Crystallization of TiO2:Nb/Ta: effect of the as-deposited film morphology and local order structure


Crystallization of TiO2:Nb/Ta: effect of the as-deposited film morphology and local order structure

Vinnichenko, M.; Junghähnel, M.; Neubert, M.; Gago, R.; Mücklich, A.; Kolitsch, A.

TiO2-based transparent conductive oxides (TCOs) are of great importance as a cost-efficient, environmentally friendly and chemically inert alternative to conventional TCOs. The two-step process, i.e. growth of amorphous TiO2:Nb (TNO) and TiO2:Ta (TTO) films on glass and their post-deposition annealing provides films with electrical resistivity below 10-3 Ω cm. Further optimization of the growth process and its up-scaling require a deeper understanding of the crystallization process of the TiO2-based amorphous films. Appropriate characterization of the film structure represents a major challenge in this case.
The TNO and TTO films were prepared using two-step process on commercial glass substrates. During the first step, amorphous intermediate material was grown by DC magnetron sputtering of electrically conducting TiO2-x-based ceramic targets. The TNO films were grown using a pilot scale in-line sputtering system with precise adjustment of the O2 flow, fO2, using mass flow controllers. The TTO films were deposited in lab-scale system at higher total pressure and applying a closed-loop feedback system based on plasma emission monitor to tune fO2. The films were subjected to post-deposition thermal annealing in vacuum at TA~450 °C for ~15-60 min. In both cases, it was found that the anatase films with low electrical resistivity (~10-3 Ω cm) and high optical transmittance (>80% in the visible) can be prepared only within very narrow window of fO2 values.
The x-ray amorphous films (those which show no diffraction peaks) upon annealing convert into the films with dramatically different structure and properties depending on the fO2 used during the deposition. In case of fO2fO2*, the as-deposited layers crystallize to high-quality anatase films with high optical transmittance, but higher than optimum electrical resistivity values. The films which form either rutile or anatase phase upon crystallization showed the same signatures characteristic of anatase-like local order in XANES spectra in as-deposited state. This is in apparent contradiction to the crystallization of undoped x-ray amorphous TiO2, when films showing nc-rutile or anatase-like signatures in XANES spectra, crystallize, to rutile and anatase phase, respectively [1]. This discrepancy was resolved by high-resolution cross-sectional TEM measurements. The as-deposited TNO/TTO films grown at fO2fO2* show only homogeneous amorphous morphology. According to TEM, the undoped TiO2 films which converted to rutile after annealing show rutile nanocrystals directly on the film surface [2]. In all cases XANES measurements were performed in total electron yield mode with the electron escape depth in the range of 10 nm, therefore this method is not sensitive to the rutile inclusions situated deeper below the film surface.
These results show that nanocrystallites of rutile present in amorphous TiO2 matrix work as seeds and induce formation of the undesirable rutile phase during annealing. In order to achieve high-quality TiO2-based TCOs, formation of the seeds has to be avoided, which can be realized by careful adjustment of the Ti/O flux ratio during the intermediate film growth.
1. R.Gago, A.Redondo-Cubero, M.Vinnichenko, and L.Vázquez, Chem. Phys. Lett. 511, 367 (2011).
2. R.Gago, M.Vinnichenko, A.Redondo-Cubero, and L.Vázquez, Plasma Process. Polym. 7, 813 (2010).

Related publications

  • Poster
    4th Symposium on Transparent Conductive Materials 2012, 21.-26.10.2012, Chersonnisos, Crete, Greece

Permalink: https://www.hzdr.de/publications/Publ-17950
Publ.-Id: 17950