US3162507A

US3162507A - Thick web dendritic growth

Info

Publication number: US3162507A
Application number: US191145A
Authority: US
Inventors: Steve N Dermatis; Jr John W Faust
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1961-03-27
Filing date: 1962-04-30
Publication date: 1964-12-22
Anticipated expiration: 1981-12-22
Also published as: GB938915A; NL276481A; CH412820A; BE631688A; GB977933A; NL292060A; DE1246683B

Description

Dec. 22, 1964 s. N. DERMATIS ETAL 3,152,507

THICK WEB DENDRITIC GROWTH 3 Sheets-Sheet l Filed April 50, 1962 Fig.3.

,.IIIIIII Il... Ilunh l5 llnm @3 WITNESS:

zw 7% www Dec. 22, 1964 s. N. DERMATIS ETAI. 3,162,507

THICK WEB DENDRITIC 'GROWTH Filed April 50, 1962 5 Sheets-Shea?l 2 Figi.

Dec. 22, 1964 s. N *DERMA-rls ETAL 3,162,507

THICK WEB DENDRITIC GROWTH Filed April 50, 1962 3 Sheets-Sheet 3 Fig.|3.

Fig.|4.

Fig.l5.

MILS. MILS.

MILS.

4.4 MILS.

Fig. I6.

United States Patent Oilice ldb Patented Dec. Z2, 1964 3,162,567 THICK WEB DENDRI'HC GROWTH Steve N. Der'rnatis, Yonngwood, and .iohn W. Faust, Jr.,

Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. Q0, 1962, Ser. No. 101,? 45 4 Claims. (Cil. 231) This invention relates generally to a grown or pulled crystalline sheet of a material crystallizing in the diamond cubic lattice structure and semiconductor materials in particular.

This application is a continuation in part of application Serial No. 98,618, tiled March 27, 1961, now U.S. 3,129,061, issued April 14, 1964, the inventors and assignee of which are the same as in the present application.

Elongated dendritic crystals of materials crystallizing in the diamond cubic lattice structure have been grown, and the crystals and process for growing them has been set forth indetail in U.S. patent application, Serial No. 844,288, filed October 5, 1959, and assigned to the same assignee as the present application.

In growing two or more dendrites of a semiconductor material simultaneously from a single seed in accordance with the process set forth in U.S. patent application, Serial No. 844,288, now U.S. 3,031,403, issued April 24, 1962, it has been found that the two dendrites sometimes grow together or the space between the several dendrites is sometimes uncontrollably partially bridged by spikes or other lateral crystal growth of solidified material from the melt. The solidied material accidentally formed between the dendrites proper has been almost invariably irregular, thick, has a rough surface, and contains a large number of dislocations. Because of these gross defects none of these grown configurations provides a body of a semiconductor material which is suitable for any use such as the fabrication of semiconductor devices. Consequently, the growth of dendrites from a seed has been so controlled as to provide for only a single dendrite being pulled at one time.

The surprising discovery has now been made that under controlled conditions an outstanding useful body of a semiconductor material can be prepared which is comprised of two or more dendritic crystals joined crystallographically by an almost perfect continuous, wide web or sheet which is rsubstantially dislocation free. The surfaces of this web may have one llat surface of approximate (111) crystal orientation.

An object of the present invention is to provide an elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two parallel elongated dendritic crystals joined crystallograpllically `into a unitary body by a web portion extending between the dendritic crystals along the length of the body.

Another object of the present invention is to provide an elongated body of a material crystallizing inthe diamond cubic lattic structure comprised of at least two parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the adjacent dendritic crystalsV over the length of the body, with a plurality of twin planes extending at least through the dendritic crystals.

Another object of the present invention is to provide an elongated body of material crystallizing in the diamond cubic lattice structure comprised of at least two parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, there being a plurality of twin planes extending through the dendritic crystal portions over the entire length of the dendrites and across the width of the entire body.

A still further object of the present invention is to provide an elongated unitary body of a semiconductor material comprising a central portion and two edge portions, said central portion being comprised of a llat sheet of substantially dislocation free, single crystal, semiconductor material crystallographically joined to the edge portions, each of said edge portions being comprised of a dendritic crystal of the semiconductor material, said central portion having a highly uniform thickness over the length of the body, and said edge portion being substantially uniformly spaced over the length of the body.

