US2889240A - Method and apparatus for growing semi-conductive single crystals from a melt - Google Patents

Description

June 2, 1959 os 2,889,240

METHOD AND APPARATUS FOR GROWING SEMI-CONDUCTIVE SINGLE CRYSTALS FROM A MELT Filed March 1, 1956 2 Sheets-Sheet 1 IN VEN TOR.

FRED D. Ros:

Byffgg June 2, 1959 F. D. ROS! METHOD AND APPARATUS FOR GROWING SEMI- 2 Sheets-Sheet 2 CONDUCTI SINGLE CRYSTALS FROM A MELT Filed March 1, 1956 INVENTOR. FRED D. Hus;

{rmmi/ nitecl tates Fred D. Rosi, Plainsboro, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application March 1, 1956, Serial No. 568,798

11 Claims. (Cl. 1481.6)

This application is a continuation-in-part of application Serial No. 535,391, filed September 20, 1955, now abandoned.

This invention relates to improved methods and apparatus for growing single crystals of semi-conductive materials. More particularly, the invention relates to improved crystal growing methods and apparatus for producing single crystals, such as germanium or silicon, having fewer structural imperfections and having high lifetime of minority charge carriers, making them more suitable for semi-conductor devices.

The growing of single crystals of a material by solidifying successive portions of the material from its molten phase is well known. Two basic techniques are widely employed. The first employs a vertical furnace in which a seed crystal of the material is contacted to the surface of a melt of the material and slowly withdrawn vertically therefrom. The molten material attaches itself to the seed and solidifies thereon as it is withdrawn from the hot melting zone of the furnace. The crystalline character and orientation of the seed determines the plane of crystallization of the solidifying material. This method is commonly known as the Kyropoulis-Czochralski technique. The second method comprises a horizontal furnace in which an elongated charge in contact with a seed crystal is successively melted and refrozen by sweeping along its length a melting zone of heat starting from the seed end of the charge. This method is usually referred to as zone-melting. In both methods a long single crystal is produced provided the seed employed is single crystalline and properly oriented.

The type of conductivity that may be established in the semi-conductor crystal is dependent upon the atomic structure of an impurity substance in relation to the atomic structure of the host crystal into which the impurity is introduced. Thus a substance whose atoms are capable of giving up electrons in a particular host crystal is termed a donor, and since there is a surplus of electrons available, the semi-conductor so doped is deemed to be of n-type (negative) conductivity. n the other hand, a substance whose atoms are capable of borrowing or accepting electrons in a particular host crystal is termed an acceptor, and since there is a shortage of electrons for current conduction, the semi-conductor so doped is deemed to be of p-type (positive) conductivity.

In n-type semi-conductors current conduction is said to take place by the available electrons; in p-type semi-conductors current conduction is said to take place by holes. Thus in n-type semi-conductors the electrons are termed majority charge carriers and holes are called minority charge carriers. In p-type semi-conductors the holes are the majority charge carriers and electrons are the minority charge carriers. Transistor action is based on the ability to introduce minority charge carriers into impurity doped semi-conductors by one means or another. The electric fields of injected holes or electrons will affect the majority charge carriers present in the semi-conductor so as to eventually result in a atent useful output current. However, electrons and holes in semi-conductor materials, being of opposite sign, tend to recombine; that is, the electrons tend to fill up the holes. In general, the longer the time before electrons and holes recombine, the more the number of uncombined charge carriers present for current conduction during that time. Thus a measure of the usefulness of a particular semi-conductor material is the lifetime of injected minority carriers by which is meant the time from injection to combination of minority charge carriers with majority charge carriers. Generally, crystals with a lifetime of about 50 microseconds and a resistivity of about 1 to 5 ohm-cm. are preferred for manysemi-conductor devices.

