Effect of elevated temperature on electrical properties and structure of ZnO:Al films with different dopant concentrations


Effect of elevated temperature on electrical properties and structure of ZnO:Al films with different dopant concentrations

Vinnichenko, M.; Rogozin, A.; Cornelius, S.; Shevchenko, N.; Gago, R.; Jimenez, I.; Kolitsch, A.; Möller, W.

Al-doped ZnO (ZnO:Al) is a cost-effective alternative to the Sn-doped indium oxide which is widely used as a material for transparent electrodes. To further reduce the production costs, it would be preferable to sputter reactively from a metal alloy target at sufficiently high partial pressure of oxygen. However, under this condition, a sufficiently low resistivity of the films can not readily be obtained, so that a deposition on heated substrates is necessary. The mechanisms of incorporation and electrical activation of the Al doping impurity into ZnO at elevated temperatures are not well understood that makes difficult improvement of reproducibility and long-term stability of the film electrical properties.
In order to have a deeper insight into these processes, polycrystalline ZnO and ZnO:Al films were grown by reactive pulsed magnetron sputtering of Zn and Zn:Al (4.7 and 8.7 at. % of Al) targets, respectively. The O2 partial pressure in Ar-O2 sputtering gas mixture was precisely monitored during deposition using a capacitance gauge. The substrate temperatures (Ts) were spanning in the 40-580 °C range. The films were characterized by Hall effect measurements, spectroscopic ellipsometry (SE), X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES). At fixed oxygen partial pressure, the electrical resistivity of ZnO:Al films shows a clear minimum at Ts=300 and 350 °C for the films grown using magnetron targets with 8.7 and 4.7 at. % of Al, respectively. The lower resistivity is due to the increased density and mobility of free electrons according to the Hall effect measurements. A maximum of the film crystallinity (grain size) is observed at temperatures of 250 and 300 °C for higher and lower Al concentration, respectively. At Ts>350 °C, the ZnO:Al film crystallinity significantly deteriorates with even stronger temperature effect at higher Al content, leading to formation of nanocrystalline ZnO:Al films at Ts>400 °C in the latter case. It is in contrast to undoped ZnO films grown at identical conditions whose crystallinity always improves by increasing Ts. XANES results show that the poorer film crystallinity and higher resistivity at high Ts can be related to a new homologous phase (ZnO)3(Al2O3) . Further, the electrical properties correlate with changes in the O(1s) absorption edge, whereas the Zn(2p) edge shows no modification with respect to undoped ZnO films.
The authors gratefully acknowledge the financial support of SMWA/SAB under the Project #11815/1854.

Keywords: transparent conducitve oxides; Al-doped ZnO; free electron mobility; reactive pulsed magnetron sputtering

  • Lecture (Conference)
    2nd International Symposium on Transparent Conductive Oxides, 22.-26.10.2008, Hersonissos, Crete, Greece

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