2023 Volume 64 Issue 6 Pages 1107-1111
A thick film scintillator with a few tens of micrometers thickness is expected to improve sensitivity and spatial resolution in radiation detection and imaging, while its production method is limited to a costly process of thinning a melt-grown single crystal ingot. Here, we demonstrated the high-speed epitaxial growth of terbium- and europium-doped yttrium aluminum perovskite (Tb3+:YAP and Eu3+:YAP) transparent thick film phosphor using laser-assisted chemical vapor deposition (CVD). The (110)-oriented YAP thick film was epitaxially grown on a (100) SrTiO3 (STO) substrate. The deposition rate was 53 µm h−1, which was 50–90 times faster than those reported for conventional thermal CVD. Under UV and X-ray irradiation, the films emitted green and orange lights originating from 4f → 4f transitions of Tb3+ and Eu3+ centers, respectively. The fluorescence decay curves of the films were fitted to 1.96 and 1.89 ms for Tb3+ and Eu3+ centers. The 9 µm-thick Eu3+:YAP thick film phosphor can be used as a scintillation screen of X-ray imaging test to see though a semiconductor storage device.
Inorganic scintillator is a phosphor that can convert radiations such as X- and α-rays into visible light and used for non-destructive inspection in the industrial, medical, and security sectors.1–3) Yttrium aluminum perovskite (YAP) has a wide band gap (7.9 eV), and a high stopping power with moderate effective atomic number (33) and density (5.37 Mg m−3). YAP is therefore promising as a host material that can incorporate rare earth elements as activating elements, not only Ce3+ for fast X-ray spectroscopy4) but also Tb3+- and Eu3+ for radiation detection and imaging.5,6) Eu3+ and Tb3+ centers have often been chosen for their red and green emissions, respectively, which are efficiently detected by CCD/CMOS cameras.7)
Single-crystal scintillators thinned to a few micrometers in thickness have recently attracted much attention after the report on the submicron resolution in synchrotron X-ray imaging using a 5-µm thick Ce3+:YAG single-crystal screen.8,9) Because inorganic scintillators including YAP have often been prepared in the form of single crystal ingot grown by melt solidification process, thinning of single crystals or sintered transparent polycrystalline ceramics is a time and cost consuming way. Therefore, the development of a direct and rapid synthesis route to the thick film scintillator is strongly demanded to develop high-resolution X-ray imaging system.
Chemical vapor deposition (CVD) method can synthesize crystalline films with conformal coverage, and controllability of microstructure and crystal orientation.10) There have been several reports on the epitaxial growth of YAP films, but the deposition rate is slow (less than 1 µm h−1) and the most films are non-doped.11–13) Since the authors have demonstrated the high-speed epitaxial growth of thick film phosphors with rare-earth-doped sesquioxide and garnet structures, such as Eu3+:Y2O3, Eu3+:Lu2O3, and Ce3+:Lu3Al5O12, at high deposition rate up to 60 µm h−1,14–17) this technique will also enable rapid synthesis perovskite-based thick film phosphors.
In the present study, we demonstrated the high-speed epitaxial growth of Tb3+ and Eu3+:YAlO3 thick film phosphor and investigated epitaxial growth mode, microstructure, optical transmittance, and photo- and radioluminescence properties of the films. X-ray imaging test was performed on a commercially available semiconductor storage card using the CVD-Eu3+:YAlO3 thick film phosphor as an X-ray scintillation screen.
The laser-assisted CVD apparatus used in this study has been reported in a previous study.18) Metal–organic precursors of aluminum tris(acetylacetonate) (Merck, USA), yttrium tris(dipivaloylmethanate) (Y(dpm)3) (Kojundo Chemical Laboratory, Japan), and Ce(dpm)4, Tb(dpm)3, and Eu(dpm)3 (Toshima Manufacturing, Japan) were maintained at temperatures of 448–443, 453–458, 483, 463, and 453 K, respectively. The resultant vapor was transported to the CVD chamber with Ar gas, and O2 gas was separately introduced into the chamber through a double-tube nozzle. The Y2O3 fraction is expressed as a ratio to Al2O3, and the Tb3+ and Eu3+ concentrations are expressed as a ratio to Y3+. The total chamber pressure was kept at 0.2 kPa. The SrTiO3 single crystal (5 × 5 × 0.5 mm3, both sides polished) was used as a substrate, and it was irradiated with a CO2 laser (wavelength: 10.6 µm; maximum laser output: 60 W; SPT Laser Technology, China) through a ZnSe window. The deposition temperature was 1173–1276 K under laser irradiation, and the deposition time was conducted at 0.3–0.6 ks.
