Single crystal-growing method and apparatus

Abstract: A process is disclosed for continuously producing a single crystal by drawing downwardly a melt of a single crystal raw material, wherein a single crystal body grown from the melt is continuously pulled downwardly, and a plurality of single crystal products are continuously formed by intermittently cutting the single crystal body being downwardly moved.

Description: Background of the Invention (1) Field of the invention The present invention relates to a process and an apparatus for producing single crystals. (2) Related Art Statement In order to grow oxide-series single crystals, a process for producing single crystal fibers by an µ pulling down process has recently attracted attention. "Electrotechnical Laboratory" No. 522, pp 4-8, July, 1993 describes the story that a single crystal fiber made of potassium lithium niobate (K 3 Li 2-2x Nb 5+x O 15+x , hereinafter referred to as "KLN") was grown by this process. According to this literature, electric power is fed to a cell or crucible made of platinum to resistively heat a raw material powder. A melt drawing hole is formed in the bottom of the cell or crucible, a rod member called a melt feeder is inserted into this drawing hole so that both a feed amount of the melt to the melt drawing hole and the state of an interface between a liquid phase and a solid phase may be controlled. A thin KLN single crystal fiber is continuously formed, while the diameter of the melt drawing hole, the diameter of the feeder, the protruded length of the feeder from the drawing hole, etc. are controlled. According to this µ pulling down process, the single crystal fiber having the diameter not greater than 1 mm can be formed, and it is easily possible to reduce thermal strain and to control the convection of the melt and the diameter of the single crystal fiber. This process also enables the production of a small size high quality single crystal suitable particularly for the blue laser device by second harmonic generation (SHG). WO 89 03902 discloses a process for manufacturing monocrystalline semiconductor plates. In this process the melt to be crystallised is flowed through a moulding shaft at the bottom of a crucible in which the melt was formed. As a monocrystal grows in the shaft it is pushed downwards towards an outlet of the shaft. When the monocrystal has reached a sufficient length it is severed. Summary of the Invention The present inventors have repeatedly studied to mass-produce KLN single crystal fibers, etc. by using the above µ pulling down process. What is the most important for the mass production technique are that a large amount of a melt is treated with an enlarged scale of the crucible, and that a long single crystal fiber is continuously pulled down from this crucible. However, a concrete technique has not yet been known, which enables the single crystal fiber to be continuously pulled down with use of such a large amount of the raw material. In view of this, the present inventors carried out the µ pulling down process by using an increased amount up to about 5 g of the raw material powder to be fed into the crucible, correspondingly enlarging the size of the crucible, and melting the raw material powder under heating through feeding electric power to the crucible. However, when the scale of the crucible was increased and the amount of the melted powder was increased, it was found extremely difficult to form the single crystal by drawing the melt through the drawing hole. More specifically, when the temperature inside a furnace in which the crucible was placed was set as low as not more than 900°C and the powder was melted inside the crucible by passing electric current mainly through the crucible, the single crystal was not excellently grown in the vicinity of the drawing hole. That is, when the electric power to be fed to the crucible was increased, the melt was kept melted and not crystallized at the drawing hole. On the other hand, when the electric power was decreased, the melt was so solidified in the vicinity of the drawing hole that the melt could not be drawn. When the above furnace temperature was set beyond 900°C, the entire crucible was largely heated by radiant heat from the furnace, so that the temperature gradient become extremely small in the vicinity of the drawing hole. In this case, the crystallization growth could not be continuously effected, either. In order to solve the above problem, the present inventors have developed a process for continuously drawing a single crystal fiber. This process will be explained later. However, it was found that even this process still had a problem particularly from the standpoint of mass-producing the single crystal fiber by continuously pulling down. That is, it is necessary to prevent changes in the quality, particularly, in the composition of the single crystal fiber. Particularly, when a material for second harmonic generation or solid laser elements is to be produced, slight change in the composition largely changes its characteristics to make the resulting product unacceptable. Therefore, it is necessary to prevent changes in the composition in continuously pulling down the single crystal fiber. In the conventional µ pulling down process, the composition of the single crystal fiber has been examined after growth. Indeed, the composition of the produced single crystal fiber can be detected by cutting off a portion of the single crystal fiber and examining its composition. But, since the single crystal fiber-growing process based on the µ pulling down process is a complicated, delicate, chemical and rheological system in which a number of various factors are correlated with one another, the composition of the single crystal fiber will often change in such a production system due to slight changes in the producing condition. As a result, since all of the numerous single crystal fibers may be unacceptable, any countermeasure is indispensable from the standpoint of the production yield. It is an object of the present invention to provide a concrete mechanism for mass-producing an oxide-series single crystal by continuously pulling down based on the µ pulling down process. It is another object of the present invention to enable the production of an oxide-series single crystal having a good quality by continuously pulling down based on the µ pulling down process, even if the size of the crucible is increased or the amount of the raw material is increased. It is a further object of the present invention to enable the mass-production of an oxide-series single crystal by pulling down based on the µ pulling down process, through continuously treating a large amount of the raw material. It is a still further object of the present invention to enable the production of an oxide-series single crystal by continuously pulling down based on the µ pulling down process, while by real-time detecting changes in the composition of the oxide-series single crystal, changes in the composition of the oxide-series single crystal are prevented and the yield thereof largery increased. The oxide-series single crystal producing process according to the present invention includes the steps of melting an oxide-series single crystal raw material in a melting crucible, continuously feeding the melted raw material to a single crystal-growing crucible having a volume smaller than that of the melting crucible (the crucible for melting the raw material) through an opening of the melting crucible, contacting a seed crystal with the melt at a drawing hole of the single crystal-growing crucible (the crucible for growing the single crystal), and growing the oxide-series single crystal body while drawing down the melt through the drawing hole. The present invention also relates to the producing apparatus for contacting the seed crystal with the melt at the drawing hole of the crucible and growing the oxide-series single crystal, while drawing down this melt through the drawing hole, and the apparatus includes the melting crucible for melting the oxide-series single crystal raw material and having the opening, the single crystal-growing crucible having the volume smaller than that of the melting crucible and provided with the drawing opening, wherein the raw material melted in the melting crucible is continuously fed to the single crystal-growing crucible through the opening of the melting crucible, the seed crystal is contacted with the melt at a drawing hole of the single crystal-growing crucible, and the oxide-series single crystal body is grown while pulling down the melt through the drawing hole. The present inventors have continued studies to enlarge the crucible so as to establish the oxide-series single crystal mass-production technique based on the µ pulling down process. However, it was revealed that since the enlargement of the crucible made it impossible to draw the single crystal fiber through the drawing hole, it was difficult to find a solution for the above enlargement problem. Having investigated causes therefor, the inventors found that