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Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals
Journal Pre-proof Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals K. Bartosiewicz, V. Babin, K. Kamada, A. Yoshikawa, S. Kurosawa, A. Beitlerova, R. Kucerkova, M. Nikl, Yu Zorenko PII:
S0022-2313(19)30131-0
DOI:
https://doi.org/10.1016/j.jlumin.2019.116724
Reference:
LUMIN 116724
To appear in:
Journal of Luminescence
Received Date: 18 January 2019 Revised Date:
13 August 2019
Accepted Date: 29 August 2019
Please cite this article as: K. Bartosiewicz, V. Babin, K. Kamada, A. Yoshikawa, S. Kurosawa, A. Beitlerova, R. Kucerkova, M. Nikl, Y. Zorenko, Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116724. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals
K. Bartosiewicz1*, V. Babin2, K. Kamada1,4, A. Yoshikawa1,3,4, S. Kurosawa3, A. Beitlerova2, R. Kucerkova2, M. Nikl2, Yu. Zorenko5 1
Institute for Material Research, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan 2 Institute of Physics, AS CR, Cukrovarnicka 10, Prague 16253, Czech Republic 3 New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, 980-8579, Japan 4 C&A corporation, T-Biz, 6-6-40 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan 5 Institute of Physics, Kazimierz Wielki University in Bydgoszcz, Powstańców Wielkopolskich 2, 85-090, Bydgoszcz, Poland * Corresponding author: [email protected]
Abstract
This research deals with the luminescence and physical properties of Gd3(Ga,Al)5O12:Ce single crystal set and it is a complementary study to analogous (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. Those three groups of materials show similar luminescence and physical properties. In this research, the influence of substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals on the luminescence and scintillation characteristics as well as thermodynamical stability of the garnet phase is studied, and it is compared to (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. The unbalanced substitution Ga for Al makes garnet phase thermodynamically unstable and leads to the formation of the multiphase system with luminescence centers in UV and visible spectral ranges. The wavelength-dispersive
X-ray
spectroscopy,
X-ray
powder
diffraction
analysis,
photoluminescence and decay kinetic measurements reviled all phase types within multiphase crystals. Temperature dependent measurements were applied to determine the quenching mechanism for Ce3+ luminescence. The timing characteristics are studied as the function of Ga content.
1
1.
Introduction
The aluminum garnets belong to the class of the oxide materials, which luminescence, scintillation, and physical properties can be easily tuned by modification of their chemical composition [1-3]. The band gap can be adjusted, the traps energy levels and their concentration can be finely tuned and their influence can be damped or on the contrary, enhanced by the specific doping for a luminescence optimization [1,4,5]. For instance, Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) activated with Ce3+ are very well know scintillators, however, with poor scintillating parameters like low light yield (LY) value and presence of the slow component in the scintillating decay time [6,7]. Composition engineering strategy based on the balanced substitution Gd for Y/Lu and Ga for Al significantly improves scintillation properties. Consequently, multicomponent garnet single crystals of general chemical formula (Y,Lu,Gd)3(Al,Ga)5O12:Ce show excellent scintillating parameters including extremely high LY and fast scintillation response [8,9]. On the other hand, unbalanced substitutions Gd for Y/Lu and Ga for Al in (Y/Lu)3(Al,Ga)5O12 single crystals lead to the formation of the multiphase systems ([10,11] and references therein). In our previous works, we have shown that unbalanced substitution Gd for Y in Y3Al5O12:Ce [10] and Gd for Lu in Lu3Al5O12:Ce [11] prevented to grow pure garnet phase single crystal. Consequently, those crystals consist of a mixture of garnet, perovskite and α-Al2O3 phases. Such multiphase crystals showed luminescence in the visible and UV spectral ranges. The visible luminescence was ascribed to Ce3+ ions in the garnet phase, whilst UV one to both Ce3+ ions in the perovskite and F+ centers and Ce3+ ions in α-Al2O3 phases. The PL measurements reviled very complex energy transfer processes between both phases as well as within the perovskite phase, for more details see Ref. 10 and 11. The phase diagram for the Al2O3-Gd2O3 system [12] shows that Gd3Al5O12 matrix melts incongruently. Consequently, during the crystallization of melted composition, the GdAlO3 and α-Al2O3 phases are formed instead of target Gd3Al5O12 one. This means that Gd3Al5O12 matrix cannot be obtained in the form of a single crystal from the melt. However, the Gd3Al5O12 matrix can be fabricated in the pure garnet phase using low-temperature synthesis methods (i.e. at temperatures below the melting point) such as liquid phase epitaxy, sol-gel combustion or solid-state sintering methods [13-16]). This work is a complementary study to analogical (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11] single crystal sets. This study completes the research on the effects 2
of substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals. The phase composition, luminescence and scintillation characteristics, as well as thermodynamical stability of the garnet phase, are studied in a function of Ga/Al ratio and they are analogical to previous works [10,11]. The unbalanced substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals leads to the formation of a multiphase system containing the mixture of the garnet phase and perovskite/α-Al2O3 (GdAlO3/α-Al2O3) eutectic phase. For simplicity, the perovskite/α-Al2O3 eutectic phase is called the secondary phase. The multiphase crystals show well-isolated luminescence properties in the UV (due to secondary phase) and visible (due to garnet phase) spectral ranges, thus, we separated them by absorption, photoluminescence excitation (PLE) and emission (PL) and radioluminescence (RL) spectra and, decay kinetic measurements. The mechanism responsible for the Ce3+ luminescence quenching in the target garnet phase was revealed by the measurements of the temperature dependence of photoluminescence decays in the prompt and delayed components. The scintillation decay times under
137
Cs γ-ray
excitation were measured in order to investigate the influence of Ga content on scintillation response in the studied crystals. All physical and luminescence properties of Gd3(Ga,Al)5O12:Ce single crystals are analogical to (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11], hence, the reader is cross-referred to Ref. 10 and 11 to better understand this topic.
2. Materials and experiment
The Ce doped Gd3GaxAl5-xO12 (x=1.5, 1.75, 2, 2.25, 2.5) single crystals were grown from the melt by the micro-pulling-down technique with the radiofrequency inductive heating [17]. The Ce content in the melt was 0.5 atomic percent (at %). An Ir crucible with a die of 2.5 mm in diameter was applied. Excess of 1 mass % of Ga2O3 was added to compensate for Ga evaporation. The Ar with 2% of O2 atmosphere was used as a growth atmosphere. The morphology of the surface of the single crystals was studied by scanning electron microscopy (SEM) using Hitachi S-3400N equipped with wavelength-dispersive X-ray spectroscopy (WDS) detector Oxford Instruments INCA WAVE500. The crystals were coated with a thin graphite layer to facilitate conductivity. The phase of the grown crystal was checked by powder X-ray diffraction analysis (XRD) with D8 DISCOVER (Bruker) diffractometer (CuKα line, 40 kV, 15 mA). The piece of crystal was grinded into powder in a mortar and its XRD pattern in the 2θ range of 20o - 60o was measured. Absorption spectra between 200 and 3
800 nm were recorded by the Shimadzu 3101PC spectrometer. The photoluminescence (PL) and radioluminescence (RL) measurements were performed with the custom-made 5000M Horiba Jobin Yvon spectrofluorometer. A deuterium steady stay-lamp (Heraus Gmbh) and Xray tube (40 kV, 15 mA) with W anode (Seifert Gmbh) were used as the excitation source in PL and RL measurements, respectively. Recorded spectra were corrected for the experimental distortions of the setup. The pulsed nanoLED, nanosecond hydrogen flash lamp or microsecond xenon flash lamp was used for fast and slow PL decay kinetic measurements, respectively. The fast decay kinetics was recorded using time-correlated single-photon counting method with Janis liquid nitrogen cryostat. Exponential fits of the decay times were obtained using the convolution of the considered function with the instrumental response and the least-square-sum-based fitting program (Spectra Solve software package from Ames Photonics). The delayed recombination (DR) light intensity was recorded with regular equipment for photoluminescence measurements. The sample was excited in the 4f→5d1 absorption band (455 nm) of Ce3+ in the garnet phase by xenon flash lamp (pulse width around 3-4 µs and flash rate of approximately 102 Hz). The emission monochromator was calibrated to the maximum of the 5d1→4f emission of Ce3+, 550 nm. The signal was recorded using a multichannel scaling method scanning the decay for approximately 20 ms. The system was programmed in such way that every excitation pulse (and corresponding measurement window opening) occurred immediately after the previous one, consequently, the monitored intervals between successive time window were due to dead time of the electronic (less than 100 µs). The accumulation time for each measurement was 5 minutes. The measurement was performed between 10 and 500 K starting from the highest temperature with a Janis Instruments closed-cycle refrigerator. The integration of the recorded decay curve, where the background is subtracted and the very first few channels that could contain prompt decay component are excluded, provides the overall intensity of delayed recombination process of an emission center [18]. The scintillation decay curves were obtained using digital oscilloscope TDS3052 under excitation by 622 keV photons from 137
Cs radioisotope.
4
3. Experimental results and discussion 3.1. Morphology of the surface of the crystals by SEM analysis
The as-grown Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce crystal rods are shown in Fig. 1. The crystals were prepared under the same conditions. The samples seem to be slightly opaque, but the opacity is just observed on the surface layer due to gallium oxide evaporation and/or thermal etching. The surface of Gd3Ga1.5Al3.5O12:Ce rod is more opaque than Gd3Ga2.5Al2.5O12:Ce one due to quite significant Ce segregation and incongruent melting. The inner part of the crystals was transparent. Interestingly, the Gd3Ga1.5Al3.5O12:Ce rod shows the irregular profile and at the end narrowed to a conical shape. Such profile is caused by incongruent melting of the Gd3Ga1.5Al3.5O12:Ce composition and formation of different phases during the crystal growth process. On the contrary, congruently melting Gd3Ga2.5Al2.5O12:Ce composition has a regular shape in a whole crystal volume due to formation only single garnet phase. Moreover, the Gd3GaxAl5-xO12:Ce crystal rods with Ga content between x=1.75 and x=2 also show irregular profile with, opaque surface and conical end. However, the crystal rods with higher Ga content show the more regular shape and less opaque surface due to shifting the phase equilibrium for the formation of the garnet phase and improved segregation of the Ce atoms within the cross-sectional direction. Interestingly, the Gd3Ga2.25Al2.75O12:Ce composition has a regular shape within the whole volume with the slightly opaque surface. In spite of the regular shape, this composition contains secondary phase inclusions, more details further in the text.
