Selector Model Based On NbO2 Phase Change Material And Design

Semiconductor memory is the core component of information processing and storage.Its market share occupies a large proportion in the whole integrated circuit industry,and it is an important part of the information technology industry.However,it is difficult for the traditional semiconductor memory to be miniaturized.Resistive random access memory?RRAM?is considered as a new semiconductor memory with great potential.It has many advantages,such as strong ability of miniaturization,good compatibility with existing semiconductor technology and simple structure,and has been widely concerned in recent years.Because of its simple structure,the integration structure of RRAM can adopt the cross array structure with the highest integration degree of 4F²in theory.However,cross array structure has the problem of crosstalk current,which will lead to information misreading.Therefore,the 1S1R?one selector one resistor?with a high nonlinearity in series can avoid the misreading caused by crosstalk,so the research based on the selector device has been widely concerned.The selector can be divided into two mechanisms:phase transition and tunneling.Tunneling can be divided into electron and Schottky tunneling.Selector not only has different mechanism,but also has different structure,such as MIM,MSI,NPN,NIPIN and MIEC.These different mechanisms and structures result in different electrical characteristics of the selector.The two most critical parameters of the selector are the threshold voltage(V_th)and nonlinearity.The selector studied in this paper is based on the phase transition mechanism,which is connected in series with unipolar and bipolar RRAM respectively to reduce the leakage current and avoid crosstalk in the cross array.In this paper,the finite element simulation software COMSOL is used to simulate the selector,a selector based on the mechanism of phase change is designed to connected to RRAM.Firstly,the conductivity,thermal conductivity and hot melt of IMT?Insulator-Metal-Transition?physical model are fitted to make the simulation results more accurate.Secondly,the electrical characteristics of VO2 selector and NbO2 selector are compared.NbO2 is selected as the material of IMT selector because the latter has low V_thh and I_off.After that,the influence of the thickness?L?and radius?R?of the conducting filaments on the electrical characteristics of the NbO2 selector is studied.It is found that with the increase of L,although V_thh is increasing,I_offff is decreasing.However,the trend of R is just the opposite of L.Finally,different sizes of selector and bipolar TiO2 RRAM were used in series,When the L and R of NbO2 selector conducting filament are 25nm and 7nm respectively,it is found that NbO2 selector are more suitable for RRAM,and the final 1S1R structure is suitable for high-density storage.NbO2 selector is a two-way threshold selector,which can not only be connected in series with bipolar RRAM,but also with unipolar RRAM.The unipolar RRAM uses a physical based metal oxide reset model.The reset process of this model is simulated by Joule heating and thermal activation defect diffusion.The distribution of conducting filament temperature,potential and defect concentration in the reset process of unipolar NiO RRAM was studied.Similar to bipolar RRAM,different sizes of selector are used in series with unipolar RRAM.When the L and R of NbO2 selector conductive filaments are 35 and 12nm respectively,it can match NiO RRAM better and greatly improve the integration.In conclusion,the optimal design of L and r for NbO2 selector of bipolar TiO2 RRAM is25 and 7nm,while the optimal design size of unipolar NiO RRAM is 35 and 12nm.