Microsc. Microanal. 21, 140–153, 2015 doi:10.1017/S1431927614013555 © MICROSCOPY SOCIETY OF AMERICA 2014 In Situ TEM Imaging of Defect Dynamics under Electrical Bias in Resistive Switching Rutile-TiO 2 Ranga J. Kamaladasa, 1 Abhishek A. Sharma, 2 Yu-Ting Lai, 1 Wenhao Chen, 1 Paul A. Salvador, 1 James A. Bain, 2 Marek Skowronski, 1 and Yoosuf N. Picard 1, * 1 Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA 2 Electrical and Computer Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA Abstract: In this study, in situ electrical biasing was combined with transmission electron microscopy (TEM) in order to study the formation and evolution of Wadsley defects and Magnéli phases during electrical biasing and resistive switching in titanium dioxide (TiO 2 ). Resistive switching devices were fabricated from single-crystal rutile TiO 2 substrates through focused ion beam milling and lift-out techniques. Defect evolution and phase transformations in rutile TiO 2 were monitored by diffraction contrast imaging inside the TEM during electrical biasing. Reversible bipolar resistive switching behavior was observed in these single-crystal TiO 2 devices. Biased induced reduction reactions created increased oxygen vacancy concentrations to such an extent that shear faults (Wadsley defects) and oxygen-deficient phases (Magnéli phases) formed over large volumes within the TiO 2 TEM specimen. Nevertheless, the observed reversible formation/dissociation of Wadsley defects does not appear to correlate to resistive switching phenomena at these length scales. These defect zones were found to reversibly reconfigure in a manner consistent with charged oxygen vacancy migration responding to the applied bias polarity. Key words: TiO 2 , resistive switching, in situ, TEM, Wadsley defects I NTRODUCTION Resistive switching devices and materials have attracted significant interest due to their promise as next generation nonvolatile dense random access memory (Waser & Aono, 2007; Yang, Strukov & Stewart, 2013). These devices are typically composed of metal–insulator–metal structures where the insulating, or functional, layer can be rendered conductive by application of bias/current. Controlled and reversible augmentation of the functional layer resistivity renders the device a resistive switch. A number of metal oxide materials exhibit resistive switching behavior (Lin et al., 2007; Szot et al., 2007; Lamperti et al., 2008; Yang et al., 2008, 2012; Waser et al., 2009; Gao et al., 2010; Huang et al., 2010; Lee et al., 2010; Oka et al., 2010; Menzel et al., 2011). It is widely accepted that resistive switching in many metal oxides is caused by the generation and redistribution of oxygen vacancies (Waser et al., 2009; Yang et al., 2013) induced by applied electrical bias. As-fabricated structures are typically highly resistive and require an “electroformation” process that lowers the device electrical resistance, often electrochemically reducing the metal oxide functional layer and generating increased concentrations of oxygen vacancies (Szot et al., 2006; Waser & Aono, 2007). The device resistance decrease is attributed to the creation and motion of positively charged oxygen vacancies responding to the bias polarity and reconfiguring within the metal oxide layer of the device. Oxygen vacancy reconfiguration can result in Schottky barrier height reduction at metal/metal oxide interfaces, thereby decreasing device resistance (Sawa, 2008). Oxygen vacancies can also aggregate into extended defects (shear faults) and eventual Magnéli phases. The formation of finite Magnéli phase regions in titanium dioxide (TiO 2 ) films creates electrically conductive filaments that can also decrease device resistance. Bipolar switching in metal oxides is distinguished by SET/RESET processes where device electrical resistivity is increased/decreased under opposite bias polarities. Hence, the modulation of barrier heights and/or reversible transforma- tion of Magnéli phase regions are generally attributed to low and high resistivity states in bipolar resistive switching. A filamentary mechanism is arguably the most com- monly proposed explanation for resistive switching in TiO 2 . This mechanism is plausible since rutile TiO 2 can transform into a variety of Magnéli phases that are more electrically conductive (Szot et al., 2011). These oxygen-deficient com- pounds are formed by crystallographic shear of the rutile TiO 2 crystal structure (Anderson & Tilley, 1970; Bursill & Hyde, 1972). Magnéli phase formation is preceded by orga- nization of oxygen vacancies into planar faults, or Wadsley defects (Bursill & Hyde, 1971). The spatial ordering of Wadsley defects with greater accumulated oxygen deficiency leads to Magnéli phase formation. Therefore, accumulation of oxygen vacancies leads to Magnéli phase formation, which in turn produces greater local electrical conductivity. Thus, Magnéli phases are often attributed to possible electrically conductive filaments in TiO 2 -based resistive switching devices (Yoon et al., 2013; Ghenzi et al., 2014). *Corresponding author. ypicard@cmu.edu Received July 3, 2014; accepted October 21, 2014