Ferroelectric field effect memory devices based on epitaxial oxide heterostructures

It is now possible to grow epitaxial heterostructures consisting of perovskite oxides. The combination of such diverse materials offers attractive possibilities to demonstrate novel device structures. When grown as a ferroelectric field effect nonvolatile memory device, the epitaxial oxide heterostructure can enhance the device performance by improving the interface quality between the ferroelectric and semiconductor layers. Although there have been several reports on a ferroelectric field effect memory device based on epitaxial heterostructures, the relatively high carrier concentration in the semiconducting oxides is likely to be the main reason that the observed ferroelectric field effects have been smaller than desired for a nonvolatile memory device. Therefore, by using LaVO₃, which can be synthesized with a relatively low ionized hole concentration (∼1018/cm3), as a channel layer, this dissertation research aimed to demonstrate a ferroelectric field effect memory device based on the epitaxial perovskite oxide heterostructures.; The first part of this dissertation research sought to establish the optimal growth conditions of LaVO₃ thin films since there is no previously published research on this topic. The oxygen partial pressure turned out to be a critical parameter in the growth of LaVO₃, unlike other perovskite oxides. Epitaxial thin films of LaVO₃ could be grown by pulsed laser deposition on a perovskite substrate in vacuum at 500°C while insulating monoclinic LaVO₄ was deposited in an oxygen ambient.; In the second part of the project, the aim was to incorporate semiconducting LaVO₃ into the ferroelectric/conductor (Pb,La)(Zr,Ti)O₃ (PLZT)/(La,Sr)CoO₃ (LSCO) multilayer. Because the growth conditions of LaVO₃ are not compatible with PLZT, a ferroelectric interlayer was required to realize a functional LaVO₃/PLZT-based heterostructure. While a paraelectric SrTiO₃ layer did not perform properly, a ferroelectric Ba_0.75Sr_0.25TiO₃ (BST) interlayer between LaVO₃ and PLZT layers enabled epitaxial LaVO₃/BST/PLZT/LSCO heterostructures to be built. The epitaxial heterostructure exhibited high remanent polarizations (>30 μC/cm2) with a 30 nm BST layer. The formation of a depletion region in the LaVO₃ layer, indicated by capacitance-voltage curves, was explained by a simple model.; In the third part, the memory function utilizing the ferroelectric field effect was demonstrated in epitaxial LaVO₃/BST/PLZT/LSCO heterostructures. A ratio of OFF/ON-state resistance up to 2.5 was observed in the LaVO ₃ channel.