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Dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals
CRYSTAL GROWTH ELSEVIER
Journal of Crystal Growth 140 (1994) 45—50
Dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals Yongfa Kong, Jinke Wen
Huafu Wang
Department of Physics, Nankai University, Tianjin 300071, People ~ Republic of China (Received 4 September 1993; manuscript received in final form 2 March 1994)
Abstract The extension and distribution of dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals at different stages of growth have been investigated using chemical etching and optical microscopy. The relations between dislocation densities, subgrain boundaries and optical quality of the crystals have been also studied. It was found that there is a core with relatively high dislocation density in the central region of the crystal shoulder. In the shouldering stage, the dislocations gathering in the core extend to its surrounding regions and the dislocation density tends to be homogeneous. Tailing increases the dislocation density in the bottom part of the crystal and causes inhomogeneous distribution of dislocations in that part. Subgrain boundaries are apt to form in high dislocation density regions, and neighbouring multiple subgrain boundaries tend to reform a more stable single subgrain boundary. The extinction ratios and conoscope images of crystals are worst in the dislocation gathering regions around the ends of subgrain boundaries, and dislocations are the basic cause of poor optical quality of crystals.
1. Introduction Magnesium-doped lithium niobate crystal has excellent optical damage resistant properties
when the dopant concentration exceeds the threshold [1,2], and it is extensively used in electro-optical, integrated optical and nonlinear optical devices. However, the growth of highly Mgdoped crystal is more difficult than that of congruent pure crystal. Furukawa et al. [31indicated that defects such as subgrain boundaries increase with MgO doping, especially in the bottom parts of their crystals. It has been reported that the
*
Corresponding author,
congruent composition of Mg-doped lithium niobate crystal is different from that of pure crystal [4], and the distribution coefficient of Mg is greater than one [5,6]. All these factors will in-
duce compositional inhomogeneity, high dislocation density and subgrain boundaries in highly Mg-doped lithium niobate crystal, which cause poor optical quality of crystals. In the present paper, we describe the characteristics of dislocations and subgrain boundaries in highly Mg-doped crystals in the different growth stages. The chemical etching method was employed to reveal defects in the crystals. Extinction ratio measurement and conoscope image observation were carried out to identify the optical quality of the crystals.
0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00004-6
46
Y Kong et a!. /Journal
Crystal Growth 140 (1994) 45—50
of
2. Experimental procedure The highly Mg-doped LiNbO
3 crystals (Li/Nb
=
48.45/51.55, 6.5 mol% MgO) were grown by
the conventional Czochralski technique with the pulling direction along the c-axis. The starting materials were Li2CO3 (4N purity), Nb205 (3N
purity) and MgO (4N purity). They were thoroughly mixed, calcined at 800°Cfor 5 h and sintered at 1100°C for 8 h. The pulling rate (3 mm/h) was kept constant by a control system through the weight loss of melt indicated by an electronic balance. During growth of the shoulder part, the crystal rotation rate was reduced continuously from 25 to 17 rpm with the increase of the crystal diameter in order to maintain a slightly convex melt—crystal interface. The temperature difference of the melt—crystal interface (20°C) and the temperature gradient (10°C/cm above the melt surface and 15°C/cmin the melt volume near the surface) were adjusted by the power of the after-furnace and the heater beneath the bottom of the crucible. The diameters and lengths
,/~\ /~‘2a
_____
\
/
2c 2d
____________
3~C\
/
•
3d
for optical observation
./
3e
3f ~
_____________
crystal A
crystal B
Fig. 1. Sketch of crystals showing the crystal preparation and the location of figures.
of the crystals were about 34 and 44 mm, respectively. Two highly MgO-doped crystals were grown for the experiments, which are denoted as crystal A and crystal B. As shown in Fig. 1, a plate,
Fig. 2. Optical micrograph~ot dt~loeation~ and ~uhgrain boundaries for crystal A: (a) and (hI br the upper part of the crystal shoulder; (c) for the bottom part of the shoulder; (d) for the constant diameter part. (a) 3211 / : (h) — (d) 160 x
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
47
r
44
(C)
~
r~”~ ~
~
*
~
4,
~
~
4
~
‘D
~
~
4
~
~
*
•‘
~..
