We have been proposing YMnO3 with low remanent polarization and permittivity as a ferroelectric gate transistor, and reported that c-oriented YMnO3 films were obtained on (111)Si with (111) oriented Y2O3 buffer layer. The ferroelectricity was confirmed by pulsed C-V measurement. However, the retention property was not satisfied because of its poor crystallinity. To improve the crystallinity of YMnO3 films, deposition conditions of Pulsed Laser Deposition (PLD) were optimized. The laser power, oxygen pressure and introducing Ozone gas are effective for maintaining the stoichiometry during the deposition. Improvement of the crystallinity of the YMnO3 film makes the retention property better. We also demonstrate the use of epitaxially grown Y2O3 buffer layer to improve the crystallinity of the YMnO3 films.

YbMnO3 (Yb/Mn = 0.96) thin films were prepared on Y2O3(111)/Si(111). Although the sample exhibited ferroelectric type C-V hysteresis, the window width changed depending on the applied bias voltage. Hence, the ferroelectric type hysteresis might include the effect of space charge. To make clear the C-V behavior caused by only ferroelectricity, and to obtain the optimal relationship between ferroelectric and insulator layer thicknesses, the C-V behavior at high frequency was computed for MFIS structure. Relationships between the counter bias voltage applied to the ferroelectric layer and the thickness of dielectric layer were also demonstrated. Compared with the calculated results, experimental C-V characteristics are discussed.

We have been proposing YMnO3 with low remanent polarization and dielectric permittivity as a transistor type FeRAM. Conventional C-V measurements are normally used to evaluate the ferroelectricity of ferroelectric thin films on Si substrates. For ferroelectric thin films with low polarization, however, there are some issues to understand the C-V hysteresis. For example, interfacial polarization and space charge affect the C-V hysteresis. The effect of interfacial polarization on C-V behavior was calculated assuming the connection of ferroelectric and dielectric layers with different resistivity. If a ferroelectric layer with a leakage current of 1×10-6 A/cm2 is connected to the dielectric layer with a leakage current of 1×10-9 A/cm2, a charge density of 10-9 C/cm2 should be generated in the period of 0.2 sec. Space charge should have a longer time constant for accumulating the charge. Pulsed C-V measurement must be effective to avoid these issues. This paper proposes a simple method to evaluate the real component of ferroelectricity in the C-V hysteresis using YMnO3 or ZnO:Li with very low remanent polarization.

We have been proposing the use of RMnO3 (R: rare earth elements) films for metalferroelectric- semiconductor field effect transistor (MFSFET)-type ferroelectric random access memories (Ferroelectric RAMs). This report describes the progress of YMnO3 and YbMnO3 films for FET type FeRAM application. Although highly (0001)-oriented YMnO3 films are easily obtained on a MgO, ZnO/Sapphire, Pt/Sapphire and Pt/Si substrates, it was very hard to obtain the crystalline films directly on Si or on SiO2/Si substrate. A Y-Mn-O buffer layer improved the crystallinity of the YMnO3 films on Si, and we got the C-V curve with ferroelectric hysteresis. The real ferroelectric component responsible for the C-V hysteresis was calculated to be just 8.4 nC/cm2 by pulse measurements. On the other hand, Y2O3 buffer layer drastically improved the dielectric properties. The window width of the C-V hysteresis does not change by changing the sweep rate and measurement frequency.

We propose the application of ZnO:X (X = Li, Mg, N, In, Al, Mn, Gd, Yb etc.) films for a monolithic Optical Integrated Circuit (OIC). Since ZnO exhibits excellent piezoelectric effect and has also electro-optic and nonlinear optic effects and the thin films are easily obtained, it has been studied as one of the important thin film wave guide materials especially for an acoustooptic device[1]. In terms of electro-optic and nonlinear optic effects, however, LiNbO3 or LiTaO3 is superior to ZnO. The most important issue of thin film waveguide using such ferroelectrics is optical losses at the film/substrate interface and the film surface, because the process window to control the surface morphology is very narrow due to their high deposition temperature. Since ZnO can be grown at extremely low temperature, the roughness at the surface and the interface is expected to be minimized. This is the absolute requirement especially for waveguide using a blue or ultraviolet laser. Recently, lasing at the wavelength of ultraviolet, ferroelectric and antiferromagnetic behaviors of ZnO doped with various exotic elements (exotic doping) have been reported. This paper discusses the OIC application of ZnO thin films doped with exotic elements.

