U.S. patent application number 09/793193 was filed with the patent office on 2001-09-06 for process and apparatus for producing oxide single crystals.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Honda, Akihiko, Imaeda, Minoru, Imai, Katsuhiro.
Application Number | 20010018888 09/793193 |
Document ID | / |
Family ID | 18578949 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010018888 |
Kind Code |
A1 |
Imai, Katsuhiro ; et
al. |
September 6, 2001 |
Process and apparatus for producing oxide single crystals
Abstract
A process is disclosed for producing an oxide single crystal,
comprising the steps of: melting a raw material for a single
crystal of an oxide inside a crucible, contacting a seed crystal
with the resulting melt, growing the oxide single crystal by
pulling-down the melt through an opening of the crucible in a given
pulling-down axis, and fixedly holding the seed crystal and then
reducing an angle of a given crystalline orientation of the seed
crystal selected for growing the single crystal to the pulling-down
axis.
Inventors: |
Imai, Katsuhiro; (Nagoya,
JP) ; Honda, Akihiko; (Nagoya, JP) ; Imaeda,
Minoru; (Nagoya, JP) |
Correspondence
Address: |
BURR & BROWN
PO BOX 7068
SYRACUSE
NY
13261-7068
US
|
Assignee: |
NGK Insulators, Ltd.
|
Family ID: |
18578949 |
Appl. No.: |
09/793193 |
Filed: |
February 26, 2001 |
Current U.S.
Class: |
117/13 |
Current CPC
Class: |
C30B 15/08 20130101;
Y10T 117/1016 20150115; C30B 29/30 20130101; Y10T 117/10 20150115;
C30B 15/00 20130101; Y10S 117/90 20130101 |
Class at
Publication: |
117/13 |
International
Class: |
C30B 015/00; C30B
021/06; C30B 027/02; C30B 028/10; C30B 030/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2000 |
JP |
2000-058, 345 |
Claims
What is claimed is:
1. A process for producing an oxide single crystal, comprising the
steps of melting a raw material for a single crystal of an oxide
inside a crucible, contacting a seed crystal with the resulting
melt, growing the oxide single crystal by pulling-down the melt
through an opening of the crucible in a given pulling-down axis,
and fixedly holding the seed crystal and then reducing an angle of
a given crystalline orientation of the seed crystal selected for
growing the single crystal to the pulling-down axis.
2. The producing process set forth in claim 1, wherein the angle of
said crystalline orientation to the pulling-down axis is controlled
to not more than 0.5.degree..
3. The producing process set forth in claim 1 or 2, wherein after
the seed crystal is fixedly held, the seed crystal is turned around
the pulling-down axis and thereby the cross sectional shape of the
seed crystal is brought into conformity with the shape of the
bottom of the nozzle.
4. The producing process set forth in claim 1 or 2, wherein the
seed crystal has a polygonal cross-sectional shape, the bottom of
the nozzle has a shape identical with or similar to the polygonal
cross-sectional shape of the seed crystal, and after the seed
crystal is fixedly held, the seed crystal is turned around the
pulling-down axis and thereby angles defined by plural pairs of
sides of the polygonal cross-sectional shape of the seed crystal
and corresponding ones of the polygonal shape of the bottom of the
nozzle, respectively, are reduced.
5. The producing process set forth in claim 4, wherein each of the
angles defined by plural pairs of the sides of the polygonal
cross-sectional shape of the seed crystal and the corresponding
ones of the polygonal shape of the bottom of the nozzle,
respectively, is not more than 0.5.degree..
6. The producing process set forth in claims 1 to 2, wherein each
of the seed crystal and the bottom of the nozzle has a rectangular
cross-sectional shape.
7. An apparatus for producing a single crystal of an oxide,
comprising a crucible for melting a raw material of the oxide
single crystal, and a stand for holding a seed crystal, said stand
comprising a holding member for fixedly holding the seed crystal,
and an inclining mechanism attached to the under side of the
holding member, and adapted to incline an angle of the holding
member to a given pulling-down axis, wherein a raw material of the
oxide single crystal is melted in the crucible, the seed crystal is
contacted with the resulting melt, the oxide single crystal is
grown by pulling-down the melt through an opening of the crucible,
and an angle of the holding member to the pulling-down direction is
reduced.
