US 3627499 A
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Dec. 14, 1971 JEAN-MARC LE U E'I'AL 3,627,499
METHOD OF MANUFACTURING A CRYSTALLINE COMPOUND Filed Jan. 21, 1969 fig.1
INVh'N'IORS JEAN-MARC LE DUC BY EMILE \DEYRIS A ENT United States Patent Oce Patented Dec. 14, 1971 METHOD OF MANUFACTURING A CRYSTALLINE COMPOUND Jean-Marc Le Duc and Emile Deyris, Caen, France, assignors to US. Philips Corporation, New York, N.Y. Filed Jan. 21. 1969, Ser. No. 792,528 Claims priority, application France, Jan. 18, 1968, 136,487 Int. Cl. B01j 17/22, 17/18 US. Cl. 23-301 SP 9 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a method of manufacturing a crystalline compound, particularly a semiconductor monocrystalline compound, in which a seed crystal which is contacted with a surface of a liquid solution of the compound is caused to grow from the solution by crystallisation of the compound and the crystalline compound is drawn from the solution, and to a device for performing the method.
A crystalline compound is to be understood to include herein mixed crystals of two or more compounds.
'It is known that certain compounds used in semiconductor technology are hard to manufacture, to clean or to obtain in a monocrystalline form according to the known methods, because the melting temperature and/ or their decomposition pressure at the melting temperature are very high. This is the case, for example, with GaP and GaAs. It has therefore been tried to obtain these compounds not by crystallisation from their melt but from a supersaturated solution. This method has been applied in various manners. A liquid solution of the compound was prepared, provided on the surface of a substrate, and then cooled. This method was used for the epitaxial deposit on disks and it produces only layers having small thicknesses and no rods.
Another known method consists in dissolving the compound in a polycrystalline form in a liquid component in which a small temperature gradient is provided. Transport of the dissolved components in the direction of said gradient and crystallisation of the compound on a seed crystal which is dipped in the above-mentioned solution take place.
With this method, small monocrystals were obtained which are less readily suitable for the industrial manufacture of semiconductor devices, since they cannot supply rods of an adequate volume and suitable shape.
In order to obtain such rods, a method has been used which is analogous to zone-melting and according to which the liquid region for-med by the solution of the compound moves along the reactor by the gradual dissolution of a usually polycrystalline rod and simultaneous crystallisation of the compound on the oppositely located side of the said region. The possibilities of such a method, however, are restricted by the dimensions of the reaction space and those of the rod from which it was started and, moreover, the crystalline deposit is constantly in touch with the walls of the space at its edges. Although the advantage of the growth from a solution is that it is performed at a lower temperature than a growth from a melt from the compound, it is of importance that the deposit or at least the interface solid-liquid of the deposit is at no instant in the contact with some source or other of impurities. On the contrary, there exists a method of crystallizing starting from a melt of the compound, in which this contact is avoided. This known method, with which monocrystalline rods can be obtained of suitable shape and dimensions, is the method by Czochralski according to which the crystal is drawn vertically from the surface of the liquid solution of the compound in which a suitable temperature gradient is maintained. If, however, this method and its devices for performing said method are used for the growth from a solution, they do not preserve all their advantages. On the one hand, the concentration of the solution is not constant as a result of the difference between the composition of the solution and that of the deposit so that the solution depletes during the formation of the deposit and the conditions of epitaxy vary which involves the necessity of constantly varying the other conditions of the deposition, particularly the temperature gradient and the rate of drawing. So it is desirable to provide a continuous supply of at least one component. On the other hand it has been found that the presence at the interface solid-liquid of an active element, for example, a voltage component in the vapour phase, with a non-negligible pressure disturbs the circumstances of the crystallisation from the solution and that it is desirable to exclude such a contact from the said interface. An addition of a component in the solid or liquid phase can also take place not in the immediate proximity of the interface solid-liquid of the deposit where this could not be effected in a sufliciently controllable and checkable manner, so as not to disturb the conditions of the crystallisation.
