Ferroelectric tunnel junctions for information storage and processing

Abstract

Computer memory that is non-volatile and therefore able to retain its information even when switched off enables computers that do not need to be booted up. One of the technologies for such applications is ferroelectric random access memories, where information is stored as ferroelectric polarization. To miniaturize such devices to the size of a few nanometres, ferroelectric tunnel junctions have seen considerable interest. There, the electric polarization determines the electrical resistance of these thin films, switching the current on and off. With control over other parameters such as magnetism also being possible, ferroelectric tunnel junctions represent a promising and flexible device design. F erroelectric random access memories (FeRAMs), in which information is encoded through the ferroelectric polarization, are commercially available products with fast write speed, large read/write cycle endurance and low-power consumption 1 . In these ferroelectric capacitors, a ferroelectric thin film (typically 100 nm thick) is sandwiched between two electrodes and the remnant polarization is switched by applying an electric field between the electrodes. The widespread development of these memories is however limited owing to the capacitive readout of the information (the polarization) that prevents the scalability of FeRAMs up to gigabit densities and necessitates the rewriting of information after readout (destructive readout) 2 . In ferroelectric diodes, the current across a thick ferroelectric can be modulated by the polarization at the ferroelectric/electrode interface, giving rise to a non-destructive, resistive readout of the information 3,4 . But in these devices, the large thickness of the ferroelectric layer results in very low readout currents, limiting their miniaturization. As the ferroelectric layer thickness is decreased to a few nanometres, electronic conduction is greatly enhanced as quantum-mechanical tunnelling through the ferroelectric becomes possible 5,6 . These devices in which two electrodes sandwich a ferroelectric tunnel barrier are called ferroelectric tunnel junctions (FTJs). The tunnel transmission may be strongly modulated by the ferroelectric polarization producing giant tunnel electroresistance (TER) with OFF/ON resistance ratios reaching 10 4 (refs 7,8). Moreover, the readout current densities are much larger than in ferroelectric diodes, thereby opening the path to possible applications as high-density ferroelectric memories with non-destructive readout.Here we review the fast progress of the nascent field of FTJs and give perspectives for new physical effects and future applications. We first summarize possible electron transport mechanisms that occur when voltage is applied across ultrathin films of ferroelectrics. We then focus on experimental reports that evidence a direct correlation between ferroelectric switching and resistive switching, from local probes experiments on ferroelectric surfaces to solid-state

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 .…”

Crystallization engineering as a route to epitaxial strain control

“…Coupled with its large electromechanical response, a visible-wavelength bandgap and thickness-and strain-engineered ferroelectric domain structure, BiFeO 3 , and its solid solutions continue to attract attention for potential applications in multiferroic, multi-state, tunneling electroresistive and optical memories, and ferroelectric photovoltaics. 14,15 In this study, we report on the epitaxial crystallization of Bi-Fe-O thin films grown by atomic layer deposition (ALD) on single-crystal (001)LaAlO 3 and (001)SrTiO 3 substrates. When grown on (001)LaAlO 3 by PLD, due to a high lattice mismatch and corresponding epitaxial strain, BiFeO 3 becomes tetragonal and exhibits a different ferroelectric behavior than its rhombohedral modification.…”

Engineering ferroelectric tunnel junctions through potential profile shaping

“…18 Various material combinations have been used to realize FTJs, giving rise to very different TER amplitudes (Ref. 19 and references therein). Fully epitaxial junctions 20,21 are based on ferroelectric tunnel barriers made of highly strained perovskite oxide layers (e.g., BaTiO 3 , (Pb, Zr)TiO 3 , and BiFeO 3 ) associated to strongly correlated oxides (e.g., Nb:SrTiO 3 , (La,Sr)MnO 3 , (La,Ca)MnO 3 , and (Ca,Ce)MnO 3 ).…”