Mimicking Synaptic Behaviors with Cross-Point Structured TiO_x/TiO_y-Based Filamentary RRAM for Neuromorphic Applications

This paper presents the fabrication and characterization of the cross-point structure 20 × 20 μm² RRAM with TiO_x/TiO_y bi-layer insulator for synaptic application in neuromorphic systems. The measured oxygen concentration of the TiO_x/TiO_y switching layers of the fabricated devices using X-ray photoelectron spectroscopy analysis showed that the oxygen concentration ratio between TiO_x and TiO_y is ~ 1.5. After electroforming at ~ 5.62 V, the on/off ratio was ~ 76 at 0.2 V with the DC sweep voltage scheme. Synaptic behaviors including long-term potentiation (LTP) and long-term depression (LTD) were performed with 50 identical pulses for the implementation of RRAM into neuromorphic systems based on convolutional neural networks. Also, linearly increased (or decreased) 25 pulses were applied to the device so that the conductance changes linearly. The resulting linear LTP and LTD characteristics were mirror-symmetric, which could maximize the accuracy. For Hebbian learning, the device also mimicked the spike-timing-dependent plasticity properties with a conductance change from − 77.79% to 96.07% using a time-division multiplexing approach.