Method for single crystal growth and growth apparatus

Disclosed is a method for the growth of a single crystal having excellent crystallinity, uniform quality in the inside thereof and hence excellently uniform optical properties, the method enabling an improvement in yields. The invention resides in a method for the growth of a single crystal of .beta.-type barium borate (.beta.-BaB.sub.2 O.sub.4), the method comprising heating a crucible 6 indirectly to grow a .beta.-BaB.sub.2 O.sub.4 single crystal 21 from a melt of barium borate (BaB.sub.2 O.sub.4) contained in the crucible and using no flux by using a seed crystal 9 of .beta.-BaB.sub.2 O.sub.4.

RELATED APPLICATION DATA
 The present application claims priority to Japanese Application No. P10
 025371 filed Feb. 6, 1998 which application is incorporated herein by
 reference to the extent permitted by law.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method and apparatus for the growth of a
 single crystal, especially, .beta.-type barium borate (.beta.-BaB.sub.2
 O.sub.4).
 2. Description of the Related Art
 There has been a demand for a laser light source with shorter wavelengths
 in recent years to satisfy the requirements of higher recording density
 and precise processing in optically technical fields, e.g. optical record
 playback, precision processing or the like.
 As one of the measures satisfying the above demand, animated research and
 development are now being made to obtain a laser light with a short
 wavelength by frequency conversion of the laser light from a conventional
 solid laser utilizing a nonlinear optical material.
 A large nonlinear optical effect and high transparency, namely, excellent
 optical properties with small light loss and high resistance to laser
 damages are demanded of the nonlinear optical materials.
 Among these nonlinear optical materials, a .beta.-BaB.sub.2 O.sub.4 single
 crystal is characterized in that it can convert into wavelengths ranging
 up to a UV (ultraviolet) region and is resistant to laser damages. Hence
 for the .beta.-BaB.sub.2 O.sub.4 single crystal, the growth of larger
 crystals of high quality is desired.
 In view of this situation, energetic studies of the .beta.-BaB.sub.2
 O.sub.4 monocrystal growth are ongoing in various fields. The barium
 borate (.beta.-BaB.sub.2 O.sub.4) crystal, though it is a congruent
 melting composition, has the feature in the presence of two phases, a high
 temperature phase consisting of .alpha.-type barium
 borate(.alpha.-BaB.sub.2 O.sub.4) and a low temperature phase consisting
 of .beta.-type barium borate (.beta.-BaB.sub.2 O.sub.4) and the
 transformation temperature between two phases is about 925.degree. C.
 (JIANG Aidong et al, Journal of Crystal Growth 79 (1986) 963-969 etc.).
 Hence the growth of the low temperature phase .beta.-BaB.sub.2 O.sub.4
 crystal is regarded to be difficult in the ordinary lifting growth method
 since the high temperature phase .alpha.-BaB.sub.2 O.sub.4 crystal is
 crystallized in this growth method.
 The crystal growth is therefore attained in customarily usual methods such
 as a TSSG (Top Seeded Solution Growth) method which is a type of flux
 method and a pulling method using a flux under the condition in which the
 low temperature phase .beta.-BaB.sub.2 O.sub.4 crystal is crystallized as
 the primary crystal by using a flux such as Na.sub.2 O or the like to
 lower the melting point or a pulling method using a flux or the like.
 However, these methods pose the problems of low productivity due to the
 extremely low crystal growth rate and of deteriorated optical properties
 due to contamination with flux components in the crystal.
 While, Japanese Patent Application Laid-Open (JP-A) No. H1-249698 proposes
 a pulling method using a self-flux in which external or different kind of
 fluxes are excluded and excess barium or boron is added to a barium borate
 composition. This method is improved in terms of exclusion of impurities
 of a different nature. It, however, has the problem that since a melt
 composition largely differs from the grown crystal composition, the melt
 composition varies with the progress of the growth and hence it is
 difficult to produce large and homogenous crystals.
 On the other hand, there has been a recent report in which a
 .beta.-BaB.sub.2 O.sub.4 single crystal can be grown by a lifting method
 even from a melt of barium borate without using a flux at all (JP-A No.
