US3394994A

US3394994A - Method of varying the thickness of dendrites by addition of an impurity which controls growith in the <111> direction

Info

Publication number: US3394994A
Application number: US545428A
Authority: US
Inventors: Jr John W Faust; Harold F John
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1966-04-26
Filing date: 1966-04-26
Publication date: 1968-07-30
Anticipated expiration: 1985-07-30

Description

United States Pate 3 s94 METHOD or vARYINi; This THICKNESS OF DEN- This invention relates to a process for varying the thickness of dendritic crystals of solid semiconductor materials by the addition of additives to the melt from which the crystals are pulled.

In the past, it has been the practice to control the thickness of dendritic crystals by varying the degree of supercooling of the melt or the rate at which the dendritic crystals are pulled from the melt.

While these techniques are generally satisfactory, they do require manipulation of the temperature control or pulling control of the furnace during growth. In addition, there is somevariation in the thickness of the crystal while the conditions are being changed.

An object of the present invention is to provide a means for controlling the thickness of a dendritic crystal by the addition of trace impurities to the melt from which the crystal is pulled.

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

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

FIGURE 1 is a view in elevation, partly in section, of a crystal growing apparatus suitable for use in accordance with the teachings of this invention;

FIG. 2 is a greatly enlarged fragmentary view of a dendritic crystal having three twin planes;

FIGS. 3 through 8 are end views, in cross section,

of a dendritic crystal in various stages of growth; and

FIG. 9 is an end view in cross section of a grown dendrite.

In accordance with the present invention, it has been discovered that crystals of solid materials may be prepared as fiatdendritic crystals having a closely controllable thickness andwith relatively precise flat parallel faces. These fiat dendritic crystals may be pulled or grown from melts comprised of a material and a thickness controlling impurity at a relatively high rate of speed of pulling of the order of one hundred times and greater than the linear pulling velocity previously employed in the art. The thickness of the crystals may be readily controlled by following the teachings of the present invention.

More particularly, in practicing the process, a melt of the material to be grown into a flat dendritic crystal and a thickness controlling impurity is prepared at a temperature slightly above the melting temperature there-- of. The surface of the melt is contacted with a previously prepared crystal having at least one twin plane and preferably three twin planes at the interior thereof, the crystal being oriented with the 2ll direction vertical to the melt surface. Other necessary or desirable crystal-- lographic and physical features of the seed crystal will be pointed out in detail herinafter. The seed crystal is dipped into surface of the melt -a suflicient period of time to cause wetting of the lower surface of the seed, usually a period of time of a few seconds to a 3,134,994 Patented July 30, 1968 can be employed, for example 0.2 inch per minute. Pulling speeds of from 10 to 25 inches per minute have given good results. The degree of supercooling and the rate of pulling can be readily so correlated that the seed crystal withdrawn from the melt comprises solidified melt material thereon of a precisely desired thickness and the desired crystallographic orientation. The present invention is particularly applicable to solid ma terials crystallizingin a diamond cubic lattice structure. Examples of such materials are the element silicon and germanium. Likewise, stoichiometric compounds having an average of four valence electrons per atom respond satisfactorily to the crystal growing process. Such compounds which have been processed with excellent results comprise substantially equal molar proportions of an element from Group III of the Periodic Table, and particularly aluminum, gallium and indium combined with an element from Group V of the Periodic Table, and particularly phosphrous, arsenic and antimony. Compounds comprising stoichiometric proportions of Group II and Group VI elements, for example ZnSe and ZuS, can be processed. These materials crystallizing in a diamond cubic lattice structure are particularly satisfactory for various semiconductor applications. Furthermore, the diamond cubic lattice structurematerials may be intrinsic or may be doped with one or more impurities to. produce N-type or P-type semiconductor materials; The crystal growing process of the present invention may be applied to all these different materials.

