US3298795A

US3298795A - Process for controlling dendritic crystal growth

Info

Publication number: US3298795A
Application number: US353709A
Authority: US
Inventors: Donald R Hamilton; O'hara Sydney; Raymond G Seidensticker
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1964-03-23
Filing date: 1964-03-23
Publication date: 1967-01-17
Anticipated expiration: 1984-01-17
Also published as: GB1043867A; AT255485B

Description

n. R. HAMILTON ETAL 3,298,795

I PROCESS FOR CONTROLLING DENDRITIC CRYSTAL GROWTH Filed March 25, 1964 Jan. 17, 1967 2 Sheets-Sheet 2 HEATING cou.

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" 4 All I Ha -Q G WL n u A H 5 v 2 3 D L H S G W L O O C COOLING COILS United States Patent 3,298,795 PROCESS FOR CONTROLLING BENDRITIC CRYSTAL GROWTH Donald R. Hamilton, Monroeville, and Sydney OHara and Raymond G. Seidensticker, Pittsburgh, Pa., as-

signors to Westinghouse Electric Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Mar. 23, 1964. Ser. No. 353,709 6 Claims. (Cl. 23301) This invention relates to apparatus and process for controlling the thickness and width of dendritic crystals grown from supercooled melts.

One method of producing members of semiconductor materials suitable for use in making solid state components is by dendritic crystalline growth from supercooled melts. This method is described in detail in Patent No. 3,031,403 of Allan I. Bennett, Jr., and consists, briefly, of supercooling a melt of the semiconductor material and immersing into the melt a seed crystal of suitable structure and withdrawing the crystal at a rate commensurate with the rateof crystal growth. By this method crystals of many feet in length may be grown. It is highly desirable to be able to control the width and thickness of the dendritic crystal in order to achieve the best possible semiconductor material for specific uses.

In accordance with the present invention it has been discovered that the Width and thickness of dendritic crystals may be controlled to a very significant extent by manipulation of the temperature of the melt. If an inverted thermal gradient is created near the surface of the melt such that within the region of dendritic growth the temperature at the surface of the melt is higher than that below the surface of the melt, a growth pattern will be established which is highly responsive to control by temperature manipulation in that if the temperature is increased slightly the dendrite will become thinner and if the temperature is subsequently restored to its original value the dendrite will then widen without thickening noticeably.

It is especially desirable to be able to produce dendritic semiconductor material in thinner and wider forms in that the thinner material is generally more uniform in electrical properties throughout its cross section and the wider material offers certain practical advantages in fabricating solid state devices.

Accordingly, it is an object of the invention to provide a method for controlling the thickness and width of semiconductor material dendritic crystal growths produced by continuous withdrawal from a supercooled melt, the control being achieved by thermal means.

Another object of the invention is to provide an apparatus suitable for thermally controlling the width and thickness of semiconductor material dendritic crystal growths in supercooled melts.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

Briefly, one aspect of the invention resides in a method of controlling thickness and width of a dendritic crystal growth in a supercooled melt which is accomplished by (1) effecting an inverted thermal gradient, near the surface of the melt, where the dendritic crystal growth is occurring such that the temperature at the surface of the melt is higher than that beneath the surface, and (2) manipulating the temperature values along this inverted thermal gradient such that in order to decrease the thickness of the crystal the temperature is increased and to increase the width of the crystal the temperature is subsequently cooled or restored to its original level.

For a better understanding of the nature and objects of the invention reference should be made to the following detailed description and drawings in which:

FIGURE 1 is a schematic elevation, in cross section, of a furnace apparatus;

, FIG. 2 is a greatly enlarged elevation, partly in cross section of a crystal exhibiting a twin plane;

FIG. 3 is a greatly enlarged elevation, partly in cross section of a crystal exhibiting three twin planes;

FIG. 4 is an elevation, greatly enlarged, of a growing dendritic crystal before the thickness control step of the process;

FIG. 5 is an end view of FIG. 4 taken along line V--V;

FIG. 6 is an end view elevation, greatly enlarged, of a growing dendritic crystal of the thickness control step of the process;

FIG. 7 is a schematic elevation, partly in cross section, of an apparatus embodying the invention;

FIG. 8 is a schematic elevation, partly in cross section, of an apparatus embodying the invention.

