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Kyropoulos growth and perfection of KNbO3 single crystal
450
Journal of Crystal Growth 24/25 (1974) 450—453 © North-Holland Publishing Co.
KYROPOULOS GROWTH AND PERFECTION OF KNbO3 SINGLE CRYSTAL T. FUKUDA, Y. UEMATSU and T. ITO Toshiba Research and Development Center, Tokyo Shibaura Electric Co., Ltd., I Komukai-Toshibacho, Kawasaki, Japan 3 in size were grown by the Kyropoulos technique. crystal, after single conversion to single domain, wasmm characterized by optical examination Colorless and wellThe defined KNbO3 crystals up to 40x40x15 and X-ray topography. Defects due to the Kyropoulos growth mechanism were observed towards the crystal edge from the seed. Growth striations were not observed. Interference patterns produced by a Twyman— Green interferometer revealed the refractive index variation of the crystal to be less than 10—s. The full width at half maximum of the X-ray rocking curves is about 14 sec, and is almost constant within the crystal.
1. Introduction
crucible. KNbO
Recently, KNbO3 single crystal has been found to have conversion efficiency 1.06 to 0.53with jim seconda large harmonic generation (SHO),ofcomparable Ba 2NaNb5O15, and to beSHG stable to an intense laser 2). The intracavity of YAG/Nd laser using beam” KNbO3 crystal was demonstrated and about 100 % 2 3 conversion was attained ). High optical quality was required for the crystal for use in such nonlinear optical devices. .
KNbO3 melts incongruently. Fluxes or self-flux are required to grow a single crystal from the melt. Various techniques have been applied to obtain large single crystals. Centimeter-size KNbO3 single crystals1’46). have been reported only by the Kyropoulos technique However, little information concerning the quality of single domain crystals has been given, because of the difficulties in poling multi-domain crystals. A poling technique has been developed and large single domain crystals were successfully obtained. This paper is concerned with the Kyropoulos growth of KNbO 3 crystals and the characterization of the single domain crystals determined by means of X-ray and optical techniques.
3 single crystal was allowed to grow laterally for ‘-.~48h to obtain an approximately 40 mm cross section using the same furnace as described in 1). The growing conditions the previous publication used are as follows: Melt composition
K 2C03 52.5 mole
.
Seed orientation Seed rotation Cooling rate of melt Atmosphere .
2
~,
5
[001] *, 30 rpm, 0.5 C/h (regulated 02 (1.5 1/mm).
‘~
±0.1 C),
As-grown crystals were poled by applying a 1 kY/cm dc field at about 180 °Cin a silicon oil bath. The polingfaces, was “{l00}” carried out oninstead the specimen cut with as-grown faces, of the specimen cut with orthorhombic ~l00} faces. The direction in which the dc field was applied was determined by comparison of the thermal expansion curves and etch patterns of the as-grown and poled crystals7). The poling completeness was confirmed by etching and SHG ex7’8). periments Optical examinations were made by means of polarized light, fringe patterns by He—Ne laser beam2) and Twyman—Green interferometer. X-ray double crystal topography was carried out with a Toshiba ADG-501 using CuKa, radiation. Specimens for optical and X-
2. Experimental Single crystals of KNbO 3 were grown by the Kyropoulos technique from melts composed of a mixture of K~C03 and Nb 2 0 5 (both grade I Johnson-Matthey Co.). The charge was contained in a 250 ml platinum
ray evaluation were polished to )/10 roughness. The angle between them was less than 5 sec. . Indices enclosed in quotation marks refer to the pseudo-cubic axes of the crystal.
*
XI — 6
KYROPOULOS GROWTH AND PERFECTION OF
Fig. I.
Typical KNbO3 as-grown crystals.
KNbO3
451
SINGLE CRYSTAL
x 0.8. Fig. 2.
KNbO3 single domain crystals.
