JP7636908B2 - SiC composite substrate - Google Patents

Description

The present invention relates to a SiC composite substrate.

In recent years, semiconductor devices using gallium nitride (GaN) and other materials have been put to practical use. GaN crystals can be formed by vapor phase growth methods such as hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD).

It is known that in such semiconductor devices, the fewer cracks and the higher the crystallinity in the material, the better the characteristics can be obtained. For example, it is known that the difference in lattice constant and thermal expansion coefficient between the base substrate used for film formation and the semiconductor film such as GaN formed on it affects the degree of cracking and the crystallinity of the semiconductor film.

In this regard, various base substrates are used for manufacturing GaN crystals, such as sapphire substrates, Si substrates, SiC substrates, Si substrates with SiC film formed thereon, and sapphire or Si substrates with a buffer layer formed thereon. Among these, for example, sapphire substrates and Si substrates have a large difference in lattice constant and thermal expansion coefficient with GaN, which causes distortion at the interface between the GaN crystal and the substrate, and the distortion causes cracks in the GaN crystal. SiC single crystal is used as the SiC substrate, and since the difference in lattice constant and thermal expansion coefficient of the SiC single crystal substrate with GaN is relatively small, it is relatively possible to suppress cracks in the GaN crystal, but it is expensive, so there is a problem that the manufacturing cost is high. As for the substrate with SiC film formed on the Si substrate, although the difference in lattice constant between the SiC on the film formation surface and GaN is small, the difference in thermal expansion coefficient between the substrate and GaN is large because the Si substrate is included in the base substrate, and there is a problem that cracks cannot be sufficiently reduced. Regarding a buffer layer provided on a sapphire or Si substrate, for example, Patent Document 1 (Patent Publication No. 5433909) describes that the degree of warping and cracking can be relatively mitigated by stacking several hundred pairs of ultra-thin films of AlN (thickness 5 nm)/GaN (thickness 20 nm) on a Si substrate. However, this type of manufacturing method has the problem of complicated processes and high costs.

For example, Patent Document 2 (Patent Publication No. 6387375) discloses a semiconductor substrate including a single crystal SiC substrate and a polycrystalline SiC substrate. By peeling off a portion of the single crystal layer into which ions have been implanted, a semiconductor substrate is obtained that includes a thin single crystal SiC substrate on a polycrystalline SiC substrate. However, when ions are implanted, the resulting single crystal SiC substrate becomes thin, and it is thought that the polycrystalline SiC will affect the GaN crystal film formation, resulting in poor crystallinity of the GaN crystal.

For example, Patent Document 3 (Patent Publication No. 3254557) discloses single crystal SiC in which a composite of an α-SiC single crystal and an amorphous or β-SiC polycrystalline substrate is bonded to each other and then heat-treated to recrystallize the amorphous or β-SiC polycrystalline substrate, thereby growing a single crystal oriented in the same direction as the crystal axis of the α-SiC single crystal substrate. However, compared to β-SiC, α-SiC has a larger difference in lattice constant with GaN, which is thought to adversely affect the degree of cracking and crystallinity of the semiconductor film.

Patent No. 5433909 Patent No. 6387375 Patent No. 3254557

As mentioned above, the difference in lattice constant and thermal expansion coefficient between the base substrate for film formation and the semiconductor film such as GaN formed thereon affects the degree of cracking and crystallinity of the semiconductor film, so a substrate capable of forming a semiconductor film with high crystallinity and suppressed cracking is desired as the base substrate. It is also desirable to reduce the manufacturing cost of the semiconductor film.

The inventors have now discovered that by fabricating a SiC composite substrate having a monocrystalline SiC layer of a given thickness on a polycrystalline SiC layer as a base substrate, and then depositing GaN crystals on the SiC composite substrate, it is possible to obtain GaN crystals that are highly crystalline and have reduced cracks at low cost.

Therefore, the object of the present invention is to provide a SiC composite substrate that can be used as a base substrate for growing GaN crystals, and that can inexpensively produce GaN crystals with high crystallinity and reduced cracks.

According to one aspect of the present invention, a SiC composite substrate is provided that includes a polycrystalline SiC layer and a single-crystal SiC layer having a thickness of 1.0 to 30 μm on the polycrystalline SiC layer.

FIG. 1 is a conceptual diagram showing the configuration of an aerosol deposition (AD) device 50.