Primary structural imperfections in the arrangement of the atoms of single crystal semi-conductive materials have a marked effect upon the lifetime of the charge carriers. A characteristic type of crystalline imperfection is an edge-type dislocation which is apparently produced, for example, by the deformation process of slipthat is, as if some force bears against the edge of a growing single crystal resulting in one portion of the crystal being sheared relative to another portion. A more complete description of crystalline structural imperfections, particularly edge-type dislocations, is found in W. T. Reads book entitled Dislocations in Crystals, published by McGraw-Hill Book Co., New York, 1953, and with special reference to Chapter 1 thereof. In general, such imperfections result in a reduction in the carrier lifetime, and while not fully understood, it is generally believed that the recombination of holes and electrons is catalyzed by these imperfections. Factors such as the growth rate of the crystal and temperature gradients in the melt and in the growing crystal may all contribute to such imperfections.

It is therefore an object of this invention to provide improved methods and apparatus for growing single crystals of semi-conductive materials with fewer structural imperfections such as edge-type dislocations and having increased charge carrier lifetime.

Another object of this invention is to provide improved methods and apparatus for growing single crystals of semi-conductive materials having increased charge carrier lifetime.

A still further object of this invention is to provide improved methods and apparatus for growing single crystals of semi-conductive materials with fewer structural imperfections such as edge-type dislocations.

Another object of the invention is to provide improved methods and apparatus for growing single crystals of germanium or silicon having improved charge carrier lifetime.

Another object of this invention is to provide improved methods and apparatus for growing single crystals of germanium and silicon with fewer structural imperfections such as edge-type dislocations and having increased charge carrier lifetime.

These objects and other advantages are achieved according to the invention by growing a crystal from a melt and establishing a relatively steep temperature gradient in the growing single crystal, which gradient results in a higher charge carrier lifetime in the crystal, and by maintaining substantially the entire interface between the melt and the solidified growing crystal perpendicular to the axis of crystal growth, which interface results in a crystal having fewer structural imperfections such as edge-type dislocations.

The steep temperature gradient is established along the length of the crystal extending from the interface of the solid face of the crystal in contact with the melt. It has been discovered that the temperature gradient maintained along the crystal in a longitudinal direction has a marked elfect upon the charge carrier lifetime of the crystal so grown. Furthermore, the steepness of the gradient is somewhat critical, the increase in lifetime being significantly less when gradients of less than 100 C. per'cm. or more than 400 C. per cm. are used. Considerable improvement in lifetime of charge carriers has been found with a temperature gradient for silicon within the range of 200 to 400 C. per cm.; for germanium the preferred range is from 150 to 250 C. per cm. To achieve a planar or flat solid-liquid interface normal to the avis of crystal growth, the temperature across the crystal diameter at its interface is maintained uniform or isothermal. Crystals grown according to the invention have dislocation densities ranging from to 100 cum- The invention will be described in greater detail With reference to the drawings in which similar reference characters designate the same or similar elements throughout.

Figure 1 is a cross-sectional, elevational view of the growing end of a single crystal showing the temperature gradient therealong according to the invention;

Figure 2 is a partial cross-Sectional, elevational view of a vertical crystal growing furnace having means to establish a particular temperature gradient in a growing crystal;

Figure 3 is a partial cross-sectional, elevational view of a vertical crystal growing furnace depicting an almost completely grown single crystal;

Figure 4 is a detailed cross-sectional, elevational view of a gas cooled seed crystal holder for a vertical crystal growing furnace such as shown in Figures 2 and 3;

Figure 5 is a detailed cross-sectional, elevational view of a liquid cooled seed crystal holder for a vertical crystal growing furnace such as shown in Figures 2 and 3;

Figure 6 is a cross-sectional, elevational View of a horizontal zone-melting crystal growing furnace having means to establish a particular temperature gradient in the growing single crystal at the beginning of the crystal growing operation; and

Figures 7a, 7b, and 7c are cross-sectional, elevational views of the growing end of three crystals each of dif ferent diameters showing the lessening of curvature and. the increasing flatness of the interface as crystal diameter is decreased.

Referring to Figure l, a seed crystal 2 of silicon is shown upon which a growing single crystal 4 is attached as the seed is withdrawn from the silicon melt 6. According to the invention, the single crystal 4 will have a markedly higher carrier lifetime if grown under a temperature gradient (as indicated) of between 200 and 400 C. per cm. This gradient is established along the first cm. of the growing crystal commencing with the interface thereof between the solid and molten phases of the silicon. Preferably the temperature gradient is about 300 C. per cm. in the case of silicon. For germanium the temperature gradient lies within the range of 150 to 250 C. per cm. and preferably is about 200 C. per cm.

