JP4466293B2 - Method for producing silicon carbide single crystal - Google Patents

Description

  The present invention relates to a method for producing a high-quality bulk single crystal of silicon carbide particularly suitable as a material for optical devices and electronic devices, and more particularly to a method capable of stable production by a liquid phase growth method.

  Silicon carbide (SiC) is about 3 times larger than silicon (Si), with a band gap of about 3 times, a breakdown voltage of about 10 times, an electron saturation speed of about 2 times, and a thermal conductivity of about 3 times. It has advantageous features. SiC is a semiconductor material that is thermally and chemically stable. In recent years, SiC has been used for power devices that break the physical limitations of Si devices and environmental devices that operate at high temperatures. Application is expected.

  On the other hand, gallium nitride (GaN) -based materials have been developed for optical device research with the aim of shortening the wavelength, but SiC has a remarkably small lattice mismatch with GaN, so it is a substrate material for GaN epitaxial growth. Is also attracting attention.

In order to use the above device or substrate material, it is necessary to produce a large, high-quality SiC bulk single crystal.
However, SiC is also famous as a substance exhibiting a crystalline polymorph (polytype). The crystal polymorphism is a phenomenon that can take many crystal structures in which the stacking mode of atoms differs only in the C-axis direction while having the same stoichiometric composition. Typical polytypes are 6H type (hexagonal system with 6 molecules as one period), 4H type (hexagonal system with 4 molecules as one period), and 3C type (cubic crystals with 3 molecules as one period). System). Two or more types of crystal polymorphs may occur even at a certain temperature, but mixing of crystal polymorphs is not preferable from the viewpoint of use as a device or substrate material.

Conventionally, as a growth method for SiC single crystal growth, a vapor phase growth method, an atchison method, and a solution growth method are known.
Vapor deposition methods include sublimation and chemical vapor deposition (CVD). In the sublimation method, silicon carbide powder is used as a raw material, which is sublimated at a high temperature of 2000 ° C. or higher, and the vapor composed of Si and C is supersaturated on the seed crystal substrate set at a lower temperature than the raw material and recrystallized. It is a method that uses that. In the CVD method, a silane gas and a hydrocarbon-based gas are used as SiC production raw materials, and a SiC single crystal is epitaxially grown on a heated substrate such as Si by a chemical reaction.

The Atchison method is a method for industrial production of artificial abrasives by heating silicic anhydride and carbon to a high temperature of 2000 ° C. or more, and single crystals are produced as by-products.
The solution growth method usually uses a graphite crucible, in which Si or Si and a metal are melted, carbon is dissolved from the graphite crucible in the produced melt, and the melt is made into an SiC solution. In this method, SiC dissolved in the melt is crystallized and grown on a seed crystal substrate placed in a low temperature portion by providing a temperature gradient. In the vicinity of the substrate, the SiC solution is supercooled, and SiC is supersaturated, so that SiC crystals are deposited on the substrate. A metal is optionally added for the purpose of increasing the dissolved concentration of carbon.

Among the above methods, it is known that a single crystal grown by a sublimation method generates lattice defects such as hollow through defects and stacking faults called micropipe defects. In the sublimation method, no gas having a stoichiometric composition of SiC exists during sublimation, and vaporizes as Si, Si ₂ C, SiC ₂ and vaporized C from a graphite jig used for growth. A large number of lattice defects are generated in the sublimation method because it is very difficult to control the partial pressure of these gases stoichiometrically and complicated reactions are involved. Furthermore, the sublimation method has a drawback that crystal polymorphism tends to occur. Conventionally, many SiC bulk single crystals have been manufactured by a sublimation method, but it has been difficult to manufacture devices with a size of several mm square with high yield.

In the CVD method, since the raw material is supplied by gas, the raw material supply amount is small, and the generated SiC single crystal is limited to a thin film, and it is difficult to manufacture a bulk single crystal for manufacturing a substrate.
In the Atchison method, there are many impurities in the raw material, and high purity cannot be achieved, and a large single crystal cannot be obtained.

