US3413098A

US3413098A - Process for varying the width of sheets of web material

Info

Publication number: US3413098A
Application number: US571617A
Authority: US
Inventors: Steven N Dermatis
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1966-08-10
Filing date: 1966-08-10
Publication date: 1968-11-26
Anticipated expiration: 1985-11-26

Description

Nov. 26, 1968 s. N. DERMATIS 3,413,098

PROCESS FOR VAHYING THE WIDTH OF SHEETS OF WEB MATERIAL Filed Aug lo, 1955 5 Sheets-Sheet l FIGI. w.

lsoTHERM 3e 34 32 J2e SRCE ...1 3o S N u 37 26`q C25q Bz z5 F162. G=;\ 1b a: N \=D Q q: 2u

:ND a=\, N \::p c' R '6 :\=D 52 :D k v 20 l I P C T 2n T i A x l 2H I p59 '2 A.

' g so 5 ol l l 5/// j [4 lX FIGB 4o 42 ,f ,l l |26 HEAT 54 55 57 SOURCE suPERcooLE R GsoN HEAT SOURCE SEED CRYSTAL INSERT FIGA.

HEAT souRcE WITNESSES |NvENToR fi/w fw sveven N. Dermafis BY J/ AKM Nov. 26, 1968 s. N. DERMATIS 3,413,098

PROCESS FOR VARYNG THE WIDTH OF SHEETS OF WEB MATERIAL Filed Aug. lO, 1966 5 Sheets-Sheet 2 Nov. 26, 1968 s. N. DERMATls 3,413,098

PROCESS FOR VR-INC THE WIDI'I OF SHEETSOF WEB MATERIAL Filed Aug. lO, 1966 5 Sheets-Sheet 3 so 2o 2o 3o 4u MILS. MILS. |50 MILS' MILS. MILS.

FIG.I5.

3o 2o -.2o 3o I4u MILS. MILS. MILS. MILS.

I 25S Mu. L3 MILS. 1 44 MILS.

I-s MlLs. MILS' 4.4 MlLs.

United States Patent O 3,413,098 PROCESS FOR VARYING THE WIDTH F SHEETS OF WEB MATERIAL Steven N. Dermatis, Waltham, Mass., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 10, 1966, Ser. No. 571,617 2 Claims. (Cl. 23-301) This invention relates generally to grown or pulled crystalline sheets of a material crystallizing in the diamond cubic lattice structure and semiconductor materials in particular, and to a process for varying the Width of such sheets.

Elongated web sheets of materials crystallizing in the diamond cubic lattice structure have been grown, and the crystals and process for growing them has been set forth in detail in U.S. Patent 3,129,061 issued Apr. 14, 1964 and assigned to the same assignee as the present application.

It has been found that the width of dendritic web sheets can be controlled by varying the pull rate and the degree of supercooling of the melt.

However, altering the temperature of the melt to either increase or decrease the degree of supercooling of the melt requires time and interrupts the continuous run.

An object of the present invention is to provide a process for widening a dendritic web sheet without changing the temperature of the melt or the pull rate.

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

For a better understanding of the nature and objects 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 top view of the melt contained within the apparatus of FIG. 1;

FIG. 3 is a greatly enlarged fragmentary View in elevation of a seed suitable for use in accordance with the teachings of this invention;

FIGS. 4 and 5 are top views of the melt contained within the apparatus of FIG. l; i

FIG. 6 is an enlarged fragmentary view in elevation of a body of material being grown in accordance with the teachings of this invention;

FIG. 7 is a crosssectional view of the body of FIG. 3 taken along the line IV-IV thereof;

FIGS. 8 an-d 9 are greatly enlarged fragmentary views in elevation of alternative seeds suitable for use in accordance with the teachings of this invention;

FIG. 10 is a fragmentary view in elevation of a body prepared in accordance with the teaching of this invention;

FIG. 11 is a fragmentary view in elevation of a modified body prepared in accordance with the teachings of this invention;

FIGS. l2, 13 and 14 are cross-sectional views of bodies of material prepared in accordance with the teachings of this invention;

FIG. 15 is a view in perspective of a body of material prepared in accordance with the teachings of the prior art; and

FIG. 16 is a view in perspective of a body of material prepared in accordance with the teachings of this invention.

