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Crystals 2017, 7(9), 262; doi:10.3390/cryst7090262

Article
LPE Growth of Single Crystalline Film Scintillators Based on Ce3+ Doped Tb3−xGdxAl5−yGayO12 Mixed Garnets
Vitalii Gorbenko 1,*, Tetiana Zorenko 1, Sandra Witkiewicz 1, Kazimierz Paprocki 1, Oleg Sidletskiy 2, Alexander Fedorov 3, Paweł Bilski 4Orcid, Anna Twardak 4 and Yuriy Zorenko 1,*Orcid
1
Institute of Physics, Kazimierz Wielki University in Bydgoszcz, 85090 Bydgoszcz, Poland
2
Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001 Kharkiv, Ukraine
3
SSI Institute for Single Crystals, National Academy of Sciences of Ukraine, 61178 Kharkiv, Ukraine
4
Institute of Nuclear Physics, Polish Academy of Sciences, 31342 Krakow, Poland
*
Correspondence:
Received: 24 July 2017 / Accepted: 25 August 2017 / Published: 30 August 2017

Abstract

:
The growth of single crystalline films (SCFs) with excellent scintillation properties based on the Tb1.5Gd1.5Al5−yGayO12:Ce mixed garnet at y = 2–3.85 by Liquid Phase Epitaxy (LPE) method onto Gd3Al2.5Ga2.5O12 (GAGG) substrates from BaO based flux is reported in this work. We have found that the best scintillation properties are shown by Tb1.5Gd1.5Al3Ga2O12:Ce SCFs. These SCFs possess the highest light yield (LY) ever obtained in our group for LPE grown garnet SCF scintillators exceeding by at least 10% the LY of previously reported Lu1.5Gd1.5Al2.75Ga2.25O12:Ce and Gd3Al2–2.75 Ga3–2.25O12:Ce SCF scintillators, grown from BaO based flux. Under α-particles excitation, the Tb1.5Gd1.5 Al3Ga2O12:Ce SCF show LY comparable with that of high-quality Gd3Al2.5Ga2.5O12:Ce single crystal (SC) scintillator with the LY above 10,000 photons/MeV but faster (at least by 2 times) scintillation decay times t1/e and t1/20 of 230 and 730 ns, respectively. The LY of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce SCFs, grown from PbO flux, is comparable with the LY of their counterparts grown from BaO flux, but these SCFs possess slightly slower scintillation response with decay times t1/e and t1/20 of 330 and 990 ns, respectively. Taking into account that the SCFs of the Tb1.5Gd1.5Al3–2.25Ga2–2.75O12:Ce garnet can also be grown onto Ce3+ doped GAGG substrates, the LPE method can also be used for the creation of the hybrid film-substrate scintillators for simultaneous registration of the different components of ionization fluxes.
Keywords:
liquid phase epitaxy; single crystalline films; scintillators; mixed garnets; Tb3+ cations

1. Introduction

The development of detectors for 2D/3D microimaging using X-ray sources and synchrotron radiation demands the creation of thin (from a few microns thick up to 20 microns) single crystalline film (SCF) scintillating screens with an extremely high ability for X-ray absorption and a micron-submicron spatial resolution [1,2,3,4]. More recently, for this purpose, the visible emitting scintillating screens based on the SCF of Ce doped Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) garnets grown by the Liquid Phase Epitaxy (LPE) method have been used and the spatial resolution of the detector in the micron range has been achieved using synchrotron radiation with energy in the 8–20 keV range [1,2]. After that, the SCFs of Eu3+, Tb3+ doped Gd3Al5O12 (GGG) and Sc3+ doped LuAG garnets [3,4], Tb3+ and double Tb3+, Ce3+ doped Lu2SiO5 (LSO) orthosilicates [5,6,7,8,9,10,11,12,13,14], Ce3+, Tb3+ and Eu3+ doped LuAlO3 (LuAP) and (Gd,Lu)AlO3 (GLAP) perovskites [15,16,17,18,19] and recently Ce3+ doped Tb3Al5O12 (TbAG) garnets [20,21], have also been successfully developed in the last decade for microimaging detectors using the LPE method.
The fabrication of screens with higher spatial resolution of X-ray images in the submicron range demands the creation of new scintillating film screens with extremely high absorption ability for X-rays—which is proportional to ρZeff4, where ρ is the density and Zeff is the effective atomic number of scintillators [2,3]—as well as the development of novel concepts for microimaging.
During the last years, two novel concepts for the creation of a detector for microtomography have been proposed [15,19,21]. The first concept is related to the engineering of K-edge of X-ray absorption multilayer-film scintillators using the solid solution of oxide compounds containing the Lu, Gd and Tb ions [15,19,21]. Indeed, the absorption ability of the film scintillator can be significantly improved in the 20–65 keV range due to the significant broadening of the K-edge of X-ray absorption in such mixed materials [15,19,21]. The second concept is based on using the complex multilayer-film scintillator with a separate pathway for registration of the optical signal from each layer and final overlapping of the images coming from the different parts of the complex scintillator [15,21]. By using such multilayer-film scintillators one can significantly improve the contrast and resolution of images even in the submicron range. Two such novel concepts also demand the fabrication of different sets of heavy and efficient SCF scintillators which can be deposited onto the same substrates.
The Ce3+ doped Lu3−xGdxAl5−yGayO12 and Gd3Al5−yGayO12 mixed garnets are related to the efficient and heavy scintillators with very high (up to 50,000 photons/MeV) light yield (LY) under γ quanta excitation [22,23,24,25]. For this reason, these compounds are also used for the fabrication of the scintillation screens with high absorption ability for X-rays [26,27,28,29,30,31,32,33]. With the aim of increasing the energy transfer efficiency from the host of mixed garnets to the Ce3+ ions, the Tb3Al5O12, Lu3−xTbxAl5O12 and Gd3−xTbxAl5O12 SCFs were also crystallized by the LPE method and their luminescent and scintillation properties were investigated [20,21,34,35]. The Ga co-doped analogues of these garnets can also be considered as very interesting matrixes for this purpose and their luminescent and scintillation properties were briefly reported by us as well [36]. At the same time, the possibility of the creation of the efficient SCF scintillators on the basis of the mentioned Tb containing garnets by the LPE method still needs the following technological and experimental evidence. First of all, estimation of the real potential of different garnet compositions for producing the scintillation screens strongly requires the LPE crystallization of these compounds in the SCF form from the different types of fluxes due to very large influence of flux related dopants on their scintillation characteristics [31,33,37,38] as well as the crystal analogs of these garnets using MPD [39] or Czochralski methods.
In this paper, we present the new results of the research on the creation of the advanced SCF scintillation screens based on Ce doped Tb1.5Gd1.5Al5−yGayO12 mixed garnets at y = 2–3.85, grown by the LPE method from the novel lead free BaO based flux (later called Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs) and compare their properties with those of Tb3−xGdxAl5−yGayO12 SCFs at x = 0–2.1 and y = 0–2.75, grown from the traditional PbO based flux [21,37] (later called Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs).
For engineering the scintillator composition we apply the combination of Ce3+ 5d-level positioning [20] and band-gap engineering [40] in the Ce doped Tb3−xGdxAl5−yGayO12 mixed garnet using the substitution by Gd3+ cations of the dodecahedral sites of Tb3Al5O15 garnet lattice with the concentration x = 1.5 and the substitution by Ga3+ ions of the Al3+ cations in both the tetrahedral and octahedral positions of the garnet host at concentration y = 2–3.85. Additionally, we can expect an increase in the energy transfer efficiency from the host of the Tb3−xGdxAl5−yGayO12 garnet to the Ce3+ ions using the sublattices of Tb3+ and Gd3+ cations.

