Research On Switching Mechanism Of Hafnium Disulfide Resistive Switching Memory

With the advent of the artificial intelligence and information era,the storage requirements of big data continue to increase.At present,big data is mainly stored in high-capacity and high-performance semiconductor memories.Semiconductor memory on the domestic market is divided into volatile semiconductor memory and non-volatile semiconductor memory.Volatile semiconductor memory needs to constantly refresh the storage unit to store a large amount of data because the charge in the storage cell will decay or even disappear in a very short time after power off,which causes a lot of energy loss.Furthermore,volatile semiconductor memory is not compatible with CMOS technology and cannot meet the needs.Therefore,the study of non-volatile memory with high density,low power consumption,high speed and low cost is imminent.As one kind of non-volatile memory,resistive random access memory?RRAM?has many advantages,such as simple device structure,low power consumption,fast speed,small storage unit size,and multilevel.RRAM is expected to become the core of the next generation of non-volatile memory.RRAM stores information through the reversible change of the resistance of the memory material between a high resistive state?HRS?and a low resistive state?LRS?.In recent years,hafnium disulfide?HfS₂?has shown extremely superior physical properties in the field of modern electronics and optoelectronics,while HfS₂ has rarely been reported in RRAM field,and its resistive switching mechanism is not yet clear.Thinking that the resistive switching was controlled by the formation and rupture of conductive filament consisted of oxygen vacancy or electrochemical active metal atom,we focuse on the role of sulfur vacancy and electrochemical active metal atom in resistive switching in dichalcogenides?TMDs?HfS₂ RRAM.Combined with the mechanism and Density functional theory?DFT?calculations,experiments are carried out to further optimize the device structure and improve the memory performance of the device.First,chemical vapor transport?CVT?was used to prepare HfS₂ crystal powder in a single temperature zone furnace,and using liquid phase ultrasonic method in CHP?N-cyclohexyl-2-pyrrolidone,N-cyclohexyl pyrrolidone?to strip HfS₂ into a single layer or a few layers nanosheet.Then,the HfS₂ nanosheet was used to prepare memory device with sandwich structure.The best experimental parameters for the preparation of HfS₂ resistive memory are summarized by exploring the influence of different preparation parameters on the resistance performance of HfS₂ resistive devices.Secondly,we studied the resistive switching characteristics and mechanism of Pt/Al/HfS₂/p⁺-Si.In order to improve the resistive performance,DFT calculation results were used to design devices with better performance of Pt/Al/HfS₂/ITO structure,and the resistive switching characteristics of the device are studied experimentally.Pt/Al/HfS₂/p⁺-Si structure device have typical bipolar resistive switching behavior.The DFT calculation results show that doping HfS₂ film with 3.8%sulfur vacancies is enough to make the p-type HfS₂ semiconductor layer into a metal layer.According to the quantitative energy band diagrams of sulfur vacancy doping and undoped state in Pt/Al/HfS₂/p⁺-Si devices,the resistive switching phenomenon can be explained by the conversion of HfS₂ layer between semiconductor and metal.Although Pt/Al/HfS₂/p⁺-Si device have relatively stable resistive switching characteristics,the window value is small and operating voltages is high.In order to solve the problems existing in Pt/Al/HfS₂/p⁺-Si device,we need to design new device structures.The DFT calculation results show that the potential barrier and affinity between the resistive layer and bottom electrode have a great influence on the window value and operating voltage of memory device.The electron affinity of ITO is almost the same as HfS₂ doped with sulfur vacancies,using ITO instead of p⁺-Si is expected to increase the window and reduce the operating voltage.Therefore,we prepared Pt/Al/HfS₂/ITO device.By comparing the I-V characteristic curves of Pt/Al/HfS₂/p⁺-Si and Pt/Al/HfS₂/ITO device,it was confirmed that Pt/Al/HfS₂/ITO device do have higher window values and lower operating voltages.Although the window value of the Pt/Al/HfS₂/ITO device is large and the operating voltage is low,it can basically meet the application requirements.But the endurance of the device is poor,which may be caused by the unpackaged device.Sulfur ions may diffuse into the air during testing of unpackaged devices,there is not enough sulfur ion compound sulfur vacancies around the device after multiple resistive switching tests,and the device cannot be reset to HRS,which eventually leads to device failure.In order to solve this problem,we use Ag electrodes instead of Al electrodes.DFT calculations indicate that Ag atoms are easily adsorbed by HfS₂films with sulfur vacancies,and the adsorption energy decreases and the system stability increases as the Ag atoms adsorbed increase.If Ag is used as the top electrode of the HfS₂ resistive device,Ag atom instead of sulfur vacant can avoid the problem of poor endurance caused by the loss of sulfur ions.The device should have good resistive performance.According to the DFT calculation results,we re-prepared the device with the structure of Ag/HfS₂/p⁺-Si,and tested the resistance performance of the device.Test results show that the device's endurance and retention has been greatly improved.The device has very stable resistive switching characteristics in1000 read and write operations,and HRS and LRS showed almost no degradation after 1×10⁴s.The switching mechanism of Ag/HfS₂/p⁺-Si memory device is controlled by the formation and breakage of conductive filaments?CFs?made of Ag atoms.Under the action of forward voltage,conductive filament composed of Ag atoms connecting the upper and lower electrodes is formed in the device,and the device is in highly conductive state.The conductive filament breaks under the action of reverse voltage and the device returns to low conductivity state.The results of these studies indicate experiments were conducted combining with the resistive switching mechanism and DFT calculation,which are helpful to design the device structure to improve and optimize the resistive switching characteristics of the memory device.In addition,Ag/HfS₂/p⁺-Si memory devices can be expanded to satisfactory number of read and write cycles,which has potential application value.