  On the other hand, since the solution growth method is crystal growth in a thermal equilibrium state, a single crystal having very good crystallinity can be obtained without generating crystal polymorphism with very few lattice defects. . However, since the dissolution concentration of carbon in the Si melt is low, the growth rate of the SiC crystal is very slow. When only Si is melted in the graphite crucible, the growth rate when the melt temperature is 1650 ° C. is said to be 5 to 12 μm / hr. This value is about two orders of magnitude smaller than the sublimation method. The solution growth method can be expected in principle to increase the concentration of carbon that can be dissolved in the melt by raising the temperature of the Si melt to 2000 ° C. or higher. However, under normal pressure, the evaporation of the Si melt is intense and practical. It ’s not right. Material Science Engineering B61-62 (1999) 29-39 shows that the carbon concentration in the solution can be increased while suppressing the evaporation of the Si melt by ultra-high pressure. There is a problem in production.

Therefore, it has been proposed to add a transition metal element to the melt in order to increase the growth rate of the SiC crystal.
For example, in Japanese Patent Laid-Open No. 2000-264790, a raw material containing at least one transition metal element and Si and C is heated and melted to form a melt, and the melt is cooled to obtain a silicon carbide single crystal. Precipitation growth is disclosed. Although the growth temperature differs depending on the additive element, it is explained that the average growth rate is 200 to 800 μm / hr at 1750 to 2150 ° C. Ar is exemplified as the atmospheric gas.

  Japanese Patent Laid-Open No. 2004-2173 discloses a method for producing a high-quality silicon carbide bulk crystal from a ternary melt containing Si, C, and M (M: one of Ti and Mn) by a solution growth method. Has been. In the embodiment, Ar is used as the atmospheric gas.

  However, in general, in the solution growth method including the methods described in these publications, bubbles are likely to be generated in the melt, and this may be taken into the crystal, and a silicon carbide single crystal is produced by liquid phase growth. It was an obstacle.

Material Science Engineering B61-62 (1999) 29-39 JP 2000-264790 A JP 2004-2173 A

  The present invention provides a high-quality bulk SiC single crystal that does not cause increase in crystal polymorphism and crystal defects and does not contain bubbles by using a solution growth method, particularly at an operationally advantageous temperature of 2000 ° C. or less. Provided is a method capable of stably producing at a reasonable growth rate.

The present invention is a solution growth method, that is, by immersing a silicon carbide seed crystal substrate in a melt containing SiC , including Si, C and Ti , and at least by cooling the solution around the seed crystal substrate. by a method for growing a silicon carbide single crystal silicon carbide in the) seed crystal substrate to supersaturated, the atmospheric gas at the time of single crystal growth, eta viscosity at single crystal growth temperature η ≦ 750μP (μP: A non-oxidizing gas that does not contain helium or a mixed gas mainly composed of helium and does not contain a gas serving as a supply source of C, and is made bubble-free by setting the crystal growth temperature to 1660 ° C. or higher in that atmosphere. This is a method for producing a silicon carbide single crystal characterized by growing silicon.

  The present inventors examined the atmospheric gas conditions and temperature conditions during crystal growth in order to reduce and eliminate bubble generation in the solution growth method. As a result, regardless of the composition of the melt species, there is a correlation between the viscosity of the atmosphere gas at the crystal growth temperature of the melt and the amount of bubbles generated, and the viscosity η of the atmosphere gas at the single crystal growth temperature is η ≦ It has been found that by using 750 μP, the generation of bubbles taken into the crystal in the solution growth method can be completely suppressed.

  “Solution supercooling at least around the seed crystal substrate” can be achieved by (1) cooling the melt or (2) providing a temperature gradient in the melt. Hereinafter, the method (1) is referred to as a “cooling method”, and the method (2) is referred to as a “temperature gradient method”. The supercooling temperature, that is, the growth temperature of the SiC single crystal is set so as not to be so low that SiC polycrystals are deposited in the melt.

  C in the melt may be supplied from a graphite crucible containing the melt, or may be added by a method such as dissolving a carbide as a raw material or blowing a gas containing carbon such as methane. is there.

  When the viscosity of the atmospheric gas during the growth of the SiC single crystal by the solution growth method is too high, bubbles generated in the melt tend to stay at the SiC crystal growth interface on the surface of the seed crystal substrate, and in the SiC single crystal as the crystal growth proceeds. As a result, the quality of the SiC single crystal is deteriorated. Therefore, the viscosity η of the atmospheric gas is η ≦ 750 μP.

The atmospheric gas is a non-oxidizing gas in order to prevent oxidation of the growing SiC silicon carbide, and can be composed of one or more gases. Examples of the non-oxidizing gas include inert gases such as He, Ne, and Ar, nitrogen, hydrogen, and the like .

