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Growth of Ca8La2(PO4)6O2 single crystals as substrates for GaN epitaxial growth
Journal of Crystal Growth 204 (1999) 302}306
Growth of Ca La (PO ) O single crystals as substrates for GaN epitaxial growth A. Yoshikawa *, V.V. Kochurikhin, N. Futagawa , K. Shimamura , T. Fukuda Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan General Physics Institute, Vavilova St. 38, 117942 Moscow, Russia Received 23 March 1999 Communicated by M. Schieber
Abstract 10 0 0 12 oriented Ca La (PO ) O (CLPA) single crystals with the apatite structure are grown by the Czochralski method. Compositional and lattice parameter uniformity of CLPA is discussed in relation to the growth conditions. Since their lattice constant have excellent matching with three times that of GaN, they are candidates as new substrates for the growth of high-quality GaN epitaxial layers. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq Keywords: Apatite structure; Czochralski growth; Substrate; GaN
1. Introduction It is known that single crystals with the apatite structure (space group P6 /m) are e!ective host materials for luminescent and laser applications [1,2]. Moreover, the existence of a variety of synthetic apatites, compounds of the rare-earth oxyapatite family such as Ca RE (PO ) O (RE"La}Lu) and Ca RE (SiO ) O allows a variety of lattice constants. This means that the rare-earth oxyapatite family has large potential for substrate materials as well.
* Corresponding author. Fax: #81-22-215-2104. E-mail address: [email protected] Yoshikawa)
(A.
In recent years, GaN has been widely investigated as a material for optoelectronic devices such as lasers and light emitting diodes (LEDs) for the blue and ultraviolet region. It has been reported that GaN thin "lms were grown on sapphire substrates by the metal organic chemical vapor deposition (MOCVD) technique for LED and laser applications [3}7]. However, dislocation densities in these "lms were found to be as high as 10 cm\ [8,9] because of the large lattice mismatch between the epitaxial layers and the substrates. Although other materials such as ZnO, MgO and SiC have also been studied as substrates, high-quality epitaxial "lms have not yet been obtained [10]. Since high-quality bulk GaN single crystals for homoepitaxy are not yet grown, it is very important to "nd an appropriate substrate for GaN heteroepitaxy.
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 9 1 - 8
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
We have found new candidates, CLPA as substrates for GaN epitaxial growth. CLPA is expected to have fairly small lattice mismatch with three times the value of the lattice constant of GaN. In the present work, we tried to determine the optimal growth conditions of large CLPA single crystals. Up to now, such crystals have been grown only by spontaneous crystallization from a solution in a melt.
2. Experimental procedure Oxyapatite single crystals were grown by the Czochralski method using an iridium crucible with a diameter of 50 mm. The heating was by induction at a frequency of 8 kHz. The temperature at the surface of the melt was measured by means of an optical pyrometer. Starting materials were prepared by mixing of 99.99% pure CaCO and La O powders and 98% pure CaHPO powder at the stoichiometric ratio of Ca La (PO ) O . Before growth, oxyapatites were synthesized in a platinum crucible in air during 24 h at 14503C. The growth atmosphere was Ar gas, or a mixture of Ar plus a small amount of O . Pulling rate was 1}2 mm/h and rotation rate was 20 rpm. Powder X-ray di!raction analysis were carried out to identify the phase of grown crystal. The X-ray source was Co Ka, the access voltage was 35 kV, and access current was 25 mA. To investigate the lattice constant of CLPA at high temperature, a RIGAKU high-temperature powder X-ray di!ractometer was used. The X-ray source was Cu Ka, the access voltage 40 kV, and access current 50 mA. The di!raction patterns were obtained in range 2h"203 to 803 at 5003, 6003, 7003, 8003, 9003, 10003, 11003, 12003, 13003, 14003 and 14503. The chemical composition was analyzed by electron microprobe analysis (EPMA). Transmittance was measured in the wavelength region from 200 to 2300 nm.
