Phosphors based on Ce:(Pb,Gd)3(Al,Ga)5O12 epitaxial films: synthesis, optical properties, application

Ce-doped (Pb, Gd)3(Al, Ga)5O12 single crystalline garnet films were grown using liquid-phase epitaxy method from supercooled PbO–B2O3-based melt solutions on substrates from Gd3Ga5O12 and Gd3Al2.26Ga2.74O12 single crystals. Optical absorption and photo- and cathodoluminescent properties of these epitaxial garnet films were studied. Ce-doped (Pb, Gd)3(Al, Ga)5O12 garnet films can be used as a fast phosphor in the design of a PIF-01 electron-optical converter.


Introduction
Gadolinium -aluminum -gallium garnet doped with cerium ions (Ce:Gd3Al2Ga3O12, Ce:GAGG), is considered as a promising scintillation material among garnet crystals. Currently, 2-in. size Ce 1%:Gd3Al2.3Ga2.7O12 single crystal grown by Czochralski method shows a scintillation light yield approximately 58,000 photons/MeV [1]. The scintillation decay time (and its relative intensity) was 172 ns (88%) for the fast component in this crystal. The emission wavelength was 516 nm. Along with bulk single crystals, single crystalline films find their application as scintillation screens [2,3]. Films can be grown by liquid-phase epitaxy (LPE) from supercooled melt solutions such as PbO -B2O3, Bi2O3 -B2O3 or BaO -B2O3 -BaF2. The difference between epitaxial films and their bulk single crystal analogs is the presence of additional optical absorption, which is caused by impurity ions, which are introduced into the films from the melt solution and crucible material. For instance, the impurity ions in the epitaxial films grown from supercooled PbO-B2O3-based melt solutions are Pb 2+ , Pb 4+ , Pt 4+ ions.
Cerium ions are the most common activator for scintillation detectors. The emission of Ce 3+ arises due to an allowed 5d-4f electronic transition. As a result Ce doped scintillators are characterized by a high scintillation light yield and a fast decay time (tens of ns). The luminescence of the Ce 3+ ion in the Gd3Ga5O12 crystal at room temperature and atmospheric pressure is completely quenched due to the location of the 5d1 level of Ce 3+ in the conduction band [4,5], but under high pressure [6] or with the partial substitution of Ga with Al in the Gd3Ga5O12 host Ce 3+ emission can be observed [7]. In this work we report the results of the study of the influence of Ga/Al ratio variation on optical absorption, photo-and cathodoluminescent properties of epitaxial films Ce:(Pb,Gd)3(Al,Ga)5O12, grown by LPE. Our main goal is to obtain samples with a maximum photoluminescence intensity of Ce 3+ ions and to investigate the applicability of epitaxial films as a fast phosphor in the design of a PIF-01 electron-optical converter.

Experimental methods
The epitaxial films Ce:(Pb,Gd)3(Al,Ga)5O12, were grown with standard isothermal LPE method. A schematic drawing of the set-up is given in the Figure 1. Before the growth of the films, the melt solution was homogenized in the platinum crucible for at least four hours. The films were characterized by X-ray diffraction on a Bruker D8 Discover A25 Da Vinsi Design X-ray diffractometer (CuKα radiation). The quantitative chemical analysis of the grown films was performed with an electron-ion scanning microscope Quanta 3 D FEG. To simplify the spectroscopic studies, we did not remove the film from the back side of the substrate. Transmission spectra of the films were measured using Perkin Elmer Lambda 900 spectrophotometer in 250-550 nm wavelength range at room temperature. The optical density D was derived from the transmission using the formula D = [ln(Ts/Tfsf)], where Ts is the transmission spectrum of the substrate and Tfsf is the transmission spectrum of the substrate with grown films on both sides. For the analysis of the absorption spectra of the films, we used the normalized optical density D/2h. It allows to compare the intensity of the absorption bands of the films with different thickness. Photoluminescence spectra of the films were measured at 300 K in the 400 -700 nm region at Eex = 165, 451 and 440 nm using specializedspectroscopic set-ups. The set-ups allowed to perform the measurements of luminescence and luminescence excitation spectra in UV and VUV spectral regions. A 150 W Xe lamp combined with monochromator MDR-206 was used as the excitation source in UV spectral region. The luminescence was detected using Oriel MS257 spectrograph equipped with Marconi 30-11 CCD detector. A 150-W deuterium discharge lamp Hamamatsu L1835 was used as an excitation source in VUV spectral region. The excitation wavelength was selected using a McPherson Model 234/302 monochromator. The luminescence spectra were registered using a Shamrock 303i-B spectrograph equipped with an ANDOR iDUS 416 CCD camera. To measure the cathodoluminescence decay kinetics of cerium ions, we selected a sample of Pb0.02Ce0.05Gd2.93Al4.29Ga0.71O12 with a maximum thickness of 49.9 μm, on which an epitaxial film was ground on one side. This sample was glued using silver paste onto the inside of the fiber-optic disk of the screen assembly of the PIF-01 type tube having a silver-oxygencesium photocathode (S1). A YAP: Nd 3+ laser, operating in the self-mode locking mode and generating a train of picosecond pulses at a wavelength of 1.08 μm, was used as a source of pulsed radiation at the input of the image intensifier tube. Се-doped (Pb,Gd)3AlxGa5-xO12 single crystalline garnet films with 2 < x  5 were grown using LPE method from supercooled PbO-B2O3-based melt solutions at concentrations С(Gd2O3) = 0.2, 0.3, 0.4 and 0.5 mol %; С(CeO2) = 0.03, 0.2 and 0.3 mol % and С(Al2O3) from 2 to 5 mol % in the mixture on single crystal garnet substrates: Gd3Ga5O12 (GGG) with the (111) orientation and Gd3Al2.26Ga2.74O12 (GAGG) with the (320) orientation.