Another object of the present invention is to provide an elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into Ia unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being at least as thick as the dendritic crystals, land there being a plurality of twin planes extending through the dendritic crystal portions over the entire length of the dendrites.

Another object of the present invention is to provide an elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being thicker than the dendritic crystal portion of the body, and there being a plurality of twin planes extending through at least the dendritic crystal por-tion of the body over the entire length of the body.

Still another object of the present invention is to provide an elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being thicker than the dendritic crystal portion of the body, at least one surface of the web portion being convex relative to the surface of the dendritic crystal portion, and there being a plurality ot' twin planes extending through at least the dendritic crystal portion of the body over the entire length of the body.

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 l 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 in elevation of a seed suitable for use in accordance with the teachings of this invention;

FIG. 3 is an enlarged fragmentary View in. elevation of a body of material being grown in accordance with the teachings of this invention;

FIG. v4 is a cross-sectional view of the body of FIG. 3 taken along the line IV-IV thereof;

FIGS. 5 and 6 yare greatly enlarged fragmentary views in elevation of alternative seeds suitable for use in accordance with the teachings of this invention;

FiG. 7 is a fragmentary View in elevation of a body prepared in accordance with the teaching of this invention;

FIG. 8 is a fragmentary view in elevation of a modified body prepared in accordance with the teachings of this invention; l

FIGS. 9, 10 and 11 are cross-sectional views of bodies of material prepared in accordance with the teachings or this invention;

FIG. l2 is a top View of the top surface of a melt prepared in accordance with the teachings of this invention;

FIGS. lf3,y 14 and 15 are cross-sectional views of bodies of material prepared in accordance with the teachings of this invention; and,

FIGS. 16, 17 and 18 are views in perspective of a body of material prepared in accordance with the teachings of this invention.

In accordance with the present invention and attainment of the foregoing objects there is provided an elongated sheet-lik`e body of a semiconductor material crystallining in the diamond cubic lattice structure comprised of at least two parallel elongated dendritic crystals spaced apart and joined crystallographically into a unitary body by a web portion ofthe same semiconductor material extending between the dendritic crystals over the entire length of the body, the web portion being at least as thick as the dendritic portion.

The elongated body of a material crystallizing in di- Iamond cubic lattice structure and particularly a semiconductor material, of this invention can be prepared by melting a quantity of the material to be grown, contacting a surface 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, the seed crystal having at least two and preferably three parallel twin planes which come into contact with the mel-t, the seed crystal being oriented with a 11l direction parallel toy the surface of the melt and a 21l direction perpendicular to the surface of rthe' melt, the -twin planes being parallel to the 2ll 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 surf-ace area of the supercooled portion being at least about 0.25 sq. Vin., initiating growthof at least two 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 approximatelyt inch per minute to 4 inches per minute whereby Ia web portion joins the parallel dendrites crystallographically.

V More particularly, in a preferred practice of the process V of thisrinventio'n, a melt of the material to be grown into melting point thereof. The surface of the melt is contacted ywith a previously prepared seed whose configuration and Yorientation will be discussed in detail hereinafter. g

y few secondstoa minute is adequate, the twin planes also bein'g'in Contact with the melt, and, then, at least that portion of the melt immediately adiacentto the seerd is` supercooled rapidly to provide a surface of an area ofat Y least about 0.25 sq. in. of supercooled liquid melt, at least two parallel dendrites should be formed on the seed, following which the seed crystal is withdrawn from the supercooled portion of the melt at a speed of about from 1A. inch to 4 inches and, preferably, at a speed of from 1/4 inch to l 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 intoa unitary body, a web or sheet portion which extends between the dendritic crystals over the entire length of the body. v

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 Yresults comprise substantially equal molar proportions of an element from Group lll of the Periodic Table, and particularly aluminum, gallium, and indium, combined with an element from Group V of the Periodic Table, and particularly phosphorous, arsenic and antimony. metric proportions of Group Il and Group VI elements, for example, ZnSe and ZnS, can be processed. These materials crystallizing in the diamond cubic lattice structure are particularly satisfactory for various semiconductor applications. Furthermore, the diamond cubic lattice structure materials may Vbe intrinsic or they may be doped with one or more impurities to produce nntype 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 1t? for practicing the teachings of this invention. The yapparatus 10.

