ZnO nanowires (NWs) were grown on Si(100) substrates at 975 °C by a vapor-liquid-solid method with ~2 nm and ~4 nm gold thin films as catalysts, followed by an argon plasma treatment for the as-grown ZnO NWs. A single ZnO NW–based memory cell with a Ti/ZnO/Ti structure was then fabricated to investigate the effects of plasma treatment on the resistive switching. The plasma treatment improves the homogeneity and reproducibility of the resistive switching of the ZnO NWs, and it also reduces the switching (set and reset) voltages with less fluctuations, which would be associated with the increased density of oxygen vacancies to facilitate the resistive switching as well as to average out the stochastic movement of individual oxygen vacancies. Additionally, a single ZnO NW–based memory cell with self-rectification could also be obtained, if the inhomogeneous plasma treatment is applied to the two Ti/ZnO contacts. The plasma-induced oxygen vacancy disabling the rectification capability at one of the Ti/ZnO contacts is believed to be responsible for the self-rectification in the memory cell.

Decreasing switching power of a memory cell to meet demands of further downsizing is feasible with several methods. However, effects of plasma treatment on switching current and power are scarcely investigated. We therefore replaced traditional single storage layer with a HfOx/ZnO bilayer and also treated its interface with argon plasma. The switching current could be suppressed to μA due to a Schottky barrier at the HfOx/ZnO interface. Additionally, argon plasma treatment on the interface enables tunability of switching power and current, which is attributed to the tunable barrier height with the absorbed oxygen species introduced by plasma treatment.

As an industry accepted storage scheme, hafnium oxide (HfO x ) based resistive random access memory (RRAM) should further improve its thermal stability and data retention for practical applications. We therefore fabricated RRAMs with HfO x /ZnO double-layer as the storage medium to study their thermal stability as well as data retention. The HfO x /ZnO double-layer is capable of reversible bipolar switching under ultralow switching current (< 3 µA) with a Schottky emission dominant conduction for the high resistance state and a Poole-Frenkel emission governed conduction for the low resistance state. Compared with a drastically increased switching current at 120 • C for the single HfO x layer RRAM, the HfO x /ZnO double-layer exhibits excellent thermal stability and maintains neglectful fluctuations in switching current at high temperatures (up to 180 • C), which might be attributed to the increased Schottky barrier height to suppress current at high temperatures. Additionally, the HfO x /ZnO double-layer exhibits 10-year data retention @85 • C that is helpful for the practical applications in RRAMs.

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