Results and discussions
X-ray diffraction analysis showed that all films have a garnet type structure [8]. X-ray diffraction patterns obtained in the θ/2θ scan mode showed only strong 444 and 888 reflections from the films and the same weak reflections from its substrate, attenuated by the film. For example, for Pb0.02Ce0.06Gd2.92Al2.22Ga2.78O12 film grown from a melt solution at concentrations of gadolinium oxides of 0.2 mol %, ceriumof 0.2 mol % and aluminumof 2.2 mol %, the unit-cell parameters of the substrate as == 12.3829 A and film af = 12.269 A were determined from the peak positions (the relative lattice mismatch (as -af)/af is 0.9%). The unit-cell parameter of the substrate agrees with JCPDS Powder Diffraction File data for GGG (card no. 76_2290). It was found that the unit-cell parameter decreases with the increasing of film thickness.
The profiles of the asymmetric 880 and 12.60 reflections indicate that the films were single crystalline and grew epitaxially on its substrates. SEM images of the surface of Ce:(Pb,Gd)3AlxGa5-  Study of the effect of the substitution of gallium ions with aluminum ions on optical absorption of cerium ions in the Ce:(Pb,Gd)3(Al,Ga)5O12 films was performed by measuring the transmission spectra (Figure 3a). Two absorption bands in the range from 300 to 360 nm (level 5d2) and from 390 to 550 nm (level 5d1), corresponding to the parity allowed electronic transitions 4f( 2 F5/2,7/2) -5d1.2 Ce 3+ ions were detected. These bands confirm the incorporation of cerium ions into the composition of the films.  Narrow absorption bands in the range from 240 to 313 nm, corresponding to electron transitions 8 S7/2  6 I15/2-7/2 Gd 3+ ions in the transmission spectra of GGG substrates were also observed.
The normalized optical density spectra are presented in Figure 3b. The absorption band in the range from 250 to 300 nm, corresponding, according to [9], to the 1 S0 → 3 P1 electronic transition in Pb 2+ (6s 2 ) ions were observed in the absorption spectra of films with thickness ≤ 32 μm. Two other broad absorption bands correspond to the 4f -5d1,2 Ce 3+ electronic transition. It was found that with the increase of C(Al2O3) from 2.1 to 5.0 mol % in the mixture at C(CeО2) = 0.2 mol % and C(Gd2O3) = 0.2 mol %, the maxima of these two absorption bands shift in the following way. When the Al/Ga concentration ratio increases from 0.74 to 98.8 in the Ce:(Pb,Gd)3AlxGa5-xO12 films (2.13  x  4.94), the shift of 4f -5d1 Ce 3+ absorption band maximum to longer wavelengths was 30 nm (from 436 to 466 nm).  (Figure 5a), respectively, at T = 300 K.
In the luminescence excitation spectra of Pb0.01Ce0.03Gd2.96Al3.14Ga1.86O12 film grown from melt solutions with C(CeO2) = 0.2 mol %, C(Gd2O3) = 0.4 mol %, C(Al2O3) = 4.5 mol % (1) four pronounced bands were observed. The bands at 448 nm and 343 nm are ascribed to electron transitions from 4f to 5d1 and 5d2 levels of Ce 3+ ions. The band at 278 nm is a superposition of two bands related to 1 S0  3 P1 and 8 S7/2  6 I in Pb 2+ and Gd 3+ ions, respectively. The latter indicates the energy transfer from Gd 3+ and/or Pb 2+ ions to Ce 3+ ion. Non-elementary broad band peaking at 215 nm can be ascribed to the superposition of several bands connected with 4f -5d3-5 transitions in Ce 3+ and also with charge transfer transitions involving Ce 4+ ions.