2. Growth of Tb3−xGdxAl5−yGayO12:Ce Single Crystalline Films

The SCFs of Tb3−xGdxAl5−yGayO12:Ce garnets were grown by the LPE method onto Gd3Al2.5 Ga2.5O12 (GAGG) substrates with a relatively high lattice constant of 12.228 Å in comparison with the traditional Y3Al5O12 (YAG) substrates with a lattice constant of 12.003 Å from supercooled melt solutions using both BaO-B2O3-BaF2 and PbO-B2O3 fluxes. Firstly, the sets of optical quality perfect SCF samples—with x values in the 0–2.1 range, y values changing in the 0–2.85 range and thickness in the 16–38 μm range—were crystallized onto GAGG substrates with a square of 1 × 1 cm2 with the (100) orientation (Figure 1, left) using the conventional PbO flux for more detail determination of the optimal ranges of Gd and Ga concentrations x and y, respectively, in comparison with work [36]. After that, other sets of optically good quality Tb1.5Gd1.5Al5−yGayO12:Ce SCF samples—with y values changing in the 2–3.8 range and thickness in the 10.5–22 μm range—were also successfully crystallized onto GAGG substrates from novel lead-free BaO based flux (see Figure 1, middle and right figures, and Table 1). The components of this flux have significantly smaller influence on their scintillation properties than in the case of PbO flux grown SCF scintillators [31,33,37,38]. At the same time, the high viscosity of this BaO based flux leads to formation of different structural macro-defects and strongly decreases the uniformity of SCF surface [31,33,37]. Such unwanted effects are also observed in the case of growing the Tb1.5Gd1.5Al5−yGayO12:Ce SCFs from BaO based flux. For this reason, using the PbO-B2O3 solvent—due to its low viscosity and good kinematic properties—is preferable for producing high quality SCF scintillators for ensuring the best structural and surface quality of screens for high-resolution X-ray imaging [6,7,18,19].
The concentration of CeO2 activating oxide was 10 and 5 mole% with respect to the garnet-forming components in the cases of SCF growth using PbO and BaO based fluxes, respectively.
The composition of SCF samples was determined using a JEOL JSM-820 electronic microscope, equipped by an EDX microanalyzer with IXRF 500 and LN2 Eumex detectors. From the microanalysis of the content of the SCF samples we have also found that the segregation coefficient of Ga3+ ions in Tb3Al5−yGayO12:Ce and Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from PbO based flux, was equal to 0.59–0.65 and 0.735–0.82, respectively (Table 2). The segregation coefficient of Gd3+ ions in Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs was equal to 0.95–1.05. The segregation coefficient of Ce3+ ions was equal to about 0.004–0.005 and 0.0095–0.02 in Tb3Al5−yGayO12:Ce SCF and Tb3−xGdxAl5−yGayO12:Ce SCFs, in the case of using PbO based flux for their growth (Table 2).
We have also found that the segregation coefficients of Ga3+ and Ce3+ ions in Tb3−xGdxAl5−yGayO12:Ce SCFs in the case of using BaO based flux were significantly larger. Specifically, the segregation coefficient of Ga3+ ions in these SCFs, grown from BaO based flux, was equal to 1.0–1.1 and was notably larger than the respective values in the SCFs grown from PbO based flux (Table 2). The segregation coefficient of Ce3+ ions in the Tb3−xGdxAl5−yGayO12:Ce SCFs was also significantly larger in the case of using BaO based flux and was equal to 0.012–0.14 in comparison with the 0.004–0.005 value in Tb3Al5−yGayO12:Ce and 0.0095–0.02 in Tb3−xGdxAl5−yGayO12:Ce SCF counterparts grown from PbO based flux (Table 2). At the same time, the segregation coefficient of Gd3+ ions, being equal to 1.0–1.1 in the case of Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from BaO based flux, was only slightly larger than that in the case of using PbO based flux (Table 2).
The XRD measurements (spectrometer DRON 4, CuKα X-ray source) were used for characterization of the structural quality of Tb1.5Gd1.5Al5−yGayO12:Ce SCFs, grown from BaO based flux (Figure 2). From the respective XRD patterns of these SCFs at y value in the 2–3.85 range (Figure 2), we can also estimate the lattice constants of the different garnet compositions and the misfit between the lattice constants of SCFs and GAGG substrate Δa = (aSCF − asub)/asub × 100% (Table 1). Namely, the lattice constant of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs at y = 2–3.85 changed from 12.069 Å for Tb1.5Gd1.5Al3Ga2O12:Ce SCFs to 12.2913 Å for Tb1.5Gd1.5Al1.2Ga3.8O12:Ce SCFs (Figure 2) and the value of misfit m changed from −1.3% to +0.52% for these SCF samples (Table 1).
The measurements of rocking curves in the ω and 2θ-ω scanning modes were applied for characterization of the structural quality of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs at different Ga content y in the 2.0–3.85 range (Figure 3a,b, respectively). As can be seen from these figures, the quality of the SCFs, which is proportional to FWHM of rocking curves, significantly increases at lower SCF-substrate misfit values m in Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs. Namely, the smallest FWHM values of 0.0182 and 0.0121 degrees are observed for Tb1.5Gd1.5Al1.5Ga3.5O12:Ce (BaO) SCFs (Figure 3a,b, respectively) grown onto GAGG substrates with the lowest SCF/substrate misfit value m = +0.05% (Table 1).

