Method for producing silicon carbide single-crystal ingot and silicon carbide single-crystal ingot

Abstract

The present invention provides a method of raising the rate of reduction of the dislocation density accompanying growth of an SiC single crystal to counter the increase in the threading screw dislocations formed near the interface of the seed crystal and grown SiC single crystal and thereby produce an SiC single-crystal ingot with a small threading screw dislocation density from the initial stage of growth. The present invention is a method for producing a silicon carbide single-crystal ingot growing a silicon carbide single crystal on a growth face of a seed crystal consisting of a silicon carbide single crystal by a physical vapor transport method so as to produce a silicon carbide single-crystal ingot, the method for producing a silicon carbide single-crystal ingot comprising forming step bunching with heights of steps of 10 ?m to 1 mm and spans of terraces of 200 ?m to 1 mm on the growth face of the seed crystal and making the silicon carbide single crystal grow on the growth face of the seed crystal by the physical vapor transport method.

Claims

1. A method for producing a silicon carbide single-crystal ingot growing a silicon carbide single crystal on a growth face of a seed crystal consisting of a silicon carbide single crystal by a physical vapor transport method so as to produce a silicon carbide single-crystal ingot, the method for producing a silicon carbide single-crystal ingot comprising forming step bunching with heights of steps of 10 ?m to 1 mm and spans of terraces of 200 ?m to 1 mm on the growth face of the seed crystal and making the silicon carbide single crystal grow on the growth face of the seed crystal by the physical vapor transport method.

2. The method for producing a silicon carbide single-crystal ingot according to claim 1 further comprising forming the step bunching by growing a thickness 0.1 mm to 3 mm silicon carbide single crystal on a growth face of a seed crystal having an off-angle by a solution growth method.

3. A silicon carbide single crystal ingot comprising a seed crystal of a silicon carbide single crystal having a growth face and a silicon carbide single-crystal region formed on the growth face, step bunching with heights of steps of 10 ?m to 1 mm and spans of terraces of 200 ?m to 1 mm being formed at the growth face, wherein a threading screw dislocation density is 500/cm.sup.2 or less in a crystal region of 60% or more of the height direction of the ingot in the silicon carbide single-crystal region.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional schematic view showing an apparatus for the fabrication of a seed crystal used for the production of an SiC single-crystal ingot of the present invention.

(2) FIG. 2 is a cross-sectional schematic view showing a single-crystal production apparatus for the production of an SiC single-crystal ingot of the present invention.

(3) FIGS. 3A and 3B are schematic explanatory views showing the state of cutting out an SiC single crystal wafer to be used for evaluating the threading dislocation densities near the interface of the seed crystal of an SiC single-crystal ingot obtained in an example and comparative example.

(4) FIG. 4 is a graph showing the state of change of the threading screw dislocation density with respect to the growth height of an SiC single-crystal ingot obtained in an example and comparative example.

(5) FIG. 5 is a graph showing the state of change of the threading edge dislocation density with respect to the growth height in an SiC single-crystal ingot obtained in an example and comparative example.

(6) FIG. 6 is an explanatory view schematically showing the state where step bunching is caused on the growth face of the seed crystal.

DESCRIPTION OF EMBODIMENTS

(7) Below, the present invention will be explained in detail. The present invention suppresses the increase in the density of threading screw dislocations formed at the interface part of a seed crystal and grown SiC single crystal when using the physical vapor transport method to grow an SiC single crystal on a seed crystal. That is, in the present invention, the ratio of reduction of the dislocation density is raised along with the growth of the SiC single crystal by the physical vapor transport method so as to obtain an SiC single-crystal ingot with a small dislocation density from the stage of the initial stage of growth.

(8) Usually, the seed crystal face is polished and step bunching is removed. The heights of the steps remaining at the seed crystal face after polishing are 3 ?m or less while the spans of the terraces are 100 ?m or less. In general, an SiC single crystal was grown on the surface of the seed crystal from which the step bunching was removed in this way. However, as explained first, the present inventors etc. discovered that by growing an SiC single crystal on the crystal growth face of a seed crystal using a seed crystal formed with step bunching on its growth surface in the crystal growth environment of an SiC single crystal by the physical vapor transport method, the threading screw dislocation density is reduced.

(9) Normally, if step bunching is formed at the growth face of a seed crystal, the crystal growth nuclei formed at the terraces of the lower side of the steps of the step bunching will no longer match at the step ends and threading screw dislocations will more easily be formed than when using a general seed crystal not having step bunching. Further, as the step bunching becomes larger, the density of the threading screw dislocations will also become larger. In this regard, however, the greater number of threading screw dislocations formed will be smaller in distances between the dislocations than usual due to their high density and the probability of pair annihilation between positive and negative dislocations will rise. Further, even if the dislocation density falls to a certain extent, dislocations will remain at close distances whereby pair annihilation will easily occur, so, it is believed, the ratio of reduction of the threading screw dislocation density becomes larger along with the growth of the SiC single crystal (below, the ratio of reduction of the threading screw dislocation density along with the growth of the SiC single crystal will be referred to as the rate of reduction of the dislocation density). In this way, the present inventors etc. came up with the reverse idea of not suppressing the increase in dislocations at the interface part of the seed crystal and the grown crystal, but rather causing greater formation of dislocations and thereby causing pair annihilation and thereby obtained an SiC single-crystal ingot with a small threading screw dislocation density.

(10) Here, the growth face of the seed crystal used in the present invention is formed with at least one step bunch having a size of heights of steps of 10 ?m to 1 mm and spans of the terrace at the bottom side of the steps of 200 ?m to 1 mm. When performing the physical vapor transport method, the surfacemost layer of the growth face of the seed crystal starts thermal decomposition in the process of the rise of temperature of the seed crystal, but if the heights of the steps are 10 ?m or more and the spans of the terraces are 200 ?m or more, step bunching will not disappear until the start of the sublimation and recrystallization of the SiC.

(11) However, if either of the heights of the steps and spans of the terraces of the step bunching exceed 1 mm, normal sublimation-recrystallization growth is inhibited and the polytype becomes unstable. For this reason, the number of step bunches where either of the heights of the steps and spans of the terraces exceeds 1 mm is preferably smaller.

(12) Further, if the heights of the steps of the step bunching are less than 10 ?m or the spans of the terraces at the bottom sides of the steps are less than 200 ?m, the effect of the present invention of the rate of reduction of the threading screw dislocation density will not become greater.

(13) FIG. 6 is an explanatory view of step bunching. It schematically shows the state where SB1, SB2, and other step bunches are formed on the growth face of a seed crystal. The formed step bunches SB1 and SB2 most preferably have step heights SBt1 and SBt2 of both 10 ?m to 1 mm and terrace spans SB1wa, SB1wb, SB2wa, and SB2wb of all 200 ?m to 1 mm.

(14) To make the rate of reduction of the threading screw dislocation density at the stage of the initial stage of growth further larger, preferably the heights of the steps of the step bunching are 30 ?m or more, more preferably 50 ?m or more. Further, the spans of the terraces at the bottom sides of the steps are preferably 400 ?m or more, more preferably 600 ?m or more.

(15) Further, as the means for forming the step bunching having such sizes on the seed crystal, preferably the solution growth method is used. In the growth of the SiC single crystal by the solution growth method, the crystal growth on the seed crystal having the off-angle becomes relatively unstable compared with crystal growth on the just face with zero off-angle, so if growing the crystal on a seed crystal having an off-angle, it is known that step bunching of a growth height of tens of ?m to 100 ?m or so will occur. By utilizing this phenomenon, that is, growing the SiC single crystal on the growth face of the seed crystal having the off-angle by the solution growth method before growth of the SiC single crystal by the physical vapor transport method, it is possible to prepare a seed crystal on which the step bunching having the above-mentioned size is formed.

(16) Here, it is possible to increase the thickness of the seed crystal to enlarge the size of step bunching on the growth face of the seed crystal. From the viewpoint of forming the desired size of step bunching, the thickness of the seed crystal for growing the SiC single crystal by the solution growth method is preferably 0.1 mm or more. However, if considering the saturation of the effect, economy, etc., the upper limit of the thickness is 3 mm. On the other hand, regarding the off-angle of the seed crystal, from the viewpoint of efficiently forming step bunching on the crystal surface etc., the off-angle is preferably 0.5 degree to 10 degrees.

(17) Further, regarding the solution growth method, it is possible to use a known method. By immersing a seed crystal having an off-angle in an Si medium in which C is dissolved (SiC solution) and forming an SiC single crystal, it is possible to form large step bunching as explained above. By arranging a seed crystal on which the step bunching having the above-mentioned size is formed in a single-crystal growth apparatus utilizing the physical vapor transport method and producing an SiC single crystal by a known method, it is possible to obtain the SiC single-crystal ingot of the present invention. Note that, the physical vapor transport method differs from the solution growth method in that no phenomenon of the off-angle of the seed crystal remarkably causing a change in the size of the step bunching can be seen, so it is difficult to use the physical vapor transport method to form large step bunching like with the solution growth method.

(18) According to the present invention, it is possible to obtain an SiC single crystal with a small threading screw dislocation density from the initial stage of growth by the physical vapor transport method. Other than using a seed crystal formed with step bunching of the above-mentioned size, for the physical vapor transport method, a known method may be used.

(19) According to the present invention, it is possible to obtain an SiC single-crystal ingot in which 60% or more of the ingot in the height direction is a high quality crystal region with a threading screw dislocation density of 500/cm.sup.2 or less in the initial stage of growth of the SiC single crystal, so it becomes possible to cut out an SiC single-crystal wafer suitable for devices even from the SiC single-crystal ingot in the initial stage of growth and to improve the yield in the production of SiC single-crystal wafers.

EXAMPLES

(20) Below, examples etc. will be used to explain the present invention more specifically. Note that the present invention is not limited to the content of the following examples.

Example 1

(21) FIG. 1 shows an apparatus for preparing a seed crystal used for crystal growth of an SiC single-crystal ingot according to an example of the present invention. This is one example of a single-crystal growth apparatus using the solution growth method. This single-crystal growth apparatus is provided with a graphite crucible 2 in which an SiC solution 1 is housed. This graphite crucible 2 is arranged inside a water-cooled stainless steel chamber 3. Further, the graphite crucible 2 is substantially sealed by a crucible lid 4 through which a seed shaft 10 passes. The outer circumference of the crucible 2 is insulated by a thermal insulation material 5. Further, a high frequency coil 6 for induction heating is provided at the outer circumference of that. The atmosphere inside the single-crystal growth apparatus is adjusted utilizing a gas introduction port 7 and a gas discharge port 8.

(22) The graphite crucible 2 was charged with Si and Ti as melt materials. Current was run through the high frequency coil 6 for induction heating to melt the materials inside the crucible and form a melt of SiTi alloy. Due to dissolution of the container, that is, graphite crucible 2, during heating, carbon melted into the high temperature solution whereby a high temperature SiC solution (SiC solution) was formed. Further, a diameter 51 mm wafer was cut out from the bulk SiC single-crystal obtained by the physical vapor transport method and polished to a mirror surface to prepare a seed crystal wafer 9 having an off-angle of 4 degrees at the (000-1) face.

(23) This seed crystal wafer 9 was dipped in an SiC solution 1 50? C. lower than the growth temperature, then was raised in temperature to a growth temperature of 1940? C. and heated for about 1 hour. After reaching the growth temperature, the solution was in an unsaturated state with carbon until carbon is sufficiently supplied to the melt by the dissolution of the crucible. For this reason, the surface layer of the seed crystal wafer 9 dissolves into the solution. In Example 1, the amount of the surface layer of the seed crystal wafer 9 dissolving (dissolved thickness) was about 30 ?m. After the carbon in the solution reached saturation, the SiC crystal grew in the solution on the seed crystal wafer 9. The crystal growth time was made 15 hours from immersion of the seed crystal wafer 9 in the solution. During that time, the graphite crucible 2 and the seed shaft 10 were made to rotate in opposite directions by 10 rpm. After growth finished, the seed shaft 10 was made to rise and the seed crystal wafer 9 was separated from the solution 1 to thereby recover the seed crystal.

(24) On the obtained seed crystal, the SiC crystal was newly grown to a thickness of about 500 to 600 ?m on the seed crystal wafer 9. The growth face of this seed crystal was observed by a laser microscope. As a result, formation of step bunching of step heights of about 30 ?m and terrace spans of about 300 ?m was observed.

(25) Further, FIG. 2 shows one example of an apparatus for producing an SiC single-crystal ingot according to an example of the present invention, that is, a single-crystal growth apparatus by the improved Rayleigh method. The crystal was grown by sublimating the SiC material 11 by induction heating and making it recrystallize on the SiC seed crystal 12. The seed crystal 12 on the growth face of which the step bunching was formed by the above solution growth method was attached to the inside surface of the graphite lid 13 of the single-crystal growth apparatus and set in the graphite crucible 14 filled with the SiC material 11. The graphite crucible 14 was covered by graphite felt 15 for shielding it against heat, then was placed on the graphite support rod 17 and set inside the double quartz tube 16.

(26) Further, the inside of the double quartz tube 16 was evacuated by an air discharge device 18, then atmospheric gas consisting of high purity Ar gas and nitrogen gas was introduced through the piping 19 while controlled by a mass flow controller 20 to make the pressure inside the quartz tube (growth atmosphere pressure) 80 kPa at the vacuum evacuation device 18. Under this pressure, current was run through the work coil 21 to raise the temperature. The temperature of the seed crystal 12 was made to rise until 2200? C. After this, the growth atmosphere pressure was reduced to 1.3 kPa over 30 minutes and the crystal was grown for 30 hours so that the (000-1) face of the seed crystal 12 on which the step bunching was formed to become the crystal growth face.

(27) After the above process, a height 9 mm, diameter 51 mm SiC single-crystal ingot was obtained. First, from this ingot, at a position of a growth height of 3 mm expressed by the height from the seed crystal 12, a wafer A for evaluation was cut out in parallel to the seed crystal face (that is, so as to have an off-angle of 4?).

(28) The obtained wafer A for evaluation was immersed for 5 minutes in 520? C. molten KOH so that the entire surface of the wafer was immersed to thereby etch the wafer by molten KOH. The surface of the etched wafer A was observed by an optical microscope (magnification: 80?) to measure the dislocation density. Here, according to the method described in J. Takahashi et al., Journal of Crystal Growth, 135, (1994), 61-70, the dislocations were classified by the etch pit shapes with shell shaped pits being deemed as basal plane dislocations, small sized hexagonal shaped pits being deemed as threading edge dislocations, and medium size and large size hexagonal shaped pits being deemed as threading screw dislocations, respectively, and then their respective dislocation densities were determined. As a result, it was confirmed that the dislocation densities were substantially uniformly distributed in the region of the wafer A minus the 5% ring-shaped region at the inside from the outer circumference by ratio of diameter (region of width 2.55 mm from outer circumference). Note that the same procedure was followed to examine the seed crystal as well, whereupon the dislocation densities were evenly distributed in the face.

(29) Next, as shown in FIG. 3A, the obtained ingot was cut so that the direction tilted by 4? in a direction opposite to the off direction of the c-axis of the seed crystal 12 became the c-axis of the crystal to obtain an wafer B having an off-angle in a direction opposite to the seed crystal at the (0001) face. At that time, the wafer B was cut out so as to pass through the center of the growth face of the seed crystal. The surface of the wafer B was made to include a seed crystal region and grown SiC single-crystal region. As shown in FIG. 3B, the angle formed by the a-side on the surface of the wafer B and the b-side on the diametrical direction of the seed crystal was 8? whereby an isosceles triangle with a height h was formed. By using this cutting method, it became possible to continuously observe changes in the dislocation density accompanying growth. Further, the change in the growth direction was enlarged to substantially 1/sin 8? (about 7?), so it was possible to measure the dislocation density with greater resolution and higher precision.

(30) The obtained wafer B, in the same way as the above wafer A, was immersed for 5 minutes in 520? C. molten KOH so that the entire surface of the wafer was immersed to thereby etch the wafer by molten KOH. While making the surface of the etched wafer follow the changes of the height (h) of the grown SiC single crystal, the measurement points on the a-side)(=h/sin 8? shown in FIG. 3B were observed by an optical microscope (magnification: 80?) to measure the threading dislocation densities. The results are shown in Table 1.

(31) TABLE-US-00001 TABLE 1 Example 1 Comparative Example 1 Threading Threading Threading Threading screw edge screw edge Growth dislocation dislocation dislocation dislocation height h density density density density (mm) (/cm.sup.2) (/cm.sup.2) (/cm.sup.2) (/cm.sup.2) ?0.2* 4.5 ? 10.sup.2 1.8 ? 10.sup.3 4.5 ? 10.sup.2 2.8 ? 10.sup.3 0.6 2.5 ? 10.sup.4 7.3 ? 10.sup.4 8.3 ? 10.sup.3 6.4 ? 10.sup.4 1.1 4.1 ? 10.sup.3 2.1 ? 10.sup.4 3.2 ? 10.sup.3 2.8 ? 10.sup.4 1.4 7.9 ? 10.sup.2 7.7 ? 10.sup.3 2.5 ? 10.sup.3 2.0 ? 10.sup.4 1.9 1.1 ? 10.sup.2 4.1 ? 10.sup.3 9.0 ? 10.sup.2 5.3 ? 10.sup.3 *In the table, ?0.2 mm shows the depth from the seed crystal face (interface of seed crystal region and grown SiC single-crystal region) to the inside of the seed crystal region.

Comparative Example 1

(32) In Comparative Example 1, without performing solution growth by an SiC solution at the crystal growth face of the seed crystal, a diameter 51 mm wafer was cut out from a bulk SiC single crystal obtained by the physical vapor transport method and polished to a mirror surface to prepare a seed crystal wafer having an off-angle of 4 degrees at the (0001) face. This was used as it is for crystal growth of the SiC single crystal by the physical vapor transport method. That is, other than not performing the solution growth step of the seed crystal, the same procedure was followed as in Example 1 to produce the SiC single-crystal ingot according to Comparative Example 1.

(33) At this time, no step height 10 ?m or more or terrace span 200 ?m or more step bunching was seen on the crystal growth face of the seed crystal wafer. The obtained SiC single-crystal ingot was cut in the same way as in Example 1 to give an off-angle in the opposite direction to the seed crystal at the (0001) face to obtain the wafer B. This was etched by KOH in the same way as Example 1 and measured for dislocation density by an optical microscope. The results are shown in Table 1.

(34) Here, the relationship between the growth height (h (mm)) and the value of threading screw dislocation density (/cm.sup.2) of the SiC single crystal at the different measurement points on the crystal growth faces of the seed crystal wafers of Example 1 and Comparative Example 1 is shown in FIG. 4. FIG. 5 shows the relationship between the growth height (h (mm)) and the value of threading edge dislocation density (/cm.sup.2) of the SiC single crystal for Example 1 and Comparative Example 1. Note that the ordinates of FIG. 4 and FIG. 5 show the log values.

(35) The solid lines and the broken lines drawn in FIG. 4 are respectively curves of primary functions approximating the date of Example 1 and the data of Comparative Example 1. As will be understood from these curves, the slope of the solid line drawn in FIG. 4 is steeper than the slope of the broken line. That is, the rate of reduction of the dislocation density in Example 1 is greater than the rate of reduction of the dislocation density in Comparative Example 1. Further, the SiC single-crystal ingot of Comparative Example 1 produced by the conventional method has a threading screw dislocation density of about 1000/cm.sup.2 at a growth height of 1.9 mm, while in the SiC single-crystal ingot of Example 1 produced using the method of production according to the present invention, this is reduced to about 100/cm.sup.2.

(36) Further, as will be understood from a comparison of the curve of the solid line in FIG. 5 (corresponding to data of Example 1) and the curve of the broken line (corresponding to data of Comparative Example 1), the method of production according to the present invention does not have any detrimental effect on changes in dislocation density of threading edge dislocations, but increases the rate of reduction of dislocation density of threading edge dislocations. In the initial period of growth near the interface with the seed crystal, in the conventional technologies, the threading dislocation density was high and the result was considered unsuitable for cutting out an SiC single-crystal wafer, while according to the present invention, it becomes possible to cut out an SiC single-crystal wafer having the same as a quality of the seed crystal or better than the quality of the seed crystal from a growth height of about 2 mm.

(37) Further, from the slope of the curve of FIG. 4, it is considered that at a growth height of 1.5 mm or more, a threading screw dislocation density of 500/cm.sup.2 or less is achieved. From the fact that the growth height of the ingot according to Example 1 is 9 mm, an SiC single-crystal ingot where a threading screw dislocation density of 500/cm.sup.2 or less is about 80% or more of the crystal region in the height direction was obtained.

REFERENCE SIGNS LIST

(38) 1. SiC solution

(39) 2. graphite crucible

(40) 3. stainless steel chamber

(41) 4. crucible lid

(42) 5. thermal insulation material

(43) 6. high frequency coil

(44) 7. gas introduction port

(45) 8. gas discharge port

(46) 9. seed crystal wafer

(47) 10. seed shaft

(48) 11. SiC material for sublimation

(49) 12. seed crystal

(50) 13. graphite lid

(51) 14. graphite crucible

(52) 15. graphite felt

(53) 16. double quartz tube

(54) 17. graphite support rod

(55) 18. vacuum evacuation device

(56) 19. piping

(57) 20. mass flow controller

(58) 21. work coil