First-Principles Study Of ZnO Resistive Memory

At present,the research on resistive memory becomes a hot research topic.The resistive memory is expected to become the leader of next generation devices,including resistive memory,phase change memory and ferroelectric memory,which are expected to become candidates for next generation memory.Compatible with CMOS process,is expected to become a leader in next-generation memory.In this paper,the physical properties of devices are studied by constructing the structural model of Ag/ZnO/Pt devices.In this paper,the world's most advanced material simulation software ATK simulation calculations,the theoretical basis for the experimental group of Pan Feng,Tsinghua University research group based on ZnO material resistance change memory.This thesis simulates and calculates by using the first principle theoretically.In order to calculate the energy band of the ZnO material accurately,the four schemes of setting the cross-correlation function as LDA,GGA,GGA + U and MGGA are respectively calculated.The method of MGGA overcomes the inaccuracy of calculating the semiconductor material by using LDA and GGA.The calculated band gap of ZnO is close to the experimental one.Adding different voltages at both ends of the device affects the forbidden band width of the ZnO material.When an electric field is applied across the device,the band gap of the ZnO decreases.As the applied voltage increases,the calculated voltage of the semiconductors The band gap of material ZnO decreases.At the same time,the density of states of ZnO is calculated.From the calculation of the density of states diagram,the contribution of d orbital in Zn is mainly analyzed for the angle of Zn element.The analysis of elemental 0 is mainly the contribution of electrons in S and P orbitals in O The band is mainly a result of the common role of the 0 element and the electron on the Zn element,which is mainly the result of the contribution of electrons on the oxygen element.Considering ZnO crystal structure,amorphous state and the impact of process environment on the material,the amorphous state of ZnO crystal is mainly studied after high temperature annealing.By comparing the radial distribution function of crystal structure with that of amorphous structure,the change of coordination number distribution function was used to determine the crystal structure of ZnO during the high temperature retreat.In the crystal structure,the coordination number of ZnO before annealing at high temperature is 4,and the coordination number after annealing at high temperature appears mainly in 4-coordination,and the coordination of 3,5 and 6 occurs at the same time.From the comparison of the radial distribution function diagrams of crystal structure and amorphous structure,it is found that the first peak appears in the crystal structure at 1.782 A,the second peak appears at 2.127 A and the third peak appears at 3.542 A.The amorphous state shows that the position of the first peak appears at 2.057A,the position of the second peak appears at 2.410A and the position of the third peak appears at 4.107A,indicating that the position of atoms in the crystal structure has changed.By calculating the formation energy of oxygen vacancies in the crystal structure and the amorphous structure,it is found that in the amorphous crystal structure,the formation of oxygen vacancies can be lower,and high writing speed and low operating voltage can be obtained.The formation of oxygen vacancies in the crystal structure can be relatively high.The formation of oxygen vacancies in the amorphous structure can be negative indicating the spontaneous formation of oxygen vacancies in the amorphous structure.The work functions of the metal surfaces of different electrode materials were calculated,including Ag(100),Ag(111),Pt(100),Pt(111),Pt(001),Cu(100)and Cu(111).Comparing the work function of the metal surface,the structure of the interface constructed in this way is easy to form ohmic contact if the work function of the two metal surfaces is similar or the work function of the surface of the middle insulator is similar to the work function of the metal.If the structure of the interface is large,the work function gap between the two interfaces is large.The surface model of the device thus constructed is easy to form non-ohmic contact and behaves nonlinearly in IV characteristics.In this thesis,the diffusion of Pt atoms on the Pt(100)surface is studied:one is the direct jumping process,in which the Pt atoms directly jump out of the Pt(100)surface from the original position;the other is the exchange diffusion,After the exchange of Pt atoms with the surrounding Pt atoms,it jumps out of the Pt(100)surface.The migration barriers in these two cases were calculated respectively,mainly based on exchange and diffusion.The influence of ZnO doped with impurity elements on resistive devices was investigated.The energy bands of ZnO,density of states,electronic local functions,IV curves and the transmission spectra of the devices were calculated.The results show that the incorporation of Ag element is conducive to the formation of metal conductive filaments,which significantly changed the device resistance characteristics.The IV characteristic curve of ZnO crystal structure and doped impurity was calculated.From the IV characteristics of the device,the impurity level was introduced after the metal element was doped,which reduced the band gap of ZnO.The calculated IV characteristic curve The performance of non-linear.The structure of Ag/ZnO/Pt device was constructed and the IV characteristic curve of the device was calculated.The influence of the system of different electrode materials is studied in detail,the influence of the work function of the metal surface is calculated,and the reference for selecting the suitable electrode material is provided.The element doping was performed on the Ag/ZnO/Pt device structure,and the IV characteristic curve of the device was calculated.From the perspective of micro-mechanism analysis of the device results of the model,with or without doping to consider the situation,the appropriate increase in metal doping concentration conducive to resistivity change device performance improvement experiments should consider the type of doping and doped Miscellaneous concentration to improve the resistive device performance.