;~t~at~
~ ‘~i
• ~ *4 4
‘4
~0~~
~
0’
Fig. 3. Optical micrographs of dislocations for crystal B: (a) and (d) for the central part of the core; (b) and (e) for the edge of the core; (c) near the edge of the crystal. (f) for the middle part of the crystal; (g) for the bottom part of the crystal. (a)—(e) 320 x ; (f), (g) 160 x.
48
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
20 mm thick, was cut from the middle part of each crystal for extinction ratio measurement and conoscope image observation using a He—Ne laser, and both sides of their c-faces were polished to optical grade. Then, both crystals were cut into 1.5 mm plates from their shoulders to their tails with the parallel faces perpendicular to the c-axis, and the c-faces of the plates were polished and etched by reagents of HF and HNO 3(HF/HNO3 = 1/2) in a water bath at 100°Cfor 30 mm. The dislocations and subgrain boundaries were observed by an optical microscope.
3. Results and discussion The typical etch patterns for crystal A are shown in Fig. 2, that for crystal B in Fig. 3, and the conoscope images are shown in Fig. 4. The corresponding parts of the crystals for observation in Figs. 2—4 are shown in Fig. 1. 3.1. Dislocations Figs. 2a and 3a—3c show the etch patterns of the upper parts of the crystal shoulders just beneath the necks of crystal A and crystal B, respectively. As shown in Figs. 3a—3c, there is a core with relatively high dislocation density in the central region of the shoulder of crystal B. The same phenomenon was also found in crystal A. Figs. 2a and 3a show the central regions of the cores, Fig. 3b the edges of the core of crystal B, and Fig. 3c out of the core, respectively. From
these figures, we can find that the dislocation density in the central region of the core of crystal A is higher than that of crystal B, and that the dislocation density of the central region of the core in crystal B is lower than that of the edge of the core. We examined the dislocation densities of the seeds of these crystals and found that the dislocation density is high (—~16 x i0~cm2) for crystal 2) for crystal B. So A and is lower density 6 X i0~cm the dislocation in the central region of the crystal shoulder is related to that in the seeds. We also compared the dislocation densities of the central regions of the cores with that of the seeds, and found that the former are much higher than the latter. The higher dislocation density in the edge of the dislocation core as in crystal B had been suggested by Xu and Zhang [71as the mismatch of seeding. This situation may be caused by the too fast pulling rate just at the beginning of necking. In the general crystal growth process, the pulling rate was ihcreased immediately from zero to a constant rate at the beginning of necking. Because the temperature difference near the melt—crystal interface is large, 20°Cin our experiment, this process will cause a marked change of temperature of the lower part of the seed, especially the edge of the lower part of the seed, which causes the appearance of lots of dislocalions. Figs. 3a—3e also show the changes of the core of crystal B in the shouldering stage. Figs. 3d and 3e were taken from the same plates of the bottom part of crystal shoulder. Figs. 3d and 3a show the central regions of the core and Figs. 3e and 3b
Fig. 4. Conoscope images showing the dislocation densities dependence of for lower dislocation density.