The preparation conditions of YMnO3 thin films by the sol-gel method using yttrium alkoxide were optimized to decrease the leakage current of the films. The leakage current of the films was decreased due to the dense microstructure of the films. Moreover, the heat treatment in hydrogen atmosphere and the zirconium doping resulted in a further decrease of the leakage current. The heat treatment in hydrogen atmosphere and the zirconium doping were effective in the decrease of carriers originating in the valence fluctuation of the Mn ions in YMnO3

We have proposed ReMnO3 (Re:rare earth) thin films, as a new candidate for nonvolatile memory devices. In this paper, we try to fabricate (0001) oriented YMnO3 films on (111)MgO, (0001)ZnO:Al/(0001) sapphire and (111)Pt/(111)MgO using rf magnetron sputtering. We succeed in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001)sapphire substrate, and polycrystalline films on (111)Pt/(1 11)MgO for the first time. Electrical property of the bottom electrode (ZnO:Al) changes with varying the deposition condition of YMnO3 films. However, we find an optimum deposition condition of ZnO:Al film such that it functions as a bottom electrode even after YMnO3 film deposition. The dielectric properties of the epitaxial and polycrystalline YMnO3 films are almost the same. The YMnO3 films show leaky electrical properties. This may be caused by a change in the valence electron of Mn from 3+.

The Orientation control of (Ca,Sr)CuO2 film has been investigated under various sputtering conditions from the view point of controlling the “self-texture” and epitaxy. The films with (001), (110) and (101) orientations were able to be obtained by changing partial gas pressure during sputtering. These orientations were observed on glass and MgO(100) single crystal substrates. The cohesive energy between the film and the MgO single crystal substrate was calculated to evaluate the change in orientation on glass and MgO substrates.

We investigated relationships between self-texture and interfacial restriction on the epitaxy of directed, bonded, thin films. Defect structures in ZnO films grown epitaxially on (0001)sapphire mere evaluated by 3—dimensional maping of k—spacing using an X—ray double crystal spectrometer. The orientation distribution and variation in lattice spacing of the films wre individually measured to elucidate the mechanism of growth. The growth mode was found to change at a film thickness of approximately 100 Å. The layer within 100Å of the interface had a small orientation distribution (1.8'), a large lattice spacing distribution, and a large cumpressive stress.The layer that was more than 100Å from the interface had a larger orientation distribution, a smaller lattice spacing distribution, and a smaller compressive stress.

ZnO film has a tendency to grow parallel to the <0001> axis. Therefore, it is difficult to obtain epitaxial or oriented films with any other orientation. Nevertheless, we obtained excellent epitaxial (1120) ZnO films on R-cut, (0112), sapphire by controlling the self-texture. Epitaxial (1120) ZnO on R-cut sapphire exhibits an anisotropic misfit at the interface. To clarify the growth mechanism, the stress relaxation was calculated by measuring the curvature of the substrate and the lattice spacings. The change in stress calculated by these two methods differs from each other. We discuss this discrepancy in terms of the growth mechanism.

It is argued that epitaxial films of ionically-bonded materials are more easily achieved than of covalently bonded materials. Good epitaxy can be achieved despite relatively large lattice mismatch with respect to the substrate. However, the strong influence of interfacial energy can result in difficulty in controlling the orientation of epitaxial films.

The crystallographic orientation of ionically-bonded LiNbO3 films was therefore studied. Growth orientation could be controlled by paying attention to the bonding between octahedra in the structure, and to the formation of the octahedra containing lithium and niobium ions. Lithium concentration could be increased by increasing the rf power, 02 partial pressure and total gas pressure, and decreasing the substrate temperature during deposition.

The orientation of the film changed from (012) to (100) via (110) by increasing the Li concentration in the film. The (012) and (100) films were epitaxial with respect to the substrate. In particular, the (100) films were of exce lent quality, being single crystalline with smooth surfaces.