US2754259A - Process and apparatus for growing single crystals - Google Patents

Description

y 10, 1956 P. ROBINSON ETAL PROCESS AND APPARATUS FOR GROWING SINGLE I CRYSTALS Filed Nov. 29, 1952 ELECTRON ELECTRON FOCUSING FOCUSING MEANS 12! MEANS INVENTORS PRESTON ROBIQISON 39 EDfi ARD D. OBRIAN 'T'H IR ATT United States Patent PROCESS AND APPARATUS FOR'GROWIN G SINGLE CRYSTALS Preston Robinson, Williamstown, Mass, and Edward D.

OBrian, Wilmington, DeL, assignors to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts Application November 29, 1952,-Serial No. 323,232

8 Claims. (Cl. 204192) The present invention relates primarily to the growing of large single crystals and especially crystals which can be used in modern electronic devices, such as rectifiers, transistors, etc.

A variety of methods of producing single crystals are given in the prior art. These include various procedures for withdrawing a thin wire from a liquid melt, depositing a layer from an electrolytic bath, or growing a large crystal at the expense of a multitude of small crystals in the solid state. A still further procedure which has been discussed, but never widely used, relates to the formation of single crystals from a vapor. Two types of procedures falling within this last category have been developed in the prior art. The first of these is exemplified by the Thompson Patent No. 1,450,464 which deals primarily with the vaporization of what is, in efiect, a refractory oxide, and a subsequent condensation of this oxide upon a suspended grid. In order to utilize this process, extreme elevated temperatures must be developed with electrical resistors, or the like, and in addition, the temperature differentials within the apparatus have to be quite carefully controlled. In many respects, the procedure of this patent is analogous to certain liquid crystallization procedures, but is much less effective thanthese procedures, and as nearly as can be determined, has never been used commercially. V

A second means of producing single crystals from a gas phase involves the thermal decomposition of a metal halide salt with the aid of hydrogen upon a hot filament. This procedure has been satisfactorily applied with tungsten, molybdenum, tantalum, iron, zirconium, and titanium. For many purposes, it produces a satisfactory single crystal of a metal, but the procedure is limited to metals alone and to the use of a hot single crystal filament as a seed nucleus. At times; polycrystalline aggregates are produced by a variation oi this process in the commercial production of zirconium.

An object of the present invention is to produce anew and more practical method of growing single crystals than is found in the above and related prior art. A further object of the inventive concept is to develop satisfactory equipment which can be used in easily and conveniently growing single crystals. These and other objects of the invention, as well as the advantages of it will be apparent from the following description and the claims, as well as the accompanying drawings in which:

Figure 1 diagrammatically pictures a partially sectional view of a crystal growing apparatus of the present invention;

Figure 2 diagrammatically indicates a partially sectional View of a modified crystal growing apparatus of the in vention; and

Figure 3 shows a material holder to be used with the apparatus of Figure 2.

In all figures, like numerals designate like components.

Briefly, the above aims of the invention are achieved through the use of a process in which a stream of electrons is used to vaporize the material to be deposited upon an adjacent single crystal base, and this base is, in

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turn, slowly moved so as to accommodate more and more material. In order to accomplish these ends, an apparatus is provided with means to produce electrons, to control the electron stream produced, to remove the base containing the single crystal produced, and to carefully and accurately control the atmosphere in which the process is carried out. Further equipment employed with the invention will be discussed in more detail.

The apparatus In order to more completely explain the process, reference is made to the drawings. In both Figures 1 and 2, the entire crystal growing procedure is carried out within a suitable glass or ceramic casing 10. The actual crystals produced are indicated as 11 and are placed upon a suitable support whose position in turn is controlled by means of threaded shaft 13 projecting through threaded bushings l4 and controlled by suitable handwheels 16 or other mechanical means not shown. The single crystal bases are grounded through the shafts and the bushings'at 17. In order to control the atmosphere with the apparatus, gas inlet pipes 18 and gas outlet pipes 19 are provided at opposite extremities of the housing 18 and are, in turn, connected to conventional vacuum apparatus which is not shown.

