Tatsuro Usuki

Red organic light-emitting diodes using an emitting assist dopant

We propose an emitting assist (EA) dopant system for obtaining organic light-emitting diodes (OLEDs) with pure red emission. The EA dopant (rubrene) did not itself emit but assisted the energy transfer from the host (Alq3) to the red emitting dopant (DCM2). The cell structure used was {indium tin oxide/hole injection layer [(20 nm), CuPc/hole transport layer (50 nm), NPB/emitting layer (40 nm), Alq3+DCM2 (2%)+rubrene (5 wt %)]/MgIn}. (CuPc: Copper (II) phthalocyanine, NPB: N, N′-Di(naphthalen-1-yl)-N, N′-diphenyl-benzidine, DCM2: 4-Dicyanomethylene- 2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran). A stable red emission (chromaticity coordinates: x=0.64, y=0.36) was obtained in this cell within the luminance range of 100–4000 cd/m2. When the cell was not doped with rubrene, the emission color changed from red to orange as the luminance increased. The EA dopant system is a promising method for obtaining red OLEDs.

Organic light-emitting diodes using a gallium complex

A gallium complex (GaMq2Cl) consisting of two 2-methyl-8-hydroxyquinolines (Mq) and a chlorine was synthesized and used for the fabrication of organic light-emitting diodes (OLEDs). The photoluminescent peak of GaMq2Cl at 492 nm was as strong in intensity as that of tris(8-hydroxyquinolinato)aluminum (Alq3). The OLED using GaMq2Cl as an emitting material showed blue-green luminance of 10 490 cd/m2. When it was used as an electron transport material in a rubrene doped cell, an OLED with a high luminance of 27 700 cd/m2 was obtained. We found that GaMq2Cl also was useful as a host material.

Development of Si monolithic (Ba, Sr)TiO3 thin film ferroelectric microbolometers for uncooled chopperless infrared sensing

We have been developing a total monolithic microbolometer technology for uncooled thermal sensing along the route from fabricating pixels of thin-film ferroelectric bolometers on micromachined Si substrates. Toward achieving this objective, sensor material of (Ba0.75Sr0.25)TiO3 (BST) has been prepared into thin-film form and been investigated to obtain a large temperature coefficient of dielectric constant (TCD) within the ambient temperature region. Operated in our proposed dynamic pulse-biased mode, the infrared responsivity (Rv) of sensor pixels is analyzed to reflect how those materials properties of BST film dominate the ultimate array performances. This new ferroelectric bolometer is expected to provide value-added merits of chopperless operation and high sensitivity enhanced by pulsed bias. In this paper, pixels of C-C balanced BST thin-film microbolometers have been fabricated by integrating Si-bulk micromachining and ferroelectric thin-film processing. Both pulsed laser deposition (PLD) and metal organic decomposition (MOD) methods have been employed in preparing BST films on those micromachined silicon substrates. Both the films show similar insufficient bolometric behavior with TCD-values smaller than 1%/K. Under pulsed bias, chopperless operation of pixels of so- fabricated microbolometers was confirmed. PLD enabled low- temperature preparation of high-quality films at 520 degree(s)C, so renders it for cutting-edge investigations to attain the TCD-value that demonstrated in BST ceramic plate by preparing large-grained, stress-free, micrometers -thick films. Meanwhile, MOD provides us those advantages of low-cost, large-area deposition and good uniformity compared to PLD method, films with TCD-value about -0.3%/K have been developed by MOD and are being geared to fabricate arrays. Finally, the future direction towards prototyping ferroelectric arrays was formulated based on the practical view of our development expeditions.

1.9-GHz-Band Surface Acoustic Wave Device Using Second Leaky Mode on LiTaO3

The propagation characteristics of a new high-phase-velocity leaky surface acoustic wave (SAW) mode, which is called the second leaky mode, in a periodic structure on LiTaO 3 (90°, 90°, 31°) are analyzed experimentally. The propagation loss is the lowest when the KH (K=wave number, H=Al electrode thickness) parameter is about 0.5. The higher mode of the second leaky mode is excited when the KH parameter is larger than 0.7. A ladder type band-pass SAW filter utilizing the second leaky mode on LiTaO 3 was designed and fabricated by applying the analysis results to the periodical structure. The pitch of the electrode fingers is 0.8 μm. The center frequency of the pass band is 1.938 GHz. The minimum insertion loss is 3.3dB. The attenuation at the center frequency ±100 MHz is 25dB. The measured characteristics of this filter can be sufficient for practical mobile communication systems.

Preparation of Aluminum Nitride Epitaxial Films by Electron Cyclotron Resonance Dual-Ion-Beam Sputtering

Aluminum nitride (AlN) thin films have been deposited on (12) sapphire by an electron cyclotron resonance (ECR) dual-ion-beam sputtering method. We have thoroughly investigated the dependence of the crystallinity and surface smoothness of the AlN thin film on the assisted conditions and sputtering conditions. The AlN thin film deposited under assisted conditions with the nitrogen ion beam energy and current density of 100 eV and 0.32 mA/cm2, respectively, and sputtering conditions with the arrival aluminum flux rate to the substrate surface (Al flux rate) and argon (Ar) ion beam energy of 42 A/min and 800 eV, respectively, is a single-crystal film with a very smooth surface. In order to prepare the epitaxial film, the Ar ion beam energy must be increased according to increases in the Al flux rate, and decreased according to decreases in the Al flux rate.

