Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric-field-driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N /H O tracer gas atmosphere combined with time-of-flight secondary-ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO -based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.
Ion transport in ceramics of the low-temperature phase of tantalum pentoxide, L-TaO, was examined by means of diffusion experiments and subsequent analysis of diffusion profiles with time-of-flight secondary ion mass spectrometry (ToF-SIMS). O/O isotope anneals were used to investigate oxygen diffusion, and oxygen tracer diffusion coefficients were obtained for the temperature range of 623 ≤ T/K ≤ 873 at an oxygen partial pressure of pO = 0.2 bar and for the oxygen partial pressure range of 10 ≤ pO/bar ≤ 10 at a temperature of T = 723 K. Cation diffusion in TaO was probed by using chemically similar niobium as the diffusant (in the absence of stable tantalum isotopes). Thin films of NbO were deposited onto TaO ceramics; diffusion anneals yielded niobium diffusion coefficients for the temperature range of 1073 ≤ T/K ≤ 1223 at an oxygen partial pressure of pO = 0.2 bar. Comparison of the measured diffusion coefficients strongly suggests that oxygen is many orders of magnitude more mobile than niobium in L-TaO at these temperatures and at pO = 0.2 bar. The electrical conductivity was also determined in the range 950 ≤ T/K ≤ 1200 and 10 ≤ pO/bar ≤ 10. Considered together with the measured diffusion coefficients, the conductivity data indicate that under oxidising conditions conduction is due to oxygen ions above T = 1090-1130 K and due to electron holes below this temperature range. Point-defect models are presented that are consistent with these transport data and with conductivity data in the literature. They suggest that under oxidising conditions oxygen interstitials are the majority ionic charge carriers in L-TaO. The implications for resistive switching devices are discussed.
The high-mobility, wide-bandgap perovskite oxide BaSnO 3 is taken as a model system to demonstrate that the native point defects present in un-doped, epitaxial thin films grown by hybrid molecular beam epitaxy can be identified and their concentrations at the ppm level determined quantitatively. An elevatedtemperature, multi-faceted approach is shown to be necessary: oxygen tracer diffusion experiments with secondary ion mass spectrometry analysis; molecular dynamics simulations of oxygen-vacancy diffusion; electronic conductivity studies as a function of oxygen activity and temperature; and Hall-effect measurements. The results indicate that the oxygen-vacancy concentration cannot be lowered below 10 17.3 cm −3 because of a background level of barium vacancies (of this concentration), introduced during film growth. The multi-faceted approach also yields the electron mobility over a wide temperature range, coefficients of oxygen surface exchange and oxygen-vacancy diffusion, and the reduction enthalpy. The consequences of the results for the lowest electron concentration achievable in BaSnO 3 samples, for the ease of oxide reduction and for the stability of reduced films with respect to oxidation, are discussed.
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