- Email: [email protected]
Growth of lithium triborate crystals by the TSSG technique
Journal of Crystal Growth 187 (1998) 455—462
Growth of lithium triborate crystals by the TSSG technique H.G. Kim!,*, J.K. Kang!, S.H. Lee", S.J. Chung! ! School of Materials Science and Engineering, Seoul National University, San 56-1, Shilim-Dong, Kwanak-Ku, Seoul 151-742, South Korea " Department of Advanced Ceramics, SsangYong Research Center, 100 Sinsung-D, Yuseong-Ku, Daejeon 305-345, South Korea Received 2 October 1997; accepted 15 December 1997
Abstract Lithium triborate (LiB O ) single crystals were grown from B O self-flux solutions. The effects of seed orientation, 3 5 2 3 rotation speed, temperature gradient in solution and crystal polarity on crystal yield were investigated. At a cooling rate of 0.96°C/day with a rotation speed of 50 rpm, relatively large crystals were grown from the seed normal to the (0 1 1) face. The growth rate and the morphology of grown crystal were affected by the polarity of seed. The quality of crystals grown from seeds of different directions was estimated by X-ray topography. Second phase has not been observed in this work under the air atmosphere without humidity control. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn; 42.70.Mp Keywords: Crystal growth; Lithium triborate; TSSG; Crystal polarity
1. Introduction The lithium triborate, LBO (LiB O ), crystal has 3 5 a wide transparency range, high damage threshold, moderate nonlinear optical coefficient and high conversion efficiency [1—3]. It is known that LBO is one of the best inorganic nonlinear optical crystals for temperature-tuned noncritical phase matching applications [4—7] and optical parametric oscillators [8—11]. However, difficulties in grow-
* Corresponding author. Fax: #82 2 884 1413; e-mail: [email protected].
ing large crystal of high quality inhibit its practical application. Since LBO decomposes at high temperature [12], LBO single crystals cannot be grown from the melt. Therefore, the widely used technique to grow LBO is top seeded solution growth (TSSG) method using B O self flux system [1,2,13—17]. However, 2 3 due to the viscosity of B O excess solution, mass 2 3 transfer in solution is hard to obtain. In addition, the cooling rate during crystal growth should be very slow, because the B O excess solution has 2 3 a supercooling tendency. To solve these problems, MoO flux system was used [18,19], but B O 3 2 3 self-flux system seems to be the best choice for the growth of higher quality LBO until now.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 0 1 7 - 7
456
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Some interesting results were shown in preliminary experiments. The crystal yield was strongly affected by growth parameters, although the cooling rate and the cooling region were fixed. In this work, the effects of seed orientation, rotation speed and temperature gradient in solution on the crystal yield were investigated. In addition, the effect of seed polarity on the morphology of grown crystal was investigated.
2. Experimental procedure All the growth experiments were carried out in a home-made TSSG furnace with a Kanthal A1 heating element controlled by a programmable PID controller. The axial temperature gradient in solution was controlled by varying the position of a crucible in the growth furnace. The solution composition was varied from Li : B"1 : 4 to Li : B"1 : 4.4 by mole ratio. Li B O and H BO powders were selected as 2 4 7 3 3 starting materials to prepare a 110 g charge after decomposition. A 70 cm3 platinum crucible was filled with pre-mixed powder melt in a specially designed charging furnace. The solution was heated to 950°C in the growth furnace, homogenized completely in 12 h and then cooled down to the saturation temperature. A seed crystal fixed to a platinum seed holder was dipped into solution and the temperature was maintained for 24 h. Then the furnace was slowly cooled down at the rate of 0.96°C/day until the end of the growth. During the growth, the crystal was unidirectionally rotated at the rate of 20 or 50 rpm. All the crystals were grown in air atmosphere without N flow nor humidity control. After cooling down 2 by 5°C, the grown crystal was pulled up from the solution and cooled down to room temperature for about 2 days. The defects of the crystal grown from the seed normal to the (0 1 1) face were observed by X-ray projection topography. The samples cut normal to the growth axis including the seed were polished to the thickness of 1.50 mm and etched by the solution of H O : HNO "10 : 1 for 3 s. The topo2 3 graphs were taken with Mo K radiation at 40 kV, a 50 mA.
