi-ties from 10¹⁹to 10²⁰cm^-3were observed. The blue shift in the band gap energy was attributed to theBurstein-Moss effect. Changes in the photocurrent response and the electronic disorder were also dis-cussed in the light of Al doping. Optical transmittances up to 60% at 550 nm were obtained, thus makingthese films suitable as transparent and conductive oxide films.