US 3507625 A
Description (OCR text may contain errors)
April 0 E. DEYRIS 3,507,625
APPARATUS FOR PRODUCING BINARY CRYSTALLINE COMPOUNDS Filed Dec. 8, 1966 INVENTOR.
EMILE DEYRIS AGEN United States Patent r 3,507,625 APPARATUS FOR PRODUCING BINARY CRYSTALLINE COMPOUNDS Emile Deyris, Caen, France, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y.,
a corporation of Delaware Filed Dec. 8,,1966, Ser. No. 600,124 Claims priority, applicastion France, Jan. 10, 1966,
2 Int. Cl. B01j 17/18; C01b 27/00 US. Cl. 23-473 4 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a method and a device for producing binary single-crystal compounds, particularly of binary semiconductor single crystals.
It is known to produce not only the generally em- 'ployed semiconductor bodies of, for example, germanium and silicon, but also binary compounds having semiconductor properties, for example gallium arsenide. Like in the case of germanium and silicon, single crystals of a high degree of purity are used in the production of binary semiconductors.
The method of producing these single crystals in known manner comprises several steps, that is to say: the reaction of the two elements for the formation of the compound, purification, if necessary, of this compound, doping and production of the single crystal.
The method of purification (by gradual crystallisation of a melt, by zone-melting in an elongated crucible by horizontal displacement or by floating zone-melting) and the method of producing the single crystal (vertical pulling from a crucible, horizontal zone-melting in an elongated crucible or by floatingzone-melting in a vertical direction) correspond with known methods of producing germanium or silicon single crystals.
In the case of binary compounds, however, it is advantageous to start from very pure elements prior to the synthesis, so that the phase of purification of the compound itself may be dispensed with, so that it is even possible to carry out simultaneously the synthesis and the formation of the single crystal.
In this case particular precautions have to be taken to ensure that the semiconductor compound is not contaminated by the atmosphere in the reaction vessel or by the material of the crucible.
The latter possibility of contamination is particularly important in view of the high temperature required for the synthesis which temperature favours parasitic reactions with the material of the crucible and the diffusion of any impurities contained in said material. The risk of contamination by the crucible is the greater, the longer is the period of contact between the binary, liquid compound at high temperature and the crucible and the larger is the contact surface between the compound and the crucible.
The reaction between gallium and arsenic for ogtaining gallium'arsenide takes place at the melting temperature See of gallium arsenide, that is to say at about 1240 C., at which temperature the quartz of the crucible is slowly reduced by the gallium, so that contamination by silicon is involved. Moreover, if the quartz employed is not additionally purified, migration of copper is likely to occur, the copper diffusing rapidly and then acting as an acceptor. The use of a graphite crucible does not provide much advantage, since graphite dissolves to a slight extent in gallium.
The influence of these impurities is great, since the formation of a given quantity of GaAs takes a long period of time. Like several other binary IIIV compounds of equal atomic quantities of an element of the Group III and of an element of the group V of the Periodical System (In, As, Ga, B, P, for example), gallium arsenide has a high dissociation pressure of about 0.9 atm. at the melting temperature. The conventional method of producing gallium arsenide utilizes the action of vapourous arsenic on liquid gallium at a temperature slightly exceeding the melting temperature of gallium arsenide, for example, at 1250 C., while the vapour pressure of arsenic is kept at a constant value of 0.9 atm., that is to say, the dissociation pressure of gallium arsenide at said temperature.
With regard to the danger involved in arsenic this reaction is usually carried out in a closed quartz ampulla, the colder portion of which is held at 600 C., at which temperature the vapour pressure of arsenic is equal to the desired pressure in the ampulla, that is to say, 0.9 atm. while the gallium is supplied in a quartz shuttle, which is heated at an appropriate temperature. At 1250 C. the reaction is performed rapidly; the resultant product is a compound of the composition GaAs, which is soluble in gallium at the same temperature. In order to form a body of, for example, 350 gs. it is very likely that, although it is common practice to maintain the reaction conditions for one hour, the reaction has finished already long before.
If during cooling no special precautions are taken the resultant body is polycrystalline.
It is known that for many uses, particularly in semiconductor techniques, single crystal semiconductor bodies are desired and that a single crystal is obtained by growth on a seed crystal. This process is performed gradually and the rate of growth depends mainly upon the transfer of heat, the uniformity of which determines the quality of the resultant crystal.
