Interface Engineering In Ferroelectric Tunnel Junctions

Recently, ferroelectric(FE) materials have attracted significant interests due to their technological application in electronic devices. Especially, FE heterostructures, such as ferroelectric tunneling junction(FTJ), have become a very promising candidate for application in FTJ-based nanoscale transducers and future non-volatile memories with high storage density, high speed, and low power consumption. The tunneling electroresistance(TER) effect is the most important effect of FTJs, and the larger the TER, the better the performance of the FTJ. It is well known, the inherent spontaneous polarization can be switched, and will lead to the asymmetry of the potential profile for the opposite polarization directions, then result in the TER effect. Interfaces between complex oxides have rich physical properties like electrical conduction, magnetism, and superconductivity. Also, interfaces have great influence on the TER effect due to the breaking of potential symmetry of the FTJs. It is proved that the FTJ with a composite barrier that combines a ferroelectric slab and a nonpolar dielectric slab exhibits an enhancement of TER. In this thesis, we investigate the interface with a pinned dipole within the composite barrier in a ferroelectric tunnel junction(FTJ) with symmetric electrodes. Based on the first-principle calculations, and within the typical FTJ SRO/BTO/SRO structure, we find that the inserted atomic monolayer can affect the polarization of the ferroelectric tunneling junction.Our main work and results are as follows1. Influence of interface within the composite barrier on the TER of FTJs.The interface with a pinned dipole within the composite barrier in a ferroelectric tunnel junction(FTJ) with symmetric electrodes is investigated within the framework of the phenomenological theory. We assume the existence of the interface between the BaTiO3 layer and the dielectric SrTiO3 layer. Different from the detrimental effect of the interface between the electrode and barrier in previous studies, an interface between the dielectric SrTiO3 slab and ferroelectric BaTiO3 slab in a FTJ will enhance the tunneling electroresistance effect. Specifically, the interface with a lower dielectric constant and larger polarization pointing to the ferroelectric slab favors the increase of TER ratio.2. First-principles study of interface engineering in ferroelectric junctionsEffect of atomic monolayer insertion on the performance of ferroelectric tunneling junction is investigated in the SrRuO3/BaTiO3/SrRuO3 junction. Based on first-principles calculations, the atomic displacement, orbital occupancy, and ferroelectric polarization are calculated. It is found that the ferroelectricity is enhanced when a(AlO2)- monolayer is inserted between the electrode SRO and the barrier BTO, where the relatively high mobility of doped holes effectively screen ferroelectric polarization. On the other hand, for the case of(LaO)+ inserted layer, the doped electrons resides at the both sides of middle ferroelectric barrier, making the ferroelectricity unfavorable, which decrease the average of the polarization. Here the interface effects are different from the charge doping effect in the bulk BTO, where the charge doping is always detrimental to the stability of the ferroelectricity of BTO. Our findings provide an alternative avenue to improve the properties of ferroelectric devices.