8. The apparatus set forth in claim 7, wherein the crucible
comprises a nozzle portion, said opening is provided at a bottom
portion of the nozzle portion, and the stand further comprises a
turning mechanism for turning the holding member around the
pulling-down axis.
9. The apparatus set forth in claim 7 or 8, wherein the bottom
portion of the nozzle portion has a polygonal shape.
10. The apparatus set forth in claim 9, wherein the bottom portion
of the nozzle portion has a rectangular shape.
11. The apparatus set forth in any one of claim 7 or 8, wherein the
stand further comprises a horizontally moving mechanism for moving
the holding member within a plane perpendicular to the pulling-down
axis.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates to a process and an apparatus
for producing single crystals of oxides.
[0003] (2) Related Art Statement
[0004] A single crystal of lithium potassium niobate and a single
crystal of lithium potassium niobate-lithium potassium tantalate
solid solution are remarked especially as single crystals for a
blue light second harmonic generation (SHG) device for a
semiconductor laser. The device can emit even the ultraviolet
lights having the wavelengths of 390 nm or so, thus the crystals
can be suitable for wide applications such as optical disk memory,
medicine and photochemical fields, and various optical measurements
by using such short-wavelength lights. Since the above single
crystals have a large electro-optic effect, they can be also
applied to optical memory devices using the photorefractive
effect.
[0005] However, for an application of a second harmonic generation
device, for example, even a small fluctuation in a composition of
the single crystal may affect the wavelength of the second harmonic
wave generated by the device. Therefore, the specification of the
range of the composition required for said single crystals is
severe, and the fluctuation in the composition should be suppressed
in a narrow range. However, since the composition consists of as
many as three or four components, growing a single crystal at a
high rate is extremely difficult to achieve while controlling the
proportions of the components to be constant.
[0006] In addition, laser beams, having a short wavelength of
around 400 nm, for example, need to be propagated inside of the
single crystal at as high an output density as possible in the case
of the optical applications, particularly the second harmonic
generation. Further, optical damage needs to be suppressed. The
good crystallinity of the single crystal is required for this
purpose.
[0007] NGK Insulators, Ltd. suggested a micro (.mu.) pulling-down
method for growing the above single crystal with the constant
compositional proportions, for example, in JP-A-8-319,191. In this
method, a raw material comprising lithium potassium niobate is put
into a platinum crucible and melted, and then the melt is pulled
down gradually and continuously through a nozzle attached to the
bottom of the crucible.
[0008] Such a micro pulling-down method is useful for growing the
above oxide single crystal having many components. However, for
widespread uses of the above single crystal, increased industrial
productivity is indispensable. For this purpose, the capacity of
the crucible needs to be increased to grow as many single crystals
as possible. To attain this, it is necessary that a driving unit is
provided under the crucible, a seed crystal is bonded to an upper
end of the driving unit, the seed crystal is contacted with the
melt in the crucible, and the seed crystal is pulled down as
straight as possible at a high precision. As such a high precision
pulling-down mechanism, a driving unit using a rail may be used,
for example.
[0009] However, inventors' mass production experiments revealed
that a single crystal having good crystallinity was successfully
grown by pulling down the seed crystal at a high precision in the
beginning, but as the grown length of the oxide single crystal
increases (with lapse of time), the crystallinity of the oxide
single crystal is deteriorate to reduce the yield.
[0010] It is an object of the present invention to prevent
deterioration in the crystallinity of a single crystal which would
accompany increase in the grown length of the single crystal, when
a raw material for the oxide single crystal is melted in a
crucible, a seed crystal is contacted with the melt, and the oxide
single crystal is grown, while being pulled down through an opening
of the crucible in a given pulling-down axis.