3. Results and discussion In Ar atmosphere, the "nal melting of the synthesized oxyapatite was observed to occur at
303
1650$103C. This value is in good agreement with the literature data [11]. However, if we merely held the melt at this temperature, even for a long time, we did not succeed in growing transparent crystals. Although, crystals were observed to be transparent during the growth process, after cooling they became cloudy and white in color, with many small cracks along the boule. This behavior was observed under both inert (pure Ar) and slightly oxidizing (Ar#1 vol% O ) atmosphere. It has been reported that in the case of phosphate apatites, the formation of poorly reactive intermediate phases like Ca P O is possible [12]. However, according to the result of powder X-ray di!raction analysis, our crystals grown as described always had a single
Fig. 1. (a) Powder X-ray di!raction pattern of CLPA boule (single crystal) produced with overheating. (b) X-ray di!raction pattern of CLPA single crystal obtained on polished plane perpendicular to the growth direction.
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A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
Fig. 2. Dependence of the lattice parameter on the temperature.
Ca La (PO ) O phase. Better results were obtained by overheating the melt to 17303C and holding it at this temperature for 2 h. Then, the melt was super-cooled to around 15503C, and the growth process was started. In that case, the crystal remained transparent without cracks after cooling. Fig. 1a shows the powder X-ray di!raction pattern of this transparent single CLPA crystal. Lattice constants were calculated from this data to be a"9.446(1) As and c"6.922(1) As . The lattice mismatch between GaN (a"3.16}3.19 As ) and CLPA is calculated as follows: a !a /3 %, !*. "0.003586 2 i.e. 0.36%. a %, Furthermore, at 10003C (the growth temperature of GaN; a"3.18}3.21 As ), the lattice constant of aaxis for CLPA is 9.561(4) As (Fig. 2). Thus, the lattice mismatch between GaN and CLPA at this temperature is only 0.089}0.71%. Fig. 1b shows the X-ray di!raction pattern observed on a cross section of the same single crystal
Fig. 3. Typical CLPA single crystal grown from overheated melt and a plate cut from the body of that crystal.
for scattering vector parallel to the growth direction. It is clear that the growth axis is controlled to be 10 0 0 12. Fig. 3 shows photographs of typical CLPA crystal grown from an overheated melt with supercooling before growth and plate cut from the body part of such a crystal. The optical transmission spectrum is shown in Fig. 4. Our CLPA shows good transmittance data from 250 to 2300 nm and no absorption was observed. We note here that there were some inclusions inside the crystals, and their distribution along the boule depended on the growth conditions. Crystals pulled at rates exceeding 2.5 mm/h always had included particles with sizes of about &10 lm collected mainly in the core region (Fig. 5). Moreover, these crystals were usually clear on the top, but the concentration of the precipitate increased towards
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
305
Fig. 4. Optical transmission spectrum of CLPA.
the bottom. In accordance with the local composition measurement by EPMA (beam diameter was 1 lm) the inclusions contained La O . However, crystals pulled at a rate of 1.5 mm/h had no detectable inclusions along the boule. With rates lower than 1.5 mm/h, the crystal quality was not improved considerably. The chemical composition and lattice parameter along the pulling direction of a crystal grown at 1.5 mm/h, is shown in Fig. 6. The lattice parameter is almost uniform along the growth axis and the chemical composition of all cations is also almost constant. The uniformity of the chemical composition agreed well with that of the lattice parameter.
4. Summary Ca La (PO ) O single crystals with apatite structure were grown by Czochralski method. The growth orientation was controlled to be 10 0 0 12 so that it can be used as a substrate for the growth of 10 0 0 12 GaN epitaxial layers. The importance of melt overheating for melting and super-cooling before growth was demonstrated. Crystals grown under optimized conditions were transparent and colorless. The lattice mismatch with GaN was calculated to be 0.089}0.71%. Considering the possibility for precise control of lattice constants, this system represents a promising candidate as a substrate material for GaN epitaxial layers with low dislocation density.