The elongated body of semiconductor material of this invention is prepared by melting a quantity of the material to be grown, contacting a surface portion of the melted material with a seed crystal of the material for a period of time to wet the seed crystal `with the melted material, said surface portion being disposed from the center of the 3,413,098 Patented Nov. 26, 1968 ice melt, the seed crystal having at least two parallel twin planes which come into contact with the melt, the seed 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 and the {lll} plane of the seed, the melt being supercooled to a temperature of at least about 5 C. below the melting point, the surface area of the supercooled portion being at least about 0.25 Sq. in., initiating growth of at least parallel dendrites from the seed, and pulling the seed crystal with at least two parallel dendrites attached thereto from the melt at a rate of from approximately 1A inch per minute to 4 inches per minute whereby a thin flat web joins the parallel dendrites crystallographically.

More particularly, in a preferred practice of the process of this invention, a melt of the material to be grown into an elongated body comprised of at least two parallel elongated dendritic crystals crystallographically joined into a unitary body by a thin web or sheet portion extending between the dendrite crystals over the entire length of the body is prepared at a temperature slightly above the melting point thereof. A portion of the surface of the melt disposed from the center of the melt is contacted with a previously prepared seed whose configuration and orientation will be discussed in detail hereinafter. The seed comprises at least two parallel twin planes disposed perpendicularly to the surface of the melt. The seed is dipped into the surface of the melt a sufficient period of time to cause wetting of the lower surface of the seed, usually a period of time of a few seconds to a minute is adequate, the twin planes also being in contact with the melt, and, then, at least that portion of the melt having a radius extending from the center of the melt to the seed is supercooled rapidly to provide a surface of an area of at least about 0.25 sq. in. of supercooled liquid melt, at least two parallel dendrites should be formed on the see-d, following which the seed crystal is withdrawn from the supercooled portion of the melt at a speed of from 1A inch to 4 inches and, preferably, at a speed of from 1A inch to 1 inch per minute. The degree of supercooling which is preferably from 5 C. to 10 C. for silicon and the rate of pulling can be readily correlated so that the seed withdrawn from the melt comprises an elongated body consisting of the parallel elongated dendritic crystals and crystallographically joining them into a unitary body, a thin web or sheet portion which extends between the dendritic crystals over the entire length of the body.

The present invention is particularly applicable to solid materials crystallizing in the diamond cubic lattice structure. Examples of such materials are the elements silicon and germanium. Likewise, stoichiometric compounds having an average of four valence electrons per atom respond satisfactorily to the crystal growing process of this invention. 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 phosphorus, arsenic and antimony. Compounds comprising stoichiometric proportions of Group Il and Group VI elements, for example, ZnSe and ZnS, can be processed. These materials crystalizing in the diamond cubic lattice structure are particularly satisfactory for various semiconductor applications. Furthermore, the diamond cubic lattice structure materials may be intrinsic or they may be doped with one or more impurities to produce n-type or p-type semiconductor materials, or bodies having a p-n-p or n-p-n cross-section. The crystal growing process of the present invention may be applied to all of these different materials.

For a better understanding of the practice of this invention, reference should be had to FIG. 1 of the drawing wherein there is illustrated apparatus for practicing the teachings of this invention. The apparatus 10' comprises a base 12 carrying a graphite support 14 for a susceptor or crucible 16 of a suitable refractory material such as graphite to hold a melt of the material which will be referred to hereinafter as silicon from which is to be grown or pulled the elongated body comprised of at least two parallel elongated dendritic crystals crystallographically joined into a unitary body by a thin web or sheet portion extending between the dendritic crystals over the entire length of the body. Molten silicon 18 is maintained within a quartz lining 19 within the susceptor 16 in the molten state by a suitable heating means such, for example, as radio frequency (RF) induction heating coil 20 disposed about the susceptor 16. Other heating means may be employed such as radiation, electron beam or a combination thereof. The best temperature control and results are realized when the RF coil extends above the top of the susceptor 16. A suitable source of energy and control means, not shown, are employed to supply an alternating electrical current, for example, from 10() kc. fto 5 megacycles, to the RF coil 20 to maintain a closely controlled temperature in the body of the melt 18. The energy input should be readily controllable so as to provide at the proper time 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 at least 1 degree below the melting temperature and preferably to supercool at least a portion of the melt from 5 to 10 C. An apertured cover 22, comprised of a suitable material such as for example, molybdenum, tantalum or tungsten, closely fitting the top of the susceptor 16 is 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 and attached growth 26. The seed 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 (not shown, but are well known in the crystal growing art).to control its upward movement at a desired uniform rate, ordinarily at a rate of from 1A inch to 4 inches per minute. A protective enclosure 32 of glass or other suitable material is disposed about the susceptor 16 and between the susceptor 16 and the RF coil 20 with a cover 34 closing the top thereof except for a sealing aperture 36 through which the pulling rod 28 passes. A heat shield 37, comprised of, for example quartz, is disposed within enclosure 34 and is mounted on a base 39, which in turn is suspended from the support 14, surrounds the susceptor 16.