[0011] A first aspect of the present invention relates to a process
for producing an oxide single crystal, comprising the steps of:
[0012] melting a raw material for a single crystal of an oxide
inside a crucible, contacting a seed crystal with the resulting
melt, growing the oxide single crystal by pulling-down the melt
through an opening of the crucible in a given pulling-down axis,
fixedly holding the seed crystal and then reducing an angle of a
given crystalline orientation of the seed crystal selected for
growing the single crystal to the pulling-down axis.
[0013] A second aspect of the present invention relates to an
apparatus for producing a single crystal of an oxide, comprising a
crucible for melting a raw material of the oxide single crystal,
and a stand for holding a seed crystal, said stand comprising a
holding member for fixedly holding the seed crystal, and an
inclining mechanism attached to the under side of the holding
member, and adapted to incline an angle of the holding member to a
given pulling-down axis, wherein a raw material of the oxide single
crystal is melted in the crucible, the seed crystal is contacted
with the resulting melt, the oxide single crystal is grown by
pulling-down the melt through an opening of the crucible, and an
angle of the holding member to the pulling-down direction is
reduced.
[0014] With respect to the above-mentioned cases where the
crystallinity of the single crystals were gradually deteriorated as
the grown length thereof increased, the inventors observed the
state of the single crystal fibers and the plates in detail. As a
result, it was found that the diameter slightly decreased in a
direction from one end to the other of the single crystal fiber. In
other words, there was tendency that the diameter of the single
crystal fiber slightly decreased in a direction from a
growth-starting end to a growth-terminating end. Further, it was
found that the single crystal plate slightly twisted.
[0015] Based on such knowledge, the inventors further made
investigations, and then considered that a way of holding the seed
crystal might be a cause. That is, the seed crystal needs to be
pulled down after being bonded to the holding member. In order to
bond the seed crystal to the holding member, a specific crystalline
orientation of the seed crystal is selected as a crystal-growing
direction, and then this crystalline orientation is aligned with
the direction of the pulling-down axis. The thus selected
crystalline orientation is ordinarily in parallel to the direction
of a specific side constituting the outer configuration of the seed
crystal. Therefore, if the seed crystal is bonded to the holding
member just straightly with this specific side being taken as a
reference, the direction of the specific side of the seed crystal
(that is, the above crystalline orientation of the seed crystal)
must be aligned with the pulling-down axis.
[0016] However, even if the seed crystal is bonded to the holding
member in the state that the above crystalline orientation of the
seed crystal is aligned with the pulling-down axis, it is
considered that the crystalline orientation of the seed crystal may
actually deviated from the pulling-down axis owing to unspecified
causes such as non-uniform shrinkage of the adhesive during curing.
In such a case, even if the seed crystal is pulled down in the
direction of the pulling-down direction, the crystalline
orientation of the seed crystal is oblique to the pulling-down
axis. The oxide single crystal grows in the direction of the
crystalline orientation of the seed crystal. Accordingly, the
single crystal-growing direction is slightly inclined to the
pulling-down axis. As a result, it is considered that although no
serious problem exists at a time when the single crystal begins to
grow, the diameter of the single crystal fiber gradually decreases
or the width of the single crystal plate gradually decreases as the
single crystal grows long.
[0017] Based on this hypothesis, the inventors tried to provide an
inclining mechanism for inclining the holding member by varying its
angle to the pulling-down axis, incline the holding member relative
to the pulling-down axis after the seed crystal is held by the
holding member, and thereby reduce the angle of the pulling-down
axis to the crystalline orientation selected for the growth of the
crystal. As a result, the inventors succeeded in preventing the
shrinkage of the diameter of the single crystal fiber and the
shrinkage of the width of the single crystal plate. In addition,
the inventors discovered that the crystallinity of the single
crystal is not deteriorated as compared with that at the time of
starting the crystal growth.
[0018] In the present invention, the angle of the crystalline
orientation of the seed crystal selected for the crystal growth to
the pulling-down axis is particularly preferably controlled to not
more than 0.5.degree., more preferably not more than
0.2.degree..
