Seishi Iida

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.

Observation of Laser Oscillation from CaGa 2S 4:Eu 2+

The first observation of laser oscillation from a single crystal of CaGa2S4:Eu2+ (1 at%) is described in this report. A liquid nitrogen cooled thin platelet (0.8 mm thick), whose one flat surface was coated with aluminum to increase the reflectivity, was excited from the other uncoated side with a pulsed dye laser beam having a photon energy of around 2.48 eV. When the input power density exceeded a threshold value of ~10 MW/cm2, a directional beam having a sharp spectral line at around 2.19 eV emerged from the sample. This laser was found to have interesting wavelength tunability extending to ~200 A, possibly related to the broad gain spectrum of Eu2+ emission in this material.

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.

P-Type Conduction in ZnS Grown by Vapor Phase Epitaxy

Low-resistivity p-type ZnS layers have been grown on GaAs substrates by vapor-phase epitaxy from a Zn-added ZnS powder source, using an open tube system under a NH3-added hydrogen flow. The simultaneous addition of NH3 and Zn has been shown to be essential for the appearance of p-type conduction. The room temperature Hall-effect measurement of a grown layer revealed the hole concentration to be 6×1018 cm-3 and the mobility to be 40 cm2/(Vs). The p-ZnS/n-GaAs heterojunction showed good rectifying behavior and exhibited an infrared emission, having a peak photon energy corresponding to the band gap energy of GaAs under forward bias.

Photoluminescence studies of p- and n-type ZnS layers grown by vapor phase epitaxy

Abstract In order to clarify the donor and acceptor levels in p- and n-type ZnS layers grown on GaAs substrates by vapor phase epitaxy, delayed photoluminescence excitation spectra were measured at some lapse of time after excitation. This method enabled us to observe a direct resonant excitation of the selective donor-acceptor pair transition having a certain Coulomb energy. Through comparison of the resonant and non-resonant portions of the excitation spectrum, the p-type layer grown under the simultaneous presence of NH 3 and excess Zn atmospheres was found to have a shallow acceptor level of either ≈60 meV or ≈100 meV ionization energy. A description is also given on the structures of the excitation spectra related to bound excitons and on decays of bound excitons.

Growth and characterization of p-type VPE ZnS layers

Abstract The conductivity type control of ZnS was demonstrated for the open tube VPE growth on GaAs substrates from a ZnS powder source. The p-type character of the layer grown under the simultaneous addition of a few at% excess Zn and a large amount of NH 3 in the growth system was revealed mainly by the sign of the Seebeck effect and electron-beam induced current profile measurements at the cleaved faces of the epitaxial layers. Various kinds of changes corresponding to the change of the conduction type were observed in the photoluminescence spectra of the edge and bound exciton emissions, and trap levels examined by the transient thermoluminescence method. The role and meaning of the simultaneous addition of Zn and NH 3 are discussed together with the observed change of the amounts of Ga and As impurities from the substrate by EPMA.

Resonant Raman scattering and free-exciton emission in CuGaS2 crystals

Origins of lines appearing in the exciton region spectra of CuGaS2 crystals at low temperatures under 4579, 4765 or 4880 A excitation of an Ar+ laser are discussed in terms of emissions from upper- and lower-branch excitonic polaritons (UBPs and LBPs) and multi-phonon Raman processes. The line at 2.500 eV is an emission from LBPs. The line at 2.504 eV seen in our samples under 4765 A excitation is shown to be interpretable as a three-phonon Raman process in resonance with UBP states, while the line reported at the same energy by Shirakata is likely to be an emission from UBPs.

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.