Within the interior of enclosure 32 is provided a suitable protective atmosphere which may be introduced through a conduit 40 and, if necessary, a Vent 42 so that a circulating current of such protective atmosphere is present. Depending on the crystal material being produced in the apparatus 10, the protective atmosphere may comprise a noble gas such as helium or argon, or a reducing gas such as hydrogen or mixtures of hydrogen and nitrogen, or nitrogen alone or mixtures of two or more such gases. In some cases, the space around the Crucible may be evacuated to high vacuum in order to insure the production of crystals free 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 con-` taining 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 crystals with respect to the component. Thus an atmosphere of arsenic may be provided when crystals of gallium arsenide are being pulled. In this case the enclosure 32 also may be suitably heated, for example, by an electrically heated jacket, 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.

Under the most ideal conditions for practicing the teachings of this invention, the entire melt 18 would be supercooled. However, this condition is not always 0btainable and in some instances only a portion of the melt is supercooled at any one time. The portion of the melt 18 most easily supercooled is the central portion directly below the aperture 24 since heat radiating from surface 45 of the melt is dissipated through the aperture instead of being reflected back. It has been found that to produce an elongated body comprised of at least two parallel elongated dendritic crystals crystallographically joined to a unitary body by a thin web or sheet portion extending between the dendritic crystals over the entire length of the body the supercooled portion of the melt must have an area of at least about 1A square inch on the melt surface 45. If it is attempted to pull the body from a melt having a lesser supercooled surface area, such as 0.12 sq. in., the web or sheet portion of the body will not form.

With reference to FIG. 2, there is shown a top View of the surface 45 of melt 18. There are a series of isotherms in the melt defining areas of varying temperature. The area in the center of the Imelt is at the lowest temperature. The temperature of each succeeding area from the center outward is at a slightly higher temperature.

Referring to FIG. i3 of the drawing there is illustrated, in a greatly enlarged Iview one type of a seed 126 which may Ibe used with considerable success in accordance with the teachings of this invention to produce the elongated sheet-like Abody of this invention. The seed 126 is a section of a dendritic crystal which was grown in accordance with the teachings of U.S. patent application Ser. No. 844,288, more fully identified hereinabove. -It will lbe understood, of course, that the seed is comprised of the sa-me material as the melt.

The seed 126 as obtained by dendritic growth cornprises two relatively flat parallel faces 50 and 52 with three intermediate parallel interior twin planes 54, 55 and 57. Examination will show that the crystallographic structure of the preferred seed on both

faces

50 and 52 is that indicated by the crystallographic direction arrows at the right and left faces, respectively, of the ligure. lIt will be noted that the horizontal directions perpendicular to the at faces 50 and 52 and parallel to the melt surfaces are (111). The direction of growth of the elongated body of this invention will be in a 211 crystallographic direction. If the faces 50 and 52 of the seed 126 were to be etched preferentially to the [111] planes, they will both exhibit equilateral triangular etch pits 58 whose vertices 59 will point upwardly while the bases 60 will be parallel to the surface of the Imelt. This etch pit orientation will exist for any seed containing a plural odd number of twin planes, and all of such seeds are excellent for the purpose of this invention. Normally the distances between successive twin planes are not the same. A seed containing two twin planes or any even number of twin planes will exhibit triangular etch pits on one face whose vertices will be pointing opposite to the direction of a triangular etch pit on the other face.

The most satisfactory crystal growth is obtained by employing seeds of the type exhibited in FIG. 3, that is, a section of a previously grown dendritic crystal wherein three twin planes are present interiorly and are continuously across the entire cross-section of the seed. It should be understood that the seed need not have ilat

exterior surfaces

50 and 52, it is only necessary that the (111) planes be parallel to the twin planes. Also, the twin planes need not be exposed at the edges of the seed, as long as they will be exposed to the melt =by melting back.

