| Recently, with the developments on technology of thin film preparation, themore prominent status of function materials, and the advantages of the ferroelectirctunnel junctions such as high density, compact and light weight, high reading andwriting speed, strong anti-radiation, and non-volatility, multi-logic-values ferroelectrictunnel junctions have attracted more and more attentions. Currently, the study onferroelectric tunnel junctions mainly concentrated on two areas: on the one hand,through preparing ferroelectric thin with better performance or using compositebarrier to improve the resistance ratio of the tunnel junctions; on the other hand,applying the properties of multiferroic that single-phase multiferroic materials orcomposite structure owed to improve the number of logical values of tunnel junctionunits.In this paper, through analyzing the possibility of multilevel polarization offerroelectric thin in theory and experiment, combining electrostatic effect offerroelectric tunnel junctions, we built up a ferroelectric tunnel junction model withmultilevel polarization, and simulated some relevant performance. In addition,combining the property of high resistance ratio on tunnel junctions with compositebarrier and the advantages of the ferroelectric tunnel junctions with multilevelpolarization, we established a tunnel junction model with composite barrier andmultilevel polarization. The results show that not only the resistance ratio but thenumber of logical values have been also improved a lot. At the same time, the inversepiezoelectric effect of the tunnel junctions has been researched. The main points andresults are shown as follows:1. A theoretical model of ferroelectric tunnel junction with multilevelpolarization states has been proposed, and the model is capable of achieving multiply(e.g., four or eight) logic states by setting different polarization states in theferroelectric thin film. The tunneling conductance that strongly depends on the barrierpotential is calculated by applying a free-electron direct quantum tunneling method.The dependences of the conductance on bias voltage, barrier width, dielectric constant,and saturated polarization are carefully investigated. Compared to mutliferroictunneling junctions which was popular in recently, the junction avoid the 2. magnetoelectric coupling, in other words, there have improved the mutualindependence of different logic vuales. At the same time, the number of logic valuescould be modulate flexibly through controlling the polarization of the ferroelectriclayer. The results may provide some insights into the realization of ultrahigh-densitymemory.3. Choosing the metal-dielectric-ferroelectric-metal structure as the study target,and researching the electrical properties by obtaining different polarization status onferreoelectric thin, the results show that the inverse piezoelectric effect of ferroelectricthin be able to improve the electrical properties of the tunnel junction. Thedependences of the electrical properties on dielectric barrier width, ferroelectricbarrier width, bias voltage, and saturation polarization are carefully investigated. Thesimulation results shows that the number of logic values could be modulate flexiblythrough controlling the polarization of the ferroelectric layer. Additions, taking intoaccount the strain effect make the simulation results more accuracy. |
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