Fig. 2 shows the SEM micrographs of Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce single crystals. Both crystals show characteristic black spots randomly distribute on the surface. The WDS elemental analysis reveals that black spots are not associated with different impurities or phases, but they belong to the garnet phase in both crystals. Those black spots are caused by the micro-cracks and bubbles which arose during the growth process and reflect the quality of the crystal. The higher content of black spots reflects the lower quality of the
5
crystal, consistently with Fig. 1. The high magnification of the side part of Gd3Ga1.5Al3.5O12:Ce crystal (Fig. 2 a) shows uniform color suggesting the lack of the impurities and secondary phases. The WDS elemental analysis confirms the presence of a pure garnet phase, see Table 1. Interestingly, the middle part of the crystal (Fig. 2 b) contains randomly distributed brighter spots. The WDS analysis of those spots reveals that they are associated with the perovskite phase significantly enriched with Al atoms. This result points to the eutectic character of this phase i.e. GdAlO3/α-Al2O3 as it is consistent with the phase diagram [12]. The distribution of the heat within the crystal volume is not homogenous. Hence, the inner part of the crystal has a significantly higher temperature than the outer one, as a result, the perovskite and α-Al2O3 phases crystallized in the inner part of the crystal as they need a higher temperature to be formed [12]. Interestingly, high magnification and WDS elemental analysis of the side and middle parts of Gd3Ga2.5Al2.5O12:Ce crystal (Fig. 2 c-d and Tab. 1) reveal the presence of the only garnet phase. Moreover, the WDS elemental analysis for the Gd3GaxAl5-xO12:Ce compositions with Ga content between x=1.75 and 2.25 also detected the GdAlO3/α-Al2O3 eutectic phase inclusions in the inner part of the cross-sectional plane of the crystals. However, increasing Ga content significantly reduces the amount of the secondary phase. Interestingly, in the Gd3Ga2.25Al2.75O12:Ce composition, the WDS elemental analysis detected only garnet phase. However, the PL measurements reveal that Gd3Ga2.25Al2.75O12:Ce composition contains secondary phase inclusion (see subsection 3.3. Photoluminescence characterization). Such disagreement between WDS elemental analysis and PL measurements is due to the difference in the detection sensitivity. PL methods are more sensitive than WDS one, consequently, there are able to detect the content of the impurities up to a few parts-per-trillion. Interestingly, the Ce content significantly varies between Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce in the cross-sectional plane of the crystals, see Tab. 1. The outer part of the crystal has a higher content of Ce than the inner one. Moreover, the increasing Ga content causes more homogenous Ce distribution within the crystal volume. This variation is caused by the segregation coefficient (keff) of Ce atoms in aluminum garnets [19-22]. It is worth to mention that the dopant distribution in the grown single crystal is affected by the dopant segregation, and large segregation is observed especially in cross-sectional (perpendicular) directions to the growth direction in single crystals grown by the µ-PD method [21,22]. Therefore, it is difficult to grow a single crystal with a highly homogenous dopant distribution in the sample with a segregation coefficient far from one by the µ-PD method. The segregation coefficient for Ce atoms in aluminum garnets depends on the chemical composition and it varies between keff=~0.05 for Y3Al5O12 and 6
keff=~0.3 for Gd3Ga3Al2O12 (GAGG). Consequently, GAGG:Ce single crystal with a larger segregation coefficient shows more uniform Ce distribution in the cross-sectional plane [19]. In the Gd3GaxAl5-xO12:Ce compositions with Ga content between x=1.75 and 2.25 the Ce content is higher in the outer part of the cross-sectional plane than in the inner one. The approximate quantity of the perovskite/α-Al2O3 euctetic phase in the Gd3Ga1.5Al3.5O12:Ce, Gd3Ga1.75Al3.25O12:Ce and Gd3Ga2Al3O12:Ce crystals is ~9, 4 and 2 %, respectively. However, for the Gd3Ga2.25Al2.75O12:Ce composition the measured signal was too weak for appropriate calculation of the perovskite/α-Al2O3 eutectic phase.
3.2. XRD, radioluminescence and absorption spectra characterization
The X-ray powder diffraction, RL and absorption spectra belong to characterization methods with relatively poor sensitivity, hence, those measurements are not appropriate to detect all types of phases and luminescence centers in the studied samples. The PL methods are very sensitive (detection sensitivity is parts-per-trillion), hence, they are applied for detailed analysis of the luminescence properties in the studied crystals, especially secondary phase [10,11], see subsection 3.3 (Photoluminescence characterization). Thus, PL experimental results are not completely consistent with WDS, XRD, radioluminescence, and absorption ones.
Fig. 3 a shows the X-ray powder diffraction analysis for the Gd3GaxAl5-xO12:Ce (x=1.5 and 2.5) crystals. The XRD analysis reveals that compositions of Gd3GaxAl5-xO12:Ce with Ga content between x=1.75 and 2.5 contain pure garnet phase. In the sample with the lowest Ga (x=1.5) content, both target garnet phase and unwanted perovskite phase are detected. Interestingly, the α-Al2O3 phase is not detected by XRD analysis. This could be due to too low content of the α-Al2O3 phase, which is below the detection limit of XRD technique. The α-Al2O3 phase can be formed in the low content due to the eutectic character of the melted composition. However, the WDS elemental analysis reveals that the secondary phase has a GdAlO3/α-Al2O3 eutectic character, see Table 1. The very sensitive PL experimental methods also suggest the presence of the α-Al2O3 phase in some compositions of the studied crystals, see subsection 3.3 (Photoluminescence characterization). Figure 3 b shows the XRD spectra for the beginning, middle and end parts of the Gd3Ga1.5Al3.5O12:Ce crystal. Interestingly, the 7
content of the perovskite phase changes significantly between the beginning and the end parts of the crystal. The beginning part of the crystal has the highest content, the middle one shows reduced content of the perovskite phase, whilst the end part exhibits a pure garnet phase. The XRD and WDS results show that low content of Ga in Al2O3-Ga2O3-Gd2O3-CeO2 system makes the target Gd3GaxAl5-xO12:Ce composition (garnet phase) thermodynamically unstable at melting temperature (incongruent melting). According to the phase diagram for Al2O3Gd2O3 system [12], the perovskite and α-Al2O3 phase inclusions are created due to incongruent melting. The Al2O3-Gd2O3 system tends to form the perovskite (GdAlO3) and αAl2O3 phases instead of garnet (Gd3Al5O12) one from the melt [15]. Based on the WDS elemental analysis, XRD and phase diagram we assume that the crystal growth process for incongruently melting compositions runs is as follows: At the first stage of the crystallization of melted Al2O3-Ga2O3-Gd2O3-CeO2 system (Al-rich system), the perovskite/α-Al2O3 eutectic phase (GdAlO3/α-Al2O3) is formed. This event shifts the melt composition towards Ga-rich (low content of Al in the local melt composition) one which prefers to form target (Gd3GaxAl5-xO12) garnet phase composition. When the local melt composition become again Al-rich, the GdAlO3/α-Al2O3 eutectic phase crystallizes and shifts the melt composition towards the Ga-rich one and again the target garnet phase crystallizes. Such a process can be repeated many times during the crystal growth. Consequently, the GdAlO3/α-Al2O3 eutectic phase is randomly incorporated within the target garnet phase (so-called stipe structure). This mechanism can be similar to the creation of the LuAl antisite defects in Lu3Al5O12 single crystal from the Lu-rich melt composition of Lu3Al5O12, whilst Al-rich melt composition of Lu3Al5O12 diminishes the content of the LuAl antisite defects [17]. The WDS and XRD results are supported by the radioluminescence spectra presented in Fig. 4. Namely, only the Gd3Ga1.5Al3.5O12:Ce composition shows luminescence at 360 nm mainly due to 5d→4f emission transition of Ce3+ ions in the perovskite phase ([10,11] and reference therein, [23-29]). However, the UV emission can be also contributed by F+ and Ce3+ emission centers in the α-Al2O3 phase as well as by perturbed Ce3+ ions located between garnet and GdAlO3/α-Al2O3 eutectic phases, this phenomenon is explained in details under PL analysis, see subsection 3.3 (Photoluminescence characterization). The crystals with Ga content greater than x>1.5 show only Ce3+ luminescence in the garnet phase located at 550 nm [23,30]. The emission lines centered at 275 and 313 nm belong to 6I7/2 → 8S7/2 and 6P7/2 → 8S7/2 emission transitions of Gd3+, respectively [11]. The low intensity of the Gd3+ related luminescence on the radioluminescence spectra is caused by the very efficient energy migration to Gd3+ and Ce3+ ions preventing the energy transfer from host to the defects and/or unwanted impurities 8
centers in the garnet lattice [31]. Sharp luminescence at 380 and 415 nm is due to 5D3 → 7F6 and 5D3 → 7F5 emission transitions belonging to the trace impurity of Tb3+ ions [32]. Fig. 5 displays RT absorption spectra of the Gd3GaxAl5-xO12:Ce (x=1.5, 2, 2,25, 2.5) single crystals. All spectra show two broad absorption bands located at ~445 and ~340 nm, which are ascribed to the allowed 4f→5d1 and 4f→5d2 transition of the Ce3+, respectively, in the garnet phase [19,20]. Interestingly, the absorption coefficient for the Ce3+ absorption transitions significantly varies among samples due to the different content of Ce among samples. This variation is caused by the segregation coefficient (keff) of Ce atoms in aluminum garnets [1922]. The absorption spectra are consistent with WDS elemental analysis, see Tab.1. The partial substitution Ga for Al changes the crystal field strength and consequently shifts the 4f→5d1 and 4f→5d2 absorption bands to higher and lower energies, respectively [30,33]. The absorption lines at 275 and 310 nm belong to the Gd3+ ions [31]. The high amplitude below 270 nm is caused by both various color and impurity related centers [32-34] and 4f→5d3,4,5 absorption transitions of Ce3+ ions in the garnet phase [34,35]. In the Gd3Ga1.5Al3.5O12:Ce single crystal, the amplitudes of 4f→5d1 and 4f→5d2 absorption bands at 445 and 340 nm, respectively, are somehow saturated due to reaching the experimental limit of the spectrometer. This observation shows that sample with the lowest Ga content (x=1.5) has very low quality due to the presence of several phases, many defects and perturbed Ce3+ ions within the crystal volume. Consequently, the Ce3+ ions are very unevenly distributed in parallel and cross-sectional directions to the growth direction of the crystal (poor segregation of Ce atoms during the crystal growth). Part of the crystal under the study has a higher than in the melt concentration of Ce causing the observed saturation, see Tab. 1. Moreover, saturation can be caused by the parasitic contribution of the Ce3+ and defects luminescence to the signal beam.