The SiC composite substrate according to the present invention comprises a polycrystalline SiC layer and a single-crystal SiC layer having a thickness of 1.0 to 30 μm on the polycrystalline SiC layer. In this way, a SiC composite substrate having a single-crystal SiC layer of a predetermined thickness on the polycrystalline SiC layer is produced as a base substrate, and GaN crystals are formed on the SiC composite substrate, thereby making it possible to obtain GaN crystals with high crystallinity and suppressed cracks at low cost.

As mentioned above, the difference in lattice constant and thermal expansion coefficient between the base substrate for film formation and the semiconductor film such as GaN formed thereon affects the degree of cracking and crystallinity of the semiconductor film, so it is desirable to have a base substrate that can produce a semiconductor film with high crystallinity and suppressed cracking. It is also desirable to reduce the manufacturing cost of the semiconductor film. In this regard, according to the present invention, a SiC composite substrate with a monocrystalline SiC layer of a predetermined thickness on a polycrystalline SiC layer is produced as a base substrate, and GaN crystals are formed on the SiC composite substrate, thereby making it possible to obtain GaN crystals with high crystallinity and suppressed cracking at low cost.

The reasons why GaN crystals with high crystallinity and suppressed cracks can be obtained inexpensively are as follows. (i) The difference in thermal expansion coefficient between the single crystal SiC layer on which the GaN crystals are formed and the GaN crystals is relatively small, and the difference in lattice constant is also small, so the GaN crystals formed on the SiC composite substrate of the present invention can be made to have high crystallinity and suppressed cracks. (ii) In addition, the SiC composite substrate of the present invention has a polycrystalline SiC layer, but since the crystal orientation of the polycrystal is not uniform, the directions of thermal expansion and thermal contraction are also not uniform. Therefore, the distortions that enter the surface of the polycrystalline SiC layer are also not uniform, which also affects the single crystal SiC layer, resulting in poor crystallinity of the obtained GaN crystals. This effect is greater the thinner the single crystal SiC layer. However, the single crystal SiC layer of the SiC composite substrate of the present invention has a thickness of 1.0 to 30 μm and is not easily affected by the polycrystalline SiC layer, so that GaN crystals with high crystallinity and suppressed cracks can be obtained. (iii) Furthermore, the SiC composite substrate of the present invention has a single crystal SiC layer on an inexpensive polycrystalline SiC layer, which can reduce manufacturing costs, compared to when an expensive single crystal SiC substrate itself is used as a base substrate. For these reasons, the SiC composite substrate of the present invention can provide a semiconductor film with high crystallinity and reduced cracks at low cost on the SiC composite substrate. This SiC composite substrate can also be used as a base substrate for crystals used as semiconductor films, such as SiC crystals, and is not limited to GaN crystals.

The thickness of the single crystal SiC layer provided in the SiC composite substrate of the present invention is 1.0 to 30 μm. This thickness is preferably 2.0 to 30 μm. By doing so, GaN crystals with high crystallinity and suppressed cracks can be effectively obtained. As described above, if the single crystal SiC layer is too thin, the crystallinity of the GaN crystals will deteriorate. The reason is unclear, but the following is thought to be the reason. Unlike single crystals, the orientation of the particles in a polycrystalline SiC layer is random, so the directions of thermal expansion and thermal contraction are different for each particle on the polycrystalline surface. Therefore, when the temperature is raised to the film formation temperature of the GaN crystals or when the temperature is lowered after film formation, it is thought that uneven distortion in the plane caused by the variation in the particle orientation of the polycrystalline part is introduced into the single crystal SiC layer surface in contact with the polycrystalline SiC layer. If the single crystal SiC layer is thin, the influence of this distortion extends to the film formation surface of the single crystal SiC layer on which the GaN crystals are formed, and as a result, it is thought that the crystallinity of the GaN crystals may deteriorate. For these reasons, the thickness of the single crystal SiC layer is 1.0 μm or more. Also, if the single crystal SiC layer is too thick, it will be disadvantageous in terms of manufacturing costs. Therefore, the thickness of the single crystal SiC layer is 30 μm or less.