if a system in which the drawing hole was provided in the bottom of the crucible and the melt was directly drawn through the drawing hole was employed and the heat capacity of the crucible and the melt increased beyond a given level, a good solid phase/liquid phase interface could not be formed in the vicinity of the drawing hole by the thermal influence of the crucible and the melt upon the vicinity of the drawing hole. Irrespective of the above finding, the inventors have looked for any process of continuously treating a large amount of the raw material for continuously pulling the single crystal fiber. During the process of the investigation, the inventors have found a technique that the single crystal-growing crucible having the volume smaller than that of the melting crucible is separately provided from this melting crucible, the single crystal raw material is melted in the melting crucible, the melted raw material is continuously fed to the single crystal-growing crucible through the opening of the melting crucible, a single crystal is contacted with the melt at the drawing opening of the single crystal-growing crucible, and the oxide-series single crystal is grown while drawing the melt downwardly. As a result, the inventors found a finding leading to the present invention, that even if the amount of the powdery raw material to be melted in the melting crucible was increased to not less than 5 g and the volume of the melting crucible was correspondingly increased, the oxide-series single crystal could be easily continuously pulled down. In addition, the present invention is not merely limited to the advantage that the single crystal fiber can be continuously pulled down through melting a large amount of the powder. The present inventors continuously pulled down a KLN single crystal fiber by using the conventional single crystal producing apparatus, while the amount of the powdery raw material in the crucible was suppressed to around 300 - 500 mg. The precise measurement of the composition of the thus obtained single crystal fiber revealed that the composition varied within about 1.0 mol%. To the contrary, according to the the present invention, even if the amount of the powdery raw material to be melted in the crucible was increased to around 30 - 50 g and the melt was continuously fed to the single crystal-growing crucible such that the amount of the melt in the growing crucible may reach around 300 - 500 mg, changes in the composition in the case of KLM single crystal fiber products were suppressed to lower values of indeed not more than 0.01 mol% showing a surprisingly high accuracy. Thus, according to the producing process and apparatus of the present invention, surprisingly high accuracy can be realized in that the homogeneity of the composition of the single crystal is improved, as compared with the conventional µ pulling down process. The reason for the above is not clear. However, according to the conventional producing apparatus based on the µ pulling down process, the powdery raw material was fed and melted in the crucible, and the single crystal fiber was pulled down through the drawing hole of the crucible. Therefore, the temperature of the melting crucible and that of the melt therein have to be kept high so that the melt might not be solidified in the vicinity of the drawing hole, and therefore it was difficult to maintain the temperature condition and the temperature gradient such that the solid crystal might be solidified at a single crystal-growing point in the vicinity of the drawing hole. Prom this reason, it is considered that even if a single crystal fiber could happened to be pulled down under a certain condition, the composition thereof unavoidably suffered changes. To the contrary, according to the present invention, since the raw material was melted in the melting crucible and then the melt is fed from the melting crucible to the single crystal-growing crucible, the single crystal-growing crucible is not thermally influenced by the high temperature of the melting crucible. Therefore, the temperature condition in the vicinity of the drawing hole of the single crystal-growing crucible can be independently set separately from that of the melting crucible. As a result, good single crystal body can be drawn out by setting a greater temperature gradient at the single crystal-growing point in the vicinity of the drawing hole of the single crystal-growing crucible. For that purpose, it is preferable to independently heat the melting crucible and the single crystal-growing crucible by separate heating mechanisms. Further, it is preferable to largely separate the melting crucible from the single crystal-growing crucible. For this, it is preferable that a nozzle portion is extended downwardly from the bottom of the melting crucible, the raw material melted in the melting crucible is continuously fed to the single crystal-growing crucible through the nozzle portion. In the present invention, the raw material is melted in the melting crucible, and the melt is continuously fed to the single crystal-growing crucible. It is preferable that the raw material is kept melted when the raw material from the melting crucible is brought into contact with the melt in the single crystal-growing crucible. However, the melt may be solidified after it is discharged through the opening of the melting crucible. In this case, it is preferable that the solidified material in the vicinity of the opening is continuously pulled downwardly or the material solidified in the vicinity of the opening is melted instantaneously after it is contacted with the melt in the single crystal-growing crucible so that the solidified raw material might not clog the opening. On the other hand, the present inventors have studied the relationship between the state of the melt in the single crystal-growing crucible and the physical properties of the single crystal, by using the above producing apparatus. As a result, the inventors have discovered that if the surface tension is more dominant than the gravity in the environment of the single crystal-growing section, the good single crystal having extremely small changes in composition can be continuously pulled down. The reason therefor is considered that a good solid phase/liquid phase interface can be formed in this case. Further, the present inventors have made researches on the above reason to find out the following finding. That is, it is considered that since the scale of the crucible in the conventional µ pulling down process was small, the single crystal fiber could be continuously pulled down. This is considered since the amount of the melt inside the crucible was small, the melt was adhered to the surface of the inner wall of the crucible through its surface tension to make the gravity acting on the drawing opening relatively small, and a solid phase/liquid phase interface which was homogeneous to some extent could be formed. However, it is considered that if the dimension of the crucible is increased, such condition that the surface tension is more dominant in the vicinity of the drawing opening was disappeared. Further, according to the present invention, it is easy to make larger the temperature gradient in the vicinity of the single crystal-growing point for the single crystal-growing crucible. By so doing, the melt can be rapidly cooled. Therefore, the present invention is particularly suitable for the production of the single crystal of a solid solution state. The single crystal of a solid solution state has a tendency that its composition ratio changes under an equilibrium condition. When the conventional µ pulling down process is employed, the equilibrium condition exists in the vicinity of the drawing hole so that the composition of the solid solution changes due to a slight temperature change or slight changes in the solidifying speed. To the contrary, according to the method and apparatus of the present invention, since the area near the single crystal-growing section can be rapidly cooled, the composition of the melt can be kept constant. As the solid solution, for example, KLN, KLTN[K 3 Li 2-2x (Ta y Nb 1-y ) 5+x ] O 15+x and Ba 1-x Sr x Mb 2 O 6 , which have tungsten-bronze structure, and Mn-Zn ferrite may be recited. Further, for the above-mentioned reason, an oxide series single crystal having a segregation composition can be produced. For example, when neodymium is added or substituted in LiNbO 3 , neodymium only in an amount smaller than that in the composition of the melt enters the single crystal because the segregation coefficient of neodium is not 1.0. For example, even if about 1.0 mol% of neodymium is contained in the melt, only about 0.3 mol% of neodymium can be substituted in the single crystal. However, according to the present invention, the single crystal having the same composition as that of the melt can be produced by rapidly cooling the melt inside the nozzle portion as mentioned above, without segregation. This can be also applied to other laser single crystals such as YAG (Y 3 Fe 5 O 12 ) partially substituted