 H2-279583). In this method, using a high frequency induction heater,
 induction current is produced in a crucible itself and the crucible is
 directly heated to set the ambient temperature condition just above the
 melt, specifically, to form a large temperature gradient just above the
 melt intentionally thereby growing a .beta.-BaB.sub.2 O.sub.4 crystal.
 Another method is proposed (JP-A No. H9-235198) in which a high frequency
 induction heater is used similarly and no flux is used. In this method,
 although the crystal is pulled from a melt of barium borate, the growth of
 .beta.-BaB.sub.2 O.sub.4 crystals by pulling can be attained even if the
 temperature gradient just above the melt is not so steep, specifically, a
 difference between the temperatures at a height of 10 mm above the melt
 surface and at the melt surface is between -165.degree. C. and
 -280.degree. C., in other words, even if the temperature gradient upward
 from the surface of the melt is as relatively gentle as 165.degree. C./cm
 to 280.degree. C./cm. Besides, by rather selecting relatively gentle
 temperature gradient in this manner, this method is improved in the
 prevention of generations of cracks and striae caused by the strain of the
 grown crystal which tends to be produced when the temperature gradient
 above the melt is made steep.
 However, any of the aforementioned crystal growth methods using no flux
 does not necessarily satisfy the internal optical uniformity of the grown
 .beta.-BaB.sub.2 O.sub.4 crystal, particularly when the crystal growth
 method intends to produce large diameter crystals.
 SUMMARY OF THE INVENTION
 The inventors of the present invention have conducted earnest studies and
 repeated investigations, and, as a result, found that such a uniformity
 problem is dependent upon the distribution of the temperature of a melt in
 a crucible.
 As described above, in the conventionally known growth methods using no
 flux, heating of the crucible to prepare a melt is performed by high
 frequency induction heating. This heating is carried out on the basis of
 the ideas that it facilitates the preparation of a desired temperature
 gradient above a melt surface.
 The inventors of the present invention clarified that when induction
 current is produced in a crucible to directly heat the crucible itself,
 the uneven induction current produced greatly affects the temperature of
 the crucible itself, which gives rise to nonuniformity between the
 temperature of the melt portion, which is directly in contact with the
 crucible wall, and the ambient temperature at the position apart from the
 melt portion.
 Based on the result of the investigation, the present invention resultantly
 provides a method and apparatus for growth of a single crystal,
 particularly, .beta.-BaB.sub.2 O.sub.4, the method and the apparatus
 enabling it possible to impart more excellent crystallinity, uniformity in
 the crystal and hence the uniformity of optical properties and to improve
 the yield.
 According to a first aspect of the present invention, there is provided a
 method for growth of a single crystal of .beta.-type barium borate
 (.beta.-BaB.sub.2 O.sub.4), the method comprising a step of heating a
 crucible indirectly to grow a single crystal of .beta.-BaB.sub.2 O.sub.4
 from a melt of .beta.-BaB.sub.2 O.sub.4 by using a seed crystal of
 .beta.-BaB.sub.2 O.sub.4 without using a flux within the crucible.
 According to another aspect of the present invention, there is provided a
 growth apparatus comprising a heating furnace provided with a heating
 means, a crucible disposed in the heating furnace and filled with barium
 borate (BaB.sub.2 O.sub.4) using no flux, a crystal pulling mechanism to
 which a seed crystal of .beta.-BaB.sub.2 O.sub.4 is attached, and a
 transfer means controlling the relative positions of the heating means and
 the crucible by relative movement of at least one of the heating means and
 the crucible.
 The heating means is such a heating means that the crucible is indirectly
 heated by the heating means.
 In the above apparatus, the relative positions of the heating means and
 crucible are controlled by the transfer means to thereby select a desired
 temperature gradient as the temperature gradient in the direction upward
 from the surface melt of barium borate (BaB.sub.2 O.sub.4) in the crucible
 and the crystal pulling means is used to grow a single crystal of
 .beta.-BaB.sub.2 O.sub.4 from the seed crystal of .beta.-BaB.sub.2 O.sub.4
 by pulling from the melt of barium borate (BaB.sub.2 O.sub.4) contained in
 the crucible and using no flux.
