Self-aligned TiO_x-based 3D vertical memristor for a high-density synaptic array

The emerging nonvolatile memory, three-dimensional vertical resistive random-access memory (VRRAM), inspired by the vertical NAND structure, has been proposed to replace NAND flash memory which has reached its integration limit. To improve the vertical ionic diffusion occurring in the conventional VRRAM structure, we propose a Pt/HfO₂/TiO₂/Ti self-aligned VRRAM with physically confined switching cells through sidewall thermal oxidation. We achieved stable bipolar switching, endurance (>10⁴ cycles), and retention (>10⁴ s) responses, and improved the interlayer leakage current issue through a distinctive self-aligned structure. Additionally, we elucidated the switching mechanism by analyzing current levels concerning ambient temperature. To utilize VRRAM for neuromorphic computing, the biological synaptic functions are emulated by applying pulse stimulation to the synaptic cell. The weight modulation of biological synapses is demonstrated based on potentiation, depression, spike-rate-dependent plasticity, and spike-timing-dependent plasticity. Additionally, we improve the pattern recognition rate by creating a linear conductance modulation with an incremental pulse train in pattern recognition simulations. The stable electrical characteristics and implementation of various synaptic functions demonstrate that self-aligned VRRAM is suitable for neuromorphic systems as a high-density synaptic device.