by Nd, Er and/or Yb, and YVO 4 partially substituted by Nd, Er and/or Yb. The producing apparatus according to the present invention is not limited to any particular heating means for the crucibles. However, it is preferable to provide a heating furnace as surrounding the crucible. In this case, it is preferable that the heating furnace is divided into an upper furnace portion and a lower furnace portion, and the melting crucible is surrounded by the upper furnace portion so that the upper furnace portion may be heated at a higher temperature to assist melting of the material powder inside the melting crucible. On the other hand, it is preferable that the lower furnace portion is arranged to surround the single crystal-growing crucible, and the temperature of the lower furnace portion is set lower so that the temperature gradient in the single crystal-growing section of the single crystal-growing crucible may be greater. Further, in order to improve the efficiency of melting the powder material inside the melting crucible, it is preferable that the melting crucible itself is made of a conductive material, and is caused to generate heat through applying electric power thereto, rather than heating the melting crucible by using only the heating furnace outside the melting crucible. Further, in order to keep the molten state of the melt flowing the nozzle portion of the melting crucible, it is preferable that the nozzle portion is made of a conductive material, and caused to generate heat through applying the electric power to the nozzle portion. Further, it is preferable that a radio frequency heating mechanism is provided to heat the melting crucible body and the nozzle portion through radio frequency induction heating. As the above conductive material, materials such as platinum, platinum-gold alloys, platinum-rhodium alloys, platinum-iridium alloys, and iridium is preferred. Since the corrosion-resistant metals such as platinum all have relatively small resistivities, the resistance of the nozzle portion have to be increased to some degree by decreasing the thickness of the nozzle portion so that the nozzle portion may be effectively heat-generated. For example, when the nozzle portion was made of platinum, it had to be formed by a thin film of about 100-200 µm. However, if the nozzle is formed by such a thin film, it is structurally weak, so that stable production of the single crystal body may be difficult due to deformation of the nozzle portion in some case. Under the circumstances, the nozzle portion may be surrounded by a heat-generating resistive member, which can be caused to generate heat through feeding electric power thereto. In this case, the nozzle portion may be made of the above corrosion-resistant metal, and this metal may also be caused to generate heat through feeding electric power thereto. In this case, no electric power may be applied to the nozzle portion. In the above case, a principal heating function may be given to the heat-generating resistive member surrounding the nozzle portion, so that a heat-generating load required for the nozzle portion becomes smaller. Since the nozzle portion may not necessarily be caused to generate heat, the mechanical strength of the nozzle portion can be enhanced by thickening the nozzle portion (for example, to not less than 300 µm), which makes the apparatus more suitable for the mass-production. According to the producing apparatus of the present invention, the raw material can be continuously or intermittently fed into the melting crucible. For, the state thermally changes inside the melting crucible due to the heat of the dissolution of the raw material, and the composition of the single crystal correspondingly changes. However, according to the invention producing apparatus, even if such thermal change occurred inside the melting crucible, the single crystal-growing crucible is less thermally influenced. Further, since the single crystal-growing section of the single crystal-growing crucible is not in an equilibrium state but in a kinetics state, the growing section is still less thermally influenced The present invention can be excellently applied to not only to the production of the single crystal fiber products but also the planar single crystal products. The invention also relates to a single crystal product-producing apparatus which includes a single crystal-growing device in which a melt of a single crystal raw material is placed and a single crystal body is grown by drawing downwardly the melt, a moving device for continuously moving downwardly the grown single crystal body, and a cutter for intermittently cutting the single crystal body being downwardly moved to continuously produce a plurality of single crystal products. The present inventors have found out that the single crystal products having uniform composition and characteristics as well as a uniform shape can be obtained by continuously pulling downwardly the single crystal body and intermittently cutting the single crystal body at a given downstream location under the pulling down step. More specifically, it was found out that the process for continuously producing a plurality of the single crystal products by continuously pulling downwardly the grown single crystal body and intermittently cutting the single crystal body being moved is a process for industrially and stably mass-producing a number of the single crystal products having a given shape. The above moving device preferably includes a pair of rotary bodies for sandwitching the single crystal body therebetween, and a driving device for rotating the rotary bodies, wherein the single crystal body is continuously downwardly moved by rotating the rotary bodies such that the single crystal body is sandwitched between-the pair of the rotary bodies. According to this embodiment, an installation space for the moving device can be reduced. Further, since a pressure can be stably applied upon the single crystal body even with the lapse of time, there is no fear that the crystallinity of the single crystal body is deteriorated upon application of a large stress upon a part of the single crystal. This construction is particularly suitable for pulling down a single crystal fiber. If the temperature of the single crystal body is high, particularly, beyond 200°C, the single crystal body may be adversely affected depending upon the material constituting the rotary bodies. Therefore, it is preferable that the rotary bodies are made of a heat-resistive material such as Teflon. Further, if a seed crystal is pulled downwardly by the above rotary bodies and then the single crystal body is continuously pulled down, it is difficult to smoothly move a portion of the single crystal body in the vicinity of a interface between the seed crystal and the single crystal body because the dimension of the seed crystal differs from that of the single crystal body. Therefore, it is preferable to pull downwardly the seed crystal by another pulling down mechanism exclusively for the seed crystal. According to the present invention, it is necessary to provide the pulling device for pulling downwardly the single crystal body. Although a method may be considered that the single crystal body is downwardly led by the self weight of the melt inside the crucible, the crystallinity of the single crystal will be deteriorated by such a method. In a preferred embodiment, the cutter cuts the single crystal body by fusing through heat-generating a heating wire. This is a method to locally fuse the single crystal body by instantaneously raising the temperature of the heater arranged near around an intended portion of the single crystal body. This method hardly applies stress upon the single crystal body with no fear of adversely affecting the crystallinity of the single crystal body. In a further preferred embodiment, the cutter cuts the single crystal body by fusing it through irradiating a laser beam upon the single crystal body. This method scarcely exerts mechanical forces upon the single crystal and does not cause deterioration of the crystallinity. Such a cutter can be easily set relative to the single crystal body. As the laser, carbon dioxide laser may be preferably used. In a further preferred embodiment, the cutter cuts the single crystal body by mechanically destructing a given portion thereof through pressing a cutting member against the single crystal body. For this purpose, in order to reduce mechanical stress applied to the single crystal body, it is preferable to use a shearing member or scissors each having a distal end of a reduced cross-sectional area. Furthermore, in a preferred embodiment, the single crystal-producing apparatus includes a feeder for automatically supplementing a fresh single crystal raw material into the melt inside the crucible. In this case, the single crystal raw material can be continuously supplemented in the crucible at a given feed rate. Further, a given amount of the single crystal raw material can be intermittently fed into the crucible every given interval. The single crystal-producing apparatus