(‘-S
COflOSCOPC
image: (a) for higher dislocation density; (b)
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
the edge of the core, respectively. These figures
49
3.2. Subgrain boundaries
show that the dislocation density of the core is
reduced with shouldering. In fact, the cross-section of the core becomes larger in the meantime, and the dislocation density of the crystal is low (‘-.‘ 7 x i0~cm~) and more homogeneous in the constant diameter part of the crystal just under its shoulder (about 3 mm), where the edge of the core has extended out of the crystal. It is known that a convex surface is profitable for the extension of dislocations from the central region of crystal to its edge, and an inversion from a convex to a concave interface is a source of subgrain boundaries [8]. Trauth and Grabmaier reported that the interface shape is strongly affected by the crystal rotation rate [9]. An empirical formula was also given: Ncr,1 = —4.ld + 41 ± 2, where Ncrit, the critical rotation rate, is in rpm, and d, the crystal diameter, in cm. In order to reduce the dislocation density and avoid the appearance of subgrain boundaries, we kept the shape of the melt—crystal interface as a little convex face in shouldering stage and a plane face with a slight convexity in uniform diameter stage. In the stage of shouldering, the diameter of the crystal shoulder is increased gradually, and in
order to keep the same shape of the melt—crystal interface, the crystal rotation rate should be reduced in the meantime according to the formula shown above. At this stage of our crystal growth, the 17rotation rate the is changing from 25 to rpm, and latter iscontinuously the crystal rotation rate in the uniform diameter stage. Figs. 3f and 3g show the effect of tailing to the dislocations in crystal B. They are etch patterns of the middle and the bottom part of crystal, respectively, and were chosen as originating from nearly equivalent positions with respect to the grown direction. It can be seen that the dislocation density in Fig. 3g is higher and more inhomogeneous than that in Fig. 3f, which indicates that tailing increases the dislocation density of
the bottom part of the crystal and causes it to be inhomogeneous. This effect extends about 8 mm into the internal parts of our crystals. The increased dislocation density was interpreted as being due to the crystal suddenly pulling apart from the melt at the end of crystal growth.
Figs. 2b—2d show the changes of subgrain boundaries with the growing of crystal A. They are etch patterns of the upper and the bottom part of the shoulder and the constant diameter part, they were chosen with as originatingrespectively, from nearlyand equivalent positions Fespectfind to that the growth axis. are From we can dislocations apt those to joinfigures and form subgrain boundaries in high dislocation density regions. The dislocation density is low in the regions boundaries and very around around the endsubgrain of a subgrain boundary, andhigh the neighbouring multiple boundaries tend to reform a more stablesubgrain single subgrain boundary. It was also found from our experiment that subgrain boundaries continuously grow longer until they extend to the edge of the crystal or form circuits, as dislocations constantly join them at their ends.
3.3. Relations between dislocation densities, sub-
grain boundaries and optical quality of crystal Fig. 4, the conoscope images, show the relation between dislocation densities and conoscope images. Fig. 4a was taken from the high dislocation 2) in crystal A and density x i0~ cm Fig. 4b region from the 2low dislocation density region 6 x i0~cm2) in crystal B. The extinction ratios measured from those regions are about 30: 1 and 1000: 1, respectively. The conoscope images and extinction ratios in the regions of subgrain boundary and the end of subgrain boundary have been also observed. It was found that the conoscope image and extinction ratio are worst in the dislocation gathering regions around the end of a subgrain boundary, and are almost similar where (‘-S
(‘-S
there are subgrain boundaries or lots of dislocations (dislocation density ‘~ 2 x cm2). From
io~
these and the above mentioned results, it is noted that dislocations are the basic factor for poor extinction ratio and conoscope image. Subgrain boundaries affect extinction ratio and conoscope image in their linear density of dislocations.
50
Y. Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
3.4. Comparison of the growth of congruent pure and highly Mg-doped LiNbO~crystals
the crystal bottom part and makes the dislocations
in
that part inhomogeneous.
Subgrain
boundaries are easily formed in high dislocation Since the composition of LiNbO3: Mg is offcongruent and the distribution coefficient of Mg is greater than one, compositional inhomogeneity and constitutional supercooling are apt to appear in the growth of highly Mg-doped crystals. Moreover, the dislocation density of the seed for a highly Mg-doped crystal is much higher than that for a congruent pure crystal. So there are usually higher dislocation density and more chance to form subgrain boundaries in the as-grown highly Mg-doped crystal. In order to reduce the dislocation density and avoid the appearance of growth striations, the melt—crystal interface for highly Mg-doped crystals must be strictly kept as a little convex face in the shouldering stage and a plane with only a slight convexity in the uniform diameter stage by changing the crystal rotation rate, and constant crystal diameter should be strictly
density regions, and neighbouring multiple subgrain boundaries tend to reform a more stable single subgrain boundary. The extinction ratios and conoscope images of crystals are worst in the dislocations gathering regions around the ends of subgrain boundaries, and dislocations are the basic factor giving rise to poor optical quality of crystals.
