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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 Å, possibly related to the broad gain spectrum of Eu2+ emission in this material.

Luminescence, decay, and thermal quenching under photonic excitation of CaGa2S4:Eu2+ mono and polycrystals have been measured. For excitation into the lower 5d states, thermal quenching starts above 350 K, which makes this material appropriate for use as a yellow phosphor under excitation by blue-emitting diodes. Thermoluminescence after 337.1 nm irradiation is explained by photoionization of Eu2+ ions. © 2002 The Electrochemical Society. All rights reserved.

The optical properties of BaAl 2 S 4 doped with Eu 2+ were investigated. The synthesis was performed in evacuated and sealed quartz ampul at 1000°C during 1 h, then at 840°C during 20 h. With absorption, excitation, and emission spectra we have determined the crystal-field splitting, the redshift, the influence of the lattice vibration properties, and the thermal quenching of the luminescence. The crystal-field splitting of Eu 2+ in BaAl 2 S 4 was estimated on the order of 8000 to 10 000 cm −1 and the Huang-Rhys parameter S = 5 ± 1 corresponds to an intermediate electron-phonon coupling regime. The mean phonon energy for BaAl 2 S 4 was evaluated by optical and Raman methods, and the obtained values were 32 ± 5 and 40 meV, respectively. These results confirm the very interesting properties of this blue BaAl 2 S 4 :Eu 2+ phosphor, the most important being the very slight thermal quenching of the luminescence.Inorganic thin-film electroluminescent ͑iEL͒ devices are expected to be very interesting candidates for full-color flat display panels. The lack of a blue phosphor with suitable color coordinates, high enough luminance and efficiency was one of the main difficulties to commercialize full color devices. These last years, binary and ternary sulfide phosphors have been investigated, like SrS:Ce, 1 ͑Ca,Sr͒Ga 2 S 4 :Ce, 2 SrS:Cu, 3 SrS:Cu,Ag, 4 and CaS:Pb. 5 Unfortunately, not one of these phosphors provides suitable performances for full-color display application.A breakthrough was made by Miura et al. in 1999, who have reported on a new thin-film blue EL phosphor, BaAl 2 S 4 doped with Eu 2+ , which gives a higher brightness of 65 cd/m 2 at 50 Hz and good color coordinates, x = 0.12 and y = 0.10. 6 Greater brightness than 700 cd/m 2 at 120 Hz has been obtained using the thick dielectric technology at iFire Company. 7 This performance allows iFire Company to announce a full-color 34-in. iEL HDTV screen for the 2006 timeframe using the Color-By-Blue ͑CBB͒ technique. 8 The first report concerning this BaAl 2 S 4 :Eu 2+ phosphor was presented by Donohue and Hanlon. 9 The cubic structure with a lattice parameter of 1.2588 nm was established. At 300 K, a blue band emission peaked at 475 nm was obtained with a decay time on the order of 0.45 s. In this pioneering paper, it was pointed that this phosphor will be efficient due to a wide bandgap leading to a low interaction between the excited states of Eu 2+ and fundamental edge. This cubic structure was summarized by Eisenmann et al. 10 and some luminescence properties were presented by Le Thi et al. 11 The radiative mechanisms of this BaAl 2 S 4 :Eu 2+ phosphor are not completely understood and their fundamental knowledge needs to be enlarged. In this paper diffuse reflectance, excitation, and emission spectra, and photoluminescence ͑PL͒ decay curves of powder samples were analyzed and the influence of temperature on the luminescence process was investigated. The results are discussed in regards to the results recently presented for BaGa 2 S 4 :Eu 2+ phosphor, 12 which h...