 The indirect heating herein is different from conventional heating methods
 in which a high frequency induction current is produced in the crucible
 itself to heat directly and is a heating method in which a material except
 for the crucible is heated and heat from the heated portion is conducted
 by radiation, convection of a heating medium or heat conduction or by heat
 ray radiation to carry out final heating.
 As described above, when the method and apparatus according to the present
 invention are used, the production of uniform portions in the inside of
 the pulled crystal is efficiently prevented. This is because ununiform
 heating of the crucible is difficult to be generated by heating the
 crucible indirectly whereby the generation of nonuniformity in the
 temperature of a melt of crystal growth materials filled in the crucible
 is restrained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The growth method of the present invention is a method for growth of a
 .beta.-type barium borate (.beta.-BaB.sub.2 O.sub.4) single crystal. In
 the growth method of the present invention, as described above, a crucible
 is indirectly heated to grow a .beta.-BaB.sub.2 O.sub.4 single crystal
 from a melt of barium borate (BaB.sub.2 O.sub.4) with no flux by using a
 seed crystal of .beta.-BaB.sub.2 O.sub.4.
 The temperature at the position above a melt of barium borate (BaB.sub.2
 O.sub.4) in the crucible in the growth of the .beta.-BaB.sub.2 O.sub.4
 single crystal is designed so that the temperature gradient in a region
 from the surface of the melt to a height of 10 mm above the liquid level
 is between 165.degree. C./cm and 600.degree. C./cm. To state it
 differently, the temperature difference between the surface of the melt
 and the position at a height of 10 mm (1 cm) above the melt surface is
 designed to be 165.degree. C. to 600.degree. C. lower than the temperature
 at the surface of the melt. The reason why the temperature gradient
 exceeding 165.degree. C./cm is selected as the temperature condition is
 because when the temperature gradient is more gentle than 165.degree.
 C./cm, the seed crystal is transformed into .alpha.-BaB.sub.2 O.sub.4 and
 hence .beta.-BaB.sub.2 O.sub.4 crystal growth cannot be attained. On the
 other hand, a steep temperature gradient exceeding 600.degree. C./cm is
 not selected because cracks occur in the grown crystal at temperatures
 above 600.degree. C.
 The temperature gradient is preferably between 165.degree. C./cm and
 410.degree. C./cm to avoid the occurrence of cracks more positively and
 more preferably between 165.degree. C./cm and 280.degree. C./cm as
 disclosed in the foregoing reference: JP-A No. H9-235198, to avoid the
 occurrence of distortion more certainly.
 The crystal growth apparatus according to the present invention by which
 the aforementioned growth method of the present invention can be practiced
 comprises a heating furnace provided with a heating means, a crucible
 disposed in the heating furnace and filled with barium borate (BaB.sub.2
 O.sub.4) using no flux, a crystal pulling mechanism to which a seed
 crystal of .beta.-BaB.sub.2 O.sub.4 is attached, and a transfer means
 adjusting the positions of the heating means and crucible by relative
 movement of at least one of the heating means and the crucible.
 The heating means is such a heating means by which the crucible is
 indirectly heated.
 The apparatus has a structure in which the relative positions of the
 heating means and crucible are adjusted by the transfer means to thereby
 select the aforementioned desired temperature gradient as the temperature
 gradient in the direction upward from the surface of the melt of barium
 borate (BaB.sub.2 O.sub.4) in the crucible and the crystal pulling means
 is used to grow a single crystal of .beta.-BaB.sub.2 O.sub.4 from the seed
 crystal of .beta.-BaB.sub.2 O.sub.4 by pulling from the melt of barium
 borate (BaB.sub.2 O.sub.4) contained in the crucible and using no flux.
 The heating furnace may be a usual resistance heating furnace structure in
 which the heating means is a usual heating furnace using electric heating.
 In this case, the crucible is indirectly heated, for example, by radiation
 from a heating furnace, heating of a heating medium, e.g. ambient
 atmosphere surrounding the crucible using a heating furnace, circulation
 or convection of a heating medium or heat conduction.
