Field induced oxygen vacancy migration in anatase thin films studied by in situ biasing TEM

Titanium dioxide TiO2 is the most prominent representative within the class of transition metaloxides and is interesting for a large number of applications due to its optical properties, memristivebehaviour, catalytic activity and electrochemical stability. Among the different polymorphs of TiO2,anatase is the preferred configuration for many applications. Even though stoichiometric anataseis a wide band gap semiconductor with an indirect optical band gap of 3.2 eV, its electronic andoptical properties are largely determined by the presence of excess electrons, which can beinduced by dopants or intrinsic defects such as oxygen vacancies (VO). VO are inherently presentin anatase and act as donors in the n-type semiconductor. Their presence induces localizedelectronic states within the band gap, correlated to the formation of Ti3+ ions [1]. Recently we wereable to show that VO form periodic oxygen concentration variations along specific crystallographicdirections without breaking the continuity of the anatase structure, contradicting the previouslyproposed formation of shear planes [2].Related to the formation of such VO superstructures is the question about the origin of thememristive behaviour of anatase. There is theoretical evidence for the mobility of VO along the[100] and [010] crystallographic direction of the crystal in an electric field [3]. The structuralimplications of such field induced VO diffusion have however not yet been studied.Here, we present an in situ biasing TEM study of the atomic structure of oxygen deficient anatasethin films, epitaxially grown on LaAlO3 substrates by Pulsed Laser Deposition (PLD). The TEMmicrographs in Figure 1 depict such a film in its initial state (a) and after increasing the voltage to3.5 V over 30 min (b). The periodic contrast variations typical for the presence of vacancysuperstructures in TiO2 films are already visible in the initial oxygen deficient state (Fig. 1a). Afterapplying 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 arrangementand the relative distances between the defective planes of the modulated structure in the observedregion.Our experimental approach enables us to apply an E-field parallel to the in-plane direction of thefilm by using a standard MEMS-based in situ biasing platform (DENS Solution Lightning), whichallows to shed light on the underlying mechanisms in electromigration and electroforming in TiO2thin films.