Modification of near band edge emission and structure with Ga-related clusters in Ga-doped ZnO nanocrystal films

Transparent conducting Ga-doped ZnO films were grown by ultrasonic spray pyrolysis with the different Ga contents of 1.0–6.5 at. %. The Ga impact on the morphology, crystal structure, photoluminescence (PL), Ga cluster formation, and electrical resistivity of ZnO nanocrystal films has been investigated. ZnO-Ga films are characterized by the hexagonal wurtzite structure with the (002) preferential orientation. It was shown that the PL intensity of near band edge (NBE) emission band A (3.18 eV) enlarges at a Ga doping of 1.0–3.0 at. % together with the decrease in electrical resistivity. Simultaneously, the XRD peaks shift to high values due to the decrease in interplanar distances, and ZnO crystallinity improves. New NBE emission band B (3.08 eV) was detected in the PL spectra of films with the Ga content ≥3.0 at. %. The PL band B was assigned to the optical transitions via Ga-related clusters formed by Ga atoms at higher Ga concentrations. Simultaneously, the 2Θ positions of XRD peaks decrease, owing to the increase in the ZnO crystal lattice parameter, as well as the fall down in the NBE emission intensity and ZnO film crystallinity. To study the Ga ion charge states and Ga cluster formation in the ZnO:Ga films, x-ray photoelectron spectra have been investigated. The optimal Ga concentration in the ZnO films has been estimated.