3. Luminescent and Scintillation Properties of Tb3−xGdxAl5−yGayO12:Ce Single Crystalline Films

For characterization of the optical properties of Ce3+ doped Tb3−xGdxAl5−yGayO12:Ce SCFs, the cathodoluminescence (CL) spectra, LY and scintillation decay kinetics measurements as well as the thermostimulated luminescence (TSL) glow curves under excitation by α-particles were performed.
The CL spectra were measured at the room temperature (RT) using an electron microscope SEM JEOL JSM-820, additionally equipped with a spectrometer Stellar Net with TE-cooled CCD detector working in the 200–1200 nm range. The scintillation LY with a shaping time of 14 μs and decay kinetics measurements were performed using the setup based on a Hamamatsu H6521 PMT, multichannel analyzer and digital TDS3052 oscilloscope under excitation by α-particles of Pu239 (5.15 MeV) source. The energy resolution (ER) of SCF scintillators is calculated as a ratio of the FWHM of the full energy peak to the peak’s centroid position: E = FWHM/centroid [%]. The TSL measurements were performed in the 300–800 K temperature range using a commercial Risoe DA-20 TL/OSL reader (Denmark) after α-particle excitation by the Am241 source which is built into the DA-20 reader. The TL glow curves were registered from 50 °C to 450 °C at the rate of 5 °C·s−1. The measurements were conducted with a Shott BG 39 green filter, with transmission from 350 to 700 nm. This filter is well adapted for the registration of Ce3+ luminescence in the SCF samples under study. Meanwhile, the spectrally resolved TSL spectra of Tb3−xGdxAl5−yGayO12 SCF with different Gd3+ and Tb3+ content (not present in the paper) show only Ce3+ luminescence in the green-yellow ranges and absence of the emission of Tb3+ or Gd3+ cations.

