Hiroya Kitahata

Ferroelectricity of YMnO3 thin films prepared via solution

We have observed the ferroelectricity at room temperature in YMnO3 thin films prepared via solution. Precursor films of YMnO3 on Pt/sapphire or Y2O3/Si substrates were heat treated in vacuum or in air for controlling the crystallinity. X-ray diffraction measurements indicated that each film was a single phase of hexagonal YMnO3. While the film heat treated in air indicated an insufficient crystallinity, the film heat treated in vacuum showed a high crystallinity with a c-axis preferred orientation. The leakage current of the film heat treated in vacuum was at least three orders of magnitude lower than that heat treated in air. The ferroelectricity of the film heat treated in vacuum was confirmed in the capacitance–voltage measurement at room temperature due to its high crystallinity with the c-axis preferred orientation and the small leakage current.

YMnO3 thin films prepared from solutions for non volatile memory devices

We demonstrated for the first time that hexagonal YMnO3 films were fabricated from solutions which were effective for lowering the formation temperature. Two different processes, a thermal decomposition method and a reflux method, were studied. In the thermal decomposition method, the solution containing Y(OAc)34H2O and Mn(OAc)24H2O dissolved in ethanol containing diethanolamine was used for coating. Firing temperatures over 900° C were needed to obtain the YMnO3 single phase film using the thermal decomposition process. In the reflux method, the precursor solution prepared from 2-ethoxyethanol as a solvent was refluxed at 125° C and the solution obtained was used for coating. The reflux process drastically lowered the firing temperatures to 700° C, and this process also improved the microstructure. Effects of reflux on the crystallization process and dielectric properties have been demonstrated.

Preparation and Dielectric Properties of YMnO3 Ferroelectric Thin Films by the Sol-Gel Method

Thin films of YMnO3 are proposed as a new candidate for non-volatile ferroelectric memory devices. They were prepared via solutions through two different processes: thermal decomposition and reflux using yttrium acetate tetrahydrate and manganese acetate tetrahydrate as starting materials. For coatings prepared by thermal decomposition process, the starting materials were dissolved in ethanol containing diethanolamine, and single phase YMnO3 was obtained with heat-treatment at 900°C. When the starting materials were refluxed using 2-ethoxyethanol as a solvent, single phase YMnO3 was obtained with heat-treatment at 800°C. Scanning electron microscopy showed that the 300 nm thick films with a stoichiometric Y/Mn ratio had many pinholes, and a very large dielectric loss, 0.83 at 100 kHz. Inclusion of 5–10% excess of Y in the coating solution produced dense structures with improved dielectric properties. The dielectric constant and loss tangent of the thin films with Y/Mn ratio of 1.00/0.90 were about 20 and 0.05 at 100 kHz, respectively.

Incubation-Free Growth of Polycrystalline Si Films by Plasma-Enhanced Chemical Vapor Deposition Using Pulsed Discharge under Near Atmospheric Pressure

By using the plasma-enhanced chemical vapor deposition (PE-CVD) under near-atmospheric pressures, we have achieved a high rate growth, 1 nm/s, of polycrystalline Si films on glass substrates without incubation layers for the first time. We have employed a short-pulse based system for a stable operation of discharge at atmospheric pressures without inert gas dilution. This feature enabled us to employ an extremely high dilution of monosilane by hydrogen, which should be the origin of the incubation-free growth of our films, in addition to the basic advantage for the high rate growth inherent in atmospheric reaction systems. The films are mainly consisted of polycrystalline Si with grain size ranging from 5 nm to above 10 nm, as observed by Raman scattering, X-ray diffractions and cross sectional transmission electron microscopy.

Lowering the Crystallization Temperature of YMnO3 Thin Films by the Sol-Gel Method Using an Yttrium Alkoxide

We have succeeded in lowering the crystallization temperature of sol-gel derived YMnO3 thin films to 500°C by using yttrium alkoxide as a starting material. The X-ray diffraction measurements indicated that the films heat treated at temperatures above 500°C had a single phase of hexagonal YMnO3. The crystallization temperature of the YMnO3 thin film prepared from yttrium alkoxide was lower by at least 300°C than that of film prepared from yttrium acetate. The film prepared from the alkoxide and heat treated at 700°C for 3 min had a high crystallinity with c-axis preferred orientation, and ferroelectricity was observed in the film. The formation temperature of the ferroelectric YMnO3 thin film prepared from yttrium alkoxide was lower by at least 100°C than that of film prepared from yttrium acetate.

Formation of Silicon Oxynitride Films with Low Leakage Current Using N2/O2 Plasma near Atmospheric Pressure

We have fabricated ultrathin silicon oxynitride films using N2/O2 plasma near atmospheric pressure. The silicon oxynitride films whose composition was Si3N1.2O4.3, as evaluated by X-ray photoemission spectroscopy, showed a leakage current as low as 1.4×10-4 A/cm2 at 5 MV/cm. Given a capacitance equivalent thickness of 1.9 nm, the leakage current is lower by two orders of magnitude than that of the reported silicon oxynitride films. The conduction mechanism of the leakage current of the film was discussed using direct-tunneling-current simulation with Webtzel-Kramers-Brillouin approximation. We concluded that an increase in the effective tunneling mass of holes, which is probably due to the nitrogen atoms incorporated to Si3N1.2O4.3 films, is responsible for the decrease in the leakage current of silicon oxynitride films.

Preparation and Ferroelectric Properties of YMnO3 Thin Films with c-Axis Preferred Orientation by the Sol-Gel Method

Hexagonal YMnO3 has a ferroelectric property with an optimal remanent polarization along the c-axis. The c-axis oriented YMnO3 thin films with a small leakage current were prepared by the sol-gel dipping method. The c-axis orientation of the films was promoted by the addition of diethanolamine to the Mn precursor solution. A heat treatment with multiple steps led to a dense film structure with fine grains. The dense structure resulted in the decrease of the leakage current. Furthermore, when the films were heat-treated in a vacuum, the leakage current became considerably small and the ferroelectricity of the YMnO3 thin films was observed even at room temperature.

Origin of Leakage Current of YMnO3 Thin Films Prepared by the Sol-Gel Method

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