The function of these critical temperature gradients in obtaining higher lifetimes is not fully understood. It is believed, however, that the presence of the relatively sharp gradient prevents the condensation or formation of clusters of vacancies. (A vacancy occurs when a crystal lattice site is not occupied by an atom of the crystal material.) It is thought that the aggregation or clustering of vacant lattice sites in the atomic arrangement of single crystals of germanium and silicon results in the ultimate collapse of disc-like holes in the crystal structure. The collapse of these holes, in turn, is thought to produce dislocations and other carrier lifetime-damaging imperfections in crystals. By establishing the above-indicated temperature gradients it is believed that the time during which vacancies are mobile and able to cluster is so short that these vacancies are frozen in situ and thus isolated. However, the invention is not predicated upon the correctness of this theory.

It is also thought that a non-uniform temperature distribution across the crystal at the interface between the liquid and solid phases in a direction normal to the axis of crystal growth results in the generation of localized' shear strains. These shearfstrains are believed to bring about deformation by slip during crystal growth which is considered to be the main source of edge-type dislocations. Hence it is necessary to maintain the liquid-solid interface as flat or planar as possible with respect to the direction of crystal growth. Therefore the thermal gradient established in the solidified crystal at the interface end thereof should be uniform across the crystal so that the temperature at the center portion of the crystal interface region is the same as the temperature at the peripheral portions of the growing crystal.

The advantages and significance of the invention can be It will be understood that the above measurements were taken on the crystal as grown and were made on the first portion to solidify. It should also be noted that in all cases a slight decrease in resistivity of the crystal was observed. This decrease in resistivity is apparently due to the loss in the final crystal by segregation of certain impurities present in the original crystal. Thus with a gradient of about 300 C. per cm. a single crystal is obtained whose lifetime is between 40-70 microseconds and whose resistivity is about 5 ohm-cm.

The effect on the dislocation density in a crystal grown according to the invention wherein the interface between the liquid-solid phases is maintained flat or planar is shown by the following data. The average edge-type dislocation density in vertically pulled crystals Where no effort is made to control the shape of the interface is between 10 and 10 cmr' Crystals have been grown according to the invention with a flat interface having dislocation densities ranging from 0 to cmr Because of the difiiculty in establishing a radially uniform temperature in a thick growing crystal, it is difiicult to establish the requisite flat or planar interface. However, by growing crystals of smaller diameters it is easier to extract heat from the interface region uniformly across the crystal and thus establish the flat interface. A silicon crystal having a diameter of 1" had an edge dislocation density of 10 cmr a silicon crystal having a diameter of 0.5" had an edge dislocation density of 10 cm.- and having a diameter of 0.25" had an edge dislocation density of substantially 0 ems- It should be appreciated that the diameter size of the crystal grown according to the invention is not the contributing factor in obtaining dislocation-free crystals. As a practical matter it was found easier to establish the flat interface in crystals of small diameter for thermodynamic reasons. According to the invention a crystal of any diameter can be grown dislocation-free if the interface between the liquid-solid phases is maintained fiat. Thus it was observed that as crystal diameter decreased the interface between the solid grown portion 4 of the crystal and the molten portion 6 became flatter and flatter as shown in Figure 7. This is due, as noted previously, to the fact that the temperature distribution across the crystal is much more uniform in the small diameter crystal as compared with the temperature distribution in larger crystals.

The necessary temperature gradients may be obtained by any number of convenient methods. It is preferred, according to the invention, to extract the heat from the growing crystal to establish the desired gradient by simultaneously cooling both the portion of the crystal immediately adjacent the interface and the far or seed end of the crystal. At the commencement of crystal growth it was found that merely cooling the seed end of the crystal was sufficient to achieve the necessary gradient. However, as the length of the crystal increased the extraction of heat from the interface region becomes more and more dependent upon thermal conduction through the crystal. Hence it becomes necessary to eventually extract the heat more directly from the region of the interface.

