4H-SiC single crystal, SiC epitaxial wafer, and SiC devices

Abstract

The SiC substrate of the present invention contains 3×10 14 cm ‑3 Above and 1×10 15 cm ‑3 Tantalum or niobium below, and 1×10 16 cm ‑3 Above and 1×10 20 cm ‑3 The following nitrogen.

Description

[0001] This application is a divisional application of the Chinese patent application with the application date of December 7, 2020, application number 202011414844.2, and invention name “Method for manufacturing SiC substrate and SiC single crystal”. Technical Field

[0002] The present invention relates to a SiC substrate and a method for manufacturing a SiC single crystal.

[0003] This application claims priority based on patent application No. 2019-222344 filed in Japan on December 9, 2019, the contents of which are incorporated herein by reference. Background Art

[0004] Compared with silicon (Si), silicon carbide (SiC) has a dielectric breakdown electric field that is one digit larger and a band gap that is three times larger. In addition, silicon carbide (SiC) has a thermal conductivity that is about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be used in power devices, high-frequency devices, high-temperature working devices, etc. Therefore, in recent years, SiC epitaxial wafers have been used in semiconductor devices such as the above.

[0005] The SiC epitaxial wafer is obtained by stacking an epitaxial layer on the surface of a SiC substrate cut from a SiC ingot. Hereinafter, the substrate before the epitaxial layer is stacked is referred to as a SiC substrate, and the substrate after the epitaxial layer is stacked is referred to as a SiC epitaxial wafer.

[0006] In recent years, along with market demands, there is a need for higher quality SiC substrates and improved production efficiency. For example, Patent Document 1 describes a silicon carbide single crystal containing metal elements other than light metals with an atomic radius larger than silicon. By adding metal elements other than light metals with an atomic radius larger than silicon, the lattice strain in the crystal is eliminated, and the quality of the silicon carbide single crystal is improved.

[0007] Prior art literature

[0008] Patent Document 1: Japanese Patent No. 3876628 Summary of the invention

[0009] However, even if the metal elements in the range described in Patent Document 1 are added, carbonization and polymorphism may occur in the SiC substrate. Here, carbonization refers to a state in which Si and C are not combined and only C is precipitated, and polymorphism refers to a state in which crystals with different polymorphs exist in a mixed state. SiC has polymorphs such as 3C-SiC, 4H-SiC, 6H-SiC, and 15R-SiC.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing a SiC substrate and a SiC single crystal having high purity and little carbonization and polymorphism.

[0011] The present inventors have found that when making a SiC ingot as a base of a SiC substrate, a large change in the C / Si ratio of the sublimation gas is one of the causes of carbonization and polymorphism. In other words, it has been found that in order to obtain a SiC substrate with high purity and little carbonization and polymorphism, it is not enough to simply eliminate the lattice strain as described in Patent Document 1, and more stringent control is required. That is, in order to solve the above-mentioned problems, the present invention provides the following means.

[0012] (1) A first embodiment of a SiC substrate comprising: 3×10 14 cm -3 Above and 1×10 15 cm -3 Tantalum or niobium, less than 1×10 16 cm -3 Above and 1×10 20 cm -3 The following nitrogen.

[0013] (2) According to the SiC substrate of the above embodiment, the content of aluminum can be less than 1×10 16 cm -3 .

[0014] (3) According to the SiC substrate of the above embodiment, the boron content can be less than 1×10 16 cm -3 .

[0015] (4) According to the SiC substrate of the above embodiment, the content of heavy metal elements can be less than 1×10 14 cm -3 .

[0016] (5) In the SiC substrate of the above aspect, the concentration of tantalum or niobium may be different on the first surface and the second surface perpendicular to the thickness direction.

[0017] (6) The SiC substrate of the above aspect may have a first surface and a second surface perpendicular to the thickness direction, and the content ratio decreases from the first surface to the second surface.

[0018] (7) A second embodiment of a method for producing a SiC single crystal employs a sublimation method in which a gas sublimated from a raw material containing tantalum or niobium is recrystallized on a surface of a seed crystal to grow a single crystal.

[0019] (8) In the SiC single crystal production method of the above aspect, the concentration of tantalum or niobium in the raw material may be substantially constant.