 Also, the heating is not limited to the resistance heating but may be
 carried out by using an infrared ray lamp or the like.
 The heating means is arranged in relation to the crucible in such a manner
 that the peak position of the heating temperature elevated by the heating
 means reaches the position lower than the surface of a melt of BaB.sub.2
 O.sub.4 in the crucible and temperatures (temperature gradient) at
 positions above the surface of the melt correspond to the foregoing
 temperature gradient. For this, the heating means may be a structure
 provided with a heating section which indirectly heats primarily the
 vicinity of the bottom of the crucible.
 Next, an embodiment of the apparatus according to the present invention
 will be explained, but the present invention is not limited to this
 embodiment.
 FIG. 1 is a schematically sectional view of an embodiment of a growth
 apparatus according to the present invention.
 This growth apparatus indicates the case of using a resistance heating
 furnace as a heating furnace 1. Stated it differently, the heating means
 is structured by using an electric heating furnace.
 The heating furnace 1 comprises, for example, an upper furnace 2 and an
 lower furnace 3. The lower furnace 2 is constituted of, for example, a
 firebrick which is formed with concave section 4 with an opening at the
 upper portion thereof.
 The lower furnace 2 is provided with a heating means 5. At the opening side
 of the concave section 4 of the lower furnace 2, located is a crucible 6
 which is indirectly heated by the heating means 5 to heat raw material of
 BaB.sub.2 O.sub.4 placed in the crucible 6 thereby forming a melt 8 of
 BaB.sub.2 O.sub.4.
 The upper furnace 3 comprises a cylinder body made of a heat resistant
 material, for example, alumina Al.sub.2 O.sub.3, disposed above the
 opening of the concave section 4 of the lower furnace 2, namely, above the
 crucible 6, the cylinder body surrounding the opening and projecting
 upward.
 A lid 7 likewise made of alumina Al.sub.2 O.sub.3, for example, and with an
 opening 7w at the center thereof is disposed at the upper end of the upper
 furnace 3.
 Besides, a crystal puller 10 provided with a seed crystal 9 of
 .beta.-BaB.sub.2 O.sub.4 is installed. The crystal puller 10 has a
 rod-like crystal pulling body made of, for instance, sapphire and a seed
 crystal 9 is attached to the bottom end of the pulling body. The crystal
 puller 10 is rotated by a rotary pulling mechanism (not shown) and pulled
 up.
 The heating means 5 comprises, for instance, a plurality of, for instance,
 four electric heaters 11 which are arranged around the vertical axis of
 the location of the crucible 6 at an equal interval, of 90 degrees in this
 example.
 These heaters 11, as shown by its perspective view of FIG. 2, respectively
 comprise, for instance, a U-like heater and the U-like part of the heaters
 11 is disposed at a fixed distance away from the crucible 6 and along the
 axis of the crucible 6 in the heating furnace 1. Each of these heaters 11
 is designed so that its both terminals 11t are led externally from the
 lower furnace 2 and the terminals 11t of these heaters 11 are connected to
 each other in such a manner that these heaters 11 are connected to each
 other in series whereby they are connected to a power source.
 These heaters 11 are made of a highly heat-resistant material, for
 instance, a molybdenum disilicate type, e.g. kantaru Super (manufactured
 by Kantaru co.,Ltd.) a silicon carbide type, e.g. Erema Specific Heating
 furnace (manufactured by Takasago Thermal Engineering Co., Ltd.) or the
 like.
 At least one of the heating means 5 and the crucible 6 is provided with a
 transfer means altering and adjusting the relative positional relation
 between them.