Acknowledgments The authors would like to thank Yansheng Tang, Ziheng Huang, Shaolin Chen and Jianhua Zhang for their assistance in crystal growth and sample preparation.
maintained in the uniform diameter stage. It is
References
essential to reduce the crystal pulling rate and increase the temperature gradient of the melt in
[1] Giguo Zhong, un Jian and Zhongkang Wu, in: Proc. 11th
order to avoid constitutional supercooling. Since
there is a high dislocation density in highly Mgdoped crystals, it is very important to choose 3 cm2)a good seed (dislocation and slowly increase the density pulling <6)< rate toi0 pull the crystal apart from the melt.
4. Conclusion There is a core with relatively high dislocation
density in the central region of the crystal shoulder. In the shouldering process, the cross-section of the core becomes larger and the dtslocation density in the core becomes lower and homogeneous. Tailing increases the dislocation density of
mt. Quantum Electronics Conf.,
IEEE. Cat. No. 80, CH 1561-0, June 1980, p. 631. [21 LetL D.A. 44 Bryan. R. 847. Gerson and H.E. Tomaschke, Phys. ~ (1984) F Nitanda and K. Ito,AppI. J. Crystal Growth 99 (1990) 832. [4] Yaifei Zhou, Jingchang Wang, Peiling Wang, Lianan Tang, Quanbao Zhu, Yaoan Wu and Haoran Tan, J. Crystal Growth 114 (1991) 87. [5] B.C. Grabmaier and F. Otto, J. Crystal Growth 79 (1986) 682. [6] L.J. Hu, Y.H. Chang, IN. Lin and SI. Yang, J. Crystal Growth 114 (1991) 191. [7] Xiuying Xu and Xingkui Zhang, Silicate Ada 16 (1988) (in Chinese). . [81542 B.C. Grabmaier, E. Willibald-Riha and E. Born Siemens Forsch. Entwickl. Ber. 17 (1988) 3. [91 J. Trauth and B.C. Grabmaier, J. Crystal Growth 112 (1991) 451.
Journal of Crystal Growth 140 (1994) 45—50
Dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals Yongfa Kong, Jinke Wen
Huafu Wang
Department of Physics, Nankai University, Tianjin 300071, People ~ Republic of China (Received 4 September 1993; manuscript received in final form 2 March 1994)
Abstract The extension and distribution of dislocations and subgrain boundaries in highly magnesium-doped lithium niobate crystals at different stages of growth have been investigated using chemical etching and optical microscopy. The relations between dislocation densities, subgrain boundaries and optical quality of the crystals have been also studied. It was found that there is a core with relatively high dislocation density in the central region of the crystal shoulder. In the shouldering stage, the dislocations gathering in the core extend to its surrounding regions and the dislocation density tends to be homogeneous. Tailing increases the dislocation density in the bottom part of the crystal and causes inhomogeneous distribution of dislocations in that part. Subgrain boundaries are apt to form in high dislocation density regions, and neighbouring multiple subgrain boundaries tend to reform a more stable single subgrain boundary. The extinction ratios and conoscope images of crystals are worst in the dislocation gathering regions around the ends of subgrain boundaries, and dislocations are the basic cause of poor optical quality of crystals.
1. Introduction Magnesium-doped lithium niobate crystal has excellent optical damage resistant properties
when the dopant concentration exceeds the threshold [1,2], and it is extensively used in electro-optical, integrated optical and nonlinear optical devices. However, the growth of highly Mgdoped crystal is more difficult than that of congruent pure crystal. Furukawa et al. [31indicated that defects such as subgrain boundaries increase with MgO doping, especially in the bottom parts of their crystals. It has been reported that the
*
Corresponding author,
congruent composition of Mg-doped lithium niobate crystal is different from that of pure crystal [4], and the distribution coefficient of Mg is greater than one [5,6]. All these factors will in-
duce compositional inhomogeneity, high dislocation density and subgrain boundaries in highly Mg-doped lithium niobate crystal, which cause poor optical quality of crystals. In the present paper, we describe the characteristics of dislocations and subgrain boundaries in highly Mg-doped crystals in the different growth stages. The chemical etching method was employed to reveal defects in the crystals. Extinction ratio measurement and conoscope image observation were carried out to identify the optical quality of the crystals.
