Shinji Okamoto

Enhancement of characteristic red emission from SrTiO3:Pr3+ by Al addition

The mechanism of enhancement on the red emission efficiency from SrTiO3:Pr3+ (1D2→3H4 emission of Pr3+) by Al addition has been investigated. The emission intensity at room temperature for the phosphor, SrTiO3:Pr3+ synthesized with 23 molu200a% Al is about 200 times higher than Al-free samples. Photoluminescence (PL) spectra at low temperatures show the progress of several red emission lines of the 1D2→3H4 transition with an increase in Al molar ratio. The lines are probably due to formation of Al-associated Pr3+ centers. The Al-associated Pr3+ PL does not exhibit thermal quenching up to room temperature. Temperature dependent PL spectra below 100 K and PL excitation spectra reveal that the energy transfer occurs from photoexcited carriers in SrTiO3 host to PL centers. The PL intensity, x-ray diffraction measurements, and high-resolution transmission electron microscope images indicate the improvement of crystallinity of SrTiO3:Pr3+ with Al addition, resulting in a decrease in defects and probably an increase...

Luminescence of rare-earth ions in perovskite-type oxides: from basic research to applications

Abstract Luminescence of a rare-earth ion in SrTiO 3 or other perovskite-type oxides was used as a probe of physical properties of a host crystal, but not as a phosphor because observed luminescence efficiency was low. Recent investigations have shown that the luminescence efficiency of SrTiO 3 :Pr 3+ is intensified up to 200 times by addition of Al ions. This paper reviews experimental results on this phenomenon and discusses mechanism of the Al-addition effect. A survey was made to find other materials, the efficiency of which can be improved by the impurity addition. Host-to-activator energy transfer by a coupling of the band-edge recombination energy and an allowed transition of a rare-earth ion is essential to provide a high efficiency. A red phosphor, Al-added SrTiO 3 :Pr 3+ , is promising for display applications.

Characteristic enhancement of emission from SrTiO3:Pr3+ by addition of group-IIIb ions

Enhancement of the emission from SrTiO3:Pr3+ by addition of group-IIIb ions (IIIb3+) has been investigated. From the photoluminescence (PL) spectra and x-ray diffraction patterns, it is speculated that one of the origins of the enhancement is charge compensation of Pr3+ at the Sr2+ site by IIIb3+ substituting for Ti4+ around Pr3+, which results in a change in the crystal field around Pr3+. The IIIb3+-dependent PL spectra can be explained qualitatively by a local distortion around Pr3+ ions due to a difference in an ionic radius between Ti4+ and IIIb3+.

Photoluminescence from surface-capped CdS nanocrystals by selective excitation

We have studied photoluminescence (PL) properties of hexagonal CdS nanocrystals whose surface is capped by pentafluorothiophenol and N,N-dimethylformamide, by means of florescence-line-narrowing spectroscopy. Both the sharp band-edge and the broad trap-state emission are strongly modified by exciton-phonon interactions enhanced by quantum confinement. The LO-phonon modified structures in the band-edge emission spectra show the existence of two excitonic states at the band edge. The broad trap-state luminescence is modified by the strong coupling between localized excitons and polar vibrations at the surface. The selectively excited luminescence properties and the coupling between electronic and vibrational excitations in CdS nanocrystals are discussed.

Photoluminescence from GaAs nanocrystals fabricated by Ga+ and As+ co-implantation into SiO2 matrices

We have fabricated GaAs nanocrystals by means of Ga+ and As+ co-implantation into SiO2 matrices and applied selective excitation spectroscopy to clarify the origin of photoluminescence (PL) from GaAs nanocrystals in SiO2 matrices. Under blue laser excitation, broad PL spectra with multipeaks are observed in the visible spectral region. Under selective excitation at energies within a certain band, fine structures are observed at low temperatures and the size-dependent PL structures are attributed to quantum confinement states in GaAs nanocrystals. The origin of visible PL from GaAs/SiO2 nanocomposites is discussed.

