US3756194A

US3756194A - Centrifugal drum for growing crystal

Info

Publication number: US3756194A
Application number: US00239097A
Authority: US
Inventors: S Lien
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1970-05-27
Filing date: 1972-03-29
Publication date: 1973-09-04
Anticipated expiration: 1990-09-04

Abstract

A vertically oriented rotatable drum includes a receptacle centrally disposed therein. Positioning means disposed within the annulus defined by the drum and receptacle side walls are adapted to confine a substrate against vertical movement while permitting lateral movement. The direction of such movement is determinable by densities of materials handled and drum speed. Means communicate the receptacle with the annular region whereby fluid material may coat alternate sides of the laterally shifting substrate.

Description

United States Patent [1 1 Lien 1451 Sept. 4, 1973 CENTRIFUGAL DRUM FOR GROWING CRYSTAL [75] Inventor:

[73] Assignee: Western Electric Company,

Incorporated, New York, NY.

Mar. 29, 1972 Suei-Yuen Paul Lien, Morrisville, Pa.

[22] Filed:

[21] Appl. No.: 239,097

Related US. Application Data [62] Division of Ser. No. 40,854, May 27, 1970, Pat. No.

[52] US. Cl. 118/52 [51] Int. Cl. 1305c 11/12 [58] Field of Search l17/101,109; 118/52-57, 19, 603

[56] References Cited UNITED STATES PATENTS 3,112,633 12/1963 Wilkinson, 11' 1l8/52X 3,213,827 10/1965 Jenkin 118/48 X 2,993,235 7/1961 Brown et a1. 117/101 X R20,444 7/1937 Hyde 117/101 3,033,159 5/1962 OBrien 117/101 X Primary Eaa mfner Morris Kaplan Attorney-W. M. Kain, J. -Rd senstoclr et al.

[57] ABSTRACT A vertically oriented rotatable: drum includes a recep' tacle centrally disposed therein. Positioning means disposed within the annulus defined by the drum and re ceptacle side walls are adapted to confine a substrate against vertical movement while permitting lateral movement. The direction of such movement is determinable by densities of materials handled and drum speed. Means communicate the receptacle with the annular region whereby fluid material may coat alternate sides of the laterally shifting substrate.

2 Claims, 14 Drawing Figures PATENTED SE? 4 I973 akrss; 194

SHEET10F4 PATENTEU SE? 4 ma SIEEIJBFQ 1 CENTRIFUGAL DRUM FOR GROWING CRYSTAL This is a division, of application Ser. No. 40,854 filed May 27, 1970 and now US. Pat. No. 3,713,883.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to growing crystals and, more specifically, to apparatus for growing crystals from a solution through the agency of centrifugal action or force.

In an even more specific sense, the present invention contemplates so-called liquid phase epitaxy (LPE), that is, the controlled precipitation and epitaxial growth of single crystalline material from a supersaturated-solution onto a seed or substrate. Such single crystalline material may be a so-called Ill-V or II-Vl semiconductive, electroluminescent compound. However, as discussed subsequently, the present invention is not necessarily limited to the growth of only such compounds.

2. Disscussion of the Prior Art Semiconductive, electroluminescent diodes and other devices, for example those made of crystal compounds such as gallium phosphide [GaP] or gallium arsenide-phosphide [GaAs P are the most efficient light sources known. Specifically, the electroluminescence of these compounds is due to the band-gap energies of their constituents being in the visible region of the radiation spectrum. More specifically, electroluminescence is caused by exciton recombination mechanism or by direct band-gap electron-hole recombination. Usually, the constituents of an electroluminescent compound are selectedas follows:

a. one or more element from Column III of the Perlodic Table one or more element from Column V of the Periodic Table; or i b. one or more element from Column lIof the Periodic Table one or more element from Column VI of the Periodic Table.

Thus, in describing electroluminescent compounds, there are derived the terms III-V and II-VI.

Further information concerning these compounds may be found in the following references: Morphology of Gallium Phosphide Crystals Grown by VLS Mechanism with Gallium as Liquid-Forming Agent, by W. C. Ellis, C. .l. Frosch and R. B. Zetterstrom in Journal of Crystal Growth, 2 (1968), pages 61-68 (printed in the Netherlands); Visible Light from Semiconductors," by Max R. Lorenz in Science, Mar. 29, I968, Vol.159, No. 3822, pages l419l423; and "Solid State Light" by A. S. Epstein and N. Holonyak in Science Journal, Jan., 1969, pages 68-33.

Besides being efficient, electroluminescent diodes and devices are more sturdy, reliable and longer-lived than, and are accordingly replacing, conventional incandescent lamps in a number of applications. Additionally, such diodes are compact, compatible with solid state circuitry and require very little power for operation.

Nevertheless, difficulties have been experienced in rapidly, efficiently and cheaply growing uniform, large area, single crystal, epitaxial compounds from which such diodes and devices are made. Such rapid, efficient an cheap crystal growth is, accordingly, one object of the present invention.

Generally, three growing methods have been used to manufacture these and other single crystal compounds. In all three methods appropriate dopants (e.g., oxygen, zinc) may be used to cause the grown crystal compound to emit light of a predetermined wavelength.

First, the compounds may be grown by a nonepitaxial, bulk method from a stochiometric or near-stochiometric melt, followed, if necessary, by zone refining. Present melt-growth methods have been found to be deficient for a number of reasons. Among these reasons are the necessity of high pressures (3045 atm), high temperatures (=1500C), and elaborate facilities; the unwanted introduction of impurities from crucibles at the pressures and temperatures necessarily utilized; and the inability to consistently grow high quality crystals.

In a second method, the compounds may be grown by epitaxial growth via vapor transport. Present vapor transport growth methods produce, at rather slow rates, crystal compounds having electroluminescent efficiencies somewhat lower than compounds grown by other methods.

