Effect of secondary phase formation on electrical and optical properties of Al-doped ZnO

It has been shown that increasing substrate temperature above its optimum value leads to an increase of Al concentration in the AZO films, which exceeds the solubility limit and triggers the formation of an insulating metastable homologous (ZnO)3Al2O3 phase. The formation of (ZnO)3Al2O3 is established for the AZO films grown in a given range of deposition conditions, while the films grown at substantially different conditions may show formation of other secondary phases (e.g. aluminium oxide or spinel). This (ZnO)3Al2O3 impedes crystal growth and causes a significant increase of free electron scattering. In turn, it leads to an increase of electrical resistivity of the films. This phase has been observed by XANES even in the films with the lowest Al concentration and the best crystallinity. Increase of this phase volume fraction with increasing Al concentration correlates with observed changes in the film Raman and optical constants spectra.