Ariyuki Kato

Optical properties and photo-induced memory effect related with structural phase transition in TlGaS2

Abstract Anomalies of the temperature coefficient of exciton absorption peak shift of TlGaS 2 were observed around 180–190 and 240–250 K. Some Raman lines were found to split at temperatures around 230–260 K. At low temperatures the light irradiation with the photon energy above the band gap energy caused a memory effect of photoluminescence quenching. This effect was observed for the emission appearing at 609 nm. Restoration of this photoluminescence quenching occurred with temperature range of 180–270 K. Complete recovery was attained at temperature above 270 K. The correspondences among the temperature ranges of the exciton absorption peak shift anomalies, the Raman line splitting, and the disappearance of the memory effect are considered to show that in TlGaS 2 successive phase transitions occur around 180–190 and 230–260 K.

Effect of Ce Co-Doping on CaGa2S4:Eu Phosphor: II. Thermoluminescence

Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of ∼2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ∼0.2 eV and ∼0.94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.

Effect of Ce Co-Doping on CaGa2S4:Eu Phosphor: I. Energy Transfer from Ce to Eu Ions

The resonance-type energy transfer process has been investigated for the first time in Ce and Eu co-doped CaGa2S4 polycrystalline samples with different dopant concentrations in the range from 0.2 at.% to 4 at.%. The energy transfer from Ce to Eu ions increases with increasing Ce and Eu concentrations. The values of the rate determined from measured Ce emission decay time constants with different dopant concentrations showed a reasonable agreement with those obtained from intensity analyses of the Ce emission which takes into account the re-absorption effect by Eu ions. In addition, the observed change of the transfer rate with dopant concentration was discussed in terms of the distance between Ce and Eu ions.

Green and Ultraviolet Emissions From Anatase TiO2 Films Fabricated by Chemical Vapor Deposition

Luminescence properties of anatase TiO2 films grown on sapphire substrates by atmospheric chemical vapor deposition have been investigated over a wide temperature range of 20 K to 300 K. An intense green emission band associated with localized exciton recombination has been observed at low temperatures under photoexcitation. The emission exhibited temperature quenching with an activation energy of ~48 meV. The shape of the emission spectrum was considerably modified upon electron beam excitation. The principal feature of this modification is a reversible redistribution of the spectrum intensity, which causes the appearance of a new UV emission band at high acceleration voltages. Details of the mutual relationship between the green and UV emission processes are considered in terms of the linear coupling configuration coordinate model. We propose that the temperature quenching of green emission is attributed to a combination of various thermally stimulated processes within the configuration coordinate diagram and energy migration through the excited state of the UV emission.

Photoluminescence characterization of rare-earth stoichiometric compounds of EuGa2S4

Abstract Photoluminescence properties of the stoichiometric compound of EuGa 2 S 4 has been investigated for the first time. The emission appearing in the region of 510–620 nm has a high quantum efficiency value reaching 100% at low temperatures. The characteristics of the emission are similar to those of CaGa 2 S 4 :Eu, and can be considered to be due to phonon-terminated transition of the Eu ion. Based on the measured luminescence data, construction feasibility of unique high gain tunable lasers is discussed.

Attempts of Homo p–n Junction Formation in ZnS by Impurity Co‐Doping with Vapor Phase Epitaxy

ZnS p-n homo junctions have been obtained for the first time with epitaxially grown layers having the structures of n-ZnS:In/p-ZnS:In, Ag, N/p-GaAs and p-ZnS:In, Ag, N/n-ZnS:In/n-GaAs. Both of these structures showed rectifying behavior which is expected for p-n junctions. The forward voltage of 3.7 V where current increases rapidly corresponds to the band gap energy of ZnS at room temperature. For reverse bias, some samples having p-ZnS:In, Ag, N/n-ZnS:In/n-GaAs structure showed backward diode type character.

Feasibility of TFEL application of Ce-doped CaGa2S4 and SrGa2S4 films prepared by flash evaporation method

Abstract Ce-doped CaGa 2 S 4 and SrGa 2 S 4 thin films were prepared for the first time by the flash evaporation method. The films were characterized before and after annealing in H 2 S(10%)+Ar gas stream by measuring photoluminescence and absorption spectra, X-ray diffraction and electron probe micro analyses. X-ray diffraction curves and absorption spectra before annealing show amorphous behaviour, whereas the annealing leads to a significant crystallization and improves the stoichiometry of the films. Based on the performance data obtained from dispersive type EL cells using CaGa 2 S 4 :Eu powder together with photoluminescence property comparison between CaGa 2 S 4 :Eu and CaGa 2 S 4 :Ce powders, the annealed films prepared by flash evaporation can be considered to become one of the candidates for TFEL flat panel devices.

Luminescence Properties of Y2O3:Tb3+ Whiskers Fabricated by Chemical Vapor Deposition

Luminescence properties of highly -oriented Y2O3:Tb3+ whiskers obtained by chemical vapor deposition on Si substrate have been investigated over a wide temperature range for different dopant concentrations. A considerable thermally stimulated increase of the green Tb3+ emission intensity was found in the temperature range of 98–433 K under 325 nm laser excitation. At temperatures higher than 433 K, the emission exhibits temperature quenching with the activation energy of ~0.98 eV. This behavior is discussed on the basis of the results of the time-decay measurements of the emission in the temperature range of 300–773 K. The thermally stimulated intensity increase is attributed to the thermal broadening of the photoluminescence excitation spectrum, whereas the temperature quenching is tentatively ascribed to the thermal activation of electrons from the excited 4f75d configuration to the conduction band. The decay time of the green Tb3+ emission exhibits shortening with increasing dopant concentration, which is considered in terms of the energy transfer processes between the Tb3+ ions.

Electrotransport Properties of p-ZnSnAs2 Thin Films Grown by Molecular Beam Epitaxy on Semi-insulating (001) InP Substrates

ZnSnAs2 thin films were prepared by molecular beam epitaxy (MBE) on semi-insulating (001) InP substrates using the same growth conditions as previously reported. High-resolution X-ray diffractometry (HRXRD) and Raman spectroscopy studies suggest the presence of both the chalcopyrite and sphalerite phases. The transport properties were measured from 5 K up to room temperature. We observed a pronounced peak in the Hall coefficient temperature dependence curve at ~130 K, similar to those observed only from chalcopyrite-phase bulk ZnSnAs2 in earlier studies. A hole concentration of p = 5.98 ×1018 cm-3, hole mobility of µ= 23.61 cm2/(Vs) and resistivity of ρ= 4.43 ×10-2 Ωcm were obtained at room temperature.

Observation of n- and p-Type Conduction of Site-selectively Zn-doped Epitaxial Layers of CuGaS2

Site-selective Zn doping for vapor phase eptaxial CuGaS2 layers on GaP substrates was tried by adding Zn vapor to each metal source supply period under completely separated alternate source feeding condition with the sources of CuCl, dietylegalliumchloride, and H2S. For the case of simultaneous Ga and Zn supply, the grown layers were found to be of n-type, while for the case of simultaneous Cu and Zn supply the layers were of p-type. The formation of a p-n homo-junction was demonstrated using these p- and n-type layers, and electroluminescence from this p-n junction was observed under pulsed forward voltage application at 77 K. This is believed to be the first successful report of conduction type control of CuGaS2 by impurity doping.