Third, the compounds may be grown from solutions (LPE). Specifically, a heated liquid-solvent phase of a solution (e.g., gallium) is used to dissolve a solid-solute phase (e.g., gallium-phosphide). The desired compound is permitted to precipitate out either randomly or controllably (the latter being onto a seed or substrate) by slowly lowering the temperature of the solution (which effects supersaturation thereof) to prevent polycrystalline growth and to encourage single crystal growth, 1

Solution growth is potentially more desirable than the other two prior art methods because it useslower temperatures (l100C) and lower pressures ambient), and producesstrain-free crystal diodes having the highest known electroluminescent efficiency (about 3X higher than the first two methods): Solution growth is rather slow (usually one-at-a-time), however, and has been found to often produce crystals having structural defects caused by, inter alia, solution concentration gradients, temperature gradients, turbulence and the capturing of undissolved dopants in the grown crystal. 1

The present invention, as noted above, is an apparatus for improvement of the third prior art method, viz., the growth of crystals from a solution (LPE).

As noted previously, the present invention is not limited to the epitaxial growth of single crystal, electroluminescent compounds. Rather, the broader scope of, and a primary object of thisinvention, is the growth of crystals from a supersaturated solution which contains a solvent and a solute, thecrystals resulting from controlled precipitation of the dissolved solute.

A simple example of the type of crystal growth improved by the present invention may entail the growth of crystals of ordinary table salt. First, the table salt (the solute) is dissolved in a solvent therefor, such as" water. If during the dissolution of the salt, the water is' heated, a saturated salt solution results upon addition:

of sufficient salt. If, now, this saturated solution is' cooled, supersaturation results and the salt precipitates as solid, particulate crystalline matter. Suchprecipita-' tion is generally random due to the generallyrandom. location of nucleating sites, salt concentration gradi-- ents and thermal gradients. If a substrate or seed is pro vided in the solution, and if the thermal properties and turbulence of the solution are appropriately adjusted and controlled, the salt will precipitate on and adhere to the substrate or seed.

Methods similar to the above-described precipitation of table salt are found in prior art methods generally used to grow from solutions epitaxial layers of semiconductor materials. Such a method is described in U.S. Pat. application Ser. No. 556,192, filed June 8, 1966, and now abandoned, assigned to Bell Telephone Laboratories, Incorporated; and in U.S. Pat. No. 3,463,680. This method is usually referred to as the tipping method.

Specifically, in the tipping method of the prior art a graphite boat is positioned in a tiltable furnace. A substrate or seed is held at one end of the boat, which is tilted to raise the substrate. The lowered diametric side of the boat has placed therein a solution which includes a solvent (e.g., gallium), saturated with a solute (e.g., (la?) plus a dopant, if desired.

The furnace-boat system is closed and the boat is heated to dissolve the solute and the dopant. When the substrate and the solution reach an appropriate temperature, the boat is tipped to cover the substrate with the heated solution. The temperature is then controllably lowered both to supersaturate the solution and to effect epitaxial deposition onto the substrate.

It should here be noted that the terms seed and substrate are used interchangeably, the term seed being the more generic term. Specifically, a seed is defined as a single crystal of a material on which it is desired to grow a crystal. A substrate, on the other hand, is a slice of a seed. Thus, the only difference between the two terms is their physical shape.

The above-described tipping method and methods derived therefrom are, at present, the best known methods of growing single crystal layers from a solution. As mentioned above, these methods are, however, plagued with difficulties which render them somewhat less than desirable.

A first difficulty involves the inability of the prior art tipping methods to effect the growth of single crystal epitaxial layers simultaneously on a large number of substrates or seeds. Specifically, as described previously, epitaxial crystal growth involves the cooling of a saturated solution to a point where supersaturation occurs and the desired material precipitates from the now supersaturated solution onto the substrate or seed. Typically, prior art tipping methods have treated one substrate at a time, which is most inefficient, and, of course, costly and slow. The reason for one-at-a-time treatment is related in part to difficult-to-analyze temperature gradients within the super-saturated solution when a large number of substrates are present. in view of the different cooling rates of the solution and of a single substrate therein after tipping, the presence of numerous substrates generates many different temperature gradients at different points within the supersaturated solution. These temperature gradients render unpredictable the exact rate at which the desired crystal precipitates therefrom, that rate being temperature dependent. Moreover, these same gradients lead to a lack of a uniform substrate temperature. This, in turn results in the crystalline layers grown on the different substrates varying in character from substrate to substrate. An object of the present invention is to eliminate these temperature gradient problems in crystal growth.

A second difficulty with prior art crystal growth tipping methods relates to the dopants which are often used and to unwanted immundities in the solution. The dopants should properly be completely dissolved in the solution along with the solute. Generally speaking, however, most dopants are not as easy to dissolve in the solvent as is the solute from which the crystal is to be grown. Consequently, such dopants, upon being partially dissolved in the solvent, may form a skin or a scum layer. This dopant skin or scum layer, as well as the unwanted immundities, often get captured between the substrate-solvent interface making the grown crystalline layer either unacceptable or unpredictable in quality. Another object of this invention, then, is to eliminate such trapping of dopants and immundities inherent in prior art crystal growing processes.

A third problem involved in prior art tipping processes is related to so-called concentration gradients. Specifically, the substrate is maintained at the bottom of or within the saturated solution as the solution is cooled to grow on the substrate the desired crystal. Such growth is due to precipitation of the solute and dopant from the cooled and now supersaturated solution. Such precipitation is not uniform throughout the solution due to temperature gradients and mobility considerations. Thus, precipitation generally first occurs from that portion of the solution immediately adjacent the substrate. Because this precipitation causes a depletion of the solute and dopant from such portions of the solution, the solute and dopant concentration in that portion is decreased.

A decrease in the solute and dopant concentration in the solution portion immediately adjacent the substrate affects the density of that portion. That is, the density of the solution portion may be rendered either greater or less than that of the rest of the solution.

The first situation is where the lower concentration portion of the solution has a density greater than the remainder of the solution. Because all of the solute and dopant that can precipitate from the supersaturated solution does precipitate from that portion of the solution immediately adjacent the substrate, further precipitation cannot take place until the solution immediately adjacent the substrate has been replenished with the solute. Such replenishment may be effected by stirring the solution. However, such stirring may cause the temperature gradients in the solution to become nonuniform and randomly located, thus, giving rise to the first problem of the tipping method discussed above.

On the other hand, replenishment by natural diffusion may be allowed to take place. This, however, requires long time intervals making the process very slow and inefficient.

The second possibility, where a portion of the solution undergoes a density change, is for that density to be less than that of the remainder of the solution. The less dense solution will tend to rise generating either the temperature gradient problem or the turbulence problems discussed below.

