Silicon carbide epitaxial substrate

Abstract

To provide a silicon carbide epitaxial substrate that can reduce surface density of triangular defects.SOLUTION: A silicon carbide epitaxial layer lies on a silicon carbide substrate. The silicon carbide epitaxial layer has a buffer layer in contact with the silicon carbide substrate, and a drift layer on the buffer layer. The drift layer has a thickness of 2 μm or more. The drift layer has a first principal face in contact with the buffer layer, and a second principal face opposite the first principal face. When carrier density of the drift layer is measured by a capacitance-voltage technique, second carrier density is divided by first carrier density to give a value of 1.3 or more where the first carrier density is one at a position 0.7 μm away from the second principal face to the first principal face and the second carrier density is one at a position 1.2 μm away from the second principal face to the first principal face.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to silicon carbide epitaxial substrates. [Background technology]

[0002] WO 2018 / 096684 (Patent Document 1) describes a silicon carbide semiconductor wafer having a carrier concentration transition layer, which has a carrier concentration gradient in the thickness direction. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 096684 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to provide a silicon carbide epitaxial substrate capable of reducing the areal density of triangular defects. [Means for solving the problem]

[0005] A silicon carbide epitaxial substrate according to the present disclosure includes a silicon carbide substrate and a silicon carbide epitaxial layer. The silicon carbide epitaxial layer is on the silicon carbide substrate. The silicon carbide epitaxial layer includes a buffer layer in contact with the silicon carbide substrate and a drift layer on the buffer layer. The drift layer has a thickness of 2 μm or more. The drift layer has a first main surface in contact with the buffer layer and a second main surface opposite the first main surface. When the carrier concentration of the drift layer is measured by a capacitance-voltage method, the carrier concentration at a position 0.7 μm away from the second main surface toward the first main surface is defined as the first carrier concentration, and the carrier concentration at a position 1.2 μm away from the second main surface toward the first main surface is defined as the second carrier concentration, the value obtained by dividing the second carrier concentration by the first carrier concentration is 1.3 or more. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide a silicon carbide epitaxial substrate capable of reducing the areal density of triangular defects. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a plan view schematically illustrating the configuration of a silicon carbide epitaxial substrate. [Figure 2] FIG. 2 is a schematic cross-sectional view taken along line II-II in FIG. [Figure 3] FIG. 3 is a schematic diagram showing the relationship between the carrier concentration in the drift layer and the position in the thickness direction. [Figure 4] FIG. 4 is a partial cross-sectional schematic diagram showing the configuration of an apparatus for measuring the carrier concentration of a silicon carbide epitaxial substrate according to this embodiment. [Figure 5] FIG. 5 is an enlarged schematic view of region V in FIG. [Figure 6] FIG. 6 is a schematic cross-sectional view taken along line VI-VI in FIG. [Figure 7] FIG. 7 is an enlarged schematic view of region VII in FIG. [Figure 8] FIG. 8 is a schematic cross-sectional view taken along line VIII-VIII in FIG. [Figure 9] FIG. 9 is a cross-sectional view schematically illustrating the configuration of an apparatus for manufacturing a silicon carbide epitaxial substrate according to this embodiment. [Figure 10] FIG. 10 is a schematic diagram showing the relationship between the flow rate of ammonia gas and time. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Summary of the embodiments of the present disclosure] First, an overview of the embodiments of the present disclosure will be described. In the crystallographic descriptions in this specification, individual orientations are represented by [ ], collective orientations by < >, individual planes by ( ), and collective planes by {}. A negative crystallographic index is usually represented by placing a "-" (bar) above the number, but in this specification, a negative crystallographic index is represented by placing a negative sign before the number.

