Research On Performance For Selector Based On TiO₂

The emergence of new nonvolatile memory(NVM)technologies—such as ferroelectric random access memory,spin-torque-transfer magnetic random access memory,phase change memory and resistance random access memory(RRAM)-has been motivated by exciting applications such as storage class memory,embedded nonvolatile memory,enhanced solid-state disks,and neuromorphic computing.Many of these applications call for such NVM devices to be packed densely in vast"cross-point" arrays offering many gigabytes if not terabytes of solid-state storage.In such arrays,access to any small subset of the array for accurate reading or low-power writing requires a strong non-linearity(NL)in the I-V characteristics,so that the currents passing through the selected devices greatly exceed the residual leakage through the non-selected devices.This NL can either be included explicitly,by adding a discrete access device at each cross-point,or implicitly with an NVM device which also exhibits a highly nonlinear I-V characteristic.If the design of peripheral circuits to avoid the leakage current,it is bound to increase the complexity and cost of the design.Therefore,the focus of this work is the generation of non-linear I-V curves of NVM devices and the improvement of their performance.The cross-point memory array is briefly outlined in the design of the circuit level,at the same time,a suitable voltage scheme is provided for the selected cell.And discussed the different device structure,in reducing the current leakage of non-selected cells play a key role,such as conventional silicon-based semiconductor devices,oxide semiconductors,ovonic threshold switching,oxide tunnel barrier,and Mixed-ionic-electronic-conduction device.It is found that the material itself,interface barrier and the combination of material and interface can produce nonlinear I-V characteristics.Therefore,the selector in series with bipolar RRAM is the focus of this work.Its cell structure can effectively reduce the sneak current of non-selected memory cells in a crossbar array,reduce static power consumption and improve integration.In this work,a first-principles calculation method is used and the selectors are studied in a cross-array of 1S1R cell structures.Its materials,structures and properties were explored and reveal the laws of its nonlinear conduction mechanisms and interface electronic properties,providing theoretical guidance and design ideas for large-scale integration and industrialization of 1S1R crossbar array.Firstly,in order to study the NL of the material,the influence of the thickness of the rutile TiO2 material on the NL is studied.The NL is studied by the change of band gap with voltage.It is found that under the applied electric field,with the increase in the thickness,the NL became stronger.Then calculating the site-projected density of states and local density of states by applying an external electric field showed that the NL of the bulk is due to the exchange of electrons between O-2p orbitals and Ti-3d orbitals.the influence of oxygen defects-oxygen vacancies(Vo),oxygen interstitial(Oi),and oxygen vacancies/oxygen interstitial(Vo-Oi)pairs(Frenkel pair defects)-on the NL of TiO2 was studied.It is found that the threshold voltage is not affected by the increase of oxygen vacancy concentration,and at the same threshold voltage,the greater the oxygen vacancy concentration,the stronger the NL.On the basis of the non-linear TiO2 material,the role of the electrode is studied from the viewpoint of the device.In this research,the first principle is used to study the selector interface of cross-point array to explore the interfacial electronic properties of metal and TiO2,which provides theoretical guidance and design ideas for the selector electrode in cross-point array.First of all,based on the first principle of density functional theory,the generalized gradient approximation was used to study the interfacial energy of the Pt/TiO2/Pt,Ni/TiO2/Ni and Ti/TiO2/Ti.By studying the interfacial energy of the three symmetrical composite structural models,it is found that Pt/TiO2 model has the lowest interfacial energy and the interfacial system is the most stable.Secondly,the influence of the metal electrode on the TiO2 material is obtained by the density of states.For the TiO2 material at the interface,the quantum states in the band gap is most affected by the electrode.The top of the valence band is contributed by the O-2p orbitals and the bottom of the conduction band is contributed by the Ti-3d orbitals,the quantum states in the band gap is generated by O-2p orbitals due to the hybridization of the O-2p orbit and the 3d orbit of the electrode metal.Finally,the calculation of differential charge density and potential in the direction perpendicular to the electrode shows that the electrode with large electronegativity has a significant electron-to-electron transfer at the interface.At the same time,the potential difference in the structure is the largest at the interface,and the presence of the bond further indicates that the interface is tightly bonded.Therefore,from the perspective of simulation,it is determined that the electrode material with large electronegativity is conducive to rectification of the device,thereby enhancing the nonlinear I-V characteristics of the device to solve the current leakage and reduce the power consumption of the device.Above all,the thickness of the material can adjust the threshold voltage,the defect concentration can improve the nonlinear,choose a suitable electrode can get a stable structure of the device.Therefore,selectors based on TiO2 materials can be flexibly applied in future 1S1R RRAM.