Single Crystalline Films of Ce3+-Doped Y3MgxSiyAl5−x−yO12 Garnets: Crystallization, Optical, and Photocurrent Properties

This research focuses on LPE growth, and the examination of the optical and photovoltaic properties of single crystalline film (SCF) phosphors based on Ce3+-doped Y3MgxSiyAl5−x−yO12 garnets with Mg and Si contents in x = 0–0.345 and y = 0–0.31 ranges. The absorbance, luminescence, scintillation, and photocurrent properties of Y3MgxSiyAl5−x−yO12:Ce SCFs were examined in comparison with Y3Al5O12:Ce (YAG:Ce) counterpart. Especially prepared YAG:Ce SCFs with a low (x, y < 0.1) concentration of Mg2+ and Mg2+–Si4+ codopants also showed a photocurrent that increased with rising Mg2+ and Si4+ concentrations. Mg2+ excess was systematically present in as-grown Y3MgxSiyAl5−x−yO12:Ce SCFs. The as-grown SCFs of these garnets under the excitation of α–particles had a low light yield (LY) and a fast scintillation response with a decay time in the ns range due to producing the Ce4+ ions as compensators for the Mg2+ excess. The Ce4+ dopant recharged to the Ce3+ state after SCF annealing at T > 1000 °C in a reducing atmosphere (95%N2 + 5%H2). Annealed SCF samples exhibited an LY of around 42% and similar scintillation decay kinetics to those of the YAG:Ce SCF counterpart. The photoluminescence studies of Y3MgxSiyAl5−x−yO12:Ce SCFs provide evidence for Ce3+ multicenter formation and the presence of an energy transfer between various Ce3+ multicenters. The Ce3+ multicenters possessed variable crystal field strengths in the nonequivalent dodecahedral sites of the garnet host due to the substitution of the octahedral positions by Mg2+ and the tetrahedral positions by Si4+. In comparison with YAG:Ce SCF, the Ce3+ luminescence spectra of Y3MgxSiyAl5−x−yO12:Ce SCFs greatly expanded in the red region. Using these beneficial trends of changes in the optical and photocurrent properties of Y3MgxSiyAl5−x−yO12:Ce garnets as a result of Mg2+ and Si4+ alloying, a new generation of SCF converters for white LEDs, photovoltaics, and scintillators could be developed.

Nonetheless, the impact of the simultaneous Mg 2+ -Si 4+ pair codoping on the optical characteristics of a single Ce 3+ -doped YAG crystal has not been investigated in detail. This is mainly connected with difficulties in the crystallization of a solid Y 3 Mg x Si y Al 5−x−y O 12 :Ce solution with conventional growth methods such as the Czochralski or micropulling down techniques. A good answer to this issue is the liquid-phase epitaxy (LPE) technique. The LPE method enables receiving a wide variety of optical materials in single crystalline film form with an extremely low concentration of host defects for the basic research of the optical properties of these materials, and the creation of various luminescent materials on their base for various applications such as laser media [19][20][21], scintillators [22][23][24][25], cathodoluminescent [26][27][28] and scintillating screens [29][30][31], thermoluminescent detectors [32][33][34], and WLED converters [35,36].
The first attempts to obtain Ce 3+ -doped Y 3−x Ca x Al 5−x Si x O 12 :Ce, Ca 3 Sc 2 Si 3 O 12 :Ce, and Ca 2 RSc 2 Si 3 O 12 :Ce ({R} = Y, Lu) garnets in SCF form using the LPE technique for the fabrication of optoelectronic components as blue LED converters or scintillators were presented in our previous works [37][38][39][40].
The optical and photovoltaic characteristics of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs with the values of x and y varying in the x = 0-0.345 and y = 0-0.31 ranges are examined in this study, along with new systematic results on growth. The LPE technique was used to produce the SCFs of these garnets onto undoped YAG substrates (see also [37][38][39][40]). Meanwhile, LPE growth methods may also be utilized to create the composite film-substrate epitaxial structures of these garnets for high-power WLED converters. Furthermore, considering that only phosphors based on YAG:Ce crystals or ceramics are available for producing high-power WLEDs under blue LED excitations, the development of this new type of phosphor is a very promising trend in solid-state lighting technology [2,3]. At the same time, we consider the rare-earth and transition metals doped of the SCFs of silicate garnet as potential raw materials for developing novel SCF cathodoluminescent screens, scintillators, and photovoltaic devices [7,8,[37][38][39][40].

Growth of Y 3 Mg x Si y Al 5−x−y O 12 :Ce Single Crystalline Films
Using the LPE technique, three sets of optically perfect Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCF samples with nominal equimolar Mg and Si contents in a melt solution equal to x, y = 1, 1.5, and 2 were grown onto YAG substrates with a (110) orientation from the supercooling melt-solution (MS) based on the PbO-B 2 O 3 flux (Table 1). Additionally, another set of samples with nominal Mg and Mg-Si contents in MS in the 0-0.1 range were grown using the LPE method in order to study the photocurrent characteristics of doped Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs ( Table 1). The initial components for LPE growth were PbO, B 2 O 3 , Y 2 O 3 , Al 2 O 3 , SiO 2 , and CeO 2 oxides of 4N purity. The SCF contents of the SCF samples were measured using an EDX detector with a SEM JEOL JSM-820 electron microscope. The measurements were performed at five different points of the samples, and the results are averaged. The contents of the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs under study and the reference YAG:Ce SCF sample are presented in Table 1.
