Al-doped ZnO films grown by reactive magnetron sputtering: properties evolution and secondary phase formation

Al-doped ZnO films grown by reactive magnetron sputtering: properties evolution and secondary phase formation

Vinnichenko, M.; Cornelius, S.; Krause, M.; Gago, R.; Munnik, F.; Kolitsch, A.; Moeller, W.

Reactive pulsed magnetron sputtering (RPMS) using high metal to oxygen flux ratio is know to provide high-quality transparent and electrically conductive Al-doped ZnO (AZO) films at low temperatures (Ts<200 °C) [1]. However, in this case electrical properties of AZO films strongly depend on deposition temperature and the films even turn insulating at Ts>350 °C [2]. It has been shown that energy deposition during growth due to the elevated TS and from the flux of energetic particles incident on the substrate causes preferential Zn desorption. This leads to a higher Al concentration in the 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) and causes a significant increase of free electron scattering which leads to observed increase of the film electrical resistivity [2]. Little is known about effect of this phase formation on the AZO film optical properties, although it is of special importance for understanding of the influence of the secondary phase formation on the fundamental and above-band gap band-to-band electron transitions in this material.

In order to clarify the problem the films with defined Al concentrations (0-20 at.%) grown by RPMS at temperatures ranging from RT to 550 °C were investigated. They were characterized by Hall-effect measurements, spectroscopic ellipsometry and Raman spectroscopy. These results were complemented by the Rutherford back scattering, elastic recoil detection analysis, X-ray diffraction and X-ray absorption near edge structure (XANES) measurements.

The comparison of undoped ZnO and AZO films with the highest crystallinity shows that an incorporation of ~1 at.% of Al leads to the best electrical properties, although (ZnO)3Al2O3 phase signature appears in its Al K-edge XANES spectra even at this low dopant concentration. This is accompanied by the broadening of the allowed ZnO Raman lines and appearance of the broad band around 565 cm-1 which is interpreted as a defect-enhanced A1 LO mode. Increase of Al concentration up to 8-10 at.% leads to deterioration of the film electrical properties accompanied by an increase of the (ZnO)3Al2O3 phase-related peaks in Al K-edge spectra. At this level of doping the allowed ZnO Raman lines are no longer detectable, but the defect-induced Raman features change their intensity distribution. Finally, increasing cFAl>10 at.% leads to formation of electrically insulating nanocrystalline films, which show even more intense (ZnO)3Al2O3 phase-related XANES peaks. The band gap of these films is significantly broader compared to that of undoped ZnO or conductive AZO layers. The latter may be understood in analogy to optical properties of the metastable wurtzite MgxZn(1-x)O alloys.

[1] B. Szyszka, Thin Solid Films 351, 164 (1999)
[2] M. Vinnichenko et al, Appl. Phys. Lett. 96, 141907 (2010).

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Publ.-Id: 15932