In this way, it is considered that the thickness of the single crystal SiC layer affects the manufacturing cost, crystallinity, and degree of cracking of the GaN crystal. Here, for example, Patent Document 2 (Patent Publication No. 6387375) discloses a semiconductor substrate including a single crystal SiC substrate and a polycrystalline SiC substrate for the purpose of improving electrical characteristics. It is described that a single crystal layer into which ions have been implanted is bonded to a polycrystalline SiC support substrate, and a part (most part) of the single crystal layer is peeled off at the ion implanted portion to obtain a semiconductor substrate including a thin single crystal SiC substrate on the polycrystalline SiC substrate. However, it is generally difficult to process a deep position of 1.0 μm or more by ion implantation. Therefore, it is estimated that the thickness of the single crystal portion of the obtained semiconductor substrate is substantially 1.0 μm or less. In other words, it is considered that the crystallinity is deteriorated due to the influence of polycrystalline SiC during GaN crystal film formation. On the other hand, according to the SiC composite substrate of the present invention, as described above, by forming a GaN crystal film on the SiC composite substrate, it is possible to obtain a GaN crystal with high crystallinity and suppressed cracking at low cost.

The polytype of the single crystal SiC layer is preferably 3C. That is, the single crystal SiC layer is preferably β-SiC single crystal. By using 3C-SiC, the difference in lattice constant between the single crystal SiC layer and the GaN crystal is small, so that GaN crystals with high crystallinity and suppressed cracks can be effectively obtained. In particular, the difference in lattice constant between the (111) plane of 3C-SiC and the (002) plane of GaN is small. Here, for example, Patent Document 3 (Patent Publication No. 3254557) discloses single crystal SiC in which a single crystal oriented in the same direction as the crystal axis of the α-SiC single crystal substrate is integrally grown by heat-treating a composite in which an α-SiC single crystal and an amorphous or β-SiC polycrystalline substrate are closely attached to each other, and the amorphous or β-SiC polycrystalline substrate is recrystallized. For the SiC composite substrate of the present invention, it is preferable to use β-SiC rather than α-SiC, and therefore it is possible to prevent growth (at all) in the same direction as the crystal axis of the α-SiC single crystal substrate. Thus, there are α-SiC and β-SiC in SiC, but the crystal structures of these are completely different, with the former being hexagonal and the latter being cubic, and they can be said to be completely different materials. For example, β-SiC has only one type of polymorph, while more than 200 types of polymorphs have been reported for α-SiC, making it difficult to obtain crystals consisting of a single polymorph. In addition, the stable temperature ranges are different, with β-SiC being stable at relatively low temperatures, while α-SiC is stable at higher temperatures than β-SiC. Thus, α-SiC and β-SiC are completely different materials.

The polycrystalline SiC layer of the SiC composite substrate of the present invention preferably further contains a rare earth element. It is believed that the polycrystalline SiC layer containing a rare earth element more effectively suppresses cracks that may occur in the GaN crystal formed on the SiC composite substrate. Although the cause of this is unclear, it is believed that the presence of an appropriate amount of rare earth elements in the polycrystalline SiC layer relieves the distortion in the polycrystalline SiC layer, thereby mitigating the warping of the SiC composite substrate and reducing the cracks in the GaN crystal. In addition, it is believed that the rare earth element is preferably contained in the polycrystalline SiC layer by using an oxide of a rare earth element as a raw material. The thermal expansion coefficient of the oxide of a rare earth element is larger than that of SiC, and the thermal expansion coefficient of GaN is larger than that of polycrystalline SiC. Therefore, it is believed that the thermal expansion coefficient of polycrystalline SiC containing an oxide of a rare earth element is closer to that of GaN compared to polycrystalline SiC without the addition of an oxide of a rare earth element. This may reduce the stress caused by the difference in thermal expansion between the SiC composite substrate and the GaN crystal, thereby reducing cracks.

When the polycrystalline SiC layer contains a rare earth element, the rare earth element is at least one element selected from the group consisting of 17 elements: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The inventors have obtained data on the thermal expansion coefficient of oxides of rare earth elements other than Pm and Tb among these rare earth elements, and have confirmed that the thermal expansion coefficient of the oxides of these rare earth elements is greater than that of SiC. The rare earth element contained in the polycrystalline SiC layer is preferably Y (yttrium) and/or Sm (samarium), and more preferably Y.

