US3243267A

US3243267A - Growth of single crystals

Info

Publication number: US3243267A
Application number: US386505A
Authority: US
Inventors: William W Piper
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1964-07-31
Filing date: 1964-07-31
Publication date: 1966-03-29
Anticipated expiration: 1983-03-29

Description

W. W. PIPER Pfg/ Filed July 51, 1964 GROWTH oF SINGLE CRYSTALS March 29, 1966 In Verv'tor'.- Wf//fd W. Piper-3 H Attorney United States Patent O 3,243,267 GROWTH F SINGLE CRYSTALS William W. Piper, Scotia, NX., assigner to General Electric Company, a corporation of New York Filed July 31, 1964, Ser. No. 386,505 Claims. (Cl. 23e-301) The present invention relates to an improved method for the production of single crystals of semiconductive materials which may not be grown by simple seed crystal withdrawal from a mol-ten bath. Such materials are, for example, useful as luminescent phosphors and as photoconducting materials. This application is a continuationin-part of my copending application Serial No. 19,295, tiled April l, 1960, and assigned to the assignee of the present invention.

In the luminescence arts, it is well known that luminescent materials such as zinc sullide, cadmium sulfide and other members of the zinc-cadmium sulffo-selenide family, as well as zinc oxide, may be made to produce visible light when suitably activate-d with acceptor type activators and donor type co-activators and are subjected -to a suitable stimulus as, for example cathode rays, ultraviolet light or the excitation of an electric eld. In most applications of these materials 'for the production of light as, for example, in the viewing face of cathode ray tubes, the luminescent phosphor is present in the form of a plurality of small or micro-crystals lwhich may be in powder form pressed, liquid settled or sprayed into a thin layer `or suspended in a suitable binder or dielectric medium.

In my U.S. Patent No. 2,841,730', it has been shown that increased luminescent etliciency may be obtained if the phosphor is in the form of a large single crystal or is composed of a plurality of members cut from a monocrystalline ingot of the phosphor material. Heretofore, however, it has been impossible to grow true single crystals of these compounds in large sizes. It has also been impossible to grow large crystals or" these substances for use in photoconducting devices. The techniques which are utilized to grow monocrystalline ingots ofi monatomic semiconductive materials, such as germanium and silicon, `are not applicable for the growth of single crystalline ingots of these compounds. The only methods previously utilized in growing true `single crystals of these compounds has been by sublimation crystal growth in which the crystals are grown in the form of small needles harving a maximum length and width dimension of the order of a millimeter. It is dillicult to construct large luminescent or photoconducting `devices or electro-optical transducers from such small crystals.

Accordingly, it is an object of the present invention to provide an improved method for the production of single crystals of compound semiconductive materials.

Another object of the `present invention is to provide a method for the production of single crystalline ingots of luminescent phosphor and lphotoconducting compounds sulliciently large so yas to facilitate the fabrication of monocrystalline luminescent, :photosensitive and electro-optical devices.

In accordance with one embodiment of the present invention, large monocrystalline ing-ots of compound semiconductive material-s are grown from the vapor phase in a closed tube having a configuration at one end thereof which kfacilitates single crystal nucleation and which is progressively moved through a stationary temperature proiile to facilitate progressive grow-th of a large single crystalline ingot by nucleation at an interface which is maintained at a constant temperature most suitable tor crystal nucleation.

In accord with another feature of the invention, the tube Within which the crystal is grown is not evacuated, but rather, is filled by an inert gas at approximately ICC one atmosphere admixed lwith vapors of the material constituting the charge from which the crystal is grown. The open end of the reaction tube, upon equalization of the pressures within land without the growing tube, is sealed by condensation of these vapors during the initial stage of operation thereby preventing undue strain due to unequal pressures across the hot, soft tube wall.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the appended drawing in which:

"FIGURE l represents a schematic illustration of an apparatus suitable for the Ipractice of the present invention,

`FIGURE Z illustrates an enlarged vertical crosssectional view of .the reaction tube =of .the apparatus of FIGURE 1, and

FIGURE 3 is a graph illustrating a typical temperature lprotile maintained along the tube of FIGURE 1 in the practice of the invention.