The segregation coefficients of Mg 2+ and Si 4+ ions in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs were defined from the microanalytical measurement of the compositions of these SCF samples grown at the nominal Mg(x) and Si(y) contents of these cations in the corresponding MS ( Figure 1). Significant changes in the segregation coefficients of Mg and Si ions in the LPE growth of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs were caused by variations in the ratio of Mg/Si/Al cations in the MS (Figure 1). In particular, when the nominal Mg and Si contents in the MS increased in the x = 1-2 range, the segregation coefficients of the Mg 2+ and Si 4+ ions in as-grown films were nonlinearly changed in the 0.08-0.155 and 0.105-0.17 ranges, respectively. As a result, the real Mg and Si ion amounts in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCF samples were varied correspondingly in the x = 0.104-0.345 and y = 0.081-0.31 ranges, respectively. Following the change in SCF growth temperature, the segregation coefficient of Ce 3+ ions in the above-mentioned garnet hosts changed from 0.017 to 0.025. As a result, the Ce content in the SCF samples was only in the range of 0.175-0.225 at. % at the average Ce concentration in the MS of around 10 mole %. Y3MgxSiyAl5−x−yO12:Ce SCFs under study and the reference YAG:Ce SCF sample are presented in Table 1. The segregation coefficients of Mg 2+ and Si 4+ ions in Y3MgxSiyAl5−x−yO12:Ce SCFs were defined from the microanalytical measurement of the compositions of these SCF samples grown at the nominal Mg(x) and Si(y) contents of these cations in the corresponding MS ( Figure 1). Significant changes in the segregation coefficients of Mg and Si ions in the LPE growth of Y3MgxSiyAl5−x−yO12:Ce SCFs were caused by variations in the ratio of Mg/Si/Al cations in the MS (Figure 1). In particular, when the nominal Mg and Si contents in the MS increased in the x = 1-2 range, the segregation coefficients of the Mg 2+ and Si 4+ ions in as-grown films were nonlinearly changed in the 0.08-0.155 and 0.105-0.17 ranges, respectively. As a result, the real Mg and Si ion amounts in the Y3MgxSiyAl5−x−yO12:Ce SCF samples were varied correspondingly in the x = 0.104-0.345 and y = 0.081-0.31 ranges, respectively. Following the change in SCF growth temperature, the segregation coefficient of Ce 3+ ions in the above-mentioned garnet hosts changed from 0.017 to 0.025. As a result, the Ce content in the SCF samples was only in the range of 0.175-0.225 at. % at the average Ce concentration in the MS of around 10 mole %. The real concentrations of Mg and Si in SCFs are not equal, even at equimolar amounts of these ions in the MS, especially at the low contents of these dopants ( Table 1). The Mg 2+ concentration was systematically higher than the Si 4+ content, as Table 1 demonstrates. This indicates that, for the local charge compensation of Mg 2+ excess, various 4+ The real concentrations of Mg and Si in SCFs are not equal, even at equimolar amounts of these ions in the MS, especially at the low contents of these dopants ( Table 1). The Mg 2+ concentration was systematically higher than the Si 4+ content, as formed, for instance, Ce 4+ ions or Pb 4+ flux-related ions. The local charge compensation of Mg 2+ excess can also occur through the formation of O − centers or oxygen vacancies [41,41]. Regarding Y 3 Mg x Si y Al 5−x−y O 12 SCFs, we could predict the presence of both forms of charge compensation of Mg 2+ excess: the dominant creation of the oxygen vacancies or O 2− centers at relatively low Mg 2+ -Si 4+ contents, and the preferential formation of Ce 4+ and Pb 4+ states at relatively high Mg-Si amounts ( Figure 1).
The structural quality of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs with varying Mg and Si contents, grown using the LPE method onto YAG substrates with (110) orientation with a lattice constant of 11.9930Ȧ, was characterized using XRD measurements, performed using a modified DRON 4 spectrometer (Cu Kα radiation) ( Figure 2). The mismatch between the lattice constants of SCF and YAG substrates as ∆a = (a SCF -a sub )/a sub × 100% being equal to 0.245% was evaluated from the respective XRD patterns of the SCF sample grown from an MS with a nominal Y 3 Mg 2 Si 2 Al 3 O 12 :Ce composition and real Y 2.96 Ce 0.04 Mg 0.345 Si 0.31 Al 4.345 O 12 content ( Figure 2). Additionally, we estimated that the lattice constant of the mentioned garnet composition from the XRD pattern was equal to 12.0224Ȧ.  [41,41]. Regarding Y3MgxSiyAl5−x−yO12 SCFs, we could predict the presence of both forms of charge compensation of Mg 2+ excess: the dominant creation of the oxygen vacancies or O 2− centers at relatively low Mg 2+ -Si 4+ contents, and the preferential formation of Ce 4+ and Pb 4+ states at relatively high Mg-Si amounts ( Figure 1). The structural quality of Y3Mgx SiyAl5−x−yO12:Ce SCFs with varying Mg and Si contents, grown using the LPE method onto YAG substrates with (110) orientation with a lattice constant of 11.9930 Ȧ, was characterized using XRD measurements, performed using a modified DRON 4 spectrometer (CuKα radiation) ( Figure 2). The mismatch between the lattice constants of SCF and YAG substrates as Δa = (aSCF -asub)/asub × 100% being equal to 0.245% was evaluated from the respective XRD patterns of the SCF sample grown from an MS with a nominal Y3Mg2Si2Al3O12:Ce composition and real Y2.96Ce0.04Mg0.345Si0.31Al4.345O12 content (Figure 2). Additionally, we estimated that the lattice constant of the mentioned garnet composition from the XRD pattern was equal to 12.0224 Ȧ.

