Epitaxial Growth of Ruby Crystal Films on Sapphire Crystal Substrates

Jun 11, 2019 - Ruby (Al2O3:Cr) crystal films were, for the first time, epitaxially grown on sapphire (Al2O3) crystal substrates via the evaporation of...
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Epitaxial Growth of Ruby Crystal Films on Sapphire Crystal Substrates and Solubility of Aluminum Oxide in Molybdenum Trioxide Flux Shunsuke Ayuzawa,†,‡ Sayaka Suzuki,§ Miki Hidaka,§ Shuji Oishi,‡,§ and Katsuya Teshima*,‡,§,∥

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Department of Science and Technology, Graduate School of Medicine, Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡ Nagano Prefecture Nanshin Institute of Technology, 8304-190 Minamiminowa, Nagano 399-4511, Japan § Department of Materials Chemistry, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ∥ Research Initiative for Supra-Materials, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

ABSTRACT: Ruby (Al2O3:Cr) crystal films were, for the first time, epitaxially grown on sapphire (Al2O3) crystal substrates via the evaporation of molybdenum trioxide (MoO3) flux. The crystal films were grown by heating the flux placed on the sapphire crystal substrates at 1100 °C for 1200 min. The films thus obtained exhibited a red color and were approximately 200 μm in thickness. Based on the X-ray diffraction and electron backscatter diffraction analyses data, the crystallographic orientation of the crystal films was found to be the same as that of the substrate crystals. A ruby crystal film with a (0001) face developed on the (0001) substrate face, and a film with a (112̅0) face developed on the (112̅0) substrate face. In addition, the solubility of Al2O3 in the flux at 1100 °C was estimated to be approximately 9.6 mol % by measuring the mass loss of the substrate and the amount of evaporated flux when the crystallization of the ruby crystals occurred.

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INTRODUCTION Ruby is an aluminum oxide (Al2O3) doped with chromium. Chromium ions impart a red color onto Al2O3. Ruby is used not only for jewelry but also for various industrial materials. Ruby has excellent mechanical, optical, and chemical properties. In particular, ruby is the second-hardest natural material known to humankind. Ruby can be used for mechanical parts that require abrasion resistance. Furthermore, the first successful solid laser consisted of a ruby crystal and was created in 1960.1 Ruby crystals are expected to be used as materials for optical devices taking advantage of being a single crystal henceforth. Various techniques such as the Verneuil, Czochralski, hydrothermal, vapor phase, and flux processes are commonly employed to grow ruby crystals.2−14 Owing to the ease of rearranging the constituent atoms, the growth of large crystals is generally performed using the liquid phase. The growth from liquid is classified into two categories: melt growth and flux growth. In melt growth, it is necessary for the crystal growth to use a temperature in excess of the melting point. However, the most notable advantage of the flux method classified as solution growth is that the crystal growth occurs at a considerably lower temperature than the melting point of the © 2019 American Chemical Society

solute. Via this method, crystal growth occurs with little environmental impact. Furthermore, flux growth close to equilibrium is useful because generation of defects such as nonluminescent centers can be inhibited. Among the flux methods, the flux evaporation method can be used to grow crystals by evaporating a flux and bringing a solute into a supersaturated state. Thus far, the authors have reported hexagonal bipyramidal ruby crystal growth and the coating of a ruby layer from molybdenum trioxide (MoO3) flux using the flux evaporation method.15−19 Ruby crystals were grown at 1100 °C, which was well below the melting point. However, there are no reports on ruby crystal films epitaxially grown on sapphire crystal substrates. If ruby crystal films can be epitaxially grown on sapphire crystal substrates using a flux method, it is possible to safely grow crystal films used for the optical device with few defects. In addition, there are no reports on measurements of the solubility of Al2O3 in the MoO3 flux. It is very important to determine the solubility of the solute in the flux to grow Received: April 11, 2019 Revised: May 28, 2019 Published: June 11, 2019 4095

