US3242015A

US3242015A - Apparatus and method for producing single crystal structures

Info

Publication number: US3242015A
Application number: US311258A
Authority: US
Inventors: Darrel M Harris
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1963-09-24
Filing date: 1963-09-24
Publication date: 1966-03-22
Anticipated expiration: 1983-03-22

Description

March 22, 1955 R s 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept. 24, 1963 4 Sheets-Sheet 1 FIG. 1

H I: I 6/ J \7V k\\5 7\ J" a 4 4A FIGZ FIG 3 I5 INVENTOR.

DARREL M, HAR RIS ATTORNEY March 22, 1966 M HARRIS 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept. 24, 1963 4 SheetsSheet 2 INVENTOR DARREL M. HARRIS BY @M}.M

ATTORNEY March 22, 1966 A s 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept. 24, 1963 4 Sheets-Sheet 5 TEMPERATURE vs 7a OF FURNACE LENGTH A I250 U L LLI Q: 3 Z 1240 0: I238 LL] 0.. E a

l l 0 20 4o 60 80 I00 To OF FURNACE LENGTH INVENTOR FURNACE GRADIENT DARREL HARR'S -INGOT GRADIENT BY ATTORNEY March 22, 1966 HARRls 3,242,015

APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Filed Sept. 24, 1963 4 Sheets-Sheet 4 TEMPERATURE VS 75 OF FURNACE LENGTH H I250 U o LIJ Q: a 1240 7 I238 LJJ Cl. 2 LL] I230 IZIO 0 2O 4O 6O 80 I00 7o OF FURNACE LENGTH INVENTOR, FURNACE GRADIENT D R HARR|5 ----THERMAL SHIELD GRADIENT BY g /L INGOT GRADIENT ATTORNEY United States Patent C) 3,242,015 APPARATUS AND METHOD FOR PRODUCING SINGLE CRYSTAL STRUCTURES Darrel M. Harris, Glendale, Mo., assignor to Monsanto Company, a corporation of Delaware lFiled Sept. 24, 1963, Ser. No. 311,258 9 Claims. (Cl. 1481.6)

This invention relates in general to certain new and useful improvements in single crystal elements and compounds and more particularly, to single crystal elements and compounds which are formed by the temperature gradient freeze process.

Today, single crystal substances have found widespread application in the electronics industry, for use in the manufacturing of semi-conductor devices such as transistors and rectifiers. The operation and performance of these semiconductor devices, however, is primarily based upon the properties of the single crystal structure. Consequently, the amount of crystal defects appearing in the structure of the crystal must be relatively small before the structure is suitable for use in semiconductor devices. Therefore, it is of strategic importance to produce a single crystal structure which is relatively pure and which is relatively free of cracks and crystal dislocations. A crystal dislocation sets up internal strains within the crystal which will eventually initiate undesirable polycrystalline growth.

One rather effective method, heretofore used in the production of single crystal elements and compounds, is the temperature gradient freeze method. This method generally consists of placing polycrystalline material in a crucible, melting the polycrystalline material in the crucible, and placing the crucible in a tube furnace which is capable of producing a temperature gradient along its length so that it is hotter at one end than at the other. As the temperature of the furnace is reduced, and the gradient is shifted, a portion of the material within the crucible will freeze causing a solid-liquid interface. Thus, when the gradient has shifted to a point below the freezing point of the material, a single crystal structure is formed within the crucible.

While the temperature gradient freeze method has been effective, it has suffered certain serious deficiencies. In the first place, it is rather dilficult tomaintain a linear temperature gradient across the entire length of the crucible. Secondly, it has been difficult to maintain even heating distribution across the entire length of the crucible. Third'ly, it has been rather difficult to control the solid-liquid interface. Recent investigations have shown that an accurate solid-liquid interface materially reduces the possibility of crystal dislocation, imperfections such as microscopic cracks and uneven crystal growth. However, there have been no effective methods devised for effectively controlling the shape of the interface.

It is, therefore, the primary object of the present invention to provide an apparatus and method for producing single crystal elements and compounds having a high degree of crystal purity and excellent crystal structure.

It is a further object of the present invention to provide an apparatus and method for controlling the liquidsolid interface when single crystal structures are produced by the temperature gradient freeze method.

It is another object of the present invention to provide an apparatus of the type stated which is cap-able of maintaining a relatively uniform temperature gradient along the longitudinal axis of the crystal bearing container.