according to the present invention preferably includes an observing device (or a shape measuring device) for observing the shape of the single crystal body, and a shape controller for controlling the shape of the single crystal body based on the information from the observing device. By so doing, if the shape of the single crystal body changes during the automatic mass-production of the single crystal products, such changes can be corrected. More specifically, if the dimension of the single crystal body increases, such a dimensional change amount is detected, and a signal corresponding to this change amount is fed to the controller. The controller may output a temperature control signal to the heater inside the furnace so that the dimension of the single crystal body may be slightly reduced by sending a signal from the controller to the heater to make the heater to slightly raise the temperature. If the heater receives a signal from the controller to slightly lower the temperature, the dimension of the single crystal body can be slightly increased. The shape measuring device may be a device for monitoring the outer shape of the single crystal body through taking a view of this outer shape with a CCD camera. However, since this device cannot detect an absolute actual dimension of the single crystal body, it is preferable to set a standard scale near the single crystal body. Further, the view of the single crystal body may be dark, it is preferable to set an illuminating device near the single crystal body. In addition, the dimension of the single crystal body may be measured by irradiating a laser beam upon the single crystal body. In this case, it is necessary that the temperature in the vicinity of the laser beam source is lowered by setting the laser beam source away from the furnace body. If the laser beam is irradiated upon the single crystal body from one direction only, the dimension can be measured only from this one direction so that the entire shape thereof cannot be measured. Therefore, preferably the dimension of the single crystal body is measured by irradiating the laser beams from at least two crossing directions. Further, if a line sensor is used, its viewing image is unfavorably dark. In this case, it is also preferable to set an illuminating device for the single crystal body. In a preferred embodiment, the single crystal-producing apparatus includes an observing device for observing the material quality of the single crystal body, and a quality controller for controlling the material quality of the single crystal body based on information from the observing device. By so doing, the single crystal products having uniform crystallinity can be mass-produced. It is preferable that a laser beam is irradiated upon the single crystal body being pulled down, an outputted light beam from the oxide-series single crystal body is measured, and the proportion ratio of components in the composition of the single crystal raw material to be fed to the crucible is controlled based on the measuremental result so that change in the composition of the raw material may be prevented even in the case of continuously pulling down a single crystal fiber. In a preferred embodiment of the present invention there is a process provided wherein a first laser beam having a wavelength greater than that corresponding to a target composition of said oxide series single crystal body and a second laser beam having a wavelength smaller than that corresponding to the target composition of said oxide series single crystal body are irradiated upon the oxide series single crystal body, and an output light beam corresponding to said first laser beam and that corresponding to said second laser beam are measured. Further, according to a preferred embodiment, a plurality of single crystal drawing holes are provided in the single crystal growing device so that single crystal bodies may be simultaneously pulled down through said plural single crystal drawing holes, respectively. As single crystals to which the present invention can be applied, oxide-series single crystals and non-oxide series single crystals may be recited. Particularly, the oxide-series single crystals are preferred. As such oxide-series single crystals, known oxide-series single crystals may be recited, and particularly oxide-series single crystals for blue laser device by SEG, such as KLN, KLTN and KN (KNbO 3 ) as well as oxide-series single crystals for ultraviolet laser device, such as CLBO (Cs 0.5 Li 0.5 B 3 O 5 ), BBO (β-BaB 2 O 4 ) and LBO (LiB 3 O 5 ) are preferred. Further, later described oxide-series single crystals in a state of solid solution are preferred. Brief Description of the Invention For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a block diagram for schematically illustrating aspects of a single crystal-producing apparatus; Fig. 2 is a sectional view of outlining a single crystal-growing device; Fig. 3(a) is a sectional view for illustrating a state of the growing device in Fig. 2 before a seed crystal is contacted with a melt; Fig. 3(b) is a sectional view for illustrating a state of the growing device in Fig. 2 in which the seed crystal is contacted with the melt; Fig. 4(a) is a plane view of a plane plate 31 with grooves 32; Fig. 4(b) is a perspective view of a nozzle portion 35 formed by combining a pair of the plane plates 31; Fig. 4(c) is a perspective view for illustrating a state in which a single crystal plate 36 is being grown through the nozzle portion 35 of a crucible 37; Fig. 5 is a view for schematically illustrating a crucible provided with plural rows of nozzle portions; Fig. 6 is a perspective view for illustrating a moving device and a cutter; Fig. 7 is a side view for illustrating a principal portion of the moving device and the cutter in Fig. 6; Fig. 8 is a front view of a holder in Figs. 6 and 7; Fig. 9 is a side view for outlining another moving device and another cutter; Fig. 10 is a perspective view for illustrating a driving mechanism for rotary bodies used in the moving device in Fig. 9; Fig. 11(a) is a schematic view for illustrating a state in which a heater 69 is contacted with a single crystal body 12; Fig. 11(b) is a schematic view for illustrating a state in which the single crystal body 12 is cut by the heater 69; Fig. 12(a) is a front view for illustrating a state in which images of the single crystal body 12 and a scale 62 are taken in a monitor 7; Fig. 12(b) is a perspective view for illustrating a state in which laser beams 64 and 65 are irradiated upon a single crystal body 63; Fig. 13 is a sectional view for outlining a single crystal-producing apparatus according to an embodiment of the third aspect of the present invention; Fig. 14 is a sectional view for outlining a single crystal-growing crucible 115 and its adjacent portion; Figs. 15(a) and 15(b) are schematic views for illustrating states of a distal end portion of the single crystal-growing crucible 115; Fig. 16 is a sectional view for outlining the construction of a crucible in a single crystal-producing apparatus according to the third aspect of the present invention; By way of explanation aspects of a single crystal-producing apparatus will now be described. Fig. 1 is a block diagram of schematically illustrating a single crystal-producing apparatus. In the single crystal-producing apparatus of Fig. 1, a heater 4A is provided inside an upper furnace 2, and a heater 4B is provided in a lower furnace 3. Temperature detectors (preferably thermocouples) 5A, 5B, 5C, 5D and 5E are arranged at specific locations of the upper and lower furnace. Signal cords are extended from each temperature detector to a controller 9. A crucible 11 is placed inside an inner space 10 of the upper furnace 2, and a single crystal body is drawn out into an inner space 18 of the lower furnace 3 through a nozzle of the crucible 11. A raw material feeder 1 is arranged above the upper furnace 2, and a feed port 1a of this raw material feeder 1 is opened to toward an upper face of the crucible 11. The raw material feeder 1 is also connected to the controller 9 through a signal cord. A photographing device 6 for the single crystal body 12 is arranged under the lower furnace 3, and connected to the monitor 7. A moving device 13 schematically shown in the form of a block is arranged under the photographing device 6, and a cutter is under the moving device 13. Both the moving device 13 and the cutter 14 are connected to the controller. A reference numeral 16 denotes a cut portion of the single crystal body, and 15 denotes a single crystal product cut with a specified shape and dimension. A transfer device 17 is arranged under the single crystal product, and the single crystal product 15 is moved into the transfer device 17 in a direction of the arrow A. The single crystal-producing apparatus is observed through a terminal unit 8, and controlled by the controller through the terminal unit. Fig. 2 is a sectional view for outlining a single crystal-growing device, and Fig. 3(a) and 3(b) are schematic views for illustrating a distal end portion