For a better understanding of the practice of the in-. vention, reference should 'be had to FIG. 1 of the drawings wherein there is illustrated apparatus 10 for practicing the process. The apparatus comprises a base 12 carrying a support 14 or crucible 16 of a suitable refractory material such as graphite to hold amelt of the material from which flat dendritic crystals are to be drawn and a thickness controlling impurity. In addition, the melt may contain one or more P-type or N-type dopants Molten material 18, for example germanium, with a suitable thickness control impurity is maintained within the crucible 16 in the molten state by a suitable heating means such, for example, as 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 temperature should be readily controllable to provide a temperature in the melt a few degres above the melting point and also to reduce heat input so that the temperature drops in a few seconds, for example in five to 10 seconds to a temperature at least one degree below the melting temperature and preferably to supercool the melt from 5 to 15 C., or lower. A cover 22 closely fitting the top of the crucible 16 may be provided in order to maintain a low thermal gradient above the top of the melt. Passing through an aperture 24 in the cover 22 is a seed crystal 26, preferably having three twin planes oriented crystallographically as will be disclosed in detail hereinafter. The crystal 26 is fastened to a :pulling rod 28 'by means of a screw 30 or the like. The pulling rod 28 is actuated by a suitable mechanism to control its upward movement at a desired uniform rate, ordinarily in excess of one inch per minute. A protective enclosure 32 of glass or other suitable material is disposed about the crucible with a cover 34 closing the top thereof except for an aperture 36 through which the pulling rod 28 passes.

Within the interior of 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. Depending on the crystal material being produced in the 3 apparatus, the protective atmosphere may comprise a nobles gas such as helium or argon, or a reducing gas such as hydrogen or mixtures of hydrogen and nitrogen, or nitrogen or the like or mixtures of two or more gases. In some cases, the space around the crucible may be evacuated to a high vacuum in order to produce crystals of material tree from any gases.

In the event that the process is applied to compounds having one component with a high vapor pressure at the temperature of the melt, a separately heated vessel containing the component may be disposed in the enclosure 32 to maintain therein a vapor of such compound at a partial pressure sufficient to prevent impoverishing the melt or the grown crystal with respect to the component. Thus an atmosphere of arsenic may be provided when crystals of gallium arsenide are being pulled. The enclosure 32 may be suitably heated, for example, by an electrically heated cover, to maintain the walls thereof at a temperature above the temperature of the separately heated vessel containing the arsenic in order to prevent condensation of arsenic thereon.

Referring to FIG. 2 of the drawings, there is illustrated in greatly enlarged view, a section of a preferred seed crystal 26 having three twin planes. Seed crystals may be obtained in various ways, for example, by supercooling a melt of solid material to a temperature at which a portion thereof solidifies, at which time some dendritic crystals having a plurality of 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 crystal a suitable section for use as a seed crystal.

The seed crystal 26 comprises two relatively fiat

parallel faces

50 and 52 with intermediate

interior twin planes

54, 55 and 57. The

faces

50 and 52 have the crystal orientation indicated by the crystallographic direction arrows at the right and left faces respectively of the figure. It will be noted that the horizontal direction perpendicular to the fiat faces 50 and 52 and parallel to the melt surface is 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 triangle etch pits 56 whose vertices 58 will be pointed upwardly while their bases will be parallel to the surface of the melt. It is an important feature of the preferred embodiment of the present invention that the etch pits of both faces 50 and 52 of seed crystal 26 have their vertices 58 pointed upwardly. The spacings or lamellae between the successive adjacent twin planes ordinarily is not unfiorm. The lamellar spacing, such as A between twin planes 54 and 55 and B between

twin planes

55 and 57 is of the order of microns, that is, from a friction of a micron to to microns or possibly greater. The ratio of A to B as determined from studies of numerous dendritic crystals is varied in the ratio of slightly more than one to as much as 18. Good seed crystals have been found to have lamellar spacings between successive twins of 5 microns and 1% microns, respectively. In all cases, all the twin planes and good seed crystals extend entirely through the seed.

parallel to the surface of the melt. Further, seed crystals Where the twin planes terminate internally, the seed crystal behaves as if no such twin plane is present insofar as pulling dendrites therewith from a melt.

The most satisfactory crystal growth is obtained by employing seed crystals of the type exhibited in FIG. 2 wherein three twin planes are present interiorly and are continuous across the entire cross section of the seed.

