Research On Fabrication And Application Of Magnesium Oxide Based Physically Transient Threshold Switching Memristors

Physically transient memristors developed in combination with physically transient characteristics can be extended for applications in in secure neuromorphic computing systems,and bio-integrated electronics.Threshold switching（TS）devices,as an important class of volatile memristors that can spontaneously convert from low resistance state to high resistance state,are potentially employed in artificial synapses,artificial neurons,and selectors for memristive crossbars.Compared with physically transient nonvolatile memristors,physically transient volatile TS memristors are less investigated.Although physically transient TS memristors based on Ag active electrodes have been developed,Ag electrode material is not degradable,which restricts the implementation of fully physically transient TS memristors.Furthermore,the realization of fully physically transient TS devices remains a challenge owing to the limitation of transient material selection.Therefore,a fully physically transient TS memristor based on Mg,W and MgO materials is prepared in this paper,and a systematic study is carried out on it.The physically transient TS device simulates key features of biological synapses and nociceptors;Based on the TS characteristics of the physically transient device,an artificial neuron is constructed to emulate the leaky integrate-and-fire（LIF）behavior of biological neurons;In addition,the reliability and durability of physically transient TS devices are optimized by co-sputtering the resistive layer with MgO and Mg,while a physically transient optoelectronic device is prepared,and a photoelectric spiking neuron is constructed by integrating them together.The main research content of this paper is summarized as follows:1.The physically transient devices with the structure of W/MgO/Mg/W are prepared by magnetron sputtering,and the devices exhibit unidirectional TS performance.The device can be cycled 300 times in voltage double sweep mode with off/on ratio of 1.1×10³ and turn on slope of 6.5 m V/dec,respectively,and can be capable of continuous voltage stress testing for 10³ s.Using a water-assisted transfer printing method,the device arrays with crossbar structure prepared by photolithography and development are transferred from a Si O_2/Si substrate to a polyvinyl alcohol（PVA）substrate,achieving a fully physically transient system,and the devices after the transfer still show good TS characteristics.Under the trigger of deionized water,the TS function of the devices is lost after 4 min,and the fully physically transient system is completely degradable after 20 min,demonstrating transient characteristics.2.Physically transient devices W/MgO/Mg/W are fabricated on an ultrathin polyimide（PI）substrate by the water-assisted transfer printing method to form the flexible memristors.The flexible memristors exhibit excellent TS characteristics under different bending conditions.Under the pulse mode,the device simulates not only the paired-pulse facilitation,paired-pulse depression,and the transition from short-term plasticity to long-term potentiation of biological synapses,but also the basic functions of threshold,relaxation,no adaptation,allodynia,and hyperalgesia of biological nociceptors.More importantly,an optoelectronic nociceptor system is constructed by the integration of a photoresistor and the transient TS memristor,demonstrating the feasibility and simplicity of the artificial nociceptive system.3.The temperature of W/MgO/Mg/W devices prepared based on magnetron sputtering is lower than the glass transition temperature of PVA substrates,and the devices are fabricated directly on a PVA substrate to create fully physical transient memristors,exhibiting volatile TS characteristics.In pulse measurement mode,the device turns on and off at 280 ns and320 ns,respectively.The device can be tested for 10⁵ times under the repeated programmable pulse,demonstrating reliable pulse endurance.An artificial neuron based on the transient TS device is constructed,which successfully realized the LIF function of a biological neuron.4.Physically transient devices W/MgO:Mg/Mg/W are prepared by co-sputtering MgO and Mg,which exhibit reliable and sustainable TS characteristics.In comparison with the W/MgO/Mg/W device,the W/MgO:Mg/Mg/W device has lower leakage current(10^-10 A),larger off/on ratio（1.6×10⁵）,steeper turn-on slope（3.8 m V/dec）,and higher endurance（10⁵cycles）.Meanwhile,physically transient photoreceptor devices Mo/Zn O/Mo are fabricated,which shows strong resistance changes in response to UV illumination with various intensities.A photoelectric spiking neuron is built based on the physically transient TS device W/MgO:Mg/Mg/W and the physically transient photoreceptor devices Mo/Zn O/Mo together.The output spike frequency of the photoelectric spiking neuron increases as the UV-365 nm light intensity increases,indicating that the photoelectric spiking neuron can edit the spike signal for different UV light intensities.