yIn FIGURE l of the drawing, a suitable apparatus within which the present invention may be practiced comprises a first high-temperature reaction tube 1 which contains a seco-nd high-temperature reaction tube `2 having a main cylindrical region 3 and a conical end region 4 terminated in a point 5. Means for closing the open e-nd of tube 2 is provided in the form of a rporous plug 6. The central region of reaction tube 1 is surrounded by a plurality of resistance yheating coils 7, I8, 9 and 1li, -to which heating currents may be applied at the respective terminals thereof. The open end of tube 1 is closed with a tapered, ground-glass stopper, suitable for makin-g a vacuum-tight seal, and which has passing therethrough a pair of tubes 12 and 13. Reaction tube 1 should be capable of withstanding very high temperatures of up to approximately 1600 C. and may be conveniently constructed of mullite, which is aluminum silicate (Al6Si2'O13). Interior reaction tube 2 should be constructed of a material which need not be yas refractory as the material from which tube 1 is constructed and preferably is of a material, such as quartz, which is not wet by condensing vapors of the compound semiccnductive material, single crystals of which are to be grown.

Plug `6 may be in the form of a variety of structures and contigui-ations which are not particularly critical but which must perform the function of Iformi-ng a passage to gas and vapors of the material being grown within tube 2 so as to allow the pressures `across the tube to equalize and to present a surface for condensation of the generated fvapors, the condens-ation or" which eventually causes that end of reaction tube 2 to be sealed. In practice, plug 6 may constitute a tightly packed mass of quartz wool. Alternatively, it may comprise a compressed mass of quartz plug having an outside dimension only slightly smaller than the inside dimension of tube 2, so that condensation of vapors of the charge takes place -upon the interior surface of tube 2 and the exterior cylindrical surface of the plug. For Iconvenience in denominating the many and varied structures which plug 6 may take, the plug may be referred to herein as closure means for allowing charge vapors and carrier gases to escape from the tube and for sealing the tube by an accumulation of condensed vapors of the charge.

This closure means is of particular importance in the present invention because it allows equalization of the pressures within and without the tube 2. The optimum material for this tube is quartz because it is readily available, workable and inert. However, at the temperatures at which the present invention is performed, quartz becomes quite soft and, under unequal pressures, may deanziane? form or even explode. The closure means of the present invention prevents such an event 'by allowing communication, and therefore equalization, of the pressures across the tube 2.

In other words, if tube 2 were sealed, the increased pressure of the inert gas due to heating and the added pressure of evolved charge vapors would be suiiicient to deform or break'the quartz tube. The closure means avoids this problem by allowing the inert gas to pass and achieve pressure equilibrium during heating and by allowing the inert gas displaced by evolved vapors to escape and maintain pressure equilibrium before the tube is `sealed ott by the condens-ation of vapors on the closure means.

The arrangement `of reaction tubes 1 and 2 and

heater coils

7, 8, 9 and 10 are such as to allow for the establishment of a suitable temperature prole, described in greater detail hereinafter to permit motion of tube 2 through the stationary profile. To this end, several variations may be utilized. Thus, for example, the coils may be wound upon tube 1 and tube 2 may be slidable therein, either on rollers or by friction at the ,relatively small line of contact between the two different diameter tubes. This arrangement makes it possible to locate the heating coils very close to the growing crystals and facilitates optimum control of the temperature profile. Alternatively,

the coils may be wound upon a third cylindrical core (not shown) and tubes 1 and 2 may be slidably moved as a unit therein, as above. This arrangement has the advantage of moving the two tubes, one of which is seated within the other, together. l