It is an addition-a1 object of the present invention to provide an apparatus of the type stated which is relatively inexpensive to manufacture.

It is another salient object of the present invention to provide a new and simple method for producing large amounts of single crystal structure by the temperature gradient freeze process.

With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement, and combination of parts presently described.

In the accompanying drawings:

FIGURE 1 is a front elevational view of an apparatus for producing single crystal structures and which is constructed in accordance with and embodying the present invention;

FIGURE 2 is a horizontal sectional view taken along line 22 of FIGURE 1;

FIGURE 3 is a longitudinal sectional view taken along line 3-3 of FIGURE 2;

FIGURE 4 is a transverse sectional view taken along line 4-4 of FIGURE 2;

FIGURE 5 is a top plan view of a modified form of an apparatus for producing single crystal structures and which is constructed in accordance with and embodying the present invention;

FIGURE 6 is a vertical sectional view taken along line 66 of FIGURE 5;

FIGURE 7 is a diagrammatical view of a temperature gradient freeze chart showing the temperature gradient over a length of ingot when the ingot was cooled in accordance with prior art methods; and

FIGURE 8 is a diagrammatical view of a temperature gradient freeze chart showing a temperature gradient over a length of ingot when the device of the present invention is employed.

Generally speaking, the present invention comprises a heat conducting shield and a heat insulating shield which are concentrically disposed about a crucible holding the polycrystalline material. The device is then placed in a temperature gradient freeze furnace for heating the polycrystalline material above its melting point to produce a liquid or so-called melt. The melt is then cooled in such a manner that a liquid-solid interface is formed and moves from one end of the crucible to the other. Through the combination of heat conducting andheat insulating shields, it is possible to obtain relatively linear heat application to the entire length of the crucible and hence maintain good crystal structure with relatively few crystal dislocations.

Referring now in more detail and by reference characters to the drawings which illustrate practical embodiments of the present invention, A designates an apparatus for producing single crystal structures which comprises an elongated boat-shaped crucible or so-called crystallizing container 1, preferably made of quartz, and which is often referred to in the art as a boat. The boat 1 includes a bottom wall 2 which integrally merges into a pair of

sidewalls

3, 4 and which are in turn integrally connected by transverse end walls 5, 6.

The boat 1 is then suitably placed in an open ended transparent ampule '7 which is then sealed in any conventional manner such as by fusing a removable end 8 thereto. The ampule 7, however, is conventional in its construction, and is therefore not described in detail herein.

The ampule 7 is concentrically disposed within an opended tubular heat conducting sleeve 9, preferably formed of silicon carbide or other heat conducting material which has a higher heat conductivity than the crucible 1 and the material being treated in the crucible 1. The heat conducting sleeve 9 is preferably formed with a wall thickness of approximately inch for a 12 inch length, the wall thickness increasing with an increase in length. A tubular heat insulating sleeve or heat insulating shield preferably formed of fibrous aluminum silicate, and having a heat conductivity of 2.26 B.t.u./hr./sq. ft.- F. at normal operating temperature of 1250 F., is concentrically disposed around and extends axially along the heat conducting sleeve 9. At these temperatures, the shield 10 is preferably formed of a fibrous material since the insulating qualities thereof are improved. The heat insulating shield 10 is preferably circular in vertical cross-section, and has an annular wall thickness of approximately inch, for a 10 inch length, the wall thickness also increasing with an increase in length. Moreover, by reference to FIGURES 2 and 3, it can be seen that the heat insulating shield 10 is slightly shorter in overall length than the heat conducting sleeve 9, thereby defining an extended heat gathering terminal portion 11 on the heat conducting sleve 9 which preferably has a length of 2 inches. At its left transverse end, reference being made to FIGURE 2, the heat insulating shield 10" is integrally provided with an inwardly struck annular flange 12 having a radial thickness which is approximately equal to the annular wall thickness of the heat conducting sleeve 9 and thereby forms a central heat dissipating aperture 13 which is aligned with the bore of the heat conducting sleeve 9. A heat insulating plug 14 preferably formed of the same material as the shield 10, is removably disposed in the open end of the heat gathering portion 11 in a closure-wise position. Fibrous calcium aluminate has also been found to be very suitable as a material of construction for the shield 10. When low melting point materials are used in the crucible 1, a metal sleeve, such as nickel, tungsten or molybdenum can be employed for the shield 10. Use of a metal of this type will give a greater heat conductivity than the material in the crucible 1 and the shield 10 will, if maintained in a reducing atmosphere, exhibit a bright heat refleeting surface and thereby reduce radiation from the sidewalls of the crucible 1. For this purpose, it is desirable to inject an inert or reducing gas such as argon, or hydrogen into the furnace 15.

