- Email: [email protected]
Growth and characterization of (La,Sr)(Al,Ta)O3 single crystals as substrates for GaN epitaxial growth
Journal of Crystal Growth 194 (1998) 209—213
Growth and characterization of (La,Sr)(Al,Ta)O single crystals as substrates for GaN epitaxial growth Kiyoshi Shimamura*, Hideo Tabata, Hiroaki Takeda, Vladimir V. Kochurikhin, Tsuguo Fukuda Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 12 July 1998
Abstract We have successfully grown 11 1 12 oriented (La,Sr)(Al,Ta)O (LSAT) mixed-perovskite single crystals by the Czochralski method. Compositional uniformity and coloring phenomena are discussed in relation to the growth conditions. Since LSAT single crystals have excellent lattice matching with GaN, they are promising as new substrates for the growth of high-quality GaN epitaxial layers. 1998 Elsevier Science B.V. All rights reserved. Keywords: (La,Sr)(Al,Ta)O ; GaN; Czochralski method; Substrate; Lattice matching; Perovskite
1. Introduction Due to its wide direct band gap and high thermal stability, GaN is a promising material for optoelectronic devices such as diodes and lasers emitting from blue to ultraviolet wavelength regions, as well as for electronic devices operating at high temperatures. It has been reported that GaN layers could be successfully grown on sapphire substrates by MOCVD, and blue light emitting diodes and lasers have been realized [1—5]. However, dislocation densities in these layers are very large, up to the level of 10—10 cm\, because of the large lattice mismatch between the epitaxial layers and the sub-
* Corresponding author. Fax: #81 22 215 2104; e-mail: [email protected].
strates [6,7]. These dislocations have recently been shown to strongly affect the lifetime of continuous wave blue laser diodes [8]. While sapphire is widely used as a substrate for GaN epitaxial growth, its large lattice mismatch with GaN (14—16%) is a limiting factor for the quality improvement of the grown epilayers [8]. Although other materials such as GaAs, SiC, MgO and ZnO have also been studied as substrates, high-quality epilayers could not be obtained because of their large lattice mismatch and/or decomposition under the reducing atmosphere used for epitaxial growth [9]. Since high-quality bulk GaN single crystals are not yet available, it is very important to find appropriate substrates for GaN heteroepitaxy. We have found new candidates, (La,Sr)(Al,Ta)O (LSAT), as substrates for GaN epitaxial growth.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 3 0 - 1
210
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
LSAT is expected to have a fairly small lattice mismatch ((0.1%) with GaN. The use of LSAT mixed-perovskite crystals as substrates for high-¹ superconductors was first reported by Mateika et al. [10]. LSAT belongs to the cubic crystal system, while GaN to the hexagonal crystal system with the space group P6 mc of the wurtzite-type structure. Although materials belonging to the hexagonal crystal system are preferable as substrates, those belonging to the cubic system may also be candidates, since the (1 1 1) of cubic crystals has hexagonal symmetry. In the present work, we demonstrate the successful growth of LSAT single crystals. We have also investigated uniformity of the chemical composition and lattice parameter, the crystal quality and coloring phenomena.
2. Experimental procedure Single crystals were grown in a conventional Czochralski (CZ) furnace driven by a 60 kW RF generator with an iridium crucible (50 mm in diameter and height). The La O , SrCO , Al O and Ta O powders of 99.99% purity were used as raw materials. Starting material was prepared by mixing these powders to the composition of La Sr Al Ta O [10], which is ex pected to have good lattice matching ((0.1% mismatch) with GaN, and then pressing to the tablet shape with dimensions of 48 mm in diameter and in height. Subsequently, they were placed in the iridium crucible, and heated to approximately 1970°C to melt them. The crystals were grown under Ar atmosphere with a gas flow rate of 1.0 l/min. The pulling rate was 1 mm/h and the rotation rate was 10 rpm. The phase identification and determination of the lattice parameter of the grown crystals were carried out by X-ray powder diffraction analysis. The chemical composition was analyzed by electron microprobe analysis. Transmittance was measured in the wavelength region from 200 to 2500 nm for two polished specimens (9;9; 1 mm) with a (1 1 1) plane cut. One specimen was annealed under Ar: 0.2% H atmosphere, and the
transmittance of this specimen was compared with that of the untreated specimen.
