Hiroko Kominami

Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation

The dependence of the structural, photoluminescent and cathodoluminescent properties of ZnO thin films deposited by electron beam evaporation on the preparation conditions has been investigated. Both as-deposited and annealed thin films deposited at substrate temperatures higher than 200°C showed c-axis orientation, and their crystallinity was improved with increasing annealing temperature. The films showed the emission with a peak at around 510 nm in photoluminescence (PL) and cathodoluminescence (CL) except for the film annealed at 800°C in air. The emission seems to be well-known blue-green emission due to ZnO:Zn phosphor. The strong green emission with a peak at around 540 nm was obtained from the film annealed at 800°C in air. The origin of the emission is not understood. The film showed CL luminance of about 60 cd/m2 under excitation of 2 kV, 400 μA/cm2. Moreover, it showed CL under excitation even at 250 V without charging-up.

Low voltage cathodoluminescent properties of phosphors coated with In2O3 by sol-gel method

Abstract The phosphors used in usual CRTs are difficult to be used in a lower voltage than 500 V, because of charging-up on their surfaces. ZnS:Ag, Cl phosphor was coated with In2O3 using indium-iso-propoxide by the sol-gel method. It was found that a very thin In2O3 conductive layer could be coated uniformly on the phosphor surface. The low voltage cathodoluminescent properties were studied and compared with those without In2O3. The luminance of the phosphor excited with an electron beam of lower voltage than 500 V was considerably improved. Moreover, it was found that the formation of the thin layer is also quite effective for the improvement of the aging characteristics of cathodoluminescence.

Strong luminescence from dislocation-free GaN nanopillars

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.

Synthesis and Characterization of Fine Particle Y 2 O 2 S : Eu Red Phosphor at Low‐Voltage Excitation

Fine‐particle red phosphor (~1 μm) with spherical shapes was synthesized from a submicron‐size yttrium (europium) oxide, obtained by controlled hydrolysis of yttrium alkoxide‐europium chloride, employing sodium thiosulfate as a sulfurizing agent. Various preparative parameters, such as activator concentration, firing temperature, and duration, were optimized for the maximum luminance at low‐voltage excitation (<1000 V), and the best phosphor was compared with a commercial phosphor. The best phosphor with an improved luminance can be obtained for the phosphor prepared at 1000°C for 6 h and doped with 4 mol % europium. The fine‐grained phosphor showed increased luminance at low current densities and decreased luminance at high current densities. The improvement in luminance is attributed to the small particle size of the phosphors and uniform distribution of the activator. Synthesized phosphors were analyzed using X‐ray diffraction and scanning electron microscopy to interpret the observed luminescent properties.

Optimization of Low-Voltage Cathodoluminescence of Electron-Beam-Evaporated Y2O3:Eu Thin Film Phosphor

In this paper, we report the properties and the optimization of Y2O3:Eu cathodoluminescent thin film phosphor fabricated by electron-beam evaporation. Films were grown on quartz substrates and showed (111) orientation of cubic yttria. The Influence of film fabrication temperature regimes on the morphology and low-voltage cathodoluminescence (CL at 0.5 kV anode voltage) was studied. It was found that increase of substrate temperature from 400 to 500 °C results in larger and more regular grains with lower surface roughness. However, when substrate temperature exceeds 500 °C, the brightness of cathodoluminescence decreased owing to the deterioration of surface evenness and supposed increase of the surface recombination rate. Annealing induced grain growth and the improvement of color purity. The CL brightness at 3.5 kV anode voltage increased with annealing temperature with 0.33 eV activation energy. The CL brightness at 0.5 kV anode voltage is maximal at 800 °C annealing temperature and significantly decreases with higher annealing temperatures. CL results were analyzed with the use of threshold voltage and steepness coefficient parameters reflecting bulk and surface properties of the films respectively.

Growth of Epitaxial ZnO Thin Film by Oxidation of Epitaxial ZnS Film on Si(111) Substrate

The growth of an epitaxial ZnO thin film on a Si substrate by oxidation of an epitaxial ZnS film is a novel method and we are reporting it for the first time. The merits of using the Si substrate are to make the driving voltage of light-emitting diodes (LEDs) lower and the cost of the LEDs less expensive than that using a sapphire substrate. In this study, the epitaxial ZnO thin film could be successfully grown on the Si substrate. The epitaxial films showed a strong near-ultraviolet emission with a peak at around 3.32 eV at room temperature under 325 nm excitation.