3.3. Photoluminescence characterization
On the contrary to RL spectra, due to high sensitivity, the PL measurements reveal UV emission centers in the following Gd3GaxAl5-xO12:Ce (x=1.5, 1.75, 2 and 2.25) crystal compositions, see Fig. 6 a-b. Interestingly, the Gd3Ga2.5Al2.5O12:Ce composition does not show emission and excitation band in the UV spectral range. This observation shows that substitution Ga for Al at least in a 50% in Gd3GaxAl5-xO12:Ce composition enables congruent 9
melting and crystallization in a pure garnet phase [15]. The RT normalized excitation spectra for emission at ~360 nm consists of a broad-bands between 200 and 320 nm (Fig. 6a), whilst, emission spectra excited at 290 nm show broad-bands between 300 and 420 nm (Fig. 6 b). The shape of the excitation and emission bands is irregular and change with increasing Ga content. However, the position and profiles of both excitation and emission bands indicate they mainly belong to the GdAlO3/α-Al2O3 eutectic phase inclusions supporting the WDS, XRD and radioluminescence results [23-29]. By analogy to (Gd,Lu)3Al5O12:Ce ([11] and references therein) crystal set and WDS elemental analysis, we can conclude that the luminescence in the UV spectral range can be caused by the overlapping of the Ce3+ emission in the perovskite phase, F+ centers in the garnet and/or perovskite and/or α-Al2O3 phases as well as Ce3+ emission centers in the α-Al2O3 phase ([11] and references therein, [36]). Moreover, it is worth to stress, that the multiphase system can create perturbed Ce3+ centers in the perovskite phase, which may influence the shape of the excitation and emission bands in the UV spectral region as well [37]. However, overlapping of the emission coming from different phases (REAlO3:Ce, α-Al2O3, α-Al2O3:Ce) hamper studying the individual components in the absolute UV emission. The PL excitation and emission spectra do not show systematic dependence due to different Ce segregation coefficient caused by changing the crystal compositions, incongruent melting and crystallization of various phases at different stages of the crystal growth process. Consequently, within the crystal, the chemical composition is strongly heterogeneous (as explained above). Such strong variation of the chemical compositions within the crystal volume disturbs systematic dependence especially in studying the secondary phase as it is randomly distributed within the crystal (i.e. part of the crystal under the study). Moreover, the luminescence centers related to the secondary phase can be located on the borders or interface between phases. Consequently, the local surrounding around the sensitive luminescence center is perturbed. Such perturbation varies among the crystals causing irregular profiles of the excitation and emission bands and thus no systematic dependence can be obtained. The decay kinetics analysis gives deeper insight into luminescence processes in the study multiphase systems. Namely, Fig. 7 shows the decay curve for luminescence centered at 360 nm in Gd3Ga1.5Al3.5O12:Ce under excitation at 290 nm. The decay kinetics is strongly nonexponential. For this reason, the double-exponential fit is used for the qualitative description of the luminescence timing properties, see Eq. 1. = ∑ −/ + , = 1 3, 10
(1)
the parameters Ii and stand for the pre-exponential factor and decay time, respectively, in the ith decay component. The double-exponential approximation with the accelerated fast τfast~5
ns and slower τslow~123 ns components can indicate the presence of different emitting
centers as well as complex energy transfer processes. Such behavior is very similar to the decay kinetics of Ce3+ luminescence in the (Lu,Y)1-xGdxAlO3:Ce single crystal perovskites, where Ce decay time exhibits non-exponential profile with the fast accelerated and significantly slower components [37]. Hence, the double-exponential character of the decay curve can be connected to the energy transfer from emission center in UV spectral range (mainly Ce3+ ions in the perovskite phase) to Gd sublattice (accelerated component, τfast~5 ns) followed by energy migration over Gd sublattice and subsequent trapping at perturbed Ce3+ centers (slower component, τslow~123 ns). Such an energy transfer mechanism has been reported in (Lu,Y)1-xGdxAlO3:Ce single crystal perovskite [32,36] and in multiphase systems containing aluminum garnet and aluminum perovskite phases [10,11]. However, the faster component of the decay times might be also contributed by the luminescence of F+ center in perovskite and/or garnet and/or α-Al2O3 phases [38], since their energy levels are located in close proximity to 5d1 energy level of Ce3+ in the perovskite phase, see Fig. 6. The decay curve analysis supports the hypothesis that UV luminescence can be caused by both Ce3+ ions in the perovskite phase and F+ center in perovskite and/or garnet and/or α-Al2O3 phases as it is consistent with the PL emission and excitation spectra (i.e. crystals under the study contain a mixture of the garnet phase and GdAlO3/α-Al2O3 eutectic phase).
Fig. 8 shows the excitation spectra for Ce3+ emission at 550 nm (garnet phase) and emission spectra under excitation at 290 nm (secondary phase) for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce single crystals. The dominant bands centered around 360 nm in the emission spectra belong to the secondary phase (mainly to Ce3+ emission in the perovskite phase) [10,11,23-29]. The broad bands centered around 345 nm and 450 nm in the excitation spectra belong to 4f→52 and 4f→5d1 excitation/absorption transitions of Ce3+ in the garnet phase, respectively. Interestingly, the UV emission bands contain a dip centered at 343 nm and 346 nm for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce, respectively. Moreover, the PL excitation and emission spectra for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce crystals show the dependence of the position of the dip in UV emission band on Ga content. This observation shows that the shift of the 4f→5d2 excitation transition of Ce3+ in the garnet phase is accompanied by the identical shift of the dip in the UV emission band. This 11
systematic dependence and overlapping of the dip with the 4f→5d2 excitation transition of Ce3+ in the garnet phase can suggest both radiative and non-radiative energy transfer processes, which have been reported in the analogous multiphase systems (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11]. Another evidence of energy transfer between UV and visible emission centers is provided by the decay kinetics of the Ce3+ luminescence centered at 550 nm in the garnet phase under excitation into absorption band in secondary phase at 290 nm, see Fig. 9. The decay curve shows three-exponential profile with the fast (τ1=70 ns) and slow (τ2=290 ns) and (τ3=1.77 µs) components. The three components of the decay time with the three significantly different decay constants might suggest that the energy transfer occurs partly by the radiative and partly by the non-radiative process via different channels including $% immediate $% sublattice )
! "# ! "#
$% → '
( "#
energy transfer and involving Gd3+
$% $% → *+,-. (( → '
( "# /
(in the perovskites the
Gd3+ ions typically excite the Ce3+ luminescence, see Ref. 39 and 40). Such complex energy transfer mechanism has been reported and discussed in detail in the similar multiphase crystals containing aluminum perovskite and aluminum garnet phases mixture, see Ref. 10 and 11. It should be stressed that another luminescence centers like F+ center in perovskite and/or garnet and/or α-Al2O3 also might contribute to the overall energy transfer process since their energy levels are located in close proximity to the 5d2 energy level of Ce3+ in the garnet phase, see Fig. 6a. 3.4. Temperature dependence characterization
The temperature dependence of the prompt and delayed radiative recombination processes were monitored to reveal the mechanism responsible for the quenching of Ce3+ emission centers in the garnet phase. Fig. 10 a-d shows examples of the prompt PL decays for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce samples recorded at 100 and 500 K. The decay curves in all set of samples exhibit single-exponential profile up to around 400 K, above this temperature decays become non-exponential, see Figs. 10 b and 10 d. Such behavior could be due to the different gap values of Ce3+ excited state and the levels of the conduction band at different Ga content and temperatures. Nevertheless, this aspect needs further study. The qualitative influence of Ga content on the Ce3+ thermal stability is visible on the decays recorded at high temperature. The decay time of the sample with the highest content of Ga (Gd3Ga2.5Al2.5O12:Ce) shows relatively slow component τ2=241 ns, whilst in the sample with 12
the low content of Ga (Gd3Ga1.5Al3.5O12:Ce), the slow component is not observed. This can suggest that in the Ga rich sample, the luminescence quenching process is assisted by the thermal ionization of the 5d1 excited state of Ce3+ center towards the conduction band [41]. Fig. 11 presents the temperature dependence of nanosecond decays for all samples. At lower and higher temperatures, the decay constants were obtained from the single and double exponential approximations, respectively, according to Equation 1 (i=1or 2), hence, instead of decay time τ, the mean time τm is used defined by Equation [10]. Analogously to other multicomponent garnets [13,30,41], increasing Ga content reduces the thermal stability of the Ce3+ luminescence centers. Consequently, the onset of luminescence quenching is shifted to a lower temperature. However, change in thermal stability of the Ce3+ luminescence is minor because the admixture of Ga in a low range only slightly affects the change of crystal field strength and bottom of the conduction band [13]. Moreover, due to the heterogeneous character of the crystals, the local chemical composition of the crystal under the study might be similar, as it is discussed above. The delayed recombination decay (DR) technique consists in the recording of the slow component in the photoluminescence decay under UV/visible excitation. When the luminescent ion is thermally ionized, charge carrier that does not decay promptly can migrate over the conduction or valence bands. After being trapped and thermally released again they return to the luminescent ion and radiatively recombine emitting delayed (at millisecond scale) light. The delayed radiative recombination light measurement is an excellent tool to study the thermally induced ionization process of the 5d1 excited state of Ce3+ center in Gd3GaxAl5-xO12:Ce crystals. This experimental method is complementary to the temperature dependence of the prompt decay technique to study the luminescence quenching mechanism. The delayed radiative recombination light measurement provides information about the thermally induced photoionization process, which influences the quenching of Ce3+ luminescence in Gd3GaxAl5-xO12:Ce crystals. It is worth to mention that quantum tunneling process also produces delayed (millisecond scale) light. However, the intensity of the delayed light produced in the quantum tunneling process is significantly lower than in the ionization one [42]. Fig. 12 shows the delayed recombination integral intensities of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals recorded under excitation into Ce3+ 4f→5d1 absorption transition at 455 nm and monitoring the Ce3+ 5d1→4f luminesce at 550 nm at temperature range 10-500 K. Interestingly, for samples with Ga content between x=1.5 and 2.25 (multiphase system 13
samples) there is a slight decrease of the intensity in the temperature range 200-500 K. This drop in the intensity could be due to decreasing amplitude of absorption in 5d1 band in favor of increasing amplitude of absorption into 5d2 with increasing temperature [43]. Alternatively, at the higher temperature, the luminescent ion can get out of resonance with some of the previously participating traps. Consequently, the total number of traps contributing to the overall delayed recombination signal would drop. In any case, there is no indication of thermal ionization of the Ce3+ excited state. For the sample with Ga content x=2.5 (pure garnet phase sample), there is a slight increase of the intensity in the temperature interval 200-400 K. Such intensity increase can indicate the thermally induced ionization of the 5d1 excited state, as it is consistent with the result presented in Fig. 10 d above. Moreover, the variety of DR decays in the study crystals confirm the plausibility of the quantum tunneling hypothesis [44]. The temperature dependence analysis shows that the increase of Ga content slightly reduces the thermal stability of the Ce3+ ions in the study Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals. The temperature dependence results show that the quenching of Ce3+ luminescence in Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals is caused by thermal relaxation from the 5d1 potential curve (parabola) to the 4f ground potential curve through the crossing point based on the configurational coordinate model [45]. However, in the Ga rich sample, the quenching mechanism could be also assisted by thermally induced ionization of the 5d1 excited state as the PL decay curve recorded at a higher temperature for Ga rich sample shows [45].