comprises a base 12 carrying a graphite support 14 for a susceptor or Crucible lo of a suitable refractory mathe susceptor lo in the molten state by a suitable heating n means such, for example, as a radio frequency (RF) induction vheating coil 20 disposed about the suseeptor` 16. Other heating means. mayV be employed such as radiation, electron beam or a combination thereof. The best temperature .control kand results are realized when the RF coil extends above the top` of the susceptor. le. A suitable source of energy and control means, not shown, are employed to supply an alternating electrical current,

for example, from Y kc.`to 5 megacycles, to the RF f coil 20 to maintain a closely controlled temperature in the body of the melt 1.3. 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 lafew seconds, for example, in 5 to. 10V seconds to a temperature at least 1 degree below the rnelt-V ing temperature and preferably to supercool at least Va 1 portion of the melt from' about'5 to 10 C. An apertur'ed cover 22, comprised of asuitable material such as forexample, molybdenum, tantalurn or tungsten, closely Y fitting the top of the susceptor f ispr'ovided in order to i maintain a low thermal gradient above the top ofthe melt.v Passing through an aperture Z4 in the. cover 22 is Also. compounds comprising stoichi- E e3 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 Ztl 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 i2 so that a circulating current of such protective atmosphere is present. Depending on the crystal material being produced in the apparatus 1t?, the protective `atmosphere may comprise a noble gas such as helium or argon, or a reducing gas such las 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 a 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 containing the component may be disposed in the enclosure 32 to maintain therein a vapor of such compound at a partial pressure suicient 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 jaclret, to maintain the walls thereof ata 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 obtainable 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 that portion directly below the Iaperture 24 since heat radiating from surface 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 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 4S. 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.

Referring to FIG. 2 of the drawing there is illustrated, in a greatly enlarged View one type of a seed 12e which may be used with considerable success in accordance with the teachings of this invention to produce the elongated sheet-like body 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 Serial No. 844,288, more fully identified hereinabove. It will be understood, of course, that the seed is comprised of the same material as the melt.y

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

faces

50 and 52 is that indillt) cated by the crystallographic direction arrows at the right and left faces, respectively, of the figure. It will be noted that the horizontal directions perpendicular to the flat faces 50 and 52 and parallel to the melt surfaces are (lll). The direction of growth of the elongated body of this invention will be in a 2ll crystallographic direction. If the faces 5d and 52 of the seed 126 were to be etched preferentially to the lll planes with a 1% to 30%, by weight, aqueous sodium hydroxide solution at from C. to 90 C., they will both exhibit equilateral triangular etch pits 58 whose vertices 59 will point upwardly while the bases 60 will be parallel to the surfaces of the melt. 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.

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.

Normally, the distances between successive twin planes are not the same. The most satisfactory results are obtained employing seeds with three internal twin planes with one twin plane being from less than l micron, for example, from about 0.5 to 0.8 micron from the central twin plane, and the third twin plane being disposed from about l to 8 microns from the central twin plane.

The most satisfactory crystal growth is obtained by employing seeds of the type exhibited in FIG. 2, that is, a section of -a previously grown dendritic crystal wherein three twin planes are present interiorly and are continuous across the entire cross-section of the seed. lt should be understood that the seed need not have hat exterior surfaces Sti and 52, it is only necessary that the (lll) 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 having an odd 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 being 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 may `not be obtainable as with the preferred three twin plane seed crystal as shown in FlG. 2. 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 bases 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. lf the seed 126 were to be inserted into the melt so that the vertices 59 pointed downwardly, very erratic growth would be produced which is not only of non-uniform dimensions but at angles of 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 be a meniscus-like contact area between the seed crystal and the surface of the melt. Such meniscus contact area should be maintained throughout the process.

@nce 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 super-cool at least be formed between the dendrites.

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 illustrated in FIG. 3, there will be observed in a period of time of the order of seconds after the heat input is reduced to the crucible, the supercooling being from about 5 C. to 10 C., an initial growth or enlargement 62 which has an elongated hexagonal horizontal cross-section and 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 10 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. Whenv the spikes are from two to three millimeters in length and the total length of the hexagonal growth is from '1A inch to as much as 1% 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 substantially less than 1A inch per minute is employed the most desired crystallographic structure may not be obtained.. If a pull rate materially greater than 4 inches per minute'y is `used, the web portion of the body ordinarily will not 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 63 and 7d formed at each end of the hexagonal area. Accordingly, two parallel dendritic crystals or dendrites are being pulled from the rnelt at one time from a single seed. Tehse 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 extends across the space between the two parallel dendrites 63 and 7h. The web or sheet 72 is crystallographically joined to the two dendrites ed and 75l, that is, the general crystal structure of the dendrites is continued through the weby 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 at the higher pull rates its thickness will not normally exceed that of the two dendrites 68 and 7d. The web 7 2 will be substantially dislocation free.