of a nozzle portion. Inside the furnace is provided a crucible 20. An upper furnace is arranged to cover the crucible 20 and an upper space 10 around it. A heater 4A is buried in the upper furnace 2. The nozzle portion 25 extends downwardly from an lower end portion of the crucible 20, and a drawing hole 25a is formed at a lowermost end of the nozzle portion 25. A lower furnace 3 is arranged to cover the nozzle 25 and a space 28 around it. A heater 4B is buried in the lower furnace. The heating furnace can of course be modified in various ways. For example, although the heating furnace is divided into two zones, i.e., the upper and lower furnaces in Fig. 2, the heating furnace may be divided into three or more zones. Both of the crucible 20 and the nozzle 25 are made of a corrosion-resistive conductive material. One electrode extending from an electric power source 22A is connected to a location B of the crucible 20, and the other electrode of the electric power source 22A is connected to a corner C of a lower bent portion of the crucible 20. One electrode extending from an electric power source 22B is connected to a location D of the nozzle portion 25, and the other electrode of the electric power source 22B is connected to a lower end of the nozzle portion 25. The electrocity-passing mechanisms mentioned above are separated from each other so that voltages applied to these mechanisms may be independently controlled. Further, an after-heater 66 is provided inside the space 18 such that heater 66 surrounds the nozzle at a given interval. Inside the crucible 20 is upwardly extended an intake tube 23, which has an intake hole 24 at its upper end. The intake hole 24 is located at a location slightly protruding into a bottom portion of a melt 21 inside the crucible 20. Alternatively, the melt intake hole 24 may be formed at the bottom of the crucible so that the intake hole 24 may not be located to protrude the intake tube 20 from the bottom of the crucible 20. In this case, no intake tube 23 is provided. However, as the crucible is used for a long time, impurities in the melt may gradually accumulate in the bottom portion of the crucible. When the intake opening 24 is provided at the upper end of the intake tube 23, the impurities at the bottom is are unlikely to enter the intake opening 24 even if impurities are accumulated on the bottom of the crucible., because the intake tube 23 protrudes in the crucible from its bottom. The upper furnace 2, the lower furnace 3 and the after heater 66 are caused to generate heat to properly set the temperature distribution inside each of the spaces 10 and 18, the raw material for the melt is fed into the crucible 20, and the crucible 20 and the nozzle portion 25 are caused to generate heat through application of electric power thereto. In this state, as shown in Fig. 3(a), the melt 21 slightly protrudes from a drawing opening 25a in a single crystal-growing section 26 at a lower end portion of the the nozzle portion 25, and is held therein by its surface tension to form a relatively flat surface 29. The gravity acting on the melt 21 inside the nozzle 25 is largely reduced through contacting of the melt with the inner surface of wall of the nozzle 25. In particular, if the inner diameter of the nozzle portion 25 was set at not more than 0.5 mm, the above-mentioned uniform solid phase/liquid phase interface (meniscus) could be formed. In this state, a seed crystal 27 is moved upwardly as shown in a direction of the arrow F, so that an end face 27a of the seed crystal 27 is contacted with the above surface 29 of the melt. Then, as shown in Fig. 3(b), the seed crystal 27 is downwardly pulled down. The uniform meniscus 30 is formed between the upper end of the seed crystal 27 and the melt 21 is drawn downwardly through the nozzle portion 25. As a result, as shown in Fig. 2, the single crystal body 12 is continuously formed above the seed crystal 27, and pulled downwardly. A reference numeral 28 denotes a mechanism for pulling down the seed crystal 27. On the other hand, if a conventional crucible is used and an amount of the raw material powder fed to the crucible is increased, an expanded portion of the melt is formed downwardly swelled from a drawing hole of the crucible. In this state, if an end face of a seed crystal is contacted with the melt, no good solid phase/liquid phase interface is formed. Next, a nozzle portion for the production of a single crystal plate will be described. As shown in Fig. 4(a), plural rows of slender grooves 32 are formed in parallel at a flat plate 31. A planar nozzle portion 35 is formed by bonding the planar plates 31 as shown in Fig. 4(b) so that plural rows of melt flow passages 33 may be formed in the nozzle portion 35. A reference numeral 34 denotes a joint. As shown in Fig. 4(c), the nozzle portion 35 is joined to the bottom of an elongated crucible 37. The melt in the crucible 37 flows out downwardly through the melt flow passages 33 of the nozzle 35. At that time, the melts flowing down through the melt flow passages 33 are combined together along the bottom face 35a of the nozzle portion 33, and the combined melt is solidified immediately below the bottom face 35a. Accordingly, the planar single crystal 36 is pulled downwardly through the nozzle portion 35. According to this type of the nozzle portion, the melt flow passages 33 each having a small diameter can be easily formed in. the nozzle portion for the production of the single crystal plate. Fig. 5 is a schematic view for illustrating a single crystal-growing device with a plurality of drawing holes. A melt 21 is received in a crucible 38, and a plurality of the drawing openings 40 are provided in the bottom of the crucible 38 so that single crystal bodies 12 may be pulled down through the respective drawing openings 40 in a direction of the arrows H. In order to supplement reduction in the amount of the melt in the crucible, a fresh single crystal raw material 39 is supplemented to the crucible in a direction of the arrow G. Fig. 6 is a perspective view illustrating an arrangement of holders and cutters, and Fig. 7 is a side view illustrating a principal portion thereof. Fig. 8 is a front view illustrating chucks of the holder and their vicinity. Feed screws 42A and 42B are provided for a pair of respective frame members 41A and 41B, and holders 44A and 44B are fixed to the respective feed screws 42A and 42B. Cutters are provided under the respective holders 44A and 44B, and each of cutting members 45A and 45B of the cutters are coupled to a cylinder 46 via a shaft 47. Motors 43A and 43B are accommodated in a base table under the frame members 41A and 41B. The motors 43A, 43B are actuated to rotate the feed screws 42A, 42B so that the holder and the cutter may be moved up or down. A construction of the holder 44A, 44B is shown in Fig. 8. A shaft 49 is connected to a cylinder 48, and a chuck 53A is fixed to the shaft 49 by a fixing member 52A. The shaft 49 is mechanically connected to a shaft 50 parallel thereto by a link mechanism 51. A chuck 53B is fixed to this shaft 50 by a fixing member 52B. The cylinder 48 can be driven in the arrow directions indicated by I. If the cylinder 48 is moved left in Fig. 8, the shaft 49 and the chuck 53A move toward the left side, whereas the shaft 50 and the chuck 53B move toward the right side. Consequently, the distance between the chucks 53A and 53B becomes greater to release grasping of the single crystal body. When the single crystal is to be grasped, the cylinder 48 is driven toward right, and the shaft 49 and the chuck 53A are moved toward the right side, whereas the shaft 50 and the chuck 53B move toward the left side. When the single crystal body is to be downwardly fed, the outer periphery of the single crystal body is first grasped by the holder 44B, and then the feed screw is driven to move the holder 44B down to a specific location. At that time, as shown in Fig. 7, the cylinder 46 is driven to protrude the cutting member 45B so that the cutting member 45B may be brought into contact with the single crystal body 12, and caused to cut the single crystal body 12 upon pushing. Thereby, a single crystal product 15 is formed. Then, while the grasping of the single crystal body with the holder 44B is released, the single crystal body is grasped by the holder 44A at a given location, and then the holder 44A is in turn moved downwardly. During this step, the holder 44B is moved up to a specific upper location. In this way, the single crystal body 12 is alternatively grasped by and released from the holders 44A and 44B, and successively moved downwardly and cut, so that the single crystal body can be successively and automatically moved and cut. The above operations of the holders, the feed screws, etc. are controlled by the controller, but the controlling itself can be made according to a known method. Fig. 9 is a side view outlining another single crystal-moving device and cutters, and Fig. 10 is a perspective view for illustrating a driving mechanism for a pair of rotary bodies. A rotary shaft 67B is fixed to