In practicing the method of the Bennett Patent 3,031,403, in producing dendritic crystals by continuous withdrawal from a superc-ooled melt of the semiconductor material an apparatus is commonly used such as that il-,

lustrated in FIG. 1. The apparatus 10 comprises a base 12 carrying a support 14 for a crucible 16 of a suitable refractory material such as graphite to hold a melt of the material from which fiat dendritic crystals are to be drawn. Molten material 18, for example, germanium is maintained within the crucible 16 in the molten state by a suitable heating means such as an induction heating coil 20 disposed about the crucible. Controls, not shown, are employed to supply an alternating electrical current to the induction coil 20 to maintain a closely controllable temperature in the body of the melt 18. The tempera ture should be readily controllable to provide a temperature in the melt a few degrees above the melting point and also to reduce the heat input so that the temperature drops in a few seconds for example in 5 to 10 seconds to a temperature of at least one degree below the melting temperature and preferably to supercool the melt from 2 to 10 C. A cover 22 is positioned on top of the crucible 16 to maintain the thermal gradient above the melt. Passing through an aperture 24 in the cover 22 is a seed crystal 26, preferably having three, or a plural odd num- 'ber, of twin planes and oriented crystallographically as is described in detail in Patent 3,031,403. The crystal 26 is fastened to a pulling rod 28 by suitable means such as a screw 30. The pulling rod 28 is actuated by a suit able mechanism to control its upward movement at a desired uniform rate, or usually in excess of one inch per minute. A protective enclosure 32 is disposed about the crucible with a cover 34 closing the top thereof except for a suitable aperture 36 through which the pulling rod and dendrite pass.

Within the interior of the enclosure 32 is provided a suitable protective atmosphere entering through a conduit 40 and, if necessary, a vent 42 may be provided for circulating a current of such protective atmosphere.

Referring to FIG. 2 of the drawing there is illustrated, in greatly enlarged view, a section of a crystal 26 having a single twin plane. Such a crystal with a single twin plane is not suitable for dendritic crystal growth, but the description thereof is given to facilitate the subsequent description.

The crystal 26 comprises two relatively flat faces 50 and 52 with an intermediate interior twin plane 54. Examination will show that the crystallographic structure of the seed on both faces 50 and 52 is that indicated by the crystallographic direction arrows at the left and right faces, respectively, of the figure.

It will be noted that i the horizontal directions perpendicular to the flat faces 50 and 52 and parallel to the melt surfaces are 1ll The direction of growth of the dendritic crystal will be in a 211 crystallographic direction. If the faces 50 and 52 of the dendritic crystal 26 were to be etched preferentially to the {111} planes they will both exhibit equilateral triangular etch pits 56 whose ver-tices 58 will point upwardly while their bases will be parallel to the surfaces of the melt. It is preferred in practicing the Bennett method that the etch pits on both faces 50' and 52 of the seed crystal 26 have their vertices 58 pointing upwardly. A non-twinned crystal or a crystal containing two twin planes, or any even number thereof, will exhibit triangular etch pits on one face whose vertices will be pointing opposite to the direction of the vertices of the other face.

Referring to FIG. 3 of the drawing there is illustrated a highly magnified portion of a seed crystal 126 which contains three

twinned planes

154, 156 and 158 extending across the entire cross section thereof. The

faces

150 and 152 have the same crystal orientation as the faces 50 and 52 of FIG. 2. The spacing or lamellae between the successive adjacent twin planes ordinarily are not uniform. The lamellar spacing, such as (A) between

twin planes

154 and 156 and (B) between

twin planes

156 and 158 is of the order of microns, that is from a fraction of a micron to 15 to 20 microns or greater. The ratio of A to B may vary from 1 to 20 or more.

As explained in detail in Patent 3,031,403 seed crystals suitable for the derived elongated dendritic growth must exhibit a plurality of twin planes. It is preferable, how ever, that the seed contain an odd plurality, that is 3, 5, 7, etc., of twin planes. The resultant dendrite will generally exhibit a propagation of the twin planes from the seed crystal. A seed crystal may be obtained in various ways, for example, by super-cooling a melt of the solid material to a temperature at which a portion thereof solidifies, at which time some dendritic crystals having one or more internal twin planes will be formed and may be removed from the melt. While these crystals may not be uniform, they are suitable for seed purposes. Also one can cut from a large twinned crystal a section suitable for use as a seed crystal.