When the polycrystalline SiC layer contains a rare earth element, the weight ratio of the content of the rare earth element to the total content of the rare earth element, Si, and C in the polycrystalline SiC layer is preferably 0.00020 to 0.20, more preferably 0.0010 to 0.15, and even more preferably 0.0020 to 0.10. By having the weight ratio of the rare earth element within such a range, it is believed that GaN crystals with suppressed cracks can be more effectively obtained on the SiC composite substrate. By having the weight ratio be 0.20 or less, it is believed that the influence of distortion caused by the presence of the rare earth element in the polycrystalline SiC layer can be prevented from becoming significant, and cracks can be more effectively reduced. In addition, it is believed that the ability to suppress the generation of a different phase of the rare earth element and SiC and the ability to suppress segregation of the rare earth element also contribute to the reduction of cracks.

The thickness of the polycrystalline SiC layer provided in the SiC composite substrate of the present invention is preferably 100 μm or more, more preferably 250 μm or more, from the viewpoint of handling the composite substrate. From the viewpoint of cost, the thickness of the polycrystalline SiC layer is preferably 5000 μm or less, more preferably 3000 μm or less. Therefore, from the viewpoint of achieving both the handleability and cost of the composite substrate, the thickness of the polycrystalline SiC layer is preferably 100 to 5000 μm, more preferably 250 to 3000 μm.

The SiC composite substrate of the present invention comprises a polycrystalline SiC layer and a single crystal SiC layer, and the boundary between the single crystal and the polycrystal can be identified by known methods. For example, it can be identified by obtaining an inverse pole figure orientation mapping of the cross section of the SiC composite substrate using an SEM equipped with an electron backscatter diffraction (EBSD) device. From the obtained inverse pole figure orientation mapping, it is possible to distinguish the oriented layer as the single crystal portion and the rest as the polycrystalline portion.

Manufacturing method of SiC composite substrate The manufacturing method of the SiC composite substrate having a polycrystalline SiC layer and a single crystal SiC layer of the present invention is not particularly limited. The polycrystalline SiC layer may be formed directly on the single crystal SiC layer, or the single crystal SiC layer and the polycrystalline SiC layer may be prepared separately and bonded.

For the single crystal SiC layer, a commercially available single crystal substrate may be used, or SiC may be epitaxially grown on a substrate such as a Si substrate.

The polycrystalline SiC layer may be a commercially available polycrystalline substrate, or may be produced by firing a SiC molded body. The SiC molded body may be a tape molded body produced by tape casting, or a green compact produced by pressure molding such as uniaxial pressing. The firing method may involve firing at normal pressure, or may involve pressurized firing such as hot pressing or HIP.

When a polycrystalline SiC layer is formed directly on a single-crystal SiC layer, examples of the formation method include solid-phase film formation methods such as AD (aerosol deposition) and HPPD (supersonic plasma particle deposition), gas-phase film formation methods such as sputtering, vapor deposition, sublimation, and various CVD (chemical vapor deposition) methods, and liquid-phase film formation methods such as solution growth. Examples of CVD methods that can be used include thermal CVD, plasma CVD, mist CVD, and MO (metal organic) CVD. Alternatively, a method of placing a SiC molded body on the single-crystal SiC layer may be used.

When preparing the single crystal SiC layer and the polycrystalline SiC layer separately and joining them, the joining method is not particularly limited, but examples include a method using an adhesive and a method by sintering. When sintering, they may be joined by applying pressure using a hot press or the like.

When preparing a polycrystalline SiC layer, the rare earth element can be contained in the polycrystalline SiC layer by making the raw material powder or raw material gas contain a rare earth compound corresponding to the rare earth element. The rare earth compound is not particularly limited, but examples include oxides, nitrides, carbides, and fluorides of at least one of the 17 rare earth elements mentioned above. As the rare earth compound, oxides of rare earth elements are preferable, and oxides of Y (yttrium oxide) and oxides of Sm (samarium oxide) are more preferable.

The above-mentioned methods for manufacturing the SiC composite substrate can all be performed under known conditions, but the following describes a method for directly forming a polycrystalline SiC layer on a single-crystalline SiC layer using the AD method or thermal CVD method, and a method for placing a previously manufactured molded body on a single-crystalline SiC layer.