3. Results and discussion 3.1. Solution composition According to the phase diagram of Li O—B O 2 2 3 system reported by Sastry and Hummel [12], LBO melts incongruently at 834$4°C. The crystallization range of LBO is from 834 to 635°C. The lower-temperature decomposition at 595$20°C, however, was disproved by Zhao et al. [2]. The appropriate lower limit of the solution temperature to grow this crystal is thought to be about 800°C because of high viscosity at low temperature. The solution composition determines the saturation temperature and the viscosity of solution. Shumov et al. [15] reported that even the solution of incongruent composition has the viscosity higher than 2000 cP. As the saturation temperature of the solution approaches the decomposition temperature, the viscosity of solution decreases, while the probability of seed decomposition increases. In this work, the Li : B mole ratio in solution was varied from 1 : 4 to 1 : 4.4. At Li : B"1 : 4, seed was not decomposed, but dome-shaped inclusions were observed in the bottom of grown crystals. Thus, all the growth experiments were performed at Li :B"1 : 4.1 for the convenience of seed dipping and the prevention of inclusion incorporation. 3.2. The effects of seed orientation Seeding in S0 0 1T direction is widely reported in Refs. [2,13,16,20]. In view of crystal symmetry, the S0 0 1T direction is a reasonable choice because this orientation has a two-fold screw component. But, alternative orientation might be taken for crystal growth as well. In this work, seeding in two different directions were compared. One was parallel to the S0 0 1T direction, and the other was normal to the (0 1 1) face. Seeding in the direction normal to widely developed faces is thought to enlarge the diameter of crystal. The (1 1 0) face is also widely developed, but it was reported that the seed normal to the (1 1 0) face makes the crystal grow asymmetrically [16]. Fig. 1 shows two LBO crystals grown under the same conditions except the seed directions. The
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
457
Fig. 1. LBO crystals grown by seeding in different directions (other conditions: 20 rpm rotation, 2°C/cm temperature gradient, each scale is 1 mm): (a) seed E S0 0 1T; (b) seed o (0 1 1).
crystals were grown until the temperature drops by 5°C with a cooling rate of 0.96°C/day and unidirectional rotation of 20 rpm. The diameter of crystal grown from the seed normal to the (0 1 1) face was larger than that of crystal grown by seeding in the S0 0 1T direction [21,22], while the thickness of crystals was almost the same. The weight of crystal was 2.36 and 1.15 g, respectively. Spurious nucleation on the solution surface was observed during the growth by seeding only in the S0 0 1T direction. The slow growth rate of the (0 1 1) face [23] is responsible for large diameter of the crystal grown by seeding normal to the (0 1 1) face. The forced convection in solution made by rotation of the same speed is affected by crystal/crucible diameters ratio. As the diameter of growing crystal increases, the forced convection in solution increases. Thus, it is thought that the rotation of the crystal grown by seeding normal to the (0 1 1) face made stronger forced convection in solution than the rotation of crystal grown by seeding in the S0 0 1T direction did. The resultant enhancement of the forced convection increases the crystal yield by increasing the mixing and the mass transfer. Markgraf et al. [16] reported that unstable growth occurred at the bottom (0 1 1) face of the
crystal grown by seeding in the S0 0 1T direction at a low rotation speed and that a flat interface promoted the unstable growth when the seed normal to (1 1 0) face was used. In our growth experiments, the unstable growth of the (0 1 1) face was not observed, although the crystal grown by seeding normal to the (0 1 1) face had flat bottom. The growth marks by solution adhesion were observed at the bottom faces. The rotation speed of 20 rpm for a crystal grown by seeding normal to the (0 1 1) face may enhance forced convection in a small crucible of 70 cm3 that was used in this work. From the results of growth experiments, it is concluded that the crystal yield was increased by enhancing the forced convection in solution. 3.3. The effects of rotation speed To verify the role of the forced convection in solution on crystal yield, the rotation speed has been increased to 50 rpm. The other growth parameters were fixed as in Section 3.2. The crystals grown at 50 rpm are shown in Fig. 2. For both seed orientations, the crystals grown at 50 rpm were larger than the crystals grown at 20 rpm. Spurious nucleation was not observed in this case. The weight of crystal grown in
458
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Fig. 2. LBO crystals grown by seeding in different directions (other conditions: 50 rpm rotation, 2°C/cm temperature gradient, each scale is 1 mm): (a) seed E S0 0 1T; (b) seed o (0 1 1).
Table 1 Crystal weights and crystal yields for various growth conditions Rotation speed Temperature gradient in solution
20 rpm 2°C/cm
50 rpm
E S0 0 1T
Weight (g) Yield (g/kg)°C)
1.15 2.09
1.77 3.22
o (0 1 1)
Weight (g) Yield (g/kg)°C)
2.71 4.93
6.69 12.16
5°C/cm
2.79 5.07
the S0 0 1T direction was 1.77 g and that normal to the (0 1 1) face was 6.69 g. The crystal weights and yields for various growth conditions are shown in Table 1. Since the temperature gradient and the seed orientations were fixed, the forced convection enhanced by increasing rotation speed seems to be the main reason for higher crystal yield. Feigelson et al. [24] reported that the natural convection is dominant in solution at low rotation speed, and that the forced convection is increased by increasing crystal rotation. As mentioned above, the increase of forced convection increases the mixing in solution and
provides more growth units of growing crystals. Thus the crystal yield was increased by the increasing rotation speed, as expected. Meanwhile the crystals grown by seeding in S0 0 1T direction were always smaller than the crystals grown by seeding normal to the (0 1 1) face at any rotation speed. Elongated inclusions along the crystal edges were observed in the crystal grown by seeding normal to the (0 1 1) face. The position of elongated inclusion was almost the same as the bottom edges of crystal grown by seeding normal to the (0 1 1) face at 20 rpm. These inclusions give the evidence to the growth of bottom side following the lateral growth for the crystal grown by seeding normal to the (0 1 1) face. The increased temperature gradient between the crystal edges and crucible wall and/or turbulent flow generated in solution may be the causes preventing the lateral growth, because grown crystal has large crystal/crucible diameters ratio. 3.4. The effect of temperature gradient in solution It was thought that enhancing the natural convection would enhance the mass transfer, as enhancing the forced convection did. Since the buoyancy-driven convection in solution strongly
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
depends on the temperature gradient in solution, temperature gradient in solution has been increased to 5°C/cm. The other growth parameters were fixed as in Section 3.3. Seeding normal to the (0 1 1) face was not examined in this condition, because higher temperature gradient would make the diameter of crystal larger than lower temperature gradient would make. The crystal grown by seeding in S0 0 1T direction is shown in Fig. 3. The weight of grown crystal was increased to 2.79 g. The morphology of grown crystal was almost the same as that reported by Zhao et al. [2]. Since all other growth conditions were fixed except the temperature gradient, the main cause of increasing yield is due to the enhanced natural convection in solution. Like the forced convection, enhanced natural convection would enhance mixing in solution and provide more growth units to growing crystal. Even though higher temperature gradient of 10°C/cm was tried to enhance natural convection. The growth result was not satisfactory. Since the temperature gradient was too high, the grown crystal was so thin as to make a crack by solidification of attached solution in the bottom of crystal during cooling to room temperature. The convex and rounded bottom of crystal was thought to imply
Fig. 3. LBO crystals grown by seeding in S0 0 1T direction at a temperature gradient of 5°C/cm and a rotation speed of 50 rpm (each scale is 1 mm).