Seed crystals having an odd number (other than 1 and 3, that is, 5, 7 and up to 13 or more) of twin planes containing the growth direction may be employed in racticing the process of this invention, due care being had to point the triangular etch pits on the outer faces of the crystal with their vertices upwardly and the bases containing an even number. of twin planes may be employed for crystal pulling, though as desirable pulled crystals will not be obtainable as with the preferred three twin plane seed crystals shown in FIG. 2. Normally, the pulled dendrite willexhibit the same twin plane structure as the seed crystal exhibits. Thus, the dendrite will have three" twin planes extending through its entire length,

and often extending from edge to edge, if the seed comprises three twin planes,

The direction of withdrawal of the seed crystal 26 having an odd numberof twin planes from the melt 18 must he with the direction of the vertices 58 of the etch pits being upward and the bases being substantially parallel to the surface of the melt. When so withdrawn, the melt will solidify at the bottom of the crystal in a satisfactory prolongation thereof. If the crystal 26 were to be inserted into the melt so that the vertices 5% pointed downwardly very erratic grown crystals will be produced which are not only of non-uniform dimensions but grow at angles of to the seed and produce very irregular spines, and generally are unsatisfactory.

When a relatively cold fiat seed crystal has been introduced into the melt which is at a temperature of only a few degrees above the melting point of the material, the melt will dissolve the tip of the seed crystal. However, there will be a meniscus-like contact between the seed crystal and the body of the melt. Such contact should be maintained by keeping the temperature of the melt close to the melting point of the material.

Upon reducing the power input to the heating coil in order to supercool the melt (or reducing the applied heat if other modes of heat application than inductive heating are employed), there will be observed in a period of time in the order of 5 seconds after the heat input is reduced to a crucible of about 2 inches in diameter and length of 2 inches, the supercooling being about 8 C., an initial elongated hexagonal growth or enlargement on the surface of the melt at the tip of the seed crystal; The hexagonal surface growth increases in area so that at approximately 10 seconds after heat input is reduced its area is approximately, three times that of the cross section of the seed crystal. At this stage, there will be evidence of spikes growing out of the two opposite hexagonal vertices lying in the plane of the seed. These spikes appear to grow at the rate of approximately '2 millimeters per second. When the spikes are from 2 to 3 millimeters in length, the seed crystal pulling mechanism is energized to pull the crystal from the melt at the desired rates. Initation of pulling is timed to the appearance and growth of the spikes for best results.

After pulling the seed crystal upwardly from the supercooled melt, it will be observed that the fiat solid diamondshaped area portion is attached to the seed crystal and that a downwardly extending dendritic crystal has formed at each end of the elongated diamond area adjacent to the spike. Accordingly, two dendritic crystals can be readily pulled from the melt at one time from a single seed crystal. By continued pulling, the two dendritic crystals may be extended to any desired length.

If the seed crystal 26 were to be pulled at aslowly increasing rate just as supercooling of the crucible is being effected by reducing'the heat input so that atthe end of about 5 to 10 seconds the full pulling rate is being applied, then only one dendritic crystal will be attached to the seed crystal.

Various other techniques are known to those skilled in tie art to ensure that only one dendrite is grown from a seed'crystal.

The seed crystal need not be flat. It may be of any suitable size or shape as long as its orientation corresponds to that shown in FIG. 2. Usually, a portion of a previously grown dendritic crystal having a plurality of twin planes will be quite satisfactory for use as a seed and ordinarily such will be used as the seed crystal. The

pull dendritic crystal need have no direct relation to the seed crystal as far as size is concerned; the pull dendritic crystal will have a size and shape depending on the pulling conditions, the degree of supercooling, and the presence or absence of thickness controlling impurities.

In growing satisfactory fiat face dendritie crystals in accordance with the present invention, the melt of the materials, including thickness controlling impurities, may be supercooled as much as 30 to 40 C. below their melting point. In practice, however, supercooling of from 5 to 15 has given best results. A greater degree of superc'ooling requires higher rates of crystal withdrawal from the melt as well as requiring more precise control of the speed of pulling.

The length of the dendritic crystals grown in accordance with the teachings of this process is limited solely by the pulling apparatus employed.

Control of the thickness of a grown dendrite has generally been limited to controlling the degree of supercooling of the melt and the pull rate. However, it has now been found that by the addition of certain impurities in predetermined amounts the thickness of a dendrite can be increased or decreased over that which would be expected for a particular degree of supercooling and pull rate.

Impurities which decrease the thickness of a dendrite are set forth in Table I. The amount of the impurity which should be present in the melt is set forth in weight percent of melt exclusive of any doping material which may be present in the melt, The impurities are generally two component alloys. The alloy composition is stated as weight percent of the alloy.

