Effects of Secondary Phase Formation on Dielectric Function of Al-doped ZnO Thin Films

Recent investigations of Al-doped ZnO (AZO) thin films using techniques based on synchrotron radiation show that formation of metastable homologous phase (ZnO)3Al2O3 is the reason for the observed deterioration of the film electrical properties at deposition temperatures higher than certain optimum value [1]. The effect of this phase formation on the AZO film optical properties has not been studied yet. Moreover, little is known about optical properties of ZnO-Al2O3 solid solutions in general and (ZnO)3Al2O3 phase in particular, although this information might facilitate in situ diagnostics of the film properties evolution. The present work is focused on characterization of AZO and undoped ZnO films by spectroscopic ellipsometry in a wide spectral range (0.73-5.8 eV). For selected samples, ellipsometric measurements were combined with reflection and transmission measurements. The films with defined Al concentrations (cFAl=0-20 at.%) grown by reactive pulsed magnetron sputtering at temperatures ranging from RT to 550 °C were investigated.

The complex dielectric function of AZO films was obtained using a general oscillator model which combines Drude term in order to account for the free electron absorption with parameterized semiconductor oscillator model (PSEMI) [2] to account for interband transitions. The comparison of undoped ZnO and AZO films with the highest crystallinity shows that an addition of ~1 at.% of Al leads to the lowest electrical resistivity and to substantial decrease of the refractive index in the whole spectral range. This is accompanied by broadening of the PSEMI oscillator around the fundamental transition energies. It is characteristic of ZnO with high defect concentrations and is consistent with observed presence of the (ZnO)3Al2O3 phase-related peaks in x-ray absorption near edge structures (XANES) of Al K-edge. The AZO films remain conductive with cFAl values increasing up to ~8-10 at.%, while their (ZnO)3Al2O3 phase-related peaks in Al K-edge spectra scale with cFAl. In this case, refractive index decreases and PSEMI oscillator broadens further which is in agreement with deteriorating film crystallinity.

Finally, increasing cFAl>10 at.% leads to formation of insulating nanocrystalline films, which show even more intense (ZnO)3Al2O3 phase-related XANES peaks. These films have the lowest refractive index, which, however, is still substantially higher than that of amorphous Al2O3. It is observed that these films have significantly lower amplitude and a blue-shifted PSEMI oscillator compared to conductive AZO films. The latter may be explained neither by the Burstein-Moss shift because the films are insulating nor by effective medium approximation using optical constants of ZnO and Al2O3. Instead, it is explained in analogy to optical properties of the metastable wurtzite Mg(x)Zn(1-x)O alloys.

[1] M. Vinnichenko, R. Gago, S. Cornelius, N. Shevchenko, A. Rogozin, A. Kolitsch, F. Munnik, and W. Möller, Appl. Phys. Lett. 96, 141907 (2010).
[2] B. Johs, C.M. Herzinger, J.H. Dinan, A. Cornfeld, and J.D. Benson, Thin Solid Films 313-314, 137 (1998).