The summary for the ferroelectric tunnel junction

Zhang Tanzhao

University of Science & Technology Beijing

FMJ is the tri layers and two peculiar interfaces hetero-structure with two electrodes and a ferroelectric barrier layer which is just like the MTJ distributed the barrier from the insulator to the ferroelectric semiconductor.It combines three significant physical quantities (spin, current, strain) into the single system and performs robust exotic properties, such as the magnetic tunnel resistance effect, ferroelectric tunnel resistance effect, magnetoelectric effect.

There are various type of the FMJ or FMTJ, classified by the cap and bottom electrodes with different materials (normal metal, ferromagnetic material, semiconductor, insulator, oxidation, doping phase, even alloys) or just change the decay rate by modulating the thickness. Whist the barrier can be roughly identified into ferroelectric semiconductor, ferroelectric insulator and multi ferroic phase. Surprisingly, there are some perovskite material embody complex phase, with the different dope stoichiometric ratio. Which make an avenue to modulate fertile phenomena simultaneously. More over these systems can be optimized by insert the dielectric layer and other methods. So they can be classified by the electrode, barrier material, whether to insert a dielectric layer, and demonstrate in the latter sections.

The main property

The signature of the property of the FTJ is that the reversal of the electric polarization in the ferroelectric barrier can produce a sizable change in resistance of the junction called TER. All other positive features just optimize this effect, with a high stabilized temperature, high resistance radio, and wider work condition.

The mechanism of the FTR effect

The external electric field can cross the cap electrode inteface performed the screen the polarization partially or just decay the field partially and the middle thin ferroelectric barrier layer performed the polarization charge, and finally get into the bottom layer. Cause of the different electrodes, the electrostatic potential can demonstrate the different ingredients in the middle layer when convert the orientation of the external field, the origin of the FTR effect, this paragraph is concluded by myself, which is convinced for me.

The band structure of the system by the first principle calculation

There more than one evanescent states that attribute the decay of the carriers in the complex insulators, like STO, BTO. The literature estimate the different bond state contribution to the decay in the band gap by the complex band structure model(CBS),and ge t a result that the largest contribution of the transmission is expected from the bands that have the lowest decay rates. More over, compared the paraelectric and the ferroelectric BTO, the literature indicates that the spontaneous polarization increases the band gap and enhances the decay rates.

This method can be optimized by the Green’s function method considering the barrier is the scattering region, while the electrode is the propagating region, which depicts the dynamics of the electron in the system.

Two methods to optimize the system

The polarization ratio of the barrier can strength the FTR, and there are two conventional methods to modulate it. 1) modulate the barrier height. The difference of the potential and the difference of the work function. 2) modulate the width of the barrier. The front can be modulated by change the thickness of the ferroelectric barrier layer considering that the breakthrough on the growth of the nanometer thick ferroelectric films. Many literatures have indicated that the TEM is positive proportional to the thickness of the barrier layer. The conventional sample is the tunnel junction based on 2nm thick BTO film grown on LSMO bottom electrode with the Au/Co cap electrodes, whose bottom electrodes are multi ferrotrics. Another method is by change the cap electrodes with the different work function. But both of the optimization indicates that the high TER is at the cost of the high off resistance.
The latter is to modulate the width of the barrier layer by the effect of the Schottky barrier and barrier metallization which deplete or accumulate the charge at the interface region of a semiconducting electrode or the reversible metallization of the interfacial region in the barrier itself. The front can’t change the length of the barrier once form the system. When the cap electrode of a system is a n type semiconductor whose main carrier is the electron, when the electric field is negative, there will be hole accumulating at the interface region, which generates the length of the barrier called Schottky barrier and on the other hand doesn’t change the length of the barrier. This phenomenon exists in the hetero structure of a metal/ferroelectric/semiconductor tunnel junction. The TER is negative exponential to the electron concentration. The conventional sample is the ferroelectric tunnel junction based on the SrRuO3/BTO/N-SrTiO3. Spontaneously, a metallic region with the ferroelectric barrier layer near the metal electrodes, reducing the tunneling barrier length.

The types of the FTJ

This section, I will classify the FTJ by the material of the electrodes and the barrier layer.

Metal/ferroelectric/metal. This system obtains the sizable FMR with the reveral of the electric field by utilizing the different metal with different work junction. The conventional subject is the Au(Co)/BTO/LSMO, M/BiFeO3/Ca0.96Ce0.04MnO (M=W, Co, Ni and Ir),
Metal/ferroelectric/semiconductor. This system acquires the large resistance ratio by the accumulation or annihilation of the charge at the interface by the Schottky barrier and metallization barrier. The conventional sample is the SrRuO3/BTO/n-SrTiO3, and Pt/BTO/Nb:STO, the concentration of the Nb vary from 0.1 to 1%.
Multi ferroic/ferroelectric/multi ferroic. The asymmetric interfaces lead to a different polarization profile when the ferroelectric field switches. The conventional sample is the system based on the two SRO electrodes and BTO barrier. The origin is the different decay rate in these three layers, the reversal field generates the different strain in the barrier. The one side, the field can cross the barrier causing of the low decay rate of the cap electrode together with the thin barrier, the other side, the field can’t get into the bottom.
Multi ferroic(inserting a dielectric layer)/ ferroelectric/ multi ferroic. This system arises a huge TER effect originated from the electrostatic potential in this layer can be sharply up or down.
Complex manganites oxidation(LSMO)/ferroelectric(BTO)/ Complex manganites oxidation(LSMO). There are fertile phases existing in the electrodes by different ratio chemical doping, and makes a transition from FM state to AFM state due to the different polarization when switches the field. This method to obtain the magnetization transition induced electrically has a important application. The conventional transition sample is the LSMO/BTO/ LCMO/LSMO FTJs with a thin layer of La0.5Ca0.5MnO (LCMO) inserted at one of the interfaces.86 The doping of the LCMO (x = 0.5) was intentionally chosen at the boundary between FM-metallic (x <0.5) and AFM-insulating (x>0.5) states.
Ferromagnetic/ferroelectric/ ferromagnetic. This system is the coexistence of the TMR and TER effects, which has four resistance states. The TMR effect generated from the electrodes when the magnetization is parallel or antiparallel. The TER effect arises from the asymmetry of the junction breaking the degneneracy between the two polarization orientation. Interestingly, both of the effect can impact each other by the complementary ferroic order. The conventional one is the Co/PbTiO3–ZrO2/Co MFTJs.