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

摘要

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/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 HfO(2)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/HfOx/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.