a rotary shaft of a motor 54 via a gear chamber 55, and a pair of wheels 68 are fixed around the rotary shaft 67B such that a gap 69 is provided between a pair of the wheels 68. By so constructing, a rotary body 66B is formed. A rotary shaft 67A is provided to be synchronized with the rotary shaft 67B via a mechanism not shown. A pair of wheels 68 are fixed around the rotary shaft 67A such that a gap 69 is provided between a pair of the wheels 68. By so constructing, a rotary body 66A is formed. The single crystal body 12 is held in the gaps between a pair of the rotary bodies 66A and 66B, and then pulled downwardly. Under the moving devices is arranged a cutter 57, which includes a cutting blade 58 connected to a driving unit. The cutting blade 58 is contacted with the single crystal body 12, and is caused to shear cut the single crystal body 12 under application of pressure. Thereby, a single crystal product not shown is obtained. This single crystal product is fallen down to a receiving box 60 along a slider 73. The single crystal body may be fused by using a heater. For example, as schematically illustrated in Fig. 11(a), a heater 189 is opposed to the single crystal body 12, and as shown in Fig. 11(b), the heater 189 is caused to generate heat, and the single crystal body 12 is locally heated and fused. Thereby, a single crystal product 27 can be obtained. At that time, a portion of the single crystal body 12 is located inside a recess 189a of the heater 189, and the heater is caused to generate heat in the state that the heater does not contact the single crystal body 12. Alternatively, the single crystal body 12 may be fused in the state that the heater 189 is brought into contact with the single crystal body 12. Referring to Fig. 12(a), a single crystal body 12 is on a screen 61 of a monitor 7. A scale 62 is arranged vertically to the single crystal 12, which enables the outer dimension of the single crystal body 12 to be measured. As illustrated in Fig. 12(b), a laser beam 64 is irradiated upon the outer periphery of a single crystal body 63, and a portion of the irradiated light beam not interrupted by the single crystal body 63 is received by a light beam-receiving device not shown. Thereby, a X-direction dimension of the single crystal body 63 can be measured. Simultaneously, a laser beam 65 is irradiated upon the the outer periphery of the single crystal body 63 in a direction orthogonal to that of the laser beam 64, and a portion of the irradiated light beam not interrupted by the single crystal body 63 is received by a light beam-receiving device not shown. Thereby, a Y-direction dimension of the single crystal body 63 can be measured. Detailed Description of the Invention Hereinafter, embodiments of the present invention will be explained in more detail with reference to the drawings. Fig.13 is a schematic cross-sectional view of the producing apparatus for growing a single crystal. Fig. 14 is an enlarged schematic cross-sectional view of the single crystal growing furnace. Figs. 15 (a) and 15 (b) are schematic cross-sectional views of the single crystal growing section. A melting crucible 112 is disposed in the interior of the furnace body. The upper furnace 2 is arranged to enclose the crucible 112 and its upper space 10. The upper furnace 2 has a heater 4A embedded therein. A nozzle portion 113 extends downwardly from the lower end of the crucible 112 and an opening 113a is provided at the lower end of the nozzle portion. A crucible 115 for growing the single crystal is disposed at immediate below an opening 113a of the nozzle portion 113. A single crystal growing crucible 115 is disposed at the neighbourhood of the boundary portion between the upper furnace 2 and the lower furnace 3. The lower furnace 3 is arranged to enclose the single crystal fiber drawing portion and its surrounding space 18. The lower furnace 3 has a heater 4B embedded therein. Of course, such an arrangement of the heating furnace may be changed variously. The melting crucible 112, the nozzle portion 113 and the single crystal growing crucible 115 are respectively made of a corrosion-resistant electrically conductive material. An electrode of the electric power source 22A is connected to a portion O of the melting crucible 112 through a leading wire and the other electrode of the power source 22A is connected to a portion P of the melting crucible 112. An electrode of the electric power source 22B is connected to the upper end T of the nozzle portion 113 and the other electrode of the power source 22B is connected to the lower end Q of the nozzle portion 113. Similarly, an electrode of the electric power source 22C is connected to an end portion R of the single crystal growing crucible 115 through a leading wire and the other electrode of the power source 22C is connected to a portion S of the single crystal growing crucible 115. These electric power supplying mechanisms are separated from each other and constructed to control its voltage independently. The after-heater 66 is arranged below the single crystal growing crucible 115 in the space 18. In case if the temperature gradient of the nozzle portion is already optimized by the furnace body (the heat-generating member and the refractive material), the after-heater 66 is not indispensable and may be omitted. The temperature distributions in the spaces 10, 18 are suitably determined by the heat-generation of the upper furnace 2 and the lower furnace 3, the raw material for the melt is fed in the melting crucible 112, and electric power is supplied to the melting crucible 112, the nozzle portion 113 and the single crystal growing crucible 115 to heat generate them respectively thereby to melt the raw material in the melting crucible 112 to form a melt 21. The melt 21 is flowed down through the passage 114 in the nozzle portion 113, while retaining its melted state by means of the heat-generation of the nozzle portion 113 and the after-heater 66. The melt is flowed in the single crystal growing crucible 115 from the opening 113a. The melt 116 is stayed in the single crystal growing crucible 115. Alternatively, the melt flowed from the opening 113a may be solidified at immediate below the opening 113a to assume a polycrystalline body, and the fiber or plate made of the polycrystalline body may be continuously fed in the single crystal growing crucible 115 in which the fiber or plate may be melted. At the stage before starting the pulling down of the single crystal fiber, the melt 116 is slightly protruded from the opening 118a at the single crystal growing section 117 existing at the end portion of the nozzle portion 113 and retained by its surface tension to form a relatively flat surface 121. Different from the melting crucible 112 which performs the melting of the raw material, the single crystal growing crucible 115 does not perform the melting of the raw material and is smaller in size than the melting crucible 112. In the small crucible 115, the melt 116 is drawn by the surface tension along the shape of the vertical inner side wall 115a and the bottom surface 115b to assume an inwardly recessed liquidus surface 120. Therefore, the gravity acting on the melt 116 in the nozzle portion 118 is largely decreased by the contact of the melt to the inner wall surfaces 115a and 115b of the single crystal growing crucible 115. At this state, the seed crystal 27 is moved upwardly as shown by the arrow T to contact its upper end surface 27a with the lower surface 121 of the melt 116. At that time, a uniform meniscus 122 is formed between the upper end of the seed crystal 27 and the lower end of the melt 116 drawn downwardly from the opening 118a of the nozzle portion 118. As a result, a single crystal fiber 12 is continuously formed on the seed crystal 27 and pulled downwardly. In this embodiment, the seed crystal 27 and the single crystal fiber 12 are pulled down by the rollers 67. Meanwhile, in case if the amount of the material powder supplied to the crucible is increased according to a conventional method, an expanded portion of the melt is formed downwardly from the opening 118a of the crucible. Moreover, if the weight of the material powder is large, the melt is flowed when the material powder is melt, and a good meniscus cannot be formed when the end surface 27a of the seed crystal 27 is contacted with the melt. Fig. 16 is a schematic cross-sectional view of another embodiment of the producing apparatus for growing the single crystal showing the shape of the crucibles. In Fig. 16 the same functional members are allotted with the same reference numerals as those of Fig.13 and the same explanations are used. The nozzle portion 113 extends downwardly from the lower end of the melting crucible 112 and has the opening 113a at the lower end. The melt or its solidified body is continuously fed from the opening 113a to the single crystal growing crucible 115. The melting crucible 112 and the nozzle portion 113 are respectively made of a corrosion resistant material. The electrodes of the electric power source 22A are connected to the upper end