  The viscosity η of the atmospheric gas varies depending on the combination of the gas type, gas pressure, and temperature. Therefore, the composition and pressure of the non-oxidizing gas are determined so that the viscosity η at the single crystal growth temperature is 750 μP or less. For example, since a gas having a large molecular weight such as Ar has a relatively high viscosity, it is difficult for Ar alone to satisfy the viscosity condition defined in the present invention.

  The lower the viscosity η of the atmospheric gas is, the more advantageous for suppressing bubbles, but in the case of hydrogen gas having the lowest gas viscosity, the viscosity is 250 μP (1500 ° C.) to 300 μP (2000 ° C.). However, considering the high melt temperature and the danger of explosion in hydrogen gas, a rare gas such as helium is easier to use industrially. As shown in the examples, even with helium or a mixed gas mainly composed of helium, the object of making the SiC single crystal bubble-free can be sufficiently achieved.

  The atmospheric gas pressure during the growth of the SiC single crystal by the solution growth method is preferably an atmospheric pressure or a pressurizing condition since the evaporation of the melt is intense if it is a negative pressure (reduced pressure). On the other hand, if the pressure is too high, the apparatus becomes large, which causes a problem in industrial production. Therefore, a more preferable atmospheric gas pressure is in the range of about 0.1 to 1 MPa.

  The melt is composed of Si and C, or Si and C and a transition metal M (M is one or more selected from Ti, Fe, Mn, and Co), and includes a melt containing Si or Si and M. This is a SiC solution in which SiC is dissolved in the liquid. If there is no adverse effect on the precipitation of the SiC single crystal, other elements can also be contained. Metal M can be added to increase the melting concentration of C. The melt composition desirably has a high carbon solubility in the melt and a range in which SiC is precipitated as primary crystals. For example, in a Si—Ti—C ternary melt in which M is Ti, the amount of Ti added is preferably up to about 25%. Even when the additive metal M is a metal other than Ti or a mixture of Ti and another metal, the metal addition amount may be determined so that the primary crystal becomes SiC. It is preferable that the melt contains the transition metal M because the crystal growth rate at a temperature of 2000 ° C. or less is increased.

  As described above, C can be contained in the melt by dissolving it in the melt from a graphite crucible. In this case, since there is no concern that undissolved carbon that serves as polycrystalline nuclei remains, a high-quality single crystal can be easily obtained, but it takes time to dissolve the carbon. On the other hand, if the melting raw material contains C or C is supplied by blowing methane gas, C can be dissolved in the melt in a shorter time. When gas blowing such as methane is used as a supply source of C, this gas may also constitute an atmospheric gas, but if the atmospheric gas is replaced with another non-oxidizing gas after the blowing is finished, the crystal The atmosphere gas at the time of growth can be made not to contain the gas used for supply of C. Therefore, the viscosity of the blowing gas for supplying C is not particularly limited.

  The supercooling temperature around the seed crystal substrate during the growth of the SiC single crystal by the solution growth method (in the present invention, this temperature is referred to as the single crystal growth temperature or the crystal growth temperature) is determined when the melt composition or the metal M is added. May be determined according to the metal element type. If it is too low, the melt will solidify or the growth rate will be slow. On the other hand, if it is too high, the evaporation of the melt becomes intense, which is problematic for industrial production. The crystal growth temperature is preferably not lower than the liquidus temperature in the melt composition to be used and not higher than 2000 ° C. In addition, this crystal growth temperature is the melt temperature after crystal growth starts to occur by cooling when the crystal growth is a cooling method.

  By controlling the viscosity of the atmospheric gas according to the present invention, it is possible to prevent bubbles from being trapped in the crystal, to make the crystal free of bubbles, and to stably produce a high-quality bulk SiC single crystal. It becomes.

FIG. 1 shows an example of a crystal growth apparatus that can be used to carry out the SiC single crystal manufacturing method of the present invention. The apparatus shown in FIG. 1 is suitable for crystal growth by a temperature gradient method.
The illustrated apparatus includes a rotatable graphite crucible 2 containing a raw material melt 1. In the melt 1, a seed crystal substrate 3 attached to a seed crystal holding jig capable of rotating in the direction opposite to the rotation direction of the graphite crucible 2 is immersed. The graphite crucible 2 is surrounded by a heat insulating material 4, and a heating coil 5 for high frequency induction heating is arranged around the heat insulating material. The whole of the above elements is accommodated in the water cooling chamber 6. The water-cooled chamber 6 includes a gas inlet 7 and a gas outlet 8, and can control the composition and pressure of the atmosphere gas for crystal growth. The side back of the graphite crucible 2 is directly measured by a pyrometer 9 such as a two-color pyrometer. A plurality of pyrometers 9 are installed on the top and bottom, and the melt temperature at different positions on the top and bottom can be measured.