The AD method is a technology in which fine particles or fine particle raw material is mixed with a gas to form an aerosol, and this aerosol is sprayed at high speed from a nozzle to collide with a substrate to form a coating, and has the characteristic that the coating can be formed at room temperature. An example of a film formation device (AD device) used in such an AD method is shown in FIG. 1. The AD device 50 shown in FIG. 1 is configured as a device used in the AD method in which raw material powder is sprayed onto a substrate under an atmosphere with a pressure lower than atmospheric pressure. This AD device 50 includes an aerosol generation section 52 that generates an aerosol of raw material powder containing raw material components, and a film formation section 60 that sprays the raw material powder onto a single crystal SiC layer 20 to form a film containing the raw material components. The aerosol generating section 52 includes an aerosol generating chamber 53 that contains raw material powder and generates aerosol by receiving a carrier gas from a gas cylinder (not shown), a raw material supply pipe 54 that supplies the generated aerosol to the film forming section 60, and a vibrator 55 that applies vibrations at a frequency of 10 to 100 Hz to the aerosol generating chamber 53 and the aerosol therein. The film forming section 60 includes a film forming chamber 62 that sprays the aerosol onto the single crystal SiC layer 20, a substrate holder 64 that is disposed inside the film forming chamber 62 and fixes the single crystal SiC layer 20, and an XY stage 63 that moves the substrate holder 64 in the X-axis and Y-axis directions. The film forming section 60 also includes an injection nozzle 66 that has a slit 67 formed at its tip and sprays the aerosol onto the single crystal SiC layer 20, and a vacuum pump 68 that reduces the pressure in the film forming chamber 62. The injection nozzle 66 is attached to the tip of the raw material supply pipe 54.

It is known that the AD method can cause pores in the film or the film to become a compact, depending on the deposition conditions. For example, it is easily affected by the collision speed of the raw material powder with the substrate, the particle size of the raw material powder, the agglomeration state of the raw material powder in the aerosol, the amount sprayed per unit time, etc. The collision speed of the raw material powder with the substrate is affected by the pressure difference between the deposition chamber 62 and the spray nozzle 66, the opening area of the spray nozzle, etc. Therefore, in order to obtain a dense polycrystalline SiC layer, it is necessary to appropriately control these factors.

In the thermal CVD method, a known film forming apparatus such as a commercially available one can be used. The source gas is not particularly limited, but silicon tetrachloride (SiCl ₄ ) gas or silane (SiH ₄ ) gas can be used as a Si source, and methane (CH ₄ ) gas or propane (C ₃ H ₈ ) gas can be used as a C source. The film forming temperature is preferably 1000 to 2200°C, more preferably 1100 to 2000°C, and even more preferably 1200 to 1900°C.

It is known that the formation of a polycrystalline SiC layer on a single-crystalline SiC layer using thermal CVD is influenced by factors such as the deposition temperature, the gas flow rates of the Si source and C source and their ratio, and the deposition pressure. The deposition temperature has a large effect, and a lower deposition temperature is preferable, preferably less than 1700°C, more preferably 1500°C or less, and even more preferably 1400°C or less. However, if the deposition temperature is too low, the deposition rate itself will decrease, so from the viewpoint of the deposition rate, a higher deposition temperature is preferable.

In the case of a method in which a SiC molded body is placed on a single-crystal SiC layer, the polycrystalline SiC layer can be produced by molding the raw material powder. For example, when press molding is used, the polycrystalline SiC layer is a press molded body. The press molded body can be produced by press molding the raw material powder of the polycrystalline SiC layer based on a known method, for example, by putting the raw material powder into a mold and pressing it at a pressure of preferably 100 to 400 kgf/cm ² , more preferably 150 to 300 kgf/cm ^2. In addition, there is no particular limitation on the molding method, and in addition to press molding, tape molding, extrusion molding, casting molding, doctor blade method, and any combination thereof can be used. For example, when tape molding is used, additives such as binders, plasticizers, dispersants, and dispersion media are appropriately added to the raw material powder to make a slurry, and the slurry is preferably discharged and molded into a sheet by passing it through a thin slit-shaped discharge port. There is no limitation on the thickness of the molded body formed into a sheet, but from the viewpoint of handling, it is preferable that it is 5 to 500 μm. If a thick polycrystalline SiC layer is required, a number of these sheets may be stacked to obtain the desired thickness. The SiC compact may be in a sintered state, and in this case, the compact may contain additives such as sintering aids in addition to the SiC raw material.

The present invention will be further illustrated by the following examples.