Fig. 5. Composition map obtained by EPMA of impurity collected mainly in the core region of crystals pulled at rates exceeding 2.5 mm/h. (a) is for La, (b) is for Ca and (c) is for P. Only La is observed.
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A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
Corporation) and also like to thank Prof. Dr. Durbin of IMR and Prof. Dr. Chai for critical reading and valuable suggestions in the manuscript. They would also like to thank Mr. Murakami of the Laboratory for Developmental Research of Advanced Materials in IMR for his support with the EPMA analysis.
References
Fig. 6. (a) Dependence of the lattice parameter on the solidi"cation fraction. Error bars are included in each marker. (b) Concentration dependence of each cation on the solidi"cation fraction, where C and C are the initial concentration and the concentration at each solidi"cation fraction, respectively. Error bars are included in each marker.
Acknowledgements The authors would like to express sincere thanks to Visiting Professor Dr. O. Oda (Japan Energy
[1] R. Mazelsky, R.C. Ohmann, K. Steinbruegge, J. Electrochem. Soc. 115 (1968) 68. [2] B.M.J. Smet, Mater. Chem. Phys. 16 (1987) 123. [3] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [4] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdolv, M. Burns, J. Appl. Phys. 76 (1994) 1363. [5] I. Akasaki, J. Amano, S. Sato, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [6] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [7] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [8] S.D. Laser, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [9] M. Osinski, D.L. Barton, Proceedings of the International Symposiom on Blue Laser and Light Emitting Diodes, 1996, p. 217. [10] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [11] K.B. Steinbruegge, T. Henningsen, R.H. Hopkins, R. Mazelsky, N.T. Melamed, E.P. Riedel, G.W. Roland, Appl. Opt. 11 (1972) 999. [12] L. Boyer, J. Carpena, J.L. Lacout, Sol. State Ionics 95 (1997) 121.
Growth of Ca La (PO ) O single crystals as substrates for GaN epitaxial growth A. Yoshikawa *, V.V. Kochurikhin, N. Futagawa , K. Shimamura , T. Fukuda Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan General Physics Institute, Vavilova St. 38, 117942 Moscow, Russia Received 23 March 1999 Communicated by M. Schieber
Abstract 10 0 0 12 oriented Ca La (PO ) O (CLPA) single crystals with the apatite structure are grown by the Czochralski method. Compositional and lattice parameter uniformity of CLPA is discussed in relation to the growth conditions. Since their lattice constant have excellent matching with three times that of GaN, they are candidates as new substrates for the growth of high-quality GaN epitaxial layers. 1999 Elsevier Science B.V. All rights reserved. PACS: 81.10.Fq Keywords: Apatite structure; Czochralski growth; Substrate; GaN
1. Introduction It is known that single crystals with the apatite structure (space group P6 /m) are e!ective host materials for luminescent and laser applications [1,2]. Moreover, the existence of a variety of synthetic apatites, compounds of the rare-earth oxyapatite family such as Ca RE (PO ) O (RE"La}Lu) and Ca RE (SiO ) O allows a variety of lattice constants. This means that the rare-earth oxyapatite family has large potential for substrate materials as well.
* Corresponding author. Fax: #81-22-215-2104. E-mail address: [email protected] Yoshikawa)
(A.
In recent years, GaN has been widely investigated as a material for optoelectronic devices such as lasers and light emitting diodes (LEDs) for the blue and ultraviolet region. It has been reported that GaN thin "lms were grown on sapphire substrates by the metal organic chemical vapor deposition (MOCVD) technique for LED and laser applications [3}7]. However, dislocation densities in these "lms were found to be as high as 10 cm\ [8,9] because of the large lattice mismatch between the epitaxial layers and the substrates. Although other materials such as ZnO, MgO and SiC have also been studied as substrates, high-quality epitaxial "lms have not yet been obtained [10]. Since high-quality bulk GaN single crystals for homoepitaxy are not yet grown, it is very important to "nd an appropriate substrate for GaN heteroepitaxy.
