Sublimation growth of silicon carbide single crystals

Abstract: The present invention relates to a method of forming large device quality single crystals of silicon carbide (33). The sublimation process is enhanced by maintaining a constant polytype composition in the source materials (40), selected size distribution in the source materials (40), by specific preparation of the growth surface of seed crystals (32), and by controlling the thermal gradient between the source materials (40) and the seed crystal (32).

Description: The present invention is a method for controlling the sublimation growth of silicon carbide to produce high quality single crystals. Silicon carbide is a perennial candidate for use as a semiconductor material. Silicon carbide has a wide bandgap (2.2 electron volts in the beta polytype, 2.8 in the 6H alpha), a high thermal co-efficient, a low dielectric constant, and is stable at temperatures far higher than those at which other semiconductor materials such as silicon remain stable. These characteristics give silicon carbide excellent semiconducting properties, and electronic devices made from silicon carbide can be expected to perform at higher temperatures, and at higher radiation densities, than devices made from the presently most commonly used semiconductor materials such as silicon. Silicon carbide also has a high saturated electron drift velocity which raises the potential for devices which will perform at high speeds, at high power levels, and its high thermal conductivity permits high density device integration. As is known to those familiar with solid state physics and the behavior of semiconductors, in order to be useful as a material from which useful electrical devices can be manufactured, the basic semiconductor material must have certain characteristics. In many applications, a single crystal is required, with very low levels of defects in the crystal lattice, along with very low levels of unwanted impurities. Even in a pure material, a defective lattice structure can prevent the material from being useful for electrical devices, and the impurities in any such crystal are preferably carefully controlled to give the desired electrical characteristics. If the impurities cannot be controlled, the material is generally unsatisfactory for use in electrical devices. Accordingly, the availability of an appropriate crystal sample of silicon carbide is a fundamental requirement for the successful manufacture of devices from silicon carbide which would have the desirable properties described above. Such a sample should be of a single desired crystal polytype (silicon carbide can form in at least 150 types of crystal lattices), must be of a sufficiently regular crystal structure of the desired polytype, and must be either substantially free of impurities, or must contain only those impurities selectively added to give the silicon carbide any desired n or p character. Accordingly, and because the physical characteristics and potential uses for such silicon carbide have been recognized for some time, a number of researchers have suggested a number of techniques for forming crystalline silicon carbide. These techniques generally fall into two broad categories, although it will be understood that some techniques are not necessarily so easily classified. The first technique is known as chemical vapour deposition ("CVD") in which reactant gases are introduced into some sort of system within which they form silicon carbide crystals upon an appropriate substrate. Novel and commercially significant improvements in such CVD techniques are discussed in currently co-pending United States applications which are assigned to us, "Growth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Serial No. 113,921, filed October 26, 1987; and "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Serial No. 113,573, filed October 26, 1987. The other main technique for growing silicon carbide crystals is generally referred to as the sublimation technique. As the designation sublimation implies and describes, sublimation techniques generally use some type of solid silicon carbide material, other than a desired single crystal of a particular polytype, as a starting material, and then heat the starting material until solid silicon carbide sublimes. The vaporized material is then encouraged to condense, with the condensation intended to produce the desired crystals. As is known to those familiar with the physical chemistry of solids, liquids and gases, crystal growth is encouraged when the seed or surface upon which a crystal is being formed is at a somewhat lower temperature than the fluid, either gas or liquid, which carries the molecules or atoms to be condensed. One technique for producing solid silicon carbide when crystal-type impurity is of little consideration is the Acheson furnace process, which is typically used to produce silicon carbide for abrasive