3. Results and discussion The CZ growth of LSAT single crystals was performed using an iridium rod as a seed. Since the grown crystal was oriented to the 10 1 12 direction, the preferred growth orientation of LSAT is supposed to be 10 1 12. For further CZ crystal growth, seed crystals were prepared from this crystal along the growth axis. Although Al O and Y Al O single crystals were also tried as seeds, LSAT crystals could not be pulled up from the melt. When the seed crystal touched the melt surface, it melted because of the formation of the other phases with lower melting temperatures than that of LSAT. Fig. 1 shows an as-grown crystal with the growth axis along 10 1 12, which corresponds to the preferred growth orientation. This crystal was transparent and free from inclusions, and was composed of a single phase with the LSAT structure. In this crystal, a large crack roughly parallel to (1 0 0) was observed. The 11 1 12 oriented single crystals are appropriate to prepare 11 1 12 oriented substrates to realize lattice matching with GaN 10 0 0 12 epitaxial layers. Therefore, 11 1 12 oriented seed crystals were prepared from the crystal shown in Fig. 1. Fig. 2 shows an as-grown LSAT single crystal with dimensions of 22 mm in diameter and 100 mm
Fig. 1. LSAT crystal grown with the preferred growth orientation 10 1 12.
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
211
Fig. 2. As-grown LSAT single crystal pulled along the 11 1 12 orientation.
in length, pulled along the 11 1 12 direction. This crystal was red-brown in color. Although a crack formed in the crystal pulled along the preferred growth orientation 10 1 12, the 11 1 12 oriented crystal was free of cracks. This may be because of the smaller thermal gradient across a (1 0 0) face in 11 1 12 oriented growth, since the (1 0 0) face is tilted from the 11 1 12 orientation, whereas it is parallel to the growth axis in the case of the 10 1 12 oriented growth. The solidification fraction, g (weight ratio of crystal to starting melt), for the crystal shown in Fig. 2 was 0.42. When the value of g exceeded 0.6, spiral growth occurred. Fig. 3a shows a cross-polarized light image of the wafer cut parallel to the growth direction at the shoulder part of the crystal shown in Fig. 2. The growth interface was fairly convex towards the melt. The existence of a core in the central region was clearly shown by the bright area in the image. Fig. 3b shows a similar image of the wafer prepared from the crystal grown under the same growth conditions, except that the rotation rate was 40 rpm rather than 10 rpm. Although the growth interface at 40 rpm was still convex towards the melt, the core observed in Fig. 3a disappeared, and the shape of the growth interface became smooth. This suggests that a higher crystal rotation speed is preferable in order to decrease the stress in the grown crystals. The red-brown color of the LSAT specimen changed to pale yellow after annealing at 1650°C for 6 h under Ar: 0.2% H gas mixtured atmo sphere. Fig. 4 shows the specimen cut perpendicular to the growth axis of the as-grown crystal, compared with that of the annealed one.
Fig. 3. Cross-polarized light image of wafers from the shoulder part of the crystal in Fig. 2, cut parallel to the growth direction. The crystal rotation rate was 10 rpm (a) and 40 rpm (b).
Fig. 4. As-grown wafer, cut perpendicular to the growth axis, with red-brown color (a), and an annealed one with pale yellow color (b). This distinct color change was observed after annealing under Ar: 0.2% H atmosphere.
212
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
Fig. 5 shows a typical transmission spectrum of the LSAT as-grown specimen compared with that of the annealed one. The as-grown specimen had an absorption edge at 258 nm and a broad absorption peak, while the annealed one had an absorption edge at 264 nm and a gentle slope without distinct absorption peaks, below the 800 nm wavelength region. The existence of the broad absorption peak and the gentle slope are the cause of red-brown and the pale yellow coloring of as-grown and annealed crystals, respectively. Annealing at 1780°C for 24 h under the same atmosphere made the LSAT specimen black. We attributed this black coloration to the valence change of Ta>, that is, Ta> or Ta> appeared by reduction of Ta> at high temperatures. Although Ta> and Ta> cations cannot exist at the site coordinated by eight oxygens, they can exist at the site coordinated by six oxygens [11]. These results, together with Fig. 5, indicate that a growth atmosphere with a lesser amount of H , such as 0.1% or 0.05%, is better for the growth of colorless LSAT crystals. The lattice parameter of the LSAT single crystal was 7.735 A> . Fig. 6 shows the dependence of the lattice parameter on the solidification fraction. The lattice parameter was almost constant along the growth axis. Fig. 7 shows the distribution coefficient of each cation composing LSAT, depending on the solidification fraction. The distribution coefficients, k, of La>, Sr>, Al> and Ta> were 1.06 (k ), 1.04 (k ), 1.05 (k ) and 0.89 (k ), respectively. * 1 2
Fig. 5. Transmission spectrum of the as-grown LSAT single crystal and that of the annealed crystal.