Accordingly, yet another object of the present invention is to obviate concentration gradient problems, such as described immediately above.

The fourth major difficulty with prior art crystal growing tipping processes is related to turbulence. Some of the difficulties caused by turbulence have been discussed previously. In addition, turbulence immediately adjacent the substrate is usually undesirable since it causes variations in the thickness of (and the chemical and electrical characteristics of) the grown crystalline layer. It has been observed that turbulence typically leads to striations in the grown crystalline layers. Another object of this invention is to eliminate the turbulence difficulties of the prior art methods.

Some workers in the art have attempted to eliminate the temperature gradient problems in the following way: The substrate is positioned at the bottom of a very deep solution mass. An appropriate temperature gradient is then imposed on the system, the hope being that the large solution mass will stabilize the thermal gradient/Such, however, has not been the case due to increased turbulence effects. Specifically, such increased turbulence is due to the so-called Rayleigh Number exceeding 1700.

The Rayleigh Number R is defined for the fluid-filled space between two parallel horizontal places as a coefficient of thermal expansion of the fluid;

0,-6 the temperature difference between the two planes;

g gravitational acceleration;

d the separation between the planes;

v= kinematic viscosity of the fluid; and

K the thermal conductivity of the fluid.

Convection currents appear when R 1700.

As discussed below, LPE is best effected with the substrate positioned within the solution at the cold end of a thermal gradient therewithin. Thus, for LPE (0 -0 may be quite large. As a consequence, all else being equal, convection currents may be realistically eliminated by minimizing the d3 term. Obviously, the deep solution mass does just the opposite, i.e., it maximizes the d term. In fact convection cells have been observed when the deep solution mass is used. Such cells create the undesirable turbulence effects noted previously, e.g., striations in the grown crystal layer. Moreover, rather than stabilizing the thermal gradient, the deep solution mass, in causing the convection cells, effects local temperature differences not only on a single substrate, 'but also from substrate to substrate, where several are used. Accordingly, another object of this invention is to prevent undesirable convection cells by minimizing the Rayleigh Number in an LPE process.

Typically, as noted above, it has been found desirable to locate the substrate at the colder end of the thermal gradient in the solution between the substrate and the ,heat source. Specifically, the most desirable position for the substrate has been found to be one wherein a surface of the substrate, on which surface crystal growth is to take place, faces the heat source heating the solution, and is, therefore, at the cold" end of the temperature gradient within the solution between that surface and the heat source. Such positioning enhances the growth of the epitaxial crystal in only one direction, namely, perpendicular to the surface of the substrate. Effecting this optimum positioning of the substrate while eliminating the above prior art difficulties has, until this invention, proved impossible.

SUMMARY OF THE INVENTION With the abovementioned and other objects in view, the present invention contemplates a new and improved apparatus. for growing crystals and more especially to a new and improved apparatus for epitaxially growing crystals from a solution.

In a preferred form of the present invention in its broadest aspects, a solvent and a solute are placed in a generally cylindrical drum or container rotatable on a major axis. The solute comprises the constituents of (and dopants in) a crystal which is to be grown. Also placed in the drum, above the solution level therein, is a substrate or a plurality thereof.

The solvent and solute are heated to dissolve the sol ute (and dopants, if any) to form a solution. While the heat is maintained, the drum is rotated at a speed sufficient to both move the solution up the side wall of the drum by centrifugal force in a so called forced vortex, and to hold the substrate against the side wall. Ultimately the substrate is covered by the solution at which time the entire system is cooled. Such cooling supersaturates the solution to effect the desired crystal growth on the substrate. When appropriate crystal growth is effected, rotation of the drum is stopped and the solution falls to its natural level below the substrate.

In a first alternate embodiment, the substrate placed within the drum is constrained against vertical movement and movement parallel to the rotation of the drum, but is horizontally movable, radially from the drum axis, between a first and a second stop member. In this first embodiment heating is performed by one of two heat sources, namely, a first heat source outside of and surrounding the drum, or, a second heat source inside the drum and surrounded thereby.

The first heat source is used when the substrate'is less dense than the solution. Specifically, the rotation of the drum initially forces the horizontally movable substrate outwardly against the first stop member via centrifugal action. When the substrate is covered by the solution (due to centrifugal force acting on the solution), the substrate floats thereon, i.e., it moves inwardly toward the drums axis until it abuts the second stop member. The order of things as viewed from the drum axis is: the inside wall of the drum, the substrate, the exposed surface thereof, the solution covering the exposed surface, the outside wall of the drum, and the first heat source. Accordingly, the exposed surface is ideally located, i.e., facing the heat source and at the cold end of the temperature gradient within the solution between the surface and the heat source.

The second heat source is used when the substrate is denser than the solution. Specifically, the rotation of the drum initially forces the horizontally movable substrate outwardly against the first stop member via centrifugal action. When the substrate is covered by the solution, itremains against the first stop member. The order of things, then, as viewed from the exterior of the drum is: the outside wall of the drum, the substrate, the exposed surface thereof, the solution covering the exposed surface, the inside wall of the drum, and the second heat source. Again, the ideal location of the exposed substrate surface is effected.

A second alternative embodiment recognizes the presence of either denser or less dense immundities in the solution. Such immundities may be the dopant skin or scum, previously referred to, or may be other unwanted and undissolved immundities.

In the second alternative embodiment the drum is divided by a cylindrical wall into an inner region and an outer annular region. The two regions are interconnected for solution flow therebetween above the level of the solution which is put in the inner region. The substrate is placed in the outer region in accordance with either the description of the broader aspects of this invention, or of the first alternative embodiment, above.

