Chemical vapor deposition of ordered structures in YAG–alumina eutectic system

Abstract

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.

1 INTRODUCTION

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).

FIGURE 1

Cross-sectional images of (A) SEM-BSE (scanning electron microscopes-backscattered electrons), (B) TEM-BF (transmission electron microscope-bright field), and (C) HAADF-STEM (high-angle annular dark-field scanning TEM) observation and STEM-EDS (STEM energy-dispersive X-ray spectroscopy) elemental mappings for Y, Al, and O K lines for the YAG–Al₂O₃ chemically deposited eutectic (CDE) film prepared on an m-cut sapphire substrate at 9 mol%Y₂O₃. (D) High-resolution TEM-BF image for the grain boundary between α-Al₂O₃ matrix and YAG rod, and corresponding fast Fourier transfer (FFT) pattern (inset) and (E) selected-area electron diffraction (SAED) pattern. Subscripts α and G denote α-Al₂O₃ and YAG phases, respectively. (F) A schematic of grain boundary between α-Al₂O₃ matrix and YAG rod.

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 $$1{\bar {1}\bar {2}}$$ YAG (Figure 1E and Figure S3, respectively), indicating that the α-Al₂O₃ (0 3 0) plane and the YAG ($$4{\bar {4}}4$$) 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 [$$4 {\bar {4}}4$$] 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 $$1{\bar {1}\bar {2}}$$ YAG || [0 0 1] α-Al₂O₃ in the direction perpendicular to the paper. The same orientation relationship of $$1{\bar {1} \bar{2}}$$ 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.

FIGURE 2

Surface SEM-BSE (scanning electron microscopes-backscattered electrons) images of YAG–Al₂O₃ chemically deposited eutectic (CDE) films prepared on m-, c-, a-, and r-cut sapphire substrates at various Y₂O₃ molar ratios. Bright contrast corresponds to the YAG phase, and dark contrast to the α-Al₂O₃ phase. Scale bars represent 5 μm. Y₂O₃ molar ratios in precursor vapor were (A) 9, (B) 23, (C) 35, and (D) 41; (E) 7, (F) 20, (G) 29, and (H) 39; (I) 8, (J) 16, (K) 31, and (L) 45; (M) 7, (N) 19, (O) 33, and (P) 41 mol%.

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 [$$1{\bar{1}\bar {1}}$$] 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 || [$$1{\bar {1}}2$$] YAG has been reported in the DSE bulks^20-24 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 3

(A) A schematic of white light emitting diode (LED) with blue and yellow light mixing method using the Ce³⁺-doped YAG–Al₂O₃ chemically deposited eutectic (CDE) phosphor, (B) operation demonstration with the CDE film grown on an a-cut substrate at 38 mol%Y₂O₃ and 3.6 mol%Ce under blue light irradiation at the center of the specimen, and (C) its emission spectrum (the dotted line represents emission spectrum of the blue LED). (D) X-ray excited radioluminescence (RL) spectrum of the Eu³⁺-doped YAG–Al₂O₃ CDE phosphor grown on an a-sapphire substrate at 35 mol%Y₂O₃ and 10 mol%Eu. Inset shows the photograph of the specimen under UV-light irradiation. (E) A schematic of an X-ray imaging test using the Eu³⁺-doped YAG–Al₂O₃ CDE phosphor and (F) a high-resolution X-ray radiograph of a semiconductor memory device taken with the CDE film grown as an X-ray scintillation screen.

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→⁷F_j (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

4 CONCLUSIONS

We demonstrated the CVD of ordered structures in the YAG–Al₂O₃ eutectic system, and the present study can be summarized as follows:

• 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 $$1{\bar {1}\bar {2}}$$ 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.

ACKNOWLEDGMENTS

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.