It is the object of the invention to avoid, at least to a considerable extent, the contact between the component to be added in the vapour phase and the interface solid-liquid. The method mentioned in the preamble is characterized in that at least one soluble component of the compound is continuously added to the solution at a place which is separated from the surface by the solution, and the component is transported to the surface by diffusion. The soluble component is preferably added in the gaseous state. With the method according to the invention, the advantages of the drawing method by Czochralski and those of the growth from a solution are simultaneously obtained and rods of large dimensions and of high crystal quality can be made.
The temperature at which crystal growth from a solution occurs may be comparatively low, dependent upon the concentration, as compared with that of the deposit from a melt of the compound, the decomposition pressures are less high and the possibility of contamination is much more restricted. The interface solid-liquid of the deposit is not in contact with any wall of the space, and the crystallisation is not disturbed by the proximity of the addition of a component in a vapour form, liquid or solid state. On the contrary, replenishing the components in which the solution depletes, is effected by diffusion in the solution, for example, over a comparatively large distance, and there exists no discontinuity of irregularity whatsoever at the interface solid-liquid of the deposit. In addition, the atmosphere in the space where drawing is carried out over the surface of the solution is substantially not reactive; it may be neutral and inert and has no influence on the crystallisation.
The most volatile component of the compound is preferably used in the solution as a solvent.
The method according to the invention has been found to give particularly favourable results when the compound used is an A B compound, preferably gallium arsenide.
Since replenishing the necessary component is effected at a distance from the surface from which it is drawn, it is the invention, to use a device which comprises two separate spaces, one for drawing, the other for the addition of the components. The invention therefore also relates to a device for carrying out the method according to the invention which is characterized by at least two spaces one of which is destined for drawing, and the other is destined for the addition of at least one of the components in a gaseous form, said spaces communicating with each other during drawing through the solution, and by a heating device by means of which during drawing a temperature gradient in the solution is maintained which is negative in the direction of the surface from which it is drawn.
In a preferred embodiment of the invention, the two spaces are separated from each other by at least one partition which, at the region where the partition in the first space is covered by the solution, is permeable to vapour from the second space to the solution in the first space. A particularly suitable device is obtained when the spaces are arranged vertically with respect to each other.
According to another preferred embodiment of the device, the spaces are separated from each other by at least one partition which, during operation of the device, separates the vapour phases in the two spaces and extends in the solution. A device in which the spaces are arranged coaxially with respect to each other has been found particularly suitable, inter alia from a point of view of space saving.
In order that the invention may be readily carried into effect, two embodiments thereof will now be described in greater detail by way of example, with reference to the accompanying drawing, in which FIG. 1 is a diagrammatic cross-sectional view of a first device for carrying out the method according to the invention,
FIG. 2 is a diagrammatic cross-sectional view of a second device for carrying out the method according to the invention.
The device shown in FIG. 1 comprises two spaces 1 and 2 of a reactor 3, constructed, for example, from quartz. The two spaces 1 and 2 are separated by a crucible 4 which bears on a partition 5. The first space 1 comprises one of the components 6 of the compound to be prepared which, for the sake of simplicity, is assumed to be binary so as to clarify the explanation of the method. The volatile component is soluble in the second component in the liquid state. This is in the crucible 4 at 7, the bottom of which is porous or perforated in such manner that it is permeable to the vapour of the first component in order to contact the second component in the liquid state, but that said second component cannot flow out of the crucible due to its surface tension.
The second space 2 comprises a gas inlet 9 and drawing means which in FIG. 1 are shown diagrammatically by a vertical stem 10 passing through an aperture 11 in the cover 12 of the reactor, which cover is provided on the Wall of the reactor by a ground joint 13. At its end in the reactor the stem 10 comprises a monocrystalline seed crystal 14 and it can be moved by a controllable slow drawing movement and simultaneously by a slow rotating movement which is also controllable. The control mechanism of the movements is not shown in the figure.
The temperatures of the component 6 and of the liquid phase are adjusted by means of a heating device 15 which comprises the desired number of heating regions to adjust the required temperatures.
