LTP–LTD transformation of unipolar pulse voltage-driven zinc oxide memristors via TiO2 thin layer incorporation

Abstract

ZnO-based memristors are considered to be promising candidates in neuromorphic computing because of their nonvolatile resistive switching characteristics. Biological synaptic plasticity processes such as long-term potentiation (LTP) and long-term depression (LTD) require the application of driving voltages with different polarity, which inescapably leads to an increase in the operational costs of memristor circuits. In this study, we have successfully introduced a strategic innovation by incorporating a TiO₂ thin layer, thereby achieving unipolar-enhanced bilayer and unipolar-weakened monolayer memristors. In the context of monolayer memristors, the unipolar weakening mechanism primarily arises from the thermal disruption of conduction filaments (CFs) caused by oxygen vacancies. Additionally, the incorporation of a thin TiO₂ layer on the ZnO layer confines Joule heat within the TiO₂ film, inhibiting the promotion of oxygen vacancy diffusion and preventing the occurrence of ZnO CF fracture. Moreover, a two transistor–two memristor (2T2R) unit based on the given memristors is introduced to achieve LTP and LTD characteristics under unipolar voltage pulse excitation. Simulation results show that the 2T2R structure unit can achieve an approximately 50% reduction in time and power consumption during the weight update process.