The aforementioned assembled components are then placed in a suitable furnace 15 which is capable of producing single crystal structures by the temperature gradient freeze process. The furnace 15 is provided with a pair of upstanding supports 16 which engage the shield 10 near each of its transverse ends. The remainder of the furnace 15 is conventional in its construction, and is therefore not described in detail herein. However, the furnace 15 is provided with a heating element 17 which is designed to cause a linear temperature differential between the two ends of the furnace 15. While the furnace 15 illustrated herein is provided with a heating element 17 with progressively increased spacing between the convolutions thereof, it should be understood that any suitable furnace such as the so-called Globar or induction furnaces could be employed.

In operation, the crucible 1 is initially charged with a suitable amount of polycrystalline material, such as gallium arsenide, to the approximate level as shown in FIG- URE 3. The crucible 1 is next placed within the ampule 7, the removable end 8 is sealed, the ampule 7 is evacuated and is in turn placed within the heat conducting sleeve 9. By reference to FIGURE 3, it can be seen that the heat conducting sleeve 9 is slightly longer than the overall length of the ampule 7, and moreover has its terminal portion 11 located within the hot end of the furnace 15. Next, the heat conducting sleeve 9 is concentrically disposed within the heat insulating shield 10 until the left transverse end of the sleeve 9 abuts the interior surface of the annular flange 12, substantially as shown in FIG- URE 3. Finally, the assembled components are suitably placed within the furnace 15. By reference to- FIGURE 1, it can be seen that the greater number of turns of the heating element 17 are located near the right transverse end of the furnace 15, and hence this is the hotter portion of the furnace 15. It can be seen that the spacing between the convolutions of the heating coil or heating element 17 increases as it traverses the length of the furnace 15. Consequently the left transverse end of the furnace 15 is cooler than the right transverse end, causing a temperature differential through out the length of the furnace, which differential is relatively linear.

The heating coil 17 is ultimately connected to a suitable source of electrical current (not shown), and through suitable control means (also not shown) is heated to a temperature above the melting point of the gallium arsenide so that the entire charge melts. The crucible 1 and the melt are then cooled so that freezing begins at the left transverse end of the crucible 1 or at the transverse end wall 6. Further cooling is carried out in the furnace 15, while still maintaining the linear temperature differential throughout the entire length thereof, so that an isothermal surface or liquid-solid interface s near the melting point passes progressively through the melt until the entire melt has solidified. In this manner a single nucleus which first forms near the end wall 6 in the crucible 1 can be made to grow and fill the entire crucible, yielding a single crystal approximately of the size and shape of the crucible itself.

The temperature of the furnace 15 is maintained at a point so that the solidification takes place at a relatively slow rate. In connection with the above, a rate of solidification where the interface s moves at approximately 0.1 to 0.3 centimeter per hour has been found to produce most desirable results. At this rate, dislocation in the crystal is materially reduced. In the past, it was difficult to maintain a relatively constant linear temperature differential across the entire length of the crucible 2. However, with the heat conducting sleeve 9, the terminal portion 11 absorbs the heat from the hot end of the furnace 15 and distributes the heat evenly across the length of the ampule 7 and the boat 1. Therefore, through the use of the sleeve 9 it has been possible to materially reduce crystal dislocations to a point where they no longer have the tendency to produce polycrystalline growth. In fact, actual tests have shown that through the above described procedures, dislocations in the single crystal structure have been reduced from 10 dislocations per square centimeter to 10 dislocations per square centimeter.