3.5. Scintillation decay times Scintillation decays recorded under γ-ray (662 keV,
137
Cs radioisotope) excitation for
Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce crystals are shown in Fig. 13 a-b. The decays show multi-exponential character and are fitted with Eq. (1), where i=2. The exponential components with the decay times of 16 and 10 ns for x=1.5 and 2.5, respectively, have negative pre-exponential factors and indicate the rise time. The decay time constants of 130 and 100 ns for x=1.5 and 2.5, respectively, are due to prompt radiative electron-hole recombination at the Ce3+ emission center. It is worth to notice that scintillation decay constants are longer that PL ones, which points to a slight slow-down in the prompt decay component undoubtedly due to the transport processes. Table 2 shows the scintillation decay
14
time components for all samples. Interestingly, the rise time and decay time constants significantly accelerate in the sample with increasing Ga content, confirming that Ga admixture reduces the electron trap activity in the scintillation process by burying them in the conduction band as well as improving the quality of the crystals (i.e. diminish content of the unwanted secondary phase) [8,9]. However, Gd3Ga2Al3O12:Ce crystal shows different properties from the rest of the set of crystals, namely it shows the longest rise time (τrise=22 ns) and fastest decay time (τrise=109 ns) constants. Such a feature could be related to poor crystal quality, which possesses higher content of charge traps than the other ones. Consequently, higher content of charge traps slow down the migration of electrons and holes towards the luminescence center as well as it can quench the Ce3+ luminescence causing lengthening in the rise time and acceleration in the decay time, respectively.
4. Conclusions In this research, the absorption, radioluminescence, photoluminescence, and scintillation decay times characteristics of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals are studied upon partial substitution Ga for Al. The obtained results are analogous to (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. The low content of Ga fraction in Gd3(Ga,Al)5O12:Ce single crystals results in the formation of a multiphase system consisting of target garnet and secondary phases. The secondary phase was detected in Gd3GaxAl5-xO12:Ce (x=1.5-2.25) crystals by WDS, XRD, radioluminescence and PL study. Only Gd3Ga2.5Al2.5O12:Ce composition showed a pure garnet phase. Increasing Ga concentration caused slight blue-shift of Ce3+ emission in the garnet phase due to the reduction of the crystal field strength. The Gd3+→Ce3+ non-radiative energy transfer in the garnet phase and Ce3+↔Gd3+ non-radiative energy transfer in the secondary phase were confirmed. Moreover, the energy transfer from secondary to garnet phase was revealed as well. The increasing Ga content slightly decreased the thermal stability of the Ce3+ luminescence. Temperature dependence of the prompt and delayed recombination decay of Ce3+ emission revealed that luminescence quenching in Gd3(Ga,Al)5O12:Ce is mainly caused by the thermally induced cross-over from the excited 5d1 state parabola to the ground state 4f parabola and can be slightly assisted by the thermally induced ionization process in the samples with higher Ga content. The high intensity of the delayed recombination integrals even at low temperatures confirm an intricate interaction between 5d1 excited state of Ce3+ and its host lattice neighborhood. The Ga admixture
15
significantly improved scintillation response in the studied crystals by reduction of the rise time and slow components.
Acknowledgment The
bilateral
ASCR-JSPS
project
and
project
No.
SOLID21
CZ.02.1.01/0.0/0.0/16_019/0000760 from Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports and 16H05986 Grant-in-Aid for Young Scientists (A), Japan Society for the Promotion of Science (JSPS) and 16H04505 Grant-in-Aid for Scientific Research(B), and Polish NCN grant No 2017/25/B/ST8/02932, are gratefully acknowledged.
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crystal composition the in melt
elements at% Gd (Lα xray photon GdF3)
Al (Kα x-ray photon Al2O3)
18
Ga (Lα x-ray photon GaP)
Ce (Lα x-ray photon CeAl2)
detected phase
picture a (Gd3Ga1.5Al3.5O12:Ce)
picture b (Gd3Ga1.5Al3.5O12:Ce)
picture c (Gd3Ga2.5Al2.5O12:Ce)
picture d (Gd3Ga2.5Al2.5O12:Ce)
28.74
54.85
15.62
0.79
garnet
46.89
52.45
0.46
0.20
perovskite/αAl2O3 (eutectic)
37.05
34.57
27.9
0.48
garnet
39.13
31.46
29.04
0.37
garnet
Tab. 1. WDS elemental analysis of Gd3Ga1.5Al3.5O12:Ce (a,b) and Gd3Ga2.5Al2.5O12:Ce (c,d) single crystals. GdF3, α-Al2O3, GaP and CeAl2 are references used in WDS elemental analysis.
crystal composition
single exponential rise time [ns]
single exponential decay time [ns]
Gd3Ga1.5Al3.5O12:Ce
τrise=16 ns
τdecay= 130 ns
Gd3Ga1.75Al3.25O12:Ce
τrise=12 ns
τdecay= 120 ns
Gd3Ga2Al3O12:Ce
τrise=22 ns
τdecay=109 ns
Gd3Ga2.25Al1.75O12:Ce
τrise= 9 ns
τdecay=116 ns
Gd3Ga2.5Al2.5O12:Ce
τrise=10 ns
τdecay=100 ns
Table 2. Scintillation decay time components of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals
Fig. 1. The as-grown crystal rods of (a) Gd3Ga1.5Al3.5O12:Ce and (b) Gd3Ga2.5Al2.5O12:Ce. Insets show cross-sectional polished plates cut from the middle part of the crystal rods.
19
Gd3Ga2.5Al2.5O12:Ce
Gd3Ga1.5Al3.5O12:Ce
d b c
a
a
c
b
d
perovskite/α-Al2O3
garnet
Fig. 2. SEM micrographs of Gd3Ga1.5Al3.5O12:Ce (a,b) and Gd3Ga2.5Al2.5O12:Ce (c,d) single crystals
20
Fig. 3. XRD spectra for (a) Gd3GaxAl5-xO12:Ce (x=1.5 and 2.5) and (b) beginning, middle and end parts of the Gd3Ga1.5Al3.5O12:Ce crystals. 21
Fig. 4. Room temperature radioluminescence spectra for Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals (X-ray excitation 40 kV, 15 mA). The luminescence centered at 360 nm in the sample with the lowest Ga content belongs to secondary phase inclusions.
Fig. 5. Absorption spectra of the Ce3+ activated Gd3GaxAl5-xO12 (x=1.5, 2, 2.25 and 2.5) crystals. 22
Fig. 6. (a) Normalized RT PLE spectra for the emission at 360 nm; (b) Normalized RT PL spectra under excitation at 290 nm in Gd3GaxAl5-xO12:Ce crystals.
23
Fig. 7. RT PL decay kinetics monitored at 360 nm under excitation at 290 nm in Gd3Ga1.5Al3.5O12:Ce.
Fig. 8. RT excitation spectra for the Ce3+ luminescence centered at 550 nm (garnet phase) and emission spectra excited at 290 nm in Gd3Ga1.5Al3.5O12 and Gd3Ga2Al3O12 single crystals
24
belonging to the secondary phase. The dips around 340 nm indicate radiative energy transfer between secondary and garnet phases.
Fig. 9. Decay kinetics of the Ce3+ related luminescence (550 nm) measured at RT excited into absorption band of the secondary phase at 290 nm in the Gd3Ga1.5Al3.5O12:Ce.
25
26
Fig. 10. PL decay kinetics for Ce3+ luminescence at 550 nm under excitation at 455 nm in (ab) Gd3Ga1.5Al3.5O12:Ce and (c-d) Gd3Ga2.5Al2.5O12:Ce samples.
27
Fig.11. The temperature dependence of photoluminescence decay times for the Ce3+ emission at 550 nm under excitation into 4f→5d1 absorption transition at 455 nm in the temperature range 77-500 K.
Fig. 12. Delayed recombination decay intensities of Gd3GaxAl5-xO12:Ce (x=1.5-2.5) as a function of temperature (λex=455 nm, λem=550 nm).
28
Fig. 13. RT Scintillation decay times for (a) Gd3Ga1.5Al3.5O12:Ce and (b) Gd3Ga2.5Al2.5O12:Ce crystals under excitation with γ-rays (137Cs radioisotope, 662 keV).
29
Highlights Instability of the garnet phase and formation of the secondary phase inclusions discussed. Temperature dependence of the decay time of Ce3+ emissions in garnet phase studied. Energy transfer processes between secondary phase inclusion and target garnet phase discussed.