Experimental Methods and Technique
The absorption (  Table 2) were recorded to characterize the optical and luminescence properties of Y3Mgx SiyAl5−x−yO12:Ce SCFs. We also measured the scintillation decay kinetics and photoelectron light yield (LY) of these SCF samples under excitation with α-particles from a 239 Pu (5.15 MeV) source (Table 3 and Figure 9). The photocurrent (PC) excitation spectra of especially prepared Y3Mgx SiyAl5−x−yO12:Ce SCFs (Samples 5-8 in Table 1)) with reduced nominal Mg 2+ and Mg 2+ -Si 4+ contents in the x, y = 0-0.1 range were investigated on a custom setup consisting of a 150 W xenon lamp (LOT Quantum Design) coupled to a grating monochromator (Omni-λ 1509) operating in the 250-1000 nm spectral range as a source of excitation; a digital electrometer (Keysight B2987A) for photocurrent measurement; an optical chopper at 5 Hz to modulate the excitation light to increase the signal-to-noise ratio; a lock-in amplifier (Signal Recovery 7270, Ametek Scientific Instruments) to extract the photocurrent signal.

Experimental Methods and Technique
The absorption (  (Figure 6) spectra, and the PL decay kinetics ( Figure 8 and Table 2) were recorded to characterize the optical and luminescence properties of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs. We also measured the scintillation decay kinetics and photoelectron light yield (LY) of these SCF samples under excitation with α-particles from a 239 Pu (5.15 MeV) source (Table 3 and Figure 9). The photocurrent (PC) excitation spectra of especially prepared Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs (Samples 5-8 in Table 1)) with reduced nominal Mg 2+ and Mg 2+ -Si 4+ contents in the x, y = 0-0.1 range were investigated on a custom setup consisting of a 150 W xenon lamp (LOT Quantum Design) coupled to a grating monochromator (Omni-λ 1509) operating in the 250-1000 nm spectral range as a source of excitation; a digital electrometer (Keysight B2987A) for photocurrent measurement; an optical chopper at 5 Hz to modulate the excitation light to increase the signal-to-noise ratio; a lock-in amplifier (Signal Recovery 7270, Ametek Scientific Instruments) to extract the photocurrent signal.      Even at the significantly large content of Mg 2+ ions in the 0.31-0.345 range, the bands that peaked at 275 and around 375 nm in the excitation spectra, related to the intrinsic transitions of F + centers [40,50], were not found in the excitation spectra of the Ce 3+ lumi nescence in the Y3MgxSiyAl5−x−yO12:Ce SCFs ( Figure 6). Such results contradict the result of Y3−xCaxSiyAl5−yO12:Ce SCFs, where the creation of F + centers was observed for the com pensation of the excess of divalent Ca 2+ ions [40]. However, the results fo Y3MgxSiyAl5−x−yO12:Ce SCFs correlate well with the investigation results of the Gd3Al5−xGaxO12:Ce,Mg crystal [41], where the emission of F + was also not found. Therefore the excess of Mg 2+ ions in the Y3MgxSiyAl5−x−yO12:Ce SCFs was compensated by other mech anisms that are probably connected with the creation of Ce 4+ states or/and O 2− -Mg 2+ pai centers [41].   [37,38,40,43,44]. As the x and y values rose, the corresponding decay curves became faster and more nonexponential. Due to this fact, the decay curves may have been extrapolated by the three components, each with a decay time value t at intensity decay levels of 1/e, 0.1, and 0.001 (Figure 7). Table 2 lists the corresponding decay times of τ1/e, τ1/10 and τ1/100.  A SEM JEOL JSM-820 electron microscope with a Stellar Net grating spectrometer operating in the 200-1120 nm spectral range was used to measure the CL spectra. An Edinburgh FS5 spectrometer was used to study the PL emission and excitation spectra, and PL decay kinetics of the SCF samples. Using a Hamamatsu H6521 PMP, multichannel analyzer, and digital TDS3052 oscilloscope setup, the scintillation LY with a shaping time of 12 s and decay kinetics were measured under excitation with α-particles of 239 Pu (5.15 MeV)   [42][43][44][45][46][47][48][49]. This suggests that the nature of this band may be related to the O 2− → Ce 4+ charge transfer transitions (CTT) [42][43][44][45][46][47][48][49]. Indeed, the Ce 3+ and Ce 4+ valence states coexist in the asgrown Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs, as confirmed by the existence of the respective absorption bands of these centers, as shown in Figure 3. The relative concentrations of these centers were highly influenced by Mg/Si/Ce contents and SCF crystallization conditions. Specifically, the intensity of the Ce 3+ E 1 band decreased with increasing Mg 2+ and Si 4+ contents in the x = 0.104-0.345 and y = 0.081-0.31 ranges, probably as a result of the Ce 3+ → Ce 4+ recharge. The beginning of the O 2− → Ce 4+ CTT in the studied SCFs could even be shifted to 400 nm, causing a large overlap of the E 2 absorption bands of the Ce 3+ ions.