The two dendrites 68 and 7@ will remain substantially parallel over the length of the complete elongated body and will thus control the width of the web or sheet portion 72. The thickness of the web or sheet portion 72 will dcpend, vas pointed out above, to a degree on the thickness of the dendrites 63 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.

lnvaccordanoe with the present invention a body is grown in which the web or sheet portion is as thick as,

, or thicker than, the dendritic portions of the body.

As pointed out above, the two important parameters in growing the material of this invention are the degree of supercooling of the melt and the pull rate. Of these two parameters, the degree of supercooling can only be varied over a relatively small range; however, the pull speed can be varied over a wide range making it the more variable parameter.

If the degree of supercooling is kept constant, it has been found that the thickness of the web or sheet portion can be increased by lowering the pull rate. For example,

it has been found that if the melt is maintained at approximately 8 of supercooling and the pull rate is decreased Vto 0.75 inch per minute or less, a body will be grown in `which the web or sheet portion is as thick as or thicker than the dendrites disposed on either side of the web or fsheet.

With reference to FIG. 4, there is illustrated a section lof the elongated body of FIG. 3 taken along the line 1V-V- This shows how the dendrites d and 7d are crystallographically joined to the web or sheet 72. Web "72 is illustrated as being thinner than the dendrites 68 and 7h. A more detailed description will be given subsequently.

While, as shown in FIG. 2, the section of a previously grown dendrite or dendritic crystal is the preferred form yof seed to institute growth of the elongated body of this invention7 other seed forms have been found satisfactory. For example, and with reference to FIG. 5 there is shown .another suitable seed 74. The seed 74 is comprised of two previously grown dendrites 76 and 73 without any web therebetween, which are joined together at one end by a hexagonal portion Sil at

spike portions

82 and 84. The seed 74 of FIG. 5 may be obtained by seeding a lsupercooled melt with a seed of the type illustrated in PEG. 2 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 FG. 6 there is illustrated another seed 86 which is essentially the structure of FIG. 3 with a web between two parallel dendrites. After a long sheetlike body has been pulled, the portion 86 shown in FIG. 6 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 web or sheet portion extending lbetween the dendritic crystals over the entire length of the ody.

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 for seeding. Such a seed 94 is shown in FIG. 7 and is comprised of edge portions comprising dendrites 96 and crystallographically joined by a web or sheet portion lf a seed such as is shown in FiG. 5 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 FlG. 8, will e comprised of four dendrites 102, 164, Mie and lltl crystallographically joined by web or sheet portions lid, 1.12 and 1M respectively disposed Vtherebetween. 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 three inches with three or more parallel dendrites, and up to one 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 sand blasting, fracturing, or electron beam cutting, or by any other similar process known to those skilled inthe art.

The body prepared in accordance with the teachings of this invention is comprised of at least two dendritic 9 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 -o'f dendritic crystals having two highly parallel at faces which may comprise ya series of fiat portions differing by steps of about 50 angstroms from each other. rlhe dendritic portions will have a thickness of from approximately 2 to 50 mils and the width across the at faces may be from mils to 400 mils and even Wider. The lsurface 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 dlat 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 0.01 mil in length of over 24 inches. A more detailed description of the dendritic portion of the bodies of this invention will be found in U.S. Patent 3,031,403, issued April 24, 1962. The dendrites set forth in this patent application are essentially identical to the dendritic portions of the body of this invention.

The web or sheet portion of the body crystallographicallly 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, and may `be as thick as .50 mil. For some purposes the web portion normally will be grown under conditions so that it is much thinner than the dendritic crystal between which it is disposed. However, by following the procedure of this invention, as pointed out above the web or sheet portion will be as thick as or thicker than the dendnitic crystals. Also the web portion will be more uniform than the dendritic portions.

When the web or sheet portion is thinner than the dendrites 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 the dendrite portions. The height of these steps when present are generally no more than 300 ang-stroms.

When the web or sheet portion is thicker than the dendrites the surfaces of the web or sheet portion will vary. With reference to FIG. 9, there is illustrated in cross-section a segment Stift of a body of material grown in accordance with the teachings of this invention.

