Hisahito Ogawa

Hall measurement studies and an electrical conduction model of tin oxide ultrafine particle films

The Hall effect has been measured for ultrafine particle films of tin oxide (particle size 50–200 A) in contact with reducing gases. Both the carrier concentration and mobility increased with an increasing concentration of reducing gas. As a result, conductivity, the product of the carrier concentration and mobility, shows a large gas concentration dependence. This means the ultrafine particle films are highly sensitive to gases. The variation of the carrier mobility with the gas concentration is interpreted in terms of a model in which the width of a long, thin channel, determined by particle size and Debye length, increases with a decrease in the number of O− ions desorbed by a chemical reaction with the reducing gases.

Pyroelectric infrared array sensors made of c-axis-oriented La-modified PbTiO3 thin films

Highly sensitive pyroelectric infrared sensors (linear array and two-dimensional array) have been fabricated by using highly c-axis-oriented La-modified PbTiO3 (PLT) thin films, together with a new process and new sensor structures. The PLT thin films were deposited by RF magnetron sputtering. The films have a high pyroelectric coefficient γ of 6.5 × 10−8 C/cm2 K and a high figure of merit for specific detectivity D* without a poling treatment. Two types of linear array sensors, reticulated and non-reticulated ones, have been fabricated. The reticulated and non-reticulated sensors have 64 elements with 0.2 mm pitch and 128 elements with 0.1 mm pitch, respectively. A remarkably high D* value of 5 × 108 cm Hz 12/W and a very low crosstalk of < 3% at 100 Hz have been attained. The linear array and discrete JFET chips are incorporated within a pin-grid package. A high performance two-dimensional sensor with 32 × 32 elements based on a new method of reading-out signals can be formed. An array sensor with a new device structure has been designed by using an optical scanning slit. On the infrared system with F = 1 optics and Δf = 20 Hz filter, a noise equivalent temperature of 0.2 K has been obtained.

Planar-optic-disk pickup with diffractive micro-optics

An integrated optical-disk pickup with a diffractive planar micro-optic system is proposed. In this device, the beam follows a zigzag optical path inside a glass substrate that is used as a light guide. To fabricate off-axis diffractive optical elements, we have recently developed an electron-beam writing system with a curve-pattern generator. It is demonstrated that a transmission off-axis objective microlens, a reflection twin-focusing beam splitter, and reflection layers were integrated on a glass substrate, and such a diffractive planar micro-optic system exhibited an excellent focusing performance and operated forfocus-error signal detection, as designed.

Diffraction-limited blazed reflection diffractive microlenses for oblique incidence fabricated by electron-beam lithography

Blazed reflection diffractive microlenses potentially have many uses. The fabrication accuracy and the optical characteristics of these microlenses can be greatly improved compared with transmission diffractive microlenses. The reflection microlenses for oblique incidence can be flexibly used without a beam splitter. The functions of the electron-beam writing system that we developed have been expanded so that the diffraction-limited microlenses for oblique incidence can be fabricated. It is demonstrated that the fabricated microlens exhibits diffraction-limited focusing characteristics with 78% high efficiency at a large oblique angle of 30 degrees . These microlenses could be used as key devices in planar optics and optical interconnections.

Hydrogenated Amorphous Silicon Carbide Photoreceptor for Photoaddressed Spatial Light Modulator

A highly photosensitive hydrogenated amorphous silicon carbide (a-Si1-xCx:H) photoreceptor with a PIN diode configuration has been prepared by means of a plasma CVD method using SiH4 and C2H2 highly diluted with He. He-diluted plasma is effective in restraining the generation of CH2, CH3 and Si-CH3 configurations in the a-Si1-xCx:H film and markedly improves photoconductivity of the film. Using this photoreceptor, a photoaddressed spatial light modulator (PASLM) which operates in a transmission mode, has been fabricated with a ferroelectric liquid crystal. The PASLM exhibits a high resolution of 180 line pairs/mm, a response time of ~50 µs and a contrast ratio of ~30:1 under a write light (λ=565 nm) of 1.5 mW/cm2 intensity.

