US3226193A

US3226193A - Method for growing crystals

Info

Publication number: US3226193A
Application number: US204116A
Authority: US
Inventors: Richard T Dolloff
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1962-06-21
Filing date: 1962-06-21
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28

Description

Dec. 28, 1965 R. T. DOLLOFF 3,226,193

METHQD FOR GROWING CRYSTALS Filed June 21. 1962 Gas 40 38 l 20 I I8 INVENTOR. RICHARD T. DOLLOF F ,4 r rok/vEr United States Patent 3,226,193 METHOD FOR GROWING CRYSTALS Richard T. Dolloif, Parma, Ohio, assignor to Union Carbide Corporation, a corporation of New York Filed June 21, 1962, Ser. No. 204,116 12 (Ilaims. (Cl. 23-191) The present application is a continuation-in-part of my application Serial No. 792,580, filed February 11, 1959, and which is now US. Patent 3,053,639 issued September 11, 1963.

This invention relates to improved methods and apparatus for growing crystals at high temperature using the Verneuil fusion method.

The Verneuil fusion method consists in starting crystal growth on a crystal the upper part of which is kept at a high temperature suflicient to produce a thin molten layer thereon. Powdered feed material for growing the desired crystal is placed in a vertical container above the growing crystal, and is allowed to fall thereon.

In the above described method, heat is provided to the crystal by means of radio frequency heaters, by a furnace, a flame, or an arc. Such heating means require the use of special containers for the crystal. In addition, where an oxyhydrogen flame or a direct are are employed, considerable impurities may be introduced into the material being melted. Less objectionable heat sources such as radio frequency require that the sample be conductive.

With a view to overcoming the above outlined limitations of the Verneuil method and apparatus, the main object of the invention is to provide a method for growing crystals characterized by intense clean heat together with a controlled atmosphere.

A related object of this invention is to provide a method for reacting materials and growing crystals therefrom characterized by intense clean heat together with a controlled atmosphere.

These and related objects, features and advantages of the present invention will be more fully understood as the description thereof proceeds, particularly when taken in conjunction with the accompanying drawing wherein FIG. 1 is a cross-sectional elevational view of crystalgrowing apparatus in accord with the invention; and FIG. 2 is a schematic view depicting an arc image furnace containing the apparatus shown in FIG. 1.

Broadly construed, the apparatus of the invention comprises a transparent enclosure having crystal support means, gas inlet and gas outlet means, and powder feed means. A source of radiant energy is associated by means of mirrors with the above apparatus in such a way that energy can be reflected and refocused to an image on the crystal support at some distance from the source.

The method of the invention broadly comprises providing a suitable crystal support means in an enclosed chamber and at the image spot of a mirror, feeding growth material into the chamber and on the support means, supplying the mirror with energy from a radiant energy source, thereby growing a crystal from the growth material, and cooling the crystal. A seed or pseudo seed crystal is preferably placed on the support means in accord with the above method to promote the formation of a crystal. As used herein, the term growth material includes those materials which react to form a product which in turn forms the crystal as well as those materials which form a crystal directly.

Referring to the drawing, FIG. 1 shows details of a crystal growth apparatus, in accord with the invention. The apparatus described comprises a transparent enclosure suitably made of quartz or other like material, and having outlet mean-s 12 at its bottom. As will be observed on FIG. 2, enclosure 10, which can be evacuated or filled with inert gases, depends from the top 14 of a chamber 16. Enclosure 10 has a flared-out top section, which cooperates with

base members

18 and 20 to secure the enclosure in place, and in air-tight engagement with the lower extremity of crystal feed mechanism 30. Depending from member 20 and extending into enclosure 10 is an adjustable crystal support 24, in this example, a bent high melting point support having a flattened-out extremity 0r pedestal 26, which may be Water-cooled if desired. The pedestal is so positioned with respect to concave mirror 27 surrounding enclosure 10 as to lie at its image spot.

Depending from the top of chamber 22 is a crystal feed mechanism comprising a funnel-shaped member 28 provided with a gas-tight cover 40 and gas inlet 38, and having an elongated tapered extremity 30 passing through member 20 and extending immediately above crystal support 26. A suitable rapping mechanism 32 communicates with perforated shaker 34 through cover in such manner that feed material contained in shaker 34 is delivered at a controllable rate through the tapered tube 30 to the growing crystal supported by pedestal 26.

The crystal growing assembly of the invention is shown in its complete form on FIG. 2. The assembly comprises a radiation chamber 36 mounted at a suitable distance from crystal-growing chamber 16.

As shown, chamber 36 contains a concave mirror 48 at the near focus of which is secured a water-cooled, high output illuminating carbon are 35. The arc image is transferred, in the manner shown by arrows in the drawing, through openings 37 and 39 in

chambers

36 and 16, respectively, to a water-cooled plane or corrected plane mirror 41 mounted at an angle of 45 in chamber 16. If desired, transparent walls will serve the same purposes as openings 37 and 39. Concave mirror 27 is vertically positioned with respect to mirror 41, so that in accord with well known laws of optics the radiant heat is concentrated on the growing crystal. Energies of 1500 watts per centimeter squared over a 1 centimeter squared area have been obtained with the described apparatus, using a single are source and a plane deflecting mirror. Ob viously, various combinations of arc sources and optical systems can provide even higher energy densities.