459
that the natural convection was too high to exceed the forced convection. Thus a temperature gradient of 5°C/cm and a rotation speed of 50 rpm were the optimum condition to enhance mass transfer for the experimental conditions in this work. 3.5. The effects of polarization The morphology of LBO crystal has been reported as symmetrical [2,23], although Markgraf et al. [16] reported that the seed normal to c-axis makes the crystal grow asymmetrically. Some crystals grown by seeding in S0 0 1T direction show quite different morphology and different growth rate, as shown in Fig. 4. Crystals had large (2 0 1) faces and small (1 0 0) faces, and solution incorporation was observed in the bottom of crystal. The morphology of crystal was similar to the morphology reported by Zhong et al. [23], while the other crystals grown in S0 0 1T direction have the morphology reported by Zhao et al. The difference between the crystals in Fig. 4 and the other S0 0 1T direction crystals in Figs. 1—3 was the direction of polarization. The crystals in Fig. 4 were grown from the seed of negative polarity. Since LBO belongs to polar point group of mm2, this crystal has spontaneous polarization along polar c-axis. Thus it was thought that the growth behavior was affected by the polarization of crystal. Zhong et al. [23] proposed that the basic growth units of LBO is (B O )~ anion group and that the 3 5 growth units with different configurations can be formed at different temperatures (or different supersaturation). If (B O )~ or other anion group 3 5 was formed in solution, Li in solution must be ionized to fulfill the charge neutrality in solution. The charge of anion growth unit depends on its configuration, while the charge of Li ion is fixed as unity. Thus, it is thought that average charge of anion growth units would be higher than that of Li ion. Since growing crystal has spontaneous polarization, the adsorption of charged growth units would be affected electrically. The electrical repulsive force between growth unit and polarization of growing crystal would decrease the growth rate by hindering the adsorption, while the electrical attractive
460
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Fig. 4. LBO crystals grown by seeding in S0 0 1T direction of negative polarity at a rotation speed of 50 rpm rotation and different temperature gradients (each scale is 1 mm): (a) 2°C/cm; (b) 5°C/cm.
force would increase the growth rate. Since the average charge of anion growth units is higher than charge of Li ion, the overall growth rate is affected by the electrical force between anion growth units and polarization of growing crystal. Therefore, the growth rate in negative polar direction is lower than that in positive polar direction. 3.6. Characteristics of grown crystals The quality of the crystal grown by seeding normal to the (0 1 1) face was compared with that in the S0 0 1T direction by X-ray topographic techniques. Fig. 5 shows the topographs taken from (1 1 0), (2 0 1) reflections of two samples. Several inclusions are distributed mostly in outer side of the (0 1 1) sample, and some bimodal dislocation images are observed. The dislocations could be assorted to three kinds as seed-induced, inclusion-induced and grown-in [25] dislocations. A strain field image near the seed and many dislocation images normal to outer (1 1 0) faces were observed in (0 0 1) sample. Because of the different orientations of two samples, the quality of the two kinds of crystals cannot be compared directly, however, the quality of crystals grown by the seeding normal to the (0 1 1) face is thought to be comparable to the quality of crystals grown by seeding in the S0 0 1T direction.
