The eutectic structure of metals and ceramics is the result of a self-organization phenomenon in which multiple solid phases solidify with an ordered structure from a liquid phase. Hence, a melt-solidification process was the only way to generate ordered structures in a ceramic eutectic system. Here, we prepared ordered structures of Y3Al5O12 (YAG)–Al2O3 composites via chemical vapor deposition in the YAG–Al2O3 eutectic system. Spatially periodic YAG rod and lamellar structures were generated in an alumina matrix homoepitaxially grown on the sapphire substrates on the Al2O3-rich side of the eutectic composition, whereas α-Al2O3 rod and lamellar structures were generated in a YAG matrix on the Y2O3-rich side. The results reveal that the pattern formation of ordered structures in a ceramic eutectic system can occur not only during in the melt-solidification process but also during the vapor deposition process. The Ce3+-doped YAG–Al2O3 composite film converts some of the blue light into yellow light, allowing some of it to pass through, and emitting white light. The Eu3+-doped YAG–Al2O3 composite film can be utilized as a scintillation screen for a high-resolution X-ray imaging test to see though a semiconductor storage device.
Directionally solidified eutectics (DSE) or melt-growth composite is known as a bulk composite in eutectic system, and it has been investigated as a high-temperature structural ceramic as the early reports on Y3Al5O12 (YAG)–Al2O3 composite indicated its excellent high-temperature strength and durability.1, 2 As rare-earth-doped YAG powder or single crystals have been used for white light emitting diode (LED)3 and scintillation crystal,4, 5 and α-Al2O3 remains a key practical material with excellent optical transparency, mechanical strength, and chemical stability, YAG–Al2O3 composite has been attracting renewed attention as composite phosphors and therefore expected to be bright and stable phosphors.6, 7
On the other hand, a micrometer-thick film form of solid-state phosphor is a potential material for high-power white LED3 and high-resolution X-ray imaging systems.8, 9 However, YAG–Al2O3 DSE in the film form is either difficult to obtain or expensive because the production method of DSE was limited to the melt-solidification process, and cost-consuming post-mechanical processing is required. In addition, YAG–Al2O3 DSE can only exhibit nonuniform microstructure, sometimes known as a Chinese script-like texture. A eutectic composite with a rodlike structure is preferred for improving spatial resolutions in lighting and imaging, where the unidirectionally grown phosphor phase is organized in an optically transparent matrix.10
Here, we propose chemical vapor deposition (CVD) of ordered structures in YAG–Al2O3 eutectic system. We have used the laser-assisted CVD and have demonstrated either the high-speed epitaxial growth of single crystalline thick films of Eu3+:HfO2 and Ce3+:Lu3Al5O12,11-13 or the phase-separated growth of composite thick films in ZrO2–Al2O3 and HfO2–Al2O3 eutectic systems.14, 15 By combining epitaxial and phase-separated growth, we can expect vapor deposition process of DSE-like unidirectional ordered structures.
We present the CVD of YAG–Al2O3 composite films with ordered structures, namely, chemically deposited eutectic system (CDE). The effects of the Y2O3 molar fraction in the precursor vapor and orientation of sapphire substrates on microstructure and orientation of the YAG–Al2O3 CDE films were studied. We also study the photo- and radioluminescence properties, including high-resolution X-ray imaging tests for the rare-earth doped YAG–Al2O3 CDE films.
2 EXPERIMENTAL METHOD
A schematic of the CVD reactor system and detailed process parameters are presented in Figure S1 and Table S1.16 A vertical cold-wall reactor with multiple precursor furnaces, double-tube precursor nozzle, hot stage, vacuum exhaust system, and laser optics has been used. Metal–organic precursors of aluminum tris(acetylacetonate) (Merck, USA), yttrium tris(dipivaloylmethane) (Y(dpm)3) (Kojundo Chemical Laboratory, Japan), Ce(dpm)4 and Eu(dpm)3 (Toshima Manufacturing, Japan) were maintained in each precursor furnace at temperatures of 448–468, 443–483, 493, and 453 K, respectively. The resultant vapor was transported to the CVD chamber with Ar gas, whereas O2 gas was separately introduced into the chamber using a double-tube nozzle. The Y2O3 fraction is expressed as a ratio to Al2O3, and the Ce3+ and Eu3+ concentrations are expressed as a ratio to Y3+. The total chamber pressure was kept at 0.2 kPa. The m-, c-, a-, and r-cut sapphire single crystals (5 × 5 × 0.5 mm3, both sides polished) were used as a substrate and heated at 1023 K with an electrical heating stage, and then it was irradiated with a CO2 laser (wavelength: 10.6 μm; maximum laser output: 60 W) through a ZnSe window. The deposition temperature was measured using an infrared radiation thermometer (CHINO IR-FAI, Japan) through a quartz glass window. It was 1064–1303 K under laser irradiation, and the deposition time was conducted at 0.18–0.30 ks.
