Defectivity of Al:ZnO thin films with different crystalline order probed by Positron Annihilation Spectroscopy

Three Positron Annihilation Spectroscopy (PAS) techniques have been employed to investigate the point defects of Al-doped Zinc Oxide (AZO) thin films grown by Radio Frequency (RF) magnetron sputtering with different substrates and deposition parameters. The films were grown with thickness varying from 100 to 300 nm, and their crystalline quality ranged from single crystalline epitaxial to partially amorphous. We found that the main defect in the crystalline samples is the 3V(Zn) - V(O) four vacancy complex, with a concentration around 10^(18) - 10^(19) cm−3. In polycrystalline films larger vacancy clusters, within 10% - 20% of the total concentration, were detected. These vacancy clusters are inferred to be most likely located at the grain boundaries. In partially amorphous films the concentration of these larger vacancy clusters, located either at grain boundaries or in the amorphous regions of the film, approached even the 40%, and also some sub-nano voids have been observed.