[0020] (9) In the SiC single crystal production method of the above aspect, the concentration of tantalum or niobium on the surface of the raw material facing the seed crystal may be lower than that in the interior of the raw material.

[0021] (10) In the SiC single crystal production method of the above aspect, the tantalum or niobium may be deposited on the surface of the SiC raw material.

[0022] (11) In the SiC single crystal production method of the above aspect, the concentration of tantalum or niobium on the surface of the raw material facing the seed crystal may be higher than that in the interior of the raw material.

[0023] (12) In the above-described method for producing a SiC single crystal, when the raw material is viewed from above, the concentration of tantalum or niobium may be higher in the center than in the outer sides.

[0024] (13) In the method for producing a SiC single crystal according to the above aspect, the amount of tantalum or niobium added may be 1 wt % or more and 8 wt % or less.

[0025] (14) The third method for producing a SiC single crystal adopts a sublimation method in which a gas sublimated from a raw material is recrystallized on a surface of a seed crystal to grow a single crystal, wherein powdered or plate-like tantalum or niobium is provided between the raw material and the seed crystal to sublime the raw material.

[0026] (15) The method for producing a SiC single crystal according to the above aspect uses a crucible having a protrusion protruding from the inner surface toward the axial center,

[0027] The powdered or plate-like tantalum or niobium may be placed on the protrusion, and the raw material may be sublimated.

[0028] The SiC substrate of the above-mentioned method has high purity and little carbonization and polymorphism. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic cross-sectional view of a manufacturing apparatus for explaining the method for manufacturing a SiC substrate according to the present embodiment.

[0030] Figure 2 It is a schematic cross-sectional view of a manufacturing apparatus for explaining a method for manufacturing a SiC substrate according to a modified example of the present embodiment.

[0031] Description of Reference Numerals

[0032] 10, 10' Crucible

[0033] 11 Inner surface

[0034] 12 Protrusion

[0035] 20 Coils

[0036] 100, 101 Manufacturing Equipment

[0037] C Single Crystal

[0038] G. Raw Materials

[0039] S Seed DETAILED DESCRIPTION

[0040] Hereinafter, the SiC substrate and the like of the present embodiment will be described in detail with reference to the accompanying drawings as appropriate. In order to facilitate understanding of the features of the present embodiment, the accompanying drawings used in the following description may sometimes enlarge the features for convenience, and the dimensional ratios of the components may sometimes be different from the actual ones. The materials, dimensions, etc. illustrated in the following description are only examples, and the present invention is not limited thereto, and may be implemented with appropriate changes within the scope of the subject matter.

[0041] The SiC substrate of this embodiment contains tantalum or niobium and nitrogen. Nitrogen is an n-type dopant. When the SiC substrate contains nitrogen, the resistance value of the SiC substrate is reduced. The nitrogen content is 1×10 16 cm -3 Above and 1×10 20 cm -3 The nitrogen content is preferably 1×10 17 cm -3 Above and 1×10 19 cm -3 Below, more preferably 1×10 18 cm -3 Above and 1×10 19 cm -3 When the nitrogen content is within this range, a SiC substrate having low resistance and excellent crystallinity can be obtained.

[0042] Tantalum or niobium in the SiC substrate exists in the SiC substrate in a simple substance or in a carbonized state. Tantalum and niobium both belong to Group 5, have similar outermost electron states, and both have the property of being easily combined with carbon. Tantalum and niobium play a role in adjusting the existence ratio of carbon during crystal growth.

[0043] The content ratio of tantalum or niobium is 3×10 14 cm -3 Above and 1×10 15 cm -3 The content ratio of tantalum or niobium is preferably 5×10 14 cm -3 Above and 8×10 14 cm -3 the following.

[0044] The content ratio of tantalum or niobium may be the same or different in the thickness direction. For example, the content ratio of tantalum or niobium may be different on the first surface and the second surface of the SiC substrate, and the content ratio may decrease from the first surface to the second surface. The first surface and the second surface of the SiC substrate are surfaces perpendicular to the thickness direction of the SiC substrate.