Raman investigation of isostructural polycrystalline MII Ga 2 (S, Se)4 compounds with MII = Pb, Sr, Eu, Yb and Ca was performed. We observed that the effect of MII atomic mass on the phonon frequencies is negligible and that the vibrations are essentially influenced by the sizes of the MII cations. As the effects of cation substitution are small we concluded that the orthorhombic thiogallates MII Ga 2 S4 exhibit nearly the same phonon energies: about 280 cm−1 (∼35 meV) and 360 cm−1 (∼45 meV) for the most intense vibration modes. The substitution of S atoms by heavier Se atoms causes the shift of the Raman spectrum to lower frequencies. The orthorhombic MII Ga 2 Se4 compounds present phonon energy at about 185 cm−1 (∼23 meV) for the most intense vibration mode. Results confirm that a molecular model is more adequate to describe the vibrations of these compounds than the factor-group analysis.

Luminescent properties of CaGa 2 S 4 :Er polycrystals in the visible and infrared spectral ranges were investigated. The presence of two different point symmetries for Er 3ϩ centers is well established by selective and polarized photoexcitation. The Ca thiogallate compound can be considered as a promising material for optical communication purposes due to the intense Er 3ϩ infrared luminescence in the 1.55 m range.Erbium-doped materials are of great interest in the thin film integrated optoelectronic technology, due to their Er 3ϩ intra-4f infrared ͑IR͒ emission at 1.55 m, a standard telecommunication wavelength in band C ͑1530 to 1560 nm͒. Dielectric thin films of different Er-doped materials can be used to fabricate planar optical amplifiers or lasers 1-5 that can be integrated with communication systems. In wide bandgap materials, Er 3ϩ ions provide also several emissions in the visible range. The ratio of the visible and IR emission intensities depend on the host material type and erbium concentration. 6 In materials with a small phonon energy the visible luminescence predominates 7 under ultraviolet ͑photoluminescence, PL͒ and electron-beam ͑cathodoluminescence, CL͒ excitations. In oxides, 8 because of the large phonon energy, the multiphonon relaxation of the excited electrons to the lower levels give rise usually to infrared and red emissions.Owing to these considerations, using small phonon energy sulfides is a possibility for practical applications. But these materials must be stable toward hydrolysis and have a high solubility for rare-earth elements.Thiogallates of the different second group elements are convenient for all the reasons mentioned above. 9 CaGa 2 S 4 and SrGa 2 S 4 crystallize in the orthorhombic system with the space group D 2h 24 -Fddd 10 with Ca and Sr ions located in three slightly different sulfur square antiprisms ͑Wyckoff positions: 8a, 8b, and 16e͒. But it has been established that for the strontium thiogallate compound doped with Nd and codoped with Na for charge compensation, there is an ordered distribution of Na ϩ and Nd 3ϩ ions: the Nd 3ϩ ions occupy the a and b polyhedra which have no common apex. 11 Therefore, the interactions responsible for concentration quenching are considerably reduced.CaGa 2 S 4 :Er luminescence properties have been intensively investigated in the visible range up to 700 nm in the past. 9,12,13 The main task of this paper is to analyze the luminescence properties of Er in the IR range and by selective laser spectroscopy and polarization methods to index the emission lines and determine the nature of the corresponding luminescence centers. MeasurementsThis paper describes the results obtained for CaGa 2 S 4 :Er polycrystals prepared by solid-phase synthesis from stoichiometric amounts of calcium ͑CaS: 99.99͒ and gallium (Ga 2 S 3 : 99.95) sulfide powders in a sulfur vapor atmosphere at 1000°C in a vacuum pumped sealed quartz bulb. Activation by Er ͑2 atom %͒ was realized using ErF 3 ͑99.9͒ doping during the synthesis process.The photon excitation at 33...

Semiconductors with the common composition II-III2-VI4 (where II are divalent cathions; III – trivalent cathions and VI – chalcogenides) are perspective materials for optoelectronics. The electrical properties, luminescence, photoconductivity and thermally stimulated depolarization of these compounds have been investigated MnGa2Se4, MnGa2S4, MnIn2S4, MnGaInS4 and PbGa2Se4 compounds possess high photosensitivity. CaGa2S4 doped with rare-earth elements have been found to be effective luminescent semiconductor materials.

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