[0019] The seed crystal preferably a cross-sectional shape
identical with or similar to but smaller than or anyway reduced in
size in any direction from that of the bottom face of the nozzle
portion. In other words, if the bottom of the nozzle portion is of
a polygonal cross-sectional shape, for example, a rectangular
cross-sectional shape, the seed crystal has a polygonal, e.g.,
rectangular cross-sectional shape identical with or similar to but
smaller than or reduced in size from that of the nozzle bottom
face. As to the last case, if the bottom face of the nozzle portion
is 50 mm wide.times.2 mm width, the cross-sectional shape of the
seed crystal may be 30 mm.times.1, for example.
[0020] In a preferred embodiment of the present invention, the
stand is provided with a turning mechanism for turning the holding
member around the pulling-down axis. After the seed crystal is held
by the holding member, the holding member is turned to match the
cross-sectional shape of the seed crystal with the bottom face of
the nozzle portion. "To match the cross-sectional shape of the seed
crystal with the bottom face of the nozzle portion" means that the
sides of the cross-sectional shape of the seed crystal are arranged
to be matched with or faced with corresponding ones of the bottom
face of the nozzle portion. By turning the seed crystal around the
pulling-down axis, angles defined by plural pairs of sides of the
polygonal cross-sectional shape of the seed crystal and
corresponding ones of the polygonal shape of the opening of the
crucible, respectively, are reduced. This angle is preferably not
more than 0.5.degree., more preferably not more than
0.3.degree..
[0021] This embodiment will be explained. For example, when a
single crystal plate is grown, the grown single crystal plate is
slightly twisted besides the above-mentioned problems. As mentioned
above, the present inventors tried to provide the turning mechanism
for turning the seed crystal around the pulling-down axis and
approach the configuration of the cross section of the seed crystal
to that of the bottom face of the nozzle by turning the holding
member. As a result, it was discovered that the twisting
deformation of the single crystal plate grown was reduced or almost
not observed and that the crystallinity was improved over the
entire length of the single crystal plate.
[0022] The configuration of the bottom face of the nozzle portion
and the cross-sectional configuration of the seed crystal are
particularly square or rectangular. In that case, the ratio between
the adjacent sides particularly preferably ranges 1:1 to 100.
[0023] In the following, an embodiment of the producing apparatus
according to the present invention will be illustrated by way of
example.
[0024] FIG. 1 is a schematically sectional view showing the
producing apparatus for the growth of single crystals, and
[0025] FIG. 2 is a front view showing a stand 30 for holding a seed
crystal.
[0026] A crucible 7 is placed in a furnace housing. An upper
furnace unit 1 is arranged to surround the crucible 7 and an upper
space 5 thereof, and has a heater 2 buried therein. A nozzle
portion 13 extends downwardly from a bottom part of the crucible 7,
and an opening 13a is formed a lower end of the nozzle 13. A lower
furnace unit 3 is arranged to surround the nozzle portion and a
lower space 6 thereof, and has a heater 4 buried therein. The
crucible 7 and the nozzle portion 13 are both made of a
corrosion-resistant conductive material.
[0027] One of electric poles of an electric power source 10 is
connected to a location A of the crucible 7 via a wire 9, and the
other similarly connected to a lower bent B of the crucible 7. One
of electric poles of another electric power source 10 is connected
to a location C of the nozzle portion 13 via a wire 9, and the
other similarly connected to a lower end D of the nozzle portion
13. These current-passing systems are separated from each other so
that their voltages can be independently controlled.
[0028] An after-heater 12 is further located in the space 6 to
surround the nozzle 13 with a distance. An intake tube 11 extends
upward in the crucible 7 and an intake opening 22 is provided at
the upper end of the intake tube 11. The intake opening 22
protrudes from a bottom portion of a melt.
[0029] The upper furnace unit 1, the lower furnace unit 3 and the
after-heater 12 are allowed to heat for setting an appropriate
temperature distribution for each of the space 5 and space 6. Then
a raw material of the melt is supplied into the crucible 7 and the
electricity is supplied to the crucible 7 and the nozzle 13 for
heating. In this state, a slight amount of the melt 8 projects
through the opening 13a at a single crystal-growing portion 35 at
the lower end portion of the nozzle portion 13.