[0045] In the SiC substrate of this embodiment, the content of aluminum may be less than 1×10 16 cm -3 In addition, in the SiC substrate of this embodiment, the boron content may be less than 1×10 16 cm -3 Aluminum and boron act as acceptors in SiC substrates. Among n-type semiconductor substrates, SiC substrates with less aluminum and boron have high purity, which is beneficial for the production of SiC epitaxial wafers and SiC devices.

[0046] In addition, the SiC substrate of the present embodiment preferably contains less heavy metal elements. Examples of heavy metal elements include vanadium, tungsten, and neodymium. The content of heavy metal elements may be, for example, less than 1×10 14 cm -3 .

[0047] Next, a method for producing a SiC substrate according to the present embodiment will be described. The SiC substrate according to the present embodiment includes a step of growing a SiC ingot and a step of slicing the SiC ingot. The SiC substrate is obtained by slicing a SiC ingot.

[0048] Figure 1 It is a cross-sectional view of a manufacturing apparatus for explaining the method for manufacturing a SiC substrate according to the present embodiment. Figure 1 An example of a process for producing a SiC ingot using a sublimation method is shown. Figure 1 The SiC ingot manufacturing apparatus 100 shown in the figure includes a crucible 10 and a coil 20. A raw material G is stored in the inner bottom surface of the crucible 10, and a seed crystal S is provided at a position opposite to the raw material G. The raw material G is heated by induction heating, thereby sublimating the raw material G. The sublimated raw material G is recrystallized on the seed crystal S, thereby growing a single crystal C. The single crystal C is taken out from the crucible 10 to form a SiC ingot.

[0049] In the method for producing a SiC substrate of the present embodiment, tantalum or niobium is added to raw material G during the growth process of the SiC ingot. Raw material G is, for example, SiC powder. Tantalum or niobium is added, for example, as powder. The concentration of tantalum or niobium in raw material G is, for example, approximately constant. In this specification, the so-called approximately constant means that the raw material G is divided into 10 parts along the height direction from the inner bottom surface of the crucible 10, and the deviation of the concentration of tantalum or niobium in each area is within 10%. The particle size of the added tantalum or niobium is preferably within the range of the particle size distribution of the SiC powder. For example, the particle size of the SiC powder is greater than 0.01 mm and less than 2.0 mm, and the particle size of the tantalum or niobium is greater than 0.1 mm and less than 1.0 mm.

[0050] The amount of tantalum or niobium added is, for example, 1 wt % to 8 wt %, and preferably 2 wt % to 6 wt %.

[0051] Single crystal C is obtained by recrystallizing SiC sublimation gas. When the carbon composition ratio in the sublimation gas is high, part of the single crystal C is carbonized. In addition, when the silicon composition ratio in the sublimation gas is high, the crystal growth of 4H-SiC is unstable, and sometimes polymorphism and / or silicon droplets are generated. The composition ratio of silicon to carbon in the sublimation gas (C / Si ratio) is not always constant, but fluctuates. If the fluctuation is large, carbonization and polymorphism are easily generated in the single crystal C.

[0052] When tantalum or niobium is added to the raw material G, tantalum or niobium adjusts the composition ratio of silicon to carbon in the sublimation gas. For example, tantalum and niobium are combined with carbon to suppress the carbon composition ratio in the sublimation gas from becoming excessive.

[0053] By adding an appropriate amount of tantalum or niobium, the C / Si ratio can be adjusted to suppress the generation of carburization and polymorphism in single crystal C. If the amount of tantalum or niobium is excessive, the silicon composition ratio in the sublimation gas increases, and polymorphism increases. If the amount of tantalum or niobium is small, the carbon composition ratio in the sublimation gas increases, and carburization occurs. Tantalum or niobium combines with carbon and is doped into the SiC ingot in the form of tantalum carbide or niobium carbide.

[0054] According to the method for manufacturing SiC ingots of this embodiment, polymorphism and carbonization can be suppressed, and high-purity crystals can be easily manufactured. Since the SiC substrate can be manufactured by slicing the SiC ingot, the manufactured SiC substrate also has high purity. In addition, the manufactured SiC substrate has high purity, which is conducive to the manufacture of SiC epitaxial wafers and SiC devices.

[0055] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the scope of the claims.