[0009] (1) A silicon carbide epitaxial substrate 10 according to the present disclosure includes a silicon carbide substrate 11 and a silicon carbide epitaxial layer 12. The silicon carbide epitaxial layer 12 is on the silicon carbide substrate 11. The silicon carbide epitaxial layer 12 includes a buffer layer 21 in contact with the silicon carbide substrate 11 and a drift layer 22 on the buffer layer 21. The drift layer 22 has a thickness of 2 μm or more. The drift layer 22 has a first main surface 1 in contact with the buffer layer 21 and a second main surface 2 on the opposite side of the first main surface 1. When the carrier concentration of drift layer 22 is measured by the capacitance-voltage method, if the carrier concentration at a position 0.7 μm away from second main surface 2 toward first main surface 1 is defined as first carrier concentration N1 and the carrier concentration at a position 1.2 μm away from second main surface 2 toward first main surface 1 is defined as second carrier concentration N2, the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 is 1.3 or more.

[0010] (2) In silicon carbide epitaxial substrate 10 according to (1) above, the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 may be 4.5 or less.

[0011] (3) In silicon carbide epitaxial substrate 10 according to (1) or (2) above, if the carrier concentration at a position 1.1 μm away from second main surface 2 toward first main surface 1 is defined as third carrier concentration N3, the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 may be 1.1 or more.

[0012] (4) In silicon carbide epitaxial substrate 10 according to (3) above, the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 may be 2.5 or less.

[0013] [Details of the embodiments of the present disclosure] Hereinafter, the details of the embodiments of the present disclosure will be described. In the following description, the same or corresponding elements will be denoted by the same reference numerals, and the same description thereof will not be repeated.

[0014] First, the configuration of silicon carbide epitaxial substrate 10 according to this embodiment will be described. Fig. 1 is a plan view schematically showing the configuration of silicon carbide epitaxial substrate 10. Fig. 2 is a cross-sectional view schematically showing the configuration of silicon carbide epitaxial substrate 10 taken along line II-II in Fig. 1.

[0015] As shown in FIGS. 1 and 2 , a silicon carbide epitaxial substrate 10 according to this embodiment has a silicon carbide substrate 11 and a silicon carbide epitaxial layer 12. The silicon carbide epitaxial layer 12 is on the silicon carbide substrate 11. The silicon carbide epitaxial layer 12 includes a buffer layer 21 and a drift layer 22. The buffer layer 21 is in contact with the silicon carbide substrate 11. The drift layer 22 is on the buffer layer 21. The drift layer 22 has a first main surface 1 and a second main surface 2. The first main surface 1 is in contact with the buffer layer 21. The second main surface 2 is on the opposite side to the first main surface 1. The second main surface 2 constitutes the surface of the silicon carbide epitaxial substrate 10.

[0016] Silicon carbide substrate 11 has a third main surface 3 and a fourth main surface 4. Fourth main surface 4 is on the opposite side to third main surface 3. Third main surface 3 is in contact with silicon carbide epitaxial layer 12. Third main surface 3 is in contact with buffer layer 21. Third main surface 3 is spaced from drift layer 22. Fourth main surface 4 forms the back surface of silicon carbide epitaxial substrate 10. Fourth main surface 4 is spaced from silicon carbide epitaxial layer 12. The polytype of silicon carbide forming silicon carbide substrate 11 is, for example, 4H. The polytype of silicon carbide forming each of buffer layer 21 and drift layer 22 is, for example, 4H.

[0017] As shown in FIG. 1 , when viewed in the thickness direction of silicon carbide epitaxial substrate 10, silicon carbide epitaxial substrate 10 has an outer peripheral edge 15. Outer peripheral edge 15 has, for example, orientation flat 13 and an arc-shaped portion 14. Orientation flat 13 extends along first direction 101. As shown in FIG. 1 , when viewed in the thickness direction of silicon carbide epitaxial substrate 10, orientation flat 13 is linear. Arc-shaped portion 14 is continuous with orientation flat 13. When viewed in the thickness direction of silicon carbide epitaxial substrate 10, arc-shaped portion 14 is arc-shaped.