In the present apparatus, the particles of feed material are symmetrically heated as they fall toward the image. Upon reaching the image area, the particles are melted by the intense radiant energy, and finally grow as a crystal or crystals on the seed surface. If desired, a rod of the material to be melted may be substituted for the powdered form, and may be fed into the image spot at a suitable regulated rate.

Since the seed is usually of the same composition as the material being melted, there is no need in these cases for crucible to withstand the very high temperature employed here. The top surface of the seed is maintained in the molten state; therefore, the only material with which the growing crystal comes in contact is usually that of its own composition. Such a construction permits the growth of crystals either under vacuum or under pressure, as desired.

The instant apparatus and method are particularly suitable for the preparation of single crystals of materials such as molybdenum. Since molten molybdenum is quite reactive particularly at the temperatures reached in crystal growing, it is most difficult to prepare pure single crystals of this material. Other workers have grown molybdenum crystals by introducing the metal in some form directly into the arc stream, and thus melting it. Again this technique may result in some contamination of the crystal and necessarily involves the use of a gaseous atmosphere in the crystal-growing region.

The use of a vacuum, as is possible with the instant apparatus, is impossible with prior art techniques.

As illustrative of the practice of the present invention, high quality single crystals of molybdenum were grown. One of the crystals measured inch in length by inch in diameter, and was of 99.9] percent purity and free from inclusions and grain boundaries. The crystal growing operation was carried out for periods of about five minutes at arc currents approaching 290 amperes in an atmosphere of argon. Feed rates were about mm. per minute. Initial melting began at about 220 amperes, and the current was steadily increased up to 290 amperes during the remainder of the experiment.

The method and apparatus of the invention may also be used to prepare high purity crystals of compounds formed by direct reaction of a refractory powder with a-selected atmosphere in the region of the image, or formed by the reaction of gaseous materials. By the use of high purity refractory metal powders such as Ti and Zr, and a purified atmosphere of nitrogen, the production of single crystals of TiN and ZrN of purities one or two orders of magnitude better than those now common in the field becomes an easy matter. Other nitrogenous atmospheres, such as ammonia, may also be used, but the purity of the crystal may be lowered somewhat due to the other constituents present. Thus, single crystals of AlN have been prepared by the reaction of Al powder in a nitrogen atmosphere with subsequent crystal growth. These crystals measured 1 mm. in length by 0.01 mm. in diameter. Their purity is high, as is indicated by their transparent nature. Doping agents may be added to the material produced, such as AlN, to provide the energy absorption necessary for the growth of larger crystals, or to provide a change in the semiconducting characteristics of the crystal. These agents may be added to the crystal in any manner which is suitable for obtaining the desired final product, such as in mixture with the feed material.

In addition, when suitable reactive gases are introduced into the chamber near the support means, the gases will react in the vicinity of the support to form a product in a powder form, and then a crystal can be grown directly from this product. Crystal growth will proceed as if the powdered product had been originally introduced in solid form. In these cases, the product may form at temperatures below those required for reasonable crystal growth rate; thus, the hot image spot should preferably be maintained as the only hot area in the system. As examples, crystals can be grown as described above after the following reactions;

It will be obvious to those in the art that many other reactions of gaseous materials will proceed in a similar manner to yield a material from which crystals can be grown.

Stoichiometry of the crystals of compounds formed by the method of the invention may be closely controlled by adjustment of the composition and pressure of the atmosphere in the growth chamber, and also by adjustment of the composition of the feed material. For example, in growing TiB a small excess of boron may be added to the feed material to counteract the preferential loss of boron from the melt, if stoichiometric or boronrich TiB crystals are desired. Alternatively, an excess of titanium in the feed material would result in titanium-rich TiB crystals.

Many other materials may be melted and crystals grown therefrom by this technique. A few examples are silicon, titanium, vanadium, germanium, tungsten can bide, silicon carbide, titanium carbide, vanadium carbide, molybdenum carbide, zirconium diboride, titanium diboride, tungsten boride, zirconium nitride and tantalum nitride. This method may be used to prepare crystals from materials having melting points up to about 4000 C., such as those discussed earlier. Crystals from metals or compounds melting at 1000 C. or lower can be grown if suitable controls such as filters or shutters are incorporated to regulate the intensity, and spectral distribution and exposure time of radiation on the sample.

What is claimed is:

1. A method for growing a crystal from materials having melting points up to about 4000" C., which method comprises providing crystal support means in an enclosed chamber and at the image spot of a mirror, supplying said mirror with energy from a radiant energy source, thereby concentrating .heat substantially solely to said image spot of said mirror, feeding growth material into said chamber and on said support means, thereby growing a crystal from said growth material, and cooling said crystal.

2. A method for growing a crystal from materials having melting points up to about 4000 C., which method comprises providing a crystal support means in an enclosed chamber and at the image spot of a mirror, placing a crystal-growth-promoting material on said support means, supplying said mirror with energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror, feeding growth material into said chamber and on said crystal growth promoting material, thereby growing a crystal from said growth material, and cooling said crystal.

