Oxygen Vacancy Creation, Drift, and Aggregation in TiO2-Based Resistive Switches at Low Temperature and Voltage

Transmission electron microscopy with in situ biasing has been performed on TiN/single-crystal rutile TiO2/Pt resistive switching structures. Three elementary processes essential for switching: i) creation of oxygen vacancies by electrochemical reactions at low temperatures (<150 degrees C), ii) their drift in the electric field, and iii) their coalescence into planar faults ( and dissociation from them) have been documented. The faults have a form of vacancy discs in {110} and {121} planes, are bound by partial dislocation loops, and are identical to Wadsley defects observed in nonstoichiometric TiO2 annealed at high temperatures. The faults can be regarded as a precursor to the formation of oxygen-deficient Magneli phases, but 3D secondary phase inclusions have not been detected. Together, the observations shed light on the behavior of oxygen vacancies in relatively low electric fields and temperatures, suggesting that, in addition to the rather accepted notion of oxygen vacancy motion during the writing processes in resistive switching devices, such motion may occur even during reading, and may be accompanied by significant oxygen vacancy creation at modest device excitation levels.