Solid-state memories based on ferroelectric tunnel junctions

Abstract

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1 × 10(6) A cm(-2)). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ∼1 × 10(4) A cm(-2) at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism.

Ferroelectric control of spin-transfer torque in multiferroic tunnel junctions

“…The TER associated with ferroelectric polarization switching has been observed experimentally in BaTiO 3 -and PbTiO 3 -based FTJs in which either La 2/3 Sr 1/3 MnO 3 or SrRuO 3 is used as one of the electrodes while the other side of the ferroelectric thin film is in contact with the tip of an atomic force microscope [24][25][26] . The predicted simultaneous TMR and TER effects 23 have also been demonstrated experimentally in La 2/3 Sr 1/3 MnO 3 /BaTiO 3 /Fe (or Co) MFTJs [27][28][29][30][31][32][33][34] . Although the majority of the work deals with low bias, the existence of TER at finite bias has been predicted based on calculations using a semiclassical approximation 19 and first-principles 35 in FTJs, and model tight-binding calculations in MFTJs 36 .…”

Tunnel electroresistance in junctions with ultrathin ferroelectric Pb(Zr0.2Ti0.8)O3 barriers

“…The change in m e,ox on polarization reversal can be explained on the basis of lattice strains 31 or a polarization dependent complex band structure. 12 The present results are a direct experimental evidence of the influence of the ferroelectric polarization on the direct tunneling in ultra-thin ferroelectric barriers. This implicitly suggests a genuine electronic mechanism of resistive switching and TER in a ferroelectric tunnel junction.…”

“…[3][4][5][6][7][8][9][10][11][12] However, there are mechanisms other than ferroelectric polarization switching, which might contribute to or generate a resistive switching effect, such as, for instance, surface electrochemical reactions. 4,13 Especially, experiments performed in ambient conditions by using a conductive atomic force microscope (AFM) tip in contact with a ferroelectric surface are prone to such effects.…”

Giant electrode effect on tunnelling electroresistance in ferroelectric tunnel junctions

“…Indeed, the presence of order parameters in these nanostructures renders possible to switch their electrical resistances by external stimuli, which opens the way for memory, logic and neuromorphic computing applications [2][3][4] . Remarkably, in layered heterostructures, where insulating layers have the thickness of a few nanometres only, the quantum mechanical electron tunnelling becomes the dominant conduction mechanism.…”

“…Based on these advancements, various FTJs have recently been successfully fabricated and the TER effect was revealed experimentally and confirmed theoretically [21][22][23][24][25][26][27][28][29][30][31] . Moreover, the functioning of solid-state memories based on FTJs has been demonstrated 3,32 and the memristive behaviour with resistance variations exceeding two orders of magnitude was observed for FTJs with the BaTiO 3 (BTO) barrier 32,33 . At the same time, multiferroic tunnel junctions, combining ferromagnetic electrodes with a ferroelectric barrier, were found to display four resistance states 21,24 and the ability to electrically switch the spin polarization 34 .…”