Electric properties of zinc oxide epitaxial films grown by ion-beam sputtering with oxygen-radical irradiation

Undoped and aluminum-doped ZnO epitaxial films were grown on (001) sapphire substrates by anion:beam sputtering method with or without the irradiation of oxygen radicals. The effect of oxygen-radical irradiation was notable in the undoped ZnO films when the growth temperature was relatively low. The irradiation improved the crystallinity and decreased the oxygen-vacancy concentration, while it induced internal stress into the films. The carrier concentration of the undoped ZnO films was decreased by the oxygen-radical irradiation, which was attributable to a decrease in the oxygen-vacancy concentration. The Hall mobility of the undoped ZnO films was as low as 1-3 cm(2) V-1 s(-1). The low mobility was explained by carrier scattering due to the potential barriers at the grain boundaries. The height of the potential barriers at the grain boundaries decreased with increasing carrier concentration. This behavior was well explained by a simple model assuming a single defect state at grain boundaries.