As it was pointed out above, it is desirable to maintain an arcuate interface which is concave into the liquid phase, that is to say the interface lies in an arcuate plane and extends into the liquid phase at the isothermal surface between the liquid and solid phases. It has been well established that this type of interface will materially reduce crystal dislocations and substantially reduce uneven crystal growth. However, in the past, it was rather difficult, if not impossible, to control the shape of the interface using the gradient freeze method of producing single crystal structures. In the past, a great deal of heat was radiated from the side walls of the ampule and boat to the furnace, especially in the colder end thereof. Since the boat and contents usually have a greater heat conductivity than the media surrounding the boat in the colder end of the furnace, the boat and its contents were at a higher temperature than the media surrounding the boat in the colder end of the furnace. Moreover, the temperature differential between the cold end of the furnace and the portion of the boat located in such end of the furnace Was quite large. These conditions resulted in a great deal of heat radiation from the material contained in the boat through the side Walls of the boat to the colder end of the furnace and hence, it was quite difficult to maintain the arcuate interface. Moreover,

since the melt did not have a uniformly linear temperature gradient across its length, some portions of the melt radiated greater amounts of heat than other portions thereof. The heat insulating shield 10 in the present invention, substantially reduced the amount of heat radiation from the side walls of the ampule 7 and boat 1. Moreover, since the heat conducting sleeve 9 surrounds the ampule 7 and has a higher heat conductivity than the ampule 7, the boat 1 is located in a media which is hotter than itself. In effect, this maintains the side walls of the boat 1 in a state of thermal isolation and the boat 1 is in effect then a thermally floating tube. The only heat received by the boat l is that heat which is conducted by the heat conducting sleeve 9. Furthermore, the only heat loss is through the left transverse end of sleeve 9, reference being made to FIGURE 1, to the colder end of the furnace IS.

The heat which is absorbed by the boat 1 moves towards the center of the melt and of the solidified portion thereof and is added to the heat of solidification which passes across the isothermal surface s and out through the left rtansverse end of the boat I and heat conducting sleeve 9. The heat which is impressed across the walls of the boat 1 in the area of the isothermal surface s is small compared to the heat of solidification passing through the center of the isothermal surface s. Consequently, this additional heat applied along the

walls

3, 4 of the crucible 1 at the isothermal surface s will tend to move aong the

walls

3, 4 before moving toward the center of the solidified mass in combining with the heat of fusion. Similarly, the heat applied at the base 2 of the crucible 1 will move along the base 2 at the isothermal surface before combining with the heat of solidification. This additional heat which is impressed across the melt adjacent to the isothermal surface s will cause a very slight temperature differential across the portion of the melt which faces the isothermal surface s so that the isothermal surface s will in effect form an arcuate face as shown in FIGURES 2 and 3. Thus, as the temperature of the furnace 15 has been lowered While maintaining the same temperature gradient between the two ends thereof, the crucible will have been progressively cooled from the left transverse end to the right transverse end thereby shifting the isothermal surface s until the entire melt has completely solidified.

While the operation of the present apparatus has been described in the production of a gallium arsenide crystal, it should be understood that the invention is not limited to this particular compound. Contemplated for use in the present invention is the production of large single crystal compounds formed from elements of Groups IIIB and VB of the Periodic System (Hubbards Chart of the Atoms). The compounds included within this group which are contemplated for use in the present invention include arsenides, phosphides and antimonides of aluminum, gallium and indium. It is also contemplated that compounds formed from the elements of Groups II and IV, and Groups I and VII of the Periodic System could be used in connection with the present invention. These elements include the sulfides, selenides and telluridesof zinc, cadmium and mercury and the chlorides, bromides and iodides of sodium, potassium, copper, rubidium, silver, cesuim and gold. It should also be understood that the apparatus of the present invention could be successfully employed for the production of single crystal elements such as selenium, tellurium, rubidium, germanium, silicon, cesium, gold, silver, etc. However, the heat conducting sleeve employed must have a higher heat conductivity than the element being crystallized.

It is possible to provide a modified form of Apparatus B for producing single crystal structures in accordance with the present invention, substantially as shown in FIGURES 5 and 6. The Apparatus B for producing single crystal structures is substantially similar to the Apparatus A and consists of an elongated boat shaped crucible 18 which is suitably placed in an open end transparent ampule 19, both of which are substantially identical to the previously described crucible 1 and ampule 7 respectively. The ampule 19 is similarly disposed within a heat conducting sleeve 20, which is, in turn, concentrically disposed within a heat insulating shield 21, the sleeve 26 also being provided with a heat insulating plug 22, which are substantially identical to the previously described sleeve 9, shield 10 and plug 14 respectively. The Apparatus B is similarly placed within a furnace 23, which is also identical to the previously described furnace 15.