In FIGURE 2 of the drawing, the portion of the apparatus in which the monocrystalline ingot 14 is grown is shown in enlarged detail. In FIGURE 2, like numerals have been Vutilized `to indicate like elements also illustrated in FIGURE 1. A charge 15 of the material from which the monocrystalline ingot isto be grown is placed immediately adjacent to plug 6. While this charge may be merely a powderedmass of the material from which the ingot is to be grown, it has been found preferable that charge 15 be a sintered mass of powdered material which has been heated to a sufficiently high temperature to cause the particles to agglomerate together, giving'a rigid form and a highderisity thereto. When the material utilized for the charge iszinc sulfide, this may be accomplished by heating powdered luminescent grade zinc suliide to a temperature of approximately 1000 C. in H25 gas at 1 atmosphere pressure vfor approximately 1/2 hour.l The resultant body has a density of approximately 3.2 to 3.5 and contains approximately to 20% voids, the remainder being solidcrystalline zinc sulde. Corresponding densities for cadmium sulfide are approximately 3.8 to 4.3 and approximately 4.8 'to 5.3 for cadmium selenide, for example. These densities, of approximately 3 to 5 for a sintered charge, compare with a representative density for powdered charges of from l to 1.5. The increased density of the charge is advantageous in that a larger amount of charge may be incorporated into the tube in a smaller space thus facilitating the task of always keeping the portion of the charge being sublimed at a sufficiently high temperature, while still keeping the temperature at which plug 6 is located at a necessary low value. Charge 15 is composed of the same material of which monocrystalline ingot 14 to 'be grown. The present invention may be practiced with semiconductive materials exhibiting luminescent and/or photoc'onductive properties and composed of compounds of elements of groups IIb and VIa of the periodic table. Such materials include simple and complex Compounds of the zinc-cadmium sulfo-selenide family including Zinc sulfide, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmium suliide, zinc-cadmium selenide, zinc sulfo-selenide,vcadmium sulfo-selenide, zinc-cadmium sulfo-selenide as well as zinc oxide and cadmium oxide. The main substantial differencein practicing the invention withy these diifer'ent materials is that the temperature profile established within the reaction tube must be adjusted upwardly or downwardly to compensate or adjust for the differing vapor pressures at a given temperature of the charge material utilized. This is because the mechanism by which the process of the'invention operates relies upon the establishment of a partial pressure of the charge material within the reaction tube 2 by sublimation. While the exact value of this partial pressure is not known with certainty, it is believed to be approximately 0.1 atmosphere. The region at which a monocrystalline ingot is` grown is maintained at a temperature only slightly below the temperature of the charge but sufficiently low enough to cause the above partial pressure to result in a suicient degree of supersaturation at point 5 to `cause nucleation. Thus it may be seen, that the specific temperature sulicient to cause the desired partial pressure within reaction tube 2 varies with the material used. It is to be noted, however, that when a complex salt 4is utilized as a charge, it is not merely a mixture of the constituent salts, but

v rather, the complex salt is present in crystalline form so as to be a solid solution of one of the constituents in another.

A temperature prole suitable for the growth of single crystals of the luminescent phosphor compounds utilized is established within the reaction tube 1 by a proper adjustment of the configuration of, and the currents passed through, heaters 7-10. Thus, for example, it has been found convenient, in the illustrated contiguration, when the outside diameter of reaction tube 1 is 18 millimeters and the length of the interior reaction tube 1 from tip 5 of conical region 4 to the beginning of the nearmost edge of plug 6 is 11 centimeters, that coils 7 and 1t) may be 21/2 in length and have 8 turns per inch of 0.030 platmum-10% rhodium wire. Similarly, under the same circumstances, coils 8 and 9 may conveniently be 3% in length and have 12 turns ,per inch of the same wire.

The voltages applied to

coils

7, 8, 9 and 10 are chosen to provide a typical temperature profile such as is illustrated in FIGURE 3 of the drawing, comparing FIG- URES 2 and 3 together. The temperature profile in FIG- URE 3 corresponds to the corresponding longitudinal position along tube 2 of FIGURE A2. This gradient is chosen to have a relatively gentle slope from beyond the innermost portion of tube k1 at the end removed from plug 11 that rises gradually, has a somewhat steeper slope beginning at point A, levels off at point B, begins to decrease at point C, and at point D, has fallen to a temperature value somewhat below the temperature at point A. The exact position of points A, B, C and D relative to tube 2 are chosen so that point N, a point midway between points A and B corresponds to the tip 5 of tapered portion 4 of tube 2 just before crystal growth is initiated. Additionally, the high temperature region between points B and C extends inwardly into the tube and includes the forward edge of charge 15. Point E, at a temperature substantially below the condensation point of the material constituting charge 15, corresponds substantially to the portion of the tube at the beginning of plug 6. Point N, between points A and B, serves as the nucleation point for the monocrystalline ingot being grown and corresponds in temperature to the temperature at which vaporized material of the charge begins to condense. The temperature of the tube in the region including points D and E and extending to the end of the tube decreases rapidly so as t`o favor rapid condensation of the charge material in or around plug 6.