3.1. Cathodoluminescence Spectra

The RT CL spectra of Tb3Al5−yGayO12:Ce and Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from PbO based flux, with close y values in the 2–2.5 range and different x values in the 0–2 range, are shown in Figure 4a,b, respectively, in comparison with the spectrum of TbAG:Ce SCF (Figure 4a, curve 1). The RT CL spectra of Tb1.5Gd1.5Al5−yGayO12:Ce SCFs, grown from BaO based flux, with different y values in the 2–3.85 range, are presented in Figure 4c, curves 1–4 in comparison with the CL spectra of standard Gd3Al2.5Ga2.5O12:Ce bulk SC (Figure 4b, curve 5).
The CL spectra of all the SCFs under study show only the dominant luminescence of Ce3+ or Tb3+ ions in the visible range without any bands in the UV range related to the luminescence of antisite defects [41,42,43,44], which typically are observed in the bulk crystal analogues of these garnets [45,46].
The results, presented in Figure 4a–c, indicate that the complicated Gd3+→Tb3+→Ce3+→Tb3+ cascade energy transfer is observed in the Tb3−xGdxAl5−yGayO12:Ce mixed garnet, with large content of Gd3+ and Ga3+ cations due to overlapping of the Gd3+ and Tb3+ emission bands and the absorption bands of Tb3+ and Ce3+ ions [47,48,49,50,51]. Namely, the change of the positions of Ce3+ and Tb3+ 4f-5d absorption and emission bands at different concentrations of Tb3+, Gd3+ and Ga3+ cations leads to a strong variation of the efficiency of Gd3+→Tb3+→Ce3+→Tb3+ energy transfer processes in Tb3−xGdxAl5−yGayO12:Ce SCFs and results in the respective changes of their CL spectra (Figure 4a–c).
The estimation of the optimal content of Gd3+ and Ga3+ cations in Tb3−xGdxAl5−yGayO12:Ce SCFs in the x = 1–1.5 and y = 2–3 ranges was firstly performed in [36]. We will explain such a choice in this work in more detail based on the results of their CL spectra (Figure 4a,b). Alloying of Ga3+ ions in Tb3Al5O12:Ce (PbO) SCF in the concentration range above x = 2 leads to a decrease of the garnet band gap value [34] and increasing the centroid shift of O-Ga bonding in comparison with O-Al bonding. That results in the subsequent blue shift of the Ce3+ emission spectra in Tb3Al5−yGayO12:Ce (PbO) SCFs (Figure 4a, curves 2 and 3) and Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs (Figure 4c, curves 1–4). It is most important here that increasing the level of Ga3+ alloying in these SCFs above y = 2–2.5 also leads to a strong decrease of the Ce3+ emission contribution to the total spectrum of the CL luminescence of these SCF samples (Figure 4a,c).
Alloying of Gd3+ ions in Tb3Al5−yGayO12:Ce (PbO) SCFs in the concentration range x = 0–1.5 has an opposite effect than the Ga3+ alloying and leads to the notable red shift of their CL spectra. The Gd3+ alloying in concentrations up to x = 1 increases the Ce3+ luminescence contribution (Figure 4b, curves 2) in comparison with the CL spectra of Gd free SCF samples (Figure 4a, curve 1). Meanwhile, the Gd3+ alloying in the concentration range above x = 1.0 in Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs also results in the notable decrease of the Ce3+ emission contribution to the total spectrum of the SCF luminescence and respective increase of the Tb3+ luminescence contribution (Figure 4a, curves 3–5). This can be caused by variation of the efficiency of Gd3+→Tb3+ and Tb3+→Ce3+ energy transfer processes in Tb3−xGdxAl5−yGayO12:Ce SCFs due to the change of the respective positions of Ce3+ and Tb3+ absorption and emission bands at different concentration of Tb3+ and Gd3+ cations.
Based on the results presented in Figure 4a,b, we can confirm here that the optimal values of the Gd3+ and Ga3+ concentrations in Tb3−xGdxAl5−yGayO12:Ce SCFs are x = 1–1.5 and y = 2–2.5 ranges, respectively. At these concentrations the respective CL spectra of Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (PbO) and (BaO) SCFs show the dominant Ce3+ emission band with relatively small contribution of the Tb3+ luminescence (Figure 4b, curves 2 and 3 and Figure 4c, curves 1 and 2).

3.2. Scintillation Decay Kinetics

The scintillation decay kinetics of Tb3−xGdxAl5−yGayO12:Ce and Tb1.5Gd1.5Al5−yGayO12:Ce SCFs, grown both from PbO (a) and BaO (b) fluxes with different x and y values in the 0–2 and 2–2.5 ranges, respectively, are shown correspondingly in Figure 5a,b. Generally, the Tb3−xGdxAl5−yGayO12:Ce SCF scintillators demonstrate the notably slower non-exponential kinetics similarly to their YAG:Ce and LuAG:Ce SCF counterparts [21,36]. Such slower decay kinetics of the Ce3+ luminescence is typical for Tb3+ and Gd3+ based SCF scintillators, where the cascade energy transfer via both sublattices of Gd3+ and Tb3+ cations is more complicated [36] in comparison with their YAG:Ce and LuAG:Ce SCF analogues, where the direct energy transfer from the garnet host to Ce3+ ions dominates [21].
The influence of Ga3+ and Gd3+ alloying on the scintillation decay of Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs was firstly considered in [36]. We will explain the choice of optimal Gd3+ and Ga3+ concentrations in these SCFs in more detail based on the results of their scintillation decay kinetics (Figure 5a). The Ga3+ alloying with the concentration up to y = 2.5 in Tb3Al5−yGayO12:Ce (PbO) SCFs leads to a significant slowdown of the decay kinetics of the Ce3+ emission in comparison with the TbAG:Ce SCF due to the lowering of the bottom of the conductive band and the arising electron transitions from the excited levels of Ce3+ to the conductive band. This results in the strong elongation of the scintillation decay of Tb3Al5−yGayO12:Ce (PbO) SCFs (not present in Figure 5a). Contrary to the influence of Ga3+ cations, the Gd3+ alloying in the concentration range x = 1.5–2 strongly accelerates the scintillation decay of Tb3−xGdxAl2.5Ga2.5O12:Ce (PbO) SCFs (Figure 5a, curves 2 and 3). It is also important to note here that the scintillation decay times t1/e and t1/20 in Tb3−xGdxAl5−yGayO12 (PbO) SCFs at high Gd and Ga concentrations (x = 1.5–2; y = 2.5) (Figure 5a, curves 2 and 3) are close or even faster with respect to the corresponding values for Tb3Al5O12:Ce SCF (see also Table 1).
The Tb1.5Gd1.5Al5−yGayO12:Ce SCF scintillators, grown from the BaO based flux (Figure 5b), demonstrate significantly more exponential kinetics than that in their SCF counterparts, grown from the PbO based flux (Figure 5a). Such a difference can be caused by eliminating the delay of energy transfer from the garnet hosts to Ce3+ ions caused by the defect centers related to the Pb2+ ions in Tb3–xGdxAl5–yGayO12:Ce (PbO) SCF scintillators. The concentration of these defect centers is significantly smaller in their analogues, prepared from the BaO based flux due to the very low contamination with Ba2+ ions.
It is necessary also to note here that the scintillation decay of Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (BaO) SCFs (Figure 5b, curves 1 and 2) is also notably faster than that in Gd3Al2.5Ga2.5O12:Ce SC counterparts (Figure 5b, curve 3) which can also be used as a substrate for producing SCF scintillators. In such a way these SCFs and the high-quality Gd3Al2.5Ga2.5O12:Ce SCs can also be used for the creation of advanced hybrid film-substrate scintillators using the LPE method for simultaneous registration of the different components of ionization fluxes [52]. In such scintillators, the separation of the signal coming from the film and crystal parts of the hybrid scintillator can be performed using the differences in their scintillation decay kinetics (Figure 5b, curves 1 and 3, respectively).