The segment 300 is comprised of two dendrites 3M and 304 crystallographically joined by a web or sheet portion Fatto. Surfaces 363 and 310 of dendrites 302 and surfaces 312 .and 3M- of dendrite .the are (111) planes. Surface 316 of the web or sheet portion 365 is convex upwardly from the surfaces 303 and 312 of dendrites 392 and d, and surface 318 of the web or sheet portion 366 i-s convex downwardly from the surfaces 310 and 31d of the

dendrites

302 and 304.

In a body such as that shown in FIG. 9, the twin planes 320, 322 and 324 within dendrit'e 302 and

twin planes

326, 32S and 330 within dendrite 304 extend entirely across the respective dendrites, whose interior edges are indicated with a dotted line for purposes of illustration only, and continue through the web or sheet portion 306 of the body.

In some cases, and as illustrated in FIG. 10, when the seed has contained two or another even number of twins the :

twin planes

421, 423, 425 and 427 and S21, 523, 525 and 527 extend only across the

dendrites

302 and 304 lil and terminate entirely `within the dendrite portions 320 and 304.

It is also possible, irrespective of the particular crosssection of the body, `to have one or more extraneous twin planes extending through at least a part of the web or sheet portion which are not extensions yof the twin planes in the dendrites. Such an extraneous twin is denoted as 62@ and illustrated in FIG. l0.

With reference to FIG. 11, there is illustrated in crosssection a segment 340 of another body of material grown in accordance with the teachings of this invention.

The segment 340 is comprised of two dendrites 342 and 341i crystallographically joined by a web or sheet portion 346.

Surfaces

348 and 350 of dendrite 342 and surfaces 352 and 354 of dendrite 344 are (lll) planes. Surface 356 of the web or sheet portion 346 -is convex upwardly from the surfaces 348 and 352 of the dendrites 342 and 34M respectively. Surface 358 of web or sheet portion 346, while resulting from non-faceted growth, is flat, and is parallel to a (111) surface. Such a surface may be described as approximating a (111) surface.

The internal twin plane st-r-ucture of the body 340 of` FIG. 11 may be the same as either that shown in FIGS. 9 or ll, `that is, the twin planes may extend only across the dendrites or they may extend across the entire crosssection of the body.

A body such as that illustrated in FIG. 9 is grown in the same manner as a body in which the web or sheet `portion is :thinner than the dendrite except that the pulling rate employed is 0.75 inch per minute or less.

To grow a body of the type illustrated in FIG. 1l, a pull rate of 0.75 inch per minute or lessl is employed and the growing body and melt are maintained in an asymmetrical thermal distribution relation.

A suitable asymmetrical thermal distribution condition can be established in one of several ways. For example, if FIG. 12 is considered to be a top View of surface 45 the melt l within the crucible 16 of FIG. 1 and the broken lines A, B, C and D denote isothermal lines, with D being the isotherm denoting the boundary of the supercooled region within the melt, then a suitable asymmetrical thermal condition, for the growth of a body having the cross-section shown in FIG. 11, can be eS- talbiished if a suitable seed is introduced into the melt at a point between E, the center of the supercooled portion of the melt and the boundary D, for example, a point on the line X.

When a body is grown in such a manner, the side nearest the isotherm D will approximate a (111) surface, such as surface 358 of body 340 of FIG. 11, and the surface nearest center point E will be convex such as surface 356 of body 340 of FIG. 11.

Another method of establishing the necessary asym` metrical thermal conditions for growing a body of the type shown in FIG. 1l consists of placing the induction heating coil 20 of FIG. 1 closer to one side of the susceptor than the other.

Other equally suitable methods of establishing asymmetrical thermal conditions within the' melt will be apparent to those skilled in the art, and the above two examples are intended only as examples and not as limitations of the teaching of this invention.

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

In order to show the differences in the thick webbed bodies of the present invention over the thin webbed bodies of the parent patent application Serial No. 98,618, reference should be had to FIGURES 13 to 16.

The internal structure of the bodies in which the web or sheet portion is thinner than the dendrites falls into three classes and these are illustrated in FIGS. 13, 14 and l5.