The phase composition of the resultant film was identified by using θ–2θ X-ray diffractometry (XRD) (Bruker D2 Phaser, USA). The microstructure and thickness of the films were observed using scanning electron microscopes (SEMs; JEOL JCM-6000, Japan and Hitachi SU-8010, Japan) and transmission electron microscope (TEM; JEOL JEM-2800, Japan). SEM-BSE (backscattered electrons) and HAADF-STEM (high-angle annular dark-field scanning TEM) images are sensitive to the difference in atomic number17; thus, in the present study, bright contrast corresponds to YAG phase and dark contrast to α-Al2O3 phase. TEM-BF (bright field) image contrast depends on atomic number, crystallinity, crystal orientation, and thickness. Fast Fourier transfer (FFT) and selected-area electron diffraction (SAED) patterns were indexed using ReciPro software.18 Schematic illustrations of crystal shape and structure were depicted using VESTA software.19 The photoluminescence and radioluminescence spectra were measured using a fluorescence spectrophotometer (JASCO FP-8300, Japan) and a multichannel spectrometer (Ocean Insight HR2000+, USA). 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 and to perform X-ray imaging test with CMOS camera (ZWO ASI224MC, China).
3 RESULTS AND DISCUSSION
Figure 1 shows the results of cross-sectional observations of the YAG–Al2O3 CDE film grown on an m-cut sapphire substrate at 9 mol%Y2O3. XRD pattern of the CDE film was indexed with YAG and α-Al2O3 phases, and (0 3 0)-oriented α-Al2O3 phase was homoepitaxially grown on an m-cut sapphire substrate (Figure S2a). SEM-BSE images show that YAG rods (bright contrast) grew vertically from the substrate in the α-Al2O3 matrix (dark contrast) (Figure 1A). The film thickness was 8.3 ± 0.1 μm, and thus, the deposition rate was calculated to be 99.6 ± 1.0 μm h−1. The width of the YAG rod (dark contrast in TEM-BF image) was approximately 300 nm (Figure 1B). HAADF-STEM and STEM-EDS images show that the Y element was detected only in the YAG phase, and Al and O elements were distributed throughout the specimen (Figure 1C).
A high-resolution TEM-BF image indicates that the interface between YAG and α-Al2O3 had a low low-angle boundary (Figure 1D). SAED and FFT patterns can be indexed with the zone axes of [0 0 1] α-Al2O3 and YAG (Figure 1E and Figure S3, respectively), indicating that the α-Al2O3 (0 3 0) plane and the YAG () plane were tilted by 5.8 degrees. The zone axis and the tilting angle of the YAG phase with respect to the α-Al2O3 phase were consistent with atomic configurations of hexagonal lattice in the α-Al2O3 phase and striped pattern in the YAG phase, as observed in Figure 1D and illustrated in part (E). The orientation relationship in the growth direction between the YAG rod and the Al2O3 matrix was  YAG || [0 3 0] α-Al2O3. The interface orientation relationship can be [4 4 0] YAG || [1 0 0] α-Al2O3 in the direction parallel to the paper and YAG || [0 0 1] α-Al2O3 in the direction perpendicular to the paper. The same orientation relationship of YAG and [0 0 1] α-Al2O3 has often been reported for the DSE bulks grown from the melt.20-24
Figure 2 summarizes typical surface SEM-BSE images of YAG–Al2O3 CDE films prepared on m-, c-, a-, and r-cut sapphire substrates at various Y2O3 molar ratios. The representative XRD patterns are shown in Figure S2. XRD confirms that all the films prepared at 5–50 mol%Y2O3 consisted of YAG and α-Al2O3 phases. The α-Al2O3 phase was homoepitaxially grown on each sapphire substrate independently of Y2O3 molar ratios and substrate planes. In contrast, the YAG phase showed preferential orientation to some planes, such as (2 1 1), (2 2 0), and (4 2 0), but no single epitaxial growth.