The present invention resides in a method and apparatus for controlling the thickness and width of the dendrites as they are grown by continuous withdrawal from a supercooled melt. The method basically consists of establishing an inverted temperature gradient in the vicinity of the surface of the melt and superimposing in this vicinity additional temperature controls. These controls assure that if the temperature is slightly increased the thickness will decrease and if the temperature is then subsequently decreased, or restored to its original value, the width of the growing dendrite will be increased to a very substantial extent without any appreciable increase in thickness. Thus, the dendritic crystal may be rendered thinner and wider.

The following explanation is offered in order to understand better how dendritic growth occurs and can be thermally controlled by the practice of the invention. Normally when a dendrite is grown in a supercooled melt by a process such as in the Patent 3,031,403 it is increased simultaneously in both thickness and width, that is, the growth is two dimensional, along its entire length of immersion in the melt. Without any inverted thermal gradient a crystal of the type shown in FIG. 4, with three distinct regions of crystal growth is observable in such a case. The innermost, or tip portion exhibits no faceting and is identified as region III. The intermediate or transition portion exhibits considerable faceting such as 312 and 314 and growth in both thickness and width and is identified as region II. The surface, or H-arm, region exhibits a marked increase in the width growth rate and is identified as region I. Thus in region I the dendrite is growing in width at a much faster rate than in thickness as opposed to region II where the width and thickness growth rate are much more comparable. However, this situation is somewhat changed when the thermal gradient within the dendrite growth regions is inverted such that the temperature near the surface of the melt is higher than in that area just beneath the surface of the melt where dendritic growth can occur. When the inverted thermal gradient is established at the surface of the melt three distinct growth crystal regions are again ob servable. However, the principal distinction is observed in region I which exhibits a marked change in that, while the high width growth rate is retained, thickness increases are very little, if any. Thus, as shown in FIGURES 4 and 5, a dendritic crystal 310 is growing in an inverted thermal gradient and has the three regions of growth. In region I the dendritic crystal grows very appreciably in width without any appreciable change in thickness. In region III, the tip region, the dendritic crystal does not exhibit faceting. In region 11 pairs of facets, forming corners, such as 312 and 314, are added to the dendritic crystal and it is here that the dendritic growth occurs in both directions, that is thickness and width. As is more clearly indicated in the side View of FIGURE 5 the dendritic crystal growth occurs in discrete relatively large thickness increments, each face such as 320 or 326 in FIGURE 5 being a perfect {111} plane and growing over the face below it. It should be noted that these thickness increasing steps occur only in region 11 of the dendritic crystal growth. The thickness increasing steps that may occur in region I are much less frequent and are of the order of 10-400 Angstrom units per cm. The dendritic crystal is widened in both regions I and II, with the greatest amount of widening occurring in region I.

Our analysis of the growth patterns in regions I and II reveals certain interesting and highly useful relationships. Since the dendritic crystalline growth is not appreciably increased in thickness in region I, twodimensional nucleation, such as that indicated by 331) in FIGURES 4 and 5, does not occur to any appreciable extent in region I. Since in region II the dendritic growth is such that thickness is being increased, the temperature in this melt region is such that the growth of a step such as 324 or 325 in FIGURE 5, over a {111} surface, 326 or 320 in FIGURE 5, occurs at an appreciably greater velocity. Thus, the level of melt super-cooling in region II is such that the growth velocity of a step over a {111} surface is appreciable; this supercooled temperature level at region II may be defined as T Since the dendritic crystalline growth appreciably increases in width in region I, the degree of melt supercooling present in region I is such that the nucleation rate in a re-entrant corner such as 316 in FIGURE 4, formed by the intersection of two {111} planes is appreciable. The melt temperature level in region I is defined as T and is a higher temperature value than T It should be noted in this respect that re-entrances exist in region II, for example 318 is a re-entrancy in region II, and the width of the dendritic crystal is appreciably increased in region II. Thus the supercooling necessary to promote appreciable growth of a thickness increment or step such as 324 over the 111} sub-surface is greater than the degree of supercooling required to promote growth in the re-entr-ancy such as 316 in FIG. 4. If the melt temperature about region 318 is T then where T is the normal melting point of the material. Therefore T T Thus T is lower, at least to some extent, than T The T and T melt temperatures are both below the normal melting point of the material forming the melt.