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100

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Figure 1. Schematic illustration showing that dissolution of sapphire crystal substrate is shifted to growth of ruby crystal via an equilibrium state. Growth of Ruby Crystal Films. The ruby crystal films were epitaxially grown via the MoO3 flux evaporation method (run nos. 1 and 2 in Table 1). The aluminum oxide was supplied from the surfaces of the sapphire crystal substrates. The substrates (Shinkosha Co., Ltd.), with well-developed (0001) (run no. 1) and (112̅0) (run no. 2) faces, were square-shaped, 20 mm × 20 mm in size, and 2 mm in thickness. The substrates were fabricated by the top-seeded meltgrowth method. The surfaces were mirror-polished. Molybdenum trioxide (Allied Material Co., Ltd.) was used for the flux. Chromium oxide (Kanto Chemical Co., Inc.) was added as an oxide dopant to give the crystals their red color. The flux (30 g) and the dopant (0.012 g) were weighed and mixed together. The substrates were placed at the bottom of a platinum crucible with a 50 cm3 capacity. The mixture of the flux and dopant was also placed in the crucible. The crucible was loosely closed by a platinum lid. The mixture-containing platinum crucible was placed in a refractory block and then inserted into an electric furnace, heated to 1100 °C at a rate of 400 °C·h−1, and held at this temperature for 1200 min. Subsequently, the platinum crucible was cooled to 500 °C at a rate of 150 °C·h−1 using a temperature program, and then allowed to cool to room temperature in the furnace. The flux in the crucible evaporated completely. The fluxgrown ruby crystal films on the sapphire crystal substrates were then taken out from the crucible. Solubility of Aluminum Oxide in the Flux. The solubility of Al2O3 in the MoO3 flux at 1100 °C was estimated. The solute concentrations in the flux were measured by changing the holding time at 1100 °C (run nos. 3−9 in Table 1). The substrates with welldeveloped (0001) faces were used as in the growth of the ruby crystal films. The surfaces were mirror-polished. The mixture-containing platinum crucible was heated to 1100 °C at a rate of 400 °C·h−1. Subsequently, the platinum crucible was removed from the electric furnace and cooled to room temperature (approximately 20 °C) in air. As MoO3 dissolves only slightly in normal water, the residual flux was removed using warm water. After removing the flux, the surfaces of the sapphire crystal substrates were observed. The Al2O3 dissolution concentrations were calculated by dividing the amount of dissolution Al2O3 by the amount of residual flux. The amount of dissolution Al2O3 was calculated using the difference in mass between the original sapphire crystal substrates and the ones with reduced mass due to dissolution. The amount of residual flux was calculated using the difference in the amount of original flux and the evaporated flux. The amount of flux evaporation was calculated using the difference in mass before and after the experiments with the platinum crucible containing the mixture. Characterization. The color and shape of the obtained crystal films were observed with the naked eye. The obtained crystal films were studied using an optical microscope (BX60, Olympus) and a 3D optical microscope (Contour GT, Bruker). The cross sections of the crystal films were studied using field-emission scanning electron microscopy (FESEM; JSM-7000F, JEOL) operated at an acceleration voltage of 25 kV. The crystallographic orientations of the crystal films were identified using X-ray diffraction (XRD; SmartLab, Rigaku) and electron backscatter diffraction (EBSD; DigiView, EDAX) patterns. An X-ray diffractometer with CuKα radiation (λ = 0.154 nm) was operated at 40 kV and 30 mA in the 2θ range of 15−85°.

crystals via the flux method. Based on the solubility data, crystals can be grown under optimum conditions. However, it is difficult to measure the solubility of Al2O3 in the MoO3 flux. The MoO3 flux is a volatile material and its mass decreases upon heating. Owing to the difficulty in determining the equilibrium, the authors supposed that the solubility could be measured by determining the moment when the dissolution of the sapphire crystal substrate shifted to the growth of the ruby crystals via equilibrium, as schematically shown in Figure 1. First, the sapphire crystal substrate was dissolved in molten MoO3 flux (Figure 1a). The flux was evaporated, and the sapphire crystal substrate continued to dissolve. The state of balance between the dissolution and deposition was equilibrated (Figure 1b). The flux was further evaporated, and the ruby crystal film grew (Figure 1c). An addition of a trace amount of chromium oxide (Cr2O3) to the MoO3 flux beforehand caused Al2O3 to incorporate chromium ions. The solubility was the solute concentration in the flux at equilibrium (Figure 1b). The solute concentration could be determined by measuring the mass loss of the sapphire crystal substrate and the amount of evaporated flux. In addition, the commencement of the crystal growth could be judged by observing the surface of the sapphire crystal substrate. The dissolution and deposition were nearly balanced at the start of the crystal growth. The purpose of this study was to obtain ruby crystal films epitaxially grown on sapphire crystal substrates at 1100 °C using the MoO3 flux evaporation method and to measure the solubility of Al2O3 in the MoO3 flux at 1100 °C.

