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Experimental evidence for the formation of titanium vacancies in TiO2 based transparent conductive oxides

Neubert, M.; Lungwitz, F.; Butterling, M.; Gebel, T.; Vinnichenko, M.; Cornelius, S.; Potzger, K.; Anwand, W.; Wagner, A.; Gemming, S.

The work is focused on understanding the physical mechanism leading to limited free electron density and mobility by variation of the oxygen content in polycrystalline tantalum doped TiO2 films. The films were prepared on glass substrates using a two-step approach involving direct-current magnetron sputtering of non-conducting amorphous films followed by annealing in vacuum. It is demonstrated that that fine tuning of the oxygen content during growth is crucial to ensure the formation of anatase films with low resistivity in the range of 10-3 Ωcm and high optical transmittance after the annealing process. An increase of the oxygen content in the anatase film leads to a decrease of the free electron density and the mobility. This dependence of the film electrical properties is discussed in terms of the effective electrical activation of the Ta dopant taking into account the formation of negatively charged acceptor like defects.
Doppler broadening positron annihilation spectroscopy was used to determine the density of negatively charged open-volume defects as a function of oxygen deficiency of the Ta-doped anatase TiO2 films. It is observed that the density of these negatively charged defects increases with increasing oxygen content in the films, which is attributed to the formation of Ti-related vacancies. These acceptor like defects are considered to counteract n-type doping by Ta resulting in a decreasing electron density with increasing oxygen content. Furthermore, due to their maximum charge state of q = -4, Ti vacancies are effective scattering centers for free electrons [1]. Thus, their presence is believed to contribute to the observed decrease of the free electron mobility with increasing oxygen content in the films. These experimental results are consistent with previously reported first-principles calculations [1] of the point defect formation enthalpies for Ti substitution by Ta and for intrinsic Ti-vacancies in anatase TiO2 in dependence of the oxygen chemical potential.

1. J. Osorio-Guillen, S. Lany, and A. Zunger, Phys. Rev. Lett. 100, 036601 (2008).

Keywords: TCO; Transparent Conductive Oxide; Titanium; Tantalum; TiO2; Positron Annihilation; negative defects

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