3. Results and discussion Colorless and well singleTypical crystals asup 3 defined in size KNbO3 were grown. to 40 x 40 x 15 mm grown crystals are shown in fig. 1. The crystal is almost a perfect parallelepiped with “{100}” planes. The asgrown “{100}” planes are macroscopically perfect, but the surface of the central region of the bottom “(001)” plane (region denoted by A in the figure) is not smooth. On the upper surface of the crystal (region denoted by B), growth ridges are observed along the “[110]” direction. The as-grown crystal is cloudy due to multidomains, but the crystal is clear when viewed along the direction of vertical growth. Any part of the crystal except region A could be poled with ease. In the specimen from region A, cracking was likely to occur during poling. It is suggested that region A contains structural defects which induce a large internal stress. Poled crystals obtained from regions B and C (see fig. 1) are shown in fig. 2. Microscopic examination ofthe poled KNbO 3 show-
x 2.3.
ed no visible as of growth fringe patterndefects of the such crystal regionstriations. B (a” x b”But x c”the = 7 x 5 x 9 mm pseudo-cubic axes), which appeared when it was placed between crossed polarizers in the He—Ne laser (6328 A), revealed optical inhomogeneity, as is seen in fig. 3 (a). The optical path direction is in the “[001]”, which is parallel to vertical growth direction. Fig. 3 (b) shows a corresponding X-ray reflection topograph of the same crystal. The topograph was taken by “(003)” reflection. A good topographic correspondence was seen between them. Figs. 3 (a) and (b) reveal the defects along the “[110]” direction, which correspond to that of a growth ridge on the crystal surface. Growth striations were not observed in the topograph. The crystal obtained from region C showed neither growth striations nor crystal defects [seefig. 3(c)] such as was observed in the crystal from region B. Optical homogeneity of the crystal was investigated
libi
by using aTwyman—Green interferometer. Fig. 4 shows
Fig. 3. The fringe pattern (a) and X-ray double crystal topograph (b) of the crystal of the region B. X-ray double crystal topograph of the crystal ofthe region C (c).
XI
—
6
452
T. FUKUDA, Y. UEMATSU AND T. ITO
b
Fig. 4. Typical Twyman-Green interference patterns of the crystal of the region C. (a) 6n~(b) ~fla~ growth axis “(3OO~
dE~fion
seed
~,
Cc)
/ \
/ ~
Fig. 5.
Fig. 6. Horizontal (a) and vertical (b) cross section of the crystal grown purposely under unsatisfactory conditions. (c) The growth model of KNbO 3 by Kyropoulos technique.
\ ~
estimated to be less than l0 which satisfied the condition of the crystal9). homogeneity needed in the nonlinear The optical degreecrystal of crystal perfection of the C region crystal, which was revealed to be the optically most homogeneous region, was characterized by rocking curves from a double crystal X-ray spectrometer. Measurement was carried out at every 0.5 mm along of the specimen (a” x b” x c” = 5 x 5 x 5 mm). A narrow X-ray beam (0.1 x 0.1 mm) was used. A typical rocking curve from the “(300)” reflection is shown in fig. 5. Line broadening at the tail of the curve is presumed to be
A typical rocking curve of the crystal of the region C.
typical interference patterns the CA)region crystal. An incident He—Ne laser beamof(6328 was polarized parallel or vertical to the c axis to evaluate the refractive index variation ~ or ~ respectively. The upper photograph in fig. 4 shows the variation of n~,~ and the lower one shows that of ~ ~ Almost no variation was observed in the crystal except the edge area in which a little variation thought to be caused from residual strain in the polishing process, was observed, From fig. 4, the index variation of this crystal was XI
—
6
KYROPOULOS GROWTH AND PERFECTION OF
due to the mechanically damaged surface layers. The full width at half maximum intensity is about 14 sec, and is almost constant within the crystal. The value for the other oxide crystal of high perfection is reported to be 10—15 sec’ 1). These results indicate that the perfecof the crystal is high. To determine the crystal imperfection due to the Kyropoulos growth mechanism, a crystal grown purposely under unsatisfactory conditions was investi-
SINGLE CRYSTAL
453
4. Conclusion KNbO3 single crystals grown by the Kyropoulos technique were characterized by optical and X-ray techniques. Defects due to the growth mechanism were observed towards the crystal edge from the seed. Growth striations were not observed. Index variation of the defect-free part was less than iO~ and high perfection was confirmed by the X-ray rocking curve. Acknowledgement
gated. Figs. 6 (a) and 6 (b) show cross sections of the crystal grown under the following conditions: temperature regulation ±10 °C,soaking temperature 1050 °Cand 1 h soaking time. Cyclic color changes between blue and clear planar bands was observed. The blue color was thought to be caused by oxygen vacancies’). These results suggest the growth model given in fig. 6 (c) which 10). is very to thatthat proposed the flux-grown It similar is assumed planarinbands are attricrystal buted to cyclic layered growth of “{100}” faces caused
The authors wish to thank Dr. S. Takasu for some X-ray measurements and useful discussions. References 1) T. Fukuda and Y. Uematsu, Japan. J. Appi. Phys. 11 (1972) 163. 2) Y. Uematsu and T. Fukuda, Japan. J. Appi. Phys. 12 (1973) 841. 3) Phys. 12 (1973) 4) Y. C. Uematsu, E. Miller, Japan. J. Appl.J. Appi. Phys. 29 (1958) 233. 1257. 5) E. Wiesendanger, Ferroelectrics 1 (1970) 141. 6) J. J. Hurst and A. Linz, Mater. Res. Bull. 6 (1971) 163. 7) T. Fukuda, T. Ito and Y. Uematsu, Oyo Buturi (in Japanese) 42 (1973) 192.