S0022-2313(19)30131-0
DOI:
https://doi.org/10.1016/j.jlumin.2019.116724
Reference:
LUMIN 116724
To appear in:
Journal of Luminescence
Received Date: 18 January 2019 Revised Date:
13 August 2019
Accepted Date: 29 August 2019
Please cite this article as: K. Bartosiewicz, V. Babin, K. Kamada, A. Yoshikawa, S. Kurosawa, A. Beitlerova, R. Kucerkova, M. Nikl, Y. Zorenko, Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116724. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ga for Al substitution effects on the garnet phase stability and luminescence properties of Gd3GaxAl5-xO12:Ce single crystals
K. Bartosiewicz1*, V. Babin2, K. Kamada1,4, A. Yoshikawa1,3,4, S. Kurosawa3, A. Beitlerova2, R. Kucerkova2, M. Nikl2, Yu. Zorenko5 1
Institute for Material Research, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai, Miyagi 980-8577, Japan 2 Institute of Physics, AS CR, Cukrovarnicka 10, Prague 16253, Czech Republic 3 New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, 980-8579, Japan 4 C&A corporation, T-Biz, 6-6-40 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan 5 Institute of Physics, Kazimierz Wielki University in Bydgoszcz, Powstańców Wielkopolskich 2, 85-090, Bydgoszcz, Poland * Corresponding author: [email protected]
Abstract
This research deals with the luminescence and physical properties of Gd3(Ga,Al)5O12:Ce single crystal set and it is a complementary study to analogous (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. Those three groups of materials show similar luminescence and physical properties. In this research, the influence of substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals on the luminescence and scintillation characteristics as well as thermodynamical stability of the garnet phase is studied, and it is compared to (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. The unbalanced substitution Ga for Al makes garnet phase thermodynamically unstable and leads to the formation of the multiphase system with luminescence centers in UV and visible spectral ranges. The wavelength-dispersive
X-ray
spectroscopy,
X-ray
powder
diffraction
analysis,
photoluminescence and decay kinetic measurements reviled all phase types within multiphase crystals. Temperature dependent measurements were applied to determine the quenching mechanism for Ce3+ luminescence. The timing characteristics are studied as the function of Ga content.
1
1.
Introduction
The aluminum garnets belong to the class of the oxide materials, which luminescence, scintillation, and physical properties can be easily tuned by modification of their chemical composition [1-3]. The band gap can be adjusted, the traps energy levels and their concentration can be finely tuned and their influence can be damped or on the contrary, enhanced by the specific doping for a luminescence optimization [1,4,5]. For instance, Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) activated with Ce3+ are very well know scintillators, however, with poor scintillating parameters like low light yield (LY) value and presence of the slow component in the scintillating decay time [6,7]. Composition engineering strategy based on the balanced substitution Gd for Y/Lu and Ga for Al significantly improves scintillation properties. Consequently, multicomponent garnet single crystals of general chemical formula (Y,Lu,Gd)3(Al,Ga)5O12:Ce show excellent scintillating parameters including extremely high LY and fast scintillation response [8,9]. On the other hand, unbalanced substitutions Gd for Y/Lu and Ga for Al in (Y/Lu)3(Al,Ga)5O12 single crystals lead to the formation of the multiphase systems ([10,11] and references therein). In our previous works, we have shown that unbalanced substitution Gd for Y in Y3Al5O12:Ce [10] and Gd for Lu in Lu3Al5O12:Ce [11] prevented to grow pure garnet phase single crystal. Consequently, those crystals consist of a mixture of garnet, perovskite and α-Al2O3 phases. Such multiphase crystals showed luminescence in the visible and UV spectral ranges. The visible luminescence was ascribed to Ce3+ ions in the garnet phase, whilst UV one to both Ce3+ ions in the perovskite and F+ centers and Ce3+ ions in α-Al2O3 phases. The PL measurements reviled very complex energy transfer processes between both phases as well as within the perovskite phase, for more details see Ref. 10 and 11. The phase diagram for the Al2O3-Gd2O3 system [12] shows that Gd3Al5O12 matrix melts incongruently. Consequently, during the crystallization of melted composition, the GdAlO3 and α-Al2O3 phases are formed instead of target Gd3Al5O12 one. This means that Gd3Al5O12 matrix cannot be obtained in the form of a single crystal from the melt. However, the Gd3Al5O12 matrix can be fabricated in the pure garnet phase using low-temperature synthesis methods (i.e. at temperatures below the melting point) such as liquid phase epitaxy, sol-gel combustion or solid-state sintering methods [13-16]). This work is a complementary study to analogical (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11] single crystal sets. This study completes the research on the effects 2
of substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals. The phase composition, luminescence and scintillation characteristics, as well as thermodynamical stability of the garnet phase, are studied in a function of Ga/Al ratio and they are analogical to previous works [10,11]. The unbalanced substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals leads to the formation of a multiphase system containing the mixture of the garnet phase and perovskite/α-Al2O3 (GdAlO3/α-Al2O3) eutectic phase. For simplicity, the perovskite/α-Al2O3 eutectic phase is called the secondary phase. The multiphase crystals show well-isolated luminescence properties in the UV (due to secondary phase) and visible (due to garnet phase) spectral ranges, thus, we separated them by absorption, photoluminescence excitation (PLE) and emission (PL) and radioluminescence (RL) spectra and, decay kinetic measurements. The mechanism responsible for the Ce3+ luminescence quenching in the target garnet phase was revealed by the measurements of the temperature dependence of photoluminescence decays in the prompt and delayed components. The scintillation decay times under
137
Cs γ-ray
excitation were measured in order to investigate the influence of Ga content on scintillation response in the studied crystals. All physical and luminescence properties of Gd3(Ga,Al)5O12:Ce single crystals are analogical to (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11], hence, the reader is cross-referred to Ref. 10 and 11 to better understand this topic.
2. Materials and experiment
The Ce doped Gd3GaxAl5-xO12 (x=1.5, 1.75, 2, 2.25, 2.5) single crystals were grown from the melt by the micro-pulling-down technique with the radiofrequency inductive heating [17]. The Ce content in the melt was 0.5 atomic percent (at %). An Ir crucible with a die of 2.5 mm in diameter was applied. Excess of 1 mass % of Ga2O3 was added to compensate for Ga evaporation. The Ar with 2% of O2 atmosphere was used as a growth atmosphere. The morphology of the surface of the single crystals was studied by scanning electron microscopy (SEM) using Hitachi S-3400N equipped with wavelength-dispersive X-ray spectroscopy (WDS) detector Oxford Instruments INCA WAVE500. The crystals were coated with a thin graphite layer to facilitate conductivity. The phase of the grown crystal was checked by powder X-ray diffraction analysis (XRD) with D8 DISCOVER (Bruker) diffractometer (CuKα line, 40 kV, 15 mA). The piece of crystal was grinded into powder in a mortar and its XRD pattern in the 2θ range of 20o - 60o was measured. Absorption spectra between 200 and 3
800 nm were recorded by the Shimadzu 3101PC spectrometer. The photoluminescence (PL) and radioluminescence (RL) measurements were performed with the custom-made 5000M Horiba Jobin Yvon spectrofluorometer. A deuterium steady stay-lamp (Heraus Gmbh) and Xray tube (40 kV, 15 mA) with W anode (Seifert Gmbh) were used as the excitation source in PL and RL measurements, respectively. Recorded spectra were corrected for the experimental distortions of the setup. The pulsed nanoLED, nanosecond hydrogen flash lamp or microsecond xenon flash lamp was used for fast and slow PL decay kinetic measurements, respectively. The fast decay kinetics was recorded using time-correlated single-photon counting method with Janis liquid nitrogen cryostat. Exponential fits of the decay times were obtained using the convolution of the considered function with the instrumental response and the least-square-sum-based fitting program (Spectra Solve software package from Ames Photonics). The delayed recombination (DR) light intensity was recorded with regular equipment for photoluminescence measurements. The sample was excited in the 4f→5d1 absorption band (455 nm) of Ce3+ in the garnet phase by xenon flash lamp (pulse width around 3-4 µs and flash rate of approximately 102 Hz). The emission monochromator was calibrated to the maximum of the 5d1→4f emission of Ce3+, 550 nm. The signal was recorded using a multichannel scaling method scanning the decay for approximately 20 ms. The system was programmed in such way that every excitation pulse (and corresponding measurement window opening) occurred immediately after the previous one, consequently, the monitored intervals between successive time window were due to dead time of the electronic (less than 100 µs). The accumulation time for each measurement was 5 minutes. The measurement was performed between 10 and 500 K starting from the highest temperature with a Janis Instruments closed-cycle refrigerator. The integration of the recorded decay curve, where the background is subtracted and the very first few channels that could contain prompt decay component are excluded, provides the overall intensity of delayed recombination process of an emission center [18]. The scintillation decay curves were obtained using digital oscilloscope TDS3052 under excitation by 622 keV photons from 137
Cs radioisotope.
4
3. Experimental results and discussion 3.1. Morphology of the surface of the crystals by SEM analysis
The as-grown Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce crystal rods are shown in Fig. 1. The crystals were prepared under the same conditions. The samples seem to be slightly opaque, but the opacity is just observed on the surface layer due to gallium oxide evaporation and/or thermal etching. The surface of Gd3Ga1.5Al3.5O12:Ce rod is more opaque than Gd3Ga2.5Al2.5O12:Ce one due to quite significant Ce segregation and incongruent melting. The inner part of the crystals was transparent. Interestingly, the Gd3Ga1.5Al3.5O12:Ce rod shows the irregular profile and at the end narrowed to a conical shape. Such profile is caused by incongruent melting of the Gd3Ga1.5Al3.5O12:Ce composition and formation of different phases during the crystal growth process. On the contrary, congruently melting Gd3Ga2.5Al2.5O12:Ce composition has a regular shape in a whole crystal volume due to formation only single garnet phase. Moreover, the Gd3GaxAl5-xO12:Ce crystal rods with Ga content between x=1.75 and x=2 also show irregular profile with, opaque surface and conical end. However, the crystal rods with higher Ga content show the more regular shape and less opaque surface due to shifting the phase equilibrium for the formation of the garnet phase and improved segregation of the Ce atoms within the cross-sectional direction. Interestingly, the Gd3Ga2.25Al2.75O12:Ce composition has a regular shape within the whole volume with the slightly opaque surface. In spite of the regular shape, this composition contains secondary phase inclusions, more details further in the text.
