| Summary: | Novel niobium heterostructure devices that integrate aluminum, hafnium, and chromium oxide are designed and constructed by sputtering and atomic layer deposition. The devices are examined for use in resistive switching (RS) memory cells. Specifically, Nb-AlO<sub>x</sub>-HfO<sub>x</sub>-AlO<sub>x</sub>-Nb is directly compared with Nb-AlO<sub>x</sub>-HfO<sub>x</sub>-CrO<sub>x</sub>-AlO<sub>x</sub>-Nb. Stable RS is observed in both cases and the conduction mechanisms are analyzed in detail. There are profound differences in the current (I-V) and capacitance (C-V) versus voltage characteristics with the insertion of CrO<sub>x</sub>, including enhanced RS at reduced voltage and current and increased non-linearity in the C-V. The I-V characteristics are fitted to established conduction models and excellent agreement is obtained considering a Poole Frenkel emission and a Fowler Nordheim tunneling component that emerges strongly with increasing voltage. Direct quantum mechanical tunneling and ionic conduction currents are apparent, but weaker as expected with 6-8 nm film. The changes subsequent to RS, are captured with significant modification of the effective trap energy level, and density of the trap states' parameters and a small change in the electrode barrier height, supporting primary physical change in the properties of the bulk of the film stack and a minor change to the electrodes. Evidence of RS processes that involve both positively charged chromium and negatively charged oxygen ionic effects that effectively modify the current conduction are apparent. The plausible physical mechanisms that contribute to the RS are discussed in terms of changes to the bonding configuration and the active roles of ions and vacancies/defects in driving the formation and growth of conduction filaments. The impacts of ferro-electric like enhancements for devices that integrate CrO<sub>x</sub> interfaced with HfO<sub>x</sub> are considered and corroborated by measurements of the ferro-electric hysteresis loops that provide direct evidence of partial ferro-electric switching. Targeting low voltage and high speed operation, pulse measurements of Nb-AlO<sub>x</sub>-HfO<sub>x</sub>-CrO<sub>x</sub>-AlO<sub>x</sub>-Nb devices, demonstrate capability to SET/RESET with <;50 ns pulses and potential for excellent endurance and nonvolatile retention times. With liquid nitrogen cooling, RS properties are retained with a consistent reduction of the thermally dependent current conduction components as compared to the tunneling.
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