by the temperature fluctuation. The defect along the “[110]” direction, as shown in fig. 3, corresponds with the direction towards the crystal edge from the seed shown in fig. 6 (c). The defect may be due to the stress of the boundary between the planar bands. However, the planar bands of the crystal grown under good conditions were scarcely observed by optical and X-ray techniques.
XI
KNbO3
8) T. Fukuda, H. Hirano, Y. Uematsu and T. Ito, Japan. J. Appl. Phys. 13(1974)1021. 9) R. L. Byer and J. F. Young, J. Appl. Phys. 41 (1970) 2320. 10) E. A. Giess, D. C. Cronomeyer, L. L. Rosier and J. D. Kuptsis, Mater. Res. Bull. 5 (1970) 495. 11) R. F. Belt, P. Moss and J. R. Latore, Mater. Res. Bull. 8 (1973) 357; S. Takasu, private communication.
—
6
Journal of Crystal Growth 24/25 (1974) 450—453 © North-Holland Publishing Co.
KYROPOULOS GROWTH AND PERFECTION OF KNbO3 SINGLE CRYSTAL T. FUKUDA, Y. UEMATSU and T. ITO Toshiba Research and Development Center, Tokyo Shibaura Electric Co., Ltd., I Komukai-Toshibacho, Kawasaki, Japan 3 in size were grown by the Kyropoulos technique. crystal, after single conversion to single domain, wasmm characterized by optical examination Colorless and wellThe defined KNbO3 crystals up to 40x40x15 and X-ray topography. Defects due to the Kyropoulos growth mechanism were observed towards the crystal edge from the seed. Growth striations were not observed. Interference patterns produced by a Twyman— Green interferometer revealed the refractive index variation of the crystal to be less than 10—s. The full width at half maximum of the X-ray rocking curves is about 14 sec, and is almost constant within the crystal.
1. Introduction
crucible. KNbO
Recently, KNbO3 single crystal has been found to have conversion efficiency 1.06 to 0.53with jim seconda large harmonic generation (SHO),ofcomparable Ba 2NaNb5O15, and to beSHG stable to an intense laser 2). The intracavity of YAG/Nd laser using beam” KNbO3 crystal was demonstrated and about 100 % 2 3 conversion was attained ). High optical quality was required for the crystal for use in such nonlinear optical devices. .
KNbO3 melts incongruently. Fluxes or self-flux are required to grow a single crystal from the melt. Various techniques have been applied to obtain large single crystals. Centimeter-size KNbO3 single crystals1’46). have been reported only by the Kyropoulos technique However, little information concerning the quality of single domain crystals has been given, because of the difficulties in poling multi-domain crystals. A poling technique has been developed and large single domain crystals were successfully obtained. This paper is concerned with the Kyropoulos growth of KNbO 3 crystals and the characterization of the single domain crystals determined by means of X-ray and optical techniques.