The temperatures and the gradients are controlled and maintained in such manner that the component 6 evaporates gradually, is transported through the bottom 8 for contacting the second component which is kept in the liquid state, and diffuses through the liquid 7 so as to reach the surface thereof at 16 at the crystallisation temperature. The seed crystal 14 is contacted with the surface of the liquid 16 and then drawn by the known method according as crystallisation proceeds.
The vapour of the component 6 which fills the space 1 is fully separated from the surface 16 of the liquid phase and cannot disturb the crystallisation in any manner. If required a neutral gas is admitted at 9 and disappears through the play in the joint 11.
The device shown in FIG. 1 can be realized with variations in shape and detail; for example, the walls of the crucible 4 may be entirely porous or perforated with the exception of the upper-most parts which are not in permanent contact with the solution; the diameter of the crucible 4 may be increased or the diameter of the reactor 3 may be decreased at the height of the crucible so as to reduce the intermediate space which enables a more accurate control of the heating of the solution.
It may be found that the optimum temperatures are hard to maintain at the various levels of the liquid 7. These temperatures determine the dissolving of the soluble component, its migration by diffusion in the solution and the rate of crystallisation.
For numerous components which require a great accuracy of the said temperatures, the device shown in FIG. 2 constitutes a preferred embodiment. According to this embodiment which, for example, will be constructed in many cases from quartz, the device comprises a horizontal space 29 and a vertical space 22. The horizontal space 21 contains one or more sources of the soluble component, for example, arsenic crystals 23, and a considerable quantity of the component which serves as a solvent, for example, liquid gallium 24. A supply tube 25 for gas and an exhaust tube 26 for gas, enable the space 27 to be cleaned by means of, for example, a neutral gas. The gas is, for example, hydrogen or argon.
A partition 28 serves to form a vessel in a part of the space 21 containing the solvent 24. This partition leaves a passage for gases and vapours between the space 27 and the liquid solvent. The vertical space 22 communicates with the horizontal space through a narrow passage 30 which is arranged so that the level of the liquid always is above said passage. The space 22 comprises a drawing device of which only the stem 31 is shown in the figure and which comprises a seed crystal 32 at its lower end, for example, a monocrystalline seed crystal of gallium arsenide. The stem 31 passes through the cover 33 of the space, via a lead-in 34, the tightness of which can be obtained, for example, by means of a ring of liquid gallium 34 in case gallium arsenide is drawn. The cover 33 is secured to the wall of the space by means of a ground joint 36 and comprises gas inlet and outlet tubes 37 and 38, respectively, with which an atmosphere and an optimum pressure, usually of inert gas, can be maintained in the interior 39 of the vertical space.
A heating device diagrammatically denoted by 40 produces the gradual evaporation of the source 23 and the temperature gradients along the space 21 and a device diagrammatically denoted by 41 adjusts the required temperature gradient in the liquid 42, the interface solidliquid 43 being kept at the crystallisation temperature. The vapour pressure of the volatile component in the space 27, the contact temperature between said vapour and the liquid at the surface 29, the diffusion gradient along the liquids 24 and 42 are coupled so as to obtain a constant supply of the component in the proximity of the interface which is adapted to the crystallisation rate which determines the speed of drawing, as in the known drawing processes for monocrystals.
The partition 28 is shown with an open upper portion but it is obvious that the passage of gas and vapour may be effected through a partition which is closed but is porous. This embodiment may be preferred in the case of compounds which tend to form a crust as a result of which the contact between the liquid and the gaseous phase is impeded.
In a case of synthesis and drawing of GaAs realized in the device shown in FIG. 2, the source 23 formed by pure arsenic crystals is kept at 450 C. in such manner that an arsenic vapour pressure of torr is obtained n the space 27. The arsenic vapour is taken along by a weak current of hydrogen which enters at 25 and disappears at 26. Along the space 21, the temperature of the arsenic vapour is increased to approximately 950 C. when it passes the partition 28 the region itself of which is kept at this temperature. In the solution 24 formed by pure gallium in which the arsenic dissolves, the temperature gradient is weakly negative and the temperature of the lower part of the space 42 is approximately 850 C.