At this point it is well to note that the various melt temperature levels defined and discussed herein refer to the melt temperature, some distance apart from the actual dendrite-melt interface. These interface temperature values vary from the melt temperature values and are impractical to measure. However, the melt temperature relationships disclosed herein and their pronounced elfect on crystal growth are observable and entirely valid.

FIGURES 4 and 5 indicate an increment in thickness 330 which might be added in region I if the temperature were reduced to the point where spontaneous two dimensional nucleation would occur. The degree of supercooling to accomplish such is much greater than that necessary to accomplish the re-entrancy growth 316 in FIGURE 4, and the thickness increment propagation 324 in FIGURE 5. The temperature at which such spontaneous two dimensional nucleation would occur is defined as T and is considerably lower than T or T Summarizing the temperature observations four temperatures may be defined:

T is the melting point of the material forming the melt.

T is the melt temperature at which the growth velocity of a step over {111} surface occurs at an appreciable rate.

T is the melt temperature at which the nucleation rate in a re-entrant corner formed by two {111} planes is appreciable.

T is the melt temperature at which spontaneous two dimensional nucleation on an already completed {111} plane occurs at an appreciable rate.

The following relationship of these temperatures is observed:

Keeping in mind the above temperature relationships it is to be noted that for an increment of thickness to be added to the dendrite region I, two dimensional nucleations such as 330 must occur on face 332. If the temperature in region I exceeds the value T such will not occur at any significant rate. That is, the level of supercooling in region I would be below that required to achieve the spontaneous two dimensional nucleation which would increase the thickness of the dendrite in region I.

If the temperatures in both regions, I and II, lie between the values T and T (T T T then the dendrite will continue to grow to thickness t If it is now desired to reduce the thickness of the dendrite from t to 1 in region I as illustrated in FIGS. 5 and 6, then the rate of growth of the planes at steps 325 in FIGURE 5 must be reduced or halted. In accordance with the invention such might be accomplished by increasing the temperatures T and T to reduce the degree of supercooling. As indicated above, a temperature greater than T will also exceed T Accordingly rate of growth will also in all probability be equally reduced or cease at the re-entrant corners, such as 316 in FIGURE 4, and the width of the resulting dendritic crystal may be reduced from an original width W to a lesser width W If after a period of time sufficient to eliminate the growth of the last thickness increment 325 of thickness t from the melt as indicated by FIGURE 6, the temperatures in regions I, II and III are returned to the original condition of inverse temperature and degree of supercooling such that they fall between T and T (i.e. T T T then the dendritic crystal will continue to grow at the thickness t but will widen appreciably to a. width determined for the most part by the temperature levels in regions I and II and the time allowed for growth. Time is an important factor since, generally speaking, the longer the growing dendritic crystal remains in a region where T T T the wider it will grow without thickening.

It should be noted that the temperatures T T and T are not rigorously constant as is T but rather depend on factors such as the time allowed for nucleation or growth to occur, that is the pull rate,- and the volume or area where the growth rate will occur. Since only T is fixed and T T and T depend on pull velocity, the temperature changes required for dimensional control may be accomplished not only by changing the absolute melt temperatures, but also by changing the pull velocity which changes the efiective values of T T and T Thus, if a dendrite is being pulled at a selected rate, for example, of 2.5 mm. per second, and has a given width and thickness, by increasing the pull rate, e.g. to 3.5 mm.' per second, the thickness of the dendrite will decrease substantially, for instance up to 25%, then after a period of seconds or minutes or even longer, the pull rate is then decreased to either the original pull rate of 2.5 mm. per second or even less, the dendrite produced at this last pull rate will retain the decreased thickness but the width will be increased up to 50% or even more. The operator by effecting these changes on the respective pull rate has produced controlled variations in the effective temperature distribution and thereby attained desired dendrite width and thickness. For example, dendrites of germanium of widths up to 2 to 3 millimeters and thickness of from 0.1 to 0.2 millimeters are regularly grown by following this practice.

It is to be understood that the above theoretical explanation and illustration of the phenomena governing crystal growth control in accordance with the invention is principally an idealized illustration and is not intended necessarily as a limitation of the invention. For example, since the term inverted gradient is relative, this invention can be practiced with a melt which is isothermal within 0.1 to 0.2 0, especially if T is much less than T which in turn is much less than T The better practice, however, would be to have an inverted temperature gradient. By the practice of this invention the control of the thickness and width of a dendritic crystal growing and being continuously withdrawn from a supercooled melt is an observable phenomenon. This is attained by the combination of (1) establishing an inverted thermal gradient in the vicinity of the surface of the melt where the dendritic crystal growth is occurring and (2) manipulating the temperature by first reducing it to provide a given degree of supercooling and thereafter increasing it to provide a smaller degree of supercooling, and finally returning to essentially the original degree of supercooling as disclosed herein. This provides a novel method of achieving improved control of width and thickness dimensions by thermal variations.