The most common, and preferred, internal structure of the web or sheet portion is the single crystal structure. With reference to FIG. 13, 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 sroe of the web or sheet portion 116. The twin planes do not extend into the web portion. In this configuration, the twinrplane structure in the dendritic portion on each s1de 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. 14, in some cases at least one twin plane, for example, twin plane 130 will extend across both dendrites 132v and 134 and through web or sheet por- Vtion 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. 15, inwhich is illustrated a body of material in which

twin Vplanes

140 and 142 originating in

dendritic portions

144 and 146 respectively, are mismatched and form an incoherent twin boundary 143 within web or sheet portion 150. Such bodies are less desirable than those'shown in FIGS. 13 and 14, but are still suitable for use in fabrication of certain semiconductor devices.

The internal structure of the web or sheet portions of y the body of material of this invention is controlled or at least influenced by (l) the twin structure of the dendritic portions on each side of the sheet, (2) the thickness of 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 can be easily made to 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 crystallographi- ,cally joined to a unitary body by a thin web or sheet portion extending between theV dendritic crystals over the entire length ofthe body of the present invention are relatively flexible 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 greaterY in continuous lengths as desired. The thinner the body the smaller the radius of the coil that may be made therefrom. Y v Y kThe Vbodies Vof materialV grown in accordance with the teachings of thisinvention in which the web or sheet portion is thinner than the dendritic portion, have surfaces both onV the dendritic portion and on the web portion of such perfection that in the case of4 semiconductive materials they Vmay be employed Vfor fabricating semicon- (111) orientation as grown and the web or sheet portion have surfacesthat very closely 'approximate (111) planes.

In thelmaking of semiconductor devices such as diodes transistors, photodiodes and other similar semiconductor- V jde'vices, the (111) surface is a, particularly desirable orientation. Y

As discussed hereinabove, the bodies of material grown in accordance with the teachings of this invention in which the web or sheet portion is as thick as or thicker than the dendritic portion has at least one surface of the web or sheet portion which is convex away from the dendrite surface. The other surface may also be convex away from the dendrite surface or it may be a at surface approximating a 111) surface.

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 specific type of semiconductivity Vand 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. .Y

One noticeable advantage obtained in practicing the present invention is that, while the previously known processes for growing crystals by the Czochralski tech-V be much closer tothe proportions in the melt using theV 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 1000 ohm-cm. Pure silicon bodies with higher resistivities have also been obtained.

The elongated bodies of thisinvention and prepared in accordance with the teachings of this invention, provide an excellent material for the ellicient and economicalV fabrication of semiconductor devices such as transistors, diodes, two and three terminal four region 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 Y have a gain (beta) of at least 150. In addition, solar cells having an eciency of from 10 to 15% have been made on silicon bodies grown in accordance with the teachings of this invention. y Y

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 sand blasting, electron beam cutting, chemical etching or the like and the device fabricated entirely upon the single crystal web or sheet portion ofthe body.

The following examples are illustrative of the practice of this invention; wherein, Examples I to IV illustrate bodies in which the dendrites are joined by thin webs or Y sheets and Examples VI and VII illustrate bodies in which the dendrites are joined by thick webs.V v.Example V illustrates the criticality of the surface area of the Vsupercooled melt.

Example I An elongated body comprising a thiniweb was pre; Y pared in accordance withthe teachingsrofV the invention A. seed comprising a section cut from a previously grownV dendrite'and having threeinterior twin planes extending entirely therethrough and oriented as in FIG. 2 of the drawings, that is, with the etch pit verticesdirected up.

Wardly, is held vertically in a holder and is'lowered un*il its lower end touches the surface of the molten silicon.

The contact with the molten siliconV is maintained until agsmall portion of the end of the' dendritic seed Vcrystal is thoroughly wetted'and is melted.V Thereafter, the tem- Gperature of the melt is lowered rapidly in a matter of were attached to the spiked ends of the hexagonal portion attached to the seed and each was of a thickness of mils and was approximately 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 150 mils. The grown dendritic crystal edge portions has substantially flat and parallel faces from end to end with (lll) 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. 16 with the major dimensions set forth.

The dendritic portions of the body were 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 flat surfaces over the entire length of the body and to be highly dislocation free.

In a similar manner sheet-like bodies are produced using a germanium seed having three twin planes. These bodies have dendritic edge portions and single crystal web portions having a thickness of 0.3 mil extending therebetween. Likewise, webbed dendritic bodies may be prepared from gallium arsenide and other Ill-V compounds.

Example Il The procedure of Example I was repeated except that the pull rate was increased to 3 inches per minute and the melt comprised of silicon was doped with two parts per billion of boron.