Fig. 6 shows the X-ray powder diffraction pattern of the grown crystal. The grown crystal clearly exhibits a single phase of LBO. Bru¨ck et al. [14] reported that LBO is unstable at elevated temperature in humid atmosphere and dry nitrogen atmosphere is required to inhibit the decomposed white coating on grown crystal surface. However, in our experiment, all crystals were grown in air atmosphere without nitrogen flow nor humidity control. It is clear that LBO crystal decomposes by the reaction with water vapor at high temperature. But, it is shown that any atmospheric control may not be required to inhibit the decomposition. In addition, the phase decomposition of LBO to Li B O 2 4 7 and Li B O reported by Nihtianova [26] was 2 8 13 not observed, even though crystals were ground at room temperature in our experiments. 4. Summary LBO single crystals were grown by TSSG method using B O as a self-flux at a cooling rate of 2 3 0.96°C/day in 70 cm3 crucible with temperature gradient of 2°C/cm in solution. The solution composition of Li : B"1 : 4.1 mole ratio was preferable for inhibiting seed decomposition and for decreasing the viscosity of the solution. Crystals grown by seeding normal to the (0 1 1) face were about two
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
461
Fig. 5. X-ray projection topographs of grown LBO crystals: (a) (1 1 0) reflection of (0 1 1) cut; (b) (2 0 1) reflection of (0 1 1) cut; (c) (1 1 0) reflection of (0 0 1) cut; (d) (2 0 1) reflection of (0 0 1) cut.
Fig. 6. Powder X-ray diffraction pattern of LBO crystal grown in this work.
times larger than those grown by seeding in the S0 0 1T direction for the game growth conditions. Enhancing the forced convection and the natural convection in solution also increased the crystal
yield. The growth rate and the morphology of crystal were affected by the polarity of seed, because the polarization of growing crystal electrically affected by the growth units in solution. The quality of
462
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
crystals grown by seeding normal to (0 1 1) face was comparable to that of crystals grown by seeding in S0 0 1T direction. Even though all crystals were grown in air atmosphere, white coating on the crystal surface and phase decomposition were not observed. Acknowledgements This work has been financially supported by HAN project of Ministry of Science and Technology. References [1] C. Chen, Y. Wu, S. Jiang, B. Wu, G. Tou, R. Li, S. Lin, J. Opt. Soc. Am. B 6 (1989) 616. [2] S. Zhao, C. Huang, H. Zhang, J. Crystal Growth 99 (1990) 805. [3] B. Wu, F. Xie, C. Chen, J. Appl. Phys. 73 (1993) 7108. [4] T. Ukachi, R.J. Lane, W.R. Bosenberg, C.L. Tang, Appl. Phys. Lett. 57 (1990) 980. [5] J.T. Lin, J.L. Montgomery, K. Kato, Opt. Commun. 80 (1990) 159. [6] F. Hanson, D. Dick, Opt. Lett. 16 (1991) 205. [7] B. Wu, F. Xie, C. Chen, J. Appl. Phys. 73 (1993) 7108. [8] J.Y. Zhang, J.Y. Huang, Y.R. Shen, C. Chen, B. Wu, Appl. Phys. Lett. 58 (1991) 213. [9] Y. Wang, Z. Xu, D. Deng, W. Zheng, B. Wu, C. Chen, Appl. Phys. Lett. 59 (1991) 531.
[10] M. Ebrahimzadeh, G. Robertson, M.H. Dunn, Opt. Lett. 16 (1991) 767. [11] V. Kubecek, T. Takagi, K. Yoshihara, G.C. Reali, Opt. Commun. 91 (1992) 93. [12] B.S.R. Sastry, F.A. Hummel, J. Am. Ceram. Soc. 41 (1958) 7. [13] T. Ukachi, R.J. Lane, W.R. Rosenberg, C.L. Tang, J. Opt. Soc. Am. B 9 (1992) 1128. [14] E. Bru¨ck, R.J. Raymakers, R.K. Route, R.S. Feigelson, J. Crystal Growth 128 (1993) 993. [15] D.P. Shumov, V.S. Nikolov, A.T. Nenov, J. Crystal Growth 144 (1994) 218. [16] S.A. Markgraf, Y. Furukawa, M. Sato, J. Crystal Growth 140 (1994) 343. [17] S.A. Guretskii, A.P. Ges, D.I. Zhigunov, A.A. Ignatenko, N.A. Kalanda, L.A. Kurnevich, A.M. Luginets, A.S. Milovanov, P.V. Molchan, J. Crystal Growth 156 (1995) 410. [18] C. Parfeniuk, I.V. Samarasekera, F. Weinberg, J. Crystal Growth 158 (1996) 514. [19] C. Parfeniuk, I.V. Samarasekera, F. Weinberg, J. Edel, K. Fjeldsted, B. Lent, J. Crystal Growth 158 (1996) 523. [20] J.K. Kang, C.H. Kim, B.M. Lim, S.J. Chung, Acta Crystallogr. A 49 Suppl. (1993) C-393. [21] H.G. Kim, J.K. Kang, S.J. Chung, Acta Crystallogr. A 52 Suppl. (1996) C-518. [22] H.G. Kim, J.K. Kang, S.J. Park, S.J. Chung, Opt. Mater. 9 (1998) 356. [23] W. Zhong, D. Tang, J. Crystal Growth 166 (1996) 91. [24] R.S. Feigelson, R.J. Raymakers, R.K. Route, J. Crystal Growth 97 (1989) 352. [25] C. Chen, Y. Wu, R. Li, Chinese Phys. Lett. 2 (1985) 388. [26] D.D. Nihtianova, D.P. Shumov, J.J. Macicek, A.T. Nenov, J. Crystal Growth 169 (1997) 527.