Fig. 2 shows the SEM micrographs of Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce single crystals. Both crystals show characteristic black spots randomly distribute on the surface. The WDS elemental analysis reveals that black spots are not associated with different impurities or phases, but they belong to the garnet phase in both crystals. Those black spots are caused by the micro-cracks and bubbles which arose during the growth process and reflect the quality of the crystal. The higher content of black spots reflects the lower quality of the
5
crystal, consistently with Fig. 1. The high magnification of the side part of Gd3Ga1.5Al3.5O12:Ce crystal (Fig. 2 a) shows uniform color suggesting the lack of the impurities and secondary phases. The WDS elemental analysis confirms the presence of a pure garnet phase, see Table 1. Interestingly, the middle part of the crystal (Fig. 2 b) contains randomly distributed brighter spots. The WDS analysis of those spots reveals that they are associated with the perovskite phase significantly enriched with Al atoms. This result points to the eutectic character of this phase i.e. GdAlO3/α-Al2O3 as it is consistent with the phase diagram [12]. The distribution of the heat within the crystal volume is not homogenous. Hence, the inner part of the crystal has a significantly higher temperature than the outer one, as a result, the perovskite and α-Al2O3 phases crystallized in the inner part of the crystal as they need a higher temperature to be formed [12]. Interestingly, high magnification and WDS elemental analysis of the side and middle parts of Gd3Ga2.5Al2.5O12:Ce crystal (Fig. 2 c-d and Tab. 1) reveal the presence of the only garnet phase. Moreover, the WDS elemental analysis for the Gd3GaxAl5-xO12:Ce compositions with Ga content between x=1.75 and 2.25 also detected the GdAlO3/α-Al2O3 eutectic phase inclusions in the inner part of the cross-sectional plane of the crystals. However, increasing Ga content significantly reduces the amount of the secondary phase. Interestingly, in the Gd3Ga2.25Al2.75O12:Ce composition, the WDS elemental analysis detected only garnet phase. However, the PL measurements reveal that Gd3Ga2.25Al2.75O12:Ce composition contains secondary phase inclusion (see subsection 3.3. Photoluminescence characterization). Such disagreement between WDS elemental analysis and PL measurements is due to the difference in the detection sensitivity. PL methods are more sensitive than WDS one, consequently, there are able to detect the content of the impurities up to a few parts-per-trillion. Interestingly, the Ce content significantly varies between Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce in the cross-sectional plane of the crystals, see Tab. 1. The outer part of the crystal has a higher content of Ce than the inner one. Moreover, the increasing Ga content causes more homogenous Ce distribution within the crystal volume. This variation is caused by the segregation coefficient (keff) of Ce atoms in aluminum garnets [19-22]. It is worth to mention that the dopant distribution in the grown single crystal is affected by the dopant segregation, and large segregation is observed especially in cross-sectional (perpendicular) directions to the growth direction in single crystals grown by the µ-PD method [21,22]. Therefore, it is difficult to grow a single crystal with a highly homogenous dopant distribution in the sample with a segregation coefficient far from one by the µ-PD method. The segregation coefficient for Ce atoms in aluminum garnets depends on the chemical composition and it varies between keff=~0.05 for Y3Al5O12 and 6
keff=~0.3 for Gd3Ga3Al2O12 (GAGG). Consequently, GAGG:Ce single crystal with a larger segregation coefficient shows more uniform Ce distribution in the cross-sectional plane [19]. In the Gd3GaxAl5-xO12:Ce compositions with Ga content between x=1.75 and 2.25 the Ce content is higher in the outer part of the cross-sectional plane than in the inner one. The approximate quantity of the perovskite/α-Al2O3 euctetic phase in the Gd3Ga1.5Al3.5O12:Ce, Gd3Ga1.75Al3.25O12:Ce and Gd3Ga2Al3O12:Ce crystals is ~9, 4 and 2 %, respectively. However, for the Gd3Ga2.25Al2.75O12:Ce composition the measured signal was too weak for appropriate calculation of the perovskite/α-Al2O3 eutectic phase.
3.2. XRD, radioluminescence and absorption spectra characterization
The X-ray powder diffraction, RL and absorption spectra belong to characterization methods with relatively poor sensitivity, hence, those measurements are not appropriate to detect all types of phases and luminescence centers in the studied samples. The PL methods are very sensitive (detection sensitivity is parts-per-trillion), hence, they are applied for detailed analysis of the luminescence properties in the studied crystals, especially secondary phase [10,11], see subsection 3.3 (Photoluminescence characterization). Thus, PL experimental results are not completely consistent with WDS, XRD, radioluminescence, and absorption ones.
Fig. 3 a shows the X-ray powder diffraction analysis for the Gd3GaxAl5-xO12:Ce (x=1.5 and 2.5) crystals. The XRD analysis reveals that compositions of Gd3GaxAl5-xO12:Ce with Ga content between x=1.75 and 2.5 contain pure garnet phase. In the sample with the lowest Ga (x=1.5) content, both target garnet phase and unwanted perovskite phase are detected. Interestingly, the α-Al2O3 phase is not detected by XRD analysis. This could be due to too low content of the α-Al2O3 phase, which is below the detection limit of XRD technique. The α-Al2O3 phase can be formed in the low content due to the eutectic character of the melted composition. However, the WDS elemental analysis reveals that the secondary phase has a GdAlO3/α-Al2O3 eutectic character, see Table 1. The very sensitive PL experimental methods also suggest the presence of the α-Al2O3 phase in some compositions of the studied crystals, see subsection 3.3 (Photoluminescence characterization). Figure 3 b shows the XRD spectra for the beginning, middle and end parts of the Gd3Ga1.5Al3.5O12:Ce crystal. Interestingly, the 7
content of the perovskite phase changes significantly between the beginning and the end parts of the crystal. The beginning part of the crystal has the highest content, the middle one shows reduced content of the perovskite phase, whilst the end part exhibits a pure garnet phase. The XRD and WDS results show that low content of Ga in Al2O3-Ga2O3-Gd2O3-CeO2 system makes the target Gd3GaxAl5-xO12:Ce composition (garnet phase) thermodynamically unstable at melting temperature (incongruent melting). According to the phase diagram for Al2O3Gd2O3 system [12], the perovskite and α-Al2O3 phase inclusions are created due to incongruent melting. The Al2O3-Gd2O3 system tends to form the perovskite (GdAlO3) and αAl2O3 phases instead of garnet (Gd3Al5O12) one from the melt [15]. Based on the WDS elemental analysis, XRD and phase diagram we assume that the crystal growth process for incongruently melting compositions runs is as follows: At the first stage of the crystallization of melted Al2O3-Ga2O3-Gd2O3-CeO2 system (Al-rich system), the perovskite/α-Al2O3 eutectic phase (GdAlO3/α-Al2O3) is formed. This event shifts the melt composition towards Ga-rich (low content of Al in the local melt composition) one which prefers to form target (Gd3GaxAl5-xO12) garnet phase composition. When the local melt composition become again Al-rich, the GdAlO3/α-Al2O3 eutectic phase crystallizes and shifts the melt composition towards the Ga-rich one and again the target garnet phase crystallizes. Such a process can be repeated many times during the crystal growth. Consequently, the GdAlO3/α-Al2O3 eutectic phase is randomly incorporated within the target garnet phase (so-called stipe structure). This mechanism can be similar to the creation of the LuAl antisite defects in Lu3Al5O12 single crystal from the Lu-rich melt composition of Lu3Al5O12, whilst Al-rich melt composition of Lu3Al5O12 diminishes the content of the LuAl antisite defects [17]. The WDS and XRD results are supported by the radioluminescence spectra presented in Fig. 4. Namely, only the Gd3Ga1.5Al3.5O12:Ce composition shows luminescence at 360 nm mainly due to 5d→4f emission transition of Ce3+ ions in the perovskite phase ([10,11] and reference therein, [23-29]). However, the UV emission can be also contributed by F+ and Ce3+ emission centers in the α-Al2O3 phase as well as by perturbed Ce3+ ions located between garnet and GdAlO3/α-Al2O3 eutectic phases, this phenomenon is explained in details under PL analysis, see subsection 3.3 (Photoluminescence characterization). The crystals with Ga content greater than x>1.5 show only Ce3+ luminescence in the garnet phase located at 550 nm [23,30]. The emission lines centered at 275 and 313 nm belong to 6I7/2 → 8S7/2 and 6P7/2 → 8S7/2 emission transitions of Gd3+, respectively [11]. The low intensity of the Gd3+ related luminescence on the radioluminescence spectra is caused by the very efficient energy migration to Gd3+ and Ce3+ ions preventing the energy transfer from host to the defects and/or unwanted impurities 8
centers in the garnet lattice [31]. Sharp luminescence at 380 and 415 nm is due to 5D3 → 7F6 and 5D3 → 7F5 emission transitions belonging to the trace impurity of Tb3+ ions [32]. Fig. 5 displays RT absorption spectra of the Gd3GaxAl5-xO12:Ce (x=1.5, 2, 2,25, 2.5) single crystals. All spectra show two broad absorption bands located at ~445 and ~340 nm, which are ascribed to the allowed 4f→5d1 and 4f→5d2 transition of the Ce3+, respectively, in the garnet phase [19,20]. Interestingly, the absorption coefficient for the Ce3+ absorption transitions significantly varies among samples due to the different content of Ce among samples. This variation is caused by the segregation coefficient (keff) of Ce atoms in aluminum garnets [1922]. The absorption spectra are consistent with WDS elemental analysis, see Tab.1. The partial substitution Ga for Al changes the crystal field strength and consequently shifts the 4f→5d1 and 4f→5d2 absorption bands to higher and lower energies, respectively [30,33]. The absorption lines at 275 and 310 nm belong to the Gd3+ ions [31]. The high amplitude below 270 nm is caused by both various color and impurity related centers [32-34] and 4f→5d3,4,5 absorption transitions of Ce3+ ions in the garnet phase [34,35]. In the Gd3Ga1.5Al3.5O12:Ce single crystal, the amplitudes of 4f→5d1 and 4f→5d2 absorption bands at 445 and 340 nm, respectively, are somehow saturated due to reaching the experimental limit of the spectrometer. This observation shows that sample with the lowest Ga content (x=1.5) has very low quality due to the presence of several phases, many defects and perturbed Ce3+ ions within the crystal volume. Consequently, the Ce3+ ions are very unevenly distributed in parallel and cross-sectional directions to the growth direction of the crystal (poor segregation of Ce atoms during the crystal growth). Part of the crystal under the study has a higher than in the melt concentration of Ce causing the observed saturation, see Tab. 1. Moreover, saturation can be caused by the parasitic contribution of the Ce3+ and defects luminescence to the signal beam.
