The Role Of Vacancy And Silver Atoms In ZnO And HfS₂ Based Resistive Random Access Memory

With the development of mobile intelligent terminal,cloud computing,Internet of Things and other technologies,it is urgent to improve the storage capacity of data centers.With the continuous improvement of integration density,non-volatile Flash memory,which dominates the solid-state storage market,has reached the limit of scalability of single node.The storage capacity of the node can no longer meet the demand of data growth,and it faces great challenges in power consumption and reliability.Therefore,new storage technology urgently needs to be developed.As a new type of non-volatile semiconductor memory,resistive random access memory(RRAM)has attracted great attention due to its low power consumption,good data retention ability,high cycle durability and so on.In this paper,by exploring the devices performance of Ag/ZnO/p⁺-Si and Ag/HfS₂/p⁺-Si RRAM.It is found that although the two have the same electrode structure,their resistive switching performance is very different,which is closely related to the functional layer performance difference of the two kinds of devices.At present,there are few studies on the resistive switching mechanism of devices with the same electrode but different functional layer.Therefore,in this paper,based on the same top electrode and bottom electrode(Ag and p⁺-Si),the resistive switching performance and mechanism of two types of RRAM devices with ZnO and HfS₂ as the functional layer are comparatively studied.Density Functional Theory(DFT)calculation to explore the role of vacancy and silver atoms in the resistive switching performance of the two types of devices,which provides an effective idea and method for in-depth study of the internal mechanism of the resistive random access memory.The details are as follows:In this paper,the parameters of Ag/ZnO/p⁺-Si and Ag/HfS₂/p⁺-Si RRAM devices prepared by Sol-Gel and Chemical Vapor Transport(CVT)methods and the effects of post-processing on device performance were investigated.The experimental results show that the oxygen content and sulfur content in the annealing atmosphere affect the resistance of the device.Comparing the electrical properties of the two kinds of devices prepared based on the optimal experimental parameters,it is found that the Ag/HfS₂/p⁺-Si device has better performance and can stable cycle 1000 times with a window value of 10~2,while the Ag/ZnO/p⁺-Si device has fewer cycles and a smaller window.In order to further analyze the reasons for the difference in device performance,two kinds of functional layer materials ZnO and HfS₂ were calculated by DFT calculation.Firstly,pure/vacancy containing ZnO structure and pure/vacancy containing HfS₂ structure were constructed for calculation according to experimental preparation conditions.By comparing the conductivity of pure/vacancy containing ZnO and HfS₂ materials,it is found that the introduction of vacancy leads to the change of band structure of materials,that means the vacancy concentration affects the conductivity of functional layer materials,and then affects the device performance.The results show that the annealing atmosphere is the main reason affecting the performance of the device.Then,the adsorption energy of Ag atoms on the surface of four pure/containing vacancies was calculated.The results show that the adsorption energy required on the surface of HfS₂ structure containing 3.8%sulfur vacancy is lower,which indicates that the existence of sulfur vacancy contributes to the adsorption of Ag atoms,enhances the stability of the system,and makes clusters between Ag atoms more likely to occur.Therefore,Ag conductive filaments are easier to form in Ag/HfS₂/p⁺-Si devices and the stability of the conductive filaments is enhanced.Because the size of the migration barrier represents the degree of migration of Ag atoms,it affects the formation speed and difficulty of conductive filaments in the device.We calculate the migration barrier of Ag atoms on the surface of four pure/vacancy structures,and find that the migration barrier of Ag atoms in 3.8%Vs@HfS₂ structure is larger,which makes the conductive filaments formed by Ag atoms accumulation more stable,resulting in the low resistance value of Ag/HfS₂/p⁺-Si device in 1000 cycles stable.However,the device operating voltage is large.Finally,we calculate the bader charge transfer and analyze the effect of Ag atom adsorption on the original structural charge of the four pure/vacancy containing structures.It is found that Ag atoms exhibit non-electric neutrality after adsorption,and the amount of charge transferred in different structures is different.The results show that Ag atoms are more easily reduced to Ag⁺in ZnO@Ag,HfS₂@Ag and 1.4%Vo@ZnO@Ag structures,which results in faster formation of Ag conductive filaments and faster operation of the device.The above results show that the study of resistive switching mechanism combined with experiment and DFT calculation is conducive to our exploration and understanding of resistance performance,and also helps to explain the physical mechanism behind resistance switching transformation,which increases the possibility of macro-regulation of resistance characteristics to a certain extent.