A vertical Kyropoulis-Czochralski type of crystalgrowing furnace is shown in Figure 2. A crucible 8, preferably of quartz, is contained within a carbon crucible 10. An electrical induction heating coil 12 surrounds the crucible assembly and is provided with an alternating electric current from any convenient power source (not shown). A mass of silicon 6 is placed within the quartz crucible and melted. The carbon crucible 10 is partially contained within the lower end of a quartz or heat-resistant glass tube 18 which extends vertically therefrom. The top end of the tube 18 is closed by a lid 20 or an air-tight metal fixture which may also be of quartz. Into the top end of the tube 18 and through an aperture in the lid 20 an elongated shank member 22 extends downwardly. The shank 22 has a seed holder 24 on its lower end to which may be attached a seed crystal 2 or" silicon. The upper end of the shank member is attached to a wire or cord 28 passing over pulleys 30 and 32 to a drum 34 driven through a chain of reduction gears within a gear box 36 by a reversible motor 38. A gear train 2123 couples the shank to a motor 25 which rotates the shank during crystal growth.

The operation of the apparatus is as follows: The crucible 8 is charged with material from which a crystal 4 is to be grown. The charge may comprise silicon in the form of fine needles, powder, or small pieces. Upon energization of the induction heating coil 12 the charge melts. When the silicon is completely molten, the seed crystal holder 24 with the seed crystal 2 attached is carefully lowered until it just touches the surface of the molten silicon. The motor 33 is then started so that it begins to slowly wind the cord 28 on the drum 34. Thus, the shank member 22 is slowly raised at a speed of about 0.5 mm. per minute. The shank 22 and hence the growing crystal 4 are rotated at a speed of about 50 r.p.m. The speed of upward movement of the shank member and seed crystal holder is adjusted so that a silicon crystal 4 grows attached to the seed crystal 2 and continues to grow as long as the silicon is available in the crucible. The temperature of the melt surface is adjusted almost to the freezing point of silicon (about 1420" C.).

In order to establish the desired temperature gradient of about 300 C. per cm. at the start of the crystal pulling operation, the seed end of the crystal is cooled by a jet of inert gas directed against the seed crystal 2. This may be accomplished by means of a hollow shank 22 supporting a hollow seed holder 24 such as shown in detail in Figure 4. The seed holder may be of metal and is tapered inwardly at its bottom end so as to suspend a seed crystal with a widened end as shown. Alternatively, the seed crystal is held in place by means of set screws (not shown). An inert gas such as argon or helium is introduced into the seed holder 24 and against the seed crystal through a pipe 27 which extends downward through the hollow shank 22. A special coupling device 29 is shown for delivering the gas from a stationary source (not shown) to the pipe 27 located within the rotating shank 22. The shank is annularly grooved. A hole is drilled from the groove into the cavity within the shank. The pipe 27 is inserted in this hole. A gas-tight stationary bushing 31 encloses the grooved portion of the shank. The bushing has a hole drilled within it which is aligned with the groove of the shank. The gas source is connected bp a pipe inserted in this hole. Thus as the shank turns, there'is always a connection from the gas source to the pipe through the groove-connected channels in the bushing and the shank.

As mentioned previously, cooling the seed end of the crystal is sufiicient to establish the required gradient while the crystal is of relatively short length. However, as the crystal grows and the distance between the cooling gas jet and the crystal interface increases it becomes increasingly diflicult to maintain the gradient. Therefore, an alternative cooling arrangement is provided which is preferably employed in combination with the cooling of the seed end of the crystal. Jets of helium or argon or another inert gas are played upon the region immediately adjacent the crystal interference by means of a jet 33 on the rotating end. The gas is supplied to the jet 33 by means of the pipe 52 which enters the crucible chamber through an aperture in the cover 20.

As shown in Figures 2 and 3, as the crystal grows, the melt becomes depleted and falls to an increasingly lower level in the crucible. Hence the jet 33 must be slowly lowered into the crucible chamber so as to maintain the jet of gas on the portion around the region of the crystal interface.

