US3416898A - Method for growing high-melting-point single crystals and an apparatus therefor - Google Patents

Description

Dec. 17, 1968 KEN|H| SHlRQKl ET AL 3,416,898

METHOD FOR GROWING HIGH-MELTING-POINT SINGLE CRYSTALS AN v AN APPARATUS THEREFOR Filed July 27, 1967 Mo TOR uN/r /l7 INVENTORS KEN/CHI SHROK/ SEI/CHI SAlfO A 7' TOR/VE X5.

United States Patent Oioce 3,416,898 Patented Dec. 17, 1968 3,416,898 METHOD FOR GROWING HIGH-MELTING- POINT SINGLE CRYSTALS AND AN AP- PARATUS THEREFOR Kenichi Shiroki and Seiichi Saito, Tokyo, Japan, assignors to Nippon Electric Company, Limited, Minato-ku, Tokyo, Japan, a corporation of Japan Filed July 27, 1967, Ser. No. 656,582 Claims priority, application Japan, July 30, 1966, 41/ 49,964 Claims. (Cl. 23-301) ABSTRACT OF THE DISCLOSURE A rod of high-melting-point substances is supported from one end thereof in the direction of its axis with the other end exposed for melting. A hollow gas flame is disposed coaxially relative to the rod, whereby the end of thel llame contacts and melts the exposed end, following which a single crystal is drawn from the melted end, the single crystal being within and surrounded by the hollow ame during the drawing.

A burner means comprised of tconcentrically arranged inner and outer annular nozzles is coaxially arranged relative to the rod supporting the rod. Located coaxially between the rod-support means and the burner is a seed crystal suport means, the rod-support means and the crystal support means being adapted to move along 4their common axis.

DETAILED DESCRIPTION OF THIS INVENTION This invention relates generally to a method for growth of single crystals of high melting point substances and an apparatus therefor and more particularly to a method for insulating single crystals comprised substantially of high-melting-point oxides.

The ame fusion technique known as the Verneuil method has been adopted,y predominantly over other crystal growth processes for the growth of high-melting- -point single crystals, -notably for the preparation of single crystals of oxides. This technique consists generally of an inverted oxhydrogen burner which opens into a muler and a support rod of refractory material located centrally in the mulle, which can -be lowered as the boule grows. As oxygen, for example, carries tine particles of the raw material of which a single crystal is to be made into the intense heat of a burner flame, they fuse and fall by gravity -on the molten upper surface of the boule so that boule growth may take place. When using a crucible for changing a raw material, the crucible material will invariably diiuse into the molten raw material. The ame fusion method is inherently free from this possibility, since no crucible is used. This advantage of the flame fusion technique is offset by the following disadvantages: In the process of 'boule growth, rapid lowering in temperature occurs at that layer of the -boule just beneath the molten upper surface in succession `which, in turn, develops internal thermal strains in that layer to become the cause of introducing crystallagrophic imperfections throughout the grown boule such as residual strain, dislocation, etc. To eliminate these imperfections, annealing of the grown crystals at high temperatures has been necessary.

Aside from this flame fusion technique, another technique has been used for growing single crystals, such as of a semiconductor, which is commonly called the Czochralski or pulling method. According to this method, the raw material is heated and melted in a Crucible and a seed crystal is lowered into the surface of the melt and then slowly withdrawn so that the melt may cling onto the seed crystal in the form of a single crystal. An advantage of the pulling method over the Verneuil is that `since the growing crystal is continuously heated by heat energy of the melt in the orucible, the temperature gradient in the growing crystal is maintained less than that encountered when using the Verneuil method, with the result that a whole single crystal with less imperfections, such as residual strain or dislocation, can be grown. A drawback of this method is the tendency of the melt contaminated by the crucible material, of intermingling of impurities into the raw material, and eventually of degrading the quality of grown single crystals. Another drawback of the pulling method is the following: Although a carbon crucible, if used, is heat-resistant to high temperatures in exces of 2,000 degrees centigrade is vacuum or in a reducing gas atmosphere, it is oxidized quickly at temperatures above 500 degrees centigrade in an oxidized atmosphere. To obviate this inconvenience, suppose that a crucible made of platinum is used. While a platinum crucible may be used at temperature above 1,500 degrees centigrade in air, it can scarcely be used at such high temperatures in a hydrogen atmosphere, because platinum is chemically attacked by the hydrogen. Such a problem imposes a limitation on both the kind of atmosphere to be employed and the usable orucible material lfor the growth of single crystals.

Accordingly, the principal object of the present invention is to provide a new and improved single-crystal growing for high-melting-point substances that would enable strain-free, dislocation-free, and containimationlfree single crystals to be grown.

Another object of this invention is to provide a new, improved, and straightforward crystal growing instrument and method to replace any one of the conventional crystal growth methods with respect to simplicity of raw material preparation, dispensability of a high-melting-point crucible, and in improved yields.

The se and other yobjects will more clearly appear when ta'ken in conjunction with the following description and the accompanying drawing which illustrates diagrammatical a cross-sectional View in elevation (not to actual proportions) of the furnace apparatus employed to eifect growth of single crystals in accordance with the invention.

An embodiment of the crystal growing method according to the principles of this invention is as follows:

The free end of a rod made of molded raw material powder is melted :by the intense heat of a ilame such Ias from an oxyhydrogen burner. A seed crystal, for instance, is lowered in the direction approximately along the axial line of the burner so that the seed crystal is caused to dip into the molten upper surface of the rod. It is then withdrawn upward gradually at a constant rate so that the melt may cling onto the seed as a single crystal.

It will be readily recognized by one skilled in the art that the present crystal growing method is advantageous over the conventional techniques in the following respects:

No Crucible being needed, there is no possibility of contamination of the growing single crystals kby :a crucible material.

Since growth of single crystals takes place by raising the growing crystal away from the molten surface of the rod, any suitable atmosphere under which crystal growth takes place can be selected in advance. For example, in case of an oxyhydrogen burner, either a hydrogen or an oxygen gas may be selected as the atmosphere surrounding directly the growing crystal.

Substantially strain-free single crystals can be grown, especially when the diameter of the rod made of molded raw material powder, or more particularly, the diameter of the molten zone is designed to be suiciently larger than that of the growing crystal, because this design permits the temperature gradient in the growing crystal to be small enough.

The apparatus adapted for carrying the method of this invention into effect comprises a special burner installed on one end of a furnace. This burner has at least two concentric annular nozzles. Each of the nozzles has one annular opening or a plurality of openings symmetrically arranged about the central axis of the nozzles. Any one of an inflamable gas and a gas for supporting combustion of the inflamable gas is fed Afrom one nozzle, while the other of the gases is fed from the other nozzle. The flow of the gas fed from the outer nozzle should be stoichiometrically much more than the flow of the gas from the inner nozzle, so that a flame formed through the combustion within the furnace has a highest temperature portion of a hollow cylindrical form enclosing an inner ame portion. At the other end of the furnace there is provided a support for `a rod of the high-melting-point substance. This support is moved by a driving means in the direction of the central axis of the nozzles, so that the end portion of the rod is kept in the highest temperature portion of the llame. Into the inner llame portion a single crystal seed of the `substance is introduced lby another support. This support is also moved in the direction of the central axis of the nozzles so as to bring at rst the seed into contact with the melting end surface of the rod and then draw up the seed apart from the melting end surface. Hence, the growing single crystal is surrounded by the inner flame portion according as the seed is drawn up.

The above-mentioned and other features of this invention, will be more clearly understood by the detailed description given of a most preferred embodiment of the single crystal growing method according to this invention in conjunction with the accompanying figure.

Referring to the drawing, it will be understood that a rod 4 made of molded alumina powder is clamped on a support 3 connected to a shaft 2 which is driven by a motor unit 1 through a gear mechanism (not shown) so as to go up and down, the major part of the alumina molded rod 4 being installed in a furnace 7. The furnace 7 has a hollow cylindrical space which is surrounded by a muffle 6 of refractory material equipped with an inspection window of transparent heat-resistant material and an exhaust pipe 22 for connecting the inside of the furnace 7 with the outside. The top of the furnace is closed by the tip of a burner 9, the lbottom being closed by a base lid 23. The burner 9 has an inner and an outer concentric annular nozzle 15 and 16 from which an oxyhydrogen llame 8 is fed into the hollow space. The burner 9 can feed oxygen and hydrogen gases supplied thereto through two inlets 10 and 11 to the furnace 7 through the two annular chambers 12 `and 13, the nozzles 15 and 16, and annular openings 14 and 14 being symmetrically arranged about the central axis of the burner 9, respectively. The ow of the hydrogen gas fed from the outer nozzle 16 is stoichiometrically much more than that of the oxygen gas fed from the inner nozzle 15 so as to form a flame whose highest temperature portion 8 is of a hollow cylindrical form. The excess hydrogen gas in the furnace 7 flows to the outside through the exhaust pipe 22 and burns. The burner 9 may have a third outermost annular concentric nozzle (not shown) or leave an annular gap between the top inner circumference of the mule 6 through either of which oxygen or another gas for supporting combustion of the hydrogen gas fed from the outer nozzle 16 is supplied in order to burn the excessive hydrogen gas. In case the hydrogen gas and the stoichiometrically less amount of oxygen gas are fed Afrom the inner and the outer nozzles 15 and 16, respectively, the excess oxygen gas ows to the outside through the exhaust pipe 22. In this case both the exhaust pipe 22 and the base lid 23 may be dispensed with.

Another shaft 19 which is movable in the axial direction and rotatable by a motor unit 17 is installed through the central portion of the burner 9 in alignment with the shaft 2. At the bottom of the shaft 19, there is a clamp for holding a seed crystal 18. At first, the seed crystal mounted at the lbottom of the shaft 19 is lowered so as to dip into the central part of the molten top end surface 20 of the alumina rod 4. With the continuous attention of an operator who observes the furnace interior through the inspection window 5, the rod is gradually raised at a constant rate. Then the melt clings onto the seed bottom in the form of a single crystal. As the growing of the single crystal 21 proceeds, the molten top 20 of the rod is gradually dissipated and the top surface of the rod is lowered. To compensate lfor the lost amount, the rod 4 should be gradually raised by means of the shaft driving unit 1.

Uniform growth of a single crystal of aluminia took place with our experiment by performing this process continuously until most of the raw material had been exhausted.

It will be seen in the present embodiment that an oxygen gas is introduced inside of the highest temperature portion 8 -constituting an oxygen-hydrogen boundary in an oxyhydrogen flame so that a single crystal of alumina may grow in the oxygen atmosphere.

As a result of conducting single crystal growth for :artificial rubies, using this furnace apparatus at the oxygen and hydrogen flow rates, respectively, of l5 litres and 37 litres per minute, a pulling speed of microns per minute, and a central flame temperature of 2,100 degrees centigrade, it was discovered that substantially strain-free and contamination-free ruby single crystals could be produced, without the need of a subsequent annealing process at high temperature for the prevention of fractures as had commonly occurred when using the Verneuil method.

It will ybe understood that the burner construction such as gas passages for the above-mentioned gases may be suitably modified depending on the quality of a single crystal to be grown.

For instance, the passages for hydrogen and oxygen gases in the above-mentioned embodiment may be interchanged in the illustration to make a hydrogen gas as the atmosphere for surrounding a `growing single crystal of manganese monoxide (MnO), for example, thereby to prevent excessive oxidation. Further, the kinds of gas to be used should by no means be restricted to an oxyhydrogen gas; any suitable combination of a combustible and an oxidizing or inert gas may lbe used, such as propane, town gas, each mixed with air, or an oxyhydrogen gas mixed with argon. Flames formed by the combustion of these gases may be circular in cross section as in the illustration or may be any suitable geometrical configuration whenever such necessity arises.

It has 'been mentioned in conjunction with the embodiment that the seed crystal is to be pulled upward, but it will be seen that it may be pulled downward in case of a furnace construction which is exactly the upside-down of the illustration because no gravitational effect is utilized in the present crystal growing method.

It will be obvious `by one skilled in the lart that the present method can find application not only in the growth of alumina single crystals, but also in that of single crystals of high-melting-point metals such as platinum, rhodium, or iridium, provided that the kinds of flame (such as chemical combustion ames for oxides and plasma flames for metals) flow rates of the gases, flame temperature, the pulling rate, and others be suitably controlled or selected according to the kind of substance to be grown.

We claim:

1. A method of growing a single crystal of a highmelting-point substance from a rod of said substance which comprises: supporting a rod of a high-melting-point substance from one end thereof in the direction of its axis; forming a hollow ame of an inflammable gas by combustion with another gas and disposing said hollow ame so that it is aligned coaxially with said rod whereby the end of said hollow flame contacts an end of said rod and melts it; and then drawing a single crystal from the melted end of said rod along the axis thereof and the axis of said hollow llame, the single crystal being completely surrounded by said hollow flame and spaced therefrom during the growing thereof.

2. The method of claim 1, wherein said hollow flame is substantially cylindrical in shape, and wherein said hollow ame is formed by feeding the inflammable and combustion gases through concentrically arranged outer and inner annular nozzles respectively, the intlammable gas being fed through the outer annular nozzle at a rate stoichiometrically greater than the gas flowing through the inner annular nozzle.

3. The method of claim 2, wherein the single crystal is drawn from the melted end of said rod Iby contacting said melted end with a seed crystal and then moving said seed crystal axially from said melted end within said hollow llame whereby to grow said single crystal.

4. An apparatus for growing a single crystal of a highmelting-point substance from a rod of said substance which comprises: a rod-support means for supporting a rod of high-melting-point substance from, one end thereof in the direction of its axis, the other end of the rod being exposed for heating; means for moving said rod-support means along the rod axis; burner means comprising concentrically arranged inner and outer annular nozzles coaxially disposed relative to said rod-support means for heating and melting the exposed end of said rod; one of said annular nozzles being adapted to feed an inllammable gas, the other nozzle a gas for combusting said inflammable gas, whereby t0 form a hollow annular flame coaxially with the rod axis, means extending centrally of and withdrawable through said nozzles for supporting a seed crystal coaxially and centrally of said nozzles and said rod-support means; and means for moving said seed crystal-support means to and from said exposed rod end along the axis of said rod, whereby to grow a single crystal from the melted end of said rod.

5. The apparatus of claim 4, including a furnace comprising a muiTle with a chamber therein coaxial with the rod-support means and the burner, wherein the rodsupport means enters the chamber from one end, wherein the concentrically arranged annular burner is mounted against the other end of the chamber, and wherein the means for supporting the seed crystal is coaxially arranged to move along the axis of said cham-ber between the rodsupport means and the burner means.

References Cited UNITED STATES PATENTS 3,190,728 6/1965 Vanderink 23-273 FOREIGN PATENTS 243,201 11/ 1925 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

GENE P. HINES, Assistant Examiner.

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