Hironaga Matsumoto

Several Blue-Emitting Thin-Film Electroluminescent Devices

Blue-emitting thin-film electroluminescent (EL) devices were studied. As the blue-emitting phosphor, thin-films in which the Tm3+ ion was doped into several hosts (ZnS, Y2O2S, CdF2, ZnF2 and YF3) and CaF2:Eu were investigated. Blue EL emission of Tm3+ ions arising from the 1D2→3H4 or 1G4→3H6 transition was observed in each Tm-doped device. The most dominant lines in these emissions varied with the kind of host materials. The CaF2:Eu thin-film also showed blue electroluminescence due to a parity-allowed 4f6(7F)5d→4f7(8S) transition of the Eu2+ ion.

DC Electroluminescence on M-I-S Structures in Thin Films

Stable d.c. electroluminescence is observed in a cell with M-I-S structures in the form of thin films (Ta–Ta2O5–ZnS: TbF3–Au). To be able to observe d.c. electroluminescence in the cell, the Ta2O5 insulating layer must be about 1000 A–5000 A thick and treated by heating in air at 250°C for about 10 minutes. Through such heat treatment, the d.c. conductance of the Ta2O5 layer increases remarkably (about 103 times). The increase is due to the transition from an anomalous Poole-Frenkel effect to a normal one in the conduction process. This heat-treated Ta2O5 layer plays an important role in the creation of hot electrons required to excite the electroluminescent cell.

Electrochromism and Electronic Structures of Nitrogen Doped Tungsten Oxide Thin Films Prepared by RF Reactive Sputtering

The doping effect of nitrogen on amorphous tungsten trioxide (a-WO3) thin films was investigated with regard to electrochromism and electronic structures. The N-doped thin films exhibit a change in electrochromic coloration from transparent yellow to black, whereas the un-doped thin films exhibit blue coloration. In addition, a new absorption peak related to nitrogen doping is observed at 2.3 eV in photoabsorption spectra during the electrochemical coloration/bleaching process. To explain these experimental results, the electronic structures of N-doped tungsten oxide were calculated by the DV-Xα molecular orbital method.

Atomic Composition and Structural Properties of Blue Emitting BaAl2S4:Eu Electroluminescent Thin Films

We have investigated the compositional and structural properties of blue emitting BaAl2S4:Eu electroluminescent thin films fabricated by switching electron-beam evaporation using two targets. The use of X-ray photoelectron spectroscopy (XPS) revealed that the ratio of barium to sulfur in thin films is 1.0:2.0, which is identical to the stoichiometry of BaAl2S4. The X-ray diffraction (XRD) peaks from the thin film were assigned to those of a BaAl2S4 crystal. The photoluminescence (PL) spectrum shows emission from the 4f65d→4f7 transition of the activated Eu2+ ions in BaAl2S4 crystals. These results indicate that the crystalline phase in thin films is only BaAl2S4. The thin films also contain a large amount of oxygen impurities, which result in the formation of an Al2O3 layer and amorphous barium aluminate.

Strong Ultraviolet-Emitting ZnF2:Gd Thin Film Electroluminescent Device

A ZnF 2 :Gd thin film electroluminescent device was fabricated, and very strong ultraviolet (UV) emission was achieved from this device. This UV emission showed a sharp line at 311.5 nm assigned to the 6 P 7/2 → 8 S 7/2 transition of Gd 3 S+ ion. Blue photoluminescence was observed from the CaF 2 :Eu single crystal excited by the ZnF 2 :Gd electroluminescent device

Effect of Heat Treatment on the Coefficient βpf for the Poole-Frenkel Effect and the Conductivity in Ta2O5 Films

The change in the coefficient βpf for the Poole-Frenkel effect and the change in the conductivity in anodized Ta2O5 films by heat treatment have been observed. Untreated films (dried at room temperature) include only neutral traps, so that the film does not exhibit the Poole-Frenkel effect and has very low conductivity. When these films are heat-treated for a few minutes in oxygen or in a vacuum, donor levels are created in the films. These films increase in conductivity by about 104 times and exhibit the Poole-Frenkel effect. When the films are heat-treated for over 10 minutes in oxygen, the conductivity decreased again. In this decreasing process, the value of βpf changed from that of an anomalous Poole-Frenkel effect into the normal one. These chances may be caused by the change in the ratio Nd (donor density)/Nt (trap density) and in the energy of the donor levels.

Electroluminescence spectra of rare-earth-doped ZnS1-xSex thin films

Abstract Electroluminescence has been measured for ZnS 1- X Se X thin films doped with rare-earth ions. As X increases the band-gap energy of the host decreases. The emission levels of trivalent rare-earth ions are not observed when the band-gap energy is narrower than the excitation levels. This is because of the energy transfer between the host and the emission center.

Electroluminescence of ZnF2 Thin-Films Doped with Rare-Earth Ions

Electroluminescent devices based on ZnF2 thin-films have been investigated. From the ZnF2 thin-films doped with rare-earth ions (except for Sc, Y, La, Pm, Yb and Lu), electroluminescence was observed. The emission in these devices was visible from the ultraviolet to the infrared region. The color and spectral region of these electroluminescences shifted to shorter wavelength compared with those of the same ions in the ZnS-based thin-film devices for many cases. To obtain information on the excitation process for rare-earth ions, the transient behavior has also been measured.

Band-Gap Energy Dependence of Emission Spectra in Rare Earth-Doped Zn(1-x)CdxS Thin Film Electroluminescent Devices

Electroluminescence has been measured for Zn(1-x)CdxS thin films doped with rare earth ions. The ZnS thin film doped with the Pr ion shows the 495 and 658 nm emissions due to the transitions from the 3P0 to 3H4 and 3F2 levels, respectively. With increasing value of X, the emission from the 3P0 level vanished and 618 nm emission arose from the transition of 1D2→3H4. The reason for this change may be related to the band-gap energy of the host lattice. From these experimental results, it is concluded that the excitation process of the rare earth ions involves the energy transfer from the host lattice for the Zn(1-x)CdxS electroluminescent devices. In addition, the transition around 800 nm emission for the Tm3+ ion is also discussed.

Optical Properties of Blue-Emitting BaAl2S4∶Eu Thin-Films for Inorganic EL Display

The optical properties of blue emitting BaAl2S4:Eu thin-films were studied. The emission peak is around 470 nm, whose FWHM (full width at half maximum) is 35 nm. The dielectric constant is 3.03 from transmission spectrum, and the optical band gap is approximately 4.6 eV. It is found that the cubic phase and orthorhombic phase exist in fabricated thin-films. Furthermore, the energy band structure of BaAl2S4:Eu thin-films from X-ray photo-electron spectra (XPS) and the absorption spectra were analyzed.