The ampule 19 is not sealed in the manner as previously described in the Apparatus A, but is connected to a similar ampule 24 also preferably formed of a transparent quartz material, through an elongated neck 25 and is therefore in communication with the ampule 24. The ampule 24 is placed within a suitable furnace 26, which is provided with a pair of upstanding supports 27 for supporting the ampule 24 near each of its transverse ends. The furnace 26 is conventional in its construction and is therefore not described in detail herein. However, the furnace is provided with a heating element 28, which has uniformly spaced convolutions and thereby maintains a linear temperature across the furnace. The furnace 26 should be capable of producing a temperature of at least 700 C. An annular heat insulating ring 29 is disposed around the elongated neck 25 and is located in endwise abutment with each of the endwise aligned

furnaces

23 and 26, in order to maintain the neck 25 at the same temperature as the interior of the furnace 26. The ring 29 is preferably formed of firebrick or similar refractory material.

The Apparatus B is employed where it is desired to react elemental gallium contained within the crucible 18 with arsenic gas contained within the ampule 24. In use, a suitable charge of liquid gallium is disposed within the crucible l8 and a suitable charge of solid arsenic is placed in the ampule 24. The crucible I8 is placed in the ampule 19 and the ampule 119 is sealed in communication with the ampule 24 through the elongated neck 25. The

ampules

19 and 24 are then evacuated to approximately 10 mm. of mercury pressure through a tube (not shown) on the ampule 24, which is ultimately sealed and thereby maintains the vacuum in each of the ampules. The furnace 26 is then heated to a temperature above the melting point of gallium arsenide or 1245 C. and the furnace 26 is heated to a temperature of approximately 610 to 620 C., thereby creating a gaseous arsenic atmosphere. The gaseous arsenic contained Within the ampule 24 will pass through the elongated neck 25 and react with the liquid gallium contained within the crucible 18 at these temperatures to form gallium arsenide.

The crucible l8 and the melt contained therein is then cooled so that freezing begins at the left transverse end of the crucible 18 in the same manner as in the Apparatus A. Further cooling is carried out in the furnace 23 while maintaining the linear temperature differential throughout so that an isothermal surface or liquid-solid interface s which is similar to the previously formed liquid'solid interface 5, passes progressively through the melt until the entire melt has solidified. In this manner, it is possible to form a single nucleus of gallium arsenide by reacting the arsenic gas with the liquid gallium in the crucible 18 so that the nucleus can grow and fill the entire crucible yielding a single crystal of gallium arsenide, which is approximately the size and shape of the crucible itself. It should also be understood that it is possible to form single crystal compounds other than gallium arsenide by employing a gas other than arsenic and a solid material other than gallium.

Contemplated for use in this embodiment is the production of large single crystal compounds formed from the elements of Groups 3B and 5B of the Periodic System. Also contemplated for use in this embodiment are compounds formed from the elements of

Groups

2 and 4 and Groups 1 and 7 of the Periodic System.

FIGURE 5 is a temperature gradient freeze chart plotting the temperature in degrees centigrade vs. the furnace length and showing the temperature gradient across the length of the furnace and the temperature gradient across the length of the crucible containing a polycrystalline gallium arsenide ingot for the production'of single crystal gallium arsenide structures. FIGURE 6 is a temperature gradient freeze chart plotting the temperature in degrees C. vs. the furnace length for the same crucible used in FIGURE 5, when the heat conducting shield and heat conducting sleeve forming part of the present invention are thus employed. Comparing FIGURE to FIGURE 6, it can be seen, that the temperature gradient across the length of the crucible very nearly approaches the temperature gradient across the length of the furnace and has almost the same slope thereof. However, with reference to FIGURE 6, when the heat conducting sleeve and heat insulating shield are employed, it can be seen that there is a greater disparity between the temperature gradient of the furnace and that of the crucible. Moreover, it can be seen that the crucible is in effect surrounded by a continually hotter surface when the heat conducting sleeve is employed. It is also to be noted, that when the heat conducting sleeve and the heat insulating shield are employed, an almost perfectly smooth temperature differential which approaches almost perfect linearity exists across the length of the crucible I. In other words, with the present invention, there is an almost completely symmetrical temperature environment in the crystalline system and hence, the resulting crystal structure produced shows no polycrystallization of the melt. The data employed to produce the temperature gradient freeze charts of FIGURES 5 and 6 is more fully described in Examples 1 and 2, hereinafter set forth.