0022-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00004-6
46
Y Kong et a!. /Journal
Crystal Growth 140 (1994) 45—50
of
2. Experimental procedure The highly Mg-doped LiNbO
3 crystals (Li/Nb
=
48.45/51.55, 6.5 mol% MgO) were grown by
the conventional Czochralski technique with the pulling direction along the c-axis. The starting materials were Li2CO3 (4N purity), Nb205 (3N
purity) and MgO (4N purity). They were thoroughly mixed, calcined at 800°Cfor 5 h and sintered at 1100°C for 8 h. The pulling rate (3 mm/h) was kept constant by a control system through the weight loss of melt indicated by an electronic balance. During growth of the shoulder part, the crystal rotation rate was reduced continuously from 25 to 17 rpm with the increase of the crystal diameter in order to maintain a slightly convex melt—crystal interface. The temperature difference of the melt—crystal interface (20°C) and the temperature gradient (10°C/cm above the melt surface and 15°C/cmin the melt volume near the surface) were adjusted by the power of the after-furnace and the heater beneath the bottom of the crucible. The diameters and lengths
,/~\ /~‘2a
_____
\
/
2c 2d
____________
3~C\
/
•
3d
for optical observation
./
3e
3f ~
_____________
crystal A
crystal B
Fig. 1. Sketch of crystals showing the crystal preparation and the location of figures.
of the crystals were about 34 and 44 mm, respectively. Two highly MgO-doped crystals were grown for the experiments, which are denoted as crystal A and crystal B. As shown in Fig. 1, a plate,
Fig. 2. Optical micrograph~ot dt~loeation~ and ~uhgrain boundaries for crystal A: (a) and (hI br the upper part of the crystal shoulder; (c) for the bottom part of the shoulder; (d) for the constant diameter part. (a) 3211 / : (h) — (d) 160 x
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
47
r
44
(C)
~
r~”~ ~
~
*
~
4,
~
~
4
~
‘D
~
~
4
~
~
*
•‘
~..
;~t~at~
~ ‘~i
• ~ *4 4
‘4
~0~~
~
0’
Fig. 3. Optical micrographs of dislocations for crystal B: (a) and (d) for the central part of the core; (b) and (e) for the edge of the core; (c) near the edge of the crystal. (f) for the middle part of the crystal; (g) for the bottom part of the crystal. (a)—(e) 320 x ; (f), (g) 160 x.
48
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
20 mm thick, was cut from the middle part of each crystal for extinction ratio measurement and conoscope image observation using a He—Ne laser, and both sides of their c-faces were polished to optical grade. Then, both crystals were cut into 1.5 mm plates from their shoulders to their tails with the parallel faces perpendicular to the c-axis, and the c-faces of the plates were polished and etched by reagents of HF and HNO 3(HF/HNO3 = 1/2) in a water bath at 100°Cfor 30 mm. The dislocations and subgrain boundaries were observed by an optical microscope.