3.3. TSL Properties

The results of the TSL investigations after irradiation by alpha particles of Tb3−xGdxAl5−yGayO12:Ce SCFs with different x and y values, grown from both PbO (a) and BaO (b) based fluxes, in the above RT range are shown in Figure 6. The TSL in these SCFs arises at the thermal liberation of electrons from traps and their recombination with the holes localized around Ce3+ ions [21,28,33,42,43]. Taking into account the low temperature of SCF preparation in oxygen containing atmosphere (air), the formation of these traps can be caused mainly by the presence of Pb2+ and Ba2+ (from flux) and Pt4+ (from crucible) contaminations in SCF samples. This leads to the creation of different locally non-compensated lattice defects, such as the oxygen or cation vacancies, around the mentioned impurities, which act as trapping centers [21,31,36,53,54].
In Tb3Al5O12:Ce (PbO) SCFs, the position of the main TSL peak is located at 440 K (Figure 6a, curve 1). Ga3+ alloying of Tb3Al5−xGaxO12:Ce SCFs in the concentration range up to y = 2 leads to the shift of the TSL peaks to 407 K and substantially decreases the TSL signal in the 350–600 K range (Figure 6a, curve 2) (see also [31,36]). Gd3+ alloying additionally shifts the TSL peak to 390 K and slightly decreases the TSL signal of Tb1.5Gd1.3Al2.5Ga2.5O12:Ce (PbO) SCFs in the 450–650 K high-temperature range in comparison with Gd free SCF samples (Figure 6a, curve 3). These results are in good correlation with the significant increase of the LY in Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (PbO) SCFs (Table 1), most probably due to elimination of the participation of high-temperature trap-related centers in the scintillation processes in the SCF samples with the mentioned optimal content.
Using the lead free BaO based flux for the growth of Tb3−xGdxAl5−yGayO12:Ce SCFs leads also to the low intensity of the TSL peaks in the above RT range in SCF scintillators (Figure 6b). Namely, the TSL intensity of Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (BaO) SCFs in the 450–600 K range is also negligible (Figure 6b). This result is in good correlation with the high LY in Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (BaO) SCFs (Table 1), due to the low Ba2+ contamination and related with them ptrapping centers as well as to the additional elimination of the trap-related phenomena in the scintillation processes in these SCFs by Ga3+ and Gd3+ alloying.
Thus, the Ga3+ alloying in the concentration range y = 2–2.5 positively affects not only the scintillation properties of melt-grown mixed garnet crystals with a large concentration of antisite defects and oxygen vacancies [22,23,24] but antisite free SCF counterparts as well [31,32,33] due to burying the trap levels by the bottom of the conductive band in the Ga-containing garnets. The additional positive effect on the elimination of the trap centers in scintillation phenomena, which is observed in Tb3−xGdxAl5−xGaxO12:Ce SCF samples, is most probably caused by burying the deeper trap levels by high-energy states of Tb3+ and Gd3+ cations in such Tb,Gd-rich garnets [36].