Example 1
(1) Preparation of polycrystalline SiC layer Commercially available fine β-SiC powder (volume-based D50: 0.7 μm) was prepared as raw material powder for AD film formation. In addition, a commercially available substrate with a diameter of about 50 mm (2 inches) was prepared, on which a 3C-SiC film (single crystal SiC layer) was epitaxially grown to a thickness of 3.5 μm on a 900 μm thick Si single crystal substrate. Using the AD device 50 shown in FIG. 1, the raw material powder was sprayed onto the 3C-SiC surface of the substrate to form an AD film.

The AD film formation conditions were as follows. First, the carrier gas was _N2 , and a ceramic nozzle with a slit of 7 mm long side x 0.6 mm short side was used for film formation. The nozzle scan conditions were a scan speed of 0.6 mm/s, 56 mm movement perpendicular to the long side of the slit and in the forward direction, 7 mm movement in the long side direction of the slit, 56 mm movement perpendicular to the long side of the slit and in the return direction, and 7 mm movement in the long side direction of the slit and in the opposite direction to the initial position, and when the nozzle moved 56 mm from the initial position in the long side direction of the slit, a scan was performed in the opposite direction to the previous direction, and a cycle of returning to the initial position was defined as one cycle, and this cycle was repeated 1200 times. In one cycle of film formation at room temperature, the set pressure of the carrier gas was adjusted to 0.04 MPa, the flow rate to 9 L/min, and the chamber pressure to 100 Pa or less. The thickness of the AD film formed in this way was about 300 μm. In this manner, a polycrystalline SiC layer (AD film) was formed on the surface of the 3C-SiC film formed on the Si single crystal substrate.

(2) Polishing of polycrystalline SiC layer The substrate on which the AD film was formed was removed from the AD device, and the polycrystalline SiC layer was ground to #6000 using a grindstone to flatten the plate surface. The plate surface was then smoothed by lapping using diamond abrasive grains. The size of the abrasive grains was gradually reduced from 3 μm to 0.5 μm to improve flatness. Then, a mirror finish was performed by chemical mechanical polishing (CMP) using colloidal silica.

(3) Removal of Si substrate Next, the Si substrate surface was ground to #6000 using a grindstone to flatten the plate surface. Then, the plate surface was smoothed by lapping using diamond abrasive grains. The size of the abrasive grains was gradually reduced from 3 μm to 0.5 μm to improve flatness. Then, a mirror finish was performed by chemical mechanical polishing (CMP) using colloidal silica, and finally the Si substrate was completely removed. Thus, a SiC composite substrate was obtained, which was provided with a 2.0 μm thick 3C-SiC film (single crystal SiC layer) and a polycrystalline SiC layer with a thickness of about 300 μm.

(4) Evaluation of polycrystalline SiC layer (4-1) Surface FE-EPMA
Using an electron probe microanalyzer (FE-EPMA) (JXA-8500F, manufactured by JEOL Ltd.), a composition analysis of the polycrystalline SiC layer surface of the obtained SiC composite substrate was carried out with a probe size of 30 μm×30 μm. This measurement was carried out at 10 points on the polycrystalline SiC layer surface. The conditions for this FE-EPMA measurement were as follows.

<FE-EPMA measurement conditions>
Acceleration voltage: 15 kV
Irradiation current: 50 nA

As a result of the surface FE-EPMA measurement, no rare earth elements were detected on the surface of the polycrystalline SiC layer, but Si, C and O were detected. The quantitative values of the detected Si and C were used to determine the weight ratio of the content of rare earth elements in the polycrystalline SiC layer to the total content of rare earth elements, Si and C (rare earth elements/(rare earth elements + Si + C)). The content value of each element is the average value obtained by measuring the above 10 locations. The results are shown in Table 1.

(5) Formation of GaN film A GaN film was formed by MOCVD on the 3C-SiC film of the SiC composite substrate prepared as described above. In growing the GaN film, TMG (trimethylgallium) was used as the Ga source gas, NH ₃ (ammonia) was used as the N source gas, and nitrogen and hydrogen were used as the carrier gas. The film formation temperature was 1100°C, and the film formation time was 1 hour. The thickness of the obtained GaN film was about 1.0 μm.

(6) Evaluation of GaN Film (6-1) Crystallinity of GaN Film Using an XRD device (D8-DISCOVER, manufactured by Bruker-AXS), XRC measurement of the (002) surface of the GaN film formed on a SiC composite substrate was carried out. In practice, 2θ, ω, χ and φ were adjusted to set the axis so that the peak of the (002) surface of GaN appeared, and then the following conditions were used: tube voltage 40 kV, tube current 40 mA, anti-scattering slit 3 mm, ω = 14.0 to 22.0° range, ω step width 0.005°, and counting time 0.5 seconds. In addition, a Ge (022) asymmetric reflection monochromator was used as the X-ray source to convert CuKα rays into parallel monochromatic light. The half-width of the XRC profile of the (002) plane of the obtained GaN film was determined by smoothing the profile and then searching for a peak using XRD analysis software ("LEPTOS" Ver. 4.03, manufactured by Bruker-AXS). The obtained results were ranked according to the following evaluation criteria and are shown in Table 1.

- Grade A: Half-width is 400 arcsec or less - Grade B: Half-width is more than 400 arcsec and less than 500 arcsec - Grade C: Half-width is more than 500 arcsec

As a result of the evaluation of the crystallinity of the GaN film, the half-width of the (002) plane XRC profile was 400 arcsec or less, indicating good crystallinity.

(6-2) Number of cracks in GaN film Using an industrial microscope (Nikon, ECLIPSE, LV150N), the eyepiece was set to 10x, the objective lens was set to 5x, and the entire surface of the GaN film was observed in the polarization mode and differential interference mode. If a crack was confirmed, the objective lens was changed to 10x, and an image was obtained. Then, only cracks with a length of 50 μm or more were counted as cracks. In addition, if the distance from one crack to another was 500 μm or less, it was considered as one crack. Regardless of the size of the GaN film, the number of cracks (pieces) on the entire surface of the film was measured and converted to the number of cracks (pieces) per film area of 20 cm ^2. The results obtained are shown in Table 1.

Example 2
In the above (3), a SiC composite substrate was produced, a GaN film was formed, and various evaluations were performed in the same manner as in Example 1, except that the thickness of the 3C-SiC film was set to 1.1 μm when removing the Si substrate. As a result of FE-EPMA measurement of the surface of the polycrystalline SiC layer, no rare earth elements were detected, but Si, C, and O were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 500 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 3
In the above (3), a SiC composite substrate was produced, a GaN film was formed, and various evaluations were performed in the same manner as in Example 1, except that the thickness of the 3C-SiC film was set to 3.0 μm when removing the Si substrate. As a result of FE-EPMA measurement of the surface of the polycrystalline SiC layer, no rare earth elements were detected, but Si, C, and O were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 4 (Comparison)
In the above (3), when removing the Si substrate, the thickness of the 3C-SiC film was set to 0.9 μm, but the same method as in Example 1 was used to prepare a SiC composite substrate, form a GaN film, and perform various evaluations. As a result of FE-EPMA measurement of the surface of the polycrystalline SiC layer, no rare earth elements were detected, but Si, C, and O were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film exceeded 500 arcsec., and the crystallinity was poorer than that of Example 1. Since the crystallinity was poor, the number of cracks was not evaluated. The results are shown in Table 1.

Example 5
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 96.0 wt % and yttrium oxide powder (volume basis D50: 0.1 μm) containing 4.0 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the polycrystalline SiC layer surface, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 6
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume standard D50: 0.7 μm) containing 99.98 wt % and yttrium oxide powder (volume standard D50: 0.1 μm) containing 0.02 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the surface of the polycrystalline SiC layer, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 7
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume standard D50: 0.7 μm) containing 99.97 wt % and yttrium oxide powder (volume standard D50: 0.1 μm) containing 0.03 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the polycrystalline SiC layer surface, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 8
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 88.0 wt % and yttrium oxide powder (volume basis D50: 0.1 μm) containing 12.0 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the polycrystalline SiC layer surface, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 9
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 76.0 wt % and yttrium oxide powder (volume basis D50: 0.1 μm) containing 24.0 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the polycrystalline SiC layer surface, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 10
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 70.0 wt % and yttrium oxide powder (volume basis D50: 0.1 μm) containing 30.0 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the polycrystalline SiC layer surface, Si, C, O, and Y as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 11
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 96.0 wt % and samarium oxide powder (volume basis D50: 0.1 μm) containing 4.0 wt % were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Except for this, the SiC composite substrate was produced, the GaN film was formed, and various evaluations were performed in the same manner as in Example 1. As a result of FE-EPMA measurement of the surface of the polycrystalline SiC layer, Si, C, O, and Sm as a rare earth element were detected. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 12
In the above (1), as the raw material powder for AD film formation, commercially available fine β-SiC powder (volume basis D50: 0.7 μm) containing 96.0 wt% and yttrium oxide powder (volume basis D50: 0.1 μm) containing 4.0 wt% were prepared, and this was mixed in ethanol for 24 hours using SiC balls in a ball mill, and dried to obtain a mixed powder. Other than this, (1) the preparation of a polycrystalline SiC layer, (2) the polishing of the polycrystalline SiC layer, and (3) the removal of the Si substrate were performed in the same manner as in Example 1. In this way, a SiC composite substrate was obtained, which was provided with a 2.0 μm thick 3C-SiC film (single crystal SiC layer) and a polycrystalline SiC layer having a thickness of about 300 μm.

