Properties, structure and phase composition of transparent conductive oxide thin films grown by magnetron sputtering


Properties, structure and phase composition of transparent conductive oxide thin films grown by magnetron sputtering

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

Understanding of the mechanisms of donor impurity incorporation, its electrical activation and charge carrier transport in transparent conducting oxides (TCO) is required for further improvement of functionality of this class of materials. The present work focuses on investigation of indium oxide (IO), Sn-doped indium oxide (ITO), ZnO, and ZnO:Al (AZO) films grown by reactive pulsed magnetron sputtering (RPMS) with a precise control of the oxygen partial pressure at substrate temperatures, Ts, ranging from RT to 550°C. In order to explore potential advantages of RPMS, the relationship between the deposition parameters and structure, phase composition and physical properties of these TCOs was investigated. The films were characterized by spectroscopic ellipsometry, Hall effect measurements, X-ray diffraction (XRD) and, in case of ZnO and AZO films, by X-ray absorption near edge structures (XANES). The Sn concentration in ITO was determined by Auger analysis, while the Al concentration in ZnO matrix was estimated by elastic recoil detection analysis and Rutherford back scattering.
The comparison of the real-time behavior of the IO and ITO film structure and electrical properties during annealing provides a direct evidence of Sn donor activation (with an estimated efficiency of 40%) in ITO due to amorphous-to-crystalline transition. The ITO film crystallinity always improves with increasing substrate temperature or during isothermal annealing, with the electrical resistivity decreasing. In contrast, the electrical resistivity of AZO films shows a clear minimum at an optimum substrate temperature (200-400 °C), which depends on metal/oxygen flux ratio and correlates with a maximum in crystallinity (grain size). In this case, the highest mobility value of 46 cm2 V-1 s-1 is comparable to the best values achieved in AZO films grown by less cost-efficient techniques. This value is achieved at the free electron density of 6x1020 cm-3 which corresponds to maximum ~30% electrical activation of Al impurity. At higher temperatures, the AZO electrical properties and crystalline quality deteriorate abruptly according to the following mechanism. 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 and causes a significant increase of free electron scattering both at grain boundaries and inclusions of this phase. In order to enable the growth of low-resistivity AZO films in a wider range of TS, lower metal/oxygen flux ratios should be used. The proposed approach to minimizing the influence of this undesirable phase may also be applied to other deposition methods of AZO involving high-energy particle bombardment.

Keywords: Al-doped ZnO; transparent conductive oxides; electrical properties; optical properties; phase composition

  • Lecture (others)
    Invited talk during visit to "Next Energy" EWE-Forschungszentrum für Energietechnologie e.V., 10.-11.02.2010, Oldenburg, Germany

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Publ.-Id: 14201