In addition to Ce 3+ -associated bands, the absorption spectra of the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs grown from PbO-based flux had bands peaking at 260-263 nm. These bands correspond to the 1 S 0 → 3 P 1 transitions of Pb 2+ ions as the main flux pollution in the SCFs grown from PbO-based flux [21]. The comparable band in the YAG:Ce SCF analogue peaked at 263 nm ( Figure 3, Curve 1).

Cathodoluminescence Spectra
The normalized CL spectra of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCF samples at RT with various Mg/Si contents are presented in Figure 4

Photoluminescence Spectra
Under stimulation in the vicinity of the E 1 Ce 3+ absorption band at 445 nm, the wide PL band in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs peaked at 550 nm for the Y 2.95 Ce 0.05 Al 5 O 12 and at approximately 536 nm for other SCFs with varied Mg/Si contents. This PL band corresponded to the radiative 5d 1 → 4f( 2 F 5/2,7/2 ) transitions of Ce 3+ ions ( Figure 5). The position of the PL emission bands and their FHWM in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs under 450 nm excitation, however, reveal a more complex dependency on the x and y values than that indicated by the CL spectra (Figure 4). In particular, the PL spectra of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs had a considerable blue shift relative to the spectra of YAG:Ce SCF, which was only 7-8 nm ( Figure 5). Furthermore, the PL spectra of these SCFs were notably narrower than the YAG:Ce counterpart. Namely (Figure 4). This further indicates the complicated nature of the Ce 3+ center formation in these garnets and the influence of some variables on the process.
Two E 1 and E 2 bands with peaks at 460 nm and in the 340-343 nm range, respectively, were observed in the excitation spectra of the Ce 3+ luminescence in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs. These bands were associated with the 4f( 2 F 5/2 ) → 5d 1,2 transitions of Ce 3+ ions in these garnets ( Figure 6). Namely, for Even at the significantly large content of Mg 2+ ions in the 0.31-0.345 range, the bands that peaked at 275 and around 375 nm in the excitation spectra, related to the intrinsic transitions of F + centers [40,50], were not found in the excitation spectra of the Ce 3+ luminescence in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs ( Figure 6). Such results contradict the results of Y 3−x Ca x Si y Al 5−y O 12 :Ce SCFs, where the creation of F + centers was observed for the compensation of the excess of divalent Ca 2+ ions [40]. However, the results for Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs correlate well with the investigation results of the Gd 3 Al 5−x Ga x O 12 :Ce,Mg crystal [41], where the emission of F + was also not found. Therefore, the excess of Mg 2+ ions in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs was compensated by other mechanisms that are probably connected with the creation of Ce 4+ states or/and O 2− -Mg 2+ pair centers [41]. Figure 7 shows the decay kinetics of the Ce 3+ ion emission in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs with various Mg and Si contents under excitation at 340 nm near the E 2 Ce 3+ ion absorption bands compared to the YAG:Ce SCF counterpart. The decay kinetics of the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs (Figure 7, Curves 2-4) was significantly nonexponential in contrast to the YAG:Ce SCF (Figure 7, Curve 1) and similar to that of other A 2+ -Si 4+ (A = Ca, Mg)-based garnets [37,38,40,43,44]. As the x and y values rose, the corresponding decay curves became faster and more nonexponential. Due to this fact, the decay curves may have been extrapolated by the three components, each with a decay time value t at intensity decay levels of 1/e, 0.1, and 0.001 (Figure 7). Table 2 lists the corresponding decay times of τ 1/e , τ 1/10 , and τ 1/100 .
Similarly to the results in [37,38,40,43,44], we assumed that the formation of Ce 4+ valence states was the primary cause of the nonexponential decay kinetics of the Ce 3+ luminescence in the as-grown Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs. The intervalence charge transfer (IVCT) transitions that cause quick nonradiative decay channels could also impact the acceleration of the Ce 3+ decay in the presence of Ce 4+ ions [51][52][53]. Recent studies described this effect for Ce 3+ /Ce 4+ couples in garnets and sulfides [52,53]. Additionally, we recently demonstrated that Ce 4+ ions, which serve as highly efficient electron trapping centers, may significantly accelerate the decay kinetics of Ce 3+ luminescence under excitation with the energies in the vicinity of O 2− → Ce 4+ CTTs [38][39][40][41][42][43][44]. The initiation of the O 2− → Ce 4+ transitions is also possible under 340 nm excitation in the E 2 Ce 3+ absorption band in garnets due to the substantial FWHM value of the mentioned CTT bands [42][43][44]. The charge transfer of Ce 4+ into the Ce 3+ state and the subsequent reverse transformation of Ce 3+ into Ce 4+ ions allowed for us to observe the luminescence of Ce 3+ ions under 340 nm excitation [38][39][40][41][42][43][44].
