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Correlation and ordering of defects in the formation of conducting nanofilaments

The correlation and ordering of defects in the formation of conducting nanofilaments is important not only from a fundamental point of view to understand the resistive switching phenomenon, but also from the promising perspective of engineering the nanofilament process in a controlled manner for high-performance devices. Here, we study the dependence of the formation energies of oxygen and metal defects on the chemical potentials of electrons as well as the atomic constituents in Ta2O5 and TiO2, and in the corresponding hypostoichiometric nanofilament composition. In studying a single defect, a metal defect (vacancy/interstitial) is found to be energetically preferred to the corresponding oxygen defect counterpart (interstitial/vacancy). This is different from the current experimental observation that oxygen defects are the main defect type in the resistive switching process. A randomly distributed multiple-defect description without the effect of correlated atomic rearrangement does not resolve this discrepancy. By noticing that the nanofilament has a certain ordered atomic structure, we propose a correlated multiple-defect description where multiple defects undergo correlated atomic rearrangement. The experimental observation of oxygen defects as the main defect type in the resistive switching process and the formation of Magneli-phase Ti4O7 nanofilaments are fully explainable within this description. The correlated atomic rearrangement effect of multiple defects can be used to predict the morphologies of nanofilaments in other resistive switching materials where experimental characterization is difficult.