Transport properties of HfO2−xbased resistive-switching memories

Abstract

Transport measurements of both the dc and the low-frequency ac are performed on Pt/HfO2$-${}x/TiN resistive-switching memory cells at various temperatures. The conductance of the pristine cells has a power law $\omega${}STN 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 frequency- and temperature-dependent conduction of HRSs indicate the existence of polarization centers which facilitate the transport and make HfO2$-${}x highly polarizable. However, the additional conductance term associated with filaments in LRS, of an order of $\sim${}105 S\hskip0.25emm$-${}1, exhibits a weak metallic behavior in temperature-dependent measurements. Properties of aligned oxygen vacancy chains on the (1\ifmmode\bar\else\textasciimacron\fi{}11) 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.