A heat source is energized. This heat source is one of the two types described above. The drum is rotated to move the solution up the outer side wall of the inner region by centrifugal action in a first forced vortex. Immundities denser than the solution remain at the bottom thereof. lmmundities less dense than the solution float on the solution stopping their upward travel at some point below the interconnection between the two regions as determined by their density and the rotational velocity of the drum.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will appear upon consideration of the following detailed description with the accompanying drawings wherein:

FIG. 1 is an elevation of a crystal layer grown on a substrate or a seed in accordance with the principles of the present invention;

FIG. 2 is a stylized representation of the prior art tipping method of growing the crystal of FIG. 1, which prior art method is improved upon the present invention;

FIG. 3 is partially cross-sectioned elevation of apparatus by which the broadest aspects of the method of the present invention are effected to grow the crystal layer shown in FIG. 1;

FIG. 4 illustrates a first alternative embodiment of the invention depicted in FIG. 3;

FIGS. 5A-5C are partial cross-sectional, elevational views illustrating the apparatus of FIG. as modified by FIG. 4 to carry out the first alternative embodiment of the method of the present invention;

FIGS. 6-6F illustrate a second alternative embodiment of FIG. 3 as well as the various stages of the present method, which stages are also carried out by, but are not specifically shown in connection with, the apparatus of FIGS. 3, 4 and SA-SB;

FIG. 7 is a partially cross-sectioned elevation of another embodiment of apparatus usable in the present invention which may be included with the apparatus depicted in FIGS. 3, 4, 5A, 5B and 6A-6F.

DETAILED DESCRIPTION Referring to FIG. I there is shown a product of the type which is produced by the present invention. The product 20 includes a crystal layer 21 grown by the method of the present invention on a substrate or seed 22, previously defined.

The crystal layer 21 may comprise either a simple crystal such as ordinary table salt or an epitaxial, single crystal of, for example, a IIIV or a lI-Vl electroluminescent compound appropriately doped. Typically, where the crystal layer 21 is an epitaxial, electroluminescent compound the constituents of the layer 21 may be gallium and arsenic [GaAs], gallium and phosphorus [GaP] or gallium, arsenic and phosphorous [GaAs,. P, The present invention is not, however, intended to be limited to such compounds and, in fact, contemplates the growth of any crystal whether or not epitaxial or electroluminescent, which can be grown from a solution.

Referring now to FIG. 2, stylized apparatus is shown for effecting the prior art tip ping method of growing the crystal layer 21, for example a gallium-phosphide [GaP] crystal layer, on the substrate 22 of FIG. 1. The prior art utilizes a furnace 23 tiltable from side-to-side on a pivot 24. The furnace 23 is heatable by any convenient heat source, such as the RF coils 26, as shown. Included within the furnace 23 is a graphite boat 28 having a convenient configuration.

One side of the boat 28 is provided with a substrate or seed holder 29 of any conventional type. The substrate or seed 22 on which it is desired to grow the crystal layer 21 is held at the bottom of the boat 28 by the holder 29.

In use, the furnace 23 is tilted, for example, to the left, to lower the end 28a of the boat 28 which is diametrically opposite the substrate 22 and the holder 29. Into this lowered end 28a of the boat 28 are placed a solvent 30, such as a gallium solution, and a particulate solute 31, which includes the constituents of the crystal to be grown as well as anydesired dopants. If the crystal layer 21 is to comprise gallium phosphide the particulate matter will include gallium phosphide particles.

The furnace 23 is sealed and the RF coils 26 are energized to heat thefurnace 23. Heating the furnace 23 aids in the dissolution of the particulate matter 31 in the solvent 30. A sufficient excess of particulate matter 31 is included so that further heating eventually produces a saturated solution 32. Next, the furnace 23 is tilted so that the heated saturated solution 32 flows over and covers the heated substrates 22. The furnace 23 and, accordingly, the solution 32 and the substrate 22 is slowly and controllably cooled by appropriated control of the RF coils 26. Such cooling, as previously described, effects the growth, epitaxial or otherwise, of the crystal layer 21 on the substrate 22 by supersaturating the solution 32 and by precipitation of the solute from the solution 32.

The above-described prior art crystal growing method is plagued with numerous difiiculties and problems which have been previously discussed. The apparatus of the present invention described immediately below is intended to obviate and eliminate all of the difficulties and problems of the prior art.

Referring now to FIG. 3, there is shown novel apparatus 38 for carrying out the present process in its broadest aspects.

The apparatus 38 includes a drum 40 located within a furnace 41 and rotatable by any convenient means (not shown), as shown by the arrow 42, on a major axis 43 thereof. The axis 43 may conveniently be generally vertically disposed. The drum 40 is heated by any convenient heating means as indicated by the arrow 44. Such heating means may include either a heat source, such as RF coils 45 surrounding the exterior of the drum 40, or a heat source, such as RF coils 46 located within a tube 47 surrounded by the drym 40. Both heat sources 45 and 46 (and the tube 47) are generally coaxial with the major axis 43 of the drum 40.

The drum 40 is partially filled with the solvent 30 and the particulate solute 31 similar to the solvent and particulate solute used in the prior art and described above in the description of FIG. 2. Moreover, the solventparticulate mixture 30-3l fills the drum 40 to some convenient height H,.

Placed within the drum 40 in any convenient manner is at least one substrate 22, but preferably a plurality thereof. The substrates 22 may be held in the drum 40 by J-shaped holders 50 at a minimum height H greater than the height H,. The holders 50 may hold the substrates 22 in either of the orientations shown in FIG. 3. Specifically, the holders 50 may expose a first surface 51 of the substrates 22 by maintaining the substrates 22 against a side wall 52 of the drum 40. Alternatively, the holders 50 may expose a second surface 53 of the substrates 22 by maintaining the substrates 22 against a radial extension 54 of the tube 47. In practice both of the holder arrangements are not used at the same time and are shown in FIG. 3 only for convenience. The substrates 22 may be vertically stacked as shown, as long as the relationship H I-I, obtains.

One of the

heat sources

45 or 46 is now energized depending on the orientation of the substrates 22. Specifically, the

heat source

45 or 46 energized is that heat source which directly faces the exposed substrate surfaces 51 and 53. If the first substrate surface 51 is exposed, the heat source 46 is energized; if the second surface 53 is exposed, the heat source 45 is energized.

Energization of the

appropriate heat source

45 or 46 heats the solvent-solute mixture 30-31 and the substrates 22. Heating is carried out until the solventparticulate mixture 30-31 and the substrates 22 reach an appropriate temperature and, if necessary, until the solvent 30 is saturated with the solute 31 to produce the solution 32.

Next, the drum 40 is rotated as shown by the arrow 42. Such rotation effects the movement of the saturated solution 32 up the side wall 52 of the drum 40 by the action of centrifugal force. The rotational speed of the drum is selected so that ultimately the solution 32 assumes a forced vortex configuration 55 shown in cross section by the dotted and dashed line. As is known forced vortices are paraboloid in cross section.