purposes. One of the first sublimation techniques of any practical usefulness for producing better crystals, however, was developed in the 1950's by J. A. Lely, one technique of whom is described in U.S. Patent No. 2,854,364. From a general standpoint, Lely's technique lines the interior of a carbon vessel with a silicon carbide source material. By heating the vessel to temperatures at which silicon carbide sublimes, and then allowing it to condense, recrystallized silicon carbide is encouraged to redeposit itself along the lining of the vessel. Although the Lely process can generally improve upon the quality of the source material, it has to date failed to produce on a consistent or repeatable basis, single crystals of silicon carbide suitable for electrical devices. Hergenrother, U.S. Patent No.3,228,756, discusses another sublimation growth technique which utilizes a seed crystal of silicon carbide upon which other silicon carbide can condense to form the crystal growth. Hergenrother suggests that in order to promote proper growth, the seed crystal must be heated to an appropriate temperature, generally over 2000° centigrade, in such a manner that the time period during which the seed crystal is at temperatures between 1800°C and 2000°C is minimized. Ozarow, U.S. Patent No.3,236,780,discusses another unseeded sublimation technique which utilizes a lining of silicon carbide within a carbon vessel, and which attempts to establish a radial temperature gradient between the silicon carbide-lined inner portion of the vessel and the outer portion of the vessel. Knippenberg, U.S. Patents Nos. 3,615,930 and 3,962,406, discuss alternative attempts at growing silicon carbide in a desired fashion. The '930 patent discusses a method of growing p-n junctions in silicon carbide as a crystal grows by sublimation. According to the discussion in this patent, silicon carbide is heated in an enclosed space in the presence of an inert gas containing a donor-type dopant atom, following which the dopant material is evacuated from the vessel and the vessel is reheated in the presence of an acceptor dopant. This technique is intended to result in adjacent crystal portions having opposite conductivity types and forming a p-n junction. In the '406 patent, Knippenberg discusses a three-step process for forming silicon carbide in which a silicon dioxide core is packed entirely within a surrounding mass of either granular silicon carbide or materials which will form silicon carbide when heated. The system is heated to a temperature at which a silicon carbide shell forms around the silicon dioxide core, and then further heated to vaporize the silicon dioxide from within the silicon carbide shell. Finally, the system is heated even further to encourage additional silicon carbide to continue to grow within the silicon carbide shell. Vodadkof, U.S. Patent No. 4,147,572, discusses a geometry-oriented sublimation technique in which solid silicon carbide source material and seed crystals are arranged in parallel close proximity relationship to one another. Addamiano, U.S. Patent No. 4,556,436, discusses a Lely-type furnace system for forming thin films of beta silicon carbide on alpha silicon carbide which is characterized by a rapid cooling from sublimation temperatures of between 2300° centigrade and 2700° centigrade to another temperature of less than 1800° centigrade. Addamiano notes that large single crystals of cubic (beta) silicon carbide are simply not available and that growth of silicon carbide on other materials such as silicon or diamond is rather difficult. Hsu, U.S. Patent No. 4,664,944, discusses a fluidized bed technique for forming silicon carbide crystals which resembles a chemical vapor deposition technique in its use of non-silicon carbide reactants, but which includes silicon carbide particles in the fluidized bed, thus somewhat resembling a sublimation technique. Some of the more important work in the silicon carbide sublimation techniques, however, is described in materials other than United States patents. For example, German (Federal Republic) Patent No. 3,230,727 to Siemens Corporation discusses a silicon carbide sublimation technique in which the emphasis of the discussion is the minimization of the thermal gradient between silicon carbide seed crystal and silicon carbide source material. This patent suggests limiting the thermal gradient to no more than 20° centigrade per centimeter of distance between source and seed in the reaction vessel. This patent also suggests that the overall vapor pressure in the sublimation system be kept in the range of between 1 and 5 millibar and preferably around 1.5 to 2.5 millibar. This German technique, however, can be considered to be a refinement of techniques thoroughly studied in the Soviet Union, particularly by Y.M. Tairov; see e.g. General Principles of Growing Large-Size Single Crystals of Various Silicon Carbide Polytypes , J. Crystal