Growth of lithium triborate crystals by the TSSG technique H.G. Kim!,*, J.K. Kang!, S.H. Lee", S.J. Chung! ! School of Materials Science and Engineering, Seoul National University, San 56-1, Shilim-Dong, Kwanak-Ku, Seoul 151-742, South Korea " Department of Advanced Ceramics, SsangYong Research Center, 100 Sinsung-D, Yuseong-Ku, Daejeon 305-345, South Korea Received 2 October 1997; accepted 15 December 1997
Abstract Lithium triborate (LiB O ) single crystals were grown from B O self-flux solutions. The effects of seed orientation, 3 5 2 3 rotation speed, temperature gradient in solution and crystal polarity on crystal yield were investigated. At a cooling rate of 0.96°C/day with a rotation speed of 50 rpm, relatively large crystals were grown from the seed normal to the (0 1 1) face. The growth rate and the morphology of grown crystal were affected by the polarity of seed. The quality of crystals grown from seeds of different directions was estimated by X-ray topography. Second phase has not been observed in this work under the air atmosphere without humidity control. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn; 42.70.Mp Keywords: Crystal growth; Lithium triborate; TSSG; Crystal polarity
1. Introduction The lithium triborate, LBO (LiB O ), crystal has 3 5 a wide transparency range, high damage threshold, moderate nonlinear optical coefficient and high conversion efficiency [1—3]. It is known that LBO is one of the best inorganic nonlinear optical crystals for temperature-tuned noncritical phase matching applications [4—7] and optical parametric oscillators [8—11]. However, difficulties in grow-
* Corresponding author. Fax: #82 2 884 1413; e-mail: [email protected].
ing large crystal of high quality inhibit its practical application. Since LBO decomposes at high temperature [12], LBO single crystals cannot be grown from the melt. Therefore, the widely used technique to grow LBO is top seeded solution growth (TSSG) method using B O self flux system [1,2,13—17]. However, 2 3 due to the viscosity of B O excess solution, mass 2 3 transfer in solution is hard to obtain. In addition, the cooling rate during crystal growth should be very slow, because the B O excess solution has 2 3 a supercooling tendency. To solve these problems, MoO flux system was used [18,19], but B O 3 2 3 self-flux system seems to be the best choice for the growth of higher quality LBO until now.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 0 1 7 - 7
456
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Some interesting results were shown in preliminary experiments. The crystal yield was strongly affected by growth parameters, although the cooling rate and the cooling region were fixed. In this work, the effects of seed orientation, rotation speed and temperature gradient in solution on the crystal yield were investigated. In addition, the effect of seed polarity on the morphology of grown crystal was investigated.
2. Experimental procedure All the growth experiments were carried out in a home-made TSSG furnace with a Kanthal A1 heating element controlled by a programmable PID controller. The axial temperature gradient in solution was controlled by varying the position of a crucible in the growth furnace. The solution composition was varied from Li : B"1 : 4 to Li : B"1 : 4.4 by mole ratio. Li B O and H BO powders were selected as 2 4 7 3 3 starting materials to prepare a 110 g charge after decomposition. A 70 cm3 platinum crucible was filled with pre-mixed powder melt in a specially designed charging furnace. The solution was heated to 950°C in the growth furnace, homogenized completely in 12 h and then cooled down to the saturation temperature. A seed crystal fixed to a platinum seed holder was dipped into solution and the temperature was maintained for 24 h. Then the furnace was slowly cooled down at the rate of 0.96°C/day until the end of the growth. During the growth, the crystal was unidirectionally rotated at the rate of 20 or 50 rpm. All the crystals were grown in air atmosphere without N flow nor humidity control. After cooling down 2 by 5°C, the grown crystal was pulled up from the solution and cooled down to room temperature for about 2 days. The defects of the crystal grown from the seed normal to the (0 1 1) face were observed by X-ray projection topography. The samples cut normal to the growth axis including the seed were polished to the thickness of 1.50 mm and etched by the solution of H O : HNO "10 : 1 for 3 s. The topo2 3 graphs were taken with Mo K radiation at 40 kV, a 50 mA.
3. Results and discussion 3.1. Solution composition According to the phase diagram of Li O—B O 2 2 3 system reported by Sastry and Hummel [12], LBO melts incongruently at 834$4°C. The crystallization range of LBO is from 834 to 635°C. The lower-temperature decomposition at 595$20°C, however, was disproved by Zhao et al. [2]. The appropriate lower limit of the solution temperature to grow this crystal is thought to be about 800°C because of high viscosity at low temperature. The solution composition determines the saturation temperature and the viscosity of solution. Shumov et al. [15] reported that even the solution of incongruent composition has the viscosity higher than 2000 cP. As the saturation temperature of the solution approaches the decomposition temperature, the viscosity of solution decreases, while the probability of seed decomposition increases. In this work, the Li : B mole ratio in solution was varied from 1 : 4 to 1 : 4.4. At Li : B"1 : 4, seed was not decomposed, but dome-shaped inclusions were observed in the bottom of grown crystals. Thus, all the growth experiments were performed at Li :B"1 : 4.1 for the convenience of seed dipping and the prevention of inclusion incorporation. 3.2. The effects of seed orientation Seeding in S0 0 1T direction is widely reported in Refs. [2,13,16,20]. In view of crystal symmetry, the S0 0 1T direction is a reasonable choice because this orientation has a two-fold screw component. But, alternative orientation might be taken for crystal growth as well. In this work, seeding in two different directions were compared. One was parallel to the S0 0 1T direction, and the other was normal to the (0 1 1) face. Seeding in the direction normal to widely developed faces is thought to enlarge the diameter of crystal. The (1 1 0) face is also widely developed, but it was reported that the seed normal to the (1 1 0) face makes the crystal grow asymmetrically [16]. Fig. 1 shows two LBO crystals grown under the same conditions except the seed directions. The
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
457
Fig. 1. LBO crystals grown by seeding in different directions (other conditions: 20 rpm rotation, 2°C/cm temperature gradient, each scale is 1 mm): (a) seed E S0 0 1T; (b) seed o (0 1 1).