The YAG–Al2O3 CDE films preferred a rodlike structure on the m-cut sapphire substrate. The YAG rod was dispersed in the α-Al2O3 matrix at 9 and 23 mol%Y2O3 (Figure 2A,B), whereas the α-Al2O3 rod was dispersed in the YAG matrix at 35 and 41 mol%Y2O3 (Figure 2C,D). On the c-cut sapphire substrate, a mixture of the rodlike and lamellar structure was formed from 7 to 39 mol%Y2O3 (Figure 2E–H). On the a-cut sapphire substrate, rodlike structures were obtained at both Al2O3-rich (8 mol%Y2O3) and YAG-rich (45 mol%Y2O3) conditions (Figure 2I,L), and a lamellar structure was formed across the eutectic composition (Figure 2J,K). Lamellar structure formation was dominant on the r-cut sapphire substrate (Figure 2N,O), whereas α-Al2O3 and YAG matrixes were faceted and roughened at Al2O3- and YAG-rich conditions (Figure 2M,P).
The atomic arrangements on m- and a-cut planes are close to a hexagonally close-packed lattice of oxygen atoms. This low anisotropy of surface oxygen arrangement might be suitable for forming smooth surfaces without disturbing the growth direction of rodlike structures in the CDE films. The r-cut plane has an anisotropic arrangement of oxygen atoms along  direction, and this in-plane anisotropy is presumably responsible for the formation of lamellae with in-plane orientation as shown in Figure 2N,O. The c-cut plane has the most symmetric oxygen honeycomb lattice, whereas the CDE films have the least regular structure and roughest surface. Because the interface orientation relationship of [0 0 1] α-Al2O3 ||  YAG has been reported in the DSE bulks20-24 and in the present CDE films, α-Al2O3 growth in the [0 0 1] direction on a c-cut sapphire substrate may be inconvenient in creating an ordered structure with YAG phase.
The results show that the CVD method can obtain a two-phase structure over a broad compositional range (5–50 mol%Y2O3), although the melt-solidification method has a narrow compositional range to obtain eutectic structure (13.5–23.5 mol%Y2O3).25 Moreover, the literature includes no report on the formation of a rodlike structure in the YAG–α-Al2O3 system via the melt-solidification method. In contrast, CDE can produce both YAG and α-Al2O3 rodlike structures.
Figure 3A illustrates a schematic of white LED with blue and yellow light mixing method. Ce3+-doped YAG powder dispersed in the resin used for the current product is replaced with the present Ce3+-doped YAG–Al2O3 CDE film. Figure 3B,C shows the Ce3+-doped YAG–Al2O3 CDE film grown on an a-cut substrate at 38 mol%Y2O3 and 3.6 mol%Ce converts some of the blue light into yellow emissions with wavelengths centered at 530 nm, and 575 nm originated from 5d–4f transitions of Ce3+ center and allowed some of it to pass through, generating white light.26-28
Figure 3D shows an X-ray excited luminescence spectrum of the Eu3+-doped YAG–Al2O3 CDE film grown on an a-sapphire substrate at 35 mol%Y2O3 and 10 mol%Eu. Red emissions originated from 5D0→7Fj (j = 1–4) transitions in the Eu3+ center were observed.29 Figure 3E illustrates a schematic of the X-ray imaging test using the Eu3+-doped YAG–Al2O3 CDE film as an X-ray scintillation screen, and part (F) presents the obtained X-ray radiograph of the semiconductor storage card (microSD card). Through-holes and tracks inside the semiconductor storage card were visualized.30
- The films prepared at 5–50 mol%Y2O3 had two-phase structures of YAG and α-Al2O3. The YAG rod and lamella grew in the α-Al2O3 matrix on the Al2O3-rich side of the eutectic composition and the α-Al2O3 rod and lamella grew in the YAG matrix on the Y2O3-rich side. It is the first report on the formation of either YAG or α-Al2O3 rodlike structure in the YAG–Al2O3 composite.
- Al2O3 phases were homoepitaxially grown on each sapphire substrate. On the m-cut sapphire, the CDE film with the YAG rodlike structure had the interface orientation relationship of YAG || [0 0 1] α-Al2O3, which has often been reported for the DSE bulk in the literature.
- The Ce3+-doped YAG–Al2O3 CDE film converts some of the blue light into yellow emissions with wavelengths centered at 530 and 575 nm originated 5d–4f transitions of Ce3+ center and allow some of it to pass through, generating white light. The Eu3+-doped YAG–Al2O3 CDE film can be used as an X-ray scintillation screen to see through a semiconductor storage device.
This study shows that the formation of ordered structures in the eutectic system can occur in the vapor deposition proces. CVD can synthesize ceramic eutectic composites as functional or protective layers over substrates without damage to the underlying layer caused by high-temperature melt.
This study was supported in part by JSPS KAKENHI Grant Numbers 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. The TEM observation (JEOL JEM-2800) was supported by the University of Tokyo Advanced Characterization Nanotechnology Platform in the Nanotechnology Platform Project sponsored by MEXT, Japan. SEM observations (Hitachi SU-8010) were 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.
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