Growth, 52 (1981) 146-150, and Progress in Controlling the Growth of Polytypic Crystals , from Crystal Growth and Characterization of Polytype Structures , edited by P. Krishna, Pergammon Press, London, 1983, p. 111. Tairov points out the disadvantages of the Lely method, particularly the high temperatures required for crystal growth (2600°-2700°C) and the lack of control over the resulting crystal polytype. As discussed with reference to some of the other investigators in patent literature, Tairov suggests use of a seed as a method of improving the Lely process. In particular, Tairov suggests controlling the polytype growth of the silicon carbide crystal by selecting seed crystals of the desired polytype or by growing the recondensed crystals on silicon carbide faces worked at an angle to the 0001 face of the hexagonal lattice. Tairov suggests axial temperature gradients for growth of between approximately 30° and 40° centigrade per centimeter. In other studies, Tairov investigated the effects of adjusting various parameters on the resulting growth of silicon carbide, while noting that particular conclusions are difficult to draw. Tairov studied the process temperatures and concluded that growth process temperature was of relatively smaller importance than had been considered by investigators such as Knippenberg. Tairov likewise was unable to draw a conclusion as to the effect of growth rate on the formation of particular polytypic crystals, concluding only that an increase in crystal growth rate statistically corresponds to an increase in the percentage of disordered structured crystals. Tairov was similarly unable to draw any conclusions between vapor phase stoichiometry and crystal growth, but pointed out that certain impurities will favor the growth of particular silicon carbide polytype crystals. For example, high nitrogen concentrations favor cubic polytype silicon carbide crystals, aluminum and some other materials favor the growth of hexagonal 4H polytype, and oxygen contributes to the 2H polytype. Tairov concluded that no understanding of the mechanisms leading to these effects had yet been demonstrated. In Tairov's experiments, he also attempted using silicon carbide single crystals of particular polytypes as the vapor source material and suggested that using such single crystals of particular polytypes as vapor sources could result in particular polytypes of crystal growth. Of course, it will be understood that although the use of single crystals as source materials is theoretically interesting, a more practical goal, particularly from a commercial standpoint, is the production of single crystals from more common sources of silicon carbide other than single crystals. Finally, Tairov concluded that the treatment of the substrate surface upon which sublimation growth was directed could affect the growth of the resulting crystals. Nevertheless, the wide variety of resulting data led Tairov to conclude that additional unidentified factors were affecting the growth he observed in silicon carbide crystals, and these unknown factors prevented him from reaching a fundamental understanding of the mechanisms of crystal growth. Therefore, in spite of the long recognized characteristics of silicon carbide, and the recognition that silicon carbide could provide an outstanding, if not revolutionary, semiconductor material and resulting devices, and in spite of the thorough investigations carried out by a number of researchers including those mentioned herein, prior to the present invention there existed no suitable technique for repeatedly and consistently growing large single crystals of desired selected polytypes of silicon carbide. EP-A-389533, from which this application is divided, describes a method of reproducibly controlling the growth of single crystals of a desired polytype of silicon carbide, by selecting the polytype, surface area or particle size distribution of an SiC powder source. The present invention consists in a method of reproducibly controlling the growth of single crystals, of a single polytype of silicon carbide, which crystals are suitable for use in producing electrical devices, the method comprising introducing a monocrystalline seed crystal of silicon carbide of a desired polytype into a sublimation system, characterised by raising the temperature of the sublimation system and heating the growth surface of the seed crystal to a temperature of at least 2150°C just below the temperature at which silicon carbide will sublime under the gas pressure conditions of the sublimation system; introducing silicon-containing and carbon-containing source gases into the sublimation system at a temperature at which they will immediately react to form Si, Si 2 C and SiC 2 in the vapour phase and independent of the presence of impurities which would otherwise control polytype growth; and generating and maintaining a substantially constant flow of vaporized