Characteristics of Surface Acoustic Wave on AlN Thin Films.

A surface acoustic wave (SAW) on a (001) plane of hexagonal aluminum nitride (AlN) deposited on R-plane sapphire ( (1\bar12)Al2O3) with an inclination of about 26 degrees to the substrate was analyzed. A Rayleigh wave and shear-horizontal-type (SH-type) wave were found. The phase velocity of these was 5400-5800 m/s and 6100-6600 m/s, respectively. To confirm this results, the propagating characteristics of both SAW modes were measured by input admittance of single interdigital transducers (IDT) and by transmission characteristics between two IDTs. The phase velocity of the Rayleigh wave and the SH-type wave was 5600 m/s and 6500 m/s, respectively, and the propagation loss was 0 dB/λ and 0.04 dB/λ, respectively. These measured characteristics approximately agree with the results of the simulation.

Property Control for High-Quality Ba1-xKxBiO3 Epitaxial Thin Films Prepared by High-Pressure Reactive rf-Magnetron Sputtering

The electrical properties of Ba1-xKxBiO3(BKBO) epitaxial thin films on SrTiO3(110) substrates have been successfully controlled using high-pressure reactive rf-magnetron sputtering under an 80-Pa discharge gas. We investigated the relationship between the target composition and the electrical properties of BKBO films. The zero resistance temperature (Tce) and the temperature dependence of resistivity above the superconducting onset temperature (Tco) were found to be primarily influenced by the ratio of Ba/K and the Bi content of the target composition, respectively. Electrical measurements showed that the highest Tce of as-grown BKBO epitaxial film was 28 K.

GHz-band surface acoustic wave devices using the second leaky mode on LiTaO3 and LiNbO3

The propagation characteristics of a new high-phase-velocity leaky surface acoustic wave (SAW) mode, which is called the second leaky mode, were analyzed experimentally in a periodic structure on LiTaO3 (90°, 90°, 31°) and LiNbO3 (90°, 90°, 37°). As regards the mode on LiTaO3, the propagation loss is lowest when KH ( K=wave number, H=Al electrode thickness) is about 0.5. The higher mode of the second leaky mode is excited when KH is more than 0.7. As regards the mode on LiNbO3, the propagation loss exhibits no distinct minimum when plotted against KH. The higher mode of the second leaky mode is excited when KH is more than 0.6. A ladder-type band-pass SAW filter utilizing the second leaky mode on LiTaO3 was designed and fabricated by applying the analysis extended results to the periodic structure. The pitch of the electrode fingers is 0.8 µ m. The center frequency of the pass band is 1.866 GHz. The minimum insertion loss is 3.3 dB. The width of the 1 dB pass band is 32 MHz. The attenuation at a center frequency of ±60 MHz is 25 dB. The measured characteristics of this filter are sufficient for practical mobile communication systems.

New High-Phase-Velocity Leaky Surface Acoustic Wave Mode on LiTaO3 and LiNbO3

A new high-phase-velocity leaky surface acoustic wave (SAW) mode, which was predicted by computer simulation, was confirmed experimentally to exist on both LiTaO3 and LiNbO3. The Euler angles of the propagation direction are (90°, 90°, 31°) on LiTaO3 and (90°, 90°, 37°) on LiNbO3. One-port SAW resonators were produced to confirm these modes. The phase velocity of the mode on LiTaO3 (90°, 90°, 31°) is 6250 m/s, and that on LiNbO3 (90°, 90°, 37°) is 7170 m/s. The temperature coefficient of the resonance frequency on LiTaO3 (90°, 90°, 31°) is -30 ppm/°C, and that on LiNbO3 (90°, 90°, 37°) is -30 ppm/°C.

Junction Characteristics of an Au/Ba1-xKxBiO3/Niobium-Doped SrTiO3 Structure

Au/Ba1-xKxBiO3 and Ba1-xKxBiO3/niobium-doped SrTiO3, two types of junctions which correspond to the emitter base and base collector in superconducting base transistors, respectively, have been fabricated and studied. The highest Tc end point (Tc0) of 28 K was obtained of thin film Ba1-xKxBiO3 by rf-sputtering method. The Au/Ba1-xKxBiO3 tunnel spectrum showed a clear gap structure and was very close to the ideal Bardeen Cooper Schrieffer theorys (BCS) form with Δ(0)=3.19 meV. The Ba1-xKxBiO3/SrTiO3(Nb) junction with heavy niobium doping (0.1 wt%) showed tunnellike I-V characteristics. The dI/dV spectrum for a high-Tc superconductor/semiconductor junction displayed the energy gap structure of a superconductor.