Single crystals of Gd3Ga3Al2O12 : Ce with different codopants were successfully grown using the Czochralski technique. Optical and scintillation properties of these codoped crystals were studied in detail including absorption, photoluminescence excitation and emission, decay time and thermoluminescence. This study revealed that while boron codoping improves the scintillation light output and energy resolution and decreases self-absorption in these crystals, calcium codoping affects their properties in the opposite manner. In addition to antisite defects, the effect of room temperature trap centres on the sensitivity of these crystals to light exposure is also reported for the first time. Light sensitivity was also found to be affected with the incorporation of codopants in the lattice. The effect of annealing in oxidizing and reducing atmospheres on the scintillation and optical properties of differently codoped crystals was also investigated in detail in order to better understand the defect structure of these crystals. All these measurements together are used to explain the effect of codoping on the crystal field and defect structure of these crystals.

http://iopscience.iop.org/article/10.1088/0022-3727/46/47/475302/pdf

Effect of codoping on scintillation and optical properties of a Ce-doped Gd 3 Ga 3 Al 2 O 12 scintillator

Phosphor particle size effect on luminous efficacy is studied for three types of phosphor converted white light-emitting diodes (WLEDs) using a Monte Carlo simulation tool. It is demonstrated for the first time that the size effect is nonmonotonic and exhibits minima in the submicrometer size range. In particular, for different LED packages, the lowest luminous efficacy all exists at the dimensionless particle size parameter (πd/λ) around 5.6. Moreover, the simulation results are verified by a theoretical model developed for the estimation of critical particle size. Finally, this study provides important technical implications for phosphor development and resolves one of the most critical issues in WLED manufacturing.

Nonmonotonic Phosphor Size Dependence of Luminous Efficacy for Typical White LED Emitters

Wavelength-resolved TSL measurements after x-ray irradiation at room temperature were performed on undoped, Ce- and (Ce, Zr)-doped Lu 3 Al 5 O 12 crystals Several glow peaks were observed in the 20-450 v °C temperature region, whose intensities and emission spectra are influenced by doping Namely, the TSL spectra are governed by defect and trace impurity-related emissions in the undoped crystal, while only the Ce 3+ emission is detected in the doped crystals Moreover, EPR measurements performed at 12 v K revealed typical spectra of Ce 3+ (4f 1 , S =1/2) occupying Lu 3+ substitutional positions In situ light irradiation (250-330 v nm) of the crystals results in a step-like decrease of the Ce 3+ EPR intensity Such a behaviour can reflect a creation of Ce 4+ ions under UV irradiation or an increase of the Ce 3+ population in the excited (5d) state

Defect states in Lu3Al5O12:Ce crystals

Samples of Ce, Er codoped yttrium aluminum garnet were annealed in ambient air or under oxygen flow and then sputter coated with Al and annealed in Ar. After this treatment Al diffused in the bulk and filled cation vacancy but this process is possible only after oxygen vacancy coupled to the cation vacancy is filled by an oxygen atom. The scintillator performance was improved from 13.2% full width at half maximum at 662keV to 3.4% for a 10×10×6mm size crystal. These findings provide an effective original approach to defect treatment in garnet crystals.

Improvement in scintillation performance of Ce, Er codoped yttrium aluminum garnet crystals by means of a postgrowth treatment

Disclosed herein are emissive ceramic elements having low amounts of certain trace elements. Applicants have surprisingly found that a lower internal quantum efficiency (IQE) may be attributed to specific trace elements that, even at very low amounts (e.g., 50 ppm or less), can cause significant deleterious effects on IQE. In some embodiments, the emissive ceramic element includes a garnet host material and an amount of Ce dopant. The emissive ceramic element may, in some embodiments, have an amount of Na in the composition less than about 67 ppm, an amount of Mg in the composition less than about 23 ppm, or an amount of Fe in the composition less than about 21 ppm.

Phosphor compositions and methods of making the same

Absorption, luminescence, ESR and ESA spectra and TL glow curves of YAG:Ce crystals codoped with Mg2+ ions and subjected to various treatments have been measured. The variations induced by irradiations in gamma and proton beams, and by thermal annealings under oxidation and reduction conditions point to large variations of the cerium ion valency. The ESA spectra of Mg-free and Mg-codoped crystals are clearly different with the peak value in the YAG:Ce, Mg sample higher and shifted towards the shorter wavelengths. Although the TL glow curves reveal the existence of the same major traps in both materials, their role in the overall deexcitation process is strongly modified upon Mg-codoping.

/pdf/changes-in-optical-properties-of-yag-ce-single-crystals-due-44wwtc8by4.pdf

Henryk Rzewuski

Papers

Changes in optical properties of YAG:Ce single crystals due to codoping and ionizing radiation treatment