Si, Si 2 C and SiC 2 per unit area per unit time to the growth surface of the seed crystal for a time sufficient to produce a desired amount of macroscopic growth of monocrystalline silicon carbide of a desired polytype upon the seed crystal. Processes in accordance with the invention at least in its preferred embodiments, can be used to achieve controlled, repeatable growth of large single crystals of silicon carbide of desired polytypes. Various embodiments of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a cross-sectional diagram of a sublimation crucible used in accordance with the method of the present invention; Figure 2 is an enlarged view of the seed crystal holder of the crucible of Figure 1; Figure 3 is a cross-sectional diagram of a sublimation furnace used in accordance with the method of the present invention; and Figure 4 is a diagram of a sublimation system showing a gas feed mechanism for introducing silicon carbide precursor materials into the sublimation system. Figure 1 illustrates a cross-sectional view of a sublimation crucible used in accordance with the method of the present invention. The crucible is broadly designated at 10 and is typically formed of graphite. Crucible 10 is generally cylindrical in shape and includes a porous graphite liner 11, a lid 12, and a seed holder 13, an enlarged view of which is illustrated in Figure 2. The remainder of the crucible is defined by the walls 14 and the floor 15. As further illustrated in Figure 1, the porous graphite liner 11 is formed in such a manner as to provide an annular chamber 16 between lower portions of the porous graphite liner 11, the crucible walls 14 and the crucible lid 12. A central sublimation chamber is illustrated at 20. In all of the apparatus described herein, the crucibles described are preferably formed of graphite and most preferably of a graphite which has approximately the same coefficient of thermal expansion as silicon carbide. Such materials are commercially available. The relative similarities of thermal coefficients of expansion are a particular requirement for materials which are being heated to the extremely high temperatures described herein and at which these processes take place. In this manner, the crucible can be prevented from cracking during the sublimation process and the lifetime of the crucible will generally be increased. Furthermore, as is recognized by those familiar with attempts at growing silicon carbide crystals, the presence of graphite in the system encourages the growth of silicon carbide by providing an equilibrium source of carbon atoms as the sublimation process takes place and by dampening variations in the flux. Furthermore, graphite is one of the few economically viable materials which can both withstand the high temperatures of these processes and avoid introducing undesired impurities into the vapor flux. The seed holder 13 is illustrated in more detail in Figure 2. A seed crystal 17 rests on upper portions of the seed holder 13 which extend into the chamber 20. A graphite washer 21 is positioned between the lower portions of the seed holder 13 and the floor of the crucible 15. Figure 2 also shows an optical opening 22, which in preferred embodiments of the invention provides optical access to the seed so that the temperature of the seed can be monitored with an optical pyrometer. A sublimation crucible such as illustrated in Figure 1 is typically used in conjunction with a sublimation furnace broadly designated at 23 in Figure 3, in which the crucible is again designated 10. Furnace 23 is generally cylindrical in shape and includes a cylindrical heating element 24, opposite portions of which are shown in the drawing. Furnace 23 is also surrounded by carbon fiber insulation 25 and includes optical ports 26, 27, and 28 through which optical pyrometers can measure the temperature of portions of the interior of the furnace. A power feed-through is generally designated at 30, and the outer housing of the furnace at 31. Although applicant does not wish to be bound by any particular theory, it is known that different polytypes of silicon carbide have different evaporation activation energies. Specifically, for cubic (3C) silicon carbide the evaporation activation energy is 452 kilojoules (kJ) per mole (108 kilocalories (kcal) per mole); for hexagonal 4H silicon carbide, 602 kJ/mole (144 kcal/mole); and for hexagonal 6H silicon carbide, 498 kJ/mole (119 kcal/mole). These differences are important, because when silicon carbide sublimes, it forms three basic vaporized materials: Si, Si 2 C, and SiC 2 . Depending upon the polytype of the source powder, the amount or "flux" of each of the species which is generated will differ. In a corresponding manner, the amount of each of the species in the overall vapor flux will tend to influence the type of polytypes which will grow when the species recondense. As used herein, the term "flux" refers to the amount of matter or energy passing through a designated plane of a given area during a given period of time. Accordingly, when used to describe the flow of vaporized species, flux can be measured and designated in units of matter, area and time such as grams per square centimeter per second (g/cm 2 /sec). It will be understood by those familiar with phase changes that sublimation and condensation are equilibrium processes, and are affected by the vapor pressure of a system as well as absolute and relative temperatures. Accordingly, it will be further understood that in the processes and systems described herein, the vapor pressures are suitably controlled in a manner which permits these processes to proceed and be controlled and adjusted based upon the temperature and thermal gradient considerations described herein. > During the sublimation growth process, gas species which contain silicon carbide evolve near the hotter top of the crucible and are transported via the thermal gradient to the seed at its respective lower temperature in the cooler lower portion of the crucible. The source material, however, is also in the thermal gradient and sublimation of the source material tends to occur at a much faster rate in the upper portion of the source material than in the lower portion. As a result if the temperature gradient remains constant, a rapid decrease in flux with time occurs as the upper source material is depleted. In a similar manner, as the crystal grows, its growth surface increases in temperature as a result of its change in position with respect to the thermal gradient. This causes a decrease in the sticking coefficient as a function of time and likewise reduces the growth rate. In another embodiment of the invention, it has been discovered that growth of the single crystal of silicon carbide can be enhanced using the methods of the present invention by providing a silicon carbide seed crystal which presents a sublimation surface which is slightly off-axis with respect to one of the Miller index faces. In effect, off-axis silicon carbide crystals tend to transfer three dimensional crystalographic information to the condensing atoms during sublimation. Accordingly, such an off axis growth surface can be used to encourage the repeatable growth of a desired specific silicon carbide polytype. This technique is particularly important when a silicon carbide crystal is being doped with an impurity during sublimation growth. As is known to those familiar with the properties of silicon carbide, particular impurities tend to encourage the growth of specific polytypes of silicon carbide. For example, doping with aluminum is known to favor growth of 4H silicon carbide, but 6H crystals of silicon carbide can be grown with aluminum doping according to the present invention if an off-axis seed is used. It has further been discovered according to the present invention that the thermal gradient control and indeed the entire process of controlling and maintaining temperatures can be enhanced by using resistance heating, rather than radio frequency (RF) induction heating, in the method of the present invention. Resistance heating offers a number of advantages in the overall sublimation process. First, resistance heating allows the process to be scaled up to larger crystal diameters than can be handled using induction heating. Induction heating techniques have several limitations which prevent any silicon carbide sublimation processes developed using induction techniques from being similarly scaled up to useful commercial scales. For example, in induction heating, the induction coil must be positioned outside of the vacuum vessel in which the sublimation takes place in order to prevent ionization of the gas (e.g. argon) present in the vessel. Secondly, if the diameter of the sublimation crucibles are increased, the coils used in induction heating tend to heat only the outside layer of the crucible resulting in an undesirable and unacceptable radial thermal gradient. Finally, induction heating requires the use of a glass vacuum vessel to transmit the RF power. As a result, in order to prevent the glass vessel from overheating, either the thermal insulation present must be increased in thickness or the glass must be cooled, typically with water. Increasing the amount of thermal insulation reduces the practical size of the crystal that can be grown, and cooling the vessel with water dramatically reduces the energy efficiency of the entire system. Alternatively, resistance heating is significantly more energy efficient than induction heating, resistance heating elements can be present within the vacuum vessel, skin heating or radial thermal gradient effects are almost entirely eliminated, and resistance heating permits improved temperature stability and repeatability of processes and control over the entire thermal gradient. Figure 4 illustrates some of the apparatus which