The solution 32 assumes the forced vortex configuration 55 and the substrates 22 are covered thereby, as shown, at an appropriate rotational velocity of the drum 40. While rotation of the drum 40 continues the

heat source

45 or 46 is appropriately adjusted to begin slow cooling of the solution 32 and of the substrates 22. Such cooling, as described previously, supersaturates the solution 32 to effect the growth of the crystal layer 2| on the substrates 22.

After the crystal layer 21 has been grown on the substrates 22, rotation of the drum 40 is stopped. In order to prevent further crystal growth, where such is not desired, a sudden stopping is used. Such stopping eliminates the centrifugal force acting on the solution 32 and the forced vortex collapses. As such collapse occurs, the solution 32 runs down the side wall 52 of the drum 40 to the bottom thereof at the height H,. Because H, is still less than H, all crystal growth ceases.

The above-described apparatus 38 eliminates the difficulties of the prior art crystal growing tipping processes. Specifically, the apparatus 38 permits the simple and expeditious growth of the uniformly good crystal layers 21 on a large number of substrates 22 in a single operation.

Because the drum 40 rotates either around or within the energized

heat soruces

45 or 46, the heating of both the substrates 22 and of the solution 32 which has moved up the side wall 52 of the drum 40 is substantially equal over the entire drum 40. That is, all of the substrates 22 and all portions of the solution 32 are exposed to the heat output of all the entire periphery of the

heat source

45 or 46 as the drum 40 rotates. Thus, an averaging or integration of the heat input to the various parts of the drum 40 takes place. In other words, the temperature gradient problem of the prior art is eliminated. Moreover, due to the fact that all of the substrates are maintained at the same temperature, with respect to each other, the growth rate of the crystal layer 21 on each substrate 22 is the same.

A second problem of the prior art eliminated by use of the apparatus 38 relates to improperly dissolved or partially dissolved dopants and to other immundities present in the solution 32 (and in the solvent 30). As discussed previously, the improperly or partially dissolved dopants often form a skin or a scum layer which may have a density greater than or less than that of the solution 32. Moreover, other immundities in the solution 32 may also have densities greater than or less than that of the solution 32.

Referring to FIG. 6D (the substrates 22 are not shown), both types of immundities, that is, those arising from undissolved dopants and those arising from other immundities are represented by particles 56a and 56b. The particles 56a are those particles of either type which are less dense than and, accordingly, float on the surface of the solution 32 (and of the solvent 30). The particles 56b are those particles of either type which are denser than and sink to the bottom of the solution 32 (and of the solvent 30). Upon rotation of the drum 40 as shown by the arrow 42 in FIG. 6E (again, the substrates 22 are not shown), the solution 32, as previously described, assumes the forced vortex configuration 55, that is, on and up the side wall 52 of the drum 40. It has been found that the particles 56a and 56b move to definitely locatable positions upon such rotation of the drum 40.

Specifically, the denser particles 56b are thrown, by centrifugal action, as is well known, against the bottom of the side wall 52 of the drum 40. Depending upon the rotational velocity of the drum 40, the denser particles 56b may tend to climb the side wall 52 in a manner similar to the solution 32; however, such rotational velocity may be empirically selected to insure that these particles 56b remain at or near the bottom of the solution 32.

The particles 56a which are less dense than the solution 32 have been found to continue to float on the solution 32 as that solution assumes the forced vortex configuration 55. The average height H, at which such particles 56a float" on the solution 32 is easily empirically determined and depends, inter alia, on both the relative densities of the particles 56a and of the solution 32 and on the rotational velocity of the drum 40. It has been found that these particles 56a never move to the maximum height X of the forced vortex solution configuration 55, but rather rise to the intermediate height H,,. Ultimately, if the height H of the substrates 22 held by the holders 50 (as shown in FIG. 3) is selected to be greater than the height H, and less than X, the impurities 56a do not interfere with the growth of the crystal layer 21.

Thus, the problems generated in prior art crystal growing processes by undissolved dopants or by other impurities are also obviated by the use of this invention.

Also, it has been found quite easy, using the apparatus 38, to eliminate the turbulence and convection cell problems of the prior art. Both of these problems are easily overcome due to the fact that the height H of the substrates 22 may quite simply bekept at a point where the thickness of the solution 32 in the forced vortex 55 thereover is quite thin. This thinness, as described earlier, minimizes the (/1 term in the Rayleigh Number formula, thus eliminating turbulence and convection cells. It is observed that near the top of the paraboloid forced vortex 55, the solution 32 is quite thin.

As can thus be seen, the apparatus 38 of FIG. 3 properly effects the necessary thermal gradient while expeditiously permitting the growth on a large number of substrates of uniform crystal layers. The thermal, turbulence, concentration and convection problems of the prior art are also resolved at the same time.

Referring now to FIG. 4, there is shown a first alternative embodiment of the invention depicted in FIG. 3. While any convenient form of substrate holder, such as the J-shaped holders 50 may be used, a holder 57, as shown in FIG. 4 may be preferred due to its versatility.

Specifically, the holder 57 may include the radial extension 54 of the tube 47 and the side wall 52 of the drum 40 defining an annular substrate-receiving groove 58 therebetween. The holder 57 further comprises some convenient means, such as upper and lower annular screens 59 or other mesh-like or porous material within the groove 58. The screens 59 prevent vertical movement of the substrates 22 within the groove 58, but permit limited movement thereof between the surface 60 of the tube 47 and the side wall 52 of the drum 40. Thus, the tube surface 60 serves as a first stop member and the drum side wall 52 serves as a second stop member. Any conventional means, such as pairs of radial members 61, may also be used in the groove 58 to limit movement of the substrates 22 parallel to the direction of rotation of the drum 40.

The holder 57 is especially convenient, when a single embodiment, such as the apparatus 38 of FIG. 3, is intended to be used with substrates 22 which are either less dense than or denser than the solution 32.