0022-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 1 9 1 - 8
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
We have found new candidates, CLPA as substrates for GaN epitaxial growth. CLPA is expected to have fairly small lattice mismatch with three times the value of the lattice constant of GaN. In the present work, we tried to determine the optimal growth conditions of large CLPA single crystals. Up to now, such crystals have been grown only by spontaneous crystallization from a solution in a melt.
2. Experimental procedure Oxyapatite single crystals were grown by the Czochralski method using an iridium crucible with a diameter of 50 mm. The heating was by induction at a frequency of 8 kHz. The temperature at the surface of the melt was measured by means of an optical pyrometer. Starting materials were prepared by mixing of 99.99% pure CaCO and La O powders and 98% pure CaHPO powder at the stoichiometric ratio of Ca La (PO ) O . Before growth, oxyapatites were synthesized in a platinum crucible in air during 24 h at 14503C. The growth atmosphere was Ar gas, or a mixture of Ar plus a small amount of O . Pulling rate was 1}2 mm/h and rotation rate was 20 rpm. Powder X-ray di!raction analysis were carried out to identify the phase of grown crystal. The X-ray source was Co Ka, the access voltage was 35 kV, and access current was 25 mA. To investigate the lattice constant of CLPA at high temperature, a RIGAKU high-temperature powder X-ray di!ractometer was used. The X-ray source was Cu Ka, the access voltage 40 kV, and access current 50 mA. The di!raction patterns were obtained in range 2h"203 to 803 at 5003, 6003, 7003, 8003, 9003, 10003, 11003, 12003, 13003, 14003 and 14503. The chemical composition was analyzed by electron microprobe analysis (EPMA). Transmittance was measured in the wavelength region from 200 to 2300 nm.
3. Results and discussion In Ar atmosphere, the "nal melting of the synthesized oxyapatite was observed to occur at
303
1650$103C. This value is in good agreement with the literature data [11]. However, if we merely held the melt at this temperature, even for a long time, we did not succeed in growing transparent crystals. Although, crystals were observed to be transparent during the growth process, after cooling they became cloudy and white in color, with many small cracks along the boule. This behavior was observed under both inert (pure Ar) and slightly oxidizing (Ar#1 vol% O ) atmosphere. It has been reported that in the case of phosphate apatites, the formation of poorly reactive intermediate phases like Ca P O is possible [12]. However, according to the result of powder X-ray di!raction analysis, our crystals grown as described always had a single
Fig. 1. (a) Powder X-ray di!raction pattern of CLPA boule (single crystal) produced with overheating. (b) X-ray di!raction pattern of CLPA single crystal obtained on polished plane perpendicular to the growth direction.
304
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
Fig. 2. Dependence of the lattice parameter on the temperature.
Ca La (PO ) O phase. Better results were obtained by overheating the melt to 17303C and holding it at this temperature for 2 h. Then, the melt was super-cooled to around 15503C, and the growth process was started. In that case, the crystal remained transparent without cracks after cooling. Fig. 1a shows the powder X-ray di!raction pattern of this transparent single CLPA crystal. Lattice constants were calculated from this data to be a"9.446(1) As and c"6.922(1) As . The lattice mismatch between GaN (a"3.16}3.19 As ) and CLPA is calculated as follows: a !a /3 %, !*. "0.003586 2 i.e. 0.36%. a %, Furthermore, at 10003C (the growth temperature of GaN; a"3.18}3.21 As ), the lattice constant of aaxis for CLPA is 9.561(4) As (Fig. 2). Thus, the lattice mismatch between GaN and CLPA at this temperature is only 0.089}0.71%. Fig. 1b shows the X-ray di!raction pattern observed on a cross section of the same single crystal
Fig. 3. Typical CLPA single crystal grown from overheated melt and a plate cut from the body of that crystal.
