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A unified microscopic principle is proposed to clarify resistive switching behaviors of transition metal oxide based resistive random access memories (RRAM) for the first time. In this unified microscopic principle, both unipolar and bipolar switching characteristics of RRAM are correlated with the distribution of localized oxygen vacancies in the oxide switching layer, which is governed by the generation and recombination with dissociative oxygen ions. Based on the proposed microscopic principle, an atomistic simulation method is developed to evaluate critical memory performance, and successfully conduct the device optimization. The experimental data are well in line with the developed simulation method.

Transport measurements of both the dc and the low-frequency ac are performed on Pt/HfO 2−x /TiN resistiveswitching memory cells at various temperatures. The conductance of the pristine cells has a power law ω S T N relationship with temperature and frequency. To account for the much larger conductance of both the high resistance states (HRSs) and the low resistance states (LRSs), an additional conductance term associated with oxygen vacancy filaments is added, which is independent of the cross-sectional area of the memory cell. This additional component of conductance in a HRS is frequency independent but temperature dependent, showing the small polaron originated transport, with an activation energy of 50 (2.1) meV at temperatures above (below) half of the Debye temperature, which agrees with the analysis of the electric field dependence data. The frequencyand temperature-dependent conduction of HRSs indicate the existence of polarization centers which facilitate the transport and make HfO 2−x highly polarizable. However, the additional conductance term associated with filaments in LRS, of an order of ∼10 5 S m −1 , exhibits a weak metallic behavior in temperature-dependent measurements. Properties of aligned oxygen vacancy chains on the ( 111) surface are calculated by first-principles simulation. Through analysis of the partial density of states and spatial distribution of the wave function of impurity states generated by oxygen vacancies, this weak metallic behavior is attributed to the delocalization of the impurity band associated with aligned oxygen vacancies.

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