cence at 530 nm in Y3MgxSixAl5−x−yO12:Ce SCFs with varying Mg and Si contents (Curves 2-4) compared to the respective PLE spectra in the YAG:Ce SCF (Curve 1). Figure 7 shows the decay kinetics of the Ce 3+ ion emission in the Y3MgxSiyAl5−x−yO12:Ce SCFs with various Mg and Si contents under excitation at 340 nm near the E2 Ce 3+ ion absorption bands compared to the YAG:Ce SCF counterpart. The decay kinetics of the Y3MgxSiyAl5−x−yO12:Ce SCFs (Figure 7, Curves 2-4) was significantly nonexponential in contrast to the YAG:Ce SCF (Figure 7, Curve 1) and similar to that of other A 2+ -Si 4+ (A = Ca, Mg)-based garnets [37,38,40,43,44]. As the x and y values rose, the corresponding decay curves became faster and more nonexponential. Due to this fact, the decay curves may have been extrapolated by the three components, each with a decay time value t at intensity decay levels of 1/e, 0.1, and 0.001 (Figure 7). Table 2 lists the corresponding decay times of τ1/e, τ1/10, and τ1/100. The existence of the fast component of the Ce 3+ luminescence in the ns range was interesting, and the nonexponential shape of the decay curves in the garnet compounds containing Ca-Mg-Si ions could have been connected to the formation of Ce 3+ multicenters [38][39][40]. The energy transfer processes between different Ce 3+ emitting centers could correspond to such a nonexponential form of the decay curves [38,39]. Nevertheless, the presence of Ce 4+ centers in the as-grown SCFs substantially masked the contribution of the above-mentioned energy transfer mechanisms to the nonexponential PL decay kinetics of the Ce 3+ luminescence. Consequently, it was only possible to analyze the impact of the energy transfer mechanisms between Ce 3+ multicenters after the elimination of Ce 4+ centers by using the thermal treatment of SCFs in a reducing atmosphere [38].

Scintillation Properties Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs
Because the majority of Ce 3+ ions in the as-grown samples had been recharged to the Ce 4+ state, Mg-Si-codoped SCFs had a low scintillation efficiency. Namely, the asgrown Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs showed a significantly reduced scintillation LY in comparison with that of the YAG:Ce SCF reference sample, which had a LY of 2600 photons/MeV under α-particle excitation with a 239 Pu (5.15 MeV) source (Table 3). In general, such scintillation properties of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs are similar to those of Y 3−x Ca x Al 5−y Si y O 12 :Ce [40] and Ca 2 YMgScSi 3 O 12 :Ce [37][38][39] SCFs, as well as (Lu,Y) 2 SiO 5 :Ce SCFs [54], where the predominant Ce 4+ valence state of cerium ions in the SCFs, grown from the PbO-based flux, causes their poor-scintillation light output. Figure 8 and Table 3 show the scintillation decay kinetics of the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs depending on Mg-Si contents. When Mg-Si concentrations increased, the scintillation response of these SCFs notably accelerate. Namely, for SCF Samples 3 and 4 with Mg/Si contents x/y = 0.186/0.141 and 0.345/0.31, respectively, the corresponding decay times were equal to t 1/e = 43 and 35 ns; and t 1/10 = 142 ns and 116 ns, respectively, in comparison with t 1/e = 68.5 ns and t 1/10 = 194 ns for the YAG:Ce SCF (Table 3). Additionally, this effect was well-correlated with the considerable drop in the LY of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs when the Mg/Si content increased ( Table 2).

Photocurrent Properties of Y3MgxSiyAl5−x−yO12:Ce SCFs
The photocurrent (PC) excitation spectra of special set of Y3MgxSiyAl5−x−yO12:Ce SCFs with reduced Mg and Si contents between 0.025 and 0.1 for x and y values are presented in Figure 9. Such reduced amounts of codopants (less than 0.05 at %), substituting the octa-and tetrahedral sites of garnet host, were used to ensure the isolated nature of the substitutional defects. In this way, the photoconductive behavior of the doped crystal was in the isolated donor/acceptor regime, analogously to doped semiconductors. The excessive concentration of donor/acceptor states may lead to the formation of quasiband states, and impairs the photoconductive response of the SCF system.
Since the photocurrent signal of the wide band-gap oxides is extremely weak, the excitation measurements were performed via the modulated light technique and extracted using a lock-in amplifier. For this reason, the absolute value of the photocurrent intensity could not be retained, but it was estimated to be in the 0.1-1 pA range.
The Mg-Si-free YAG:Ce SCF did not show any visible PC under excitation in the 250-600 nm range (Figure 9, Curve 1). However, the single Mg 2+ and double Mg 2+ -Si 4+ codoping of YAG:Ce SCF led to the appearance of PCs, and the value of such PCs increases with increasing Mg 2+ and Si 4+ concentrations in the films. The maxima of the complex PC excitation band in Y3MgxSiyAl5−x−yO12:Ce SCFs were observed in the 345-365 range (Figure 9). Such complex bands consist of at least two low-energy and high-energy sub-bands. Interestingly, the maximum of the complex PC band was slightly shifted from 360 to 345 nm at Mg and Mg-Si concentration x = 0.025, and later shifted to 358 and 365 nm when Mg-Si content increased to x = 0.05 and 0.1, respectively ( Figure 9).