Various means and apparatus may be employed to achieve the inverted thermal gradient in the vicinity of the surface of the melt where the dendritic crystal growth occurs. One suitable apparatus 410 is shown in FIGURE 7 where 414 is a crucible of a suitable refractory material such as graphite which holds the melt 415 of a material such as silicon or germanium from which a dendritic crystal is to be drawn. A heat retaining cover 416 with a central aperture closes the top of the crucible. The molten material, for example germanium, is maintained within the crucible in the molten state by a suitable heating means, for example an induction heating coil 412 having turns disposed about and above the crucible. No external heat is provided at the bottom 430 of the crucible which functions as a heat sink or heat dissipating means. An external radiation shield 434 may be provided and may offer some advantages in heat retention in the melt and in minimizing the thermal gradient along the length of the dendrite after it is withdrawn from the melt. A portion 432 of the shield may be adjustable to aid in controlling heat dissipation from the crucible bottom 430. Thus heat is supplied to the crucible at the top and sides thereof and heat is dissipated at the bottom thereof. This creates an inverted temperature gradient such that the temperature at the surface of the melt is higher than at a point below the surface of the melt. Isotherms such as 420 and 422 indicated in FIGURE 7 illustrate the general temperature pattern in this type of apparatus where isotherm 421i is the highest and 422 and the others below are progressively lower. The temperature at and near the surface of the melt, such as indicated by isotherm 420, is higher than in those portions beneath it.

In a preferred embodiment of the apparatus of the invention, a heat dissipating means may be provided at some point other than the bottom, that is at some point along the side of the crucible. Such an apparatus is illustrated in FIGURE 8. The groove 52% reduces the efi'ective heating thereof by the coil 530 so that heat is dissipated by radiation at the groove. The advantages of using such an apparatus are that the desired inverted thermal gradient is restricted to the vicinity of the dendritic crystal growth as opposed to extending all the way to the bottom of the crucible. In the FIGURE 7 apparatus it is possible in some cases that the temperature at the bottom will be so low as to result in spontaneous nucleation resulting in the melt freezing. This potential difiiculty is alleviated by the apparatus illustrated in FIGURE 8.

Another advantage of the apparatus illustrated in FIG- URE 8 is that the position of the heat dissipating means along the length of the crucible that is with respect to the depth of the melt, may be varied, for example by varying the position, size and number of collars such as 535, 537 and 539, thus to some extent affording an additional control on the dendritic crystal growth. That is, if the heat dissipating means, the circumferential groove in FIGURE 8, is lowered slightly below the center of the melt this will tend to elongate the depth of regions I and II thus enhancing the probability of achieving wider dendritic growths.

Preferably, the heating means, such as radio frequency induction heating coils 526 and 528 are disposed at the top and bottom of the crucible. This is especially important where the crucible and melt depth are small since it ensures a symmetrically desirable inverted heat flow pattern into the top and bottom of the melt and out through the groove heat dissipating means on the side of the melt. Additional heating coil elements such as 530 and 534- may be furnished about the sides of the melt, but in most cases will not be necessary adjacent the heat dissipating means, groove 520.

Various means may be utilized as the heat dissipating means. For example, in the FIGURE 8 apparatus cooling coils 552 or other heat absorbing facilities may be provided in lieu of or in combination with the groove 520. Further, many schemes of achieving the inverted temperature gradient in practicing the invention Will suggest themselves to those skilled in the art. For example, merely heating at the surface will probably induce the inverted gradient without any specific heat dissipating provision. In most cases a mere absence or decrease of heat input to a given portion of a crucible renders it a heat dissipation site.