3.4. Photoelectron Light Yield Measurements

The scintillation LY of Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from PbO and BaO based fluxes at different content of Gd3+ and Ga3+ cations, measured with a shaping time of 14 μs under excitation by α-particles of Pu239 (5.15 MeV) source, is shown in Table 1 and Figure 7.
In principle, the simultaneous influence of Gd3+ and Ga3+ doping of TbAG:Ce SCFs can also result in a strong increase in the LY of Tb3−xGdxAl5−yGayO12:Ce SCF scintillators at x = 0–1.5 and y = 2–3 [21,36]. Thus, the determination of the optimal content and ratio between the Gd3+ and Ga3+ cations in the TbAG:Ce garnet host is the most important task for the optimization of the properties of Tb3−xGdxAl5−yGayO12:Ce SCF scintillators in the case of growth from both PbO and especially lead free BaO based flux.
We have observed that Ga3+ doping in the concentration range y = 2–2.5 leads to the increase of the LY of Tb3Al5−yGayO12:Ce (PbO) SCF scintillators. Indeed, with the Ga3+ doping at the concentration y = 2.5, the LY of these scintillators notably (up to 20%) overcomes the LY of the Tb3Al5O12:Ce SCF sample (Table 1). This is caused mainly by the elimination of trap-related phenomena in Tb3Al5−yGayO12:Ce SCF scintillators due to the decrease of the Tb3Al5O12 band gap in the case of Ga alloying in the mentioned concentration range (see Part 3.3 for details).
In addition to the positive trend caused by the doping with Ga3+ ions, the significant increase of the LY is observed in Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs due to Gd3+ doping at the concentration x = 1–1.5 (Table 1). This effect can be caused by the deepest localization of the Ce3+ emitting levels inside the band gap and better separation of them with respect to the levels of conductive band. Namely, the Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) SCF possess excellent scintillation properties (Table 1 and Figure 7). The photoelectron LY of this SCF sample significantly overcomes the LY of the best samples of LuAG:Ce SCF [53], Lu1.5Gd1.5Al2.75Ga2.25O12:Ce SCF [31] and Tb3Al5O12:Ce SCF [21], grown from PbO based flux, up to 1.85, 2.62 and 1.95 times, respectively (see Table 1). This LY is the highest one ever obtained in our group for the garnet SCF scintillators grown by the LPE from traditional PbO-B2O3 based flux [21,31,36,38,53].
The elimination of Pb2+ contamination by using the lead-free BaO based flux in principle can result in the improvement of the LY and energy resolution of Tb3−xGdxAl5−yGayO12:Ce SCF scintillators (Table 1 and Figure 7). Indeed, the Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCF sample also possess excellent scintillation properties. Probably due to the elimination of the quenching and trap related phenomena caused by lead ions, the best LY and energy resolution in these SCF is observed at lower Ga concentration y = 2 in the Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCF sample than in the case of using the PbO based flux (Table 1). The LY of these SCFs also notably (up to 10%) exceeds the LY of the best Lu3−xGdxAl5−yGayO12:Ce (BaO) and Gd3Al5−yGaxO12:Ce (BaO) SCF scintillators [31] (Table 1). Without doubt, this is the highest LY for garnet SCF scintillators, grown by the LPE method from BaO based flux in our group [31]. Hoverer, the increase of the LY of Tb3−xGdxAl5−yGayO12:Ce SCFs due to the elimination of negative influence of Pb2+ flux related dopants is relatively small and is not so significant as in the case of producing Lu1−xGdxAl5−yGayO12:Ce and Gd3Al5−yGayO12:Ce SCFs scintillators from PbO and BaO based fluxes [31]. This can be caused by a more favourable situation in terms of the energy transfer phenomena via the sublattice of Tb3+ cations to Ce3+ ions in the case of Tb3−xGdxAl5−yGayO12:Ce (PbO) SCFs which enables the production of these scintillators with high LY and excellent structural quality from the traditional PbO based flux.
Thus, the main reason for such an increase of the LY of Tb3−xGdxAl5−yGayO12:Ce SCFs is the optimized cation content with respect to the crystal field strength and energy transfer efficiency to Ce3+ ions directly from the mixed garnet host and via the sublattice of Tb3+ and Gd3+ cations. Most probably, the relative position of Ce3+ levels with respect to the levels of Tb3+ and Gd3+ cations is optimal in Tb1.4–1.5Gd1.6–1.5Al3–2.5Ga2–2.5O12:Ce garnet hosts from the point of view of efficiency of the complicated Gd3+→Tb3+→Ce3+→Tb3+ energy transfer in these matrixes. The details of such a transfer are presented in the separate papers [34,35,49,50,51].
It is also important to note that the LY of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) and Tb1.5Gd1.5Al3 Ga2O12:Ce (BaO) SCFs is comparable with that in the high-quality Gd3Al2.5–2Ga2.5–3O12:Ce SC samples (Table 1 and Figure 7). This also enables producing the hybrid film-substrate detectors using the LPE method with high LY both in film and substrate scintillators with optimized contents, taking into account the requirement for the decay times of the signals coming from the film and substrate components of the hybrid scintillator differs by a factor of at least two [52]. Such a demand is fully realized in the case of Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCF/Gd3Al2.5Ga2.5O12:Ce SC hybrid scintillators (Figure 4b). These scintillators can be proposed for different types of application, especially for registration of the components of mixed ionizing flux [52] and multi-layer screens for visualization of X-ray images [21].

4. Conclusions

In this work, we report the creation of advanced single crystalline film (SCF) screens with excellent scintillation properties based on the Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce mixed garnet compounds grown by the LPE method from both novel lead free BaO and traditional PbO based fluxes onto Gd3Al2.5Ga2.5O12 (GAGG) substrates.
The optimization of Gd3+ and Ga3+ content in the Tb3−xGdxAl5−yGayO12:Ce garnet at x = 1.5 and y = 2–2.5 results in the strong improvement of the energy transfer efficiency from the Tb3+-Gd3+ based matrix to Ce3+ ions due to the modification of the band gap value and Ce3+ energy structure, as well as the elimination of the TSL peaks above room temperature. Namely, the Tb1.5Gd1.5Al3Ga2O12:Ce SCFs grown from BaO based flux under α-particle excitation possess the highest LY values among all the LPE grown garnet SCF scintillators obtained in our group, which exceeds by at least 10% the LY of the best samples of the recently developed Lu1.5Gd1.5Al2.75Ga2.25O12:Ce and Gd3Al2.75Ga2.25O12:Ce SCF scintillators grown from BaO based flux [31]. The photoelectron LY of these SCF scintillators under excitation by 239Pu (5.15 MeV) source is comparable with that in high-quality Gd3Al2.5−3Ga2.5−3O12:Ce reference bulk crystal analogue with a photoelectron LY of 1370 phels/MeV (light yield of about 10,000 photons/MeV). Tb1.5Gd1.5Al3Ga2O12:Ce SCFs also have a relatively fast scintillation response in the hundred ns range under α-particle excitation with decay times t1/e and t1/20 of 230 and 730 ns, respectively. Meanwhile, the structural uniformity and optical quality of these SCF scintillators are strongly influenced by the high-viscosity of BaO based melt.
The SCFs of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce garnets, grown from PbO based flux onto GAGG substrates, possess very high structural quality and excellent scintillation properties. Under α-particle excitation, the LY of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) SCFs is comparable with that of their analogues grown from BaO flux and these SCFs possess only slightly slower scintillation response with decay times t1/e and t1/20 of 330 and 990 ns, respectively. It is important to note that the negative quenching influence of the Pb2+ flux related dopants is not so significant during manufacturing the Tb1.5Gd1.5Al2.5Ga2.5O12:Ce SCF scintillators as in the case of Lu1−xGdxAl5−yGayO12:Ce and Gd3Al5−yGayO12:Ce SCFs analogues grown from PbO based fluxes [31]. This enables the production of Tb3−xGdxAl5−yGayO12:Ce SCF scintillators with high LY and excellent structural quality from the traditional PbO based flux. Most probably, this positive trend is observed only in the Tb containing scintillators, where very complicated but efficient energy transfer from Tb3+ and Gd3+ cation sub-lattices to Ce3+ ions can be realized in comparison with Lu- and Gd-containing scintillators where such transfer is absent.
We have also found that the scintillation decay of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) and especially Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCFs in the 0–2 μs range is notably faster (at least by 2 times) than that in Gd3Al2.5Ga2.5O12:Ce SC counterparts which can also be used as a substrate for producing SCF scintillators. In such a way these SCFs and the high-quality Gd3Al2.5Ga2.5O12:Ce crystals can be used for the creation of hybrid film-substrate scintillators using the LPE method for simultaneous registration of the different components of ionization fluxes. In such types of hybrid scintillators, the separation of the signal coming from the film and crystal parts can be performed using the differences in the scintillation decay kinetics.