Effects of Al Addition on Photoluminescence Properties in Rare‐Earth Ion‐Doped SrTiO3

Enhancement of emission intensity of rare-earth ion-doped SrTiO 3 by Al addition has been investigated. In Pr 3+ and Tb 3+ , addition of 23 mol % Al intensifies emission by more than 200 times. On the other hand, in other rare-earth ions, the addition of 20 mol % Al intensifies emission at most by three times. This observation implies that 4f-5d transition plays an important role in the emission enhancement. In SrTiO 3 doped with a rare-earth ion other than Pr 3+ or Tb 3+ , a photoluminescence (PL) spectrum is changed, probably due to a decrease in the effective concentration of the ion in SrTiO 3 . The temperature dependence of PL spectra shows that the energy transfer from carriers to Pr 3+ or Tb 3+ ions is much more efficient than that to other rare-earth ions in SrTiO 3 . It can be speculated that the energy transfer in SrTiO 3 :Pr 3+ or Tb 3+ occurs from carriers to Pr 3+ or Tb 3+ ion via 4f-5d transitions, which are much higher in oscillator strength than 4f-4f transitions.

Blue luminescent SrGa2S4:Ce thin films grown by molecular beam epitaxy

Abstract SrGa 2 S 4 polycrystalline thin films, currently attracing much attention as host material for blue electroluminescence, are grown by MBE employing Sr metal and Ga 2 S 3 compound as source materials. The stoichiometry of this film can be controlled by changing the Ga 2 S 3 /Sr flux ratio. A single-phase SrGa 2 S 4 layer with high crystallinity is obtained in a flux ratio of 60 at the growth temperature of 560°C. A Ce-doped SrGa 2 S 4 phosphor film shows efficient blue photoluminescence (PL) whose chromaticity is comparable to that of blue CRT phosphors.

Atmospheric pressure vapor-phase growth of ZnO using a chloride source

Abstract Atmospheric pressure vapor-phase growth was demonstrated for the growth of zinc oxide (ZnO) films on sapphire (0xa00xa00xa01) substrates. The X-ray diffractogram showed a typical pattern of ZnO having a hexagonal structure, and a full-width at half-maximum (FWHM) of 23.3xa0min was obtained in the X-ray diffraction profile. The growth rate of the ZnO film increased from 0.1 to 8.0xa0μm/h with increasing growth temperature and input partial pressures of ZnCl 2 and O 2 , respectively. A strong band edge emission at 370.0xa0nm was observed in the photoluminescence spectra at 20xa0K.

Dependence on Ce Concentration of Blue Emission and Crystallographic Properties in SrGa2S4:Ce Electroluminescent Thin Films Grown by Molecular Beam Epitaxy

The Ce-doping dependence of blue emission properties has been investigated for SrGa2S4:Ce electroluminescent (EL) thin films grown by molecular beam epitaxy (MBE). EL spectra, EL luminance, photoluminescence (PL) spectra, X-ray diffraction (XRD) intensity, XRD rocking curves, spacing of lattice planes, EL and PL decay times were measured in samples with Ce-doping concentrations ranging from 0.87 mol% to 8.37 mol%. EL luminance reached a maximum at 2.40 mol%. In particular, EL chromaticity remained within the pure blue region even with higher Ce-doping concentrations.

SrS:Ce Thin-Film Electroluminescent Devices Fabricated by Post-Annealing Technique and Their Electrical Properties

A postannealing technique has been used to make double-insulating SrS:Ce thin film electroluminescent (EL) devices. This technique has the effect of lowering the substrate temperature during SrS:Ce deposition, and also of improving the luminance and emission color with blueshift. Using this technique, the controllability of thin film processing and reproducibility of the devices have also been improved. A suitable annealing temperature is about 720°C. The device has a luminance of 320 cd/m2 and improved color coordinates, x=0.18 and y=0.35, using a 1-kHz drive frequency. The devices have also been evaluated as to EL properties and film crystallinity. In particular, a novel way of directly measuring the transient electric field across the active layer has been proposed.