[0056] For example, when tantalum or niobium is added to raw material G, the existence ratio of tantalum or niobium in the raw material may be different in the whole raw material G. For example, the concentration of tantalum or niobium on the surface of raw material G may be lower than that inside raw material G. The vapor pressure of silicon is higher than that of carbon, and silicon is easier to sublimate than carbon in the early stage of crystal growth. When the amount of tantalum or niobium sublimated in the early stage of crystal growth increases, the silicon composition ratio in the sublimation gas becomes higher, and polymorphism is easily generated. When the concentration of tantalum or niobium on the surface of raw material G is low, the amount of tantalum or niobium sublimated in the early stage of crystal growth becomes less. In the early stage of crystallization, the silicon composition ratio tends to become higher, and by reducing the amount of carbon adsorbed by tantalum or niobium, the generation of polymorphism can be further reduced. In this case, in the single crystal C grown by crystallization, the concentration of tantalum or niobium is lower on the first surface close to the seed crystal S than on the second surface opposite to the first surface.

[0057] In addition, for example, the concentration of tantalum or niobium on the surface of the raw material G can be made higher than that in the interior of the raw material G. For example, when the raw material G is divided into two parts in the height direction, tantalum or niobium can be added only to the upper layer on the seed crystal side. The presence of tantalum or niobium near the surface of the raw material G can improve the sublimation efficiency of tantalum or niobium.

[0058] In addition, for example, when the raw material G is viewed from above, the concentration of tantalum or niobium in the center can be made higher than that in the outside. SiC gas flows from the high temperature area to the low temperature area. Since the crucible 10 is heated from the outside, the temperature of the outside is higher than that of the center. That is, part of the sublimated SiC gas flows from the outside to the center. In addition, by allowing the sublimated SiC gas to pass through the area with a high concentration of tantalum or niobium, the C / Si ratio can be appropriately adjusted.

[0059] In addition, for example, the raw material G may be divided into a portion composed of SiC and a portion composed of tantalum or niobium.

[0060] For example, tantalum or niobium may be deposited on the surface of the SiC material. The presence of tantalum or niobium near the surface of the raw material G can improve the sublimation efficiency of tantalum or niobium. In addition, the C / Si ratio can be appropriately adjusted by the sublimated SiC gas passing through the layer composed of tantalum or niobium.

[0061] In addition, for example, tantalum or niobium may be filled in the central region, and SiC raw material may be filled in the peripheral region thereof when looking down at the raw material G. As described above, a part of the sublimated SiC gas flows from the outside to the central side, and therefore, by arranging tantalum or niobium at the position where the sublimated SiC gas passes, the C / Si ratio can be appropriately adjusted.

[0062] In addition, for example, tantalum or niobium may be provided in a portion other than the raw material G. Figure 2 FIG. 1 is a cross-sectional view of a SiC ingot manufacturing apparatus 101 according to a modified example, showing an example in which tantalum or niobium is provided in a portion other than the raw material G. FIG. Figure 2 An example of a manufacturing process of a SiC ingot using a sublimation method is shown. In the manufacturing device 101, the same reference numerals are given to the same structures as those of the manufacturing device 100, and their descriptions are omitted. The surface parallel to the axial direction of the crucible 10' of the manufacturing device 101 is called the inner surface 11. The manufacturing device 101 has a protrusion 12 protruding from the inner surface 11 toward the axial center of the crucible 10'.

[0063] For example, powdered or plate-like tantalum or niobium may be placed at any position between the raw material G and the seed crystal S in the height direction. For example, powdered or plate-like tantalum or niobium may be placed above the protrusion 12 protruding inward from the inner surface 11 of the crucible. Figure 2In the embodiment, the powdered or plate-like tantalum or niobium is represented by symbol G2. The protrusion 12 provided with tantalum or niobium may or may not have a portion overlapping with the seed crystal S in a plan view. By arranging tantalum or niobium at a position where the sublimated SiC gas passes, the C / Si ratio can be appropriately adjusted.

[0064] [Example]

[0065] (Example 1)

[0066] 3 wt% Ta powder was added to SiC raw material powder with Ta content below the detection limit and placed in a crucible. Then, a seed crystal was set at a position opposite to the raw material placed in the crucible, and a single crystal was grown by sublimation. The tantalum content of the obtained single crystal was 5×10 14 cm -3 The same experiment was repeated 20 times. Then, the single crystallization rate of Example 1 was determined. The single crystallization rate is the ratio of the number of experiments in which no heteropolymorphism occurs in the single crystal. The single crystallization rate of Example 1 was 95%.