Seed crystals hafving an old number (other than 3, that is, 5, 7 and up to 13 or more) twin planes containing the growth direction may be employed in practicing the process of this invention, due care Ibeing had to point the triangular etch pits on the outer faces of the crystals with their vertices upwardly and the bases parallel to the surface of the melt. Further, seeds containing an even number of twin planes may be employed for crystal pulling, though as desirable pulled bodies will not be obtainable as with the preferred three twin plane seed crystal as shown in FIG. 3. Normally, the pulled body will exhibit the same twin plane structure in the dendritic portion as the seed exhibits.

The direction of withdrawal of the seed having an odd number of twin planes from the melt 18 must be with the direction of the vertices 59 of the etch pits being upwardly and the fbases being substantially parallel to the surface of the melt. When so withdrawn, the melt will solidify in indefinitely prolonged growth, at the bottom of the seed in the vertical direction. If the seed 4126 were to ybe inserted into the melt so that the vertices 59 pointed downwardly, very erratic growth will be produced which is not only of non-uniform dimensions but at angles of 120 to the (211) direction and with very irregular spines in the dendritic portion of the body thereby resulting in bodies which are generally unsatisfactory.

When a relatively cold flat seed crystal has been introduced into the melt which is at a temperature at the melting point or only a few degrees above the melting point of the material, the melt will wet and dissolve the tip of the seed and expose the interior twin planes if they do not extend to the surface. There will tbe a meniscuslike contact area between the seed crystal and the surface of the melt. Such contact area should be maintained throughout the process.

Once good wetting of the seed is obtained and the twin planes are in Contact with the melt, the power input to the heating coil is reduced in order to supercool at least that portion of the melt adjacent the seed (or reducing the applied heat if other modes of heat application than RF inductive heating are employed). As iillustrated in FIG. 4, there will be observed in a period of time of the order of 5 seconds after the heat input is reduced to the crucible, the supercooling lbeing from 5 C. to 10 C., an initial growth or enlargement 62 which has an elongated hexagonal horizontal cross-section occurs on the surface of the melt attached to the tip of the seed crystal. The hexagonal surface growth increases in area so that in approximately seconds after heat input is reduced its area is approximately -3 times that of the cross-section of seed from a dendrite. At this stage, there will be

evident spikes

64 and 66 growing at the ends of the hexagonal growth. These spikes appear to grow at the rate of approximately 2 millimeters per second. When the spikes are from two to three millimeters in length and the total length of the hexagonal growth is from about 1A inch to as much as 3A inch, the seed crystal pulling mechanism is energized to pull the seed with its attached hexagonal growth from the melt at the desired rate of from 1A; inch to 4 inches per minute and preferably from 1A: inch to l inch per minute. If a pull rate of significantly less than 1A inch per minute is employed the desired crystallographic structure is not obtained. If a pull rate materially greater than 4 inches per minute is used, the web portion of the body will not be formed between the dendrites. The initiation of pulling is timed to the appearance and size of the' hexagonal growth with the spikes. After pulling the seed crystal upwardly from the supercooled portion of the melt, it will be observed that from the spikes of the solid hexagonal shaped area portion 62 attached to the seed crystal 126 there are downwardly extending

dendrites

68 and 70 formed at each of the hexagonal area. Accordingly, two parallel dendritic crystal or dendrites are being pulled from the melt at one time from a single seed. These dendrites are parallel to each other and their faces are parallel to the faces of the seed crystal.

When pulled at this critical rate from the melt, a thin web or sheet 72 of solidified material from the melt CII extends across the space between the two

parallel dendrites

68 and 70. The web or sheet 72 is crystallographically joined to the two

dendrites

68 and 70, that is, the general crystal structure of the dendrites is continued through the web or sheet 72. However, the web or sheet portion 72 will generally be single crystal material whereas the dendrites will have twin planes extending therethrough. The web usually will have a thickness of at least 0.1 mil, but its thickness will not exceed that of the two

dendrites

68 and 70. The web 72 will be substantially dislocation free.

The two

dendrites

68 and 70 will remain substantially parallel over the length of the complete elongated body and will thus control the width of the lweb or sheet portion 72.

The thickness of the web or sheet portion 72 will depend, as pointed out above, to a degree on the thickness of the

dendrites

68 and 70, and in addition, by the degree of supercooling of the melt and the pull rate. The higher the degree of supercooling and the slower the pull rate, the thicker the web or sheet portion.

