Takao Tohda

Optical nonlinearities of Au/SiO2 composite thin films prepared by a sputtering method

Third‐order nonlinear optical properties of Au/SiO2 composite thin films have been investigated by means of a degenerate four‐wave mixing at room temperature. In the optical‐absorption spectra the absorption peak due to the surface plasmon resonance of Au particles is observed at the wavelength around 530 nm. With increasing the mean diameter of Au particles, the absorption at the peak is increased and the full width at half‐maximum of the absorption band is decreased from 130 to 80 nm. The third‐order nonlinear susceptibility χ(3) exhibits a peak at the wavelength of the absorption peak and the maximum value of χ(3) is obtained to be 2.0×10−7 esu. The size‐dependent enhancement of the χ(3) in Au particles with the mean diameter of 3.0–33.7 nm has been also investigated. The value of χ(3)/α (α: absorption coefficient) for the films is increased upon an increase of mean diameter of Au particles. This is explained by the size dependences of the local‐field factor and the imaginary part of dielectric constan...

Nanometer‐scale recording on chalcogenide films with an atomic force microscope

A nanometer‐scale recording technique has been demonstrated on an amorphous GeSb2Te4 film with an atomic force microscope (AFM). Data are recorded by locally changing the electrical property of the film with a conductive AFM probe. The conductance of the film is able to be increased more than one hundred times by applying a pulse voltage between the probe and the film. The recorded data are read by detecting the change of the conductance with the probe. The simultaneous measurement of the topographic and conductance images with the AFM shows that the surface topography of the recorded regions is not changed during the recording process. The smallest recorded region is 10 nm in diameter, which corresponds to a data storage density of 1 Tbit/cm2.

Effects of heat treatment on Ag particle growth and optical properties in Ag/SiO 2 glass composite thin films

Small Ag particles were embedded in SiO 2 glass thin films by a multi-target sputtering method. The mean diameter of Ag particles in the as-deposited film with 28.0 at. % of Ag was estimated to be 4.4 nm and it was increased to 24.0 nm when the film was heat-treated at 700 °C for 3 h. The diameter was proportional to the cube root of the heat-treatment time, suggesting that the Ag particles grew in the supersaturated solid solution. In the optical absorption spectra of the heat-treated films, the absorption peak due to the surface plasmon resonance of Ag particles was observed about 410 nm. The peak intensity became large and the full width at half maximum of the absorption band was decreased with increasing the diameter of Ag particles.

Highly‐oriented ZnO films by rf sputtering of hemispherical electrode system

Highly oriented ZnO films (c‐axis orientation) were fabricated on glass and fused‐quartz substrates by an rf‐sputtering system with a hemispherical electrode configuration. The sputtered films show a high longitudinal mode electromechanical coupling factor, kt=0.23–0.24. Initial results suggest that the hemispherical electrode configuration may provide improved reproducibility over planar sputtering systems in the preparation of ZnO films for acoustic device applications.

Highly‐reliable temperature sensor using rf‐sputtered SiC thin film

A SiC thin-film thermistor for high-temperature use has been developed by using rf-sputtered SiC thin films. This thermistor can be used for industrial and consumer use within an operating temperature range of -100 to 450 degrees C. By using SiC thin films, the thermistor maintains high electrical stability. The resistance change is less than 3% after exposure to heat at 400 degrees C for 2000 h. In addition, the film growth technique made possible the production of a high-accuracy thermistor, i.e., thermistor coefficient < +/-0.5%, thermistor resistance < +/-1.5%.

Atomic force microscopy using ZnO whisker tip

We have developed an atomic force microscope (AFM) using a zinc oxide (ZnO) whisker crystal as a probing tip. The ZnO whisker crystal is tetrapodal in shape, with each leg having a length of 5–30 μm, a radius of curvature less than 10 nm, and a cone half angle of 1°–2°. Polyimide thin films rubbed with cloths as liquid‐crystal aligning films were employed for AFM imaging. Due to the needle shape of the probing tip, the AFM was able to resolve the tiny grooves (3–5 nm deep, 60–80 nm apart) on these films more clearly than that using a conventional pyramidal tip. The new AFM will be available for precise evaluation of surfaces on which fine structures are microfabricated in nanometer scale.

Nanometer-Scale Erasable Recording Using Atomic Force Microscope on Phase Change Media.

We have demonstrated a nanometer-scale recording technique using an atomic force microscope (AFM) with an amorphous GeSb2Te4 film as a phase change medium. Data are recorded by changing the local electric properties of the film using a conductive AFM probe. The conductance of the film can be increased by more than one hundred times by applying a pulse voltage between the probe and the film. The recorded data are read by detecting the change in the conductance using the probe. The smallest possible recording region is 10 nm in diameter, which corresponds to a data storage density of 1 Tbit/cm2. The data can be erased by applying a negative DC voltage to the probe while scanning the probe over the film. The mechanisms of the reversible conductance change in the film are also discussed.

Preparation and Nonlinear Optical Properties of Ag/SiO2 Glass Composite Thin Films

A multitarget sputtering method was applied to prepare Ag particles embedded in SiO2 glass thin films. In the optical absorption spectra of the films, the absorption peak due to the surface plasmon resonance of Ag particles was clearly observed at the wavelength of 390–406 nm. The full width at half-maximum of the absorption band is decreased with increasing diameter of Ag particles. The third-order nonlinear susceptibility at 400 nm was estimated to be 1.6×10-8 esu for the film with 9.0 at% of Ag by means of a degenerated four-wave mixing.

Electrical Properties of Perovskite Type Oxide Thin-Films Prepared by RF Sputtering

Oxide thin-films with perovskite structure are prepared by rf magnetron sputtering in argon-oxygen mixed gas at substrate temperature below 500°C using the Sr(Zr, Ti)O3 and Ba(Sn, Ti)O3 solid solution ceramic targets. The dielectric properties and the microstructure of the sputtered thin-films depend on the substrate temperature, oxygen partial pressure, and target compositions. The dielectric constant and the electrical breakdown strength of the thin-films sputtered by using Sr(Zr0.2Ti0.8)O3 and Ba(Sn0.2Ti0.8)O3 targets is about 100 and 3 MV/cm, respectively. These thin-films with the high dielectric constant and the high electrical breakdown strength are useful for the dielectic layer of the ac TFEL panel.

New efficient phosphor material ZnS:Sm,P for red electroluminescent devices

Thin‐film electroluminescent devices employing a new phosphor material ZnS:Sm,P have been found to exhibit bright red emission. Luminous efficiency of ZnS:Sm,P phosphor films is higher than that of ZnS:Sm phosphor films in the range of annealing temperature above 500 °C. A brightness of 1000 cd/m2 and an efficiency of 8×10−2 lm/W have been obtained in the devices with ZnS:Sm(1 at. %),P(0.5 at. %) phosphor films annealed at 600 °C. These results indicate that P is an efficient co‐activator for Sm in ZnS.