[0018] 1, when viewed in the thickness direction of silicon carbide epitaxial substrate 10, second main surface 2 extends along first direction 101 and second direction 102. When viewed in the thickness direction of silicon carbide epitaxial substrate 10, first direction 101 is a direction perpendicular to second direction 102.

[0019] The first direction 101 is, for example, the <11-20> direction. The first direction 101 may be, for example, the [11-20] direction. The first direction 101 may be a direction obtained by projecting the <11-20> direction onto the first main surface 1. From another perspective, the first direction 101 may be, for example, a direction including a <11-20> direction component.

[0020] The second direction 102 is, for example, the <1-100> direction. The second direction 102 may be, for example, the [1-100] direction. The second direction 102 may be, for example, a direction obtained by projecting the <1-100> direction onto the first main surface 1. From another perspective, the second direction 102 may be, for example, a direction including a <1-100> direction component.

[0021] As shown in FIG. 1, the diameter (first width W1) of silicon carbide epitaxial substrate 10 is not particularly limited, but is, for example, 100 mm (4 inches) or more. First width W1 may be 125 mm (5 inches) or more, or 150 mm (6 inches) or more. There is no particular upper limit to first width W1. First width W1 may be, for example, 200 mm (8 inches) or less.

[0022] In this specification, 4 inches refers to 100 mm or 101.6 mm (4 inches x 25.4 mm / inch). 5 inches refers to 125 mm or 127.0 mm (5 inches x 25.4 mm / inch). 6 inches refers to 150 mm or 152.4 mm (6 inches x 25.4 mm / inch). 8 inches refers to 200 mm or 203.2 mm (8 inches x 25.4 mm / inch).

[0023] Second main surface 2 of silicon carbide epitaxial substrate 10 may be inclined, for example, at an off angle of 8° or less with respect to the {0001} plane. Specifically, second main surface 2 may be inclined at an off angle of 8° or less with respect to the (0001) plane or the (0001) plane. First main surface 1 may be inclined at an off angle of 8° or less with respect to the (000-1) plane or the (000-1) plane.

[0024] The upper limit of the off angle is not particularly limited, but may be, for example, 6° or less, or 4° or less. The lower limit of the off angle is not particularly limited, but may be, for example, 2° or more, or 1° or more. The off direction is not particularly limited, but may be, for example, the <11-20> direction. <0001> It may be a direction.

[0025] The silicon carbide substrate 11 contains n-type impurities such as nitrogen (N). The conductivity type of the silicon carbide substrate 11 is, for example, n-type (first conductivity type). In this case, the carriers are electrons. The carrier concentration of the silicon carbide substrate 11 is, for example, 1×10 19 m -3 More than 1×10 20 cm -3The thickness (fourth thickness T4) of silicon carbide substrate 11 is not particularly limited, but is, for example, not less than 200 μm and not more than 500 μm.

[0026] The buffer layer 21 contains n-type impurities such as nitrogen (N). The conductivity type of the buffer layer 21 is, for example, n-type (first conductivity type). In this case, the carriers are electrons. The carrier concentration of the buffer layer 21 is, for example, 1×10 18 cm -3 More than 1×10 19 cm -3 The thickness (first thickness T1) of buffer layer 21 is not particularly limited, but is, for example, not less than 0.1 μm and not more than 10 μm.

[0027] The drift layer 22 contains n-type impurities such as nitrogen (N). The conductivity type of the drift layer 22 is, for example, n-type (first conductivity type). In this case, the carriers are electrons. The carrier concentration of the drift layer 22 is, for example, 1×10 14 cm -3 More than 1×10 17 cm -3 The average carrier concentration of drift layer 22 may be lower than the average carrier concentration of buffer layer 21. The average carrier concentration is the average value of the carrier concentration in the thickness direction of silicon carbide epitaxial layer 12. In the thickness direction of silicon carbide epitaxial layer 12, the interval between the measurement positions of the carrier concentration is, for example, 0.1 μm.