crystals were grown until the temperature drops by 5°C with a cooling rate of 0.96°C/day and unidirectional rotation of 20 rpm. The diameter of crystal grown from the seed normal to the (0 1 1) face was larger than that of crystal grown by seeding in the S0 0 1T direction [21,22], while the thickness of crystals was almost the same. The weight of crystal was 2.36 and 1.15 g, respectively. Spurious nucleation on the solution surface was observed during the growth by seeding only in the S0 0 1T direction. The slow growth rate of the (0 1 1) face [23] is responsible for large diameter of the crystal grown by seeding normal to the (0 1 1) face. The forced convection in solution made by rotation of the same speed is affected by crystal/crucible diameters ratio. As the diameter of growing crystal increases, the forced convection in solution increases. Thus, it is thought that the rotation of the crystal grown by seeding normal to the (0 1 1) face made stronger forced convection in solution than the rotation of crystal grown by seeding in the S0 0 1T direction did. The resultant enhancement of the forced convection increases the crystal yield by increasing the mixing and the mass transfer. Markgraf et al. [16] reported that unstable growth occurred at the bottom (0 1 1) face of the
crystal grown by seeding in the S0 0 1T direction at a low rotation speed and that a flat interface promoted the unstable growth when the seed normal to (1 1 0) face was used. In our growth experiments, the unstable growth of the (0 1 1) face was not observed, although the crystal grown by seeding normal to the (0 1 1) face had flat bottom. The growth marks by solution adhesion were observed at the bottom faces. The rotation speed of 20 rpm for a crystal grown by seeding normal to the (0 1 1) face may enhance forced convection in a small crucible of 70 cm3 that was used in this work. From the results of growth experiments, it is concluded that the crystal yield was increased by enhancing the forced convection in solution. 3.3. The effects of rotation speed To verify the role of the forced convection in solution on crystal yield, the rotation speed has been increased to 50 rpm. The other growth parameters were fixed as in Section 3.2. The crystals grown at 50 rpm are shown in Fig. 2. For both seed orientations, the crystals grown at 50 rpm were larger than the crystals grown at 20 rpm. Spurious nucleation was not observed in this case. The weight of crystal grown in
458
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Fig. 2. LBO crystals grown by seeding in different directions (other conditions: 50 rpm rotation, 2°C/cm temperature gradient, each scale is 1 mm): (a) seed E S0 0 1T; (b) seed o (0 1 1).
Table 1 Crystal weights and crystal yields for various growth conditions Rotation speed Temperature gradient in solution
20 rpm 2°C/cm
50 rpm
E S0 0 1T
Weight (g) Yield (g/kg)°C)
1.15 2.09
1.77 3.22
o (0 1 1)
Weight (g) Yield (g/kg)°C)
2.71 4.93
6.69 12.16
5°C/cm
2.79 5.07
the S0 0 1T direction was 1.77 g and that normal to the (0 1 1) face was 6.69 g. The crystal weights and yields for various growth conditions are shown in Table 1. Since the temperature gradient and the seed orientations were fixed, the forced convection enhanced by increasing rotation speed seems to be the main reason for higher crystal yield. Feigelson et al. [24] reported that the natural convection is dominant in solution at low rotation speed, and that the forced convection is increased by increasing crystal rotation. As mentioned above, the increase of forced convection increases the mixing in solution and
provides more growth units of growing crystals. Thus the crystal yield was increased by the increasing rotation speed, as expected. Meanwhile the crystals grown by seeding in S0 0 1T direction were always smaller than the crystals grown by seeding normal to the (0 1 1) face at any rotation speed. Elongated inclusions along the crystal edges were observed in the crystal grown by seeding normal to the (0 1 1) face. The position of elongated inclusion was almost the same as the bottom edges of crystal grown by seeding normal to the (0 1 1) face at 20 rpm. These inclusions give the evidence to the growth of bottom side following the lateral growth for the crystal grown by seeding normal to the (0 1 1) face. The increased temperature gradient between the crystal edges and crucible wall and/or turbulent flow generated in solution may be the causes preventing the lateral growth, because grown crystal has large crystal/crucible diameters ratio. 3.4. The effect of temperature gradient in solution It was thought that enhancing the natural convection would enhance the mass transfer, as enhancing the forced convection did. Since the buoyancy-driven convection in solution strongly
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
depends on the temperature gradient in solution, temperature gradient in solution has been increased to 5°C/cm. The other growth parameters were fixed as in Section 3.3. Seeding normal to the (0 1 1) face was not examined in this condition, because higher temperature gradient would make the diameter of crystal larger than lower temperature gradient would make. The crystal grown by seeding in S0 0 1T direction is shown in Fig. 3. The weight of grown crystal was increased to 2.79 g. The morphology of grown crystal was almost the same as that reported by Zhao et al. [2]. Since all other growth conditions were fixed except the temperature gradient, the main cause of increasing yield is due to the enhanced natural convection in solution. Like the forced convection, enhanced natural convection would enhance mixing in solution and provide more growth units to growing crystal. Even though higher temperature gradient of 10°C/cm was tried to enhance natural convection. The growth result was not satisfactory. Since the temperature gradient was too high, the grown crystal was so thin as to make a crack by solidification of attached solution in the bottom of crystal during cooling to room temperature. The convex and rounded bottom of crystal was thought to imply
Fig. 3. LBO crystals grown by seeding in S0 0 1T direction at a temperature gradient of 5°C/cm and a rotation speed of 50 rpm (each scale is 1 mm).