3 single crystal was allowed to grow laterally for ‘-.~48h to obtain an approximately 40 mm cross section using the same furnace as described in 1). The growing conditions the previous publication used are as follows: Melt composition
K 2C03 52.5 mole
.
Seed orientation Seed rotation Cooling rate of melt Atmosphere .
2
~,
5
[001] *, 30 rpm, 0.5 C/h (regulated 02 (1.5 1/mm).
‘~
±0.1 C),
As-grown crystals were poled by applying a 1 kY/cm dc field at about 180 °Cin a silicon oil bath. The polingfaces, was “{l00}” carried out oninstead the specimen cut with as-grown faces, of the specimen cut with orthorhombic ~l00} faces. The direction in which the dc field was applied was determined by comparison of the thermal expansion curves and etch patterns of the as-grown and poled crystals7). The poling completeness was confirmed by etching and SHG ex7’8). periments Optical examinations were made by means of polarized light, fringe patterns by He—Ne laser beam2) and Twyman—Green interferometer. X-ray double crystal topography was carried out with a Toshiba ADG-501 using CuKa, radiation. Specimens for optical and X-
2. Experimental Single crystals of KNbO 3 were grown by the Kyropoulos technique from melts composed of a mixture of K~C03 and Nb 2 0 5 (both grade I Johnson-Matthey Co.). The charge was contained in a 250 ml platinum
ray evaluation were polished to )/10 roughness. The angle between them was less than 5 sec. . Indices enclosed in quotation marks refer to the pseudo-cubic axes of the crystal.
*
XI — 6
KYROPOULOS GROWTH AND PERFECTION OF
Fig. I.
Typical KNbO3 as-grown crystals.
KNbO3
451
SINGLE CRYSTAL
x 0.8. Fig. 2.
KNbO3 single domain crystals.
3. Results and discussion Colorless and well singleTypical crystals asup 3 defined in size KNbO3 were grown. to 40 x 40 x 15 mm grown crystals are shown in fig. 1. The crystal is almost a perfect parallelepiped with “{100}” planes. The asgrown “{100}” planes are macroscopically perfect, but the surface of the central region of the bottom “(001)” plane (region denoted by A in the figure) is not smooth. On the upper surface of the crystal (region denoted by B), growth ridges are observed along the “[110]” direction. The as-grown crystal is cloudy due to multidomains, but the crystal is clear when viewed along the direction of vertical growth. Any part of the crystal except region A could be poled with ease. In the specimen from region A, cracking was likely to occur during poling. It is suggested that region A contains structural defects which induce a large internal stress. Poled crystals obtained from regions B and C (see fig. 1) are shown in fig. 2. Microscopic examination ofthe poled KNbO 3 show-
x 2.3.
ed no visible as of growth fringe patterndefects of the such crystal regionstriations. B (a” x b”But x c”the = 7 x 5 x 9 mm pseudo-cubic axes), which appeared when it was placed between crossed polarizers in the He—Ne laser (6328 A), revealed optical inhomogeneity, as is seen in fig. 3 (a). The optical path direction is in the “[001]”, which is parallel to vertical growth direction. Fig. 3 (b) shows a corresponding X-ray reflection topograph of the same crystal. The topograph was taken by “(003)” reflection. A good topographic correspondence was seen between them. Figs. 3 (a) and (b) reveal the defects along the “[110]” direction, which correspond to that of a growth ridge on the crystal surface. Growth striations were not observed in the topograph. The crystal obtained from region C showed neither growth striations nor crystal defects [seefig. 3(c)] such as was observed in the crystal from region B. Optical homogeneity of the crystal was investigated
libi
by using aTwyman—Green interferometer. Fig. 4 shows
Fig. 3. The fringe pattern (a) and X-ray double crystal topograph (b) of the crystal of the region B. X-ray double crystal topograph of the crystal ofthe region C (c).
XI
—
6
452
T. FUKUDA, Y. UEMATSU AND T. ITO
b
Fig. 4. Typical Twyman-Green interference patterns of the crystal of the region C. (a) 6n~(b) ~fla~ growth axis “(3OO~
dE~fion
seed
~,
Cc)
/ \
/ ~
Fig. 5.