Fig. 6. Dependence of the lattice parameter on the solidification fraction.
Fig. 7. Concentration dependence of each cation on the solidification fraction, where C and C are the initial concentration and the concentration at each solidification fraction, respectively. Error bars are included in each marker.
The values of k , k , and k were larger than * 1 unity, while that of k was less than unity. How2 ever, each cation was distributed almost uniformly along the growth axis. The uniformity of chemical composition agreed well with that of the lattice parameter. This is because g is small and k is near to unity. If a crystal is grown from La>-, Sr>- and Al>-enriched and Ta>-deficient melt composition, compared with that of La Sr Al Ta O , perhaps, we predict a crystal with distri bution coefficient of unity would result. It is known that the color of crystals has an effect on the growth behavior, since radiative heat transfer within the crystal significantly affects the
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
temperature distribution in crystals with high melting temperatures [12,13]. The tendency for unstable growth (e.g. spiral growth) in such crystals has been explained on the basis of the optical absorption in the infrared, together with Wien’s displacement law [14,15]. However, the case of LSAT does not fit with this explanation. In the wavelength region from 800 to 2500 nm, LSAT shows constant transmittance without any distinct absorption peaks (Fig. 5). We believe the origin of the spiral phenomenon, which appeared at g"0.6 during the growth of LSAT single crystals to be due to the change of the melt composition. As LSAT crystals grow, the composition change in the melt proceedes in the direction of Ta> enrichment. When the melt composition reaches a critical point, which may correspond to g"0.6, stable crystal growth cannot continue, and spiral growth starts. The (1 1 1) plane of the LSAT crystals is a closepacked plane of the anion framework, which forms the primitive cubic lattice, and thus the 11 1 12 direction is the body-diagonal direction of this cubic cell. The surface structure, therefore, has hexagonal symmetry. The lattice parameter “a ” of the LSAT (1 1 1) plane, considered as a 2-dimensional hexagonal crystal, is given by a "(6/3a
(1)
where a is the conventional lattice parameter of the cubic crystal. Since the measured a of LSAT was 7.735 A> , a was calculated to be 6.316 A> , which was close to twice the corresponding lattice parameter of GaN (3.160—3.190 A> ). The lattice mismatch was calculated to be 0.06—1.00%.
4. Summary LSAT single crystals of the cubic crystal system were successfully grown by the conventional Czochralski method. The growth orientation was controlled to be 11 1 12 so the crystals could serve as substrates for the growth of 10 0 0 12 GaN epitaxial layers. The lattice parameter was 7.735 A> , constant along the growth axis. This agreed with the uniformity of the chemical composition. Asgrown crystals were red-brown in color, while crys-
213
tals annealed under Ar: 0.2% H atmosphere be came pale yellow in color. Because of the small lattice mismatch with GaN, LSAT single crystals have high potential as substrates for the growth of high-quality GaN epitaxial layers with low dislocation density.
Acknowledgements The authors would like to express sincere thanks to Visiting Professor O. Oda (Japan Energy Corporation) and Associate Professor S. Durbin of the Institute for Materials Research, Tohoku University, for their critical reading of the manuscript and fruitful discussions. The authors are also indebted to Mr. Y. Murakami of the Institute for Materials Research, Tohoku University, for his help in carrying out the chemical composition analysis.
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [4] I. Akasaki, H. Amano, S. Sato, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [5] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [6] S.D. Lester, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [7] M. Osinski, D.L. Barton, Proc. of the Int. Symp. on Blue Laser and Light Emitting Diodes, 1996, p. 217. [8] H. Amano, N. Sawaki, I. Akazaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [9] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [10] D. Mateika, H. Kohler, H. Laudan, E. Vo¨lkel, J. Crystal Growth 109 (1991) 447. [11] R.D. Shannon, Acta Crystalogr. A 32 (1976) 751. [12] S. Brandon, J.J. Derby, J. Crystal Growth 110 (1991) 481. [13] S. Brandon, J.J. Derby, J. Crystal Growth 121 (1992) 473. [14] Y. Terada, T. Maeda, V.V. Kochurikhin, K. Shimamura, S. Uda, T. Fukuda, J. Crystal Growth 178 (1997) 518. [15] R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, 2nd ed., McGraw-Hill, New York, 1981, p. 24.