[0028] The thickness of drift layer 22 (second thickness T2) is 2 μm or more. The lower limit of second thickness T2 is not particularly limited, but may be, for example, 4 μm or more, or 6 μm or more. The upper limit of second thickness T2 is not particularly limited, but may be, for example, 100 μm or less, or 50 μm or less.

[0029] 3 is a schematic diagram showing the relationship between the carrier concentration and the position in the thickness direction in drift layer 22. In Fig. 3, the vertical axis represents the carrier concentration in drift layer 22. In Fig. 3, the horizontal axis represents the position in the thickness direction of drift layer 22. In the thickness direction of silicon carbide epitaxial layer 12, fifth position P5 corresponds to second main surface 2.

[0030] The carrier concentration at a position (first position P1) that is 0.7 μm away from the second main surface 2 toward the first main surface 1 is a first carrier concentration N1. The carrier concentration at a position (second position P2) that is 1.2 μm away from the second main surface 2 toward the first main surface 1 is a second carrier concentration N2. The carrier concentration at a position (third position P3) that is 1.1 μm away from the second main surface 2 toward the first main surface 1 is a third carrier concentration N3. In the thickness direction of the drift layer 22, the first position P1 is between the fifth position P5 and the third position P3. In the thickness direction of the drift layer 22, the third position P3 is between the first position P1 and the second position P2.

[0031] 3, the carrier concentration of the drift layer 22 increases, for example, monotonically from the first position P1 toward the third position P3. The carrier concentration of the drift layer 22 increases, for example, monotonically from the third position P3 toward the second position P2. The third carrier concentration N3 is lower than the second carrier concentration N2. The first carrier concentration N1 is lower than the third carrier concentration N3. The third carrier concentration N3 is a concentration between the first carrier concentration N1 and the second carrier concentration N2.

[0032] The value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 is 1.3 or more. The lower limit of the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 is not particularly limited, but may be, for example, 1.5 or more, or 1.7 or more. The value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 may be, for example, 4.5 or less. The lower limit of the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 is not particularly limited, but may be, for example, 4.0 or less, or 3.5 or less.

[0033] The value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 may be, for example, 1.1 or more. The lower limit of the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 is not particularly limited, but may be, for example, 1.2 or more, or 1.3 or more. The value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 may be, for example, 2.5 or less. The upper limit of the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 is not particularly limited, but may be, for example, 2.4 or less, or 2.3 or less.

[0034] The carrier concentration at a position (fourth position P4) 0.9 μm away from second main surface 2 toward first main surface 1 is a fourth carrier concentration N4. In the thickness direction of drift layer 22, fourth position P4 is located between first position P1 and third position P3. Between first position P1 and fourth position P4, the carrier concentration of drift layer 22 hardly changes. Fourth carrier concentration N4 is approximately the same as first carrier concentration N1.

[0035] Specifically, the value obtained by dividing the fourth carrier concentration N4 by the first carrier concentration N1 is, for example, 0.95 or more and 1.05 or less. The value obtained by dividing the fourth carrier concentration N4 by the first carrier concentration N1 may be, for example, 0.97 or more and 1.03 or less. The fourth carrier concentration N4 may be higher or lower than the first carrier concentration N1. Between the first position P1 and the fifth position P5, the carrier concentration is approximately constant.

[0036] Next, a method for measuring the carrier concentration of silicon carbide epitaxial substrate 10 according to this embodiment will be described. Fig. 4 is a partial cross-sectional schematic view showing a method for measuring the carrier concentration of silicon carbide epitaxial substrate 10 according to this embodiment.

[0037] The carrier concentration of silicon carbide epitaxial substrate 10 can be measured using, for example, a CV (capacitance-voltage) measurement device (model number: CVmap92A) manufactured by Four Dimensions. As shown in FIG. 4 , mercury probe 110 is disposed so as to face second main surface 2 of silicon carbide epitaxial substrate 10. Mercury inside mercury probe 110 is sucked up by vacuum suction, bringing the mercury into contact with second main surface 2. The measurement diameter of mercury probe 110 is approximately 1.2 mm. The measurement speed is approximately 1 minute per point. Electrode 111 is connected to fourth main surface 4 of silicon carbide epitaxial substrate 10, for example. A voltage is applied to silicon carbide epitaxial substrate 10 using mercury probe 110 and electrode 111. This forms a depletion layer in drift layer 22. Next, the capacitance between mercury probe 110 and electrode 111 is measured.

