LAUSR

HfOx-based resistive random-access memory devices have shown great potential in next-generation non-volatile memory devices, but they are not optimized as synaptic devices for neuromorphic systems. In this paper, the fabrication and biological synaptic behavior of HfO2/HfOx bilayered ultrathin memory devices is studied. 4 nm non-stoichiometric HfOx films were prepared on the TiN-coated Si substrate by plasma-enhanced atomic layer deposition (PEALD) using Hf[N(C2H5)CH3]4 (TEMAH) and hydrogen plasma. 2 nm stoichiometric HfO2 films were then deposited by thermal atomic layer deposition (TALD) using TEMAH and H2O precursors. X-ray photoelectron spectroscopy and electron energy loss spectroscopy were used to analyze the oxygen vacancy concentrations in HfO2/HfOx bilayer films. The effect of oxygen vacancy concentration in non-stoichiometric HfOx layers on resistive switching characteristics of Pt/HfO2/HfOx/TiN bilayered memristive devices has been explored. The memristor with 12.1% oxygen vacancy content in the HfOx layer shows better comprehensive properties with the optimal pulse energy consumption, reset switching speed and DC endurance and retention properties. Each operation is evaluated in a range of approximately ten-picojoules. Some important biological synaptic functions such as nonlinear transmission characteristics, short-term and long-term plasticity, paired-pulse facilitation, spike-timing-dependent plasticity and conditioned reflex have also been demonstrated in optimized memristive devices. This HfO2/HfOx ultrathin memristor is used for an attractive large-scale hardware implementation of neuromorphic simulations.