[0067] (Example 2)

[0068] 1 wt% Ta powder was added to SiC raw material powder with Ta content below the detection limit and placed in a crucible. Then, a seed crystal was set at a position opposite to the raw material placed in the crucible, and a single crystal was grown by sublimation. The tantalum content of the obtained single crystal was 3×10 14 cm -3 The same experiment was repeated 20 times. Then, the single crystallization rate of Example 2 was determined. The single crystallization rate is the ratio of the number of experiments in which no heteropolymorphism occurs in the single crystal. The single crystallization rate of Example 2 was 90%.

[0069] (Example 3)

[0070] 8 wt% Ta powder was added to SiC raw material powder with Ta content below the detection limit and placed in a crucible. Then, a seed crystal was set at a position opposite to the raw material placed in the crucible, and a single crystal was grown by sublimation. The tantalum content in the obtained single crystal was 1×10 15 cm -3 The same experiment was repeated 20 times. Then, the single crystallization rate of Example 3 was determined. The single crystallization rate is the ratio of the number of experiments in which no heteromorphism occurs in the single crystal. The single crystallization rate of Example 3 was 90%.

[0071] (Comparative Example 1)

[0072] This is different from Example 1 in that the mass ratio of Ta powder added to the SiC raw material powder having a Ta content below the detection limit was set to 10 wt %. The tantalum content in the obtained single crystal was 2×10 15 cm-3 The same experiment was repeated 20 times. Then, the single crystallization rate of Comparative Example 1 was determined. The single crystallization rate is the ratio of the number of experiments in which no heteropolymorphism occurs in the single crystal. The single crystallization rate of Comparative Example 1 was 80%.

[0073] (Comparative Example 2)

[0074] This is different from Example 1 in that the mass ratio of Ta powder added to the SiC raw material powder having a Ta content below the detection limit was set to 0.5 wt %. The tantalum content in the obtained single crystal was 1×10 14 cm -3 The same experiment was repeated 20 times. Then, the single crystallization rate of Comparative Example 2 was determined. The single crystallization rate is the ratio of the number of experiments in which no heterogeneous polymorphism occurs in the single crystal. The single crystallization rate of Comparative Example 2 was 80%.

Claims

1. A 4H-SiC single crystal, comprising: 3×10 14 cm -3 Above and 1×10 15 cm -3 Tantalum, as follows; and 1×10 16 cm -3 Above and 1×10 20 cm -3 The following nitrogen, The aluminum content is less than 1×10 16 cm -3 , The boron content is less than 1×10 16 cm -3 , The content of vanadium, tungsten and neodymium is less than 1×10 14 cm -3 . 2 . The 4H—SiC single crystal according to claim 1 , wherein the SiC single crystal is an ingot.

3. A SiC epitaxial wafer comprising a 4H-SiC substrate and a SiC epitaxial layer stacked on the 4H-SiC substrate, The 4H-SiC substrate contains 3×10 14 cm -3 Above and 1×10 15 cm -3 The following tantalum, and 1×10 16 cm -3 Above and 1×10 20 cm -3 The nitrogen content is less than 1×10 16 cm -3 , the boron content is less than 1×10 16 cm -3 , the content of vanadium, tungsten and neodymium is less than 1×10 14 cm -3 . 4 . The SiC epitaxial wafer according to claim 3 , wherein the concentration of tantalum is different between the front surface and the back surface of the 4H-SiC substrate.

5. A SiC device comprising a 4H-SiC substrate and a SiC epitaxial layer stacked on the 4H-SiC substrate, The 4H-SiC substrate contains 3×10 14 cm -3 Above and 1×10 15 cm -3 The following tantalum, and 1×10 16 cm -3 Above and 1×10 20 cm -3 The nitrogen content is less than 1×10 16 cm -3 , the boron content is less than 1×10 16 cm -3 , the content of vanadium, tungsten and neodymium is less than 1×10 14 cm -3 .

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