A more detailed description of the invention is set forth in the following examples which have reference to the foregoing specification and the accompanying drawings:

Example 1 Approximately 450 grams of surface cleaned gallium arsenide is placed in a quartz crucible having a length of approximately 5 inches and a diametral cross section of approximately 33 mm. The crucible and its contents are then placed in a quartz ampule having a length of approximately 8 inches and a diametral cross section of 38 mm. The ampule is heated to 150 C., then subjected to a high vacuum and evacuated to a pressure of approximately 5 10 mm. of mercury, and is then sealed. The ampule is then cleaned and placed in a silicon carbide tube having a length of 10 inches and a diametral cross section of 2 inches, and a wall thickness of inch. The ampule is axially centered within the silicon carbide tube and located in such a position that one transverse end of the crucible is located approximately 1 inch internally of the transverse end of the silicon carbide tube which is located in the cold end of the furnace. The carbide tube is then wrapped in calcium aluminate paper commercially known as Fiberfrax, until a thickness of inch is attained. The silicon carbide tube is then plugged with a sufficient amount of the Fiberfrax paper at the hot end thereof in order to prevent direct heating of the ampule in the crucible contained within the silicon carbide tube.

The aforementioned assembly is then placed in a temperature gradient freeze furnace and disposed upon supports within the furnace, which make a minimum physical contact with the insulated tube. The furnace is sized so that there is a V; inch clearance between the furnace wall and the insulated silicon carbide tube. Thermo couples are so placed in the silicon carbide tube that temperatures can be obtained for each inch of ingot length. The furnace is then heated until the coolest end of the ingot has reached a minimum temperature of 1242 C., the melting point of the charge, and the furnace is maintained at this temperature for at least 1 hour in order to assure complete melting thereof. The temperature of the furnace is reduced over a period of 18 hours 8 until the hottest portion of the charge in the crucible is below the melting point of gallium arsenide or 1238 C. The data thus obtained is used in the production of the temperature gradient freeze chart of FIGURE 6.

Approximately 425 grams of a single crystal of gallium arsenide is thus obtained. Oriented slices of the single crystal structure on the 1l1 plane (Miller indices) are removed from the end of the crystal which first froze in the crucible. Similarly, oriented slices are removed on the same lll plane (Miller indices) from the end of the crystal which was located in the hotter portion of the furnace. These oriented slices are then mechanically polished and treated with a polish etch consisting of 4 parts of water, 3 parts of concentrated nitric acid, and 1 part of 48% hydrochloric acid in order to remove the mechanically worked surfaces thereof. The polished slices are then treated with an etch consisting of 2 parts of Water and 1 part of concentrated nitric acid to develop etch pits. The slices from the end of the crucible which was located in the colder portion of the furnace are examined under a microscope and found to contain 1500 etch pits per square centimeter. The slices from the crystal which was located in the hotter portion of the furnace are similarly examined and found to contain 6100 etch pits per square centimeter.

Example 2 The above process used in Example 1 is repeated with approximately 450 grams of surface clean gallium arsenide in the same quartz boat. However, in this example, the silicon carbide tube and the calcium aluminate insulating paper is not employed. The data thus obtained is used in the production of the temperature gradient freeze chart of FIGURE 5.

Oriented slices thus removed from the end of the single crystal located in the colder end of the furnace, in the same manner as in the above example when examined are found to contain approximately 15,000 etch pits per square centimeter. Oriented slices removed from the end of the crystal which was located in the hotter portion of the furnace, when examined are found to contain approximately 65,000 etch pits per square centimeter.

Example 3 Example 4 Approximately 220 grams of silicon is charged into the crucible used in Example 1, and the procedure of Example 2 is followed. Thus, in this case the silicon carbide tube and the calcium aluminate paper are not employed. Oriented slices removed from the end of the crystal located in the colder portion of the furnace reveal approximately 100,000 etch pits per square centimeter whereas oriented slices removed from the end of the crystal located in the hotter portion of the furnace reveal approximately 500,000 etch pits per square centimeter.