With reference to FIG. 4, if the seed 26 contacts the melt in the central isothermic region, which is entirely below the supercooled temperature or if the seed is centrally disposed within the melt, the opposed thermal forces, indicated as F1, F2, F3 and F4 in FIG. 4, are equal, that is F1 equals F3 and F2 equals F4 and the web dendritic sheet will be grown, assuming a uniform pull rate with a particular thickness and ywidth for a particular pull rate.

However, and with reference to FIG. 5, if a seed 226 contacts the melt in a location disposed from the center of the melt relative to the width of the supercooled area, but still completely in an area that is supercooled, thermal force F3 will be greater than thermal force F1, because it is near the heat source, and the web dendritic sheet will widen into colder central region against force F1. Thus a wider dendritic sheet will be produced, employing the same pull rate and degree of supercooling, than if the seed had contacted the melt at the center of the melt.

With reference to FIG. 7, there is illustrated a section of the elongated body of FIG. 3 taken along the line IV-IV. This shows how the

dendrites

68 and 70= are crystallographically joined to the web or sheet 72. A more detailed description will be given subsequently.

While, as shown in FIG. 3, the section of a previously grown dendrite or dendritic crystal is the preferred form of seed to institute growth of the elongated body of this invention, other seed forms have been found satisfactory. For example, and with reference to FIG. 8 there is shown another suitable seed 74. The seed 74 is comprised of two previously grown

dendrites

76 and 78 without any web therebetween, which are joined together at one end by a hexagonal portion 80 at spike portions -82 and 84. The seed 74 of FIG. 8 may be obtained by seeding a supercooled melt with a seed of the type illustrated in FIG. 3 and pulling at a rate in excess of 4 inches per minute whereby only the dendrites grow individually and no web or sheet portion forms.

With reference to FIG. 9 there is illustrated another seed 86 which is essentially the structure of FIG. 6 with a web between two parallel dendrites. After a long sheetlike body has been pulled, the portion 86 shown in FIG. 9 is severed at 88 and used as a seed to initiate further growth. This seed 86 can be used over and over again for initiating satisfactory growth of a successive series of elongated bodies comprised of at least two parallel elongated dendritic crystals crystallographically joined into a unitary body by a thin Iweb or sheet portion extending between the dendritic crystals over the entire length of the body.

In addition, any complete transverse section cut from a previously grown elongated body prepared in accordance with the teaching of this invention is also satisfactory 7 for seeding. Such a seed 94 is shown in FIG. 10 and is comprised of edge

portions comprising dendrites

96 and 98 crystallographically joined by a web or sheet portion 100.

IIf a seed such as is shown in FIG. 8 comprised of double dendritic crystals attached to the original seed is introduced into the same or another melt at or slightly above the melting temperature and after supercooling the melt, on pulling the double dendritic crystal from the surface, there will be formed two separate hexagonal shaped areas attached to each of the dendrites and four dendritic crystals will be pulled--two attached to each of the original dendrites. The area between each of the adjacent dendrites will, in accordance with the teachings of this invention, be lled in with a web or sheet portion. The resulting body, a fragmentary view of which is shown in FIG. 11 Iwill be comprised of four

dendrites

102, 104, 106 and 108 crystallographically joined by web or sheet portions 110, 112 and 114 respectively disposed therebetween. Sheet-like bodies with three dendrites and two web portions also have been obtained.

A body of the material prepared in accordance with the teachings of this invention may vary from less than one inch to many feet in length. The width may be up to one inch with three or more parallel dendrites, and up to inch for two dendrites. The web portion has been obtained in widths of 1/2 inch and more. Segments or sections of any desired length can be cut from the grown or pulled elongated bodies by Sandblasting, fracturing, or electron beam cutting, or by any other similar process -known to those skilled in the art.

The 'body prepared in accordance with the teachings of this invention is comprised of at least two dendritic crystals, at the edges, which extend the entire length of the body, crystallographically joined by a web or sheet portion over their entire length.

The dendritic portions of the body of material will be comprised of dendritic crystals having two highly parallel fiat faces which may comprise a series of flat portions differing by steps of about 50 angstroms from each other. The dendritic portions will have a thickness of from approximately 2 to 25 mils and the width across the fiat faces may be from mils to 200 mils and even wider. The surface of the flat faces will exhibit essentially perfect 111 orientation. The dendritic -crystals will contain two or more twin planes which will usually extend the entire length of the dendritic crystal and will be parallel to the two parallel at faces. In addition, the dendritic portions of the body will be of substantially uniform thickness over the entire length of the body, at the extreme not varying as much as 0.1 mil in length of over .2 inch. A more detailed description of the dendritic portion of the bodies of this invention will be found in U.S. patent application Ser. No. 844,288. The dendrites set forth in this patent application are essentially identical to the dendritic portion of the body of this invention.

