US2998335A

US2998335A - Method and apparatusfor growing single crystals from molten bodies

Info

Publication number: US2998335A
Application number: US637388A
Authority: US
Inventors: Dehmelt Friedrich Wilhelm
Original assignee: Telefunken AG
Current assignee: Telefunken AG
Priority date: 1956-02-04
Filing date: 1957-01-31
Publication date: 1961-08-29
Anticipated expiration: 1978-08-29

Description

Aug. 29, 1961 F. w. DEHMr-:LT

METHOD AND APPARATUS FOR GROWING SINGLE CRYSTALS FROM MOLTEN BODIES Filed Jan. 3l, 1957 a Il. F

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INVENTOR: Y FRIEDRHCH WILHELM DEHMELT vl B United States Patent O many Filed Jan. 31, 1957, Ser. No. 637,388 Claims priority, application Germany Feb. 4, 1956 9 Claims. (Cl. 14S-1.6)

The present invention relates to a method for growing single crystals lfrom molten bodies and, particularly, from molten bodies of semi-conductors.

Methods of growing single crystals of various substances :have previously been k-nown. One method of growing a single crystal comprises melting a body of a substance and slowly withdrawing the single crystal from the melt with the `aid of a seed crystal. Another method is the sor-called zone-melting system, wherein a bar shaped body of a substance is associated at one end with a seed crystal and is caused to melt in a cross-sectional zone in the region adjacent to the seed crystal. This relatively narrow molten zone, which extends through the entire cross-section of the bar, is then 'allowed to travel in the longitudinal direction of the bar. This technique is usually carried out by placing the bar in a carbon or quartz crucible which is dimensioned to conform to the shape.

of the bar.

Although this method has the advantage over the iirst aforementioned method of yielding single crystals which have a substantially constant diameter and wherein the regions of impurity content are relatively ,evenlyv distributed, it has not been satisfactory because the sur-V faces of the crucible are apt to exercise a. detrimentaleffect upon the growth of the crystals and the lattice structure of such crystals.

The object of the present invention is to provide a method that overcomes these disadvantages and makes possible the production of single crystals which have throughout their length, or at least throughout the major portion thereof, a homogeneous gradient-free distribution of the regions of impurities and which possess throughout the length thereof a uniform cross-section.

The above aims of the invention are achieved by the combination of the two previously known methods described above. This is accomplished, according to the invention, by disposing |a charge of the semi-conductor material in the form of a bar within a crucible, then causing the melting of a cross-sectional zone of the bar in the region adjacent the seed crystal, yand. transferring the molten zone at a constant rate of speed.

along the bar. During this transfer of the molten zone, the seed crystal and the new crystal portion adhering thereto are withdrawn from the melt at a constant rate of speed, such that the crystal withdrawn from the melt no longer contacts the walls of the crucible.

Due to the simultaneous withdrawal of the crystal and the transfer of the molten zone along the bar-shaped base material, it is possible to grow a single crystal having a substantially uniform diameter throughout its entire length. This surprising result is achieved by an automatic thermal control of the crystal growing procedure in carrying out the method of the present invention which may be explained as follows:

Despite constant withdrawal and crucible transfer speeds and constant heat supply, it is not possible to obtain a constant temperature at the surface of the melt. It is, therefore, important to consider the conditions resulting from such small temperature fluctuations occurring at the surface of the melt. If the temperature at the surface of the melt drops below a normal value, i.e., if the melt surface is somewhat colder, the diameter of the growing crystal will become larger. Since its with# drawal speed lremains constant, it follows that the volume of molten material withdrawn from the melt during a given period will also be increased. This, in turn, causes the level of the melt surface to be lowered. The surface of the melt passes, therefore, into a region of higher temperature fas it is displaced in a direction toward the center of the heating coil. Thereafter, the increase of temperature at the surface of the melt will result in a decrease of the diameter off the crystal withdraum from the melt. From this, it appears that the fluctuation of temperature and the accompanying fluctuation in the diameter of the growing crystal at the surface of the melt is compensated by the fluctuation in the height of the melt Surface and, consequently, the temperature prevailing at this place. The melt surface is constantly displaced into a hotter or colder region,

so that the temperature fluctuations become equalized. The end result is that the fluctuation in the diameter, i.e., the size of the withdrawn crystal, is suppressed. and the crystal obtained has la constant diameter throughout.

Still further objects and the entire scope of applicabil ity of the present invention will become apparent from the detailed description given hereinafter', it should be understood, however, that the detailed description and specific example, while indicating the preferred embodiment of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those` skilled in the art from this detailed description.