Acknowledgments

The work was performed in the framework of NCN No 2016/21/B/ST8/03200 and NCBR NANOLUX2014 No 286 projects and also partly supported by the Ministry of Education and Science of Ukraine in the framework of SF-20 F projects.

Author Contributions

Vitalii Gorbenko performed the SCF growth experiments and wrote growth part of paper, Tetiana Zorenko performed the scintillation LY and decay kinetic measurements, Kazimierz Paprocki performed the CL spectra measurements; Alexander Fedorov performed the XRD investigations and analysis of SCF structural quality, Oleg Sidletskiy performed the GAGG substrates preparation; Sandra Witkiewicz collected and analyzed the SCF optical properties; Paweł Bilski and Anna Twardak performed the TSL measurements of the SCF samples and Yuriy Zorenko analyzed experimental materials and wrote the Introduction, Third part and Conclusion of the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Images of undoped Gd3Al2.5Ga2.5O12 (GAGG) substrate (left), Tb3Al2.5Ga2.5O12:Ce (PbO) (middle) and Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) (right) SCF scintillators, grown by the LPE method onto GAGG substrates.
Figure 1. Images of undoped Gd3Al2.5Ga2.5O12 (GAGG) substrate (left), Tb3Al2.5Ga2.5O12:Ce (PbO) (middle) and Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) (right) SCF scintillators, grown by the LPE method onto GAGG substrates.
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Figure 2. XRD patterns of (1200) planes of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs at y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4). The film/substrate lattice misfit m lies in the −1.3% < m < +0.5% range.
Figure 2. XRD patterns of (1200) planes of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs at y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4). The film/substrate lattice misfit m lies in the −1.3% < m < +0.5% range.
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Figure 3. Rocking curves of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs grown onto GGAG substrates at different y values: y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4) recorded in ω (a) and 2θ-ω (b) scanning modes.
Figure 3. Rocking curves of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs grown onto GGAG substrates at different y values: y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4) recorded in ω (a) and 2θ-ω (b) scanning modes.
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Figure 4. Normalized CL spectra at 300 K of Tb3Al5−yGayO12:Ce (PbO) (a), Tb3−xGdxAl5−yGayO12:Ce (PbO) (b) and Tb3−xGdxAl5−yGayO12:Ce (BaO) (c) SCFs with different x and y values (see legend of the figure) in comparison with CL spectra of Tb3Al5O12:Ce (PbO) SCF (1a) and Gd3Al2.5Ga2.5O12:Ce bulk SC (5b).
Figure 4. Normalized CL spectra at 300 K of Tb3Al5−yGayO12:Ce (PbO) (a), Tb3−xGdxAl5−yGayO12:Ce (PbO) (b) and Tb3−xGdxAl5−yGayO12:Ce (BaO) (c) SCFs with different x and y values (see legend of the figure) in comparison with CL spectra of Tb3Al5O12:Ce (PbO) SCF (1a) and Gd3Al2.5Ga2.5O12:Ce bulk SC (5b).
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Figure 5. (a) The normalized scintillation decay kinetics of Tb3−xGdxAl2.5Ga2.5O12:Ce (PbO) SCFs (curves 2, 3) with different Gd and Ga contents in comparison with the decay kinetics of the Tb3Al5O12:Ce (PbO) SCF counterpart (curve 1); (b) the normalized scintillation decay kinetics of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs (curves 1, 2) with different Ga content in comparison with the decay kinetics of Gd3Al2.5Ga2.5O12:Ce bulk SC (curve 3).
Figure 5. (a) The normalized scintillation decay kinetics of Tb3−xGdxAl2.5Ga2.5O12:Ce (PbO) SCFs (curves 2, 3) with different Gd and Ga contents in comparison with the decay kinetics of the Tb3Al5O12:Ce (PbO) SCF counterpart (curve 1); (b) the normalized scintillation decay kinetics of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs (curves 1, 2) with different Ga content in comparison with the decay kinetics of Gd3Al2.5Ga2.5O12:Ce bulk SC (curve 3).