Specifically, assuming centrifugal action to have already filled the groove 58 with the solution upon rotation of the drum 40, a less dense substrate floats" on the solution 32 (which passes upwardly through the screens 59) similar to the particles 56a. Such floating forces the substrate 22 toward the axis 43 and against the first stop member, i.e., the surface 60 of the tube 47 to expose the first substrate surface 51. Where this is the case, the heat source utilized is the RF coils 45 exterior of the drum 40. Thus there is effected the previously described favorable sustrate orientation, viz., the surface 51 on which the crystal layer 21 is to be grown is at the cold end of the thermal gradient in the solution 32.

A more dense substrate 22, on the other hand, similar to the particles 56b, is forced by the centrifugal action away from the axis 43 and against the second stop member, i.e., the side wall 52 of the drum 40. In this position of the substrate 22, the second surface 53 thereof is exposed. Here, the heat source 46 is used. Again the favorable substrate orientation in the thermal gradient is effected.

The substrate 22 does not, in reality, assume the position shown in FIG. 4 (nor in FIGS. 68 and 7). Rather, as indicated by the double-headed arrow 62, the substrate 22, moves against either the first or the second stop member or 52, respectively, depending on the density thereof.

Referring now to FIGS. 5A and 5B, there are shown two modifications derived from the embodiments shown in FIGS. 3 and 4 and which embody the principles thereof. The same reference numerals as in FIGS. 3 and 4 denote the same or similar elements in FIGS. 5A and 5B.

The drum 40 is mounted for rotation within the furnace 41. The drum 40 is rotatable as shown by the arrow 42 by apparatus (not shown). SUrrounding the furnace 41 is the heat source, which may comprise the RF heating coils 45. The tube 47 is mounted within the drum 40 coaxially of the axis 43 thereof. In FIG. 5A, the tube 47 includes at a lower end thereof an outwardly' directed, flange-like, annular, radial extension 54. The extension 54' includes an outside surface 60'. The extension 54 also includes an annular tongue-like member 63 which abuts the sidewall 52 of the drum 40. Mounted to the side wall 52 of the drum 40 is an annular and inwardly extending inverted cup-like member 64. The member 64 may be vertically removable from the drum 40 The inside wall 65 of the member 64 and the outside surface 60' of the extension 54' define an annular, substrate-receiving compartment 58 similar to the groove 58. The substrates 22 are held within this compartment 58' by any convenient means such as the .I-shaped holders 50, or more preferably a holder permitting the same limited substrate movement as the holder 57 of FIG. 4.

Also as shown in FIG. 5B the outside surface 60' of the extension 54 may have, rather than a regular, annular configuration, a polygonal configuration such as an octagon. In this case the apexes of the octagon contact the inside surface 65 of the member 64 to define a plurality of sector-shaped compartments 58", which confine the substrates 22 in a direction parallel to the rotation of the drum 40. Here, the J-shaped holder 50 or the holder 57 may, of course, be used to constrain the substrates 22 vertically.

The bottom of the drum 40 includes a region 66 depressed below the bottom of the extension 54'. This depressed region 66 includes a solution-containing well 67 into which either the solvent-solute mixture 30-31 or the saturated solution 32 is placed. The depressed region 66 communicates with the substrate-receiving compartment 58 (or 58") via a plurality of holes 68 formed through the tongue-like member 63.

In operation, either the solvent-solute mixture 30-31 or the saturated solution 32 is placed in the well 66 and, as before, the RF coils 45 maintain the system in a hightemperature condition until the solution 32 results. Rotation of the drum 40 is initiated to cause, via centrifugal action, the solution 32 to move out across the bottom of the depressed region 65 up through the holes 67 and into the

compartment

58 or 58". Ultimately, the substrates 22 within the compartment 58' or 58" are covered by the solution 32. The RF coils 45 are then controlled to lower the temperature slowly, thus growing the crystal layers 21 on the substrates 22 as previously described.

When proper crystal growth has been effected, the drum 40 is stopped, the solution 32 returning to the well 67 via the holes 68.

In the modification shown in FIG. A it is assumed that the substrates 22 are less dense than the solution 32. Accordingly, when that solution fills the

compartment

58 or 58" the substrates 22 float thereon and move inwardly toward the axis 43 of the drum 40 and against the surface 60 of the extension 54, which surface 60 serves as the first stop member. Such movement exposes the first surface 51 of the substrates 22. It is on this surface 51 that the crystal layer 21 is to be grown. Moreover, as shown in FIG. 5A if the substrates 22 are less dense than the solution 32, the heat source 45 exterior of the drum 40 is used. This positions the first surface 51 of the substrates 22 at the cold end of the thermal gradient existing in the solution 32 occupying the compartment 58' or 58". The order of things as viewed from the drum axis 43 is: the outside surface 60 of the extension 54, the substrates 22, the first surface 51 thereof, the solution 32 within the compartment 58' or 58" which covers the first surface 51, the inside wall 65 of the member 64, and the heat source 45. As previously mentioned, this is the ideal location for the first substrate surface 51.

The modification depicted in FIG. 5C is very similar to that of FIGS. 5A and 5B with the exception that the heat source 46 located within the tube 47 is used. The reason for the use of heat source 46 is that the substrates 22 shown in FIG. 5C are denser than the solution 32. Accordingly, upon rotation of the drum 40 the solution 32 moves up through the holes 68 into the

compartment

58 or 58". The denser substrates 22 move against the surface 65 of the angled member 64 exposing the second surface 53 thereof. Thus, the second surface 53 faces the heat source 46. The order of things, then, as viewed from the exterior of the drum 40 is: the surface 65 of the member 64, the substrates 22, the second exposed surface 53 thereof, the solution 32 covering the second surface 53, the outside surface 60' of the extension 54' and the heat source 45. Thus, again, the ideal location of the second substrate surface 53, namely at the cold end of the thermal gradient within the solution 32, is effected.

In the modification shownin FIGS. 5A-5C any convenient form of substrate holder may be used. To iterate, such holders preferably (but not necessarily) permit generally horizontal movement of the substrates 22 either toward or away from the axis 43 of the drum 40. The holders should, however, constrain the substrates 22 in the vertical direction and also in a direction parallcl with the rotational movement of the drum 40. Such a holder 57 is shown in FIG. 4 and may comprise the mesh-like members, such as the screens 60, therein depicted.

