Dependence of Ce3+ - related photo- and thermally stimulated luminescence characteristics on Mg2+ content in single crystals and epitaxial films of Gd3(Ga,Al)5O12:Ce,Mg

doi:10.1016/j.optmat.2018.05.087

Optical Materials

Volume 83, September 2018, Pages 290-299

https://doi.org/10.1016/j.optmat.2018.05.087 Get rights and content

Highlights

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Single crystals and epitaxial films of Gd₃(Ga,Al)₅O₁₂:Ce, Mg are studied.
•
Mg²⁺-induced changes in characteristics of crystals and films are different.
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Mg²⁺ ions influence the energy levels of the neighbouring Ce³⁺ ions.
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Optical quenching of Ce³⁺ luminescence in {Ce³⁺ - Mg²⁺} pairs is suggested.
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Mg²⁺ ions induce afterglow and TSL reduction and afterglow acceleration.

Abstract

Photoluminescence and thermally stimulated luminescence of Gd₃(Ga,Al)₅O₁₂:Ce single crystals and epitaxial films co-doped with different concentrations of Mg²⁺ ions are investigated in the 77–500 K temperature range under excitation in the 4f - 5d₁ and 4f - 5d₂ absorption bands of Ce³⁺ ions. Influence of Mg²⁺ ions on the photoluminescence intensity, spectrum, decay kinetics, temperature dependence of the photoluminescence intensity, and the activation energy of luminescence thermal quenching is observed. Co-doping with Mg²⁺ is shown to result in a drastic reduction of the afterglow and thermally stimulated luminescence and in a strong shortening of the afterglow decay kinetics. It influences also defects creation spectra and the activation energy of the photostimulated defects creation. The Mg²⁺ - induced changes in the characteristics of the Gd₃(Ga,Al)₅O₁₂:Ce, Mg single crystals and epitaxial films are found to be strongly different, and possible reasons of these differences are discussed.

Introduction

Ce - doped single crystals, ceramics and epitaxial films of Gd₃Ga_xAl_5-xO₁₂:Ce (GAGG:Ce) with different Ga contents (x) were intensively studied as promising scintillator materials for medical imaging application because of their extremely high light yield, good energy resolution, relatively high density (6.63 g/cm³), fast scintillation response, high radiation stability, and high hardness (see, e.g. [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]). In Ref. [11], the light yield of 58 000 ph/MeV and a good energy resolution of 4.2%@662 keV were achieved for the single crystals of Gd₃Ga_xAl_5-xO₁₂:Ce with x = 2.7 and x = 2.4, respectively. The light yield of 74 000 ± 7400 ph/MeV and energy resolution of 7.8% were reported in Ref. [23] for Gd₃Ga₃Al₂O₁₂:Ce single crystals under ¹³⁷Cs (γ-ray, 662 keV) excitation. The preparation of Ce - doped garnets at much lower temperatures by the liquid phase epitaxy (LPE) method allowed to noticeably decrease the concentration of antisite- and vacancy-related defects which act as effective traps for electrons. Scintillation characteristics of the LPE films of GAGG:Ce prepared with the use of a BaO-B₂O₃-BaF₂ flux were studied in Ref. [18]. Photoluminescence and thermally stimulated luminescence (TSL) characteristics of the LPE films and single crystals (SC) of GAGG:Ce were reported in Refs. [24] and [25], respectively.

A considerable improvement of scintillation characteristics (but accompanied with the decrease of the scintillation light yield) was achieved by the co-doping of GAGG:Ce SC with divalent alkali-earth ions, e.g., with Mg²⁺ or Ca²⁺ [5,14,15,19,26,27]. The Mg²⁺ co-doping was found to have much better influence on the scintillation characteristics of GAGG:Ce as compared with the Ca²⁺ co-doping. Indeed, the GAGG:Ce, Mg SC showed considerable improvement of the timing performance (much faster decay and rise times of the scintillation pulse with consequent improvement of the coincidence time resolution) and comparatively smaller (about 1.3 times) light output decrease with the increasing Mg concentration [15,19,26,27]. A strong Mg²⁺ - induced reduction of the TSL intensity was observed in Ref. [28], despite a small Mg content in the available GAGG:Ce, Mg crystal.

Co-doping of various Ce - doped oxide materials with divalent ions resulted in the change of cerium charge state from Ce³⁺ to Ce⁴⁺ (see, e.g. [[29], [30], [31], [32], [33], [34]], and references therein). Both the Ca²⁺ and Mg²⁺ co-doping stimulates the appearance of Ce⁴⁺ centers also in GAGG:Ce SC [5,14,15,19]. The presence of Ce⁴⁺ is manifested by the appearance of a broad absorption band in the 4–6 eV energy range attributed to the electron transfer from the valence band towards the Ce⁴⁺ ion [35,36]. A strong suppression of slow scintillation decay components and reduction of the afterglow and TSL intensity was explained just by the presence of Ce⁴⁺ ions. Indeed, being excellent electron traps, Ce⁴⁺ ions prevent electron capture at various intrinsic defects, thus decreasing the concentration of intrinsic electron centers, and provide an alternative channel for fast radiative recombinations (for more details, see, e.g. [14,15,19,28,32], and references therein).

In Ref. [32], luminescence and scintillation characteristics of Mg²⁺ co-doped multicomponent garnet LPE films were investigated for the first time. The results obtained at the study of the (Lu,Gd)₃(Al,Ga)₅O₁₂:Ce, Mg films with various Mg contents showed that the Mg²⁺ co-doping can significantly improve the timing characteristics of these materials and reduce the afterglow down to extraordinarily low values for a garnet scintillator. The LPE films of GAGG:Ce, Mg have never been investigated so far. It was expected that the combination of the Mg²⁺ co-doping with the LPE technology can allow to make the next step in the improvement of the GAGG:Ce characteristics.

In the present work, photo- and thermally stimulated luminescence characteristics of six single crystals and four LPE films of GAGG:Ce,Mg, containing different concentrations of Mg²⁺ ions (varying from 0 to 0.045 at.%), were investigated in the 77–500 K temperature range under selective excitation in the Ce³⁺ - related 4f – 5d₂ and 4f – 5d₁ absorption bands. This allowed us to consider separately the excitation process, where an electron is released from the excited level of Ce³⁺ into the conduction band (CB), and the process, where no electron release takes place. The aim of the work was to investigate an influence of Mg²⁺ ions on the photo- and thermoluminescence characteristics of GAGG:Ce in order to understand the mechanism of the processes, resulting in the light yield reduction in GAGG:Ce,Mg, and to clarify the influence of Mg²⁺ ions on the photoluminescence and photostimulated defect creation processes, characteristics of electron and hole traps, energy levels of Ce³⁺ ions, and recombination processes in GAGG:Ce,Mg. The participation of Gd³⁺ levels in the defects creation processes was considered as well.

Section snippets

Experimental procedure

In this work, six Gd₃Ga_xAl_5-xO₁₂:Ce (x = 2.46–2.83) single crystals and four Gd₃Ga_xAl_5-xO₁₂:Ce (x = 2.7) epitaxial films co-doped with different concentrations of Mg²⁺ ions, varying from 0 to 0.045 at.%, were investigated. The composition of the samples in stoichiometric formula units, determined with the use of an electron probe microanalysis (EPMA) and glow discharge mass spectroscopy (GDMS), is shown in Table 1.

The single crystalline films were grown by isothermal dipping liquid phase

Experimental results and discussion

Absorption spectra of the investigated SC and LPE films of GAGG:Ce, Mg presented in Fig. 1a and b, respectively, indicate that the co-doping of GAGG:Ce with Mg results in the reduction of the Ce³⁺ - related absorption, arising from the 4f - 5d₂ (∼3.65 eV) and 4f - 5d₁ (∼2.8 eV) electron transitions of Ce³⁺ ions, and appearance of the Ce⁴⁺ - related absorption in the E > 3.8 eV energy range (see also, e.g. [14]). The reduction of Ce³⁺ concentration is much stronger in the single crystals (Fig. 1

Conclusions

As a result of the study of the photostimulated luminescence and defects creation processes in the Gd₃Ga_xAl_5-xO₁₂:Ce single crystals and epitaxial films, containing different concentrations of Mg²⁺ ions (0–0.045 at%), the following conclusions can be made:

(i)
The presence of two types of Ce³⁺ - related centers in GAGG:Ce,Mg: the single Ce³⁺ ions and the Ce³⁺ ions perturbed by the neighboring Mg²⁺ ions, appears in the dependence of the emission spectrum position on the excitation energy, in

Acknowledgments

The work was supported by the Institutional Research Funding IUT02-26 of the Estonian Ministry of Education and Research and the project 16-15569S of the Czech Science Foundation.

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Dependence of Ce3+ - related photo- and thermally stimulated luminescence characteristics on Mg2+ content in single crystals and epitaxial films of Gd3(Ga,Al)5O12:Ce,Mg

Highlights

Abstract

Introduction

Section snippets

Experimental procedure

Experimental results and discussion

Conclusions

Acknowledgments

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Nucl. Instrum. Meth. Phys. Res. A

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Nucl. Instrum. Meth. Phys. Res. A

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Dependence of Ce³⁺ - related photo- and thermally stimulated luminescence characteristics on Mg²⁺ content in single crystals and epitaxial films of Gd₃(Ga,Al)₅O₁₂:Ce,Mg