Some properties of RF sputtered Al sub 2 O sub 3 -Ta sub 2 O sub 5 composite thin films

Al{sub 2}O{sub 3}-Ta{sub 2}O{sub 5} composite dielectric thin films were prepared by RF plasma sputtering under various preparation conditions. Secondary ion mass spectroscopy and x-ray photoelectron spectroscopy were used to investigate the impurities and the oxygen vacancies in the films. The main impurity of the film was Na, but the sodium can be reduced by the use of high purity (5N) Al-Ta metal target. Oxygen vacancies were mainly related to Ta atoms and were dependent on the deposition rate of the films. Films sputtered at around 50 {Angstrom}/min were found to be relatively good for thin film devices, i.e., they have a high dielectric constant ({epsilon}{sub r}: {approximately}19), a low leakage current ({approximately}30 mA/F, at 10 V), and fewer oxygen vacancies. In this paper the preparation, details of the structure, and some electrical properties of the Al{sub 2}O{sub 3}-Ta{sub 2}O{sub 5} composite dielectric thin films are described.

Visible Photoluminescence in Highly Photoconductive Hydrogenated Amorphous Silicon Deposited by Glow Discharge of SiH4 Highly Diluted with He

A wide-band-gap (> 2.0 eV) hydrogenated amorphous silicon (a-Si:H) film showing visible photoluminescence (PL) is obtained by rf glow-discharge decomposition of SiH4 highly diluted with He. The PL and the structural properties of the a-Si:H are investigated. The He dilution leads to the extension of the band gap and the increase of the SiH2 and ( SiH2)n configurations in the film. The PL increases rapidly and blue-shifts consistently with the increase of the SiH2 and (SiH2)n configurations. By observation using transmission electron microscopy and Raman scattering, it is found that the film consists of uniform amorphous structure and includes no microcrystallites. The film exhibits a photoconductivity (under AM-1 light of 100 mW/cm2) to dark conductivity ratio of over 106 in addition to a visible PL at room temperature. This new a-Si:H film has potential for application to an optical device with characteristics of both light emission and photosensitivity.

Electrophotographic and structural studies on novel photoconductive polyimide films

Novel photoconductive polyimides have been developed, whose monomer units are composed of a series of thiophenylene moieties, ‐C6H4S‐, and imide groups. The most photosensitive polyimide shows an excellent photosensitivity of less than 1.0 μJ/cm2 in the visible light region, 400–570 nm. These photosensitive polyimide films are prepared by polymerizing the cast prepolymer films and annealing at the melting points of the polyimides. In this process their photosensitivities remarkably increase with the increase of crystallinity. From the structural studies by ultraviolet visible absorption, x‐ray diffraction, infrared absorption, and x‐ray photoelectron spectroscopy measurements, it is proposed that the coplanary conformation takes place in the crystallizing. The overlapped π orbitals, perpendicular to the polymer chains, lead to the more expanded conjugate system.

Photoaddressed spatial light modulator using transmissive and highly photosensitive amorphous-silicon carbide film

A novel photoaddressed spatial light modulator with a highly photosensitive hydrogenated amorphoussilicon carbide photoreceptor and a ferroelectric liquid-crystal layer has been developed that operates in a transmission mode for visible light. The hydrogenated amorphous-silicon carbide photoreceptor shows a high photosensitivity comparable with that of hydrogenated amorphous silicon, which is prepared by a plasma chemical vapor deposition method with helium dilution of the source gases SiH(4) and C(2)H(2) An image is input to the photoacdressed spatial light modulator by blue or green light, and an output is read out by red light as a transmitted image. The photoaddressed spatial light modulator exhibits a response time of ~50 µs and a contrast ratio of 30:1 under a write light (λ = 565 nm) of 1.5 mW/cm(2) intensity and a resolution of 90 line pairs/mm.

Reflection aspherical microlenses for planar optics fabricated by electron-beam lithography

Reflection aspherical microlenses are proposed for planar optics. These microlenses have the structure of a nonconcentric elliptical concave mirror. Broadband light can be used for these microlenses because these microlenses have no chromatic aberration. The microlens fabricated by electron-beam lithography and Ag deposition was found to have a smooth surface as designed. The focal length of the microlens was constant independent of the wavelength. The measured spot sizes agreed with diffraction-limited values at two different wavelengths. By using the array of this microlens, multiple images with excellent contrast were obtained under incoherent white-light illumination.