3. Results and discussion The typical etch patterns for crystal A are shown in Fig. 2, that for crystal B in Fig. 3, and the conoscope images are shown in Fig. 4. The corresponding parts of the crystals for observation in Figs. 2—4 are shown in Fig. 1. 3.1. Dislocations Figs. 2a and 3a—3c show the etch patterns of the upper parts of the crystal shoulders just beneath the necks of crystal A and crystal B, respectively. As shown in Figs. 3a—3c, there is a core with relatively high dislocation density in the central region of the shoulder of crystal B. The same phenomenon was also found in crystal A. Figs. 2a and 3a show the central regions of the cores, Fig. 3b the edges of the core of crystal B, and Fig. 3c out of the core, respectively. From
these figures, we can find that the dislocation density in the central region of the core of crystal A is higher than that of crystal B, and that the dislocation density of the central region of the core in crystal B is lower than that of the edge of the core. We examined the dislocation densities of the seeds of these crystals and found that the dislocation density is high (—~16 x i0~cm2) for crystal 2) for crystal B. So A and is lower density 6 X i0~cm the dislocation in the central region of the crystal shoulder is related to that in the seeds. We also compared the dislocation densities of the central regions of the cores with that of the seeds, and found that the former are much higher than the latter. The higher dislocation density in the edge of the dislocation core as in crystal B had been suggested by Xu and Zhang [71as the mismatch of seeding. This situation may be caused by the too fast pulling rate just at the beginning of necking. In the general crystal growth process, the pulling rate was ihcreased immediately from zero to a constant rate at the beginning of necking. Because the temperature difference near the melt—crystal interface is large, 20°Cin our experiment, this process will cause a marked change of temperature of the lower part of the seed, especially the edge of the lower part of the seed, which causes the appearance of lots of dislocalions. Figs. 3a—3e also show the changes of the core of crystal B in the shouldering stage. Figs. 3d and 3e were taken from the same plates of the bottom part of crystal shoulder. Figs. 3d and 3a show the central regions of the core and Figs. 3e and 3b
Fig. 4. Conoscope images showing the dislocation densities dependence of for lower dislocation density.
(‘-S
COflOSCOPC
image: (a) for higher dislocation density; (b)
Y Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
the edge of the core, respectively. These figures
49
3.2. Subgrain boundaries
show that the dislocation density of the core is
reduced with shouldering. In fact, the cross-section of the core becomes larger in the meantime, and the dislocation density of the crystal is low (‘-.‘ 7 x i0~cm~) and more homogeneous in the constant diameter part of the crystal just under its shoulder (about 3 mm), where the edge of the core has extended out of the crystal. It is known that a convex surface is profitable for the extension of dislocations from the central region of crystal to its edge, and an inversion from a convex to a concave interface is a source of subgrain boundaries [8]. Trauth and Grabmaier reported that the interface shape is strongly affected by the crystal rotation rate [9]. An empirical formula was also given: Ncr,1 = —4.ld + 41 ± 2, where Ncrit, the critical rotation rate, is in rpm, and d, the crystal diameter, in cm. In order to reduce the dislocation density and avoid the appearance of subgrain boundaries, we kept the shape of the melt—crystal interface as a little convex face in shouldering stage and a plane face with a slight convexity in uniform diameter stage. In the stage of shouldering, the diameter of the crystal shoulder is increased gradually, and in
order to keep the same shape of the melt—crystal interface, the crystal rotation rate should be reduced in the meantime according to the formula shown above. At this stage of our crystal growth, the 17rotation rate the is changing from 25 to rpm, and latter iscontinuously the crystal rotation rate in the uniform diameter stage. Figs. 3f and 3g show the effect of tailing to the dislocations in crystal B. They are etch patterns of the middle and the bottom part of crystal, respectively, and were chosen as originating from nearly equivalent positions with respect to the grown direction. It can be seen that the dislocation density in Fig. 3g is higher and more inhomogeneous than that in Fig. 3f, which indicates that tailing increases the dislocation density of
the bottom part of the crystal and causes it to be inhomogeneous. This effect extends about 8 mm into the internal parts of our crystals. The increased dislocation density was interpreted as being due to the crystal suddenly pulling apart from the melt at the end of crystal growth.
Figs. 2b—2d show the changes of subgrain boundaries with the growing of crystal A. They are etch patterns of the upper and the bottom part of the shoulder and the constant diameter part, they were chosen with as originatingrespectively, from nearlyand equivalent positions Fespectfind to that the growth axis. are From we can dislocations apt those to joinfigures and form subgrain boundaries in high dislocation density regions. The dislocation density is low in the regions boundaries and very around around the endsubgrain of a subgrain boundary, andhigh the neighbouring multiple boundaries tend to reform a more stablesubgrain single subgrain boundary. It was also found from our experiment that subgrain boundaries continuously grow longer until they extend to the edge of the crystal or form circuits, as dislocations constantly join them at their ends.