The production of single crystals of binary compounds with a marked dissociation pressure at the melting temperature requires, in order to avoid dissociation of the compounds in the molten state, that the reactions should be carried out in an appropriate atmosphere in a closed vessel, the temperature of the colder portion of which usually determines the vapour pressure. In the case of gallium arsenide, for instance, the vessel employed for the synthesis of the semiconductor compound may be used, while a crucible is employed which is gradually cooled from one end to the other. In order to obtain satisfactory results, this process has to be carried out accurately and for a very long period of time. A crystal of 350 gs. takes, for example, about 48 hours, during which time the liquid gallium arsenide absorbs a growing amount of impurities.
In pulling a single crystal from a melt with the aid of a seed crystal the quantity solidifying per second depends upon the quantity of heat transferrable per second from the solid substance to the solid liquid interface and hence also upon the geometric proportions of the system and upon the pressure, the temperature and the heat exchange with the ambience.
During the pulling operation it is preferred to rotate the crystal in order to avoid asymmetric crystal growth. In order to maintain a satisfactory hermetic closure the vessel in which the crystal is formed is often sealed beforehand, the required control of the moving parts then being carried out with the aid of magnetic means; it is not .diflicult to obtain in this way a sufficiently uniform movement. In other known devices a mechanical controlmember is passed through the wall of the vessel by means of a cylindrical, ground through-connection, but in such devices arsenic may leak out, which invcolves the known danger and difficulties. Therefore, liquid gallium seals are also used as an alternative.
All these known devices for crystal pulling have the disadvantage that during the Whole process the melt remains in contact with the crucible at a high temperature.
In floating zone-melting, in which a floating molten zone is held by surface tension between two aligned solid rod portions of gallium arsenide, a contact between the molten zone and the wall of the crucible is avoided, but this method requires much care, while during the synthesis the semiconductor material may have been contaminated.
The present invention has for its object inter alia to mitigate the aforesaid disadvantages.
According to the invention the method of producing a binary monocrystalline semiconductor compound, on the one'hand, by the reaction of pure, volatile constituents in a closed space, the vapour pressure of which is kept substantially equal to the dissociation pressure of said compound at the melting temperature thereof, with a pure molten constituent, at a reaction temperature which exceeds said melting temperature of the compound, and on the other hand by crystallizing out the compound in the form of a single crystal by pulling, is characterized in that only a zone of the molten, continuously replenished constituent located at the surface near the place where the crystal is growing during pulling is heated at the reaction temperature by means of an auxiliary heating member, while the molten compound obtained by synthesis is directly withdrawn from the melt by the pulling process.
The quantity of molten constituent taking part in the reaction, which may be contaminated by the high temperature heating, is only small, so that the method according to the invention provides a compound in the form of a single crystal of a high degree of purity.
The quantity of heat supplied by the auxiliary heating member and the rate of pulling are controlled so that the rate of formation of the compound corresponds to the rate of growth of the crystal.
The level of the molten constituent in the reaction space is preferably held constant.
For this purpose the vessel for the melt can be connected by a duct taken through the wall of the reaction space in a gas-tight manner with a reservoir of the molten constituent, the surface of which is exposed to a chemically controllable pressure of an inert gas, which does not affect the molten constituent.
The invention will now be described more fully with reference to the drawing; the figure is a vertical sectional view of apparatus embodying the invention for carrying out the method.
Hereinafter the production of single crystals of gallium arsenide is chosen by way of example; as a matter of course, other binary semiconductor compounds can be obtained by means of the method and the device according to the invention.
The device shown in the figure comprises a first space 4, termed the reaction space, which is formed by a hollow, vertical cylinder 1, a second space 13 and a duct 8 between said spaces.
The duct 8 forms at the same time the horizontal limb of a U-shaped tube 26, a vertical limb 30 of which opens out inside the space 4 in the form of a vessel having a widened portion 18 from which the Single crystal is 4 pulled up, whereas the other vertical limb 9, which is widened to form a reservoir, opens out inside the space 13.
The open ends of the vertical limbs of the tube 26 are preferably located in the one horizontal plane, so that the heights of the levels 27 and 28 of the liquids in this tube can be easily observed in the reservoir 9 and in the widened portion 18 through the walls, which are transparent at least at the relevant areas.
The through-connections 32 and 31 of the duct 8 through the walls of the spaces 4 and 13 are sealed by known means, for example, by melting, if quartz glass is used.
