Resistive Switching And Neuromorphic Functions Achieved With Memristive Devices Based On Binary Metal Oxides

With the rapid development of society,the explosive growth of information drives the development of large storage capacity devices and highly effective information processing systems.While the silicon-based semiconductor industry has progressed significantly following Moore's law in the past few decades,it is approaching the ultimate physical limit.Memristor,as the fourth fundamental passive circuit element,apart from resistor,capacitor and inductor,has very unique nonlinear electrical characteristics,and is very promising for information storage,logic operation and neuromorphic compuation.The research of memristors can potentially make significant breakthroughs in the semiconductor industry.In this thesis,thin film and one-dimensional structured memristive devices based on binary metal oxides,such as WO3 and MoO3,are investigated.The performances and mechanisms of resistive switching are studied.Some neuromorphic functions,such as synaptic plasticity and metaplasticity,are mimicked.The main research works are summarized as follows:1.Pt/WO3/Pt devices with the crossbar structure are fabricated using magnetron sputtering technique.The volatile bipolar resistive switching(BRS)with negative differential resistance(NDR)effect is realized in such devices.Based on the rectifying effect,fitting of conduction mechanisms and equivalent circuit analysis,the resistive switching behaviors can be ascribed to the trap/detrap of electrons at the interface between the top Pt electrode and WO3.2.With the modification of the device fabrication parameters,the Pt/WO3/Pt memristive devices show multi-functional BRS after electroforming.Gas bubbles are observed during the electroforming process,likely due to the forming of Magneli phase in WO3.By proper electrical operation,three types of resistive switching are obtained.Two of them can be transformed reversibly,and this performance offers a potential for irregular multi-resistance storage.The resistance state of the third resistive switching shows partial non-volatibility and can be adjusted consecutively by proper pulse stimuli,which can be applied for mimicking the synaptic plasticity of neuron cells.With the systematical study,it is found that such resistive switching behaviors are due to oxygen vacancy based filament rupture/formation,the trap/detrap of electrons at the defect state and the modification of Schottky barrier height at the interface between Pt and WO3 by oxygen vacancies.3.The study of the failure and revival of memristive devices helps in improving the endurance of resistive switching.The revival of a failed memristive device is investigated on Pt/WO3/Pt devices.It is believed that inappropriate operation with high-voltage pulse creates an ordered structure of oxygen vacancies and such an ordered structure makes the normal reset process fail.A precisely controlled operation can disrupt the ordered structure by Joule heating and make the device work again.4.Neuromorphic functions,such as synaptic plasticity including short term plasticity,long term plasticity and spike-timing-dependent plasticity(STDP),are mimicked with Pt/WO3/Pt memristive devices as artificial synapses.Moreover,metaplasticity is implemented based on the conventional plasticity for the first time.The effects of metaplasticity on STDP-based neuromorphic functions are systematically discussed in terms of activity sequence and stimulus interval.Metaplasticity represents a major form of adaptation that helps keeping larger neural networks in appropriate state for learning.This work provides guidance for future neuromorphic computing systems using memristive devices.5.One-dimensional nanomaterials,synthesized by bottom-up approaches,have the advantage of easy scaling-down and exhibit the potential for high-denisty devices.In this work,Pt/WO3/Pt and Au/MoO3/Au memristive devices based on one-dimensional nanostructure are studied.WO3 and MoO3 nanobelts are synthesized through hydrothermal method.Memristive devices based on these one-dimensional nanostructure are fabricated using photolithography.It is found that Pt/WO3/Pt devices show volatile BRS,which probably results from the trap/detrap of electrons at defect states.However,Au/MoO3/Au devices exhibit unipolar resistive switching with the Ron/Roff ratio about 103,which could be accounted for by the formation/rupture of MoO2 filaments formed at dislocations or stacking faults with the help of electrical field and Joule heating.