3.3. Photoluminescence characterization
On the contrary to RL spectra, due to high sensitivity, the PL measurements reveal UV emission centers in the following Gd3GaxAl5-xO12:Ce (x=1.5, 1.75, 2 and 2.25) crystal compositions, see Fig. 6 a-b. Interestingly, the Gd3Ga2.5Al2.5O12:Ce composition does not show emission and excitation band in the UV spectral range. This observation shows that substitution Ga for Al at least in a 50% in Gd3GaxAl5-xO12:Ce composition enables congruent 9
melting and crystallization in a pure garnet phase [15]. The RT normalized excitation spectra for emission at ~360 nm consists of a broad-bands between 200 and 320 nm (Fig. 6a), whilst, emission spectra excited at 290 nm show broad-bands between 300 and 420 nm (Fig. 6 b). The shape of the excitation and emission bands is irregular and change with increasing Ga content. However, the position and profiles of both excitation and emission bands indicate they mainly belong to the GdAlO3/α-Al2O3 eutectic phase inclusions supporting the WDS, XRD and radioluminescence results [23-29]. By analogy to (Gd,Lu)3Al5O12:Ce ([11] and references therein) crystal set and WDS elemental analysis, we can conclude that the luminescence in the UV spectral range can be caused by the overlapping of the Ce3+ emission in the perovskite phase, F+ centers in the garnet and/or perovskite and/or α-Al2O3 phases as well as Ce3+ emission centers in the α-Al2O3 phase ([11] and references therein, [36]). Moreover, it is worth to stress, that the multiphase system can create perturbed Ce3+ centers in the perovskite phase, which may influence the shape of the excitation and emission bands in the UV spectral region as well [37]. However, overlapping of the emission coming from different phases (REAlO3:Ce, α-Al2O3, α-Al2O3:Ce) hamper studying the individual components in the absolute UV emission. The PL excitation and emission spectra do not show systematic dependence due to different Ce segregation coefficient caused by changing the crystal compositions, incongruent melting and crystallization of various phases at different stages of the crystal growth process. Consequently, within the crystal, the chemical composition is strongly heterogeneous (as explained above). Such strong variation of the chemical compositions within the crystal volume disturbs systematic dependence especially in studying the secondary phase as it is randomly distributed within the crystal (i.e. part of the crystal under the study). Moreover, the luminescence centers related to the secondary phase can be located on the borders or interface between phases. Consequently, the local surrounding around the sensitive luminescence center is perturbed. Such perturbation varies among the crystals causing irregular profiles of the excitation and emission bands and thus no systematic dependence can be obtained. The decay kinetics analysis gives deeper insight into luminescence processes in the study multiphase systems. Namely, Fig. 7 shows the decay curve for luminescence centered at 360 nm in Gd3Ga1.5Al3.5O12:Ce under excitation at 290 nm. The decay kinetics is strongly nonexponential. For this reason, the double-exponential fit is used for the qualitative description of the luminescence timing properties, see Eq. 1. = ∑ −/ + , = 1 3, 10
(1)
the parameters Ii and stand for the pre-exponential factor and decay time, respectively, in the ith decay component. The double-exponential approximation with the accelerated fast τfast~5
ns and slower τslow~123 ns components can indicate the presence of different emitting
centers as well as complex energy transfer processes. Such behavior is very similar to the decay kinetics of Ce3+ luminescence in the (Lu,Y)1-xGdxAlO3:Ce single crystal perovskites, where Ce decay time exhibits non-exponential profile with the fast accelerated and significantly slower components [37]. Hence, the double-exponential character of the decay curve can be connected to the energy transfer from emission center in UV spectral range (mainly Ce3+ ions in the perovskite phase) to Gd sublattice (accelerated component, τfast~5 ns) followed by energy migration over Gd sublattice and subsequent trapping at perturbed Ce3+ centers (slower component, τslow~123 ns). Such an energy transfer mechanism has been reported in (Lu,Y)1-xGdxAlO3:Ce single crystal perovskite [32,36] and in multiphase systems containing aluminum garnet and aluminum perovskite phases [10,11]. However, the faster component of the decay times might be also contributed by the luminescence of F+ center in perovskite and/or garnet and/or α-Al2O3 phases [38], since their energy levels are located in close proximity to 5d1 energy level of Ce3+ in the perovskite phase, see Fig. 6. The decay curve analysis supports the hypothesis that UV luminescence can be caused by both Ce3+ ions in the perovskite phase and F+ center in perovskite and/or garnet and/or α-Al2O3 phases as it is consistent with the PL emission and excitation spectra (i.e. crystals under the study contain a mixture of the garnet phase and GdAlO3/α-Al2O3 eutectic phase).
Fig. 8 shows the excitation spectra for Ce3+ emission at 550 nm (garnet phase) and emission spectra under excitation at 290 nm (secondary phase) for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce single crystals. The dominant bands centered around 360 nm in the emission spectra belong to the secondary phase (mainly to Ce3+ emission in the perovskite phase) [10,11,23-29]. The broad bands centered around 345 nm and 450 nm in the excitation spectra belong to 4f→52 and 4f→5d1 excitation/absorption transitions of Ce3+ in the garnet phase, respectively. Interestingly, the UV emission bands contain a dip centered at 343 nm and 346 nm for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce, respectively. Moreover, the PL excitation and emission spectra for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2Al3O12:Ce crystals show the dependence of the position of the dip in UV emission band on Ga content. This observation shows that the shift of the 4f→5d2 excitation transition of Ce3+ in the garnet phase is accompanied by the identical shift of the dip in the UV emission band. This 11
systematic dependence and overlapping of the dip with the 4f→5d2 excitation transition of Ce3+ in the garnet phase can suggest both radiative and non-radiative energy transfer processes, which have been reported in the analogous multiphase systems (Y,Gd)3Al5O12:Ce [10] and (Gd,Lu)3Al5O12:Ce [11]. Another evidence of energy transfer between UV and visible emission centers is provided by the decay kinetics of the Ce3+ luminescence centered at 550 nm in the garnet phase under excitation into absorption band in secondary phase at 290 nm, see Fig. 9. The decay curve shows three-exponential profile with the fast (τ1=70 ns) and slow (τ2=290 ns) and (τ3=1.77 µs) components. The three components of the decay time with the three significantly different decay constants might suggest that the energy transfer occurs partly by the radiative and partly by the non-radiative process via different channels including $% immediate $% sublattice )
! "# ! "#
$% → '
( "#
energy transfer and involving Gd3+
$% $% → *+,-. (( → '
( "# /
(in the perovskites the
Gd3+ ions typically excite the Ce3+ luminescence, see Ref. 39 and 40). Such complex energy transfer mechanism has been reported and discussed in detail in the similar multiphase crystals containing aluminum perovskite and aluminum garnet phases mixture, see Ref. 10 and 11. It should be stressed that another luminescence centers like F+ center in perovskite and/or garnet and/or α-Al2O3 also might contribute to the overall energy transfer process since their energy levels are located in close proximity to the 5d2 energy level of Ce3+ in the garnet phase, see Fig. 6a. 3.4. Temperature dependence characterization
The temperature dependence of the prompt and delayed radiative recombination processes were monitored to reveal the mechanism responsible for the quenching of Ce3+ emission centers in the garnet phase. Fig. 10 a-d shows examples of the prompt PL decays for Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce samples recorded at 100 and 500 K. The decay curves in all set of samples exhibit single-exponential profile up to around 400 K, above this temperature decays become non-exponential, see Figs. 10 b and 10 d. Such behavior could be due to the different gap values of Ce3+ excited state and the levels of the conduction band at different Ga content and temperatures. Nevertheless, this aspect needs further study. The qualitative influence of Ga content on the Ce3+ thermal stability is visible on the decays recorded at high temperature. The decay time of the sample with the highest content of Ga (Gd3Ga2.5Al2.5O12:Ce) shows relatively slow component τ2=241 ns, whilst in the sample with 12
the low content of Ga (Gd3Ga1.5Al3.5O12:Ce), the slow component is not observed. This can suggest that in the Ga rich sample, the luminescence quenching process is assisted by the thermal ionization of the 5d1 excited state of Ce3+ center towards the conduction band [41]. Fig. 11 presents the temperature dependence of nanosecond decays for all samples. At lower and higher temperatures, the decay constants were obtained from the single and double exponential approximations, respectively, according to Equation 1 (i=1or 2), hence, instead of decay time τ, the mean time τm is used defined by Equation [10]. Analogously to other multicomponent garnets [13,30,41], increasing Ga content reduces the thermal stability of the Ce3+ luminescence centers. Consequently, the onset of luminescence quenching is shifted to a lower temperature. However, change in thermal stability of the Ce3+ luminescence is minor because the admixture of Ga in a low range only slightly affects the change of crystal field strength and bottom of the conduction band [13]. Moreover, due to the heterogeneous character of the crystals, the local chemical composition of the crystal under the study might be similar, as it is discussed above. The delayed recombination decay (DR) technique consists in the recording of the slow component in the photoluminescence decay under UV/visible excitation. When the luminescent ion is thermally ionized, charge carrier that does not decay promptly can migrate over the conduction or valence bands. After being trapped and thermally released again they return to the luminescent ion and radiatively recombine emitting delayed (at millisecond scale) light. The delayed radiative recombination light measurement is an excellent tool to study the thermally induced ionization process of the 5d1 excited state of Ce3+ center in Gd3GaxAl5-xO12:Ce crystals. This experimental method is complementary to the temperature dependence of the prompt decay technique to study the luminescence quenching mechanism. The delayed radiative recombination light measurement provides information about the thermally induced photoionization process, which influences the quenching of Ce3+ luminescence in Gd3GaxAl5-xO12:Ce crystals. It is worth to mention that quantum tunneling process also produces delayed (millisecond scale) light. However, the intensity of the delayed light produced in the quantum tunneling process is significantly lower than in the ionization one [42]. Fig. 12 shows the delayed recombination integral intensities of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals recorded under excitation into Ce3+ 4f→5d1 absorption transition at 455 nm and monitoring the Ce3+ 5d1→4f luminesce at 550 nm at temperature range 10-500 K. Interestingly, for samples with Ga content between x=1.5 and 2.25 (multiphase system 13
samples) there is a slight decrease of the intensity in the temperature range 200-500 K. This drop in the intensity could be due to decreasing amplitude of absorption in 5d1 band in favor of increasing amplitude of absorption into 5d2 with increasing temperature [43]. Alternatively, at the higher temperature, the luminescent ion can get out of resonance with some of the previously participating traps. Consequently, the total number of traps contributing to the overall delayed recombination signal would drop. In any case, there is no indication of thermal ionization of the Ce3+ excited state. For the sample with Ga content x=2.5 (pure garnet phase sample), there is a slight increase of the intensity in the temperature interval 200-400 K. Such intensity increase can indicate the thermally induced ionization of the 5d1 excited state, as it is consistent with the result presented in Fig. 10 d above. Moreover, the variety of DR decays in the study crystals confirm the plausibility of the quantum tunneling hypothesis [44]. The temperature dependence analysis shows that the increase of Ga content slightly reduces the thermal stability of the Ce3+ ions in the study Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals. The temperature dependence results show that the quenching of Ce3+ luminescence in Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals is caused by thermal relaxation from the 5d1 potential curve (parabola) to the 4f ground potential curve through the crossing point based on the configurational coordinate model [45]. However, in the Ga rich sample, the quenching mechanism could be also assisted by thermally induced ionization of the 5d1 excited state as the PL decay curve recorded at a higher temperature for Ga rich sample shows [45].