Photocurrent Properties of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs
The photocurrent (PC) excitation spectra of special set of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs with reduced Mg and Si contents between 0.025 and 0.1 for x and y values are presented in Figure 9. Such reduced amounts of codopants (less than 0.05 at %), substituting the octa-and tetrahedral sites of garnet host, were used to ensure the isolated nature of the substitutional defects. In this way, the photoconductive behavior of the doped crystal was in the isolated donor/acceptor regime, analogously to doped semiconductors. The excessive concentration of donor/acceptor states may lead to the formation of quasiband states, and impairs the photoconductive response of the SCF system.
Since the photocurrent signal of the wide band-gap oxides is extremely weak, the excitation measurements were performed via the modulated light technique and extracted using a lock-in amplifier. For this reason, the absolute value of the photocurrent intensity could not be retained, but it was estimated to be in the 0.1-1 pA range.
The Mg-Si-free YAG:Ce SCF did not show any visible PC under excitation in the 250-600 nm range (Figure 9, Curve 1). However, the single Mg 2+ and double Mg 2+ -Si 4+ codoping of YAG:Ce SCF led to the appearance of PCs, and the value of such PCs increases with increasing Mg 2+ and Si 4+ concentrations in the films. The maxima of the complex PC excitation band in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs were observed in the 345-365 range (Figure 9). Such complex bands consist of at least two low-energy and high-energy subbands. Interestingly, the maximum of the complex PC band was slightly shifted from 360 to 345 nm at Mg and Mg-Si concentration x = 0.025, and later shifted to 358 and 365 nm when Mg-Si content increased to x = 0.05 and 0.1, respectively (Figure 9).
Taking into account the advanced Mg 2+ concentrations in the SCFs under study with respect to the content of Si 4+ ions and partial compensation of such Mg 2+ advance by Ce 4+ formation (Table 1), and the absence of F + and F-related centers formation in these samples, the observed bands in the PC excitation spectra could probably be connected with the creation of Mg 2+ -Ce 4+ and Mg 2+ -O 2− centers with local and charge compensation. The above-mentioned shift in the maxima of PC bands could have been caused by the relative concertation of the mentioned pair centers at different contents of Mg and Si ions.
ing of YAG:Ce SCF led to the appearance of PCs, and the value of such PCs increases with increasing Mg 2+ and Si 4+ concentrations in the films. The maxima of the complex PC excitation band in Y3MgxSiyAl5−x−yO12:Ce SCFs were observed in the 345-365 range (Figure 9). Such complex bands consist of at least two low-energy and high-energy sub-bands. Interestingly, the maximum of the complex PC band was slightly shifted from 360 to 345 nm at Mg and Mg-Si concentration x = 0.025, and later shifted to 358 and 365 nm when Mg-Si content increased to x = 0.05 and 0.1, respectively (Figure 9).  Table 1).
The mechanism of charge and volume compensation of the Mg 2+ excess in Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs was different than that in LPE-grown Y 3−x Ca x Al 5−y Si y O 12 :Ce SCFs (see [40] for details), where the observed Ca 2+ advance in the SCF samples was compensated with the Ce 4+ and F + center formation.

Optical Properties of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs Annealing in Reducing Atmosphere
The optical characteristics of Sample 2 (Y 2.965 Ce 0.035 Mg 0.104 Si 0.081 Al 4.815 O 12 SCF) were also investigated after 12 h of thermal treatment (TT) at 1000-1300 • C in a 95% N 2 -5% H 2 reducing atmosphere (Figures 10-12). The relative concentrations of the Ce 4+ and Ce 3+ centers in the above-mentioned SCF sample were significantly altered by the TT in such a reducing atmosphere as a result of the O 2− + 2 Ce 4+ → V O + 2 Ce 3+ reaction, where Vo is the oxygen vacancy, as can be seen from the absorption spectra of the as-grown and annealed samples of this film in Figure 10.  Table 1).
Taking into account the advanced Mg 2+ concentrations in the SCFs under study with respect to the content of Si 4+ ions and partial compensation of such Mg 2+ advance by Ce 4+ formation (Table 1), and the absence of F + and F-related centers formation in these samples, the observed bands in the PC excitation spectra could probably be connected with the creation of Mg 2+ -Ce 4+ and Mg 2+ -O 2− centers with local and charge compensation. The above-mentioned shift in the maxima of PC bands could have been caused by the relative concertation of the mentioned pair centers at different contents of Mg and Si ions.
The mechanism of charge and volume compensation of the Mg 2+ excess in Y3MgxSiyAl5−x−yO12:Ce SCFs was different than that in LPE-grown Y3−xCaxAl5−y SiyO12:Ce SCFs (see [40] for details), where the observed Ca 2+ advance in the SCF samples was compensated with the Ce 4+ and F + center formation.