 For instance, a transfer means 12 which can move the crucible 6 in the
 direction of its axis is installed. The transfer means 12, as shown in,
 for instance, FIG. 1, comprises, for instance, a vertically movable
 rod-like support 14 which extends downward along the axis of the crucible
 6 through or not through a pedestal 13 made of, for example, a
 heat-resistant brick which supports the crucible 6, the bottom end of the
 support being externally led from the lower furnace 2. Moreover, for
 instance, either a thread groove is formed on or a material formed with a
 thread groove is attached to the outer periphery of the lower section of
 the support 14. On the other hand, for instance, a rotor 15 formed with a
 thread groove, which is to be engaged with the above thread groove, on the
 inner periphery thereof is disposed in the condition that its movement in
 the direction of the axis is limited. The rotor 15 is designed to be
 rotated by a driving motor 17 via a rotation transfer mechanism 16. The
 rotation of the rotor 15 moves the support 14 vertically as shown by the
 arrow a thereby moving the crucible 6 vertically.
 The heating means 5 is designed to be adjusted in its movement both in a
 direction along the axis of the crucible 6 and in a direction
 perpendicular to the axis as shown by the arrows b and c respectively. A
 transfer means 18 for the movement of the heating means 5 is provided with
 a rectangular hole 19 drilled into the section of the lower furnace 2 from
 which both ends of each heater 11 are led out, the aperture 19 having a
 width of the extent enough to penetrate the ends of the heater 11 and the
 longitudinal axist direction to the direction of the axis of the crucible
 6. In the condition that the ends of each heater 11 are moved under
 adjustment in the directions of the arrows b and c in the rectangular
 aperture 19, a heat-resistant material 20 is inserted into the rectangular
 aperture 19 to arrange to position of each heater 11 and to close the
 rectangular aperture 19.
 In order to attain the growth of a target single crystal of
 .beta.-BaB.sub.2 O.sub.4 by pulling with the above-mentioned apparatus,
 first the relative positions of the crucible 6 and the heating means 5 are
 selected by the transfer means 12 and 18 mentioned above and further the
 inside diameter and height of the upper furnace 3, the inside diameter of
 the opening 7w of the lid 7 and the like are properly selected such that
 the temperature gradient in a vertical direction upward from the surface
 of the melt 8 corresponds to the foregoing desired temperature gradient.
 At this time, the surface temperature of the BaB.sub.2 O.sub.4 melt 8 is
 designed to be about 1050.degree. C.
 Then the crystal puller 10 is used to lower the seed crystal 9 and thereby
 to bring the seed crystal 9 into contact with the surface of the melt or
 to dip the seed crystal 9 in the melt. Then, the seed crystal 9 is pulled
 while it is rotated thereby growing a .beta.-BaB.sub.2 O.sub.4 single
 crystal 21 from the seed crystal 9.
 Next, an example of the method for growth of .beta.-BaB.sub.2 O.sub.4
 crystal according to the present invention will be explained in detail,
 which is not intended to be limiting of the present invention thereto.
 EXAMPLE 1
 This example shows the case of growing a .beta.-BaB.sub.2 O.sub.4 single
 crystal by using the growth apparatus of the present invention using a
 resistance heating furnace shown in FIGS. 1 and 2, namely, a radiation-ray
 heating furnace.
 In this case, BaCO.sub.3 of 99.99% purity and B.sub.2 O.sub.3 of the same
 99.99% purity as starting materials were mixed in a ratio by mol of 1:1
 and temporarily baked. About 700 g of the mixture was filled in a platinum
 crucible 6 with a diameter of 80 mm and a depth of 40 mm. An inspection of
 the temporarily baked product by means of an X-ray powder diffraction
 method revealed that it included, other than a .beta.-BaB.sub.2 O.sub.4
 structure, an .alpha.-BaB.sub.2 O.sub.4 structure and non-crystal
 materials, which, however, may be acceptable if these are BaB.sub.2
 O.sub.4 compositions in a molten state.
 Then the crucible 6 was heated by heat radiation emitted from the heating
 means 5 to melt the raw materials in the crucible 6 and to form a
 temperature gradient of 250.degree. C./cm in a vertical direction ranging
 from the surface of the melt 8 to a height of 10 mm above the melt
 surface.
 In this condition, a c axis seed crystal 9 of .beta.-BaB.sub.2 O.sub.4 with
 each side dimension of 2 mm was grown for 10 hours at a rotation rate of 5
 rpm and a pulling rate of 2 mm/hour. At this time, a crystal with a
 diameter of about 50 mm and the length of the straight cylinder portion
 being about 15 mm was grown.