TABLE I Impurity: Wt. percent of melt P-l-Sn alloy (.1% to 5% P+99.9% to95% Sn) .1 2 Sb-l-Ag alloy (.1 to Sb+99.9% to 90% Ag) .1-2 Sb+Au alloy (.1% to 10% Sb+99.9% to 90% Au) .l-2 Ag .01-2

Impurities which enhance or increase the thickness of a dendrite are set forth in Table II. The amount of the impurity which should be present in the melt is set forth in weight percent of melt exclusive of any doping material which may be present in the melt. When the impurity is a two component alloy, the alloy composition is stated as weight percent of the alloy.

TABLE II Impurity: Wt. percent of melt Ag-l-Ga alloy (99% to 85% Ag+1% to Ga) .l-Z Zn .l2

Sn .01-2 Au-l-Ga alloy (99% to 85% Au+1% to 15% Ga) .l-2 Au .l-2

To understand how the impurity either retards or enhances the thickness of the growing dendrite, it is necessary to consider the growth process of a dendrite.

The growth process of the dendrite itself can for the purpose of this discussion be considered as taking place in four steps. However, it should be realized that these four steps blend almost indistinguishnbly, one into the other and actually occur with great rapidity. The first step is the formation of a core or central region 59 as illustrated in FIG. 3 wherein the seed 26 is viewed in cross section a short distance behind the tip. It can be seen that the core or central region 59 containing the twin planes 54, 55 and 57 is propagated readily ahead of other growth and assumes a cruciform structure, with well defined growth regions 60 and 62 perpendicular to the twin planes 54, 55 and 57 and

regions

64 and 66 which are parallel to the twinplanes- The regions 60 and 62 determine the thickness T ofthe final dendrite and

regions

64 and 66 determine, to a limited extent the width W of the final dendrite. During this first step, the cruciform structure, which forms the core of the ultimate dendrite may reach an appreciable fraction of the final thickness of the dendrite while achieving only a small fraction of the final width of the dendrite.

The impurities set forth above in Table I and Table II control the final thickness of the dendrite by either retarding or enhancing the growth of regions 60 and 62 of the cruciform structure of FIG. 3.

Growth of the regions 60 and 62 is in the 1l1 direction. The impurities of Table I poison or otherwise hinder growth in the 1ll direction without effecting growth in the or ll2 direction. The growth direction of

regions

64 and 66 is 110 and this controls the width of the final dendrite.

With reference to FIG. 4, there is shown a cruciform structure 159 grown from a melt containing at least one of the impurities of Table I. The impurity from Table I comprises from .1% to 2%, by weight, of the melt with the exception of Ag which varies from .0l% to 2% by weight.

The impurity poisoned or stunted the growth in the 1l1 direction and the regions 160 and 162 which control the thickness did not grow to the same extent as regions 60 and 62 in FIG. .3. FIG. 3 shows a cruciform structure of a dendrite drawn from a melt free of any of the impurities of Table I or Table II.

Such poisoning takes place over the whole 111 surface underneath the melt; thus the dendrite does not have an opportunity to thicken during any part of the growth cycle.

With reference to FIG. 5, there is shown a cruciform structure 259 drawn from a melt containing at least one of the impurities of Table II. The impurities from Table II comprise from .I% to 2%, by weight, of the melt with the exception of Ga and Sn which vary from .01 to 2% by weight.

The impurities of Table II enhance growth in the 1ll direction and the

regions

260 and 262, which control thickness of the final dendrite, grew to a greater extent than the regions 60 and 62 of FIG. 3.

When the growth of regions 60 and 62 of FIG. 3 and 160 and 162 of FIG. 4 and 260 and 262 of FIG. 5 perpendicular to the twinplanes has progressed to a degree that T is a substantial proportion of the final thickness of the dendrite, well defined growth facets form along the outer edges of the regions 60 and 62 of the cruciform 59 of FIG. 3 as shown in FIG. 6. These facets are designated as 68, 70, 72 and 74 respectively in FIG. 6. The dendrite now has an I-I-shaped cross sectional configuration with the

facets

68, 70, 72 and 74 forming the legs .of the H and the core 59 forming the cross-bar of the H. The lateral growth of the

facets

68, 70. 72 and 74 proceeds rapidly outwardly from the core 59 and is independent of the growth of the core 59.