From the lower part along the vertical space the temperature still decreases somewhat. At'the level of the surface 44 from which there is drawn up to the interface solid-liquid 32 the gradient must be suificiently large in order that the crystallisation can take place under normal circumstances. The gradient is, for example, 100 C. per cm. and the temperature at the interface solid-liquid is 800 C., at which temperature the solution in this region of the liquid phase is supersaturated.
In thses circumstances the deposit is epitaxial and continuous while the crystallized compound is stoichiometric. It is obvious that non-stoichiometric compounds can be obtained, if desired by varying the conditions of crystallisation and the supply of the components. Monocrystalline rods having considerable dimensions have been manufactured while the operation was carried out in a reasonable time.
The profile of the temperature gradients as described above is a function of the shape and dimensions of the spaces as well of the physical properties of the components, of the solution and of the compounds formed. The atmosphere in the interior of the two spaces is also chosen in accordance with the said properties. It is often desirable to perform the activities in a closed and tight space in order that the vapour phases of the volatile com ponent are present only in the space 27. In a variation of the embodiment of the method, the component or components are supplied in the form of a gaseous compound; for example, arsenic may be provided in the space 27 in the form of a hydride or a halogen compound.
In another variation of the method according to the invention the solvent components and the dissolved components are added by the gradual dissolution of the compound manufactured previously, for example, in a polycrystalline form.
In another variation of the method the solvent component or components are gradually supplied, in addition to the dissolved component or components which are supplied according to the above described method, so that the level and the volume of the solution are kept constant. For example, a spare quantity of solvent is provided in the space 21 and the solvent is introduced into the vessel 24 at a constant pressure through a narrow pipe. In this case the possibilities of the device are restricted only by the rate of drawing.
The invention may be applied to a semiconductor binary compound, for example, GaAs and GaP and to mixed crystals, for example Ga In P and Ga In As for which, for example, a mixture of gallium and indium may form the solvent, as well as to mixed crystals, for example, GaP As for which more than one gaseous component is dissolved in the solvent.
What is claimed is:
1. A method of manufacturing a crystalline compound comprising the steps of contacting a liquid solution of the compound with a seed crystal which grows by crystallisation of the compound from the solution, adding at least one soluble component of the compound in vaporform to the solution at a place which is separated from the surface by the solution, transporting the component to the surface of the solution by diffusion, and drawing the crystalline compound from the solution.
2. A method as claimed in claim 1, wherein the soluble component is added in a gaseous state.
3. A method as claimed in claim 1, wherein the least volatile component of the compound is used in the solution as a sols/ent.
4. A method as claimed in claim 1 wherein the compound is an A B compound.
'5. A method as claimed in claim 4, wherein GaAs is the A B compound.
6. A device comprising means to draw a crystal from a solution of a crystalline compound, a vessel having at least two connected chambers one of which contains the solution of the crystalline compound, and the other of which contains a supply of at least one of the components in a vapour form wherein the chambers are separated from each other by at least one partition which, at the region where the partition in the first chamber is covered by the solution, is permeable to vapour from the second chamber to the solution in the first chamber, and a heating device for maintaining a negative temperature gradient in the solution during drawing in the direction of the surface from which the crystal is drawn.
7. A device as claimed in claim, 6 wherein the chambers are coaxial with respect to each other.
8. A device as claimed in claim 6, wherein the chambers are vertically disposed relative to each other.
9. A device as claimed in claim 6, wherein the chambers are separated from each other by at least one partition which, during operation of the device, separates the vapour phase in the two chambers from each other and extends in the solution.
References Cited UNITED STATES PATENTS 3,206,286 9/1965 Bennett Jr. et al. 23-301 3,260,573 7/1966 Ziegler 23-301 3,265,469 8/1966 Hall 2330l 3,488,157 1/1970 Koffer 23301 NORMAN YUDKOFF, Primary Examiner S. S'ILVERBERG, Assistant Examiner US. Cl. X.R. 23-273 SP 3%;" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N Dated December 14,
lnventofls) JEAN-MARC LE DUC and EMILE DEYRIS It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 3, line 3, before "the invention" insert of advantage for carrying out the method according to-;
Column 5, line 23, change "thses" to read these-.
Signed and sealed this 13day of June 197 (SEAL) Attest:
EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents