Photo- and Radioluminescence Properties of Ce3+-doped Lu3Al5O12 Thick Film Grown by Chemical Vapor Deposition

We demonstrate the rapid synthesis of Ce3+-doped Lu3Al5O12 (Ce3+:LuAG) thick film phosphor grown by laser-assisted chemical vapor deposition. The radioluminescence properties of the film are compared with those of Ce3+:LuAG single crystals. The (100) Ce3+-LuAG thick film was epitaxially grown on a (100) Y3Al5O12 substrate at a deposition rate of 16 nm s−1. Under UV and X-ray irradiation, the film emitted a yellow-green light originating from 5d–4f transitions of Ce3+ ions. Under α-ray excitation from an 241Am source, the scintillation decay curve of the Ce3+:LuAG thick film was fitted to two time constants, 32 and 666 ns, associated with the Ce3+ centers and antisite defects in garnet structures, respectively.

Metal-organic chemical vapor deposition (MOCVD) is a promising process for growing thick films because it can synthesize functional ceramics at a high deposition rate (10-300 μm h −1 ). (13,14) We have prepared transparent thick films of HfO 2 and Lu 2 O 3 phosphors and Y 3 Fe 5 O 12 and SrFe 12 O 19 magneto-optic crystals by the MOCVD technique. (15)(16)(17)(18) In addition, the high dopant concentration with low segregation during vapor deposition will generate sufficient light intensity.
The present study demonstrates the high-speed epitaxial growth of Ce 3+ :LuAG thick film phosphor. The growth mode, microstructure, and photo-and radioluminescence properties of the film are examined, and the scintillation properties of the obtained thick film phosphor are compared with those of Ce 3+ :LuAG single crystals.

Materials and Methods
The laser-assisted CVD apparatus has been described elsewhere. (19) Metal-organic compounds of lutetium tris(dipivaloylmethanate) (Strem Chemicals, Inc., USA), aluminum tris(acetylacetonate) (Sigma-Aldrich Co. LLC, USA), and cerium tetrakis(dipivaloylmethanate) (Toshima Manufacturing Co., Ltd., Japan) were maintained at temperatures of 453, 453, and 493 K, respectively, in the precursor furnaces. The resultant vapor was transferred to the CVD chamber using Ar (purity: 99.9999%) as the carrier gas, and O 2 gas (99.5%) was separately introduced to the chamber through a double-tubed nozzle. The molar ratio in the precursor vapor was estimated from the mass change in each precursor before and after deposition, and it was calculated to be Ce:Lu:Al = 3:33:64 (9 mol%Ce). The total chamber pressure was maintained at 0.2 kPa.
The substrate was (100) Y 3 Al 5 O 12 (YAG; 5 × 5 × 0.5 mm 3 ) polished on both sides. The substrate was preheated to 1000 K on a heating stage, then irradiated with a CO 2 laser (wavelength: 10.6 μm; maximum laser output: 60 W; SPT Laser Technology Co., Ltd., China) through a ZnSe window. The laser irradiation heated the substrate to 1193 K. The deposition time was 0.3 ks.
The phase composition of the resultant film was determined by X-ray diffraction (XRD; Bruker D2 Phaser, USA), and the in-plane orientation was studied by X-ray pole figure measurements (Rigaku Ultima IV, Japan). The microstructure was observed using a scanning electron microscope (SEM; JEOL JCM-6000, Japan). An energy dispersive X-ray spectrometer attached with a SEM (SEM-EDX; JEOL JSM-6510LA) was used to determine the composition of the films. The in-line transmittance was measured using a UV-visible spectrophotometer (JASCO V-630, Japan) in the 190-1100 nm wavelength range. The photoluminescence (PL) and PL excitation (PLE) spectra were measured using fluorescence spectrophotometers (JASCO FP-8300 and FP-8500, Japan).
The X-ray-excited luminescence spectra were recorded using a spectrometer (Ocean Insights HR2000+, USA) under Cu Kα irradiation generated at 40 kV and 40 mA. Under α-ray excitation from a sealed source of 241 Am, the scintillation decay curves were measured in a photomultiplier tube (PMT; Hamamatsu Photonics R329-02, Japan) connected to an oscilloscope (Keysight DSOX3034T, USA). For this purpose, the specimens were mounted on the window of the PMT with a thin layer of optical silicone grease (Adhesive Materials Group, V-788, USA). For comparison, a commercially available Ce 3+ :LuAG single crystal (5 × 5 × 1 mm 3 , Epic Crystal Co., Ltd., China) was evaluated in the same setup.

Results and Discussion
The out-of-plane XRD pattern of the resultant film was indexed to the LuAG structure (ICSD No. 182354; space group: 3 Ia d and a = 1.193 nm) with a (100) orientation [ Fig. 1(a)]. X-ray pole figures of the LuAG {110} and YAG {110} planes presented fourfold patterns at the same azimuthal angle [ Fig. 1(b)]. The in-plane orientation relationship was [001] Ce 3+ :LuAG || [001] YAG. Figure 2 shows cross-sectional and surface SEM images of the Ce 3+ :LuAG film grown on the YAG substrate. The cross section of the Ce 3+ :LuAG film exhibited a dense structure with a smooth surface. The film was 4.8 μm thick, implying a deposition rate of 16 nm s −1 . The Ce molar content in the film was measured to be 5 mol%. The Ce 3+ :LuAG film was transparent and    yellowish-green (Fig. 3, inset). Its in-line transmittance was 63% that of the raw substrate, as depicted in Fig. 3. Figure 4 shows the PL-PLE contour plots of the Ce 3+ :LuAG film grown on the YAG substrate. The broad emission peak in the 500-600 nm range was attributed to 5d→ 2 F 5/2 and 5d→ 2 F 7/2 transitions of Ce 3+ ions. The excitation bands around 250, 340, and 450 nm were contributed by energy transitions from the ground state (4f) to 2 T 2g , 2 E 2 , and 2 E 1 , respectively. The Ce 3+ :LuAG film grown on the YAG substrate emitted intense yellow-green emission under UV light irradiation from a low-pressure mercury-vapor lamp (Fig. 4, inset). The weak emission peak at 590 nm was attributed to 5 D 0 → 7 F 1 transitions of Eu 3+ ions, which were probably contaminants from the CVD chamber. Figure 5 shows the scintillation spectra of the Ce 3+ :LuAG film and a Ce 3+ :LuAG single crystal under X-ray irradiation. Both samples produced a broad emission (at 470-650 nm) from 5d-4f transitions of Ce 3+ ions. In the spectrum of the Ce 3+ :LuAG film, the two broad peaks at 310 and 380 nm were associated with defect-related emission from LuAG and YAG hosts (20) and the sharp peaks around 590-620 nm were attributed to Eu 3+ contamination. (21) Figure 6 shows the scintillation decay curves under α-ray excitation of the Ce 3+ :LuAG film and Ce 3+ :LuAG single crystal. The decay curves of the Ce 3+ :LuAG film and single crystal were both fitted to two decay time constants: 32 and 666 ns for the film and 55 and 768 ns for the crystal. The fast and slow decay time constants have been attributed to Ce 3+ centers (50-70 ns) and Lu Al antisite defects (600-1000 ns), respectively. (8) The fast decay constant of the grown film was lower than that in the single crystals (50-70 ns) and was comparable to that of the thin film (33 ns). (22) Because dopant elements can be more concentrated in films (10 mol%Ce in a previous study (22) and 5 mol%Ce in the present study) than in single crystals (0.03-0.12 mol%Ce), (23) the short lifetime of the Ce 3+ centers in the film was attributed to Ce 3+concentration quenching. (22)