[0038] Next, based on the capacitance (C) and voltage (V), the carrier concentration of the drift layer 22 is estimated by calculation. The carrier concentration of the drift layer 22 is found using the following Equation 1.

[0039] 1 / C 2 ={2 / (e×ε0×ε S )}×(VV d )×N d (Equation 1) C: Capacitance (F) measured by mercury probe 110 e: Elementary charge = 1.602×10 -19 (C) ε S : relative dielectric constant of drift layer 22 ε0: Dielectric constant of vacuum = 8.854 × 10 -14 (F / cm) V: voltage (V) applied to silicon carbide epitaxial substrate 10 V d :Reference potential (V) N d : Carrier concentration (cm) of the drift layer 22 -3 ) As described above, the carrier concentration of the drift layer 22 is measured by the contact capacitance-voltage method (CV method). When the carrier concentration of the drift layer 22 is measured by the capacitance-voltage method, the carrier concentration at a position 0.7 μm away from the second main surface 2 toward the first main surface 1 is defined as the first carrier concentration N1, and the carrier concentration at a position 1.2 μm away from the second main surface 2 toward the first main surface 1 is defined as the second carrier concentration N2. The value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 is 1.3 or more. The carrier concentration of the drift layer 22 may also be measured using the non-contact capacitance-voltage method. An example of a non-contact capacitance-voltage device is a CV (capacitance-voltage) measurement device (model number: FAaST210) manufactured by SEMILAB Corporation.

[0040] 5 is an enlarged schematic diagram of region V in FIG. 1. As shown in FIG. 5, the second main surface 2 may have a first triangular defect 30. When viewed in a direction perpendicular to the second main surface 2, the first triangular defect 30 has a triangular outer shape. When viewed in a direction perpendicular to the second main surface 2, the length of the first triangular defect 30 in a first direction 101 is a first length L1. When viewed in a direction perpendicular to the second main surface 2, the length of the first triangular defect 30 in a second direction 102 is a second length L2. The second length L2 is not particularly limited, and may be, for example, 0.5 to 2 times the first length L1.

[0041] Fig. 6 is a schematic cross-sectional view taken along line VI-VI in Fig. 5. As shown in Fig. 6, first triangular defect 30 may have a first portion 31, a second portion 32, and a first starting point 33. First starting point 33 is located on third main surface 3, which is the boundary between silicon carbide substrate 11 and silicon carbide epitaxial layer 12. First triangular defect 30 includes, for example, a stacking fault.

[0042] The first portion 31 extends continuously from the first starting point 33 to the second main surface 2. The first portion 31 is continuous with both the first starting point 33 and the second main surface 2. The first portion 31 extends along the basal plane. The first portion 31 may be, for example, a basal plane dislocation. The second portion 32 extends continuously from the first starting point 33 to the second main surface 2. The second portion 32 is continuous with both the first starting point 33 and the second main surface 2. The second portion 32 may extend along a direction perpendicular to the second main surface 2.

[0043] If the thickness of silicon carbide epitaxial layer 12 at the center of second main surface 2 is a third thickness T3, then the length of first triangular defect 30 in first direction 101 (first length L1) is ideally (T3 / tan θ). The thickness of silicon carbide epitaxial layer 12 varies in the in-plane direction. In silicon carbide epitaxial substrate 10 of this embodiment, a triangular defect whose first length L1 is equal to or greater than 0.9 × (T3 / tan θ) and equal to or less than 1.1 × (T3 / tan θ) is defined as a first triangular defect 30.