As examples of temperatures which may be utilized in performing the process of the invention utilizing the described apparatus and typical members of the zinccadmium sulo-selenide family of materials, lfor zinc suled point A is approximately 1520 C. For cadmium sulr'ide it is approximately 1220 C., and for cadmium selenide it is approximately 1l20 C. Point B for zinc suliide is approximately 1550 C., while for cadmium sulfide it is approximately 1250 C. and for cadmium selenide it is approximately 1150 C. Point D is approximately 1400 C. for zinc sulfide, 1100 C. for cadmium sulde and a 1000 C. for cadmium selenide.

In practicing the invention, one difficulty which must be overcome is that of cooling the growing ingot sufficiently so tha-t the heat of the fusion is removed therefrom so as to maintain the interface temperature at a value which is optimum for nucleation. One method of accomplishing this is illustra-ted by the addition, to tube 2 of glass rod 16 which extends outward of the field of infiuence of heater coils 9 and 10 and constitutes a heat sink. Other means for creating a heat sink near crystal 14 for removing heat therefrom may be utilized. Thus, for example, a blast of cool air may be directed upon the adjacent portion of tube 1. Alternatively a cooling coil may encircle tube 1 in this region. Other well known cooling means may be provided.

In performing the growth of large single crystals of compound semiconductive materials of the zinc-cadmium sulfo-selenide family and off theoxides of zinc and cadmium in accord with the present invention, a charge is made by sintering, as described hereinbefore. The charge is placed within tube 2 and plug 6 is inserted at the end thereof. Tube 2 is then inserted in tube l. Plug 11 is inserted in the end of tube 1 and the atmosphere therein is evacuated to a pressure which may conveniently be less than 1 micron of mercury while the tubes are heated to a temperature of approximately 500 C. to remove therefrom any atmospheric constituents which may react with, or contaminate the vapors of, the charge. This evacuation may conveniently be conducted through tube 12 by a vacuum pump, not shown. When a suitable value of low pressure has been obtained within the charnber, a suitable non-reactive gas such as argon or any other of the noble gases is admitted to tube 2 at a pressure which may conveniently be approximately 1 atmosphere, through tube 13 and is allowed to escape therefrom through tube 12. Alternatively, the evacuation step may be dispensed with, and the atmosphere within tube 1 may merely be purged by fiushing, while heating, a suitable -gas such as argon into tube 13 and allowing the gas to escape through tube 12, for a suitable period of time, which may for example be 5 hours, to remove non-reactive atmospheric contaminants.

After a suita-bly pure atmosphere of a non-reactive gas has been established within tube 1, suitable voltages are applied to

coils

7, 8, 9 and 10 to establish a temperature profile such as that illustrated in FlGURE 3 and described hereinbefore. Since the temperature within tube 1 at which the inner edge of the sintered charge 15 is located is higher than the temperature at which the material of the charge sublimes, vapors of the charge material rapidly fill the interior of tube 1 to a suitable partial pressure of, for example, approximately 0.1 atmosphere. TheV temperature at the vicinity of charge `15 is, however, maintained at a temperature only slightly in excess of that necessary to cause sublimation so that the vapor pressure of the charge material does not become too high. As -the interior of tube 1 is filled with vapors of charge 15 the vapors begin to leak out through or past plug 6. Since the temperature in the vicinity of plug 6 falls rapidly as the end of the tube is approached, the vapors escaping through or past plug 6 rapidly begin to condense thereupon. After a relatively short time, these vapors have deposited in or around plug 6 to the extent that the end of the tube is sealed, thus preventing the escape therefrom of any further vapors. By that time impurities, air and other undesirable vapors have been flushed from the system.

As previously noted, the use, in accord with the invention, of a sealable plug 6 or other closure means is of importance. All materials known to be suitable for use as reaction tubes become soft `at the temperatures required to sublime IIb-Vla materials and may deform if a pressure differential exists between the interior of tube 1 and tube 2. Accordingly, the novel closure means allow gas expansion and vapor evolution of the charge to force out sufficient inert gas to equalize the pressures across tube 2 and thereafter the temperature gradient established is maintained so that the pressure differential between the inside and outside of reaction tube 2 does not exceed a small value of, for example, 50 millimeters of mercury.

It has been found in performing this invention that sublimation of the sintered powder occurs back into the cool region adjacent the plug thereby forming a solid, high density, polycrystalline boule. This results in a first purification of the material. When the leading edge of the solid charge enters the sublimation zone, vapors evolve therefrom and a second purification occurs. If, upon completion of crystal growth, a final portion of the boule adjacent the plug is not sublimed, the resultant crystal is found to be much cleaner than is the case if the entire boule is vaporized.