[0030] In this state, a seed crystal is moved upwardly, so that an
upper face 15c of the seed crystal 15 is contacted with the melt 8.
Then, the seed crystal 15 is downwardly pulled. At that time, a
uniform solid phase/liquid phase interface (meniscus) is formed
between the upper end of the seed crystal 15 and the melt 8 being
pulled out downwardly from the nozzle 13. As a result, a single
crystal 14 is continuously formed at the upper side of the seed
crystal 15, and drawn out downwardly.
[0031] As the seed crystal, those having various shapes may be
used. In this embodiment, for example, a planar seed crystal, that
is, a single crystal plate is used as the seed crystal. As shown in
FIGS. 2, 3(a) and 3(b), the seed crystal has a pair of wide main
planes 15a, a pair of side planes 15b, an upper face 15c to contact
the melt, and a bottom face 15d opposite to the upper face 15c.
[0032] A stand 30 is arranged on a fitting table 24. The fitting
table 24 is attached to an external driving mechanism not shown.
The driving mechanism pulls down the entire stand 30 in a direction
of a pulling-down axis P.
[0033] The bottom 15d of the seed crystal 15 is bonded to an upper
end of a holding member 17 with an adhesive 16. The lower end of
the holding member 17 is chucked, via a chuck 18, to an adjusting
mechanism under the chuck. The entire adjusting mechanism is
attached onto the fitting table 24. The adjusting mechanism
comprises a turning mechanism 19, an inclining mechanism 20 and a
horizontally moving mechanism 25.
[0034] The rotary mechanism 19 can turn the holding member around
the pulling-down axis P. The inclining mechanism 20 can incline the
holding member 17, relative to the pulling-down axis, in an
arbitrary direction. The horizontally moving mechanism 25 includes
an X-axis moving mechanism 23 and a Y-axis moving mechanism 21. The
X-axis moving mechanism 23 can arbitrarily move the holding member
in the X-axis direction as shown in FIG. 3(b). The Y-axis moving
mechanism 21 can arbitrarily move the holding member in the Y-axis
direction. The Z-axis in FIG. 3(b) corresponds with the
pulling-down axis P.
[0035] In a preferred embodiment of the present invention, a single
seed crystal 15 is bonded to the holding member, and then a
plurality, for example, ten of single crystals are successively
grown. Therefore, for example, after growth of the first single
crystal fiber or plate is terminated, changes in dimensions such as
the diameter and the width of the cross section of the single
crystal are examined in detail. If any deviation is observed, the
mechanisms 19 and 20 are operated depending upon such deviations.
Thereafter, a next growing is performed.
[0036] For example, if the diameter of the single crystal fiber
decreases in a direction from one end to the other, it is
considered that the angle 0 between the crystallization orientation
L (See FIG. 3(a)) selected as the growing direction and the
pulling-down axis P is large. Therefore, the angle of the growing
face of this single crystal fiber is measured to determine the
direction of the crystallization orientation L from this angle of
the crystallizing face. Then, the crystallizing orientation L and
the pulling-down axis P are compared with each other to calculate
this angle .theta.. The inclining mechanism 20 is inclined to meet
this angle .theta., and next growing is effected. Similar
adjustment is effected in a case where the width of the single
crystal plate decreases in a direction of one end to the other.
[0037] If the single crystal plate is twisted, the holding member
is turned around the pulling-down axis P (Z-axis) as indicated by
.alpha. within a X-Y plane (a plane vertical to the pulling-down
axis) as shown in FIG. 3(b), and then next growing is effected.
[0038] Although the oxide single crystal is not limited to any
particular one, but recitation may be made of lithium potassium
niobate (KLN), lithium potassium niobate-lithium potassium
tantalate solid-solution (KLTN:
[K.sub.3Li.sub.2-x(Ta.sub.yNb.sub.1-y).sub.5+xO.sub.15+2x]),
lithium niobate, lithium tantalate, lithium niobate-lithium
tantalate solid-solution, Ba.sub.1-xSr.sub.xNb.sub.2O.sub.6, Mn--Zn
ferrite, yttrium aluminum garnet substituted by Nd, Er and/or Yb,
YAG and YVO.sub.4 substituted by Nd, Er and/or Y, for example.