In order to provide a stream of electrons, cathodes 21 are connected to a suitable source of high voltage low amperage current by means of leads Z0 projecting through the casing 10. The'opposed anode '23 in Figure l is also connected to the same source by means of a lead 22. This anode'23 is of an annular shape surrounding the center bushing 14. In Figure 2, the anode 24' is of a substantially cylindrical shape and disposed completely around the single crystal 11 above the bushing 14. Here it is shown also projecting above the top edge of the single crystal.

To control the electron flow between the anode and cathode, electrostatic electrodes 39 are shown in Fig ure 1. In Figure 2 electromagnetsSl are used to accomplish this sameend.

The material to be vaporized in the embodiment of the invention shown in the' initial figure is held within the appropriate support 32. In Figure 2 it is held'within the support 33 which is shown in greater detail in Figure 3 as being composed of smaller wedge shape'section D of one material3 and a larger section of a second material 38. This support 33 is adapted to be rotated around a center shaft 37 by means of gears 36 connected to handwheel 34 through a bushing 35 which projects into the center cavity of the housing 19.

p The actual cathode employed with the invention may be composed with a variety of essentially stable materials. Either the so-called hot or the so-called cold cathodes known to the electronic industry may be employed with the invention. However, it is preferred to utilize an essentially cold cathode consisting of aluminum or an aluminum alloy capable of emitting electrons of relatively low potentials. Such alloy electrodes contain a restricted percentage of alkali or alkali earth metals. An alloy of aluminum'and magnesium presently appears to be preferred. It is to be understood that the actualcathode shape is comparatively immaterial, although for many applications and specific designs of equipment, certain shapes will show amarked superiority over others. The presence of an adherent film of oxide such as an aluminum oxide does not appear to inhibit the emission of electrons from the underlying surface of a cathode and is hence unobjectional. It will be realized by those skilled in the art that such an adherent film frequently protects the cathode with regards to gases, such as oxygen, hydrogen, nitrogen, halogen, and the like, which are apt to be present within the apparatus either prior to use or during operation of the device. The cathode during use is subjected to a certain amount of positive ion bombardment which at times can exercise a detrimental effect upon its longevity.

The anode material employed with the invention must also be highly stable under conditions of use and during'exposure to gases such as are indicated in the preceding paragraph. Here again, the preferred materials are aluminum or aluminum alloys which can be readily formed into the desired shapes required and which can also be formed with a thin adherent film of an oxide which is relatively diflicultly decomposed by electron bombardment. Other alloys, such as certain iron and steel materials containing silicon, chromium, nickel, and the like, and various scarcer metals, such as tantalum, titanium, etc. also form satisfactory anodes.

Those skilled in the art will realize that no material is completely satisfactory for all applications, and frequently, it will be necessary to replace both the anodes and cathodes used with the invention. For this reason, they preferably should be manufactured in such a way as to be easily replaced and should be relatively cheap.

The base 12 containing a single crystal seed is capable of a number of specific constructions. For convenience in the drawings this base is shown consisting essentially of a metallic stud to which is attached, as by soldering, a layer of single crystals. In a preferred embodiment of the invention, a single crystal wafer is attached to this stud as by soldering in such a manner that a single face of the crystal is exposed to the material vaporized by an electron beam. This is so that essentially all of the material deposited upon the single crystal base will be deposited upon this face, and so that the crystal may be readily cut across planes parallel to the growing face by techniques such as are employed in the diamond cutting industry into a plurality of wafers. Also, by this expedient, it is possible to obtain blocking layers which are substantially at right angles to the axis of the single crystal produced. In order to prevent extraneous growth, the crystal wafer initially employed preferably projects beyond the edges of the base stud 12. Collecting means (not shown) can be employed to pick up those portions of the vapor produced by the electron beam which are not deposited upon the single crystal. If desired, various extraneous heat producing means, such as, for example, infra-red lamps can be used to heat the growing face of a single crystal 11 in order to help control the rate of crystalline growth.

