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 (TiO2). Resistive switching devices were fabricated from single-crystal rutile TiO2 substrates through focused ion beam milling and lift-out techniques. Defect evolution and phase transformations in rutile TiO2 were monitored by diffraction contrast imaging inside the TEM during electrical biasing. Reversible bipolar resistive switching behavior was observed in these single-crystal TiO2 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 TiO2 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.
Transient currents associated with electroforming TiO2-based resistive switching devices were measured using three distinct circuits designed to limit them, and they were correlated with physical changes in the top electrode using scanning electron microscopy. A transient current more than 10 times greater than expected was observed when only using the source meter to limit the current via the compliance set point. The large excursion arose from equipment delays and resulted in significant physical changes to the top electrode. An external resistor was used to decrease the excess transient current value to nearly zero, as long as parasitic capacitive discharges were also suppressed. Simultaneously, the physical changes to the top electrode were completely suppressed, indicating physical damage was related to Joule heating from the excess forming currents. The switching characteristics of all devices were similar, implying damage during electroformation of functional switches can be avoided by device/circuit design.
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