EXAMPLES AND COMPARATIVE EXAMPLES
Comparative Example 1
[0039] A single crystal plate of lithium potassium niobate was
produced according to the present invention by using a single
crystal-producing apparatus as shown in FIG. 1. More specifically,
the temperature of the entire furnace was controlled with an upper
furnace unit 1 and a lower furnace unit 3. The temperature gradient
near the single crystal-growing portion 35 was controlled by
supplying the electric power to a nozzle portion 13 and generating
heat from an after-heater 12. A single crystal pulling-down
mechanism was mounted to pull down the single crystalline plate in
a vertical direction, while controlling the pulling-down velocity
evenly within a range from 2 to 100 mm/h.
[0040] A raw material for starting growth was prepared by mixing
potassium carbonate, lithium carbonate and niobate oxide at a mole
ratio of 30:25:45. About 10 g of this growth-starting raw material
was filled in a platinum crucible 7, and placed at a given
location. The temperature of the space 5 inside the upper furnace
unit 1 was adjusted in a range of 1100 to 1200.degree., and the
material was melted inside the crucible. The temperature of the
space 6 in the lower furnace unit 3 was uniformly controlled in a
range of 500 to 1000.degree.. Given electric powers were supplied
to the crucible 7, the nozzle portion 13 and the after-heater 12,
thereby growing the single crystal. At that time, the temperature
of the single crystal-growing portion could be set at 980 to
1150.degree., and the temperature gradient in the single
crystal-growing portion at 10 to 150.degree./mm. During growing the
single crystal, another raw material powder was fed continuously to
the crucible from an external raw material feeder not shown. As
this raw material powder, a powder prepared by mixing potassium
carbonate, lithium carbonate and niobium oxide at a mole ratio of
30:19:51 was used.
[0041] As the nozzle portion 13, a planar nozzle portion made of
platinum was used. The dimensions of the cross-section of the inner
space of the nozzle portion were 1 mm.times.50 mm with the length
of 10 mm. The configuration of the crucible was rectangular
parallelepiped. The width and the length of the inner space of the
crucible was 10 mm and 50 mm, respectively, with the depth of 10
mm. In this state, a single crystal plate was pulled down in a
direction <110> (crystallizing orientation selected for
growth) at a rate of 20 mm/h. The cross section of the single
crystal plate has a rectangular shape of 1 mm.times.30 mm.
[0042] As a seed crystal, a planar single crystal of lithium
potassium niobate was used. The seed crystal had a rectangular
cross-sectional shape of 1 mm.times.30 mm. The half-width value of
an X-ray locking curve of the seed crystal was 50 seconds
(Measuring apparatus: MRD diffractometer manufactured by Philips,
measured reflection: 004).
[0043] Then, a bottom face 15d of a seed crystal 15 was bonded to a
holding member 17 with a heat-resistant inorganic adhesive as shown
in FIG. 2. Only a horizontally moving mechanism 25 and a fitting
table 24 were arranged under a chuck 18.
[0044] As a result, a single crystal plate having a length of 100
mm was grown. This single crystal plate had a width of 30 mm and a
thickness of 1.0 mm at a growth-starting point and a width of 20 mm
and a thickness of 0.7 mm at a growth-terminating point. The plate
was visually slightly twisted. A sample was cut from a site of this
single crystal plate at 50 mm from the growth starting point, and
the sample was observed by the X-ray diffraction method to give a
half-width value of an X-ray locking curve of not less than 100
seconds. Example 1
[0045] After Comparative Example 1 was performed, the single
crystal plate was removed from the seed crystal. At this point of
time, the seed crystal 15 still remained bonded to the holding
member 17. Subsequently, a holding member was fitted to a stand 30
as shown in FIG. 2 without removing the seed crystal 15 from the
holding member 17. Then, the inclining mechanism 20 was inclined to
adjust an angle between a direction <110> in Comparative
Example 1 and a pulling-down axis to not more than 0.5.degree..