The web or sheet portion of the body crystallographically joins the two or more dendritic crystals which comprise the dendritic portion of the body, and extends the entire length of the body.

The web or sheet portion will have a thickness of at least approximately 0.1 mil, usually from 0.3 to 0.5 mil to 1 mil. The web portion normally will be much thinner than the dendritic crystal between which it is disposed. Also the web portion will be more uniform than the dendrite portions.

The surfaces of the web or sheet portion will be substantially parallel and will approximate very closely the 111 planes. Examination by optical and interference microscopy shows the surfaces to be extremely smooth in nearly all cases when grown properly. However, in some cases the surfaces will be smooth in the central part but will contain reverse steps on other parts especially in the area near thedendrite portions. The height of these steps when present are generally no more than 300 angstroms.

The web or sheet portions of the bodies of this invention are substantially dislocation free and silicon bodies have been prepared having less than 450 dislocations per square centimeter.

The internal structure of the web or sheet portion falls into three classes and these are illustrated in FIGS. 12,

13 and 14.

The most common, and preferred, internal structure of the web or sheet portion is the single crystal structure. With reference to FIG. 12, when web or sheet portion 116 is single crystal, all twin planes, for example,

twin planes

118, 120 and 122 are present only in and terminate at the edge of the dendrites 124 and 128 disposed on each side of the web or sheet portion 116. The twin planes do not extend into the web portion. In this configuration, the twin plane structure in the dendritic portion on each side of the web or sheet is asymmetric with respect to the web, that is, it does not extend entirely across the width of the dendrite, the web or sheet portion will be single crystal.

With reference to FIG. 13, in some cases at least 011e twin plane, for example, twin plane will extend across both

dendrites

132 and 134 and through web or sheet portion 136. Since, relative to electrical properties, material containing twin planes behaves essentially the same as single crystal material, the fact that at least one twin plane extends across the web or sheet portion does not detract from the usability of the material for the fabrication of electrical devices, especially semiconductor devices such as transistors, diodes, solar cells and the like.

Occasionally, there is found the configuration shown in FIG. 14, in which is illustrated a body of material in which twin planes and 142 originating in dendritic portions 144 and 145 respectively, are mismatched land form an incoherent twin boundary 148 within web or sheet portion 150. Such bodies are less desirable than those shown in FIGS. 12 and 13, but are still suitable for use in fabrication of certain semiconductor devices.

The internal structure of the web or sheet portions of the body of material of this invention is controlled or at least inuenced by (1) the twin structure of the dendritic portions on each side of the sheet, (2.) the thickness 0f the web or sheet, and (3) thermal distribution in the melt.

When the process is carried out to produce the thinner web or sheet portions, for example, webs of from 0.3 mil to 3 mils, these will normally be single crystal because the twin planes will be asymmetric with respect thereto at the edge of the dendritic portion of the body and therefore the twin planes do not extend into the thin sheet. The growth of single crystal web portions can be assured by using seeds wherein the twin planes are closer to one surface of the seed than the other.

The elongated sheet like bodies comprised of at least two parallel elongated dendritic crystals crystallographically joined to a unitary Vbody by a thin web or sheet portion extending between the dendritic crystals over the entire length of the body of the present invention `are relatively exible and may be bent on a circle of a radius of about 4 inches or even less without breaking. Consequently, crystals may be continuously drawn from the melt and wound on a cylinder of a radius of this order or greater in continuous lengths as desired. The thinner the body the smaller the radius of the coil that may be made therefrom.

The bodies of material grown in accordance with the teachings of this invention have surfaces both on the dendritic portion and on the web portion of such perfection that in the case of semiconductive materials they may -be employed for fabricating semiconductor devices simply by applying to the surfaces desired alloys or solders without any intermediate polishing, lapping or etching. In fact, in general, etching results in a degradation of the perfection of the crystal surfaces. In all cases the dendritic portion surface has a perfect 111 orientation as grown and the web or sheet portion have surfaces that Very closely approximate 111 planes. In the making of semiconductor devices such as diodes, transistors, photodiodes and other similar semiconductor devices, the 111 surface is a particularly desirable orientation.