It is apparent that the modification of FIGS. 5A and 5C could be easily combined by having available for use both

heat sources

45 and 46. Centrifugal action due to rotation of the drum 40 and/or the relative densities of the substrates 22 and the solution 32 position the substrates 22 against either the first stop member (the surface 60) or the second stop member (the surface 65) depending on the density of the substrate 22. Preknowledge of the density of the substrates 22 then permits energizing the appropriate heat source depending upon which surface 51 or 53 of the substrates 22 will be exposed.

Referring now to FIGS. 6A-6F, there is shown a second alternative embodiment of the present invention.

Contained within the furnace 41 is the drum 40 rotatable upon its major axis 43 as shown by the arrow 42. Both types of

heat sources

45 and 46 may be included depending upon the substrate density considerations above-described.

Surrounding the drum 40 is a second drum generally coaxial therewith. The

drums

40 and 70 thus define an inner region 71 and an outer annular region 72. If the tube 47 is present within the drum 40 the inner region 71 is also annular. If the tube 47 is not used, the inner region is not, of course, annular. The

regions

71 and 72 intercommunicate via a plurality of passageways 73 formed through the wall of the first drum 40. The passageways 73 are located at a height H, which is greater than the height I-I, to which the less dense floating impurity particles 56a rise upon rotation of the

drums

40 and 70, but is no higher than (and is preferably lower than) a height X to which the solution 32 is able to rise in the forced vortex configuration 55 (FIGS. 3 and 6E). The floating of the particles 56a was described above in the description of FIG. 3.

The substrates 22 are mounted within the outer annular region 72 by any convenient means. Such mounting means may (as in FIG. 58) comprise sector-shaped compartments 58" or (as in FIG. 6A) comprise the .I- shaped holders 50, previously described, which may hold the substrates 22 against either the exterior wall 74 of the drum 40 (right-hand side of FIG. 6A) or the outer, interior wall 75 of the drum 70(left-hand side of FIG. 6A). Preferably, the mounting means (as shown in FIG. 68) comprises the type of holder 57 shown in FIG. 4. As described with reference to FIG. 4 the holder 57 permits horizontal movement of the substrates 22 but constrains the substrates 22 vertically and in a direction parallel to the rotation of the

drums

40 and 70. In this instance, if the holder 57 is used, when the outer annular region 72 is filled with the solution 32 the substrates 22 (a) move inwardly toward the axis 43 against the wall 74 of the drum 40 if they are less dense than the solution 32 and (b) move outwardly away from the axis 43 against the wall 75 of the drum 70 if they are denser than the solution 32. Thus, the

walls

74 and 75 serve, respectively, as the first and second stop surfaces.

FIGS. 6A and 6D depict the situation prior to initiation of rotation of

drums

40 and 70. FIG. 6D, as mentioned previously, shows the location of immundity particles 56a and 56b prior to rotation.

FIG. 6B shows an intermediate stage in the process of this invention wherein rotation of the

drums

40 and 70 has been initiated and the solution 32 has deformed into an intermediate forced vortex configuration 55' tending toward the forced vortex configuration 55 shown in FIG. 3. As shown in FIG. 6E, the immundity particles 56a and 56b assume the positions previously described. It should be noted that the height H, to which the immundities 56a rise is below the height H, of the passageways 73.

Accordingly, as shown in FIG. 6C, continued rotation of the drums40 and 70 cuases the solution 32 to rise up to the passageways 73 and to begin flowing (numeral 76) therethroughinto the outer annular region 72. Because of the location of the immundities 56a and 56b, as shown in FIG. 6E, substantially immundity-free solution 32 passes into the outer annular region 72.

Ultimately, as shown in FIG. 6F, the solution 32 in the outer annular region 72 is moved up the inner wall 75 of the drum 70 by centrifugal force into a second forced vortex configuration 77. Such movement of the solution 32 covers the substrates 22 therewith. Moreover, if the type of holder 57 depicted in FIG. 4 is used, either the first or the second surface 51 or 53 (see FIG. 63), respectively, of the substrates 22 will be covered by and exposed to the solution 32 within the outer annular region 72, as indicated by the arrow 62 in FIG. 68. Control of the

appropriate heat source

45 or 46 to cool the system is now effective to grow the crystal layer 21 of the substrates 22.

In the embodiment of FIGS. 3 and A-5C, after the crystal layer 21 has been grown, cessation of the rotation of the drum caused the solution 32 to retreat from the substrates 22. In FIG. 3 the relation I-I I-I, is always true; in FIGS. 5A-5C, the solution 32 returns to the well 67 via the holes 68. In FIGS. 6A6F either approach may be taken. As shown in FIG. 6F the height H of the lowest substrates may be chosen so that upon the

drums

40 and 70 stopping I-I H, where H, is the height of the solution 32 in the outer region 72.

On the other hand, as shown in FIG. 6C another approach may be taken. Specifically, coaxially mounted to the drum 70 is a cup 90 defining an annular solution receptacle 91 therebetween. A passageway 92 runs through the drum 70 and between the outer region 72 and the receptacle 91. The passageway 92 is selectively opened and closed by a valve 93 operated by appropriate circuitry (not shown).

The valve 93 is kept closed during growth of the crystal layer 21 and is opened only when the

drums

40 and 70 stop. Thus, the height H may be any convenient height and need not be greater than H, in the outer region 72.

A modification of the second alternative embodiment of FIG. 6 is depicted in FIG. 7. This modfication is not limited to the apparatus of FIG. 6, however, and may easily be adapted to the embodiments shown in FIGS. 3 and 5.

In the modification of FIG. 7 the substrates 22 are mounted by any appropriate holder, such as the J- shaped holder 50 (left-hand side of FIG. 7) or the holder 57 (right-hand side of FIG. 7) within an annular chamber 77 ofa mesh-like cage 78. The cage 78 comprises a pair of

coaxial mesh cylinders

80 and 81 defining the chamber and joined by a mesh bottom 82. The cage bottom 82 contains a hole 83 large enough to fit over the drum 40.

The cage 78 is designed to fit into the outer annular region 72 and to rotate along with the

drums

40 and 70 by any convenient means, for example, by a key-in-slot arrangement (not shown). The substrates 22 are loaded into the chamber 77 of the cage 78 which initially resides in an upraised position as shown in FIG. 7. After such loading the cage 78 is moved downwardly by means (not shown) into the outer annular region 72, as shown by the arrows 84. Operation of the apparatus of FIG. 7 then proceeds as in the description of FIGS. 6A6F. After the crystal layers 21 have been grown on the substrates 22 the cage 78 is lifted out of the outer annular region 72, the substrates 22 being easily transported without being contaminated within the cage 78. Where the holder 57 is used, the walls of the

cylinders

80 and 81 serve, respectively, as the first and second stop members.