459
that the natural convection was too high to exceed the forced convection. Thus a temperature gradient of 5°C/cm and a rotation speed of 50 rpm were the optimum condition to enhance mass transfer for the experimental conditions in this work. 3.5. The effects of polarization The morphology of LBO crystal has been reported as symmetrical [2,23], although Markgraf et al. [16] reported that the seed normal to c-axis makes the crystal grow asymmetrically. Some crystals grown by seeding in S0 0 1T direction show quite different morphology and different growth rate, as shown in Fig. 4. Crystals had large (2 0 1) faces and small (1 0 0) faces, and solution incorporation was observed in the bottom of crystal. The morphology of crystal was similar to the morphology reported by Zhong et al. [23], while the other crystals grown in S0 0 1T direction have the morphology reported by Zhao et al. The difference between the crystals in Fig. 4 and the other S0 0 1T direction crystals in Figs. 1—3 was the direction of polarization. The crystals in Fig. 4 were grown from the seed of negative polarity. Since LBO belongs to polar point group of mm2, this crystal has spontaneous polarization along polar c-axis. Thus it was thought that the growth behavior was affected by the polarization of crystal. Zhong et al. [23] proposed that the basic growth units of LBO is (B O )~ anion group and that the 3 5 growth units with different configurations can be formed at different temperatures (or different supersaturation). If (B O )~ or other anion group 3 5 was formed in solution, Li in solution must be ionized to fulfill the charge neutrality in solution. The charge of anion growth unit depends on its configuration, while the charge of Li ion is fixed as unity. Thus, it is thought that average charge of anion growth units would be higher than that of Li ion. Since growing crystal has spontaneous polarization, the adsorption of charged growth units would be affected electrically. The electrical repulsive force between growth unit and polarization of growing crystal would decrease the growth rate by hindering the adsorption, while the electrical attractive
460
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
Fig. 4. LBO crystals grown by seeding in S0 0 1T direction of negative polarity at a rotation speed of 50 rpm rotation and different temperature gradients (each scale is 1 mm): (a) 2°C/cm; (b) 5°C/cm.
force would increase the growth rate. Since the average charge of anion growth units is higher than charge of Li ion, the overall growth rate is affected by the electrical force between anion growth units and polarization of growing crystal. Therefore, the growth rate in negative polar direction is lower than that in positive polar direction. 3.6. Characteristics of grown crystals The quality of the crystal grown by seeding normal to the (0 1 1) face was compared with that in the S0 0 1T direction by X-ray topographic techniques. Fig. 5 shows the topographs taken from (1 1 0), (2 0 1) reflections of two samples. Several inclusions are distributed mostly in outer side of the (0 1 1) sample, and some bimodal dislocation images are observed. The dislocations could be assorted to three kinds as seed-induced, inclusion-induced and grown-in [25] dislocations. A strain field image near the seed and many dislocation images normal to outer (1 1 0) faces were observed in (0 0 1) sample. Because of the different orientations of two samples, the quality of the two kinds of crystals cannot be compared directly, however, the quality of crystals grown by the seeding normal to the (0 1 1) face is thought to be comparable to the quality of crystals grown by seeding in the S0 0 1T direction.
