Studies On The Synaptic Characteristics Of WO_3-x Based Memristors

The human brain performs learning and memory functions through a large number of synapses and neurons,it can process and store a large amount of information at the same time.Compared with traditional computers,the brain has lower power consumption and better stability.Synapse is the basic unit of signal modulation and transmission.The signal of the former neuron is sent to the post neuron through the synaptic gap,and the information is processed and stored at the same time.Therefore,the development of a device similar to biological synapses will be a prerequisite for the development of neuromorphic systems,and synaptic devices have become a hot topic in recent years.The memristor is similar to the biological synapse in structure and information transmission.Memristor with simple structure,low manufacturing cost,low power consumption,good compatibility with traditional semiconductor process and high integration density,is the most suitable device for simulating biological synaptic function.At present,memory synaptic devices have been successfully used in neuromorphic computing circuits,which can realize the function of graphic recognition.Nevertheless,the performance of memristor synaptic devices still needs to be improved,such as limited conductance range,asymmetric conductance response,stability issues,so it is of great practical significance for the study of synaptic devices and their performance optimization.In this paper,amorphous and nanocrystalline structure tungsten oxide films are successfully deposited by HWCVD（hot wire CVD method）under the condition of pure oxygen at low temperature and low pressure.A thin layer of oxygen deprivation is grown on the upper surface of the film by improving the process of WO_3-Xlayer deposition thus forming an ohmic contact at the W/WO_3-Xinterface.Based on this,the synaptic performance of three WO_3-Xbased memristor is studied.Firstly,the W/m-WO_3-X/ITO structure memristor based on amorphous tungsten oxide film is prepared and studied.The test shows that the ohmic W/m-WO_3-Xcontact makes the device exhibit excellent symmetry.Under the action of positive/negative DC voltage or pulse voltage,the device exhibits almost the same degree of current weakening or enhancement based on the oxygen vacancy injected into the WO_3-Xand its migration.The conductance of the device can be adjusted by changing the parameters of the pulse,such as amplitude,pulse width and the number of pulses,that is,controllable synaptic weight adjustment.The device can also simulate paired pulse facilitation and paired pulse depression,as well as the function of transition between them.Subsequently,the Ag/m-WO_3-X/p⁺-Si structure memristor based on amorphous tungsten oxide film is studied.Using the same scanning voltage as the first device,the device does not exhibit synaptic characteristics but exhibits switching characteristics at higher voltages.The migration of Ag⁺ions in m-WO_3-Xis not obvious at low voltage when oxygen vacancy injection is not enough to cause conductance change,so there is no synaptic characteristic.At higher voltage,the formation and fracture of metal filament channels related to Ag⁺ion migration leads to switching process.Finally,the W/nc-WO_3-X/ITO structure memristor based on nanocrystalline tungsten oxide film is prepared and studied.This device exhibits better synaptic properties.It has a smaller I-V hysteresis area and a higher resolution due to the denser structure of nano tungsten oxide.The I-V curve fitting indicates that the carrier has the same transport mechanism under the positive/negative voltage action,so the I-V curve has a better symmetry.The impedance spectrum test results further show that the carrier transport in the non-oxygen vacancy injection region determines the device characteristics.From the variable temperature test,the trap energy level in tungsten oxide is 0.98 e V under the conduction band,which is consistent with the trap energy level related to the oxygen vacancy in the report.