The temperature of tip 5 is so selected that the partial pressure of charge vapors in the reaction tube 1 creates a supersaturated condition at that point and nucleation begins at the pointed tip 5 of tapered end 4 to tube 2. One single crystal does not necessarily nucleate to the exclusion of all others. Several crystals may actually nucleate simultaneously. Should such a condition exist, however, one or two crystals crowd out the remainder and these continue to grow and fill the entire diam-eter of the tube. If several crystals grow, a bi-crystal or tri-crystal results, but the ingot is so large that two more crystals may readily be cut apart. Once nucleation has been commenced and crystal 14 begins to grow, tube 2 is moved longitudinally within tube 1 so that the growing crystal interface is always located at a point corresponding to point N intermediate to points A and B on the curve of FGURE 3 of the drawing, which point corresponds to the most favorable nucleation temperature for the material of which the single crystal is being grown and the degree of supersaturation.

As the tube 2 is moved through the temperature profile, plug 6 enters into a region having a temperature higher than initially. This is, however, not important since the tube has already been sealed and the increased temperature present at plug 6 is controlled so as to be insufficient to allow the tube to be opened by sublimation. It is necessary, however, should an extremely long crystal be grown, that as plug 6 progresses inwardly, the temperature profile be adjusted so that the condensed material in and around plug 6 does not sublime, thus destroying the seal of tube 2.

The rate at which tube 2 is moved corresponds to the rate of crystal growth. Although this rate is not critical within an order of magnitude, it has been found that it is relatively slow. For example, if tube 1 has a crosssectional diameter of 13 milli-meters and the material being grown is zinc sulfide or cadmium sulfide, it has been found that the rate of travel, and consequently the rate of crystal growth, should be controlled to be Within the ranges of from 0.1 to 2 millimeters per hour. Under these conditions and with these materials, it has been found that ideal conditions obtain and maximum size single crystals are grown when the rate of crystal growth is approximately 0.5 millimeter per hour.

Crystal growth is continued as described above until substantially all of the charge is exhausted. After the growth of the crystal has ceased, the coils are disconnected from the voltage supply and the reaction apparatus allowed to cool. Since the compounds of the zinc-cadmiu-m sulfo-selenide family as well as Zinc and cadmium oxides do not readily wet a quartz tube and since the coefficients of thermal expansion of these materials are greater than that of quartz, when the tube cools the single crystal shrinks away from the Walls of tube 2 and may readily be removed therefrom.

Utilizing the method of crystal growth of the present invention, single crystals of zinc or cadmium oxides, as well as zinc sulfide, zinc selenide, cadmium sulfide, cadmium selenide, and complex salts therebetween having diameters up to 13 millimeters and lengths as high as 20 millimeters have been grown. These crystals have been found to have excellent crystal perfection, high purity, and, when suitably activated, may be made photo-sensitive or luminescent when energized by cathode' rays, ultraviolet light or an electric field. Such activation may readily be accomplished by cutting a single crystalline wafer from the grown ingot and diffusing suitable luminescence activators such as copper, silver, gold, and manganese, and suitable luminescence co-activators such as chlorine, bromine, iodine, aluminum, galliu-m, and indium substantially uniformly throughout the wafer. In one means for accomplishing this, the activator and co-activator may be vacuum evaporated upon one surface of the crystal and the crystal heated for a temperature and time suitable to diffuse the activator and co-activator uniformly throughout the crystal or throughout a wafer cut from the crystal. Additionally, if the charge is an activated luminescent phosphor, it has been found possible to sublime and condense, in monocrystalline form, the entire activated phosphor in the same proportion of host and activator.

In producing monocrystalline ingots in accord with the present invention, it has been found essential that the crystals be grown in an atmosphere of an inert gas with a partial vapor pressure of material from which the crystal is to be grown and that this vapor pressure be established in a tube which is initially opened and which is closed by condensation of vapors of the charge upon a region in which condensation is sufficient to close off the reaction area, once a suflicient pressure of the charge substance has been obtained. Thus, for example, it has been found that a similar apparatus may not be utilized is first evacuated and sealed off and then the material isV heated to cause a vapor pressure of the substance. One critical reason for this is that without the balance of pressure supplied by the inert gas within tube 2, all materials known to be suitable for reaction tubes are so soft at the operating temperature that the interior tubes collapse. In this invention, this is avoided by the use of the porous plug or similar closure means. Additionally, the inert gas, by supplying a source of convection currents and a mediu-m of reasonable thermal conductivity, facilitates the essential features of removal of heat of fusion from the growing crystal and maintaining close control over the temperature profile within tube 2, which profile must change with respect to tube 2 as the tube is moved relative to the profile.

It is further important, in the practice of the present invention, that the maximum temperature within the reaction vessel be only slightly above the temperature at Which the vapors of the charge condense to form the single crystalline ingot. This is due to two factors. Greater differentials in pressure may cause the establishment of two high a vapor pressure within the chamber and cause a deformation of quartz tube 2., as noted above. Additionally, single crystal nucleation seems to be favored by only a slight temperature gradient between the region in which vapors are formed and the region upon which the vapors condense to form the single crystal.

The following specific examples of the practice of the present invention are set forth for purposes -of explanation only and are not to be construed in a limiting sense.

Example 1 The apparatus illustrated in FIGURES 1 and 2 was utilized. In .this apparatus a mullite tube havingian inside diameter of 3i land a length of 30 centimeters was utilized as tube 1. Tube 2 was of quartz and had an outside diameter of 5/s and a wall thickness of 1/16.

A charge of 50 grams of luminescent zinc sulfide activated with approximately 0.01 weight percent each of copper and galliu'm was sintered by heating in a 1/2" diameter Crucible for approximately 1/2 hour at ll00 C. in 1 atmosphere of hydrogen sulfide gas. The length of the sintered charge was l2 centimeters. This charge was inserted into tube 1, nearly abutting against the tapered portion of region 4. A lecentimeter long, 1/2 outside diameter quartz plug was inserted into the interior of tube 2. The tube was sealed, as illustrated, and evacuated to a pressure of approximately l micron. The temperature of the tube was raised by energizing all coils uniformly to approximately 700 C. while pumping continued for 1 hour. Argon gas was introduced into the tubes at a pressure of 1 atmosphere while the temperature profile was established by connecting to coil 7 a .20-volt voltage source causing 6.8 amperes of current to exist in coil 7. `Coil 8 was connected to a 60-volt supply causing 8.9 amperes to existv therein. Coil 9 was connected to a 55-volt voltage supply causing a current of 8.0 amps to exist therein and coil 10 was connected to a 20,-volt supply causing 6.8 amperes of current to exist therein. After the temperature profile had been established with the tip S of tube 2 at a temperature of 1470" C., the assembly of tubes 1 and 2 was pushed at a rate of 0.8 millimeter per hour into the prole (tothe right in FIGURE 2)y for a period'of 50 hours. At the end of that time it was found that a single crystalline ingot of zinc sulde having a length of 1.8l centimeters had been formed within tube 2. This single crystal was removed and wafers cut therefrom. These wafers were found to luminescence green under ultraviolet excitation of 3650 A.U., under cathode ray excitation and when connected to an electric eld of approximately ,volts per centimeter strength.

Example 2 The apparatus described in Example l was utilized. A cadmium sulde charge was for'med by heating 50 grams of cadmium sulfide ,activated -with 0.01% by weight of copper and gallium in a |vertical 1/2 diameter crucible at a temperature of 1000 C. for 1/2 hour. The resultant charge wa-s inserted into tube 2, almost abut ting against tapered portion 4. The same plug utilized in Example l was inserted intotube 2 and tube 1 was closed after tube 2 had beeninserted therein. Both tubes were evacuated to a pressure of approximately 1 micron, at which time the temperature along the entire length of tube 2 was raised uniformly to approximately 600 C. This temperature maintained while pumping for a period of 10 hours. AfterY the foregoing period of time, a temperature profile was established within tube 2 by connecting coil 7 to a 20,-volt supply to cause 6.8 amperes of current to exist therein. Coil 8 was connected to a 48-volt supply to cause 5.4 amperes to exist therein. Coil 9 was connected to 32-volt supply to cause 4.2 amperes to exist. therein. Coil 10 was connected to a 22-volt supply to cause 6.8 amperes of current to exist therein. The temperature at the tip of tu-be 2 was approximately 1230 C. After thermal equilibrium had been established, tubes 1 and 2 were pushed into the stationary temperature profile (to the right in FIGURE 2) at a rate of 0.3 millimeter per hour. After 60 hours, the coils were disconnected and the tubes allowed to cool. A single crystal 1.5 centimeters in length was readily removed from the quartz tube. Wafer-s cut from this crystal exhibited photoconductivity characteristics and, when irradiated by 3650 A.U. excitation or exposed cathode ray excitation, emitted in the infrared portion of the electromagnetic spectrum.

While the invention has been set forth herein with respect to certaink embodiments and specific examples thereof many modifications and changes will readily occur to those skilled in the art.. Accordingly, I intend by the appended claims, to cover all such modifications and changes as fall within the true spirit and scope of the invention. A l

What I claim as new and desire to secure by Letters Patent of the United -States is:

1. The method-of growing single crystals of a compound semiconductive material of the zinc-cadmium sulfo-selenid-e group comprising a cation of group IIb of the periodic, table and an anion of group VIa of the periodic table, which method comprises:

(a) placing a charge of said material in a partially lclosed tube;

l, (b) establishing a preselected temperature profile along said tube such that the te-mperature at a iirst portion of said tube is suiciently high to vaporize said material and the temperature at a second portion of said tube is sufficiently low as to cause condensation of said vaporized material thereat;

(c) maintaining said profile until said vapors fill said 1- tube 'and seal said partially closed tube by condensation and a single crystal of said compound begins to grow at said second portion of said tube; and

(d) moving said tube with respect to said temperature profile so that the interface between said growing c rystal and lvapors of said charge is always at a temperature at which vapors of said charge just begin to condense.

2. The method of growing large single crystals of a compound semiconductive material of the zinc-cadmium sulfo-selenide group comprising a cation of group IIb of the periodic table and an anion of group Vla of the periodic table which sublimes appreciably before melting which method comprises:

(a) placing a charge of said compound within a iirst reaction tube having a tapered end;

(b) closing the remaining end of said first tube with closure means for allowing vapors of said charge to escape from said tube and for sealing said tube by accumulation of condensed vapors of said charge;

(c) inserting said iirst tube in a second reaction tube;

(d) establishing within both of said tubes an atmosphere of a gas which is non-reactive with vapors of said charge;

(e) establishing along said irst tube a temperature profile such that the temperature thereof in a region encompassing at least a portion of said charge is suiiiciently high to cause sublimation thereof, the tapered end thereof is substantially at a temperature at which said vapors can condense, and said closure means is in a region of rapidly decreasing temperature below the temperature at which vapors of said charge condense;

(f) maintaining said temperature proiile until a partial vapor pressure of said charge is established in said first tube, causing vapors thereof to escape from said first tube through said closure means and become condensed thereupon to thereby seal said irst tube; and

(g) further maintaining said profile to cause at least one single crystal of said charge material to nucleate at the pointed tip of said first tube and grow thereupon while longitudinally moving said rst tube with respect to -said temperature profile to keep the interface between said growing crystal and vapors of said charge always at a temperature at which supersaturation exists and vapors of said charge just begin to condense.

3. The method of growing single crystals of a compound semiconductive material selected from the group consisting of Zinc sulde, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmium selenide, zinc-cadmium suliide, Zinc salio-selenide, cadmium sulfo-selenide, Zinc oxide and cadmium oxide and which method comprises:

(a) placing a charge of said material in a partially closed tube;

(b) establishing a preselected temperature profile such that the temperature at a -tirst portion of said tube is sufiiciently high to vaporize said material and the temperature .at a second portion of said tube is sufliciently low as to cause condensation of said vaporized material thereat;

(c) maintaining said prole until said vapors iill said tube and seal said partially closed tube by condensation and at least one single crystal of said compound begins to grow at said second portion of said tube; and

(d) moving said tube with respect 'to said temperature profile so that the interface between said growing crystal and vapors of said charge is always at a temperature at which vapors of said charge just begin to condense.

4. The method of growing large single crystals of a compound semiconductive material selected :from the group consisting of zinc sulfide, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmium selenide, zinccadmium sulfide, zinc sul'fo-selenide, cadmium sulfo-selenide, zinc oxide and cadmium oxide and which method comprises: v .-V(a) placing a charge of saidmaterial within a -frst reaction tube having a tapered end;

(b) closing the remaining end of said first tube with closure means -for allowing vapors of said charge to escape from said tube and for sealing said tube by accumulation of condensed vapors of s-aid charge;

(c) inserting said first tube in a second reaction tube;

(d) establishing within both of said tubes an atmosphere of a gas which is non-reactive with vapors o-f said char-ge;

(e) establishing along s-aid rst tube a temperature profile such that the temperature thereof in a region encompassing at least a portion of said charge is sufficiently high to cause sublimation thereof, the tapered end thereof is substantially at a temperature at which said vapors can condense, and said closure means is in a region of rapidly decreasing temperature below the temperature at which vapors of said charge condense;

(f) maintaining said temperature proile until a partial vapor pressure of said charge is established in said first tube, causing vapors thereof t0 escape from s-aid first tube through said closure means and become condensed thereupon to thereby seal said first tube; and

(g) further maintaining said profile to cause at least one single crystal of said charge material to nucleate the pointed tip of said first tube and grow thereupon While longitudinally moving said iirst tube with respect to said temperature proiile to keep the interface between said growing crystal and vapors of said charge always at a temperature at which supersaturation exists and vapors of said charge just begin to condense.

5. The method of growing large single crystals of a compound semiconductive material selected from the group consisting of zinc sulfide, cadmium sulfide, zinc selenide, cadmium selenide, zinc-cadmium selenide, Zinccadmium sulfide, zinc -sulfo-selenide, cadmium sulfo-selenide, zinc oxide and cadmium oxide and which method comprises:

(a) placing a sintered powder charge of said material in a first reaction tube having a iirst, tapered end and a second end;

(b) closing said second end of said first tube with closure means for allowing vapors of said charge to escape from said tube and for sealing said tube by accumulation of condensed vapors of said charge;

(c) inserting said first tube in a second reaction tube;

(e) establishing a preselected temperature profile cornprising:

(aa) a irst region and a second region wherein the temperature is suiciently low as to condense vapors of said material therein, and

(bb) a third region intermediate said first and second regions wherein the temperature is sufciently high as to vaporize said material;

(f) position-ing said yfirst tube in said prole in such a manner` that said rst, tapered end and atleast a portion of said charge lie in said third region and said second end lies in said rst region so that said portion of said charge vaporizes and lthe vapors conyden-se in said rst region to equalize the pressure within said rst and second tubes, seal said closure means and form a dense boule in said 'rst tube;

(g) moving said rst tube with respect to said profile so that said boule enters said third temperature region causing evolution of vapors therefrom and said first, tapered yend enters said second temperature region causing nucleation and growth' of a single crystal of said material thereat, while maintaining said second end in said rst temperature region;

(h) continuing said movement so lthat vthe interface be- References Cited by the Examiner UNITED STATES PATENTS 2,890,939 6/1959 Ravich t 23-294 2,947,613 8/1960 Reynolds et al. 23-294 3,019,092 1/1962 Rosi et a1. 23-294 FOREIGN PATENTS 1,163,905 10/1958 France.

OTHER REFERENCES Lawson et al.: Preparation of Single Crystals, Butterworth Publishing Co., 1958, pp. 21-25, 89-91.

NORMAN YUDKOFF, Primary Examiner.

G. HINES, A. l. ADAMCIK, Assistant Examiners.

Claims

1. THE METHOD OF GROWING SINGLE CRYSTALS OF A COMPOUND SEMICONDUCTIVE MATERIAL OF THE ZINC-CADMIUM SULFO-SELENIDE GROUP COMPRISING A CATION OF GROUP IIB OF THE PERIODIC TABLE AND AN ANION OF GROUP VIA OF THE PERIODIC TABLE, WHICH METHOD COMPRISES: (A) PLACING A CHARGE OF SAID MATERIAL IN A PARTIALLY CLOSED TUBE; (B) ESTABLISHING A PRESELECTED TEMPERATURE PROFILE ALONG SAID TUBE SUCH THAT THE TEMPERATURE AT A FIRST PORTION OF SAID TUBE IS SUFFICIENTLY HIGH TO VAPORIZE SAID MATERIAL AND THE TEMPERATURE AT A SECOND PORTION OF SAID TUBE IS SUFFICIENTLY LOW AS TO CAUSE CONDENSATION OF SAID VAPORIZED MATERIAL THEREAT;