In the drawing:

FIGURE 1 illustrates schematically a longitudinal section through an apparatus for carrying out the method according to the present invention;

FIGURE 2 is a schematic showing on an enlarged scale of a longitudinal section through la part of the apparatus of FIGURE l, illustrating the beginning of the withdrawing step;

FIGURE 3 is the same kind of showing as ElGUREl 2, illustrating a later phase of the operation than that.

shown in FIGURE 2.

Referring in more detail to the drawings, FIGURE 1 shows a substantially conventional crystal growing 'ap-- para-tus for making crystalline land semi-conductors wh1ch1 nected to terminals -6 of the coil 2. The crucible 1 is provided with a charge of material which, because of the considerable axial length of the Crucible, is in the form of a bar-shaped 'body 3. The material 3 is melted by means of the heating coil 2 in a relatively narrow regionA 4. A seed crystal 5, oriented in a particularly desired manner, is then immersed into the aforesaid mol-tenv region yil. As the operation described herein continues, the heating device 2, which maintains the melting zone 4 at a temperature preferably only slightly above the melting point of the material 3, is moved in `the direction of the arrow at a speed vs. which is usually held substantially constant, is thereby transferred slowly from one end of the body 3 to the At the same time, the seed crystal 5 is slowlyv withdrawn in the opposite direction -at a speed vk, so as to other.

grow a crystal portion of molten material attached to said seed crystal and correspondingly oriented thereby.

The melting zone, the extent of4 In order to obtain the most uniform heat distribution in the melting zone -4 and, particularly, to have it independent of the geometrical position of the heating coil 2, it is preferred -to have at least the seed crystal 5 or the crucible 1 with the charge 3 and 4 rotate about the axis 7. However, it is also possible to have both, the seed crystal 5 and the crucible 1, rotate in the same direction `about this axis 7, i.e., at the same angular velocity, since, by this expedient, the growing crystal would be withdrawn from a melt apparently in the state of repose and the crystals resulting therefrom would be particularly uniform.

The inventive method is especially suitable for the production of crystals of mixed composition, as for example those consisting of a low melting component, hereinafter refered to as base material, and a higher melting component, which will be refered to as the addition material. In this connection, germanium, having a melting point of 930 C., is contemplated as base material for use with silicon as addition material, having la melting point of l430 C. This addition material has a segregation constant greater than l in the melt. Therefore, the mixture Will have a higher melting point or solidiiication point than the pure base material. In this regard, it is also contemplated to use addition which, in turn, may be cornposed of several single components.

In the preparation of crystals of mixed composition, it is preferable -to use a seed crystal consisting of pure base material. This may be desirable for the reason that the melt, upon initiating the Withdrawal procedure, is `apt to depart in any event. `as far as its composition is concerned. from the melt in the later stages of the melting procedure.

It is a particularly important advantage of the above system that it permits the withdrawal of a crystal of mixed composition `and uniform growth in a single operation. This is especially difficult since the gradient of the laddition material concentration in the withdrawn crystal body must be everywhere only of small magnitudes. If it is too great at a given point, the crystal body withdrawn would necessarily be polycrystalline in nature.

Preliminary to starting to grow such a mixed crystal, an initial zone of the bar consisting of pure base material is first melted and the crystal is then withdrawn therefrom. It is essential in carrying out the inventive method, that the temperature gradient within the melt extends from ya maximum temperature in the center of the heating coil to a temperature at which the pure base material solidiiies. The maximum temperature will be chosen in such a way that the liquid aggregate condition is maintained, even with the final composition of the melt.

After the seed crystal has contacted the surface of the melt and is withdrawn therefrom, the new crystal portion is caused to grow in the zone where the solidiiication takes place. The portion of addition material incorporated with the melt is noW continuously increased from a zero value -to a constant end value. This takes place because the melting zone passes from the initial zone which consists only of pure base material into the region of the bar that includes the aforementioned addition material. The increase of the portion of -addition material in the melt is accompanied by an increase in its soliditication temperature. Since the solidication region lies in the temperature gradient which includes the appropriate temperature range, this solidication region will be displaced along the gradient into the range of higher temperature where it adjusts itself in accordance with the solidification temperature varying according to the content of the addition material. This will insure the most efficient crystallization.

