Preprints of Annual Meeting of The Ceramic Society of Japan<br> Preprints of Fall Meeting of The Ceramic Society of Japan

We have investigated Y₂O₃ thin films as a buffer layer to grow ferroelectric films on a Si substrate for ferroelectric gate FETs which are expected to be a novel non-volatile memory device. Since good dielectric properties and high crystallinity are required for the buffer layer, epitaxially grown Y₂O₃ films are desired. In this study, Y₂O₃ epitaxial films were grown by two step process to prevent the oxidization of the Si surface. Although the dielectric properties indicate the formation of the low dielectric layer, the oxidization of the Si surface could be suppressed by the substrate treatment using (NH₄)₂SO₃. Other investigations to suppress the formation of the low dielectric layer are also discussed.

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We have proposed ferroelectric gate piezoelectric-semiconductor thin-film transistors (TFTs) as a novel non-volatile logic device. It can be expected that the spontaneous polarization and piezoelectricity of the piezoelectric semiconductor and ferroelectric layers enhance the carrier confinement at the ferroelectric/piezoelectric semiconductor interface. In this study, ZnO and YMnO₃ were selected as a channel layer and a ferroelectric gate insulator, respectively. The Au/Ti/ZnO/YMnO₃/Pt/sapphire epitaxial capacitor exhibited a ferroelectric-type C-V hysteresis. Furthermore, the ferroelectric gate TFT using a ZnO/YMnO₃ hetero structure exhibited the on/off ratio of more than 10³ and a ferroelectric-type hysteresis with a memory window of 1.1V in the I_D-V_G measurement.

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Ferroelectric-gate field-effect-transistors (FETs) have been investigated for the applications to nonvolatile memory devices. We have proposed YMnO₃/Y₂O₃/Si structure for Metal/ Ferroelectric/Insulator/Semiconductor (MFIS) type ferroelectric gate FET, because YMnO₃ has suitable properties for this application such as small spontaneous polarization and low permittivity. Moreover, we have proposed several characterization methods such as pulsed C-V measurement, polarization-electric field (P-E) measurement for MFIS capacitors. In this study, electrical properties of epitaxially grown Pt/ YMnO₃/Y₂O₃/Si capacitors were characterized by the methods described above as well as conventional C-V measurement to discuss the ferroelectric characteristics, optimum structure and the effect of space charge on the electrical properties. The leakage current mechanisms and the effect of the leakage current on the memory retention characteristics are also discussed.

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One-dimensional (1D-) metallic and semiconducting nanostructures (nanowires,nanorods, and nanotubes) have attracted considerable attentions due totheir striking fundamental properties and potential applications innano-scale electronic devices including field-effecttransistors\cite{melosh:electronic},opto-electronic devices\cite{huang:laser}, sensors\cite{cui:sensor}, and light-emittingdiodes\cite{bjork:diode}. In the recent years, 1D-piezoelectric/ferroelectricnanostructures are also stimulating more researches for fundamentalfinite size effects and potential applications in such as nanoactuators,nanoelectromechanical systems. In contrast to the template-assistedpreparation of polycrystalline ferroelectric 1D-nanostructures\cite{cao:review,morrison1}, this paperreports one two-step hydrothermal route to synthesize lead-freehigh Curie temperature piezoelectric BaTi$_2$O$_5$\cite{tillmanns} single crystalline nanowiresthrough monitoring nucleation and growth processes bymeans of controlling the shape of nucleus. For a typical case ofBaTi$_2$O$_5$ nanowires prepared at 90$^o$C for 1 h with titaniananotubes prepared at 170$^o$C for 17.5 h as titanium resource,BaTi$_2$O$_5$ nanowires exhibited a diameter between 20-500 nm andlength of 1-10 $\mu m$. The formation mechanism of BaTi$_2$O$_5$nanowires was discussed from the viewpoint of lattice structure. The piezoelectric coefficient for theBaTi$_2$O$_5$ nanowires was preliminarily measured as about -9 pm/V at room temperature.This method does not require any template, membrane,surfactant or applied external field to create anisotropicnanoparticle or to control their growth orientation, whichenables the growth of ferroelectric titanate nanowires atlarge-scale and very low temperatures.

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