Low-Power Forming Free TiO2–x/HfO2–y/TiO2–x-Trilayer RRAM Devices Exhibiting Synaptic Property Characteristics

The insertion of an HfO
_2-y
layer within TiO
_2-x
/HfO
_2-y
/TiO
_2-x
resistive random access memory (RRAM) yields in low set power of 50 nW (10 nA at 5 V), low reset power of 3 nW (1 nA at -3 V), and good cycling variability (σ/μ <; 0.5). In addition, under pulse experiments, fast switching time of 1 μs, good 107 cycling endurance and retention performance at 150 °C, was demonstrated. The confinement of the switching effect into the HfO
_2-y
film, which has the highest oxygen content and deeper oxygen vacancy energy levels compared with the adjusting two layers of TiO
_2-x
which act as two series resistances, can explain the low switching energy. The gradual modulation of the resistance permits also the manifestation of longterm potentiation synaptic plasticity, induced by the application of a train of pulses with different repetition intervals. A quantitative model was applied in order to reproduce the analog SET/RESET responses of the trilayer configuration and highlight the role of the local distribution of oxygen vacancies. These effects in conjunction with the room temperature fabrication process used and the forming-free nature of the thin films are considered as an optimization route toward high-density RRAM design.