 The resulting grown crystal had no crack generated and it was confirmed by
 a powder X-ray diffraction method that it was a .beta.-BaB.sub.2 O.sub.4
 crystal.
 COMATIVE EXAMPLE 1
 In this example, using a customary growth apparatus provided with a high
 frequency induction heating furnace, specifically, a type of heating
 furnace designed to heat a platinum crucible directly by high frequency
 induction heating, instead of indirect heating in the aforementioned
 Example 1 and using the same raw materials, the same temperature gradient
 above the surface of the melt as in Example 1 was formed to grow a
 .beta.-BaB.sub.2 O.sub.4 crystal with a diameter of about 50 mm.
 It was confirmed by the powder X-ray diffraction method that the crystal
 grown in Comparative Example 1 was also a .beta.-BaB.sub.2 O.sub.4
 crystal. These .beta.-BaB.sub.2 O.sub.4 crystals produced in Example 1 and
 Comparative Example 1 were cut in each side of 14 mm by 20 mm and 8 mm
 wide by a phase matching plane of the fourth harmonic generation of YAG
 laser. Both surfaces of the crystal were processed by mirror-finish
 polishing and then a variation in the refractive index in the crystal was
 evaluated by using an optical interferometer. In the measurement, a laser
 interferometer (Model No. SI-10, manufactured by ZYGO) was used and a
 light source, a He--Ne laser was made into linear polarized light by using
 a polarizer. FIGS. 3 and 13 show interference patterns when measuring a
 variation in the refractive index of the inside of a cylinder part with a
 diameter of 12 mm and a thickness of 7.5 mm, corresponding to the center
 of each crystal, in the direction of an extraordinary ray. FIG. 3 shows
 the interference pattern of the crystal grown in Example 1 and FIG. 13
 shows the interference pattern of the crystal grown in Comparative Example
 1.
 That is, according to the method of the present invention, the interference
 pattern is linear with a uniform interval, showing that the grown crystal
 is uniform in quality. On the contrary, according to the method heating
 the crucible itself by high frequency induction heating, the interference
 pattern curves significantly with ununiform intervals. In other words, in
 the method of the present invention compared with the conventional method,
 a significantly uniform crystal and hence a crystal which is uniform in
 terms of optical properties is grown.
 Next, the output of the fourth harmonic (a wavelength of 266 nm) of YAG
 laser using the crystals obtained in Example 1 and Comparative Example 1
 was measured. FIG. 4 shows a structural view of the optical measuring
 system.
 In this case, a high output laser light, specifically, a laser light, with
 a wavelength of 1064 nm, a pulse width of 30 ns and a repetitive frequency
 of 7 kHz, from a Q switch Nd:YAG laser unit 41 was entered into a wave
 length conversion element 42 made of a LBO (LiB.sub.3 O.sub.5), whose
 optical properties required for second harmonic generation were confirmed,
 to output a laser light with an average power of 2.5 W and a wavelength of
 532 nm. This laser light was restricted by a lens L to form an elliptical
 beam with a minor axis of 70 .mu.m and a major axis of 180 .mu.m and the
 beam was introduced into a test sample 43 processed from each of the
 crystals prepared in Example 1 and Comparative Example 1 for fourth
 harmonic generation. The beam was then separated by a prism 44 into laser
 lights 45, 46 and 47 with wavelengths of 1064 nm, 532 nm and 266 nm
 respectively. The laser light 47 with a wavelength of 266 nm was measured
 by a power meter 48. In this case, the output from any of the crystals
 prepared in Example 1 and Comparative Example 1 was 550 mW. It was
 understood that the crystal prepared by the method according to the
 present invention stood comparison with the conventional products with
 respect to the wave conversion properties.
 EXAMPLE 2
 A crystal with a diameter of 50 mm was grown in the same condition as in
 Example 1 except that only the temperature gradient in a region from the
 level of the melt to a height of 10 mm above the melt surface was altered
 to 350.degree. C./cm. There was no crack generated in the crystal grown in
 this manner and it was confirmed by the powder X-ray diffraction method
 that it was a .beta.-BaB.sub.2 O.sub.4 crystal.