portion O and the lower bent portion P. A circular heat-generating member 114 is arranged around the nozzle portion 113. An electrode of the electric power source 22B is connected to the upper end portion T of the heat-generating member 114 and the other electrode of the electric power source 22B is connected to the lower end Q of the heat-generating member 114. These electric power supplying mechanisms are separated from each other and constructed to control its voltage independently. A radio frequency induction heating mechanism not shown may be provided around the nozzle portion 113 to heat the same thereby to control the supply of the melt. In the above described embodiments, a circular shaped nozzle portion was used as the nozzle portion of the melting crucible as well as as the nozzle portion of the single crystal growing crucible. However, such a circular shaped nozzle portion is generally much expensive in processing. For example, the nozzle portion having a small inner diameter is difficult to produce by processing a material made of a noble metal, such as, platinum. Therefore, the inventors produced the nozzle portion by forming a groove in a corrosion-resistant member made of a corrosion-resistant metal or a corrosion-resistant ceramics, and adhering or joining the grooved member to the other corrosion-resistant member. In such a nozzle portion, the groove serves as the elongated minute diameter passage for the melt. The grooved member should have a flat surface on the side in which the groove is formed, and is preferably a flat plate shaped. At that time, the passage for the melt may be prepared by forming the groove in both the flat plates and integrally uniting the grooves when adhering the flat plates. Alternatively, the passage may be prepared by forming the groove in one flat plate, while leaving the other flat plate as it is, and adhering the two flat plates to obtain the passage formed by the groove in the flat plate. The nozzle portion prepared in this way can be used for the crucible for melting the raw material as well as for the single crystal growing crucible. In addition, in using the single crystal growing crucible the inventors could produce a single crystal plate by forming a plurality of grooves in parallel to each other in the nozzle portion thereby to form the passages for the melt, and drawing the melt from the passages. If the nozzle portion is used for the single crystal growing crucible, preferably the grooves have respectively a width of 0.01-0.5 mm, and a spacing of 0.1-10 mm. The grooves may have a square, a rectangular, V-shape or a half circular shape. Hereinafter, the present invention will be explained with reference to more concrete experimental results. Example 1 Using the single crystal producing apparatus shown in Fig.13, a KLN single crystal fiber was produced according to the present invention. The temperatures in the whole furnace were controlled by means of the upper furnace 2 and the lower furnace 3. The temperature gradient in the neighbourhood of the single crystal growing section was controlled by the supply of electric power to the single crystal growing crucible 115 and heat-generation of the after-heater 66. For pulling down the single crystal fiber a mechanism was used which pulls the single crystal fiber downwardly at a controlled uniform pulling rate in a range of 2-100 mm/hr in the vertical direction. Potassium carbonate, lithium carbonate and niobium oxide were reciped in a mol ratio of 30 : 20 : 50 to prepare a raw material powder. Around 50 g of the material powder were fed in the melting crucible 122 made of platinum and the melting crucible 122 was set at a desired position in the furnace. The nozzle portion 113 had outer and inner cross-sections of circular shapes and an outer diameter of 1 mm, an inner diameter of 0.3 mm and a length of 20 mm. The melting crucible had a circular shape in plan view and a diameter of 30 mm and a height of 30 mm. The single crystal growing crucible had a size capable of containing 1g of the raw material. A tube of an outer diameter of 1.2 mm, an inner diameter of 0.8 mm and a length of 2 mm were used for the nozzle portion. The temperature in the space 10 of the upper furnace 2 was adjusted to a temperature of 1,100-1,200°C to melt the raw material in the crucible 112. The temperature in the space 18 of the lower furnace 3 was uniformly controlled to a temperature of 500-1,000°C. Desired electric powers were supplied to the melting crucible and the nozzle portion to control the supply of the melt to the single crystal growing crucible 115. Electric power was controlledly supplied to the after-heater 66 and the single crystal growing furnace. As a result, the single crystal growing section could be controlled to a temperature of 1,050-1,150°C and to a temperature gradient of 10-150°C/mm. At this state, the single crystal fiber was pulled downwardly at a pulling rate of 20 mm/hr to find out that a good KLN single crystal fiber could be drawn down. The thus grown single crystal fiber having a longitudinal and lateral size of 1 mm x 1 mm and a length of 100 mm was examined with respect to the composition distribution of the single crystal fiber viewed in the length direction (grown direction). Concretely explaining, the single crystal fiber was irradiated at various portions in the longitudinal direction by a light beam, and the wavelength of the light beam emitted therefrom was measured to detect an SHG phase matched wavelength. If there is even a slight variation in the composition of the KLN single crystal fiber, the SHG phase matched wavelength of the emitted light beam is varied by the variation in the composition. The result of the measurement shows that the wavelength was controlled within not more than 1 nm. So, the composition of the single crystal fiber could be controlled with a high precision of not more than 0.01 mol % of composition, which is a high precision never attained before as a KLN single crystal. The wavelength conversion efficiency of the KLN single crystal fiber was substantially the same with the theoretical value with an error of not more than ± 2 % which is within the range of measuremental error. Example 2 In the same manner as in Example 1, a KLN single crystal fiber was grown except that a raw material feeding mechanism was used in the furnace, which intermittently feeds the raw material in the melting crucible 112. Also, a cutting mechanism was arranged below the furnace, which intermittently cuts the single crystal fiber to a desired length to continuously grow the single crystal fiber. With the progress of the single crystal fiber, the raw material powder was periodically fed in the melting crucible in an amount corresponding to the amounts of the components pulled down out and evaporated from the melting crucible. In this way, a single crystal fiber of a length of around 10 m was continuously formed and the variation in the composition thereof was measured in that same manner as in Example 1. As a result, the variation in the composition of the single crystal fiber could be controlled within not more than 0.01 mol % over the entire length of around 10 m. Example 3 The present invention was applied to a method of growing a single crystal of neodymium substituted in LiNbO 3 . However, the amount of substituted neodymium in this system was around 0.3 mol %, if a method, for example, a CZ method was used. Neodymium oxide, lithium carbonate and niobium oxide were reciped in a mol ratio of 1 : 49 : 50 to prepare a raw material powder. The same apparatus as in Example 1 for producing a single crystal fiber was used. Around 50 g of the material powder were fed in the melting crucible 112 and the melting crucible 112 was set at a desired position in the furnace. The temperature in the space 10 of the upper furnace 2 was adjusted to a temperature of 1,250-1,350°C to melt the raw material in the melting crucible 112. The temperature in the space 18 of the lower furnace 3 was uniformly controlled to a temperature of 500-1,200°C. The temperature gradient in the neighbourhood of the single crystal growing section was controlled by the supply of electric power to the nozzle portion 113, the single crystal growing furnace 115 and the heat-generation of the after-heater 66. At that time, the single crystal growing section was controlled to a temperature of 1,200-1,300°C and to a temperature gradient of 10-150°C/mm. At this state, the single crystal fiber was pulled downwardly at a pulling rate of 20 mm/hr to find out that a good KLN single crystal fiber could be pulled down. The thus grown single crystal fiber having a longitudinal and lateral size of 1 mm x 1 mm and a length of 100 mm was elementary analyzed by RPMA with respect to the composition distribution of the single crystal fiber viewed in the length direction (grown direction). As a result, it was found that, the proportion of neodymium in the composition of the single crystal fiber could be controlled to 1.0 mol % with an error of not more than ± 2 %. which is a high precision within a detectable extent.