[0044] The surface density of the first triangular defects 30 on the second main surface 2 is, for example, 0.3 defects / cm 2 The upper limit of the surface density of the first triangular defects 30 on the second main surface 2 is not particularly limited, but may be, for example, 0.2 defects / cm 2 May be less than 0.1 pieces / cm 2 The lower limit of the surface density of the first triangular defects 30 on the second main surface 2 is not particularly limited, but may be, for example, 0.005 defects / cm 2 It may be more than 0.01 pieces / cm 2 It may be more than that.

[0045] 7 is an enlarged schematic diagram of region VII in FIG. 1. As shown in FIG. 7, the second main surface 2 may have a second triangular defect 40. When viewed in a direction perpendicular to the second main surface 2, the outer shape of the second triangular defect 40 is a triangle. When viewed in a direction perpendicular to the second main surface 2, the length of the second triangular defect 40 in the first direction 101 is a third length L3. When viewed in a direction perpendicular to the second main surface 2, the length of the second triangular defect 40 in the second direction 102 is a fourth length L4. The fourth length L4 is not particularly limited, and may be, for example, 0.5 to 2 times the third length L3.

[0046] When viewed in the thickness direction of silicon carbide epitaxial substrate 10, the area of ​​second triangular defect 40 is smaller than the area of ​​first triangular defect 30. Specifically, third length L3 is smaller than first length L1. Fourth length L4 is smaller than second length L2.

[0047] Fig. 8 is a schematic cross-sectional view taken along line VIII-VIII in Fig. 7. As shown in Fig. 8, second triangular defect 40 may have a third portion 41, a fourth portion 42, and a second starting point 43. Second starting point 43 is spaced apart from third main surface 3, which is the boundary between silicon carbide substrate 11 and silicon carbide epitaxial layer 12. Second starting point 43 is located in silicon carbide epitaxial layer 12. Second starting point 43 may be located in drift layer 22 or buffer layer 21.

[0048] The third portion 41 extends continuously from the second starting point 43 to the second main surface 2. The third portion 41 is continuous with both the second starting point 43 and the second main surface 2. The third portion 41 extends along the basal plane. The third portion 41 may be, for example, a basal plane dislocation. The fourth portion 42 extends continuously from the second starting point 43 to the second main surface 2. The fourth portion 42 is continuous with both the second starting point 43 and the second main surface 2. The fourth portion 42 may extend along a direction perpendicular to the second main surface 2.

[0049] If the thickness of second defect 40 is defined as a fifth thickness T5, the length of second triangular defect 40 in first direction 101 (third length L3) is (T5 / tan θ). If the thickness of silicon carbide epitaxial layer 12 at the center of second main surface 2 is defined as a third thickness T3, (T5 / tan θ) is smaller than (T3 / tan θ). In silicon carbide epitaxial substrate 10 of the present embodiment, a triangular defect whose third length L3 is less than 0.9 × (T3 / tan θ) is defined as a second triangular defect 40.

[0050] The surface density of the second triangular defects 40 on the second main surface 2 is, for example, 0.08 defects / cm 2 The upper limit of the surface density of the second triangular defects 40 on the second main surface 2 is not particularly limited, but is, for example, 0.04 defects / cm 2 May be less than 0.02 pieces / cm 2 The lower limit of the surface density of the second triangular defects 40 on the second main surface 2 is not particularly limited, but may be, for example, 0.001 defects / cm 2 It may be more than 0.005 pieces / cm 2 It may be more than that.

[0051] 9 is a cross-sectional view schematically illustrating the configuration of an apparatus for manufacturing silicon carbide epitaxial substrate 10 according to this embodiment. As shown in FIG. 9, the apparatus for manufacturing silicon carbide epitaxial substrate 10 is, for example, a hot-wall horizontal CVD (Chemical Vapor Deposition) apparatus. The apparatus 200 for manufacturing silicon carbide epitaxial substrate 10 mainly includes a chamber 201, a gas supply unit 235, a control unit 245, a heating element 203, a quartz tube 204, a heat insulating material (not shown), and an induction heating coil (not shown).