During the growth of the legs or-facets, step. three takes place during which the area between the legs is filled in by material solidifying inwardly from the legs. The growth directions are designated by the arrows C and D in FIG. 6. The resultant fully grown dendrite is illustrated in FIG. 9.

With reference to FIG. 7, there is shown growth facets I68, 170, 172 and 174 growing along the outer edges impurities of either Table I or Table 11 do not afiect the rest of the growth of the dendrite.

The final step, which is the addition of layers of microscopic thickness to the exterior surfaces of the legs, which gives the dendrite an atomically fiat mirror-like surface, then takes place, just as the dendrite is pulled from the melt. The complete dendrite is shown in FIG. 9.

The impurities set forth in Table I and Table II affect the thickness of a dendrite grown from silicon, germanium, Groups III-V compounds and Groups II-VI compounds. In addition, certain other impurities affect the growth of particular materials. In the case of III-V compounds, tellu-rium has been found to enhance nucleation on {111} planes and zinc has been found to suppress nucleation on {111} faces. More specifically, in the case of indium antimonide, mercury and to some extent zinc decrease the tendency for {111} nucleation to occur.

The following examples are illustrative of the present invention:

Example I An apparatus similar to FIG. 1, the graphite crucible containing a quantity of intrinsic germanium was heated by the induction coil to a temperature several degrees above the melting point of the germanium, the temperature being about 938 C., until the entire quantity formed a molten pool. A dendritic seed crystal having three interior twin planes extending entirely therethrough and oriented as FIG. 2 of the drawings, held vertically in a holder was lowered until its lower end touched the surface of the molten germanium. The contact with the molten germanium was maintained until a small portion of the, end of the dendritic seed crystal had melted. Thereafter the temperature of the melt was lowered rapidly in a matter of 5 seconds by reducing current to the coil 20, to a temperature 8" C. below the melting point of the germanium so that the melt was supercooled (about 928 C.). After an interval of approximately seconds at this temperature, the germanium seed crystal was pulled upwardly at a rate of 7 inches per minute. A dendritic seed crystal was attached to the seed and had a thickness of 7 mils and was approximately 2 millimeters in width. The grown dendritic crystal had substantially flat and highly parallel faces from end to end with 111 orientation. The germanium dendritic crystal so grown was found to have no surface imperfections except for a number of microscopic steps differing by about 50 Angstroms and was of a quality suitable for semiconductor applications.

Example II An apparatus similar to FIG. 1, a graphite crucible containing a quantity of intrinsic germanium and .2%, by weight, of an alloy consisting of .l%, by weight, P and 99.9% by weight, Sn an impurity of Table I, was heated by the induction coil to a temperature several degrees above the melting point of germanium, the temperature being about 938 C., until the entire quantity formed a molten pool. A dendritie-seed crystal having three interior twin planes extending entirely therethrough and oriented as in FIG. 2 of the drawings, held vertically in a holder was lowered until its lower end touched the surface of the molten germanium. Thecontact with the molten germaniumwas maintained until a small portion of the end of the dendritic seed crystal had melted. Thereafter, the temperature of the melt was lowered rapidly in a matter of 5 seconds by reducing current to the coil 20, to a temperature 8" C. below the melting point of germanium so that the melt was supercooled (about 928 C.). After an interval of approximately 10 seconds at this temperature, the .ger-manim seed crystal was pulled upwardly at a rate of. 7 inches per minute. A dendritic crystal was attached to the seed which had a thickness of 4 mils and was approximately 2 millimeters in width.

Example III Example IV The procedure of Example I was repeated except that the melt consisted of intrinsic germanium and .1%, by

weight, of an alloy consisting of 2%, by weight, Ga and.

98% by weight, Ag, an impurity of Table II. The dendrite so grown had a thickness of 9 mils and was approximately 2 millimeters in width.

Example V The procedure of Example I was repeated except that the melt consisted of germanium and .1% by weight of Sn, an impurity of Table II. The dendrite so grown had a. thickness of 11 mils and was approximately 2 millimeters in Width.