The elongated body of this invention may be grown in an intrinsic form from a melt free of doping impurities, or the bodies may be grown doped to a specic type of semiconductivity and resistivity from a melt containing either acceptor or donor impurities. Examples of acceptor impurities include aluminum, boron, gallium, and indium. Examples of donor impurities include phosphorus, arsenic, and antimony.

One noticeable advantage obtained in practicing the present invention is that, while the previously known processes for growing crystals by the Azochralski technique result in a crystal in which the proportions of the doping impurities are usually radically different from the proportions of the doping impurities in the melt, it has been found in general the proportion of doping impurities in the pulled crystalline bodies of this invention will be much closer to the proportions in the melt using the process of this invention.

As a result of doping, sheet-like bodies of silicon have been prepared in accordance with the teachings of this invention having resistivities varying from less than 0.01 ohm-cm to greater than 200 ohm-cm. Pure silicon bodies with higher resistivities have also been obtained.

The elongated bodies of this invention and prepared in accordance with the teachings of this invention, provide an excellent material for the efficient and economical fabrication of semiconductor devices such as transistors, diodes, two and three terminal four regions devices, solar cells and related devices. Transistors have been prepared from silicon bodies prepared in accordance with the teachings of this invention and have been found to have a gain (beta) of at least 50. 1n addition, solar cells having an eiliciency of from l to 15% have been made on silicon bodies grown in accordance with the teachings of this invention.

When fabricating semiconductor devices prepared in accordance with the teachings of this invention, the edge dendritic portions can be left on the bodies or they may be removed by Sandblasting, electron beam cutting, chemical etching or the like and the device fabricated entirely upon the single crystal web or sheet portion of the body.

The following examples are illustrative of the practice of this invention:

EXAMPLE I An elongated body was prepared in accordance with the teachings of this invention in the following manner.

ln apparatus similar to FIG. 1, a quartz lined graphite Crucible containing a quantity of intrinsic silicon is heated by the induction coil to a temperature a few degrees above the melting point of silicon, the temperature being about 1430 C., until the entire quantity forms a molten pool. A seed comprising a section cut from a previously grown dendrite and having three interior twin planes extending entirely therethrough and oriented as in FIG. 3 of the drawings, that is, with the etch pit vertices directed upwardly, is held vertically in a holder and is lowered until its lower end touches the surface of the molten silicon at the center point of the surface. The contact with the molten silicon is maintained until a small portion of the end of the dendritic seed crystal is thoroughly wetted and is melted. Thereafter, the temperature of the melt is lowered rapidly in a matter of 5 seconds by yreducing current to the coil 20, to a temperature 8 C. below the melting point of the silicon so that the melt is supercooled (-temperature about 14l9 C.). After an interval of approximately seconds at this temperature until an elongated hexagonal portion about 1A: inch long formed, the seed crystal is pulled upwardly at the rate of 1 inch per minute from the supercooled portion of the melt which supercooled portion has a surface area of approximately 1A sq. inch. Two dendritic crystals were attached to the spiked ends of the hexagonal portion attached to the seed and each was of a thickness of 25 mils and was approximately 30 mils in width. The outside edges of the dendrite portions were approximately 0.25 inch apart. A sloping portion of about 20 mils extended inwardly to a central web portion of a width of about mils. The grown dendritic crystal edge portions has substantially flat and parallel faces from end to end with (111) orientation.

The two dendrite portions were crystallographically joined by a single crystal `thin web having a thickness of approximately 3 mils. The surfaces of the web portion very closely approximated (111) planes.

The body as grown was comprised of two dendrite portions crystallographically joined along :their entire length by the web or sheet portion. The body was grown to a length of about 14 inches and is shown schematically in FIG. 15 with the major dimensions set forth.

The dendritic portions of the body was found to have no visual microscopic surface imperfections except for a number of microscopic steps dilering by about 50 angstroms. The web or -sheet portion was found to have essentially at surfaces over the entire length of the body and to be highly dislocation free.

The procedure of Example I was repeated exa-ctly except that the seed contacted the melt at a point equal to one-half the radius measured outwardly from the center of the melt.

The resultant web sheet was comprised of two dendritic crystals each of which was 25 mils thick and approximately 30 mils wide.

A -sloping portion of about 20 mils extended inwardly to a central web portion of a width of about 200 mils. The grown dendritic crystal edge portions has substantially at and parallel faces from end to end with (111) orientation.