After passing through the intermediate zone, the mixed crystal Vwill continue to grow at a constant rate which is accomplished by the incorporation of addition material in constantly uniform amounts based upon the diameter of the base material body. Moreover, it is important that the withdrawal speed, displacement of the crucible and heat output be maintained constant. There is, of course, no particular problem in maintaining a mechanical movement constant. However, special care has to be taken fto maintain the heat output at a constant value. For lthis reason, such values yare preferably adjusted in the initial melting zone for the pure base material and at the beginning of the withdrawal operation. This will compensate for irregularities and changes in the temperature distribution encountered 4in the wi-thdrawal of the -mixed crystal portion proper. Thus, at the beginning the melt volume is somewhat larger, since the level of the melt will extend along the temperature gradient beyond the heating coil.

A preferred way of incorporating the addition material is to introduce it through a bore or like opening hollowed out of the base material bar, as shown in the examples in FIGURES 2 and 3 of the drawing. No addition material is present in the initial melt zone. With a view to obtaining a continuous transfer of the melt composition in the intermediate zone, the addition material will be present in the semi-conductor bar in the form of a cone.

FIGURE 2 shows the beginning of the withdrawal operation in the case of a mixed crystal. The crucible 1 is again surrounded by a heating coil 2 which is to melt only a certain zone of the crucible content. The material to be melted is placed in the crucible 1 in the form of a bar-shaped body 3 consisting of base material. This body, which has a cylindrical bore having a pointed head, contains a rod-like body 8, consisting of addition material, inserted in said bore. At the beginning of the withdrawal operation, the initial melting zone consisting of pure base material 4 is caused to melt. It will further expand, due to the low melting point of the base material, and extend somewhat beyond the heating coil 2 into a colder region where, at its melting temperature, the seed crystal 5 is applied. The highest temperature prevails at the center of the heating coil 2. It decreases inthe direction of the axis of the crucible 1 which coincides with the direction of withdrawal.

FIGURE 3 shows the continuation of the withdrawal operation after the stationary condition is achieved. The solidication region has now passed into a hotter zone and the mixed crystal 5 lwill grow in this zone. In the melting zone, the constant heat output and the constant movement will cause the melting of both the base material 3 and the addition material 8 at a uniform rate. The growth of the mixed crystal under such stationary condition is, therefore, necessarily homogeneous and gradientfree.

I claim:

1. A method for obtaining a single crystal from a melt with the aid of a seed crystal, comprising the steps of placing a semi-conductor starting material in the form of a bar in a crucible, melting said bar so as to establish a melting zone in the upper cross-sectional region of the bar, contacting said upper cross-sectional region of the bar with a seed crystal, moving said melting zone downwardly along said bar at a constant speed, and withdrawing said seed crystal by moving the same upwardly at a constant speed together with a new crystal portion adhering thereto, said withdrawal of said crystal being concurrent with the movement of said melting zone, whereby the crystal withdrawn from the melt is out of contact with the walls of the crucible.

2. A method according to claim 1, characterized in that the heat output is maintained at a constant value during the withdrawal operation.

3. A method according to claim l, characterized in that for the purpose of obtaining a mixed crystal the composition of the melt s continuously modified during the withdrawal operation by the successive incorpo ration of an addition material.

4. A method according to claim 1, wherein between the seed crystal and the melt at least one is caused to rotate about its longitudinal axis.

5. A method according to claim 3, wherein the addition material necessary for obtaining a mixed crystal is incorporated in said bar-shaped starting material in the form of a solid rod-shaped body having a pointed head and located below the initial upper end of said bar, whereby the end of said addition material lying in the direction of the melting zone has a diameter which continuously decreases until it reaches zero.

6. A method according to claim 3, wherein the addition material is incorporated in said semi-conductor bar below the initial upper end of said bar, whereby the melted zone at the beginning of the withdrawal operation is free of addition material.

7. A method according to claim 3, wherein said addition material is tted into an opening provided in said 20 semi-conductor bar.

6 8. A method according to claim 3, wherein the da sociation coeicient of said addition material with respect to the base material is greater than 1.

9. A method according to claim 3, wherein between the seed crystal and the melt at least one is caused to rotate about its longitudinal axis.

References Cited in the tile of this patent UNITED STATES PATENTS 1,937,065 Moore Nov. 28, 1933 2,375,107 Hopkins May 1, 1945 2,493,951 Druyvesteyn et al. Jan. 10, 1950 2,665,318 Herres Jan. 5, 1954 2,747,971 Hein May 29', 1956 2,768,914 Buehler i Oct. 30, 1956 2,904,512 Horn Sept. 15, 1959 FOREIGN PATENTS 522,097 Belgium Aug. 31, 1953 1,127,036 France Aug. 6, 1956 754,767 Great Britain Aug. 15, 1956