Relatively thin sheets of metals or compounds can be used as material to be vaporized in either of the layers 32 or 33. When metals are used, they must be in thin enough sheets so that these sheets are electron permeable. Layers of compounds must also meet the same test. For best results, a porous mass, such as is produced by sintering a plurality of small crystals or by powdered metallurgical techniques is employed. Alternatively, pressed metal wool can be used as the electron beam target. Obviously, a target such as is shown in 32 or 33 can comprise a mixture of a relatively stable material and a relatively unstable material with these terms being used with reference to electron stability. Frequently, it is desirable to use a stable screen upon which is placed a mass of porous powder. A suitable stable material for a number of applications is zinc.

In Figure 3, the target 33 is composed of two separate types of material 38 and 39. The apparatus of Figure 2 is, as readily seen from the drawing, adapted so that this target 33 may be rotated so as to expose either material 38 or material 39 to the electron beam. Other similar arrangements can be used.

Although in Figure 1 only two sets of electrodes are shown and in Figure 2 only two electromagnets are pic'- tured,'it is to be understood that electrostatic or electromagnetic focusing means are used, as indicated by the legends for example. By their use,

the electrons from an extremely large emission source or area can be concentrated into a relatively small area according to a preferred embodiment so as to obtain a much more intense effect than could previously be produced.

The apparatus shown in the figures of the drawing operate most effectively at less than 2.0 millimeters of mercury pressure. For most applications, it is desired to have the little residual gas within the equipment be of an inert nature, such as, for example, helium, argon, neon, krypton, or the like. For specialized purposes, however, as for the production of oxide blocking layers, or the production of oxide single crystals, or the production of various interstitial alloys, various atmospheres, such as, oxygen, hydrogen, nitrogen, etc., can be used provided the amount of these gases does not exceed the limit for a high vacuum specified above.

The operation In operation, the various pieces of equipment specified are assembled substantially as is shown in the drawing. The space within the housing 10 is exhausted to the extent that the available equipment permits and is filled with a sweeping gas, such as argon, krypton, or the like, and again exhausted to 2.0 millimeters of mercury pressure. At this point, a high D. C. potential is used to establish a discharge between the anode and the cathode. The precise amount of this potential, as well as the current employed, depends upon a large number of factors,

such as, for example, the size of the apparatus, the degree of concentration, position of elements, the vacuum, the temperature, the material being treated, etc. Usually, with metals, best results are obtained when the D. C. potential is arranged from 3000 to 20,000 volts, together with a current of from about 10 to 200 milliamps. Frequently, during the initial stages of use of the apparatus, it is necessary to adjust the electron focusing means in accordance with established procedures. As the process herein described continues, it is necessary to slowly withdraw the single crystal 11 by the use of the handwheel 16 so as to keep the growing face of the crystal in a subsequently unchanged position. Also, with the embodiment of the invention shown in Figure 2, at various stages during the process, it is advantageous to rotate the target 33 so as to present either a new batch of the same material to be vaporized and/or a second compo nent material to be vaporized for a short period of time. As will be further seen, at predetermined periods oxidizing, reducing, or other gases may be introduced into the apparatus. When compounds are being vaporized which tend todecompose, preferred results are obtained by the use of a sweeping stream of an inert gas in order to remove the volatile decomposition products.

