Secondary phase formation and AZO film electrical properties
In contrast to the tin-doped indium oxide, the electrical resistivity of the much cheaper Al-doped ZnO (AZO) often increases significantly during either annealing or growth at temperatures above a certain optimum value (typically, between 150 and 300 ºC), which limits the practical applications of AZO films. Outdiffusion of Al from substitutional sites in the ZnO wurtzite lattice and subsequent segregation of Al2O3 are believed to be the main reason for the degradation of film electrical properties. However, experimental observations supporting this assumption are quite limited.
Therefore, the electrical properties of AZO films were investigated with respect to their bonding structure and phase composition probed by X-ray absorption near edge structure (XANES) and X-ray diffraction (XRD) using synchrotron radiation.
Secondary phase formation in AZO
XANES and XRD results point to formation of a new metastable homologous phase (ZnO)3(Al2O3) at substrate temperatures around or above the optimum value. The representative structure of this phase includes Zn and O layers which are similar to those of wurtzite ZnO with certain sheets of Zn replaced by Al sheets, either completely (coordination number, CN=6) or partially (CN=4, 5). This phase is detected even in the films with the lowest electrical resistivity obtained at the lowest Al concentration cFAl~1.2 at%. Increasing TS above its optimum value leads to a higher Al concentration in the AZO films, which exceeds the solubility limit and triggers the formation of an insulating metastable homologous (ZnO)3Al2O3 phase. This phase impedes crystal growth (decreasing the grain size L) and causes a significant increase of free electron scattering which leads to an increase of the film electrical resistivity.
It is shown that lower (Zn + Al)/O flux ratios should be used during deposition in order to enable the growth of low-resistivity AZO films in a wider range of TS. The proposed approach to minimizing the influence of this undesirable phase may also be applied to other growth methods of AZO films involving high-energy particle bombardment.
M. Vinnichenko, R. Gago, S. Cornelius, N. Shevchenko, A. Rogozin, A. Kolitsch, F. Munnik, and W. Möller: Establishing the mechanism of thermally induced degradation of ZnO:Al electrical properties using synchrotron radiation. Appl. Phys. Lett. 96, 141907 (2010).