 The crystal prepared in this example was likewise evaluated by an optical
 interferometer in the same manner as in Example 1 and as a result the
 interference pattern shown in FIG. 5 was obtained. It was thus confirmed
 that the crystal grown in Example 2 was also uniform in quality. Moreover,
 the crystal was measured in terms of the output of the fourth harmonic
 generation (a wavelength of 266 nm) of YAG laser in the same manner as in
 Example 1 and as a result the same output as in Example 1 was obtained.
 EXAMPLE 3
 A crystal with a diameter of 50 mm was grown in the same condition as in
 Example 1 except that only the temperature gradient in a region from the
 surface of the melt to a height of 10 mm above the melt surface was
 altered to 410.degree. C./cm. There was no crack generated in the crystal
 grown in this manner and it was confirmed by the powder X-ray diffraction
 method that it was a .beta.-BaB.sub.2 O.sub.4 crystal.
 The crystal prepared in this example was likewise evaluated by an optical
 interferometer in the same manner as in Example 1 and as a result the
 interference pattern shown in FIG. 6 was obtained. It was thus confirmed
 that the crystal grown in Example 3 was also uniform in quality. Moreover,
 the crystal was measured in terms of the output of the fourth harmonic
 generation (a wavelength of 266 nm) of YAG laser in the same manner as in
 Example 1 and as a result the same output as in Example 1 was obtained.
 It is easily understood that the apparatus and method of the present
 invention are not limited to the forgoing embodiments. For instance, in
 the structure of the radiation-ray heating furnace 1 used in the apparatus
 of the present invention, the heating means 5 thereof may be varied in its
 structure. For instance, in order to obtain a desired temperature gradient
 above the crucible 6, heating means 5 may have a structure in which the
 heater 11 constituting the heating means 5 is disposed opposite to the
 bottom of the crucible 6 as shown, for instance, in FIG. 7 which shows a
 schematically sectional view of the major part of the heating furnace 1
 and in FIG. 8 which shows the lateral sectional view of FIG. 7. In FIGS. 7
 and 8, parts corresponding to those in FIG. 1 are represented by the same
 symbols and redundant explanations thereof are omitted. As shown in FIG.
 8, this example is the case where the heater 11 is bent in zigzags so that
 primarily the bottom of the crucible 6 can be uniformly heated by
 radiation. In this case, the support 14 which vertically moves the
 crucible 6 may be provided with a slit 14S penetrating the heater 11
 therethrough.
 Also in this case, as shown in FIGS. 7 and 8, the heater 11 may be provided
 with the transfer means 18 enabling the movement in the direction b along
 the axis of the crucible 6 and in the direction c perpendicular to the
 direction b. The transfer means 18 may have a structure in which an
 elongated aperture 19 is formed in the wall of the lower furnace 2 to lead
 out the end of the heater 11 and, in the state in which the position of
 the heater 11 is selected, the elongated aperture 19 is filled with a
 heat-resistant material 20 while the penetrating portion of the heater 11
 is enclosed in the elongated aperture 19.
 Moreover, the transfer means 12 of the crucible 6 may have the same
 structure as that shown in FIG. 1, though it is not shown. This is an
 embodiment in which the pedestal 13 of the crucible 6 is omitted.
 As shown by the major part of the heating furnace 1 in FIG. 9, the heating
 means 5 may be structured such that a band-like heater 11 formed of SiC is
 disposed so as to wind around the location of the crucible 6 as shown by a
 plan view and a side view in FIGS. 10A and 10B.
 In this case, also the transfer means 18 enabling the heater 11 to move in
 the direction of the axis of the crucible 6 may have the same structure as
 those described in FIGS. 1, 7 and 8. The transfer means 12 of the crucible
 6 may have the same structure as that shown in FIG. 1 and the like, though
 it is not shown.
 The heater 11 may be made into the ring-like form and the terminal 11t may
 be led out from both opposite sides of the heating furnace as shown by a
 plan view and a side view in FIGS. 11A and 11B respectively.