Through the upper portion of the space 13 is taken 2T slanting tube 10 in an airtight manner. The upper wall of the space 13 is provided for this purpose with a tubular portion 34 for passing the tube 10, while at 25 the passage of the tube 10 is sealed in an airtight manner. Inside the space 13 the tube 10 is prolonged so that at 33 it opens out above or in the opening of the reservoir 9. The other end of the tube 10, outside the space 13, is provided with a bulb 12, into which a quantity of gallium 11 can be introduced, which is sufiicient to completely fill the member 26 after being melted, while it can be continuously replenished during the pulling operation.
The space 13 can be connected through the tube 29 with the outlet duct 14 of an exhausting member or with an inlet duct 16 for an inert gas. By means of cocks 15 and 17 the ducts 14 and 16 can be closed wholly or partly, since the pressure of the inert gas inside the space 13 has to be variable at will.
The closed vessel 6 is connected at 24 in an airtight manner with the space 4 through a duct 5 on the side wall of the cylinder 1. The vessel 6 is provided with an adequate quantity of crystalline arsenic 7 for obtaining and maintaining in the reaction space 4, subsequent to sublimation, an atmosphere of arsenic of appropriate pressure during the whole duration of the reaction.
The upper portion of the cylinder 1 has a vertical, cylindrical, narrowed portion 19, the axis of which coincides with the axis of the vertical limb 30 of the U-shaped tube 26 and in which a cylindrical piston 2 is arranged so as to be vertically displaceable and/or rotatable about its axis. The lower end of the piston is adapted to receive a monocrystalline seed 3. The lateral surface of the piston 2 and the inner wall of the narrowed portion 19 are preferably ground so that the piston can slide or rotate substantially without friction. However, in spite of a most accurate machining such cylindrical contact surfaces are not completely gas-tight, which applies particularly to vapourous arsenic. In the device according to the invention the ground seal is improved by a liquid seal formed by an annular trough 20, formed by the cylinder 1 directly above the narrowed portion 19, and containing a liquid of suitable viscosity, for example liquid gallium, if gallium arsenide has to be produced, so that soiling of the outer surface of the piston 2 and contamination of the atmosphere in the space 4 and hence of the growing crystal are avoided.
The length of the ground portion of the piston 2 and the height of the cylinder 1 are sufiicient to allow a vertical translatory movement over a distance corresponding to the length of the single crystal to be produced.
The vertical translatory movement and the rotary movement of the piston 2 can be carried out in known manner by means of a mechanism (not shown), arranged outside the tube 1. The rates of these movements are preferably variable and uniform. The pulling rate is adapted to the process which is the slower in the widened portion 18, that is to say to the solidification of the gallium arsenic, which is slower than the synthesis of the gallium arsenide. The solidification is determined by the heat transfer during the crystallisation and hence by the geometric conditions of the crystal-pulling system.
A furnace 21 provides and maintains the temperature required in the space 4, the value thereof always exceeding the temperature corresponding to the vapour pressure of the arsenic in the vessel 6. The furnace 21 surrounds the space 4, the narrowed portion 19 and the parts of the ducts 5 and 8 adjacent the space 4 so that the temperature at any point of the space 4 is not lower than the temperature maintained in the vessel 6 for the sublimation of the arsenic. A furnace 22 of known type provides and maintains the temperature of the vessel 6 and surrounds the vessel 6 and the portion of the duct 5 extending up to the portion heated by the furnace 21. The temperature of the vessel 6 can be maintained at a value which is sufficient for evaporating the arsenic until the resultant vapour pressure corresponds to the dissociation pressure of gallium arsenide at the melting temperature thereof. This temperature of the vessel 6 determines the pressure of the arsenic in the reaction space.
Moreover, a heating element, which is preferably formed by a coil of a single winding 23, traversed by a high-frequency current, is capable of heating the central part of the widened portion 18 of the tube 26 and the upper portions of the liquid contained therein at a temperature which exceeds that of the remaining part of the space 4, and at which the gallium arsenide is liquid. In accordance with the geometric conditions of the system and upon the materials employed it may be advantageous to provide, above the winding 23, a second winding (not shown) in order to avoid that the heat transfer along the single crystal 3 produces an excessively strong cooling of the liquid portions, where the compound is being formed. Melting of the charge 11 in the bulb 12 by means of a suitable heating element, for example, a simple burner, may be performed in one step or in several stages. If melting is performed gradually, the bulb 12 is provided internally with a member capable of dosing the outlet of the molten compound. This member is formed by a vertical partition 35, having an opening of a given size on the lower side or by a different, appropriate member.