  The high frequency induction heating by the heating coil 5 is controlled based on the measured value of the back surface temperature of the graphite crucible. Due to the relative positions of the graphite crucible and the heating coil, a temperature gradient is formed in the vertical direction of the graphite crucible, and the temperature gradient can be changed by controlling the coil winding interval and the number of turns. In order to obtain a large temperature gradient, it is effective to forcibly cool the low temperature portion of the graphite crucible with a water cooling jig. In general, it is advantageous to provide a temperature gradient so that the upper portion of the graphite crucible becomes a low temperature portion, whereby a single crystal can be grown by pulling up the seed crystal substrate.

  Examples of the present invention are shown below. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention. It goes without saying that various modifications are possible within the scope of the present invention (including the equivalent range).

In the following Reference Examples, Examples, and Comparative Examples, bulk SiC single crystal growth experiments were conducted by the temperature gradient method using the same apparatus as shown in FIG. In the experiment, the non-oxidizing atmosphere gas composition, the atmosphere gas pressure, and the solution growth temperature were used as variables. The temperature gradient was formed so that the upper part of the graphite crucible became a low temperature part. The temperature difference between the low temperature part and the high temperature part was in the range of 40-100 ° C. The temperature in the vicinity of the seed crystal substrate immersed in the low temperature part is determined by measuring the temperature of the melt that has been heated under the same conditions as the growth experiment by inserting a thermocouple in advance in the melt separately from the growth experiment. Asked. The atmosphere gas in the crystal growth apparatus was adjusted using the gas inlet and the gas outlet.

  In the experiment, Si or Si and a predetermined additive metal element were charged into a high-purity graphite crucible, and the raw material was melted by heating so as to form a predetermined temperature gradient in the melt, and the heating state was maintained for 5 hours. Then, C was dissolved from the graphite crucible. Thereafter, a SiC seed crystal substrate (10 mm × 10 mm, 6H—SiC (0001) on axis) held in a graphite seed crystal holding jig was immersed in the low temperature part (upper part of the melt) of the melt. When a predetermined time passed after the immersion of the seed crystal substrate, the seed crystal holding jig was raised and the seed crystal substrate was pulled up from the melt. During the heating, the graphite crucible containing the melt and the holding jig for holding the seed crystal substrate were rotated in opposite directions.

  Thereafter, the graphite crucible was gradually cooled to room temperature, and the seed crystal substrate on which the SiC single crystal was grown was collected from the holding jig. The seed crystal substrate was washed with hydrofluoric acid to remove the adhering melt coagulum. The cross section and surface of a SiC single crystal newly grown on the seed crystal were observed with an optical microscope, and the state of bubbles confined in the crystal was examined. Since this single crystal is transparent, the presence or absence of bubbles could be reliably identified by changing the focus and observing the generation of bubbles inside the crystal. Judgment was made when the bubble-free one was marked with ◎, and when even one bubble was found in the observation field area of 10 mm × 10 mm, it was marked with ×.

  As the viscosity η of the atmospheric gas, the value obtained by the capillary viscosity measurement method for calculating the gas viscosity from the difference in gas pressure between the inlet side and the outlet side when a gas having the same composition was flowed through the capillary was used.

( Reference Example 1 )
The graphite crucible was filled with Si and heated to a crystal growth temperature (temperature of the low temperature part) of 1630 ° C. in a He atmosphere and atmospheric pressure to melt Si. The above heating was maintained for 5 hours so that C was dissolved to the saturation concentration from the inner wall of the graphite crucible. Thereafter, the seed crystal substrate held by the holding jig was immersed in the melt in the low temperature portion of the graphite crucible, and after the immersion time of 20 hours had elapsed, the holding jig was raised and the substrate was pulled up from the melt. . Next, the furnace temperature was cooled to room temperature, the seed crystal substrate was recovered, and the presence or absence of bubbles was determined.

( Example 1 )
A SiC single crystal was produced in the same manner as in Reference Example 1 except that a raw material of Si _0.77 Ti _0.23 was filled in a graphite crucible and the crystal growth temperature was 1675 ° C.

( Example 2 )
A SiC single crystal was produced in the same manner as in Example 1 except that the atmospheric gas species was changed to a mixed gas of He (90%) and N ₂ (10%) and the crystal growth temperature was 1660 ° C.