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EXPERIMENTAL SECTION

The experimental conditions for the growth of the ruby crystal films and the measurement of the solubility of Al2O3 in MoO3 flux are summarized in Table 1.

Table 1. Experimental Conditions for Growth of Ruby Crystal Films and Measurement of Solubility of Al2O3 in MoO3 Flux run no.

holding time/min

cooling rate/°C·h−1

orientation of substrate

1 2 3 4 5 6 7 8 9

1200 1200 30 60 100 120 130 140 160

150 150 air cooling air cooling air cooling air cooling air cooling air cooling air cooling

(0001) (112̅0) (0001) (0001) (0001) (0001) (0001) (0001) (0001) 4096

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100

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Article

of the ruby crystal films and substrates depicted in Figure 2b,c, respectively. The thickness of the ruby crystal films was found to be approximately 200 μm. The interfaces between the ruby crystal films and the substrates were relatively smooth. Based on the growth patterns and the interfaces, the ruby crystal films were presumed to have epitaxially grown on the substrates. The orientations of the ruby crystal films were investigated. Figure 3a−e shows the XRD patterns of the ruby crystal film

RESULTS AND DISCUSSION Ruby crystal films were successfully prepared on sapphire crystal substrates via the isothermal evaporation of the MoO3 flux (run nos. 1 and 2 in Table 1). First, the ruby crystal films were grown by heating the mixture of MoO3 and Cr2O3 powders at 1100 °C for 1200 min in the platinum crucible. The substrates used are shown in Figure 2a. When the

Figure 3. XRD patterns of the (a) ruby crystal film on the substrate (0001) face, (b) sapphire crystal substrate (0001) face, (c) ruby crystal film on the substrate (112̅0) face, (d) sapphire crystal substrate (112̅0) face, and (e) Al2O3 ICDD PDF.20

on the substrate (0001) face, the sapphire crystal substrate (0001) face, the ruby crystal film on the substrate (112̅0) face, the sapphire crystal substrate (112̅0) face, and the Al2O3 ICDD PDF,20 respectively. The diffraction angles of ruby and sapphire should be almost the same because their lattice parameters are almost the same, since ruby is an aluminum oxide doped with a small amount of chromium. The orientations in which the ruby crystals grow with the lowest energy are similar to those of the sapphire crystal substrates. In Figure 3a,b, only the diffraction lines of the (0006) plane are predominant. In Figure 3c,d, only the diffraction lines of the (112̅0) and (224̅0) planes are predominant. Diffraction lines other than those of the substrate were not observed. This clarified that the ruby crystal films had epitaxially grown on the substrate as a seed crystal. The homogeneity of orientation over the entire ruby crystal film was also investigated with regard to the substrate with (112̅0) faces. The SEM image of the cross-section of the ruby crystal film on the sapphire crystal substrate with a welldeveloped (112̅0) face is shown in Figure 4a. Figure 4b shows the EBSD image of the corresponding part shown in Figure 4a.

Figure 2. (a) Photographs of sapphire crystal substrate, and ruby crystal films grown on the substrates with (b) (0001) and (c) (112̅0) faces. Optical micrographs of surfaces of the ruby crystal films grown on the substrates with (d) (0001) and (e) (112̅0) faces. Optical micrographs of cross sections of the center portions of the ruby crystal films and substrates with (f) (0001) and (g) (112̅0) faces.

experiments were completed, the sapphire crystal substrates were covered with transparent-red ruby crystal films. The ruby crystal films on the substrates with well-developed (0001) and (112̅0) faces are shown in Figure 2b,c, respectively. Figure 2d,e show optical micrographs of the center portions of the ruby crystal films depicted in Figure 2b,c, respectively. The ruby crystal film on the substrate with (0001) faces exhibits layer growth patterns (Figure 2d). Based on the uniformity of the step intervals of the layer growth patterns, the ruby crystal film was estimated to have grown under stable growth conditions. The ruby crystal film on the substrate with (112̅0) faces exhibits striation patterns (Figure 2e). Figure 2f,g shows optical micrographs of the cross sections of the center portions 4097

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100

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Figure 4. (a) Cross-sectional SEM image and (b) corresponding EBSD image of the ruby crystal film and substrate.