As an example of the present invention, a single crystal of approximately /z inch square was soldered using conventional techniques to a brass stud corresponding to the stud 12 so as to have one face of the crystal being substantially parallel to this stud. The apparatus shown was assembled utilizing an electrode target 32, ,5 inch thick, of a pressed, sintered mass of initially mesh germanium metal particles. The degree of sintering was barely enough to firmly hold the individual particles in a unitary fixed relationship. At this point, the equipment was exhausted to a pressure of 1.5 millimeters of mercury, filled with argon and again exhausted to a pressure of 1.0 millimeter of mercury. Next, a D. C. potential of 11,000 volts with a current of 40 milliamps was applied between the anode and the cathode so as to start electron flow. In the apparatus used, this cathode was approximately 12 inches from the anode and 4 inches from the target material 32 which, in turn, was positioned roughly 1.5 centimeters from the top growing face of the germanium crystal. The process was continued for a period of 2 hours, the growing crystal face being held in substantially the same position throughout the period by means of manual control. 7

Blocking layers every 5 millimeters in-distance-throughout the length of the crystal produced as in the preceding paragraph were formed in a separate run by introducing' an amount of oxygen equivalent td-the amount required to exhibit a partial pressure of .5 millimeter of mercury within the apparatus at appropriate periods.

A single-crystal was further grown using thesame relative distances, currents, etc. as given in the second preceding paragraph by employing as a target a disc, such as is shown in Figure 3 of the drawing. The main portion of this disc was composed of sintered germanium powder as above, while the remainder of it contained sintered lead powder of the same size, which was prepared in substantially the same manner. At periods corresponding to every millimeters of crystal growth, this target was turned away from the germanium portion so as to present the lead portion of the electron beam for a period of approximately 10 seconds.

The inventive concept herein involved is not limited to the formation of metallic crystals and/or the formation of crystals from a comparatively pure metallic substance. Thus, with the invention, it is possible to utilize a chloride, oxide, sulfide, or similar compounds as the target material and by either directly vaporizing this compound, or decomposing this compound, and then vaporizing the remainder. The exact result accomplished depends to a large extent upon the precise potential applied to the electrodes, as well as the other factors previously indicated.

As an example of the procedure in which a single crystal is formed from a chloride, the following data is given. In an apparatus substantially the same as was used in the formation of a germanium crystal indicated in the first example above, a target was employed containing finely divided silver sulfide which had been pressed into a compact pellet /2 inch thick. At a D. C. potential 1800 volts between the anode and the cathode with a current of 30 milliamps, the compound was substantially decomposed to a large extent leaving behind a mass of silver. As the voltage was increased to 4,000 volts with a current of 35 milliamps, this silver was vaporized and deposited upon a single crystal base as indicated. The same pressures and atmospheres as used with the initial germanium example were employed herewith.

The apparatus used in the initial example was employed to vaporize a pressed layer of 100 mesh barium oxide particles A inch thick under the atmospheric conditions and with the various pieces of apparatus positioned as indicated. A voltage of about 7000 volts, together with a current of approximately 40 milliamps was employed.

The spacing of the single crystal base with respect to the material being vaporized is in many cases a critical feature of the instant invention, best results being obtained with this crystal face being positioned as closely as conveniently possible to this material, but with the face being out of contact with it. If the material being vaporized is in contact with the single crystal, various impurities present therein, which otherwise would be left with the unvaporized material, are apt to be found in the final single crystal. If the growing face of the single crystal is more than about 5 centimeters from the nearest particle of material being vaporized, this material will tend to have lost a substantial portion of its initial energy, and external means, such as the heating means referred to earlier, are apt to have to be used in order to obtain a proper crystal formation.

Those skilled in the art will realize that the present inventive concept is capable of wide variation, and may be employed with a variety of materials and conditions, and different apparatuses patterned after the specific equipment pictured. Such modifications, changes, and ramifications of the invention are to be considered part of the present inventive concept insofar as they are defined by the following claims.