The phase composition and in-plane orientation of the resultant film were identified by θ–2θ XRD (Bruker D2 Phaser, USA) and pole figure XRD (Rigaku Ultima IV, Japan). The crystal structures were schematically illustrated using the VESTA software package.19) The microstructure and thickness of the films was observed using scanning electron microscopes (JEOL JCM-6000 and Hitachi SU-8010, Japan). The optical absorption spectra and photoluminescence properties were measured using a UV–visible spectrophotometer (JASCO V-630, Japan) and a fluorescence spectrophotometer (JASCO FP-8300, Japan), respectively.
Commercially available X-ray source (Cu target operated at an acceleration voltage of 40 kV and applied current of 40 mA installed in Rigaku Ultima VI, Japan) was used to record X-ray excited luminescence spectrum a multichannel spectrometer (StellarNet SLIVER-Nova, USA) and to take radiograph with the CMOS camera (ZWO ASI224MC, China) with objective lens (magnification: 10×, OLYMPUS MPLFLN10X, Japan).
The θ–2θ XRD pattern of the resultant film prepared with 46 mol%Y2O3 at deposition temperature of 1238 K was indexed to the (110)-oriented YAP structure with an orthorhombic structure (ICSD No. 236590; space group: Pbnm, lattice parameters: a = 5.172 nm, b = 5.327 nm, and c = 7.361 nm) (Fig. 1(a)). X-ray pole figures of the {111} YAP and {101} STO planes presented fourfold patterns at the same azimuthal angle (Fig. 1(b)), and the in-plane orientation relationship was determined to be [001] YAP || [010] STO, in which lengths cYAP = 0.7361 nm and (aYAP2 + bYAP2)1/2 = 0.7425 nm are consisted with length 2aSTO = 0.7807 nm, as illustrated in Fig. 2. Broadening of the {111} YAP peaks might be associated with the existence of 90°-rotated domains or (101) and (011) oriented domains due to the orthorhombic structure that slightly distorted from an ideal cubic structure.
(a) θ–2θ XRD pattern and (b) X-ray pole figure of the non-doped YAP thick film grown on (100) STO substrate at deposition temperature of 1238 K and 46 mol%Y2O3. An asterisk indicates Kβ line.
A plan view of (110)-oriented YAP domain epitaxially grown on (100) STO plane. Solid and dotted lines represent unit cells of YAP and STO, respectively.
Cross-sectional and surface SEM images show that the YAP thick film had a dense structure with a smooth surface (Fig. 3(a)). The thickness of the film was 4.4 µm, and deposition rate was calculated to be 53 µm h−1. The in-line transmittance of the film at the wavelength of 600 nm was 97% for the STO substrate (Fig. 3(b)). Table 1 summarizes preparation of YAP films using CVD method with β-diketone precursors.11–13) The YAP films preferred (001) orientation on the (110) LaAlO3 and (110) SrTiO3 substrates. The YAP film obtained on (100) STO substrate had (001) and (110) co-orientations although the YAP film prepared in the present study showed (110) epitaxial growth. The reported deposition rates were 0.6–1.0 µm h−1, and thus the deposition rate achieved in the present study was 50–90 times faster than those reported in the literature. In the present CVD process, laser irradiation on the film surface promotes a chemical reaction between the vapor phase and the film surface in the chemical vapor deposition.10,20)
(a) Cross-sectional SEM image of the non-doped YAP thick film grown on (100) STO substrate at deposition temperature of 1238 K and 46 mol%Y2O3. Inset shows the surface SEM image with the scale bar of 5 µm. (b) In-line transmittance spectra of the non-doped YAP films grown on STO substrate (solid line) and an STO raw substrate (dotted line). Inset shows photographs of the non-doped YAP film and an STO raw substrate under room light.