( Example 3 )
A SiC single crystal was produced in the same manner as in Example 1 except that the gas pressure was changed to 0.9 MPa and the crystal growth temperature was 1660 ° C.

( Example 4 )
A SiC single crystal was produced in the same manner as in Example 1 except that the crystal growth temperature was changed to 1950 ° C.

(Comparative Example 1)
A SiC single crystal was produced in the same manner as in Example 1 except that the atmospheric gas species was changed from He to Ar and the crystal growth temperature was changed to 1650 ° C.

(Comparative Example 2)
A SiC single crystal was produced in the same manner as in Example 2 except that the atmospheric gas species was changed from He to Ar and the crystal growth temperature was changed to 1650 ° C.

(Comparative Example 3)
A SiC single crystal was produced in the same manner as in Example 2 except that the atmospheric gas species was changed to a mixed gas of He (80%) and Ar (20%) and the crystal growth temperature was 1670 ° C.

Table 1 below shows the determination results of the presence or absence of bubbles in the bulk SiC single crystal obtained in the above Reference Examples, Examples, and Comparative Examples together with various crystal growth conditions. In addition, cross-sectional and optical micrographs of the SiC single crystals on the substrates obtained in Reference Example 1 and Comparative Example 1 are shown in FIGS. 2 (a) and (b) and FIGS. 3 (a) and (b), respectively. Show. As can be seen from these figures, the presence or absence of bubbles can be reliably determined from both micrographs of the cross section and the surface.

As can be seen from the results of Reference Example 1, Examples 1 to 4 and Comparative Examples 1 to 3, by controlling the gas viscosity η according to the present invention regardless of the melt type, gas type and gas pressure used, It is possible to eliminate bubbles trapped in the single crystal and eliminate the bubbles of the single crystal. This is presumably because when the gas viscosity is low, bubbles generated in the melt are less likely to remain at the SiC crystal growth interface.

From the results of Example 3 , it is possible to eliminate the bubbles confined in the crystal even under pressurized conditions. This is presumably because there is almost no difference between the atmospheric pressure (0.1 MPa) and the pressure of about 1 MPa. Although it is possible to eliminate bubbles even under pressurized conditions, pressurization increases the size of the apparatus, which is disadvantageous for industrial production.

From the results of Example 4 , it is possible to eliminate bubbles contained in the crystal even when the crystal growth temperature is high, and an increase in the speed of SiC crystal growth can be expected.
The SiC single crystals produced in Reference Example 1, Examples 1 to 4 and Comparative Examples 1 to 3 were all the same 6H—SiC single crystals as the substrate. The growth temperature is 2000 ° C or less, and there is little deterioration of the growth furnace, so there are few problems in equipment, and there is almost no problem such as evaporation of the melt. Was possible.

It is explanatory drawing which shows an example of the crystal growth apparatus which can be utilized for the solution growth method. 2 (a) and 2 (b) are optical micrographs showing the cross section and surface of the SiC single crystal produced in Reference Example 1 , respectively. 3 (a) and 3 (b) are optical micrographs showing the cross section and surface of the SiC single crystal produced in Comparative Example 1, respectively.

Explanation of symbols

1: melt, 2: graphite crucible, 3: seed crystal substrate, 4: heat insulating material, 5: heating coil, 6: water cooling chamber, 7: gas inlet, 8: gas outlet, 9: pyrometer

Claims (3)

A method in which a silicon carbide seed crystal substrate is immersed in a melt containing SiC , including Si, C, and Ti, and a silicon carbide single crystal is grown on the seed crystal substrate by solution supercooling at least around the seed crystal substrate. In addition, the atmosphere gas during single crystal growth is a gas that is a helium or helium-based mixed gas and has a viscosity η at a single crystal growth temperature satisfying η ≦ 750 μP (μP: micropoise) and serving as a C supply source A method for producing a silicon carbide single crystal comprising growing a non-bubbled single crystal in a non-oxidizing gas containing no oxygen and having a crystal growth temperature of 1660 ° C. or higher in the atmosphere .   The method for producing a silicon carbide single crystal according to claim 1, wherein the pressure of the atmospheric gas is set to atmospheric pressure or a pressurizing condition.   The method for producing a silicon carbide single crystal according to claim 1 or 2, wherein the crystal growth temperature is 2000 ° C or lower and the liquidus temperature of the melt or higher.