[0052] The heating element 203 has, for example, a cylindrical shape, and defines a chamber 201 therein. The heating element 203 is made of, for example, graphite. The heating element 203 is provided inside a quartz tube 204. A heat insulating material surrounds the outer periphery of the heating element 203. The induction heating coil is wound around, for example, the outer periphery of the quartz tube 204. The induction heating coil is configured so that an alternating current can be supplied to it from an external power source (not shown). This causes the heating element 203 to be induction heated. As a result, the chamber 201 is heated by the heating element 203.

[0053] The chamber 201 is formed by being surrounded by an inner wall surface 205 of a heating element 203. A susceptor 210 that holds a silicon carbide substrate 11 is provided in the chamber 201. The susceptor 210 is made of, for example, silicon carbide. The silicon carbide substrate 11 is placed on the susceptor 210. The susceptor 210 is placed on a stage 206. The stage 206 is rotatably supported by a rotation shaft 209. When the stage 206 rotates, the susceptor 210 rotates.

[0054] Manufacturing apparatus 200 for silicon carbide epitaxial substrate 10 further includes gas inlet 207 and gas outlet 208. Gas outlet 208 is connected to an exhaust pump (not shown). Arrows in FIG. 9 indicate the flow of gas. Gas is introduced into chamber 201 from gas inlet 207 and exhausted from gas exhaust outlet 208. The pressure within chamber 201 is adjusted by balancing the amount of gas supplied and the amount of gas exhausted.

[0055] The gas supply unit 235 is configured to be able to supply a mixed gas containing a source gas, a dopant gas, and a carrier gas to the chamber 201. Specifically, the gas supply unit 235 includes, for example, a first gas supply unit 231, a second gas supply unit 232, a third gas supply unit 233, and a fourth gas supply unit 234.

[0056] The first gas supply unit 231 is configured to be able to supply a first gas containing, for example, carbon atoms. The first gas supply unit 231 is, for example, a gas cylinder filled with the first gas. The first gas is, for example, propane (C3H8) gas. The first gas may also be, for example, methane (CH4) gas, ethane (C2H6) gas, acetylene (C2H2) gas, etc.

[0057] The second gas supply unit 232 is configured to be able to supply a second gas containing, for example, silane gas. The second gas supply unit 232 is, for example, a gas cylinder filled with the second gas. The second gas is, for example, silane (SiH4) gas. The second gas may be a mixed gas of silane gas and a gas other than silane.

[0058] The third gas supply unit 233 is configured to be able to supply a third gas containing, for example, ammonia gas. The third gas supply unit 233 is, for example, a gas cylinder filled with the third gas. The third gas is a doping gas containing N (nitrogen atoms). Ammonia gas is more susceptible to thermal decomposition than nitrogen gas, which has a triple bond. The third gas may be nitrogen gas.

[0059] The fourth gas supply unit 234 is configured to be able to supply a fourth gas (carrier gas) such as hydrogen, etc. The fourth gas supply unit 234 is, for example, a gas cylinder filled with hydrogen.

[0060] The control unit 245 is configured to be able to control the flow rate of the mixed gas supplied from the gas supply unit 235 to the chamber 201. Specifically, the control unit 245 may include a first gas flow rate control unit 241, a second gas flow rate control unit 242, a third gas flow rate control unit 243, and a fourth gas flow rate control unit 244. Each control unit may be, for example, an MFC (Mass Flow Controller). The control unit 245 is disposed between the gas supply unit 235 and the gas inlet 207. In other words, the control unit 245 is disposed in a flow path connecting the gas supply unit 235 and the gas inlet 207.

[0061] (Method for manufacturing silicon carbide epitaxial substrate) Next, a method for manufacturing silicon carbide epitaxial substrate 10 according to this embodiment will be described.

