Theory Research On The Materials And Resistance Mechanism Of Memristor

Memristor is one of the most promising next-generation non-volatile memories.It has obvious advantages in device structure,erasing speed,scalability,three-dimensional high-density integration,etc.Currently,the memristor is still far away from industrialization because of there are still many critical issues that need to be addressed.In this thesis,first-principles calculations based on density functional theory(DFT)will be employed to theoretically study on some basic problems in memristor,such as materials computational method,commonly used materials and resistance mechanism.The main research and innovation achievements are as follows:Firstly,based on the in-depth analysis of the original energy band calculation method DFT-1/2(LDA/GGA-1/2),creatively proposed an improved method of a self-potential trim method using a spherical shell model instead of the original simple spherical model,named as shell-DFT-1/2(shDFT-1/2).The improved sh DFT-1/2 calculation method has greatly improved the energy band calculation of group IV,III–V,and II–VI covalent semiconductors,especially the band calculation results of Ge,which is in good agreement with the experiments.In addition,we have prescribed a generic rule for correctly applying DFT-1/2 band calculations for the first time.The improved sh DFT-1/2 technique can be compatible with both ionic and covalent semiconductors and of great significant for the research of silicon-based microelectronics.Secondly,we have analyzed the crystal structures of five important binary oxide memristor materials(?-Al₂O₃,r-TiO₂,m-ZrO₂,m-Hf O₂and Ta₂O₅).Especially for the recently predicted triclinic?-phase ground state Ta₂O₅ by Yang and Kawazoe,we found that it can be assigned a much more symmetric I4₁/amd space group,namely tetragonalg-Ta₂O₅.Then,we have investigated the influence of five exchange-correlation functional(PBE,PBEsol,PW91,rev PBE and AM05)flavors on the(sh)GGA-1/2 band structures,taking four important binary oxide memristor materials(?-Al₂O₃,r-TiO₂,m-ZrO₂and m-Hf O₂)as benchmark examples.We have found that the performance of(sh)GGA-1/2 is comparable to state-of the-art GW and generally superior to the HSE06 hybrid functional.Among the five GGA functionals,PBEsol yields the best results in general.The subsequent analysis of the bandgap calculation results of Ta₂O₅ also shows that the DFT-1/2 calculation has a great advantage in the calculation of its electronic structure.This work not only provides a highly symmetric ground state phase structure for the study of Ta₂O₅,but also provides a guide when applying the(sh)DFT-1/2 method to metal oxides.Finally,the resistance mechanism of memristor have been studied combining with the energy band calculation method(sh)GGA-1/2.In the Ox RAM,taken Ta₂O₅ and Hf O₂memristors as an example,based on the high symmetry tetragonal phase together with computationally efficient GGA-1/2 method,we found that the filament-to-dielectric in Ta₂O₅ form Ohmic or quasi-Ohmic contacts,while in the HfO₂ there is a Schottky contact.Our work explains the difference in the ON/OFF resistance ratio of the two typical memristors from the atomic level and provide a theoretical guidance for the selection of memristors dielectric layer.On the other hand,in the CBRAM,based on charge density difference,The physical mechanism behind the different growth modes of Ag and Cu conductive filaments in eight common CBRAM materials(Ge Se,Ge Te,Ge₂Sb₂Te₅,Al₂O₃,HfO₂,Ga₂O₃,MoO₃ and Ta₂O₅)have been explored based on Bader charge analysis,ion migration barrier and interface barrier calculation.It is found that the stability,ion migration barrier,and polarization of Ag or Cu ions in the material play a key role in the growth mode of conductive filaments.Finally,the growth model of conductive filaments in CBRAM with double-layer structure such as Cu/GeTe/Al₂O₃/Pt are also analyzed,which may have significance impact for experimental application.