Figure 5. Time dependences of the amounts of (a) residual flux and (b) dissolution Al2O3 shown as the results of run nos. 3−9 in Table 1.

after 130 min, a ruby crystal grows on the sapphire crystal substrate. The commencement of crystal growth was judged by observing the surfaces of the sapphire crystal substrates. Figure 6a shows photographs of the substrates obtained under the respective conditions in run nos. 3−9 in Table 1. For example, no. 3 corresponds to the result of run no. 3 in Table 1. The substrates of nos. 3−6 were colorless and transparent. The pale-red color peculiar to ruby could be confirmed on the substrate of no. 7. The red color deepens in the order of no. 8 < no. 9. Figure 6b shows optical micrographs of the substrate surfaces. Figure 6c shows 3D optical micrographs of the corresponding parts shown in Figure 6b. Only etch pits were observed on the surfaces of nos. 3−6. Dissolution of Al2O3 occurred for nos. 3−6. Growth hillocks were observed on the edges of the etch pits (no. 7). The growth hillocks first grew and covered the etch pit edges (no. 8). The growth hillocks then grew further and completely covered the etch pits (no. 9). These results suggested that the moment the dissolution of the sapphire crystal substrate shifted to the growth of ruby crystals was that for no. 7. The dissolution and deposition were nearly balanced for no. 7. The time dependence of the Al2O3 dissolution concentration is shown in Figure 7. The numbers shown in Figure 7 correspond to the run nos. in Table 1. As the holding time increased, the Al2O3 dissolution concentration increased

The same color indicates the same orientation. Both the ruby crystal film and sapphire crystal substrate exhibit a blue color. The blue color indicates that the cross-section of the film and substrate are oriented toward the (11̅00) face. This implies that the orientation of the grown ruby crystal film is homogeneous. In addition, the interface between the film and substrate was smooth. This showed that the ruby crystal film was oriented in the same direction throughout the interface with the sapphire crystal substrate and was epitaxially grown. Moreover, we concluded that the ruby film was singlecrystal because no grain boundary was observed in the crosssectional SEM images. The solubility of Al2O3 in the MoO3 flux was investigated (run nos. 3−9 in Table 1). The time dependence of the amount of residual flux is shown in Figure 5a. As the holding time increased, the amount of residual flux decreased. The flux became more difficult to evaporate with time, owing to factors such as change in the solute concentration and crucible shape. The rate of decrease in the amount of residual flux decreased with an increase in the holding time. The time dependence of the amount of dissolution Al2O3 is shown in Figure 5b. The amount of dissolution Al2O3 increased with the holding time up to 130 min; beyond 130 min, it decreased with an increase in the holding time. This implies that up to a holding time of 130 min, the sapphire crystal substrate is dissolved; however, 4098

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100

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Figure 6. (a) Digital photographs of the substrates, (b) optical micrographs of the surfaces, and (c) 3D optical micrographs after heating at respective conditions of run nos. 3−9. (c) Magnified images of the parts marked in (b).

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linearly. The dissolution and deposition were nearly balanced for no. 7. The solubility of Al2O3 in MoO3 at 1100 °C was estimated to be approximately 9.6 mol %, as shown by C0 in Figure 7. The high-temperature solution of Al2O3 (9.6 mol %)−MoO3 (90.4 mol %) at 1100 °C was at equilibrium. For the growth of high-quality ruby crystal films, it is desirable to use a mixture containing close to 9.6 mol % solute. Solute concentration well above 9.6 mol % creates many nuclei. If the solute concentration is close to 9.6 mol %, the ruby crystal film epitaxially grows on the sapphire crystal as a seed.

CONCLUSIONS

Ruby crystal films were epitaxially grown on sapphire crystal substrates via the evaporation of an MoO3 flux. Based on the XRD and EBSD analyses data, the crystallographic orientation of the crystal films was found to be the same as that of the substrate crystals. A ruby crystal film with a (0001) face developed on the (0001) substrate face, and a film with a (112̅0) face developed on the (112̅0) substrate face. It was found that the solubility of Al2O3 in the MoO3 flux at 1100 °C 4099