An important alternative method for cooling the seed end of the crystal is shown in Figure 5. Here the hollow shank 22 has its lower end closed off and in intimate contact with the upper end of the seed crystal 2. Water is introduced and circulated through the hollow shank by means of the pipe 54 which runs down through the shank. The outlet of the pipe 54 into the hollow shank is located down near the region of the shank which contacts the seed crystal. Thus relatively cool water enters the pipe 54 and leaves through the shank 22. Heat is extracted by the water from the seed crystal 2 by conduction through the shank wall in contact therewith.

The coupling device for admitting and removing the water is based on the same principle as the gas coupling device (29) described in connection with Figure 4. The hollow shank 22 contains an inner pipe 54. The hollow seed holder 24 may be threaded onto the lower end of the shank 22. The pipe 54 extends downwardly from within the shank and into the seed holder so as to deliver the water adjacent the end of the seed crystal 2. The upper end of the hollow shank has a passage drilled through its wall which connects the internal cavity of the shank with an annular groove on its outside surface. The inlet pipe 54 is threaded into the smaller diameter portion of the upper part of the shank and a passage is drilled through the shank to connect this pipe to a second annular groove in the outer surface of the shank. The stationary bushing 35 has two passages drilled through it so as to connect water inlet and outlet pipes 37-39, with the respective inlet and outlet grooves of the shank 22. In this manner Water or any other coolant may be introduced into and circulated within the rotating seed holder 24.

The practice of the invention both as to the requisite temperature gradient and interface flatness may also be carried out when growing crystals by the horizontal zone melting technique. Referring to Figure 6, an elongated boat-like crucible 8 of graphite, for example, is charged with a rod shaped piece of germanium 14, for example. At one end of the germanium rod a seed crystal 2 of germanium is placed.

The crucible 8 is placed within a quartz tubular enclosure 58 and is gradually propelled at about 2.5 inches per hour or less, for example, through a ring shaped induction heating element 12 starting at the end of the crucible where the seed crystal 2 is placed. The crucible is supported within the quartz tube 58 by means of a ring shaped member 61. When the temperature reaches the melting point of germanium (about 940 C.), the initial end of the germanium rod melts as well as a portion of the seed crystal 2. As the crucible with its contents is propelled through the heating element 10 successive segments of the rod 14 are melted and refrozen. The refrozen portions grow as a single crystal extension of the seed crystal.

In order to establish a temperature gradient of about 200 C. per cm. along the length of the growing crystal jets of an inert gas such as argon are played upon the end of the seed crystal 2 and against the region of the growing crystal immediately adjacent the interface. This may be accomplished by means of the nozzles 60 and 62 which are connected to a source of inert gas (not shown). The jet of gas which is being directed against the region of the crystal interface should be carefully controlled so as to not strike the molten portion 64. If the gas strikes the molten portion, the surface thereof will probably freeze in advance of the solid-liquid interface and will increase the difiiculty of propagating single crystalline growth due to the undesired nucleating center formed by the solidified surface.

There thus has been shown and described an improved method and apparatus for growing single crystals of semi-conductive materials such as germanium and silicon, having greatly increased carrier lifetime as well as being freer of edge-type dislocations. The invention should not be limited to the specific apparatus shown and described for the purpose of illustrating the invention. Many modifications of the embodiments described can be made without departing from the spirit of the invention which modifications include the establishment of a particular temperature gradient range and a flat or planar interface between the liquid-solid phases of a growing crystal in order to obtain greatly improved single crystals of semiconductive materials. With respect to maintaining a flat interface when growing crystals, it should be understood that the invention is concerned with the physical or crystallographic structure of materials rather than upon the chemistry thereof. Hence this aspect of the invention may be practiced to advantage in growing crystals of any material and is not limited to the specific exemplary materials described in this specification, namely germanium and silicon.

What is claimed is:

l. The method of growing a single crystal of a semiconductive material comprising the steps of: melting at least a portion of said material, contacting a seed crystal of said material to said molten portion, relatively moving said seed crystal with respect to said molten portion whereby successive portions of said molten portion are frozenout on said seed crystal, establishing and maintaining in said frozen-out portion a temperature gradient of between 150 and 400 C. per cm. adjacent the interface between said frozen solid portions and said melt, and maintaining the temperature across said frozen solid portions radially uniform whereby said interface is planar in a direction normal to the direction of crystal growth.