Further, a turning mechanism 19 was turned by 1.degree. in such a
direction as to remove the twisting of the single crystal plate.
After that, a single crystal plate of lithium potassium niobate was
grown again by using the apparatus in FIG. 1 in the same manner in
Comparative Example 1.
[0046] As a result, the single crystal plate having a rectangular
shape of 1 mm.times.30 mm was grown with a length of 100 mm. This
single crystal plate had a width of 30 mm and a thickness of 1.0 mm
at a growth-starting point and a width of 30 mm and a thickness of
1.0 mm at a growth-terminating point. The plate was not visually
twisted. A sample was cut from a site of this single crystal plate
at 50 mm from the growth starting point, and the sample was
observed by the X-ray diffraction method to give a half-width value
of an X-ray locking curve of 40 seconds.
[0047] With respect to a single crystal plate of lithium potassium
niobate-lithium potassium tantalate solid solution, the same
results as mentioned above were obtained.
Comparative Example 2
[0048] Next, a single crystal plate of lithium potassium niobate
was grown, while a fibrous single crystal of lithium potassium
niobate was used as a seed crystal. The seed crystal had a
cross-sectional square shape of 1 mm.times.1 mm. The longitudinal
direction of the fiber was directed to a <110> direction,
which was coincident with the crystallizing orientation for the
crystal growth. The half-width value of an X-ray locking curve of
the seed crystal was 80 seconds.
[0049] A single crystal-producing apparatus was prepared in the
same manner as in Comparative Example 1, and a single crystal plate
began to be grown. The pulling down speed for the seed crystal was
20 mm/h. The width and the thickness of the growing single crystal
at a growth-starting point were the same as those of the seed
crystal, that is 1 mm.times.1 mm. As the seed crystal was pulled
down, the growth of the single crystal proceeded, so that the
single crystal gradually increased in width to form a shoulder
portion. When the crystal reached a width of about 15 mm, a
positional deviation between the end of the crystal and the bottom
of the nozzle portion became larger to cause twisting. As a result,
it was difficult to further increase the width. When the crystal
was grown continuously up to a length of 100 mm in this state, the
crystal had a width of 10 mm and a thickness of 0.7 mm at the
growth-terminating point. Twisting was observed in the entire
single crystal plate. A sample was cut out from a site of this
single crystal plate at 50 mm from the growth starting point, and
the sample was observed by the X-ray diffraction method to give a
half-width value of an X-ray locking curve of not less than 100
seconds.
Example 2
[0050] After Comparative Example 2 was performed, the single
crystal plate was removed from the seed crystal. At this point of
time, the seed crystal 15 still remained bonded to the holding
member 17. Subsequently, a holding member was fitted to a stand 30
as shown in FIG. 2 without removing the seed crystal 15 from the
holding member 17. Then, the inclining mechanism 20 was inclined to
adjust an angle between a direction <110> in Comparative
Example 2 and a pulling-down axis to not more than 0.5.degree..
Further, a turning mechanism 19 was turned by 1.degree. in such a
direction as to remove the twisting of the single crystal plate.
After that, a single crystal plate of lithium potassium niobate was
grown again by using the apparatus in FIG. 1 in the same manner in
Comparative Example 2.
[0051] In Example 2, the width of the crystal increased at the
shoulder portion without being twisted, and reached 30 mm. In this
state, the crystal was grown continuously to a length of 100 mm,
and had a width of 30 mm and a thickness of 1.0 mm at that time.
The plate was not visually twisted. A sample was cut from a site of
this single crystal plate at 50 mm from the growth starting point,
and the sample was observed by the X-ray diffraction method to give
a half-width value of an X-ray locking curve of 40 seconds.
[0052] As mentioned above, according to the present invention, when
the raw material of the oxide single crystal is melted in the
crucible, the seed crystal is contacted with the resulting melt,
and the oxide single crystal is grown while the melt is being
pulled down through the opening of the crucible in a predetermined
direction, the deterioration in the crystallinity of the single
crystal which would follow increase in the grown length of the
single crystal can be prevented.
* * * * *