Growth and characterization of (La,Sr)(Al,Ta)O single crystals as substrates for GaN epitaxial growth Kiyoshi Shimamura*, Hideo Tabata, Hiroaki Takeda, Vladimir V. Kochurikhin, Tsuguo Fukuda Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 12 July 1998
Abstract We have successfully grown 11 1 12 oriented (La,Sr)(Al,Ta)O (LSAT) mixed-perovskite single crystals by the Czochralski method. Compositional uniformity and coloring phenomena are discussed in relation to the growth conditions. Since LSAT single crystals have excellent lattice matching with GaN, they are promising as new substrates for the growth of high-quality GaN epitaxial layers. 1998 Elsevier Science B.V. All rights reserved. Keywords: (La,Sr)(Al,Ta)O ; GaN; Czochralski method; Substrate; Lattice matching; Perovskite
1. Introduction Due to its wide direct band gap and high thermal stability, GaN is a promising material for optoelectronic devices such as diodes and lasers emitting from blue to ultraviolet wavelength regions, as well as for electronic devices operating at high temperatures. It has been reported that GaN layers could be successfully grown on sapphire substrates by MOCVD, and blue light emitting diodes and lasers have been realized [1—5]. However, dislocation densities in these layers are very large, up to the level of 10—10 cm\, because of the large lattice mismatch between the epitaxial layers and the sub-
* Corresponding author. Fax: #81 22 215 2104; e-mail: [email protected].
strates [6,7]. These dislocations have recently been shown to strongly affect the lifetime of continuous wave blue laser diodes [8]. While sapphire is widely used as a substrate for GaN epitaxial growth, its large lattice mismatch with GaN (14—16%) is a limiting factor for the quality improvement of the grown epilayers [8]. Although other materials such as GaAs, SiC, MgO and ZnO have also been studied as substrates, high-quality epilayers could not be obtained because of their large lattice mismatch and/or decomposition under the reducing atmosphere used for epitaxial growth [9]. Since high-quality bulk GaN single crystals are not yet available, it is very important to find appropriate substrates for GaN heteroepitaxy. We have found new candidates, (La,Sr)(Al,Ta)O (LSAT), as substrates for GaN epitaxial growth.
0022-0248/98/$ — see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 7 3 0 - 1
210
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
LSAT is expected to have a fairly small lattice mismatch ((0.1%) with GaN. The use of LSAT mixed-perovskite crystals as substrates for high-¹ superconductors was first reported by Mateika et al. [10]. LSAT belongs to the cubic crystal system, while GaN to the hexagonal crystal system with the space group P6 mc of the wurtzite-type structure. Although materials belonging to the hexagonal crystal system are preferable as substrates, those belonging to the cubic system may also be candidates, since the (1 1 1) of cubic crystals has hexagonal symmetry. In the present work, we demonstrate the successful growth of LSAT single crystals. We have also investigated uniformity of the chemical composition and lattice parameter, the crystal quality and coloring phenomena.
2. Experimental procedure Single crystals were grown in a conventional Czochralski (CZ) furnace driven by a 60 kW RF generator with an iridium crucible (50 mm in diameter and height). The La O , SrCO , Al O and Ta O powders of 99.99% purity were used as raw materials. Starting material was prepared by mixing these powders to the composition of La Sr Al Ta O [10], which is ex pected to have good lattice matching ((0.1% mismatch) with GaN, and then pressing to the tablet shape with dimensions of 48 mm in diameter and in height. Subsequently, they were placed in the iridium crucible, and heated to approximately 1970°C to melt them. The crystals were grown under Ar atmosphere with a gas flow rate of 1.0 l/min. The pulling rate was 1 mm/h and the rotation rate was 10 rpm. The phase identification and determination of the lattice parameter of the grown crystals were carried out by X-ray powder diffraction analysis. The chemical composition was analyzed by electron microprobe analysis. Transmittance was measured in the wavelength region from 200 to 2500 nm for two polished specimens (9;9; 1 mm) with a (1 1 1) plane cut. One specimen was annealed under Ar: 0.2% H atmosphere, and the
transmittance of this specimen was compared with that of the untreated specimen.
3. Results and discussion The CZ growth of LSAT single crystals was performed using an iridium rod as a seed. Since the grown crystal was oriented to the 10 1 12 direction, the preferred growth orientation of LSAT is supposed to be 10 1 12. For further CZ crystal growth, seed crystals were prepared from this crystal along the growth axis. Although Al O and Y Al O single crystals were also tried as seeds, LSAT crystals could not be pulled up from the melt. When the seed crystal touched the melt surface, it melted because of the formation of the other phases with lower melting temperatures than that of LSAT. Fig. 1 shows an as-grown crystal with the growth axis along 10 1 12, which corresponds to the preferred growth orientation. This crystal was transparent and free from inclusions, and was composed of a single phase with the LSAT structure. In this crystal, a large crack roughly parallel to (1 0 0) was observed. The 11 1 12 oriented single crystals are appropriate to prepare 11 1 12 oriented substrates to realize lattice matching with GaN 10 0 0 12 epitaxial layers. Therefore, 11 1 12 oriented seed crystals were prepared from the crystal shown in Fig. 1. Fig. 2 shows an as-grown LSAT single crystal with dimensions of 22 mm in diameter and 100 mm
Fig. 1. LSAT crystal grown with the preferred growth orientation 10 1 12.
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
211
Fig. 2. As-grown LSAT single crystal pulled along the 11 1 12 orientation.
in length, pulled along the 11 1 12 direction. This crystal was red-brown in color. Although a crack formed in the crystal pulled along the preferred growth orientation 10 1 12, the 11 1 12 oriented crystal was free of cracks. This may be because of the smaller thermal gradient across a (1 0 0) face in 11 1 12 oriented growth, since the (1 0 0) face is tilted from the 11 1 12 orientation, whereas it is parallel to the growth axis in the case of the 10 1 12 oriented growth. The solidification fraction, g (weight ratio of crystal to starting melt), for the crystal shown in Fig. 2 was 0.42. When the value of g exceeded 0.6, spiral growth occurred. Fig. 3a shows a cross-polarized light image of the wafer cut parallel to the growth direction at the shoulder part of the crystal shown in Fig. 2. The growth interface was fairly convex towards the melt. The existence of a core in the central region was clearly shown by the bright area in the image. Fig. 3b shows a similar image of the wafer prepared from the crystal grown under the same growth conditions, except that the rotation rate was 40 rpm rather than 10 rpm. Although the growth interface at 40 rpm was still convex towards the melt, the core observed in Fig. 3a disappeared, and the shape of the growth interface became smooth. This suggests that a higher crystal rotation speed is preferable in order to decrease the stress in the grown crystals. The red-brown color of the LSAT specimen changed to pale yellow after annealing at 1650°C for 6 h under Ar: 0.2% H gas mixtured atmo sphere. Fig. 4 shows the specimen cut perpendicular to the growth axis of the as-grown crystal, compared with that of the annealed one.
Fig. 3. Cross-polarized light image of wafers from the shoulder part of the crystal in Fig. 2, cut parallel to the growth direction. The crystal rotation rate was 10 rpm (a) and 40 rpm (b).
Fig. 4. As-grown wafer, cut perpendicular to the growth axis, with red-brown color (a), and an annealed one with pale yellow color (b). This distinct color change was observed after annealing under Ar: 0.2% H atmosphere.
212
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
Fig. 5 shows a typical transmission spectrum of the LSAT as-grown specimen compared with that of the annealed one. The as-grown specimen had an absorption edge at 258 nm and a broad absorption peak, while the annealed one had an absorption edge at 264 nm and a gentle slope without distinct absorption peaks, below the 800 nm wavelength region. The existence of the broad absorption peak and the gentle slope are the cause of red-brown and the pale yellow coloring of as-grown and annealed crystals, respectively. Annealing at 1780°C for 24 h under the same atmosphere made the LSAT specimen black. We attributed this black coloration to the valence change of Ta>, that is, Ta> or Ta> appeared by reduction of Ta> at high temperatures. Although Ta> and Ta> cations cannot exist at the site coordinated by eight oxygens, they can exist at the site coordinated by six oxygens [11]. These results, together with Fig. 5, indicate that a growth atmosphere with a lesser amount of H , such as 0.1% or 0.05%, is better for the growth of colorless LSAT crystals. The lattice parameter of the LSAT single crystal was 7.735 A> . Fig. 6 shows the dependence of the lattice parameter on the solidification fraction. The lattice parameter was almost constant along the growth axis. Fig. 7 shows the distribution coefficient of each cation composing LSAT, depending on the solidification fraction. The distribution coefficients, k, of La>, Sr>, Al> and Ta> were 1.06 (k ), 1.04 (k ), 1.05 (k ) and 0.89 (k ), respectively. * 1 2
Fig. 5. Transmission spectrum of the as-grown LSAT single crystal and that of the annealed crystal.
Fig. 6. Dependence of the lattice parameter on the solidification fraction.
Fig. 7. Concentration dependence of each cation on the solidification fraction, where C and C are the initial concentration and the concentration at each solidification fraction, respectively. Error bars are included in each marker.
The values of k , k , and k were larger than * 1 unity, while that of k was less than unity. How2 ever, each cation was distributed almost uniformly along the growth axis. The uniformity of chemical composition agreed well with that of the lattice parameter. This is because g is small and k is near to unity. If a crystal is grown from La>-, Sr>- and Al>-enriched and Ta>-deficient melt composition, compared with that of La Sr Al Ta O , perhaps, we predict a crystal with distri bution coefficient of unity would result. It is known that the color of crystals has an effect on the growth behavior, since radiative heat transfer within the crystal significantly affects the
K. Shimamura et al. / Journal of Crystal Growth 194 (1998) 209–213
temperature distribution in crystals with high melting temperatures [12,13]. The tendency for unstable growth (e.g. spiral growth) in such crystals has been explained on the basis of the optical absorption in the infrared, together with Wien’s displacement law [14,15]. However, the case of LSAT does not fit with this explanation. In the wavelength region from 800 to 2500 nm, LSAT shows constant transmittance without any distinct absorption peaks (Fig. 5). We believe the origin of the spiral phenomenon, which appeared at g"0.6 during the growth of LSAT single crystals to be due to the change of the melt composition. As LSAT crystals grow, the composition change in the melt proceedes in the direction of Ta> enrichment. When the melt composition reaches a critical point, which may correspond to g"0.6, stable crystal growth cannot continue, and spiral growth starts. The (1 1 1) plane of the LSAT crystals is a closepacked plane of the anion framework, which forms the primitive cubic lattice, and thus the 11 1 12 direction is the body-diagonal direction of this cubic cell. The surface structure, therefore, has hexagonal symmetry. The lattice parameter “a ” of the LSAT (1 1 1) plane, considered as a 2-dimensional hexagonal crystal, is given by a "(6/3a
(1)
where a is the conventional lattice parameter of the cubic crystal. Since the measured a of LSAT was 7.735 A> , a was calculated to be 6.316 A> , which was close to twice the corresponding lattice parameter of GaN (3.160—3.190 A> ). The lattice mismatch was calculated to be 0.06—1.00%.
4. Summary LSAT single crystals of the cubic crystal system were successfully grown by the conventional Czochralski method. The growth orientation was controlled to be 11 1 12 so the crystals could serve as substrates for the growth of 10 0 0 12 GaN epitaxial layers. The lattice parameter was 7.735 A> , constant along the growth axis. This agreed with the uniformity of the chemical composition. Asgrown crystals were red-brown in color, while crys-
213
tals annealed under Ar: 0.2% H atmosphere be came pale yellow in color. Because of the small lattice mismatch with GaN, LSAT single crystals have high potential as substrates for the growth of high-quality GaN epitaxial layers with low dislocation density.
Acknowledgements The authors would like to express sincere thanks to Visiting Professor O. Oda (Japan Energy Corporation) and Associate Professor S. Durbin of the Institute for Materials Research, Tohoku University, for their critical reading of the manuscript and fruitful discussions. The authors are also indebted to Mr. Y. Murakami of the Institute for Materials Research, Tohoku University, for his help in carrying out the chemical composition analysis.
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [4] I. Akasaki, H. Amano, S. Sato, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [5] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [6] S.D. Lester, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [7] M. Osinski, D.L. Barton, Proc. of the Int. Symp. on Blue Laser and Light Emitting Diodes, 1996, p. 217. [8] H. Amano, N. Sawaki, I. Akazaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [9] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237. [10] D. Mateika, H. Kohler, H. Laudan, E. Vo¨lkel, J. Crystal Growth 109 (1991) 447. [11] R.D. Shannon, Acta Crystalogr. A 32 (1976) 751. [12] S. Brandon, J.J. Derby, J. Crystal Growth 110 (1991) 481. [13] S. Brandon, J.J. Derby, J. Crystal Growth 121 (1992) 473. [14] Y. Terada, T. Maeda, V.V. Kochurikhin, K. Shimamura, S. Uda, T. Fukuda, J. Crystal Growth 178 (1997) 518. [15] R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, 2nd ed., McGraw-Hill, New York, 1981, p. 24.