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Figure 6. TSL (in the log scale) of Tb3−xGdxAl5−yGayO12:Ce (PbO) (a) with a different content of Gd3+ and Ga3+ cations (see legend of the figure) and Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (BaO) (b) SCFs after irradiation by Am241 α-particles and registration of the Ce3+ luminescence.
Figure 6. TSL (in the log scale) of Tb3−xGdxAl5−yGayO12:Ce (PbO) (a) with a different content of Gd3+ and Ga3+ cations (see legend of the figure) and Tb1.5Gd1.5Al3–2.5Ga2–2.5O12:Ce (BaO) (b) SCFs after irradiation by Am241 α-particles and registration of the Ce3+ luminescence.
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Figure 7. Pulse height spectra of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) (2) (LY = 1368 phels/MeV (380%); E = 20%) and Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) (3) (LY = 1370 phels/MeV (380%); E = 12.1%) in comparison with YAG:Ce SCFs (1) (LY = 360 phels/MeV (100%), E = 12.4%) and Gd3Al2.5Ga2.5O12:Ce SC (4) (LY = 1375 phels/MeV (381%); E = 6.9%) excited by α-particles of 239Pu (5.15 MeV) source and registered with a shaping time of 14 μs.
Figure 7. Pulse height spectra of Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) (2) (LY = 1368 phels/MeV (380%); E = 20%) and Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) (3) (LY = 1370 phels/MeV (380%); E = 12.1%) in comparison with YAG:Ce SCFs (1) (LY = 360 phels/MeV (100%), E = 12.4%) and Gd3Al2.5Ga2.5O12:Ce SC (4) (LY = 1375 phels/MeV (381%); E = 6.9%) excited by α-particles of 239Pu (5.15 MeV) source and registered with a shaping time of 14 μs.
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Table 1. Growth conditions, luminescent and scintillation properties of Tb3−xGdxAl5−yGayO12:Ce SCFs. M—SCF/substrate misfit, λmax—maximum of CL spectra, t1/e and t1/20 scintillation decay times to 1/e and 1/20 levels, respectively; LY—photoelectron light yield under α-particle excitation by 239Pu (5.15 MeV) source with respect to the standard YAG:Ce SCF with a photoelectron LY of 360 phels/MeV (light yield of 2650 photons/MeV) [38] and reference Gd3Al2–2.5Ga3–2.5O12:Ce bulk crystals with a photoelectron LY of 1300–1370 phels/MeV (light yield of about 10,100 photons/MeV).
Table 1. Growth conditions, luminescent and scintillation properties of Tb3−xGdxAl5−yGayO12:Ce SCFs. M—SCF/substrate misfit, λmax—maximum of CL spectra, t1/e and t1/20 scintillation decay times to 1/e and 1/20 levels, respectively; LY—photoelectron light yield under α-particle excitation by 239Pu (5.15 MeV) source with respect to the standard YAG:Ce SCF with a photoelectron LY of 360 phels/MeV (light yield of 2650 photons/MeV) [38] and reference Gd3Al2–2.5Ga3–2.5O12:Ce bulk crystals with a photoelectron LY of 1300–1370 phels/MeV (light yield of about 10,100 photons/MeV).
Content of SCF SamplesFluxSubstratem, %λmax, nmt1/e/t1/20, nsLY, %Reference
YAG:CePbOYAG-53567.3100[21,31]
LuAG:CePbOYAG−0.8250952.8205[21,31]
Tb1.5Gd1.5Al3Ga2O12:CeBaOGAGG−1.30553228/728380
Tb1.5Gd1.5Al2.5Ga2.5O12:CeBaOGAGG−0.79543201/893270
Tb1.5Gd1.5Al1.5Ga3.5O12:CeBaOGAGG+0.05543183/728160
Tb1.5Gd1.5Al1.15Ga3.85O12:CeBaOGAGG+0.5543103/86850
Tb3Al5O12:CePbOGAGG−1.29560306/1795195[21]
Tb3Al3Ga2O12:CePbOGAGG−0.49543435/1340200[36]
Tb3Al2.5Ga2.5O12:CePbOGAGG−0.37543456/1368235[36]
Tb2GdAl2.5Ga2.5O12:CePbOGAGG−0.20543291/883254[36]
Tb1.5Gd1.5Al2.5Ga2.5O12:CePbOGAGG−0.12543333/990380[36]
TbGd2Al2.5Ga2.5O12:CePbOGAGG−0.04543299/88160[36]
Gd3Al2.5Ga2.5O12:Ce SC---547441/1536381[21,31]
Gd3Al2Ga3O12:Ce SC---549240/876365[21,31]
Table 2. Segregation coefficients of different ions in Tb3−xGdxAl5−yGayO12:Ce SCFs grown onto GAGG substrates.
Table 2. Segregation coefficients of different ions in Tb3−xGdxAl5−yGayO12:Ce SCFs grown onto GAGG substrates.
Garnet ContentType of FluxSegregation Coefficient
Gd3+Ga3+Ce3+
Tb3Al52.9Ga02.1O12:CePbO 0.59–0.650.004–0.005
Tb20.9Gd12.1Al2.252.4Ga2.752.6O12:CePbO1.0–1.10.735–0.820.0095–0.02
Tb1.5Gd1.5Al31.15Ga23.85O12:CeBaO1.0–1.051.0–1.10.012–0.14
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