3.5. Scintillation decay times Scintillation decays recorded under γ-ray (662 keV,
137
Cs radioisotope) excitation for
Gd3Ga1.5Al3.5O12:Ce and Gd3Ga2.5Al2.5O12:Ce crystals are shown in Fig. 13 a-b. The decays show multi-exponential character and are fitted with Eq. (1), where i=2. The exponential components with the decay times of 16 and 10 ns for x=1.5 and 2.5, respectively, have negative pre-exponential factors and indicate the rise time. The decay time constants of 130 and 100 ns for x=1.5 and 2.5, respectively, are due to prompt radiative electron-hole recombination at the Ce3+ emission center. It is worth to notice that scintillation decay constants are longer that PL ones, which points to a slight slow-down in the prompt decay component undoubtedly due to the transport processes. Table 2 shows the scintillation decay
14
time components for all samples. Interestingly, the rise time and decay time constants significantly accelerate in the sample with increasing Ga content, confirming that Ga admixture reduces the electron trap activity in the scintillation process by burying them in the conduction band as well as improving the quality of the crystals (i.e. diminish content of the unwanted secondary phase) [8,9]. However, Gd3Ga2Al3O12:Ce crystal shows different properties from the rest of the set of crystals, namely it shows the longest rise time (τrise=22 ns) and fastest decay time (τrise=109 ns) constants. Such a feature could be related to poor crystal quality, which possesses higher content of charge traps than the other ones. Consequently, higher content of charge traps slow down the migration of electrons and holes towards the luminescence center as well as it can quench the Ce3+ luminescence causing lengthening in the rise time and acceleration in the decay time, respectively.
4. Conclusions In this research, the absorption, radioluminescence, photoluminescence, and scintillation decay times characteristics of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals are studied upon partial substitution Ga for Al. The obtained results are analogous to (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. The low content of Ga fraction in Gd3(Ga,Al)5O12:Ce single crystals results in the formation of a multiphase system consisting of target garnet and secondary phases. The secondary phase was detected in Gd3GaxAl5-xO12:Ce (x=1.5-2.25) crystals by WDS, XRD, radioluminescence and PL study. Only Gd3Ga2.5Al2.5O12:Ce composition showed a pure garnet phase. Increasing Ga concentration caused slight blue-shift of Ce3+ emission in the garnet phase due to the reduction of the crystal field strength. The Gd3+→Ce3+ non-radiative energy transfer in the garnet phase and Ce3+↔Gd3+ non-radiative energy transfer in the secondary phase were confirmed. Moreover, the energy transfer from secondary to garnet phase was revealed as well. The increasing Ga content slightly decreased the thermal stability of the Ce3+ luminescence. Temperature dependence of the prompt and delayed recombination decay of Ce3+ emission revealed that luminescence quenching in Gd3(Ga,Al)5O12:Ce is mainly caused by the thermally induced cross-over from the excited 5d1 state parabola to the ground state 4f parabola and can be slightly assisted by the thermally induced ionization process in the samples with higher Ga content. The high intensity of the delayed recombination integrals even at low temperatures confirm an intricate interaction between 5d1 excited state of Ce3+ and its host lattice neighborhood. The Ga admixture
15
significantly improved scintillation response in the studied crystals by reduction of the rise time and slow components.
Acknowledgment The
bilateral
ASCR-JSPS
project
and
project
No.
SOLID21
CZ.02.1.01/0.0/0.0/16_019/0000760 from Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports and 16H05986 Grant-in-Aid for Young Scientists (A), Japan Society for the Promotion of Science (JSPS) and 16H04505 Grant-in-Aid for Scientific Research(B), and Polish NCN grant No 2017/25/B/ST8/02932, are gratefully acknowledged.
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crystal composition the in melt
elements at% Gd (Lα xray photon GdF3)
Al (Kα x-ray photon Al2O3)
18
Ga (Lα x-ray photon GaP)
Ce (Lα x-ray photon CeAl2)
detected phase
picture a (Gd3Ga1.5Al3.5O12:Ce)
picture b (Gd3Ga1.5Al3.5O12:Ce)
picture c (Gd3Ga2.5Al2.5O12:Ce)
picture d (Gd3Ga2.5Al2.5O12:Ce)
28.74
54.85
15.62
0.79
garnet
46.89
52.45
0.46
0.20
perovskite/αAl2O3 (eutectic)
37.05
34.57
27.9
0.48
garnet
39.13
31.46
29.04
0.37
garnet
Tab. 1. WDS elemental analysis of Gd3Ga1.5Al3.5O12:Ce (a,b) and Gd3Ga2.5Al2.5O12:Ce (c,d) single crystals. GdF3, α-Al2O3, GaP and CeAl2 are references used in WDS elemental analysis.
crystal composition
single exponential rise time [ns]
single exponential decay time [ns]
Gd3Ga1.5Al3.5O12:Ce
τrise=16 ns
τdecay= 130 ns
Gd3Ga1.75Al3.25O12:Ce
τrise=12 ns
τdecay= 120 ns
Gd3Ga2Al3O12:Ce
τrise=22 ns
τdecay=109 ns
Gd3Ga2.25Al1.75O12:Ce
τrise= 9 ns
τdecay=116 ns
Gd3Ga2.5Al2.5O12:Ce
τrise=10 ns
τdecay=100 ns
Table 2. Scintillation decay time components of the Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals
Fig. 1. The as-grown crystal rods of (a) Gd3Ga1.5Al3.5O12:Ce and (b) Gd3Ga2.5Al2.5O12:Ce. Insets show cross-sectional polished plates cut from the middle part of the crystal rods.
19
Gd3Ga2.5Al2.5O12:Ce
Gd3Ga1.5Al3.5O12:Ce
d b c
a
a
c
b
d
perovskite/α-Al2O3
garnet
Fig. 2. SEM micrographs of Gd3Ga1.5Al3.5O12:Ce (a,b) and Gd3Ga2.5Al2.5O12:Ce (c,d) single crystals
20
Fig. 3. XRD spectra for (a) Gd3GaxAl5-xO12:Ce (x=1.5 and 2.5) and (b) beginning, middle and end parts of the Gd3Ga1.5Al3.5O12:Ce crystals. 21
Fig. 4. Room temperature radioluminescence spectra for Gd3GaxAl5-xO12:Ce (x=1.5-2.5) crystals (X-ray excitation 40 kV, 15 mA). The luminescence centered at 360 nm in the sample with the lowest Ga content belongs to secondary phase inclusions.
Fig. 5. Absorption spectra of the Ce3+ activated Gd3GaxAl5-xO12 (x=1.5, 2, 2.25 and 2.5) crystals. 22
Fig. 6. (a) Normalized RT PLE spectra for the emission at 360 nm; (b) Normalized RT PL spectra under excitation at 290 nm in Gd3GaxAl5-xO12:Ce crystals.
23
Fig. 7. RT PL decay kinetics monitored at 360 nm under excitation at 290 nm in Gd3Ga1.5Al3.5O12:Ce.
Fig. 8. RT excitation spectra for the Ce3+ luminescence centered at 550 nm (garnet phase) and emission spectra excited at 290 nm in Gd3Ga1.5Al3.5O12 and Gd3Ga2Al3O12 single crystals
24
belonging to the secondary phase. The dips around 340 nm indicate radiative energy transfer between secondary and garnet phases.
Fig. 9. Decay kinetics of the Ce3+ related luminescence (550 nm) measured at RT excited into absorption band of the secondary phase at 290 nm in the Gd3Ga1.5Al3.5O12:Ce.
25
26
Fig. 10. PL decay kinetics for Ce3+ luminescence at 550 nm under excitation at 455 nm in (ab) Gd3Ga1.5Al3.5O12:Ce and (c-d) Gd3Ga2.5Al2.5O12:Ce samples.
27
Fig.11. The temperature dependence of photoluminescence decay times for the Ce3+ emission at 550 nm under excitation into 4f→5d1 absorption transition at 455 nm in the temperature range 77-500 K.
Fig. 12. Delayed recombination decay intensities of Gd3GaxAl5-xO12:Ce (x=1.5-2.5) as a function of temperature (λex=455 nm, λem=550 nm).
28
Fig. 13. RT Scintillation decay times for (a) Gd3Ga1.5Al3.5O12:Ce and (b) Gd3Ga2.5Al2.5O12:Ce crystals under excitation with γ-rays (137Cs radioisotope, 662 keV).
29
Highlights Instability of the garnet phase and formation of the secondary phase inclusions discussed. Temperature dependence of the decay time of Ce3+ emissions in garnet phase studied. Energy transfer processes between secondary phase inclusion and target garnet phase discussed.