3.3. Relations between dislocation densities, sub-
grain boundaries and optical quality of crystal Fig. 4, the conoscope images, show the relation between dislocation densities and conoscope images. Fig. 4a was taken from the high dislocation 2) in crystal A and density x i0~ cm Fig. 4b region from the 2low dislocation density region 6 x i0~cm2) in crystal B. The extinction ratios measured from those regions are about 30: 1 and 1000: 1, respectively. The conoscope images and extinction ratios in the regions of subgrain boundary and the end of subgrain boundary have been also observed. It was found that the conoscope image and extinction ratio are worst in the dislocation gathering regions around the end of a subgrain boundary, and are almost similar where (‘-S
(‘-S
there are subgrain boundaries or lots of dislocations (dislocation density ‘~ 2 x cm2). From
io~
these and the above mentioned results, it is noted that dislocations are the basic factor for poor extinction ratio and conoscope image. Subgrain boundaries affect extinction ratio and conoscope image in their linear density of dislocations.
50
Y. Kong et a!. /Journal of Crystal Growth 140 (1994) 45—50
3.4. Comparison of the growth of congruent pure and highly Mg-doped LiNbO~crystals
the crystal bottom part and makes the dislocations
in
that part inhomogeneous.
Subgrain
boundaries are easily formed in high dislocation Since the composition of LiNbO3: Mg is offcongruent and the distribution coefficient of Mg is greater than one, compositional inhomogeneity and constitutional supercooling are apt to appear in the growth of highly Mg-doped crystals. Moreover, the dislocation density of the seed for a highly Mg-doped crystal is much higher than that for a congruent pure crystal. So there are usually higher dislocation density and more chance to form subgrain boundaries in the as-grown highly Mg-doped crystal. In order to reduce the dislocation density and avoid the appearance of growth striations, the melt—crystal interface for highly Mg-doped crystals must be strictly kept as a little convex face in the shouldering stage and a plane with only a slight convexity in the uniform diameter stage by changing the crystal rotation rate, and constant crystal diameter should be strictly
density regions, and neighbouring multiple subgrain boundaries tend to reform a more stable single subgrain boundary. The extinction ratios and conoscope images of crystals are worst in the dislocations gathering regions around the ends of subgrain boundaries, and dislocations are the basic factor giving rise to poor optical quality of crystals.
Acknowledgments The authors would like to thank Yansheng Tang, Ziheng Huang, Shaolin Chen and Jianhua Zhang for their assistance in crystal growth and sample preparation.
maintained in the uniform diameter stage. It is
References
essential to reduce the crystal pulling rate and increase the temperature gradient of the melt in
[1] Giguo Zhong, un Jian and Zhongkang Wu, in: Proc. 11th
order to avoid constitutional supercooling. Since
there is a high dislocation density in highly Mgdoped crystals, it is very important to choose 3 cm2)a good seed (dislocation and slowly increase the density pulling <6)< rate toi0 pull the crystal apart from the melt.
4. Conclusion There is a core with relatively high dislocation
density in the central region of the crystal shoulder. In the shouldering process, the cross-section of the core becomes larger and the dtslocation density in the core becomes lower and homogeneous. Tailing increases the dislocation density of
mt. Quantum Electronics Conf.,
IEEE. Cat. No. 80, CH 1561-0, June 1980, p. 631. [21 LetL D.A. 44 Bryan. R. 847. Gerson and H.E. Tomaschke, Phys. ~ (1984) F Nitanda and K. Ito,AppI. J. Crystal Growth 99 (1990) 832. [4] Yaifei Zhou, Jingchang Wang, Peiling Wang, Lianan Tang, Quanbao Zhu, Yaoan Wu and Haoran Tan, J. Crystal Growth 114 (1991) 87. [5] B.C. Grabmaier and F. Otto, J. Crystal Growth 79 (1986) 682. [6] L.J. Hu, Y.H. Chang, IN. Lin and SI. Yang, J. Crystal Growth 114 (1991) 191. [7] Xiuying Xu and Xingkui Zhang, Silicate Ada 16 (1988) (in Chinese). . [81542 B.C. Grabmaier, E. Willibald-Riha and E. Born Siemens Forsch. Entwickl. Ber. 17 (1988) 3. [91 J. Trauth and B.C. Grabmaier, J. Crystal Growth 112 (1991) 451.