Optical Properties of Y3MgxSiyAl5−x−yO12:Ce SCFs Annealing in Reducing Atmosphere
The optical characteristics of Sample 2 (Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF) were also investigated after 12 h of thermal treatment (TT) at 1000-1300 °C in a 95% N2-5% H2 reducing atmosphere (Figures 10-12). The relative concentrations of the Ce 4+ and Ce 3+ centers in the above-mentioned SCF sample were significantly altered by the TT in such a reducing atmosphere as a result of the O 2− + 2 Ce 4+ → VO + 2 Ce 3+ reaction, where Vo is the oxygen vacancy, as can be seen from the absorption spectra of the as-grown and annealed samples of this film in Figure 10.  Figure 10. Influence of thermal treatment at 1000 and 1300 °C in a 95% N2-5% H2 atmosphere on the absorption spectra of the as-grown Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF sample (curve 1). Curves 4 and 5 represent the difference in the spectra of the untreated and annealed samples at 1000 °C (Curve 4) and 1300 °C (Curve 5).
The structure of the emission and excitation bands connected to various Ce 3+ centers noticeably change because of annealing of this SCF sample in the reducing atmosphere. In particular, the maximum of the Ce 3+ emission band was located at 532 nm in the untreated Y2.965Ce0.035Mg0.104 Si0.081Al4.815O12 SCF, and excited in the bands that peaked at 382 and 456 nm (Figure 11, Curve 1). We assumed that the above-described bands may have been connected with the Ce1 center. The difference between the locations of the E1 and E2 excitation bands for such a Ce1 center could be equal to 0.526 eV. The Stokes shift was proportional to the difference between the emission and low-energy excitation bands, and for the Ce1 center, it was equivalent to 76 nm (0.388 eV).  Figure 11. Influence of thermal treatment on the emission spectra (Curves 1-3) and excitation spectra (Curves 1′-3′) of Ce 3+ luminescence in the Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF in an N2 95% + H2 5% atmosphere at 1300 °C.
However, as a result of the TT of the Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF at 1300 °C, the maximum of the Ce 3+ emission spectrum was noticeably shifted to 560 nm, the intensity of the excitation band peaked at 382 nm significantly decreases, and a new excitation band appears with a maximum at 339 nm. The mentioned shift in emission and excitation spectra can be attributed to an increase in the relative concentration of Ce2 centers in the SCF sample after TT. The difference between the locations of the E1 and E2 excitation bands for such a Ce2 center was equal to 0.972 eV. Therefore, due to the larger crystal field strength of Ce2 centers than that of Ce1 centers, the position of the emission band of the Ce2 center was red-shifted relative to the Ce1 center. The difference between the positions of the emission and low-energy excitation bands was proportional to the Stokes shift and equal to 98 nm (0.469 eV) for the Ce2 center.    Figure 11. Influence of thermal treatment on the emission spectra (Curves 1-3) and excitation spectra (Curves 1′-3′) of Ce 3+ luminescence in the Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF in an N2 95% + H2 5% atmosphere at 1300 °C.
However, as a result of the TT of the Y2.965Ce0.035Mg0.104Si0.081Al4.815O12 SCF at 1300 °C, the maximum of the Ce 3+ emission spectrum was noticeably shifted to 560 nm, the intensity of the excitation band peaked at 382 nm significantly decreases, and a new excitation band appears with a maximum at 339 nm. The mentioned shift in emission and excitation spectra can be attributed to an increase in the relative concentration of Ce2 centers in the SCF sample after TT. The difference between the locations of the E1 and E2 excitation bands for such a Ce2 center was equal to 0.972 eV. Therefore, due to the larger crystal field strength of Ce2 centers than that of Ce1 centers, the position of the emission band of the Ce2 center was red-shifted relative to the Ce1 center. The difference between the positions of the emission and low-energy excitation bands was proportional to the Stokes shift and equal to 98 nm (0.469 eV) for the Ce2 center.  The structure of the emission and excitation bands connected to various Ce 3+ centers noticeably change because of annealing of this SCF sample in the reducing atmosphere. In particular, the maximum of the Ce 3+ emission band was located at 532 nm in the untreated Y 2.965 Ce 0.035 Mg 0.104 Si 0.081 Al 4.815 O 12 SCF, and excited in the bands that peaked at 382 and 456 nm (Figure 11, Curve 1). We assumed that the above-described bands may have been connected with the Ce1 center. The difference between the locations of the E 1 and E 2 excitation bands for such a Ce1 center could be equal to 0.526 eV. The Stokes shift was proportional to the difference between the emission and low-energy excitation bands, and for the Ce1 center, it was equivalent to 76 nm (0.388 eV).
However, as a result of the TT of the Y 2.965 Ce 0.035 Mg 0.104 Si 0.081 Al 4.815 O 12 SCF at 1300 • C, the maximum of the Ce 3+ emission spectrum was noticeably shifted to 560 nm, the intensity of the excitation band peaked at 382 nm significantly decreases, and a new excitation band appears with a maximum at 339 nm. The mentioned shift in emission and excitation spectra can be attributed to an increase in the relative concentration of Ce2 centers in the SCF sample after TT. The difference between the locations of the E1 and E2 excitation bands for such a Ce2 center was equal to 0.972 eV. Therefore, due to the larger crystal field strength of Ce2 centers than that of Ce1 centers, the position of the emission band of the Ce2 center was red-shifted relative to the Ce1 center. The difference between the positions of the emission and low-energy excitation bands was proportional to the Stokes shift and equal to 98 nm (0.469 eV) for the Ce2 center.