Example 5 Approximately 400 grams of germanium are charged into the crucible used in Example 1 and the procedure of Example 1 is followed. In this case, the silicon carbide tube and the calcium aluminate paper are disposed around the crucible containing the germanium. Oriented slices removed from the end of the crystal located in the colder portion of the furnace reveal approximately etch pits per square centimeter and oriented slices removed from the end of the crystal located in the hotter portion of the furnace reveal approximately 650 etch pits per square centimeter.

Example 6 Approximately 400 grams of germanium are charged into the crucible used in Example 1 and the procedure of Example 2 is followed. In this case, the silicon carbide tube and the calcium aluminate paper are not employed. Oriented slices removed from the end of the crystal located in the cooler portion of the furnace reveal approx imately 750 etch pits per square centimeter, and oriented slices removed from the end of the crystal located in the hotter portion of the furnace reveal approximately 3050 etch pits per square centimeter.

It should be understood that changes and modifications in the form, construction, arrangement and combination of parts presently described and pointed out can be made and substituted for those herein shown without departing from the nature and principal of my invention.

Having thus described my invention what I desire to claim and secure by Letters Patent is:

1. An apparatus for producing single crystal substances by the temperature gradient freeze method which comprises in combination, a crystallizing container, a heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, and heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container, said heat conducting element having a terminal end which extends beyond one transverse end of the heat insulating member, the terminal end of said element having a length sufficient to draw heat from the heating means for transmitting the heat across the length of the heat conducting element.

2. An apparatus for producing single crystal substances by the temperature gradient freeze method which comprises in combination, a crystallizing container located in a temperature gradient atmosphere Where one end thereof is at a higher temperature than the other end of said container, an open ended heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, said heat insulating member being of sufiicient length to surround the crystallizing container for its entire length, a heat insulating plug disposed within the open end of the heat conducting element which is proximate to the end of the crystallizing container at the higher temperature and is of sufiicient thickness to prevent direct heating through the open end of the heat conducting element, and heating means surrounding said heat conducting elernent to apply a uniform temperature gradient across said crystallizing container.

3. An apparatus for producing single crystal substances which comprises in combination, a crystallizing container, a heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container, and means in heat conductive association with said heat conducting element for drawing heat from the hot end of the heating means and transmitting the heat along the entire length of the heat conducting element, said last named means being located at the hotter end of said gradient and having suflicient surface area to draw the required amount of heat for transmission along the length of the heat conducting element.

4. An apparatus for producing single crystal substances which comprises in combination, .a crystallizing container, a heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, and heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container, said heat conducting element having a greater length than the heat insulating member and having a portion which extends beyond one end of the heat insulating member, said portion being located at the hotter end of said temperature gradient and having a length suflicient to draw heat from the heating means for transmitting the heat along the length of the heat conducting element.

5. An apparatus for producing single crystal substances which comprises in combination, a crystallizing container, a heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, and heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container, said heat conducting element having a greater length than the heat insulating member and having a portion which extends beyond one end of the heat insulating member, said last named portion extending into the hotter end of the temperature gradient and having a length suflicient to draw heat from the heating means for transmitting the heat along the length of the heat conducting element, said heat insulating member having an annular flange which engages one transverse end of the heat conducting element.

6. A process for the production of single crystal substances by the temperature gradient freeze method which comprises melting a polycrystalline form of such substance in a container disposed within a crystallizing zone to produce a melt, adjusting the temperature within said crystallizing zone, to provide a substantially linear temperature gradient across the entire length of the polycrystalline form of such substance, cooling said crystallizing zone incrementally from one end at a slow uniform rate to initiate crystallization of said melt, thereby forming a crystal in the cooled portion of the crystallizing zone, continually applying heat to the walls of the container along its entire length but at the substantially linear temperature gradient so that heat flows through the contamer from the hotter end thereof to the colder end thereof, applying heat to the walls of said container at a rate which is less than the rate of movement of heat through the center of the crystal to maintain an arcuate interface between the crystal and the melt, with a temperature differential thereacross, said arcuate interface bemg concave with the liquid phase of the melt, continually cooling said crystallizing zone until the entire melt has crystallized, and recovering a crystal from the container.

7. An apparatus for producing single crystal substances which comprises in combination a crystallizing container, a first reacting element in said crystallizing container, 21 reactant container containing a second reacting element, and being 111 communication with the reacting element in said crystallizing container, 21 heat conduct-ing element surrounding said crystallizing container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conduct-ing element to prevent radiatron from the longitudinal surface of said crystallizing heat conducting element having a portion beyond one end of said heat insulating member, said last named portion having a length which is sufficient to gather heat and transmit heat along the length of the heat conducting element, first heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container, and second heating means surrounding said reactant container.

