Effect of oxygen on active Al concentration in ZnO:Al thin films made by PLD

Highlights: • C-axis oriented ZnO:Al thin films were made by pulsed laser deposition. • The nominal Al doping concentration was between 1 and 10 at%. • Films were deposited in oxygen atmosphere and in vacuum. • The effective Al concentration was influenced by deposition ambient. • Vacuum-deposited films had much higher electron concentrations. - Abstract: Al doped ZnO is used as a material for transparent conductive electrodes in solar energy and display screen applications, as well as semiconducting material in electronic and photonic devices. For effective use it is essential to control the electrical and optical properties of ZnO:Al thin films. In order to investigate the influence of oxygen environment on effective Al solubility and intrinsic defects introduced at high doping levels during the film growth, ZnO:Al thin films were deposited in vacuum and oxygen background by pulsed laser deposition method. Films were doped with varying Al concentrations by using targets with Al doping levels of 1–10 at%. In vacuum, substantially increased free electron concentrations were observed for all Al doping levels, which indicates that the formation of acceptor-type defects, acting as electron killer centers, was largely suppressed during the growth in oxygen-poor conditions. The dependence of carrier mobility from Al concentration was also greatly influenced by oxygen conditions during the film growth, suggesting that ionized impurity concentrations in the films deposited in vacuum and oxygen background were significantly different. The results were interpreted in the context of intrinsic acceptor-type defects V_Zn (zinc vacancy), which concentration is strongly modified by the presence of oxygen during the film deposition. These vacancies are assumed to influence free electron concentration and electron mobility by acting as deep electron acceptors and charged electron scattering centers (V_Zn²⁻)