Lastly, we could assume that the Ce1 and Ce2 centers had been formed when Ce 3+ ions replaced Y 3+ cations with different local environment caused by the nonuniform distribution of the Mg 2+ and Si 4+ cations in the octahedral and tetrahedral position of the garnet host. This assumption about the nature of the Ce1 and Ce2 centers was supported by the corresponding changes in the absorption and PL excitation spectra, and by the PL emission spectra and decay kinetics of PL ( Figure 12, Table 4) in the Y 2.965 Ce 0.035 Mg 0.104 Si 0.081 Al 4.815 O 12 SCF, which were related to the change in the concentration of the Ce 4+ and Ce 3+ centers after reducing TT in the 1000-1300 • C range. Since Ce2 centers in the as-grown samples had the predominant Ce 4+ valence state, it was difficult to record these centers in the PL emission and excitation spectra, and decay kinetics of the Ce 3+ luminescence (Figures 3-7). However, when the Ce 4+ ions recharged to the Ce 3+ states during the TT at temperatures between 1000 and 1300 • C, it was also possible to observe the Ce2 centers in the PL spectra and the decay kinetics of the Ce 3+ emission (Figures 10-12). Table 4. Parameters of three exponential approximations of the decay curves presented in Figure 12. The accuracy of the decay time parameter determination was about ±5%.

Conclusions
The single crystalline films (SCFs) of Y 3 Mg x Si y Al 5−x−y O 12 :Ce garnet at x and y changing from 0 to 0.345 and 0.31, respectively, were crystallized using the LPE growth method from a melt solution based on the PbO-B 2 O 3 flux onto Y 3 Al 5 O 12 (YAG) substrates at the SCF-substrate misfit from 0 up to 0.245%. The segregation coefficients of Mg and Si ions in these SCFs were varied in the 0.08-0.155 and 0.105-0.17 ranges, respectively, when the nominal concentration of these dopants in the melt solution was changed in the x, y = 0-2 range. Additionally, Mg 2+ excess was systematically present in the as-grown Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs, which was presumable compensated by the Ce 4+ ion and Mg 2+ -O 2− center formation. Especially prepared Mg 2+ /Mg 2+ -Si 4+ codoped YAG:Ce SCFs with low concentrations of manganese and silicon ions also demonstrated the appearance of a photocurrent that increased with rising Mg 2+ and Si 4+ contents in the films.
The absorption and luminescence properties of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs were studied and compared with the properties of the reference YAG:Ce SCF sample. As a result of the Mg 2+ -Si 4+ pair codoping, the cathodoluminescence spectra of Ce 3+ ions in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs were noticeably extended in the red range compared to those of the YAG:Ce SCFs due to the Ce 3+ multicenter formation in the dodecahedral sites of the lattice of the mentioned mixed garnets. Furthermore, we confirmed the formation of two types of Ce 3+ centers of Y 3 Mg x Si y Al 5−x−y O 12 :Ce in the emission and excitation spectra of the Ce 3+ photoluminescence in the SCFs of these garnets. These two centers (Ce1 and Ce2) possessed various local surroundings due to replacement with the Mg 2+ and Si 4+ ions of Al 3+ cations in the octahedral and tetrahedral sites of the garnet host and were characterized by differing spectral behaviors.
The as-grown Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCF samples exhibited poor scintillation properties. Under α-particle excitation through the 239 Pu (5.15 MeV) source, these SCFs had a fast scintillation response with decay times in the t 1/e = 30-43.5 ns and t 1/20 = 79-148 ns ranges, but a relative low light yield (LY) of 14-19% in comparison with the reference YAG:Ce SCF. However, the LY of Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs could increase after their annealing in a reducing atmosphere (95% N 2 + 5% H 2 ) at a temperature above the SCF growth temperature.
The simultaneous formation of the Ce 4+ and Ce 3+ valence states was also observed in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs due to the nonuniform distribution of the Mg 2+ and Si 4+ cations and charge compensation requirement. The presence of Ce 4+ ions in the as-grown SCFs was confirmed via the presence of the O 2− → Ce 4+ absorption band that peaked at 247 nm. The Ce 4+ centers were also responsible for the acceleration of the initial stage of the cerium photoluminescence decay profiles, and for the presence of fast components with a lifetime in the range of a few ns in these SCFs. The annealing of the samples in the reducing atmosphere at temperatures over 1000 • C resulted in the Ce 4+ → Ce 3+ recharge in the Y 3 Mg x Si y Al 5−x−y O 12 :Ce SCFs, and also led to the more exponential-like decay kinetics of the Ce 3+ luminescence in these SCFs. This allows for studying the energy transfer processes between different Ce 3+ centers in this garnet.
Author Contributions: V.G., SCF growth of and writing Section 2; T.Z., absorption and scintillation property measurements of the films; A.S., measurements of the PL decay kinetics of the samples; P.P., analysis of the photocurrent results and paper preparation; A.O. and M.B., influence analysis of the thermal treatment on the optical properties of films; A.F. XRD measurements; S.M., T.L. and N.M., film photovoltaic property investigation; Y.Z., conception of the main paper idea, analysis of the whole experimental materials, and paper writing and correction. All authors have read and agreed to the published version of the manuscript.