8. An apparatus for producing single crystal substances by the temperature gradient freeze method which comprises in combination, a crystallizing container located in a temperature gradient atmosphere where one end thereof is at a higher temperature than the other end of said container, an open ended heat conducting element surrounding said container and being in heat exchange relation thereto, a heat insulating member surrounding said heat conducting element to prevent heat radiation from the longitudinal surface of said crystallizing container, said heat insulating member being of sufficient length to surround the crystallizing container for its entire length, a heat insulating plug disposed within the open end of the heat conducting element which is proximate to the end of the crystallizing container at the higher temperature, said heat insulating member having an annular flange which engages the transverse end of the heat conducting element which is proximate to the end of the crystallizing container at the lower temperature, and heating means surrounding said heat conducting element to apply a uniform temperature gradient across said crystallizing container.

9. A process for the production of single crystal substances by the temperature gradient freeze method which comprises melting a polycrystalline form of such substance in a container disposed within said crystallizing zone to produce a melt, gathering heat from the hotter end of the temperature gradient and conducting the heat along a heat conductive element surrounding the crystallizing zone, transmitting the heat by radiation to the container within the crystallizing zone, preventing re-radiation of the heat from the container and radiation of heat from the conductive element by an insulating element to provide a substantially linear temperature gradient across the entire length of the polycrystalline form of such substance, cooling said crysta-llizing zone incrementally from one end at a slow uniform rate to initiate crystallization of said melt, thereby forming a crystal in the cooled portion of the crystallizing zone, continually applying heat to the walls of the container along its entire length but at the substantially linear temperature gradient so that heat flows through the container from the hotter end thereof to the colder end thereof, applying heat to the walls of said container at a rate which is less than the rate of movement of heat through the center of the crystal to maintain an arcuate interface between the crystal and the melt with a temperature differential thereacross, said arcuate interface being concave with the liquid phase of the melt, continually cooling said crystallizing zone until the entire melt has crystallized, and recovering a crystal from the container.

References Cited by the Examiner UNITED STATES PATENTS 2,475,810 7/1949 Theuerer 148-15 2,789,039 4/1957 Jensen 23273 2,837,618 6/1958 Gildart 148-1.5 2,871,377 1/1959 Tyler et a1. 148-173 2,902,350 9/1959 Jenny et al 1481.6 3,012,865 12/1961 Pellin 23-273 3,121,619 2/1964 Scholte 148-1.6

OTHER REFERENCES Braun et al.: Article in the Journal of the Electrochemical Society, October 1961, pages 969-973.

Miller: Gradient Freeze Single-Crystal Growth, Compound Semi-Conductors, Reinhold Publishing Corp, New York, vol. 1, pp. 274-279.

DAVID L. RECK, Primary Examiner.

Claims

1. AN APPARATUS FOR PRODUCING SINGLE CRYSTAL SUBSTANCES BY THE TEMPERATURE GRADIENT FREEZE METHD WHICH COMPRISES IN COMBINATION, A CRYSTALLIZING CONTAINER, A HEAT CONDUCTING ELEMENT SURROUNDING SAID CONTAINER AND BEING IN HEAT EXCHANGE RELATION THERETO, A HEAT INSULATING MEMBER SURROUNDING SAID HEAT CONDUCTING ELEMENT TO PREVENT HEAT RADIATION FROM THE LONGITUDINAL SURFACE OF SAID CRYSTALLIZING CONTAINER, AND HEATING MEANS SURROUNDING SAID HEAT CONDUCTING ELEMENT TO APPLY A UNIFORM TEMPERATURE GRADIENT ACROSS SAID CRYSTALLIZING CONTAINER, SAID HEAT CONDUCTING ELEMENT HAVING A TERMINAL END WHICH EXTENDS BEYOND ONE TRANSVERSE END OF THE HEAT INSULATING MEMBER, THE TERMINAL END OF SAID ELEMENT HAVING A LENGTH SUFFICIENT TO DRAW HEAT FROM THE HEATING MEANS FOR TRANSMITTING THE HEAT ACROSS THE LENGTH OF THE HEAT CONDUCTING ELEMENT.