can be used to accomplish the methods of the present invention. Figure 4 shows a silicon carbide seed crystal 32 upon which a growing crystal 33 has epitaxially attached. The respective crystals 32 and 33 are maintained upon a graphite seed holder 34 which in turn is positioned upon a shaft 35. The remainder of the crucible is defined by graphite walls 36 and a porous graphite barrier 37. An optical sight hole 45 is also illustrated, and can be used to either monitor the temperature of the growing crystal 33 using an optical pyrometer or to determine the exact position of the crystal with respect to the silicon carbide source powder 40 at the top of the high density graphite cylinder 44. In certain embodiments of the invention, the shaft can be pulled in a manner which moves the growth face of the growing crystal 33 away from, or if desired towards, the silicon carbide source. In yet another embodiment of the invention, the shaft can be rotated to ensure that the temperature profile across the growth face is constant. In such a manner, the crystal can be encouraged to grow symmetrically as the effect of flux variations are dampened out and the growing crystal can be prevented from attaching itself to the graphite enclosure. According to the present invention, the silicon carbide which recondenses as the growing crystal, is not supplied as a powder, but instead is introduced into the system by providing respective gas feeds of silane (SiH 4 ) and ethylene (C 2 H 4 ) into the system at a temperature at which they will immediately react to form silicon carbide vapors which will then migrate in the manner in which vapors generated from source powders will migrate through the porous graphite barrier and onto the growing crystal. Instead of a bed of silicon carbide source powder, therefore, the system includes a silane gas feed 52 and an ethylene gas feed 53. In order to keep these molecules from dissociating under the high temperatures of the system, they are insulated in a water cooled molybdenum jacket until they reach a point in the sublimation system where the temperature is maintained at approximately 2400°C and at which the materials are released and immediately react to form silicon carbide. Once the silane and ethylene have left the jacket 54 and have reacted to form silicon carbide containing species, they behave in the same manner as would silicon carbide containing species which had sublimed from a source powder. They pass through the porous graphite barrier 37 and lodge upon the growth face of the growing crystal 33. The use of such a gas feed system for sublimation purposes offers several advantages, the primary one being the delivery of a constant flux of SiC vapor to the growing crystal surface. Another advantage is the high purity in which silane and ethylene can be obtained in commercial quantities so that a resultingly pure crystal results from this technique.

Claims: A method of reproducibly controlling the growth of single crystals, of a single polytype of silicon carbide, which crystals are suitable for use in producing electrical devices, the method comprising: introducing a monocrystalline seed crystal (32) of silicon carbide of a desired polytype into a sublimation system; characterised by : raising the temperature of the sublimation system and heating the growth surface of the seed crystal to a temperature of at least 2150°C and just below the temperature at which silicon carbide will sublime under the gas pressure conditions of the sublimation system; introducing silicon-containing and carbon-containing source gases into the sublimation system at a temperature at which they will immediately react to form Si, Si 2 C and SiC 2 in the vapour phase and independent of the presence of impurities which would otherwise control polytype growth; and generating and maintaining a substantially constant flow of vaporized Si, Si 2 C and SiC 2 per unit area per unit time to the growth surface of the seed crystal for a time sufficient to produce a desired amount of macroscopic growth of monocrystalline silicon carbide of a desired polytype upon the seed crystal. A method according to claim 1 wherein the step of introducing source gases comprises introducing silane and ethylene gases. A method according to claim 1 or 2 wherein the step of elevating the temperature of the sublimation system and the growth surface of the seed crystal comprises elevating the temperature to between 2150° and 2250° centigrade. A method according to any preceding claim wherein the single seed crystal of silicon carbide presents a sublimation surface which is slightly off axis with respect to one of the Miller index faces.

Inventors: CARTER CALVIN H JR, DAVIS ROBERT F, HUNTER CHARLES ERIC, CARTER, CALVIN H., JR., DAVIS, ROBERT F., HUNTER, CHARLES ERIC
Applicants: UNIV NORTH CAROLINA, NORTH CAROLINA STATE UNIVERSITY
Application year: 1988
Main classification: H01L21
IPC codes: C30B23