Claims: A process for producing an oxide-series single crystal comprising the steps of providing melt (116) of a raw material for the oxide series single crystal in a first crucible (115), contacting a seed crystal to the melt emerging at a drawing hole (118) of the first crucible (115), and growing the single crystal body, while drawing the melt (116) downwardly, wherein a free top surface of the melt (116) is maintained in the first crucible (115), characterised by melting the raw material in a second crucible (112) and continuously feeding the melt of the raw material to the first crucible (115), which has a volume smaller than that of the second crucible (112), through an opening (113a) in the second crucible. The process claimed in claim 1, wherein surface tension is more dominant than the gravity acting on the melt in the single crystal-growing location. The process claimed in claim 1 or 2, wherein fresh single crystal raw material is periodically or continuously fed into the second crucible (112) while the melt (116) is being drawn downwardly, and thereby the oxide-series single crystal body is continuously grown. The process claimed in any one of claims 1 to 3, wherein said oxide-series single crystal body has a solid-solution composition. The process claimed in any one of claims 1 to 3, wherein said oxide-series single crystal body is a single crystal body having segregation. The process claimed in any one of claims 1 to 5, wherein a laser beam is irradiated upon the oxide-series single crystal body, an output light beam from the oxide-series single crystal body is measured, and a composition ratio of the raw material to be fed to the crucible is controlled in dependence on the measured result. The process claimed in any one of claims 1 to 6, wherein a single crystal body is continuously moved downwardly, and a plurality of single crystal products are formed by intermittently cutting the single crystal being downwardly moved. The process claimed in any one of claims 1 to 6, wherein the temperature of the first crucible (115) and the temperature of the second crucible (112) are independently controlled. The process claimed in claim 6, wherein a first laser beam having a wavelength greater than that corresponding to a target composition of said oxide series single crystal body and a second laser beam having a wavelength smaller than that corresponding to the target composition of said oxide series single crystal body are irradiated upon the oxide series single crystal body, and an output light beam corresponding to said first laser beam and that corresponding to said second laser beam are measured. The process claimed in claim 6 or 9, wherein the cross-sectional shape of the oxide series single crystal body is measured by an optical sensor. The process claimed in claim 1, wherein the first and second crucibles are independently located. Apparatus for producing an oxide-series single crystal comprising a melting crucible (112) for melting a raw material for an oxide-series single crystal and having an outlet opening (113a), and a single crystal-growing crucible (115) having a volume smaller than that of the melting crucible (112) and provided with a drawing hole (118) from which in use melt is drawn to form the single crystal body, said outlet opening (113a) being arranged to feed a melt of the raw material in use to the single-crystal growing crucible (115) while a melt of the raw material having a free top surface is maintained in the single-crystal growing crucible (115). The apparatus according to claim 12, having means for independent heating of the melting crucible (112) and the growing crucible (115). The apparatus according to claim 12, wherein said melting crucible (112) comprises a nozzle portion (113) made of a conductive material and extending downwardly from a main crucible portion, and an electric power supply (22B) for applying electric power to the nozzle portion (113) to cause the nozzle portion to generate heat, said outlet opening (113a) being provided at a lower end portion of said nozzle portion (113). The apparatus according to claim 12, wherein said melting crucible comprises a nozzle portion (113) extending downwardly from a main crucible portion and having said outlet opening (113a) at a lower portion thereof, a heat-generating resistive member (114) arranged to surround said nozzle portion (113), and an electric power supply (22B) for applying electric power to the heat-generating resistive member to make the heat-generating resistive member generate heat. The apparatus according to claim 12, wherein said melting crucible (112) comprises a nozzle portion (113) extending downwardly from a main crucible portion and having said outlet opening (113a) at a lower end portion thereof, and a radio frequency induction heating mechanism for heating the nozzle portion (113) through radio frequency induction. The apparatus according to any one of claims 13 to 16, which is adapted to produce an oxide series single crystal body of planar shape, wherein a tip end face of an outlet nozzle portion of the single crystal growing nozzle (15) has a flat surface having shape corresponding to the cross-sectional shape of the planar single crystal body, plural rows of melt flow passages are formed in this nozzle portion and open at said flat surface of the tip end face thereof, whereby the melt is simultaneously drawn downwardly through said melt flow passages, and the flows thus drawn downwardly are integrally combined together along said flat surface. The apparatus according to any one of claims 14 to 17, including means for independently controlling the temperature of the melting crucible (115) and the temperature of the nozzle portion (112). The apparatus according to any one of claims 12 to 18, including means for independently controlling the temperature of the melting crucible (112) and the temperature of the crystal growing crucible (115). The apparatus according to any one of claims 12 to 19, which further includes a moving device for continuously pulling downwardly the grown single crystal body, and a cutter for intermittently cutting the single crystal being downwardly moved to continuously produce a plurality of single crystal products. The apparatus according to claim 20, wherein the said moving device comprises a pair of rotary bodies for sandwiching the single crystal body therebetween, and driving device for rotating at the rotary bodies, the single crystal body being continuously downwardly pulled by rotating the rotary bodies in a state in which the single crystal body is sandwiched between a pair of the rotary bodies. The apparatus according to claim 20 or claim 21, wherein said cutter comprises a heating wire. The apparatus according to claim 20 or 21, wherein said cutter comprises a laser beam irradiator, and the single crystal body is cut by fusing it through irradiating a laser beam upon a single crystal body with the irradiator. The apparatus according to claim 20 or 21, wherein said cutter comprises a cutting member, and the single crystal body is cut by mechanically destroying a portion thereof through pressing a cutting member against the single crystal body. The apparatus according to any one of claims 12 to 24, further including a feeder for automatically supplementing a fresh single crystal raw material into the melt inside said melting crucible. The apparatus according to any one of claims 12 to 25, further comprising a laser beam source for irradiating a laser beam upon the oxide series single crystal body, a measuring device for measuring an output light beam from the oxide series single crystal body, and a controller for controlling a composition ratio of the raw material to be fed to the crucible in dependence on the output signal from the measuring device. An apparatus claimed in claim 12, which further comprises a raw material feeder for feeding the raw material to the crucible, a moving device for pulling down the oxide-series single crystal body from the crucible, a laser beam source for irradiating the laser beam upon the oxide-series single crystal body, a measuring device for measuring an output light beam from the oxide-series single crystal body, and a controller for controlling a composition ratio of the raw material to be fed to the crucible based on an output signal from the measuring device.

Inventors: HONDA AKIHIKO, IMAEDA MINORU, IMAI KATSUHIRO, IMANISHI YUICHIRO, KOKUNE NOBUYUKI, SOGO SHOJI, TANIUCHI TETSUO, YAMAGUCHI KAZUAKI, HONDA, AKIHIKO, IMAEDA, MINORU, IMAI, KATSUHIRO, IMANISHI, YUICHIRO, KOKUNE, NOBUYUKI, SOGO, SHOJI, TANIUCHI, TETSUO, YAMAGUCHI, KAZUAKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: C30B15
IPC codes: C30B15