The temperatures produced by the furnaces 21 and 22 are controlled and checked by known means (not shown), formed for example by suitably arranged thermo-elements and a suitable electric arrangement; especially the temperature of the envelope 6 has to be controlled very accurately. Pulling of the single crystal is checked visually through a window in the furnace 21; this window must not bring about any reduction of temperature in the space 4.
It is advantageous to add a member for fixing and controlling the level 28 of the liquid. This member may be formed by an optical indicator operating with a fine light beam or by a detector of passing rays (for example X- rays) and it permits of keeping said level constant by acting upon the control-member for the pressure of the inert gas in the reservoir 13 with greater accuracy, so that a uniform single crystal is obtained.
The assembly formed by the cylinder 1, the vessels 6 and 12, the reservoir 9, the widened portion 18 and the ducts arranged between them as described above is preferably made of quartz of the purest possible quality. The through-connections 31, 32, 34 and the connections 24 and 25 are preferably welded.
The method according to the invention to be described hereinafter by way of example and carried out by means of the device shown in the figures relates to the production of a single crystal of gallium arsenide.
The gallium arsenide seed crystal 3 is fastened to the lower end of the quartz piston 2, which is inserted into the quartz cylinder 1. Then one compound, the arsenic, in the crystalline form is put into the quartz vessel 6, which is then hermetically connected at 24 with the quartz duct 5. The required quantity of arsenic depends upon the size of the desired single crystal; since the gallium arsenide to be produced must have 50% of the atoms of each constituent, the weight of the arsenic in the compound is equal to 1.074-times the weight of gallium in the compound. Above the narrowed portion 19 an adequate quantity of liquid gallium is provided for an effective operation of the seal 20.
The quartz bulb 12 is provided with the gallium charge 11 in the form of an ingot and the quartz duct 10 is hermetically welded at 25 to said envelope. The cock 17 is opened and the cock 15 is closed so that the whole system can be evacuated at a low temperature for example, for two hours. After degassing the cock 15 is closed. The furnace 21 is switched on and the gallium 11 is partly heated at the melting temperature of about 29 C., for example, by means of a burner until the liquid gallium fills the reservoir 9 through the duct 10, after which it flows through the duct 8 into the widened portion 18. The space 4 is thus sealed by the liquid gallium in the duct 26.
The furnaces 21 and 22 then heat the vessel 6 and the arsenic contained therein at a temperature of 600 C., which is kept constant with a tolerance of 0.25 C. This temperature determines the pressure of the arsenic in the reaction space, which is heated as a whole at a tem perature of, for example, 630 C. by the furnace 21. According as the temperature of the arsenic in the vessel 6 increases, the vapour pressure of the arsenic in the reaction space increases until a value of 0.9 atm. is attained at a temperature of 600 C. of the arsenic. In order to avoid that this pressure urges the liquid gallium from the widened portion 18 into the reservoir 9, an equal counter-pressure is exerted on the surface 27 of the liquid gallium in the reservoir 9 by means of an inert gas, for example, argon, which is supplied through the duct 16 after the cock 17 is opened. The pressure of the inert gas is controlled by known means at the inlet 16 so that the level 28 of the liquid gallium in the widened portion 18 is located slightly above the height of the coil 23. Instead of argon nitrogen may be used, since in the device according to the invention the gas is in contact with the gallium only at a temperature which is sufliciently low not to give rise to a reaction.
The coil 23 is then connected to a current of, for example, 5 mc./s., so that the gallium in the widened portion 18 is heated locally at the surface at a temperature of 1250 C. This temperature is attained only in the central portion of the liquid surface, since the surroundings are slightly cooled by the wall of the widened portion. The zone heated at 1250" C., which value slightly exceeds the melting temperature of gallium arsenide, is thus restricted to a small surface at the centre of the widened portion 18 in line with the piston 2 and the seed 3. Owing to the vapour pressure of the arsenic in the space 4 and to the presence of the liquid gallium of 1250 C., a small quantity of gallium arsenide is formed at the surface only in a central zone.
The seed crystal 3 is slowly moved downwards until it just touches the liquid surface. The high-frequency current through the coil 23 is again switched on to melt the end of the seed crystal, so that a drop is formed, which joins the surface of the liquid, at this place by gallium arsenide in statu nascendi.
Pulling of the crystal proper is carried out by slowly and uniformly moving the piston 2 upwards, the piston being simultaneously rotated in order to obtain a uniform crystal. The pulling rate is, for example, 2 cms./ hour and the speed of rotation is, for example, 20 rev./ min. During the pulling process the liquid gallium is available at the surface of 1250 C., where locally liquid gallium arsenide is immediately formed, so that the risk of contamination is slight, since the gallium arsenide is not at any moment in contact with the quartz or any other material.
The supply of gallium from the widened portion 18 from the reservoir 9 via the duct 8 is performed according to need by melting the gallium charge 11, while a hours for obtaining a single crystal of a length of 10 cms. under the aforesaid conditions.
After pulling, the temperatures can be lowered and the whole assembly can be brought to the ambient temperature, while the gallium of the seal 20 is maintained in the liquid state in order to permit of removing the piston 2 and the single crystal attached thereto.
By the choice of the dimensions of the various parts of the device and of the time of pulling a single crystal of the desired dimensions can be obtained, which has the desired purity, uniformity and homogeneity.
Although in the foregoing the manufacture of single crystals of gallium arsenide is described, the invention is, of course, not restricted thereto; the device may also be used for the production of single crystals of other binary semiconductor compounds, one of the constituents being supplied in the form of the vapour and the other or a combination of both in the form of a liquid, within the scope of the invention.
The device according to the invention may also be employed for the production of single crystals with impurities or admixtures, supplied in the vapourous form.
What is claimed is:
1. Apparatus for producing by pulling a monocrystalline binary semiconductor compound, comprising a closed envelope, within the envelope means for supporting a supply of a volatile constituent of the compound, means for heating the supply of the volatile constituent at a first temperature producing within the envelope a vapor pressure of the volatile constituent approximately equal to the dissociation pressure of the compound at its melting temperature, within the envelope a tubular vessel for holding a molten supply of a non-volatile constituent of compound, a duct connected at one end at the bottom of the tubular vessel, a reservoir for the non-volatile constituent connected to the opposite end of the duct, said reservoir being arranged in a closed space isolated from the said closed envelope, means arranged above the tubular vessel for supporting a seed crystal and for moving the seed crystal to the melt in the tubular vessel and then pulling the seed crystal from the melt, heating means for maintaining the space within the envelope around the pulling means at a second temperature, heating means for maintaining only the surface of the melt in the vicinity of the seed crystal at a third temperature exceeding the melting temperature of the compound and at which the melt constituent reacts with the vapor to form the compound which grows on the seed crystal while the later is being pulled, means for feeding material in liquid form from the reservoir through the duct into the tubular vessel and thus beneath the melt surface to replenish the melt material lost to the growing crystal while avoiding reaction between the said vapor and the replenishing material before the latter becomes part of the melt supply, and means provided above the reservoir for supporting and melting an ingot of the non-volatile constituent and for causing the melt to flow into the reservoir.
2. Apparatus as set forth in claim 1 wherein the heating means for maintaining the third temperature includes a radio-frequency heating coil surrounding the envelope at the level of the melt surface.
3. Apparatus for producing by pulling a monocrystalline binary semiconductor compound, comprising a closed envelope within the envelope means for supporting a supply of a volatile constituent of the compound, means for heating the supply of the volatile constituent at a first temperature producing within the envelope a vapor pressure of the volatile constituent approximately equal to the dissociation pressure of the compound at its melting temperature, within the envelope a tubular vessel for holding a molten supply of a non-volatile constituent of the compound, a duct connected at one end at the bottom of the tubular vessel, a reservoir for the non-volatile constituent connected to the opposite end of the duct, means arranged above the tubular vessel for supporting a seed crystal and for moving the seed crystal to the melt in the tubular vessel and then pulling the seed crystal from the melt, heating means for maintaining the space within the envelope around the pulling means at a second temperature, heating means for maintaining only the surface of the melt in the vicinity of the seed crystal at a third temperature exceeding the melting temperature of the compound and at which the melt constituent reacts with the vapor to form the compound which grows on the seed crystal while the latter is being pulled, means for feeding material in liquid form from the reservoir through the duct into the tubular vessel and thus beneath the melt surface to replenish the melt material lost to the growing crystal while avoiding reaction between the said vapor and the replenishing material before the latter becomes part of the melt supply, said reservoir being arranged in a closed space isolated from the said closed envelope, and associated with the reservoir closed space means for exhausting the closed space and means for filling same with an inert gas at a controlled pressure. I
4. Apparatus as set forth in claim 3 wherein the pulling means extends through a wall portion of the envelope sealed off by a melt of the non-volatile constituent.
References Cited UNITED STATES PATENTS 9/1959 Horn 2330l 2/1963 Enk et al. 23-204 US. Cl. X.R. 23204, 301