Fig. 6 shows the X-ray powder diffraction pattern of the grown crystal. The grown crystal clearly exhibits a single phase of LBO. Bru¨ck et al. [14] reported that LBO is unstable at elevated temperature in humid atmosphere and dry nitrogen atmosphere is required to inhibit the decomposed white coating on grown crystal surface. However, in our experiment, all crystals were grown in air atmosphere without nitrogen flow nor humidity control. It is clear that LBO crystal decomposes by the reaction with water vapor at high temperature. But, it is shown that any atmospheric control may not be required to inhibit the decomposition. In addition, the phase decomposition of LBO to Li B O 2 4 7 and Li B O reported by Nihtianova [26] was 2 8 13 not observed, even though crystals were ground at room temperature in our experiments. 4. Summary LBO single crystals were grown by TSSG method using B O as a self-flux at a cooling rate of 2 3 0.96°C/day in 70 cm3 crucible with temperature gradient of 2°C/cm in solution. The solution composition of Li : B"1 : 4.1 mole ratio was preferable for inhibiting seed decomposition and for decreasing the viscosity of the solution. Crystals grown by seeding normal to the (0 1 1) face were about two
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
461
Fig. 5. X-ray projection topographs of grown LBO crystals: (a) (1 1 0) reflection of (0 1 1) cut; (b) (2 0 1) reflection of (0 1 1) cut; (c) (1 1 0) reflection of (0 0 1) cut; (d) (2 0 1) reflection of (0 0 1) cut.
Fig. 6. Powder X-ray diffraction pattern of LBO crystal grown in this work.
times larger than those grown by seeding in the S0 0 1T direction for the game growth conditions. Enhancing the forced convection and the natural convection in solution also increased the crystal
yield. The growth rate and the morphology of crystal were affected by the polarity of seed, because the polarization of growing crystal electrically affected by the growth units in solution. The quality of
462
H.G. Kim et al. / Journal of Crystal Growth 187 (1998) 455–462
crystals grown by seeding normal to (0 1 1) face was comparable to that of crystals grown by seeding in S0 0 1T direction. Even though all crystals were grown in air atmosphere, white coating on the crystal surface and phase decomposition were not observed. Acknowledgements This work has been financially supported by HAN project of Ministry of Science and Technology. References [1] C. Chen, Y. Wu, S. Jiang, B. Wu, G. Tou, R. Li, S. Lin, J. Opt. Soc. Am. B 6 (1989) 616. [2] S. Zhao, C. Huang, H. Zhang, J. Crystal Growth 99 (1990) 805. [3] B. Wu, F. Xie, C. Chen, J. Appl. Phys. 73 (1993) 7108. [4] T. Ukachi, R.J. Lane, W.R. Bosenberg, C.L. Tang, Appl. Phys. Lett. 57 (1990) 980. [5] J.T. Lin, J.L. Montgomery, K. Kato, Opt. Commun. 80 (1990) 159. [6] F. Hanson, D. Dick, Opt. Lett. 16 (1991) 205. [7] B. Wu, F. Xie, C. Chen, J. Appl. Phys. 73 (1993) 7108. [8] J.Y. Zhang, J.Y. Huang, Y.R. Shen, C. Chen, B. Wu, Appl. Phys. Lett. 58 (1991) 213. [9] Y. Wang, Z. Xu, D. Deng, W. Zheng, B. Wu, C. Chen, Appl. Phys. Lett. 59 (1991) 531.
[10] M. Ebrahimzadeh, G. Robertson, M.H. Dunn, Opt. Lett. 16 (1991) 767. [11] V. Kubecek, T. Takagi, K. Yoshihara, G.C. Reali, Opt. Commun. 91 (1992) 93. [12] B.S.R. Sastry, F.A. Hummel, J. Am. Ceram. Soc. 41 (1958) 7. [13] T. Ukachi, R.J. Lane, W.R. Rosenberg, C.L. Tang, J. Opt. Soc. Am. B 9 (1992) 1128. [14] E. Bru¨ck, R.J. Raymakers, R.K. Route, R.S. Feigelson, J. Crystal Growth 128 (1993) 993. [15] D.P. Shumov, V.S. Nikolov, A.T. Nenov, J. Crystal Growth 144 (1994) 218. [16] S.A. Markgraf, Y. Furukawa, M. Sato, J. Crystal Growth 140 (1994) 343. [17] S.A. Guretskii, A.P. Ges, D.I. Zhigunov, A.A. Ignatenko, N.A. Kalanda, L.A. Kurnevich, A.M. Luginets, A.S. Milovanov, P.V. Molchan, J. Crystal Growth 156 (1995) 410. [18] C. Parfeniuk, I.V. Samarasekera, F. Weinberg, J. Crystal Growth 158 (1996) 514. [19] C. Parfeniuk, I.V. Samarasekera, F. Weinberg, J. Edel, K. Fjeldsted, B. Lent, J. Crystal Growth 158 (1996) 523. [20] J.K. Kang, C.H. Kim, B.M. Lim, S.J. Chung, Acta Crystallogr. A 49 Suppl. (1993) C-393. [21] H.G. Kim, J.K. Kang, S.J. Chung, Acta Crystallogr. A 52 Suppl. (1996) C-518. [22] H.G. Kim, J.K. Kang, S.J. Park, S.J. Chung, Opt. Mater. 9 (1998) 356. [23] W. Zhong, D. Tang, J. Crystal Growth 166 (1996) 91. [24] R.S. Feigelson, R.J. Raymakers, R.K. Route, J. Crystal Growth 97 (1989) 352. [25] C. Chen, Y. Wu, R. Li, Chinese Phys. Lett. 2 (1985) 388. [26] D.D. Nihtianova, D.P. Shumov, J.J. Macicek, A.T. Nenov, J. Crystal Growth 169 (1997) 527.