3. The method of claim 2 wherein the interior of said chamber is maintained under pressure.

4. The method of claim 2 wherein the interior of said chamber is maintained under a vacuum.

5. The method of claim 2 wherein said chamber contains an inert gas.

6. A method for reacting materials having melting points up to 4000 C. and growing a crystal therefrom, which method comprises providing a crystal support means in an enclosed chamber and at the image spot of a mirror, feeding said materials into said chamber and on said support, supplying said mirror with energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said materials and growing a crystal therefrom, and cooling said crystal.

7. A method for reacting materials having melting points up to about 4000 C. and growing a crystal therefrom, which method comprises providing a crystal support means in an enclosed chamber and at the image spot of a mirror, placing a crystal-growth-promoting material on said support means, feeding said materials into said chamber and on said support, supplying said mirror with energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said materials and growing a crystal therefrom, and cooling said crystal.

8. A method for reacting materials having melting points up to about 4000 C. and growing a crystal therefrom, which method comprises placing a crystal-growthpromoting material on a crystal support means in an enclosed chamber and at the image spot of a mirror, introducing into said chamber a reactive gas and another material reactive with said gas, supplying said mirror with energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said gas and said other material and growing a crystal therefrom, and cooling said crystal.

9. A method for producing crystalline titanium nitride, which method comprises placing a crystal-growth-promoting material on a crystal support means in an enclosed chamber and at the image spot of a mirror, introducing a nitrogenous atmosphere into said chamber, feeding powdered titanium on the surface of said growth-promoting material, supplying said mirror with energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said titanium with said nitrogenous atmosphere and growing a crystal therefrom, and cooling said crystal.

10. A method for producing crystalline Zirconium nitride, which method comprises placing a crystal-growthpromoting material on a crystal support means in an enclosed chamber and at the image spot of a mirror, introducing a nitrogenous atmosphere into said chamber, feeding powdered zirconium on the surface of said growth-promoting material, supplying said mirror with radiant energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said zirconium with said nitrogenous atmosphere and growing a crystal therefrom, and cooling said crystal.

11. A method for producing crystalline aluminum nitride, which method comprises placing a crystal-growthpromoting material on a crystal support means in an enclosed chamber and at the image spot of a mirror, introducing a nitrogenous atmosphere into said chamber, feeding powdered aluminum on the surface of said growth-promoting material, supplying said mirror with radiant energy from a radiant energy source, thereby concentrating heat substantially solely to said image spot of said mirror and thereby reacting said aluminum with said nitrogenous atmosphere and growing a crystal therefrom, and cooling said crystal.

12. The method defined in claim 11 wherein a doping agent is introduced into said chamber during crystal growth.

References Cited by the Examiner UNITED STATES PATENTS 2,461,019 2/1949 Alexander 23191 2,929,126 3/1960 Bollack et a1. 23--191 X 2,973,349 2/1961 Weber et a1. 23191 X 2,993,763 7/1961 Lewis 23-2235 3,086,856 4/1963 Siebertz 23301 X MAURICE A. BRINDISI, Primary Examiner.

Claims

1. A METHOD FOR GROWING A CRYSTAL FROM MATERIALS HAVING MELTING POINTS UP TO ABOUT 4000*C., WHICH METHOD COMPRISES PROVIDING CRYSTAL SUPPORT MEANS IN AN ENCLOSED CHAMBER AND AT THE IMAGE SPOT OF A MIRROR, SUPPLYING SAID MIRROR WITH ENERGY FROM A RADIANT ENERGY SOURCE, THEREBY CONCENTRATING HEAT SUBSTANTIALLY SOLELY TO SAID IMAGE SPOT OF SAID MIRROR, FEEDING GROWTH MATERIAL INTO SAID CHAMBER AND ON SAID SUPPORT MEANS, THEREBY GROWING A CRYSTAL FROM SAID GROWTH MATERIAL, AND COOLING SAID CRYSTAL.

9. A METHOD FOR PRODUCING CRYSTALLINE TITANIUM NITRIDE, WHICH METHOD COMPRISES PLACING ACRYSTAL-GROWTH-PROMOTING MATERIAL ON A CRYSTAL SUPPORT MEANS IN AN ENCLOSED CHAMBER AND AT THE IMAGE SPOT OF A MIRROR, INTRODUICNG A NITROGENOUS ATMOSPHERE INTO SAID CHABMER, FEEDING POWDERED TITANIUM ON THE SURFACE OF SAID GROWTH-PROMOTING MATERIAL, SUPPLYING SAID MIRROR WITHENERGY FROM A RADIANT ENERGY SOURCE, THEREBY CONCENTRATING HEAT SUBSTANTIALLY SOLELY TO SAID IMAGE SPOT OF SAID MIRROR AND THEREBY REACTING SAID TITANIUM WITH SAID NITROGENOUS ATMOSPHERE AND GROWING A CRYSTAL THEREFROM, AND COOLING SAID CRYSTAL.