The resulting sheet-like body resembled that of Example I, except the web portion was thinner, and had a p-type semiconductivity and a resistivity of 200 ohms-cm.

Example III The procedure of Example I was repeated except that the pull rate was increased to 4 inches per minute and the melt comprised of grams of silicon was doped with .00067 gram of arsenic.

The resulting body was similar to that of Example I, the web being thinner, and the dendritic portions were 7 mils thick, had n-type semiconductivity, and a resistivity of .01 ohm-cm.

Example IV Example V The procedure of Example I was repeated except that the surface area of the supercooled melt was limited to ld 1/s sq. inch. The dendritic growth was observed lbut there was no web or sheet portion between the two dendrites.

Example VI The procedure of Example I was repeated except that the pull rate was 0.50 inch per minute. The melt was supercooled 8 C.

The body as grown was comprised of two dendrite portions of 30 mils thickness crystallographically joined by a web having a thickness, at the thickest point, of 40 mils. The surfaces of the web were convex away from the dendritic portion of the body.

The body was grown to a length of about 18 inches and is shown schematically in FIG. 17 with the major dimensions being as set forth.

Example VII The procedure of Example I was repeated except the pull rate was 0.45 inch per minute. The melt was supercooled 8. The seed was introduced olf center relative to the axis of the supercooled portion of the melt as illustrated in FIG. 12.

The body as grown was comprised of two dendrite portions crystallographically joined by a web having a thickness, at the thickest point, of 50 mils. One surface of the web portion was convex away from dendrite portion of the body. The other surface of the web portion was flat and approximated a (111) surface.

The body was grown to a length of about l2 inches and is shown schematically in FIG. 18 with the major dimensions set forth.

It will be understood that while it is a preferred embodiment of this invention to produce elongated bodies having liat surfaced portions of (111) orientation disposed between at least two dendrites, the dat surface portions can also be grown with or (100) surface orientation. It is also possible to grow the elongated bodies of this invention by pulling properly oriented seeds from the melt in the 1l0 and 100 directions.

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.

We claim as our invention:

1. An elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being thicker than the dendritic crystals, and there being a plurality of twin planes extending at least through the dendritic crystal portions over the entire length of the body. 2. An elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being thicker than the dendritic crystals, at least one surface of the web portion Vbeing convex relative to the surface of the dendritic crystal portion of the body, and there being a plurality of twin planes extending at least through the dendritic crystal portions over the entire length of the body.

3. An elongated body of a material crystallizing in the diamond cubic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined crystallographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body, the web portion being thicker than the dendritic crystals, both surfaces 'of the web portion being convex relative to the surface of the dendritic crys-L tal portion of the body, and there being a plurality of t5. twin planes extending at least through the dendritic crystal' portions over the entire length of the body.

4. An elongated body of a material crystalli-zing in the diamond cnbic lattice structure comprised of at least two substantially parallel elongated dendritic crystals joined erystailographically into a unitary body by a web portion extending between the dendritic crystals over the length of the body,- the web portion being thicker than the dendritic crystals,y one surface of the web portion being convex relative to the surface of the dendritic crystal portion and one surfaceV of the web portion being at and approximatinga (lll) surface, andfthere being a plurality of twin planes extending at least through the UNTED- STATES PATENTS 3O4l,403 4/62 Bennett 23-223.5"X

OTHER REFERENCES Acta Metallurgical, vol. 5, 1957, pages 53:h and 54;

Acta` Crystallographica, vol; 8', part 6, June 1955,V

pages 353 and 354. y MAURICE A. BRINDISI, Primary Examiner.-

Claims

1. AN ELONGATED BODY OF A MATERIAL CRYSTALLIZING IN THE DIAMOND CUBIC LATTICE STRUCTURE COMPRISED OF AT LEAST TWO SUBSTANTIALLY PARALLEL ELONGATED DENDRITIC CRYSTALS JOINED CRYSTALLOGRAPHICALLY INTO A UNITARY BODY BY A WEB PORTION EXTENDING BETWEEN THE DENDRITIC CRYSTALS OVER THE LENGTH OF THE BODY, THE WEB PORTION BEING THICKER THAN THE DENDRITIC CRYSTALS, AND THEE BEING A PLURALITY OF TWIN PLANES EXTENDING AT LEAST THROUGH THE DENDRITIC CRYSTAL PORTIONS OVER THE ENTIRE LENGTHOF THE BODY.