for scattering vector parallel to the growth direction. It is clear that the growth axis is controlled to be 10 0 0 12. Fig. 3 shows photographs of typical CLPA crystal grown from an overheated melt with supercooling before growth and plate cut from the body part of such a crystal. The optical transmission spectrum is shown in Fig. 4. Our CLPA shows good transmittance data from 250 to 2300 nm and no absorption was observed. We note here that there were some inclusions inside the crystals, and their distribution along the boule depended on the growth conditions. Crystals pulled at rates exceeding 2.5 mm/h always had included particles with sizes of about &10 lm collected mainly in the core region (Fig. 5). Moreover, these crystals were usually clear on the top, but the concentration of the precipitate increased towards
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
305
Fig. 4. Optical transmission spectrum of CLPA.
the bottom. In accordance with the local composition measurement by EPMA (beam diameter was 1 lm) the inclusions contained La O . However, crystals pulled at a rate of 1.5 mm/h had no detectable inclusions along the boule. With rates lower than 1.5 mm/h, the crystal quality was not improved considerably. The chemical composition and lattice parameter along the pulling direction of a crystal grown at 1.5 mm/h, is shown in Fig. 6. The lattice parameter is almost uniform along the growth axis and the chemical composition of all cations is also almost constant. The uniformity of the chemical composition agreed well with that of the lattice parameter.
4. Summary Ca La (PO ) O single crystals with apatite structure were grown by Czochralski method. The growth orientation was controlled to be 10 0 0 12 so that it can be used as a substrate for the growth of 10 0 0 12 GaN epitaxial layers. The importance of melt overheating for melting and super-cooling before growth was demonstrated. Crystals grown under optimized conditions were transparent and colorless. The lattice mismatch with GaN was calculated to be 0.089}0.71%. Considering the possibility for precise control of lattice constants, this system represents a promising candidate as a substrate material for GaN epitaxial layers with low dislocation density.
Fig. 5. Composition map obtained by EPMA of impurity collected mainly in the core region of crystals pulled at rates exceeding 2.5 mm/h. (a) is for La, (b) is for Ca and (c) is for P. Only La is observed.
306
A. Yoshikawa et al. / Journal of Crystal Growth 204 (1999) 302}306
Corporation) and also like to thank Prof. Dr. Durbin of IMR and Prof. Dr. Chai for critical reading and valuable suggestions in the manuscript. They would also like to thank Mr. Murakami of the Laboratory for Developmental Research of Advanced Materials in IMR for his support with the EPMA analysis.
References
Fig. 6. (a) Dependence of the lattice parameter on the solidi"cation fraction. Error bars are included in each marker. (b) Concentration dependence of each cation on the solidi"cation fraction, where C and C are the initial concentration and the concentration at each solidi"cation fraction, respectively. Error bars are included in each marker.
Acknowledgements The authors would like to express sincere thanks to Visiting Professor Dr. O. Oda (Japan Energy
[1] R. Mazelsky, R.C. Ohmann, K. Steinbruegge, J. Electrochem. Soc. 115 (1968) 68. [2] B.M.J. Smet, Mater. Chem. Phys. 16 (1987) 123. [3] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [4] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdolv, M. Burns, J. Appl. Phys. 76 (1994) 1363. [5] I. Akasaki, J. Amano, S. Sato, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [6] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [7] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [8] S.D. Laser, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [9] M. Osinski, D.L. Barton, Proceedings of the International Symposiom on Blue Laser and Light Emitting Diodes, 1996, p. 217. [10] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [11] K.B. Steinbruegge, T. Henningsen, R.H. Hopkins, R. Mazelsky, N.T. Melamed, E.P. Riedel, G.W. Roland, Appl. Opt. 11 (1972) 999. [12] L. Boyer, J. Carpena, J.L. Lacout, Sol. State Ionics 95 (1997) 121.