Optimization of oxygen vacancy concentration in HfO2/HfOx bilayer-structured ultrathin memristors by atomic layer deposition and their biological synaptic behavior

Abstract

HfO_x-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/HfO_x bilayered ultrathin memory devices is studied. 4 nm non-stoichiometric HfO_x 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/HfO_x bilayer films. The effect of oxygen vacancy concentration in non-stoichiometric HfO_x layers on resistive switching characteristics of Pt/HfO₂/HfO_x/TiN bilayered memristive devices has been explored. The memristor with 12.1% oxygen vacancy content in the HfO_x 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/HfO_x ultrathin memristor is used for an attractive large-scale hardware implementation of neuromorphic simulations.