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100

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(8) Teshima, K.; Kondo, H.; Suzuki, T.; Oishi, S. Growth of large bipyramidal ruby crystals by the evaporation of molybdenum trioxide flux. J. Ceram. Soc. Japan. 2005, 113, 733−735. (9) Teshima, K.; Miyajima, A.; Kondo, H.; Mochizuki, K.; Suzuki, T.; Oishi, S. Growth of bipyramidal ruby crystals by the evaporation of A2O (A = Na, K)−MoO3 fluxes. J. Ceram. Soc. Japan. 2005, 113, 758−760. (10) Teshima, K.; Tomomatsu, D.; Suzuki, T.; Ishizawa, N.; Oishi, S. Growth of Na2Ta4O11 crystals from a Na2Mo2O7 flux. Cryst. Growth Des. 2006, 6, 18−19. (11) Teshima, K.; Yubuta, K.; Sugiura, S.; Fujita, Y.; Suzuki, T.; Endo, M.; Shishido, T.; Oishi, S. Selective growth of calcium molybdate whiskers by rapid cooling of a sodium chloride flux. Cryst. Growth Des. 2006, 6, 1598−1601. (12) Teshima, K.; Kikuchi, Y.; Suzuki, T.; Oishi, S. Growth of ErAl3(BO3)4 single crystals from a K2Mo3O10 flux. Cryst. Growth Des. 2006, 6, 1766−1768. (13) Teshima, K.; Yubuta, K.; Ooi, S.; Suzuki, T.; Shishido, T.; Oishi, S. Environmentally friendly growth of calcium chlorapatite whiskers from a sodium chloride flux. Cryst. Growth Des. 2006, 6, 2538−2542. (14) Teshima, K.; Horita, K.; Suzuki, T.; Ishizawa, N.; Oishi, S. Flux growth and characterization of layered K4Nb6O17 crystals. Chem. Mater. 2006, 18, 3693−3697. (15) Teshima, K.; Takano, A.; Suzuki, T.; Oishi, S. Unique coating of ruby crystals on an aluminum oxide wall by flux evaporation. Chem. Lett. 2005, 34, 1620−1621. (16) Teshima, K.; Matsumoto, K.; Kondo, H.; Suzuki, T.; Oishi, S. Highly crystalline ruby coating on α-Al2O3 surfaces by flux evaporation. J. Ceram. Soc. Japan 2007, 115, 379−382. (17) Oishi, S.; Teshima, K.; Kondo, H. Flux growth of hexagonal bipyramidal ruby crystals. J. Am. Chem. Soc. 2004, 126, 4768−4769. (18) Teshima, K.; Kondo, H.; Oishi, S. Growth of hexagonal bipyramidal ruby crystals by the evaporation of a Li2O−MoO3 flux. Bull. Chem. Soc. Jpn. 2005, 78, 1259−1262. (19) Teshima, K.; Kondo, H.; Oishi, S. Growth of hexagonal bipyramidal ruby crystals by the evaporation of molybdenum trioxide flux. J. Gemmol. 2005, 29, 450−454. (20) ICDD PDF 46-1212.

Figure 7. Time dependence of Al2O3 dissolution concentration.

was approximately 9.6 mol %. The solubility could be measured by determining the moment when the dissolution of the sapphire crystal substrate shifted to the growth of ruby crystals via an equilibrium. Ruby crystal films were grown under stable growth conditions close to equilibrium. This can be applied to a method for conveniently growing crystal films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sayaka Suzuki: 0000-0002-0186-1959 Katsuya Teshima: 0000-0002-5784-5157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS Grants-in-Aid for Scientific Research (C) (No. 18K05272), JSPS Grants-in-Aid for Young Scientists (B) (No. 17K14809), a grant from the Society for the Promotion of Nanshin-Koka-Tandai, a ShinshuMethod for Regional Innovation Ecosystem by Industrial Implementation of Innovative Inorganic Crystal Material Technology of the Program on Regional Innovation and Ecosystem Formation Program from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and Program on Open Innovation Platform with Enterprises, Research Institute and Academia. The authors gratefully thank Ms. Kikuchi, Mr. Horita, and Mr. Yamakami for their help in the EBSD analysis.

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REFERENCES

(1) Maiman, T. H. Stimulated optical radiation in ruby. Nature 1960, 187, 493−494. (2) White, E. A. D. A new technique for the production of synthetic corundum. Nature 1961, 191, 901−902. (3) Elwell, D. Man-Made Gemstones; Ellis Horwood Ltd.: Chichester, UK, 1979. (4) Elwell, D.; Scheel, H. J. Crystal Growth from High-Temperature Solutions; Academic Press: London, 1975. (5) Linares, R. C. Growth of refractory oxide single crystals. J. Appl. Phys. 1962, 33, 1747−1749. (6) Nelson, D. F.; Remeika, J. P. Laser action in a flux-grown ruby. J. Appl. Phys. 1964, 35, 522−529. (7) Stephens, D. L.; Alford, W. J. Dislocation structures in singlecrystal Al2O3. J. Am. Ceram. Soc. 1964, 47, 81−86. 4100

DOI: 10.1021/acs.cgd.9b00483 Cryst. Growth Des. 2019, 19, 4095−4100