While the above description emphasizes the application of the present invention to semiconductor materials, it will be understood that it may be employed for producing grown dendritic crystals from any metal or alloy or compound of zinc blend structure and growable from a melt. By the practice of the present invention, flat crystals of control thickness and having a high perfection of orientation may be produced by the practice of the process disclosed herein.

It will be understood that the above description and drawings are only illustrative and not limiting.

We claim as our invention:

1. A process for producing fiat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium and stoichiometric compounds having an average of four valence electrons per atom, the steps comprising forming a melt of the material to be grown into a dendritic crystal and an impurity capable of affecting growth in the 111 direction, the impurity being at least one impurity selected from the group consisting of phosphorustin alloy, antimony-silver alloy, antimony-gold alloy, silver, silver-gallium alloy, zinc, gallium, tin, gold-gallium alloy and gold, and such impurity constitutes from 0.1% to 2% by weight of the melt, except when said impurity is one selected from the group consisting of gallium, tin and gold, in which case said impurity constitutes from .01 to 2%, by weight, of the melt, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melt with a seed crystal of the material to be grown into a dendritic crystal for a period of time to wet the seed crystal with the melt, the

seed crystal having a plural odd number of parallel interior twin planes, the crystal being oriented with a 11l direction parallel to the surface of the melt and a 2l1 direction perpendicular to the surface of the melt, the twin planes being parallel to the 2l1 direction, the seed when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed perpendicularly upward with respect to the melt surface, supercoolin'g the melt to a selected temperature, and pulling the seed crystal at a rate of the order of at least one inch a minute with respect to the met surface while maintaining the selected temperature whereby the material from the melt solidifies on the seed crystal and produces an elongated flat dendritic crystal.

2. A process for producing flat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium and stoichiomctric compounds having an average of four valence electrons per atom, the steps comprising forming a melt of the material to be grown into a dcndritic crystal and an impurity capable of affecting growth in the ll1 direction, the impurity being at least one impurity selected from the group consisting of 0.1% to 5%, by weight, phosphorus and 99.9% to 95%, by weight, tin; 0.1% to by weight, antimony and 99.9% to 90%, by weight, silver; 0.1% to 10%, by weight, antimony and 99.9% to 90% gold; silver; 99% to 85%, by weight, silver and 1% to by weight, gallium; zinc; gallium; tin; 99% to 85%, by weight, gold and 1% to 15% gallium, and gold, bringing the melt to a temperature slightly above the melting point of the material, contacting -a surface of the melt with a seed crystal of the material to be grown into a dendritic crystal for a period of time to wet the seed crystal with the melt, the seed crystal having a plural odd number of parallelinterior twin planes, the crystal being oriented with a l11 direction parallel to the surface of the melt and a 2l1 direction to the surface of the melt, the twin planes being parallel to the 211 direction, the seed when etched exhibiting triangular etch pits on both faces with the verticesof the triangular etch pits being directed perpendicularly upward with respect to the melt surface, supercooling the melt to a selected temperature, and pulling the seed crystal at a rate of the order of at least one inch a minute with respect to the melt surface while maintaining the selected temperature whereby the material from the melt solidifies on the seed crystal and produces an elongated fiat dendritic crystal.

3. A process for producing flat crystals of a solid material crystallizing in the diamond cubic lattice structure selected from the group consisting of silicon, germanium and stoichiometric compounds having an average of four valence electrons per atom, the steps comprising forming a melt of the material to be grown into a dendritic crystal and an impurity capable of afl'ecting growth in the 111 direction, the impurity being at least one impurity selected from the group consisting of 0.1% to 5%, by weight, phosphorus and 99.9% to 95%, by weight, tin; 0.1% to 10%, by weight, antimony and 99.9% to 90%, by weight, silver; 0.01% to 10%, by weight, antimony and 10 99.9% to 90% gold; silver; 99% to by weight, silver and 1% to 15%, by weight, gallium; zinc; gallium; tin; 99% to 85%, by weight, gold and 1% to 15% gallium, and gold and such impurity constitutes from 0.1% to 2% by weight of the melt, except when said impurity is one selected from the group consisting of gallium, tin and gold in which case said impurity constitutes from 01% to 2%, by weight, of the melt, bringing the melt to a temperature slightly above the melting point of the material, contacting a surface of the melt with a seed crystal of the material to be grown into a dendritic crystal for a period of time to wet the seed crystal with the melt, the seed crystal having a plural odd number of parallel interior twin planes, the crystal being oriented with a 11l direction parallel to the surface of the melt and a 21l direction perpendicular to the surface of the melt, the twin planes being parallel to the 2l1 direction, the seed when etched exhibiting triangular etch pits on both faces with the vertices of the triangular etch pits being directed perpendicularly upward with respect to the melt surface, supercooling the melt to a selected temperature, and pulling the seed crystal at a rate of,