Fig. 6. Horizontal (a) and vertical (b) cross section of the crystal grown purposely under unsatisfactory conditions. (c) The growth model of KNbO 3 by Kyropoulos technique.
\ ~
estimated to be less than l0 which satisfied the condition of the crystal9). homogeneity needed in the nonlinear The optical degreecrystal of crystal perfection of the C region crystal, which was revealed to be the optically most homogeneous region, was characterized by rocking curves from a double crystal X-ray spectrometer. Measurement was carried out at every 0.5 mm along of the specimen (a” x b” x c” = 5 x 5 x 5 mm). A narrow X-ray beam (0.1 x 0.1 mm) was used. A typical rocking curve from the “(300)” reflection is shown in fig. 5. Line broadening at the tail of the curve is presumed to be
A typical rocking curve of the crystal of the region C.
typical interference patterns the CA)region crystal. An incident He—Ne laser beamof(6328 was polarized parallel or vertical to the c axis to evaluate the refractive index variation ~ or ~ respectively. The upper photograph in fig. 4 shows the variation of n~,~ and the lower one shows that of ~ ~ Almost no variation was observed in the crystal except the edge area in which a little variation thought to be caused from residual strain in the polishing process, was observed, From fig. 4, the index variation of this crystal was XI
—
6
KYROPOULOS GROWTH AND PERFECTION OF
due to the mechanically damaged surface layers. The full width at half maximum intensity is about 14 sec, and is almost constant within the crystal. The value for the other oxide crystal of high perfection is reported to be 10—15 sec’ 1). These results indicate that the perfecof the crystal is high. To determine the crystal imperfection due to the Kyropoulos growth mechanism, a crystal grown purposely under unsatisfactory conditions was investi-
SINGLE CRYSTAL
453
4. Conclusion KNbO3 single crystals grown by the Kyropoulos technique were characterized by optical and X-ray techniques. Defects due to the growth mechanism were observed towards the crystal edge from the seed. Growth striations were not observed. Index variation of the defect-free part was less than iO~ and high perfection was confirmed by the X-ray rocking curve. Acknowledgement
gated. Figs. 6 (a) and 6 (b) show cross sections of the crystal grown under the following conditions: temperature regulation ±10 °C,soaking temperature 1050 °Cand 1 h soaking time. Cyclic color changes between blue and clear planar bands was observed. The blue color was thought to be caused by oxygen vacancies’). These results suggest the growth model given in fig. 6 (c) which 10). is very to thatthat proposed the flux-grown It similar is assumed planarinbands are attricrystal buted to cyclic layered growth of “{100}” faces caused
The authors wish to thank Dr. S. Takasu for some X-ray measurements and useful discussions. References 1) T. Fukuda and Y. Uematsu, Japan. J. Appi. Phys. 11 (1972) 163. 2) Y. Uematsu and T. Fukuda, Japan. J. Appi. Phys. 12 (1973) 841. 3) Phys. 12 (1973) 4) Y. C. Uematsu, E. Miller, Japan. J. Appl.J. Appi. Phys. 29 (1958) 233. 1257. 5) E. Wiesendanger, Ferroelectrics 1 (1970) 141. 6) J. J. Hurst and A. Linz, Mater. Res. Bull. 6 (1971) 163. 7) T. Fukuda, T. Ito and Y. Uematsu, Oyo Buturi (in Japanese) 42 (1973) 192.
by the temperature fluctuation. The defect along the “[110]” direction, as shown in fig. 3, corresponds with the direction towards the crystal edge from the seed shown in fig. 6 (c). The defect may be due to the stress of the boundary between the planar bands. However, the planar bands of the crystal grown under good conditions were scarcely observed by optical and X-ray techniques.
XI
KNbO3
8) T. Fukuda, H. Hirano, Y. Uematsu and T. Ito, Japan. J. Appl. Phys. 13(1974)1021. 9) R. L. Byer and J. F. Young, J. Appl. Phys. 41 (1970) 2320. 10) E. A. Giess, D. C. Cronomeyer, L. L. Rosier and J. D. Kuptsis, Mater. Res. Bull. 5 (1970) 495. 11) R. F. Belt, P. Moss and J. R. Latore, Mater. Res. Bull. 8 (1973) 357; S. Takasu, private communication.
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6