The cage 78 may, accordingly, be viewed either as a handling expedient, as an alternative to the passageway-valve-receptacle 92-93-91 arrangement of FIG. 6C (upward movement of the cage 78 may terminate crystal layer growth notwithstanding the relation of H to 1-1,), or both.

Thus, there has been described a method, and apparatus for effecting that method, which method permits the convenient growth of crystal layers of any type from a solution on one or on a plurality of substrates simultaneously while eliminating all of the difficulties of prior art processes and apparatus. It should be noted that the above described embodiments of this crystal growing method are simply illustrative of the principles of the present invention. Numerous other arrangements and modifications may be devised by one skilled in the art without departing from the spirit and scope of this invention. For example, the forced vortex 55 may be generated by an impeller arrangement (not shown) within and generally coaxial with either of the

drums

40 and 70. SUch impeller arrangement may be similar to the impeller of a centrifugal pump or of a conventional cream separator.

Moreover, it is not necessary that the forced vortex 55 be generated simultaneously with the cooling of the saturated solution 32. Specifically, as described previously, if the saturated solution 32 is cooled to the point of super-saturation without the substrate or seed being present, solid, particulate crystalline matter randomly precipitates therein. If the seed or substrate 22 is present in the solution 32 at supersaturation, the crystal layer 21 is grown thereon. Many saturated solutions 32 have been found to possess a property whereby the temperature at which random precipitation occurs is lower than the temperature at which the controlled growth of the crystal layer 21 occurs. Accordingly, the present invention may be used in the fllowing manner: The particulate solute 31 is dissolved in the solvent 30 at an elevated temperature to produce the saturated solution 32. The saturated solution 32 is then cooled to a point where super-saturation occurs but the random precipitation does not occur. The substrate 22 is next placed in either the drum 40 or the drum and is maintained at a temperature at which crystal growth thereon will take place. The forced vortex configuration 55 may now be imposed on the supersaturated solution so that the substrate 22 is at least momentarily covered thereby. Growth of the crystal layer 21 occurs.

What is claimed is:

1. Apparatus for growing crystal layers on substrates from a saturated solution rich in a solute made up of the crystals constituents, the solution also including immundities having densities greater than or less than the solution, which apparatus comprises:

a. drum means rotatable on a major axis, said drum means including a receptacle defining an inner solution-receiving region and an outer substratereceiving region defined by an annulus which includes the outer wall of said receptacle, both regions including side walls generally coaxial with said axis;

b. means interconnecting said regions at a position above the level of the solution within said inner region for permitting solution flow from said inner region into said outer region;

c. positioning means, for mounting at least one substrate within said outer region, comprising:

first and second stop members horizontally aligned on a radius of said axis; and

means for constraining the substrate against vertical movement and for permitting horizontal radial movement of the substrate toward and away from said axis between said first and said second stop members;

so that when the substrate is contacted by the solution due to rotation of said drum means by said rotating means, (1) the substrate moves toward said axis and against said first stop member to expose a first surface thereof facing away from said axis, if the substrate is less dense than the solution, and (2) the substrate moves away from said axis and against said second stop member to expose a second surface thereof facing toward said axis, if the substrate is denser than the solution;

d. means for rotating said drum means until centrifugal action moves the solution in a forced vortex on said side wall of said inner region, through said interconnecting means and into said outer region to contact the substrate with the solution, the denser immundities remaining at the bottom of the solumeans further includes:

a first heat source generally coaxial with said axis and surrounding the exterior of said drum means;

a second heat source surrounded by said drum means and generally coaxial with said axis; and

means (1) for selectively energizing said first heat flux source if the substrate is less dense than the solution to orient said first surface at the cold end of the thermal gradient within the solution contacting the substrate and 2) for selectively energizing said second heat flux source if the substrate is denser than the solution to orient said second surface at the cold end of the thermal gradient within the solution contacting the substrate.

Claims

1. Apparatus for growing crystal layers on substrates from a saturated solution rich in a solute made up of the crystal''s constituents, the solution also including immundities having densities greater than or less than the solution, which apparatus comprises: a. drum means rotatable on a major axis, said drum means including a receptacle defining an inner solution-receiving region and an outer substrate-receiving region defined by an annulus which includes the outer wall of said receptacle, both regions including side walls generally coaxial with said axis; b. means interconnecting said regions at a position above the level of the solution within said inner region for permitting solution flow from said inner region into said outer region; c. positioning means, for mounting at least one substrate within said outer region, comprising: first and second stop members horizontally aligned on a radius of said axis; and means for constraining the substrate against vertical movement and for permitting horizontal radial movement of the substrate toward and away from said axis between said first and said second stop members; so that when the substrate is contacted by the solution due to rotation of said drum means by said rotating means, (1) the substrate moves toward said axis and against said first stop member to expose a first surface thereof facing away from said axis, if the substrate is less dense than the solution, and (2) the substrate moves away from said axis and against said second stop member to expose a second surface thereof facing toward said axis, if the substrate is denser than the solution; d. means for rotating said drum means until centrifugal action moves the solution in a forced vortex on said side wall of said inner region, through said interconnecting means and into said outer region to contact the substrate with the solution, the denser immundities remaining at the bottom of the solution in said inner region against said side wall thereof, the less dense impurities floating on the solution as it moves on said side wall of said inner region at a level away from said interconnecting means; and e. means simultaneously operable with said rotating means for controlling the temperature of the solution and substrate therewithin to effect growth of the crystal layer on the substrate.

2. The apparatus of claim 1 wherein said heating means further includes: a first heat source generally coaxial with said axis and surrounding the exterior of said drum means; a second heat source surrounded by said drum means and generally coaxial with said axis; and means (1) for selectively energizing said first heat flux source if the substrate is less dense than the solution to orient said first surface at the cold end of the thermal gradient within the solution contacting the substrate and (2) for selectively energizing said second heat flux source if the substrate is denser than the solution to orient said second surface at the cold end of the thermal gradient within the solution contacting the substrate.