the order of at least one inch a minute with respect to the melt surface while maintaining the selected temperature whereby the material from the melt solidifies on the seed crystal and produces an elongated flat dendritie crystal.

References Cited UNITED STATES PATENTS 2,841,559 7/1958 Rosi 252-623 2,898,528 8/1959 Patalong 25262.3 2,990,372 6/1961 Pinter 252-62.3 3,129,061 4/1964 Dermatis 23-223.5 3,162,507 12/1964 Dermatis 234-301 NORMAN YUDKOFF, Prima'ry 'Exan'liner.

G. P. HINES, Assistant Examiner.

Claims

1. A PROCESS FOR PRODUCING FLAT CRYSTALS OF A SOLID MATERIAL CRYSTALLIZING IN THE DIAMOND CUBIC LATTICE STRUCTURE SELECTED FROM THE GROUP CONSISTING OF SILICON, GERMINIUM AND STOICHIOMETRIC COMPOUNDS HAVING AN AVERAGE OF FOUR VALENCE ELECTRONS PER ATOM, THE STEPS COMPRISING FORMING A MELT OF THE MATERIAL TO BE GROWN INTO A DENDRITIC CRYSTAL AND AN IMPURITY CAPABLE OF AFFECTING GROWTH IN THE <111> DIRECTION, THE IMPURITY BEING AT LEAST ONE IMPURITY SELECTED FROM THE GROUP CONSISTING OF PHOSPHORUSTIN ALLOY, ANTIMONY-SILVER ALLOY, ANTIMONY-GOLD ALLOY, SILVER, SILVER-GALLIUM ALLOY, ZINC, GALLIUM, TIN, GOLD-GALLIUM ALLOY AND GOLD, AND SUCH IMPURITY CONSTITUTES FROM 0.1% TO 2% BY WEIGHT OF THE MELT, EXCEPT WHEN SAID IMPURTY IS ONE SELECTED FROM THE GROUP CONSISTING OF GALLIUM, TIN AND GOLD, IN WHICH CASE SAID IMPURITY CONSTITUTES FROM .01% TO 2%, BY WEIGHT, OF THE MELT, BRINGING THE MELT TO A TEMPERATURE SLIGHTLY ABOVE THE MELTING POINT OF THE MATERIAL, CONTACTING A SURFACE OF THE MELT WITH A SEED CRYSTAL OF THE MATERIAL TO BE GROWN INTO A DENDRITIC CRYSTAL FOR A PERIOD OF TIME TO WET THE SEED CRYSTAL WITH THE MELT, THE SEED CRYSTAL HAVING A PLURAL ODD NUMBER OF PARALLED INTERIOR TWIN PLANES, THE CRYSTAL BEING ORIENTED WITH A <111> DIRECTION PARALLEL TO THE SURFACE OF THE MELT AND A <211> DIRECTION PERPENDICULAR TO THE SURFACE OF THE MELT, THE TWIN PLANES BEING PARALLEL TO THE <211> DIRECTION, THE SEED WHEN ETCHED EXHIBITING TRIANGULAR ETCH PITS ON BOTH FACES WITH THE VERTICES OF THE TRIANGULAR ETCH PITS BEING DIRECTED PERPENDICULARLY UPWARD WITH RESPECT TO THE MELT SURFACE, SUPERCOOLING THE MELT TO A SELECTED TEMPERATURE, AND PULLING THE SEED CRYSTAL AT A RATE OF THE ORDER OF AT LEAST ONE INCH A MINUTE WITH RESPECT TO THE MELT SURFACE WHILE MAINTAINING THE SELECTED TEMPERATURE WHEREBY THE MATERIAL FROM THE MELT SOLIDIFIES ON THE SEED CRYSTAL AND PRODUCES AN ELONGATED FLAT DENDRITIC CRYSTAL.