The two dendrite portions were crystallographically joined by a single crystal thin web having a thickness of approximately 3 mils. The ysurfaces of the web portion very closely approximated (111) planes.

The body as grown was comprised of two dendrite portions crystallographically joined along their entire length by the web or sheet portion. The body was grown to a length of about 14 inches and is shown schematically in FIG. 16 with the major dimensions set forth.

The dendritic portions of the body was found to have no visual microscopic surface imperfections except for a number of microscopic steps differing by about 50 angstroms. The web or sheet portion was found to have essentially at surfaces over the entire length of the body and to be highly dislocation free.

Since certain changes in carrying out the above process and in the product embodying the invention may be made without departing from its scope, it is intended that the accompanying description and drawings be interpreted as illustrative and not limiting.

I claim as my invention:

1. A process for producing an elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two parallel elongated dendritic crystals, said dendritic crystals being joined crystallographically into a unitary body by a thin web portion of the same semiconductor material extending between the dendritic crystals, the steps comprising melting a quantity of the material, bringing the melt to a temperature at approximately the melting point of the material, contacting a point on a surface of the melted material displaced from the center of the melt surface with a least a surface of a seed `crystal in which the seed contacts the melt at a point equal to approximately one-half the radius of the supercooled area from the center of the melt of the material for a period of time to wet the -seed crystal with the melted material, the seed crystal having at least two twin planes extending across at least a portion of the surface in contact with the melt, the seed 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, supercooling at least that portion of the melt extending from the center of the melt and encompassing the seed to a.temperature at least 5 C. below the melting point, the surface area of the supercooled portion being at least 0.25 sq. inch, and pulling the seed crystal from the melt at a rate of approximately 1A inch per minute to 4 inches per minute, whereby the material from the supercooled portion of the melt solidies on the seed crystal and produces an elongated body comprised of at least two dendritic crystals crystallographically joined by a thin web of material.

2. The process 0f claim 1 in which the seed contacts the melt at a point equal to approximately one-half the radius of the supercooled area from :the center of the melt.

References Cited UNITED STATES PATENTS 3,162,507 12/1964 Dermatis 23-301 FOREIGN PATENTS 882,570 11/1961 Great Britain.

NORMAN YUDKOFF, Primary Examiner.

G. P, HINES, Assistant Examiner.

Claims

1. A PROCESS FOR PRODUCING AN ELONGATED BODY OF A MATERIAL CRYSTALLIZING IN THE DIAMOND CUBIC LATTICE STRUCTURE COMPRISED OF AT LEST TWO PARALLEL ELONGTED DENDRITIC CRYSTALS, SAID DENDRITIC CYRSTALS BEING JOINED CYRSTALLOGRAPHICALLY INTO A UNITARY BODY BY A THIN WEB PORTION OF THE SAME SEMICONDUCTOR MATERIAL EXTENDING BETWEEN THE DENDRITIC CRYSTALS, THE STEPS COMPRISING MELTING A QUANTITY OF THE MATERIAL, BRINGING THE MELT TO A TEMPERATURE AT APPROXIMATELY THE METLING POINT OF THE MATERIAL, CONTACTING A POINT ON A SURFACE OF THE MELTED MATERIAL DISPLACED FROM THE CENTER OF THE MELT SURFACE WITH A LEAST A SURFACE OF A SEED CRYSTAL IN WHICH THE SEED CONTACTS THE MELT AT A POINT EQUAL TO APPROXIMATELY ONE-HALF THE RADIUS OF THE SUPERCOOLED AREA FROM THE CENTER OF THE MELT OF THE MATERIAL FOR A PERIOD OF TIME TO WET THE SEED CRYSTAL WITH THE MELTED MATERIAL, THE SEED CRYSTAL HAVING AT LEAST TWO TWIN PLANES EXTENDING ACROSS AT LEAST A PORTION OF THE SURFACE IN CONTACT WITH THE MELT, THE SEED 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 SUPERCOOLING AT LEST THAT PORTION OF THE MELT EXTENDING FROM THE CENTER OF THE MELT AND ENCOMPASSING THE SEED TO A TEMPERATURE AT LEAST 5*C. BELOW THE MELTING PONT, THE SURFACE AREA OF THE SUPERCOOLED PORTION BEING AT LEAST 0.25 SQ. INCH AND PULLING THE SEED CRYSTAL FROM