After that, a 3C-SiC film was further formed on the 3C-SiC film of the SiC composite substrate by the CVD method. In growing the 3C-SiC film, trichlorosilane and ethylene were used as the source gases, and hydrogen was used as the carrier gas. The film formation temperature was 1380°C, and the 3C-SiC film was formed until it reached a thickness of 40 μm.

Next, the 3C-SiC film was ground to #6000 using a grindstone to flatten the plate surface. The plate surface was then smoothed by lapping using diamond abrasive grains. Flatness was improved while the abrasive grain size was gradually reduced from 3 μm to 0.5 μm. After that, a mirror finish was performed using chemical mechanical polishing (CMP) using colloidal silica, and finally a SiC composite substrate was obtained with a 30 μm-thick 3C-SiC film (single crystal SiC layer) and a polycrystalline SiC layer approximately 300 μm thick.

The SiC composite substrate thus obtained was subjected to deposition of a GaN film and various evaluations in the same manner as in Example 1. FE-EPMA measurement of the surface of the polycrystalline SiC layer detected Si, C, O, and Y as a rare earth element. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1.

Example 13 (Comparative)
A commercially available substrate with a diameter of about 50 mm (2 inches) was prepared, in which 3.0 μm of 3C-SiC (single crystal SiC layer) was epitaxially grown on a 900 μm thick Si single crystal substrate. A GaN film was formed on the 3C-SiC film of this substrate by MOCVD. In growing the GaN film, TMG (trimethylgallium) was used as the Ga source gas, NH ₃ (ammonia) as the N source gas, and nitrogen and hydrogen as the carrier gas. The film formation temperature was 1100° C., and the film formation time was 1 hour. The thickness of the obtained GaN film was about 1.0 μm.

The GaN film thus obtained was evaluated in the same manner as in Example 1. When the crystallinity of the GaN film was evaluated, the half-width of the (002) plane XRC profile of the GaN film was 400 arcsec or less, indicating good crystallinity. The results are shown in Table 1. As described above, in Example 13, a polycrystalline SiC layer was not formed, and the GaN film was formed on a composite substrate in which 3C-SiC (single crystal SiC layer) was formed on a Si single crystal substrate, so the polycrystalline SiC layer was not evaluated.

Claims (7)

A SiC composite substrate comprising: a polycrystalline SiC layer; and a single-crystal SiC layer having a thickness of 1.0 to 30 μm on the polycrystalline SiC layer , the polycrystalline SiC layer further containing a rare earth element . The SiC composite substrate according to claim 1, wherein the thickness of the single crystal SiC layer is 2.0 to 30 μm. The SiC composite substrate according to claim 1 or 2, wherein the polycrystalline SiC layer has a thickness of 100 to 5000 μm. The SiC composite substrate according to any one of claims 1 to 3, wherein the polytype of the single crystal SiC layer is 3C. The SiC composite substrate according to any one of claims 1 to 4 , wherein the rare earth element is Y and/or Sm. The SiC composite substrate according to any one of claims 1 to 5 , wherein the rare earth element is Y. The SiC composite substrate according to any one of claims 1 to 6 , wherein the weight ratio of the content of the rare earth element to the total content of the rare earth element, Si and C in the polycrystalline SiC layer is 0.00020 to 0.20.

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