The thermal conditions in the melt induced by the crucible geometry indicated in FIGURE 8 are illustrated by the isotherm lines such as 542 and 544. The isotherms represent higher temperatures at the top and bottom of the melt with the isotherm such as 546 in the vicinity of the heat dissipating means representing lower temperatures. That is, the temperatures at 542 and 554 exceed those at 546. For example the temperature at the surface of the melt may be one-half or one degree centigrade below the normal melting point of the melt and the temperature level at isotherm 546 in the vicinity of the heat dissipating means may be at a level of about 4 or 5 C. below the normal melting point of the melt. A thermal saddle point 545 is exhibited near the center of the melt.

Means for controlling the heat input to the crucible must be provided such that the temperature manipulation discussed earlier may be achieved. Various means may be employed for such as are well known to those practicing the art. For example, in FIGURE 8 the principal heating means may be disposed at the top and bottom of the crucible that is heating tube coils 526 at the top and 528 at the bottom of the crucible provide the main heating source. Auxiliary heating tube coil such as 530 and 531 at the sides of the crucible may be utilized for fine temperature control effects and may be independently controlled, that is 530 may be controlled independently of 526. An external radiation shield such as 522 may be provided and may ofier some advantages in heat retention in the melt and in minimizing the thermal gradient along the length of the dendrite after it is withdrawn from the melt. The shield may be provided with port holes 521 to aid heat dissipation in the vicinity of the groove 520.

A better understanding of the method of the invention is best achieved by reference to a specific illustrative example as follows. In apparatus such as that generally used in producing dendritic crystalline growths by continuous withdrawal thereof from a supercooled melt, such as that illustrated in FIGURE 1 but as modified by utilizing a crucible as illustrated in FIGURE 8 as to the heat input and heat dissipating means, a supercooled melt of germanium is prepared. The melt is first heated to a point above its melting point of approximately 935 C., for example it may be heated to 940 C., and then cooled gradually to a point approximately 2 to 5 C. below the melting point at the coldest point in the melt which is near the heat dissipating means. A suitable seed crystal, preferably containing three twin planes is prepared. In order to properly orient the seed, it may be preferentially etched and the triangular etch pits revealed by the etching oriented with their vertices directed upwardly and bases parallel to the surface of the melt. The seed is attached by suitable means such as a screw to a pulling rod such as 28 in FIGURE 1. The seed crystal is immersed to a depth of about one-half millimeter for a period of time in the order of about one to five seconds and then withdrawn at a rate of about .05 to .5 centimeter per second. It should be noted that these parameters of depth, time and pulling rate are relative and are determined by trial and error for the particular melt temperatures and apparatus employed. For a better understanding of the factors involved in establishing continuous dendritic growth reference should be made to U.S. Patent 3,031,- 403. Once dendritic growth has been achieved and the dendrite is being withdrawn from the melt its thickness and width are observed and measured. In this specific example the thickness is observed to be ,2 millimeter and the width is observed to be only .1 millimeter that is, the dendritic growth is twice as thick as it is wide, the width being the dimension parallel to the twin planes. At this point the temperature is increased by increasing the power to the coils above, and, if desired, to the coils below the crucible. The temperature is increased to an extent of about only one-fourth to one-half degree centigrade. The crystal dendrite is continually withdrawn at the same pull rate for a period of time of about 5 to 10 seconds. At this point it will be observed that the dendrite has decreased in thickness to a value of about .07 to .1 millimeter thickness. The operator of the apparatus may vary the temperatures and thepull rates to some degree, using care that the dendritic growth is not terminated.

It will be observed that while the thickness of the dendrite has diminished to a value of .07 to .1 millimeter thus rendering it more suitable for semiconductor applications, the width also may have diminished to a point less than .1 millimeter, its original value. In order to widen the dendritic crystal the temperature is lowered such that it is restored, that is cooled to a point somewhere near its original value. This wall permit the dendritic crystal to widen, particularly in growth region I, without any appreciable increase in thickness over the .07 to .1 millimeter value achieved earlier. That is, referring now to FIGURE 6, the last increment 325, or

increments, in thickness have been withdrawn from the melt. Thus the temperature may be decreased to any point substantially above the point at which spontaneous two dimensional nucleation will occur on a completed {111} surface (temperature T without any appreciable increase in thickness occurring on the dendritic growth. However, the dendritic crystal will widen to an appreciable extent especially in region I. In such a manner the dendrite may be widened to about 3 millimeters.