Althoughthe present invention is primarily intended to be used in the formation of crystals .ofsemi-conducting metals,'such as germanium, silicon,,etc.,,it can satisfactorily'be applied to the production of singlecrystals of metals such as iron, nickel; copper,-vanadium, platinum, tungsten, tantalum, molybdenum, zirconium, titanium, etc. It is contemplated that further work will prove that crystals of such non-metals as carbon can-be grown from an amorphous carbon target by the process herein disclosed. When the herein described apparatus and process is applied to the vaporization of either target compounds or elements contained in such compounds, it is preferred that these starting materials are solely binary compounds, such as sulfides, oxides, chlorides, fluorides, bromides, etc. in order that there be as little difiiculty as possible through the formation of undesired vaporization products. Mixtures of two or more compounds of the indicated categories can be used with the present invention. Particularly suitable binary compounds contain common metallic ions, such as, for example, ions of the metals indicated above. With either metals and/or compounds, it is possible to obtain a material degree of purification in growing single crystals in accordance with the present invention by appropriate control of the electron potential. For any application, those skilled in the art will be able to easily and conveniently determine the correct voltage.

What is claimed is:

1. A process for producing single crystals which are suitable for use with a variety of electrical applications which comprises establishing electron conductance under vacuum between an anode and cathode, focusing the electrons from said cathode into a beam which intersects a target material and which vaporizes at least a portion of said material, condensing said vapor upon a single crystal base positioned adjacent to said target means and slowly withdrawing said single crystal base so as to maintain the growing face of said crystal in a substantially fixed position.

2. A process as defined in claim 1 above wherein a minute quantity of a gas capable of reacting with said vaporized material is introduced at predetermined periods during the growth of said crystal.

3. A process as defined in claim 1 above wherein said target material is changed at a plurality of predetermined periods during the growth of said single crystal so as to expose a diiferent material to said electron beam and to provide impurities in said single crystal.

4. An apparatus for growing single crystals, said apparatus having a vacuum chamber, electron beam generating structure including an anode, cathode and focusing means connected to supply a focused concentrated beam of electrons in said chamber, target securing structure to hold a target material from which the single crystals are to be grown, said securing structure being mounted in the path of the concentrated beam so that the material is heated by the beam and converted to vapor, a support positioned immediately adjacent the target securing structure for holding a single crystal seed material adjacent the target material to grow by condensation of the vaporized material, and withdrawal elements connected to the single crystal support for gradually withdrawing this support away from the target structure in a substantially linear direction as the growth progresses.

5. A process for growing single crystals, which process comprises subjecting to a focused concentrated electron beam in an evacuated space a material from which a single crystal is to be grown to cause some of the material to become vaporized, holding immediately alongside the vaporizing material a single crystal to be grown, so that the vaporizing material condenses on the crystal and causes it to grow, and continuously adjusting the location of the growing crystal to position the growth site.

6.. The process of claim 5 in which the single crystal is 2,103,623 Kott ec, 28, '1937 'a. semiconductor. I H 2,160,981 4 V OBrien June 6, 1939 7; The process of claim 5 in which the single crystal 2,239,642 Burkhardt V Apr. 22, 1941 is germanium. 2,527,747 Lewis 'Oct. 31, 1950 8. The process of claim 5 in which the single crystal 5 7 V V FOREIGN PATENTS is silicon. a

868,386 France Sept. 29, 1941 References Cited in the file of this patent UNITED STATES PATENTS 1,450,464 Thomson Apr. 3, 1923 10

Claims (1)

1. A PROCESS FOR PRODUCING SINGLE CRYSTALS WHICH ARE SUITABLE FOR USE WITH A VARIETY OF ELECTRICAL APPLICATIONS WHICH COMPRISES ESTABLISHING ELECTRON CONDUCTANCE UNDER VACUUM BETWEEN AN ANODE AND CATHODE, FOCUSING THE ELECTRONS FROM SAID CATHODE INTO A BEAM WHICH INTERSECTS A TARGET MATERIAL AND WHICH VAPORIZES AT LEAST A PORTION OF SAID MATERIAL, CONDENSING SAID VAPOR UPON A SINGLE CRYSTAL BASE POSITIONED ADJACRNT TO SAID TARGET MEANS AND SLOWLY WITHDRAWING SAID SINGLE CRYSTAL BASE SO AS TO MAIN-

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