Titanium dioxide TiO₂ is the most prominent representative within the class of transition metal oxides and is interesting for a large number of applications due to its optical properties, memristive behaviour, catalytic activity and electrochemical stability. Among the different polymorphs of TiO₂, anatase is the preferred configuration for many applications. Its electronic and optical properties are largely determined by the presence of excess electrons, which can be induced by dopants or intrinsic defects such as oxygen vacancies (VO) [1]. VO have recently been shown to form periodic superstructures along certain crystallographic orientations [2]. The presence of such VO is also related to the question of the memristive behaviour of anatase. The structural implications of field induced VO diffusion in anatase have however not yet been studied.

Here, we present an in situ biasing TEM study of the atomic structure of oxygen deficient anatase thin films, epitaxially grown on LaAlO₃ substrates by Pulsed Laser Deposition (PLD). The TEM micrographs in Figure 1 depict such a film in its initial state (a) and after increasing the voltage to 3.5 V over 30 min (b). The periodic contrast variations typical for the presence of vacancy superstructures in TiO₂ films are already visible in the initial oxygen deficient state (Fig. 1a). After applying an E-field along the [100] orientation they are, however, found to significantly increase. This finding points towards an increase of VO which preserve the overall structural arrangement and the relative distances between the defective planes of the modulated structure in the observed region.

Our experimental approach enables us to apply an E-field parallel to the in-plane direction of the film by using a standard MEMS-based in situ biasing platform (DENS Solution Lightning), which allows to shed light on the underlying mechanisms in electromigration and electroforming in TiO₂ thin films.

References

[1] B. Gobaut et al., ACS applied materials & interfaces, 2017, 9,23099.

[2] D. Knez et al., Nano letters, 2020, 20, 6444.

[3] A. Paris, S. Taioli, J. Phys. Chem. C, 2016, 120, 22045.