If desired, the steps may be repeated. After the temperature has been restored to its original level and the dendrite has widened appreciably the temperature may again be increased so that an additional thickness decrease occurs. This repetition of temperature manipulations will serve to restore the diminishing widths as the thickness is diminished. For example, in the above illustrative discussion relative to controlling the width and thickness of a germanium dendrite it is possible that the width might decrease excessively before the desired decrease of thickness is obtained. In extreme cases this couldresult in a termination of the crystal growth. Thus, there will be cases where the temperature manipulation, that is, the increase and subsequent decrease in temperature will be repeated a number of times in order to achieve the final desired dendritic dimensions.

In a manner very similar to that in which germanium is grown in dendritic form by a continuous withdrawal from a supercooled melt, similar crystals of indium antimonidemay be grown. Generally the parameters of growth are very similar to the case of germanium except that indium antimonide melts at a much lower temperature, about 525 C. In practicing the invention in such an operation the parameters above mentioned as typical for the germanium case would also be typical for the case of indium antimonide. Similarly many other materials may be grown from supercooled melts and controlled dimensionally by the practice of the invention.

It should be noted that the particular parameters, the degree to which temperature is manipulated and the time for which the temperature deviation is maintained will vary greatly depending on the particular material from which the crystal is grown together with the pull rate, crucible design and size, and various other considerations. Thus, for a given system the operating parameters will have to be determined by trial and error in accordance with the guide lines disclosed herein.

The above description is to be considered as illustrative of and not in limitation of the invention.

We claim as our invention:

1. In the process of producing elongated bodies of dendritic material of selected width and thickness by continuous withdrawal of dendritic crystals from a supercooled melt of the material, the improvement comprising the steps of (1) initiating dendritic crystal growth from the melt,

(2) establishing an inverted temperature gradient in the melt so that the surface where the dendritic crystal leaves the melt is hotter than portions of the melt directly below wherein the melt is supercooled to a selected degree of supercooling about and adjacent to regions of dendritic crystal growth, whereby at a given rate of crystal withdrawal from the melt a dendritic crystal of a given width and thickness is produced, said given thickness being greater than said selected thickness,

(3) increasing the temperature of the melt in the region of dendritic crystal growth to provide a lesser degree of supercooling for a period of time while maintaining an inverted temperature gradient, and while maintaining substantially the given rate of dendritic crystal withdrawal whereby there results a decrease of the thickness of the dendritic crystal being withdrawn from the melt to the selected thickness, and

(4) thereafter decreasing the temperature of the supercooled portion of the melt to provide a greater degree of, supercooling while maintaining the inverted temperature gradient and while maintaining substantially the given rate of withdrawal so that the width of the dendritic crystal increases to the selected width without substantially increasing the thickness over the selected thickness.

2. The process of claim 1 wherein steps (3) and (4) are repeated at least once until a dendritic crystal of the selected width and thickness is withdrawn from the melt.

3. In the process of producing semiconductor material by continuously withdrawing dendritic crystals thereof from a melt of the said semiconductor material supercooled in the region of crystal growth to a given temperature the improvement for controlling the width and thickness of the dendrite comprising the steps of (1) inserting a seed into the region of maximum supercooling and withdrawing it at a substantially constant rate,

(2) establishing an inverted temperature gradient in the region of the crystal growth so that the surface of the melt is hotter than the melt below the surface adjacent the region of crystal growth, whereby a dendrite of given thickness and width grows,

(3) increasing the temperatures within the supercooled range suitable for dendritic crystal growth adjacent the region of crystal growth for a period of time but maintaining an inverted temperature gradient whereby the thickness of the dendrite being withdrawn decreases, and

(4) thereafter decreasing the temperature adjacent the region of crystal growth to substantially the given supercooled temperature values along the inverted temperature gradient of step (2), the temperature of the melt about the dendrite adjacent the surface has a degree of supercooling so that the dendrite will grow in width but not substantially in thickness, whereby the width of the dendrite increases without any substantial increase in thickness of the dendrite.

4. The process of claim 3, wherein the rate of withdrawal of the dendritic crystal from the melt during steps (1), (2), (3) and (4) is substantially constant.