 In FIGS. 9 to 11, parts corresponding to those in FIGS. 1, 7 and 8 are
 represented by the same reference numerals and redundant explanations
 thereof are omitted.
 As shown by a schematically sectional view of the major part of a heating
 furnace in, for instance, FIG. 12, the heating means may have the
 structure as shown in FIGS. 7 and 8 and explained in FIGS. 9 to 11,
 namely, the structure in which the heater 11 is each disposed at the
 bottom and the outer periphery of the crucible 6.
 Also, a variety of structures may be adopted for the heater 11. For
 instance, an electric heating wire is disposed and wound as a coil-like
 around the crucible 6 and the heating temperature elevated by the heating
 means reaches a peak at the position sufficiently lower than the level of
 the fused liquid contained in the crucible 6.
 As described above, the crystal produced by using the method and apparatus
 of the present invention is surely grown as a .beta.-BaB.sub.2 O.sub.4
 single crystal uniform in quality and hence a .beta.-BaB.sub.2 O.sub.4
 single crystal whose optical properties are uniform can be obtained. It is
 considered that this is because, in the present invention, the crucible is
 heated by indirect heating such as emission of heat radiation rays or the
 like to avoid direct heating of the crucible itself by electricity which
 is turned on by high frequency induction current whereby ununiform heating
 is caused in the crucible with difficulty which restricts the occurrence
 of ununiform temperatures in the melt of crystal growth materials placed
 in the crucible.
 The above embodiment shows the case where it uses a radiation ray heating
 furnace in which the heating means has a so-called resistance heating
 furnace structure using electric heating and any section other than the
 crucible, specifically, the heater 11 is heated. Heat from the heater 11
 is either utilized for so-called radiation heating or used to heat
 surrounding gas around the crucible which gas is used as a heating medium
 thereby indirectly heating the crucible 6. However, in addition to the
 above heating furnace, various indirect heating furnace structures may be
 adopted. For instance, a radiation-ray heating furnace using a heating
 means emitting infrared rays, e.g. a lamp, may be adopted.
 The transfer means for moving the crucible 6 or the heater 11 is not
 limited to the foregoing embodiments. For instance, the transfer means 12
 for moving the crucible 6 may comprise a supporting base, e.g.
 heat-resistant brick, for supporting the crucible 6 disposed under the
 crucible 6 disposed by changing and adjusting the thickness of the
 supporting base, the relative position to the heater 11 is moved and
 adjusted.
 As aforementioned, in the present invention, the crucible is indirectly
 heated so that .beta.-BaB.sub.2 O.sub.4 which is uniform in quality and
 has excellent crystallinity can be produced. Hence, when the crystal of
 the present invention is used as an optical crystal used as, for example,
 a frequency conversion element, the yield and hence mass production
 efficiency thereof can be improved. Thus the crystal and method of the
 present invention contribute to a reduction in cost and has an
 industrially large effect.
 Having described preferred embodiments of the invention with reference to
 the accompanying drawings, it is to be understood that the invention is
 not limited to those precise embodiments and that various changes and
 modifications could be effected therein by one skilled in the art without
 departing from the spirit or scope of the invention as defined in the
 appended claims.
 FIG. 1
 2 LOWER FURNACE
 3 UPPER FURNACE
 4 CONCAVE SECTION
 5 HEATING MEANS
 6 CRUCIBLE
 7 LID
 7w OPENING
 8 FUSED LIQUID OF BaB.sub.2 O.sub.4
 9 SEED CRYSTAL OF .beta.-BaB.sub.2 O.sub.4
 10 CRYSTAL LIFTING MECHANISM
 11 HEATING FURNACE
 11t TERMINAL
 12 TRANSFER MEANS
 13 PEDESTAL
 14 MOVING SUPPORT
 15 ROTOR
 16 ROTATION TRANSFER MECHANISM
 17 DRIVING MOTOR
 18 TRANSFER MEANS
 19 ELONGATED APERTURE
 20 HEAT-RESISTANT MATERIAL
 21 MONOCRYSTAL OF .beta.-BaB.sub.2 O.sub.4