Figure 4 shows photoluminescence and photoexcitation luminescence spectra of the films. For the Tb3+:YAP thick film prepared with 46 mol%Y2O3 and 9 at%Tb3+ at deposition temperature of 1276 K, broad peaks centered at 215 and 275 nm were observed in the excitation spectrum, originating from the 4f → 5d transition (Fig. 4(a)). In the emission spectrum, the 5D4 → 7Fj (j = 3–6) transition peak was observed between 490 and 650 nm.21) For the Eu3+:YAP thick film prepared with 45 mol%Y2O3 and 10 at%Eu3+ at deposition temperature of 1177 K, a charge transfer band (CTB)-derived broad peak centered at 245 nm was observed in the excitation spectrum (Fig. 4(b)). In the emission spectrum, the 5D0 → 7Fj (j = 1–4) transition peak was observed between 590 and 720 nm.22)
Photoluminescence and photoexcitation luminescence spectra of (a) Tb3+:YAP thick film grown on STO substrate at 46 mol%Y2O3 and 9 mol%Tb at 1276 K, and (b) Eu3+:YAP thick film grown on STO substrate at 45 mol%Y2O3 and 8 mol%Eu at 1177 K. Insets show photographs of the films under UV-light irradiation.
The fluorescence decay curves of the films were fitted to 1.96 ms for the emission wavelength of 590 nm (5D4 → 7F4 transition of Tb3+ center) at excitation wavelength of 485 nm (Fig. 5(a)) and 1.89 ms for the emission wavelength of 615 nm (5D0 → 7F2 transition of Eu3+ center) at excitation wavelength of 395 nm (Fig. 5(b)). These decay constants are in good agreement with previously reported values for Tb3+:YAP and Eu3+:YAP powders.23,24)
Fluorescence decay curves of (a) the Tb3+:YAP thick film excited at 485 nm and monitored at 590 nm, and (b) the Eu3+:YAP thick film excited at 395 nm and monitored at 615 nm.
In the X-ray excited luminescence spectra, sharp peaks originated from 4f–4f transitions from Tb3+ and Eu3+ centers were observed at 490–650 nm and 580–710 nm, respectively. The observed emission peaks were in good agreement with the photoluminescence spectra (Fig. 6). Since the resolution of the multichannel spectrometer (1 nm) is smaller than that of the fluorescence spectrophotometer (5 nm), the emission peaks were measured without superimposition. The Eu3+:YAP thick film was used to perform X-ray imaging test, and through-holes and metal patterns inside the semiconductor storage card are visualized as shown in Fig. 7.
X-ray excited luminescence spectra of the (a) the Tb3+:YAP thick film and (b) the Eu3+:YAP thick film.
X-ray radiograph of the commercially available microSD card taken with the Eu3+:YAP thick film as an X-ray scintillation screen.
We demonstrated the high-speed epitaxial growth of the (110)-oriented YAP thick film on the (100) STO substrate with the in-plane orientation relationship of [001] YAP || [010] STO. The deposition rate was 53 µm h−1, which was 50–90 times faster than those reported for conventional thermal CVD. The film was the dense in cross section with the smooth surface. The in-line transmittance at the wavelength of 600 nm was 97% for STO substrate.
Under UV and X-ray irradiation, the Tb3+:YAP and Eu3+:YAP films emitted green and orange lights originating from 4f → 4f transitions of Tb3+ and Eu3+ centers, respectively. The fluorescence decay curves of the films were fitted to 1.96 and 1.89 ms for Tb3+ and Eu3+ centers, respectively. The Eu3+:YAP thick film can be used as an X-ray scintillation screen to see though the semiconductor storage device.
Thick-film phosphors synthesized by the high-speed epitaxial growth technique using the laser CVD method have sufficient transparency and radioluminescence emission, and therefore this technique is useful as a rapid production route to obtain scintillation crystals for radiation detection and high-resolution X-ray imaging.
This study was supported in part by JSPS KAKENHI Grant Number JP17H03426, JP20H02477, JP20H05186, 21H01825, 21H05199, and 21J11881. This study was also supported in part by the JST SCORE University Promotion Type, Grant Number JPMJST2078, Japan, and NEDO Project Number JPNP20004, Japan. This study was also supported in part by Yokohama Kogyokai, Japan, and “Joint Research Project B” and “Joint Research Project C” from the Graduate School of Environment and Information Sciences, Yokohama National University, Japan. X-ray pole figure measurement (Rigaku Ultima IV) was carried out at Instrumental Analysis Center, Yokohama National University. We would like to thank Drs. Kaoru Dokko and Yosuke Ugata for using X-ray tube.