Improvement of Low Voltage Cathodoluminescent Properties of Zinc Sulfide Phosphors by Sol-Gel Method

The phosphors used in conventional cathode ray tubes (CRTs) are difficult to use at voltages lower than 500 V, because of charge buildup on their surface. To overcome this drawback a ZnS:Ag,Cl phosphor was coated with a very thin In2O3 conductive layer by the sol-gel method using tri-iso-propoxyindium. Properties of the cathodoluminescence at voltages ranging from 50–500 V were studied and compared with those of the as-received ZnS:Ag,Cl phosphor. It was found that the luminance of the In2O3-coated phosphor was far better than that of the as-received phosphor when they were excited using an electron beam with an energy of less than 500 eV. Furthermore, the aging characteristics of the cathodoluminescence showed marked improvement.

Cathodoluminescence properties of blue-emitting SrGa2S4:Ce thin-films grown by low-temperature process

The cathodoluminescence (CL) properties of Ce-activated strontium thiogallate (SrGa2S4:Ce) thin films have been investigated aiming towards phosphor screen application in an emissive display under low to medium accelerating voltage (<5 kV) excitation. The SrGa2S4:Ce thin films were grown by multisource deposition using a molecular beam epitaxy system. The growth temperature of the CL phosphor thin films was 472.5 °C, which is the lowest value ever achieved. Several related properties were observed, including the CL spectra, luminance, luminous efficacy, and surface morphology of the thin films. A luminance of 1000 cd/m2 and an efficacy of 1 lm/W were obtained at an accelerating voltage of 5 kV with a current density of 60 μA/cm2. The light-outcoupling efficiency (ηout) of the film was 0.0531. In spite of this small ηout of the films compared to that of powder phosphors, the luminance and efficacy were comparable to the conventional ZnS:Ag, Cl powder phosphor. The average grain size of the films was 0.14 μ...

Green Cathodoluminescence Properties of Zinc Oxide Films Prepared by Excimer Laser Irradiation of a Sol-Gel-Derived Precursor.

Green cathodoluminescent (CL) zinc oxide (ZnO) films were successfully prepared at a low substrate temperature (473 K) in air by KrF-excimer laser irradiation of a sol–gel-derived precursor (ELISG method). Structures and CL properties of the films irradiated by the laser at different energy fluences (Ef) were studied and compared with those of a film prepared by conventional heat treatment in a reducing atmosphere. The prepared ZnO films showed a green CL band peaking at around 510 nm, in addition to an ultraviolet CL band. The maximum intensity of the green CL was obtained from a film produced by laser irradiation at an Ef of 130 mJ/cm2. Near-IR spectroscopy suggested that the green CL intensity depends on free-carrier concentration of the films and is maximized at an appropriate concentration. The formation process of the green CL films by the ELISG method was discussed in terms of thermal and photoinduced effects induced by the laser irradiation.

Influence of Preparation Conditions on the Structural and Luminescent Properties of Blue-Emitting SrGa2 S 4 : Ce Thin-Film Phosphors

Blue-emitting ternary strontium thiogallate doped with Ce (SrGa 2 S 4 :Ce) thin-film phosphor has been prepared by a multisource deposition technique by employing Sr, Ga 2 S 3 , and CeCl 3 as the source materials. The as-deposited films show a growth of single-phase polycrystalline strontium thiogallate with poor crystallinity. Various preparation conditions such as annealing temperatures, times, and Ce concentrations have been studied with respect to structural and luminescent properties of SrGa 2 S 4 :Ce thin films. The film annealed at elevated temperature shows an increase in crystallinity of the SrGa 2 S 4 , which leads to dissociation to SrS on the surface. The photoluminescence spectra show a typical blue emission at 445 nm independent of annealing temperatures, whereas the cathodoluminescence spectra show blue emission at 800°C and a very broad band green emission peaked at 540 nm at 900°C. The results are discussed on the basis of formation of thiogallate, stability at high temperature, luminescent properties as a function of temperature, annealing atmosphere, and Ce concentrations.