5. In the process of producing semiconductor material by continuously withdrawing dendritic crystals at a selected pull rate thereof from a melt of the said semiconductor material supercooled to a given temperature in the region of dendritic crystal growth the improvement for controlling the width and thickness of the dendrite comprising the steps of (1) initiating growth of a dendritic crystal from the melt,

(2) establishing an inverted temperature gradient in the region of the crystal growth so that the surface of the melt is hotter than the melt below the surface adjacent the region of crystal growth, whereby a dendrite of given thickness and width grows at a given pull rate,

(3) increasing the pull rate of the dendrite while maintaining the inverted temperature gradient and degree of supercooling whereby the effective temperature distribution is changed and thereby the thickness of the dendrite being withdrawn from the melt is decreased, and

(4) thereafter decreasing the pull rate of the dendrite while maintaining the inverted temperature gradient and degree of supercooling whereby the effective temperature distribution is again changed and the dendrite now being grown is of increased width and having the said decreased thickness.

6. The process of producing elongated dendrites of selected width and thickness by continuous withdrawal of dendritic crystals of a semiconductor material from a supercooled melt of the material comprising the steps of (1) establishing a supercooled region of the melt with an inverted temperature gradient in such region such that the surface of the melt is hotter than the portions of the melt directly below,

(2) initiating dendritic crystal growth from the supercooled melt having the inverted temperature gradient by immersing a seed into the melt whereby dendritic growth occurs, the 'dendritic growth beginning at a portion within the melt having a higher degree of supercooling than the surface, and the dendrite characterized by substantially flat surfaces and re-entrant angles at the sides, and

(3) withdrawing the growing dendrite at a rate such forms a crystal of a given thickness and width while the melt solidifies on the dendrite on the surface portion of the melt substantially only at the reentrant angles on the sides so that a wide dendrite is produced but substantially none of the material deposits on the flat faces of the dendrite because of the lower degree of supercooling.

References Cited by the Examiner FOREIGN PATENTS 650,166 10/1962 Canada.

NORMAN YUDKOFF, Primary Examiner.

that the growing tip of the dendrite in the material 15 HINES Assistant Examiner-

Claims

1. IN THE PROCESS OF PRODUCING ELONGATED BODIES OF DENDRITIC MATERIAL OF SELECTED WIDTH AND THICKNESS BY CONTINUOUS WITHDRAWAL OF DENDRITIC CRYSTALS FROM A SUPERCOOLED MELT OF MATERIAL, THE IMPROVEMENT COMPRISING THE STEPS OF (1) INITIATING DENDRITIC CRYSTAL GROWTH FROM THE MELT, (2) ESTABLISHING AN INVERTED TEMPERATURE GRADIENT IN THE MELT SO THAT THE SURFACE WHERE THE DENDRITIC CRYSTAL LEAVES THE MELT IS HOTTER THAN PORTIONS OF THE MELT DIRECTLY BELOW WHEREIN THE MELT IS SUPERCOOLED TO A SELECTED DEGREE OF SUPERCOOLING ABOUT AND ADJACENT TO REGIONS OF DENDRITIC CRYSTAL GROWTH, WHEREBY AT A GIVEN RATE OF CRYSTAL WITHDRAWAL FROM THE MELT A DENDRITIC CRYSTAL OF A GIVEN WIDTH AND THICKNESS IS PRODUCED, SAID GIVE THICKNESS BEING GREATER THAN SAID THICKNESS, (3) INCREASING THE TEMPERATURE OF THE MELT IN THE REGION OF DENDRITIC CRYSTAL GROWTH TO PROVIDE A LESSER DEGREE OF SUPERCOOLING FOR A PERIOD OF TIME WHILE MAINTAINING AN INVERTED TEMPERATURE GRADIENT, AND WHILE MAINTAINING SUBSTANTIALLY THE GIVEN RATE OF DENDRITIC CRYSTAL WITHDRAWAL WHEREBY THERE RESULTS A DECREASE OF THE THICKNESS OF THE DENDRITIC CRYSTAL BEING WITHDRAWN FROM THE MELT TO THE SELECTED THICKNESS, AND (4) THEREAFTER DECREASING THE TEMPERATURE OF THE SUPERCOOLED PORTION OF THE MELT TO PROVIDE A GREATER DEGREE OF SUPERCOOLING WHILE MAINTAINING THE INVERTED TEMPERATURE GRADIENT AND WHILE MAINTAINING SUBSTANTIALLY THE GIVEN RATE OF WITHDRAWAL SO THAT THE WIDTH OF THE DENDRITIC CRYSTAL INCREASES TO THE SELECTED WIDTH WITHOUT SUBSTANTIALLY INCREASING THE THICKNESS OVER THE SELECTED THICKNESS.