2. The method according to claim 1 wherein said semiconductive material is silicon and said temperature gradient is between 200 and 400 C. per cm.

3. The method according to claim 1 wherein said semiconductive material is germanium and said temperature gradient is between 150 and 250 C. per cm.

4. The method of growing a single crystal of a semiconductive material comprising the steps of: melting at least a portion of said material, contacting a seed crystal of said material to said molten portion, relatively moving said seed crystal with respect to said molten portion whereby successive portions of said molten portion are frozen-out on said seed crystal, and establishing and maintaining in said frozen-out portion a temperature gradient of between and 400 C. per cm. adjacent the interface between said frozen solid portions and said melt.

5. The method according to claim 4 wherein said semiconductive material is silicon and said temperature gradient is between 150 and 250 C. per cm.

6. The method according to claim 4 wherein said semiconductive material is germanium and said temperature gradient is between 150 and 250 C. per cm.

7. In the vertical pulling method for growing single crystals of a semi-conductive material selected from the class consisting of germanium and silicon, the method of growing high lifetime substantially dislocation-free crystals of said material comprising the steps of: forming a melt of said material, contacting a seed crystal of said material to the surface of said melt, slowly withdrawing said seed crystal at a predetermined rate so as to grow a solidified crystal portion of said molten material attached to said crystal, establishing a constant temperature gradient along the length of said solid crystal portion and said seed crystal of between 150 and 400 C. per cm. from the interface between said solid crystal portion and said melt, maintaining the temperature across said solidified crystal portion radially uniform whereby said interface is planar in a direction normal to the direction of crystal growth, and continuing to withdraw said growing crystal portion to continue the growth thereof while maintaining said gradient and said planar interface.

8. In the vertical pulling method for growing single crystals of a semi-conductive material selected from the class consisting of germanium and silicon, the method of growing high lifetime crystals of said material comprising the steps of: forming a melt of said material, con tacting a seed crystal of said material to the surface of said melt, slowly withdrawing said seed crystal at a predetermined rate so as to grow a solidified crystal portion of said molten material attached to said crystal, establishing a constant temperature gradient along the length of said solid crystal portion and said seed crystal of between 150 and 400 C. per cm. from the interface be tween said solid crystal portion and said melt, and continuing to withdraw said growing crystal portion to continue the growth thereof while maintaining said gradient.

9. The method of claim 1 wherein said step of relatively moving said seed crystal with respect to said molten portion is accomplished by horizontally zone melting said semiconductive material.

10. The method of claim 9 wherein said semiconductive material is silicon and said temperature gradient is between 200 and 400 C. per centimeter.

11. The method of claim 9 wherein said semiconductive material is germanium and said temperature gradient is between 150 and 250 C. per centimeter.

Physical Review (2nd series), vol. 33, 1929, pages 81 to 89. Published by the American Physical Society, Minneapolis, Minn.

Schmid et al.: Plasticity of Crystals, page 31, January 1950.

Claims (1)

1. THE METHOD OF GROWING A SINGLE CRYSTAL OF A SEMICONDUCTIVE MATERIAL COMPRISING THE STEPS OF: MELTING AT LEAST A PORTION OF SAID MATERIAL, CONTACTING A SEED CRYSTAL OF SAID MATERIAL TO SAID MOLTEN PORTION, RELATIVELY MOVING SAID SEED CRYSTAL WITH RESPECT TO SAID MOLTEN PORTION WHEREBY SUCCESSIVE PORTIONS OF SAID MOLTEN PORTION ARE FROZEN-OUT ON SAID SEED CRYSTAL, ESTABLISHING AND MAINTAINING SAID FROZEN-OUT PORTION A TEMPERATURE GRADIENT OF BETWEEN 150 AND 400*C. PER CM. ADJACENT THE INTERFACE BETWEEN SAID FROZEN SOLID PORTIONS AND SAID MELT, AND MAINTAINING THE TEMPERATURE ACROSS SAID FROZEN SOLID PORTIONS RADIALLY UNIFORM WHEREBY SAID INTERFACE IS PLANAR IN A DIRECTION NORMAL TO THE DIRECTION OF CRYSTAL GROWTH.

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