[0062] First, a silicon carbide substrate 11 is prepared. The silicon carbide substrate 11 is obtained by slicing a silicon carbide ingot manufactured by, for example, sublimation. The silicon carbide substrate 11 is made of, for example, silicon carbide of polytype 4H. The silicon carbide substrate 11 contains, for example, nitrogen (N) as an n-type impurity. The conductivity type of the silicon carbide substrate 11 is, for example, n-type (first conductivity type).

[0063] Next, silicon carbide substrate 11 is placed in chamber 201. Silicon carbide substrate 11 is placed on susceptor 210 in chamber 201. Next, chamber 201 is heated to a temperature of, for example, 1600°C or higher and 1700°C or lower. Next, a mixed gas containing, for example, silane, propane, ammonia, and hydrogen is introduced into chamber 201. As a result, silicon carbide epitaxial layer 12 is formed on silicon carbide substrate 11.

[0064] FIG. 10 is a schematic diagram showing the relationship between the flow rate of ammonia gas and time. As shown in FIG. 10, a buffer layer 21 is formed on a silicon carbide substrate 11 between a first point in time C1 and a second point in time C2. Specifically, a mixed gas containing silane, propane, ammonia, and hydrogen is introduced into a chamber 201, thereby forming the buffer layer 21 on the silicon carbide substrate 11. In the step of forming the buffer layer 21, the flow rate of the silane gas is adjusted to, for example, 46 sccm. The flow rate of the propane gas is adjusted to, for example, 14 sccm. The flow rate of the hydrogen gas is adjusted to, for example, 120 slm. Between the first point in time C1 and the second point in time C2, the flow rate of the ammonia gas is adjusted to a first flow rate B1. The first flow rate B1 is, for example, 0.7 sccm.

[0065] Next, drift layer 22 is formed on buffer layer 21. Drift layer 22 is formed between second point C2 and fifth point C5. In the process of forming drift layer 22, the flow rate of silane gas is adjusted to, for example, 115 sccm. The flow rate of propane gas is adjusted to, for example, 37.5 sccm. The flow rate of hydrogen gas is adjusted to, for example, 120 slm.

[0066] At the second point in time C2, the flow rate of the ammonia gas introduced into the chamber changes from the first flow rate B1 to the second flow rate B2. The second flow rate B2 is, for example, 0.23 sccm. From the second point in time C2 to the third point in time C3, the flow rate of the ammonia gas introduced into the chamber is maintained at the second flow rate B2. Next, the flow rate of the ammonia gas introduced into the chamber gradually decreases. Specifically, from the third point in time C3 to the fourth point in time C4, the flow rate of the ammonia gas changes from the second flow rate B2 to the third flow rate B3. The third flow rate B3 is, for example, 0.17 sccm. The time from the third point in time C3 to the fourth point in time C4 is, for example, 7 minutes. From the fourth point in time C4 to the fifth point in time C5, the flow rate of the ammonia gas is maintained at the third flow rate B3. In this manner, the drift layer 22 is formed on the buffer layer 21.

[0067] Next, the effects of silicon carbide epitaxial substrate 10 according to this embodiment will be described.

[0068] Triangular defects may occur on the surface of silicon carbide epitaxial substrate 10. Triangular defects grow from origins at the boundary between silicon carbide substrate 11 and silicon carbide epitaxial layer 12 or inside silicon carbide epitaxial layer 12, and are exposed on the surface of silicon carbide epitaxial layer 12. Triangular defects that originate at the boundary between silicon carbide substrate 11 and silicon carbide epitaxial layer 12 (first triangular defects 30) grow larger than triangular defects that originate inside silicon carbide epitaxial layer 12 (second triangular defects 40).

[0069] The inventors have conducted extensive research into ways to reduce the areal density of triangular defects and have come to the following conclusions: Specifically, the inventors have found that the areal density of triangular defects (particularly, second triangular defects 40) can be reduced by providing a specific carrier concentration gradient in the drift layer 22.

[0070] In silicon carbide epitaxial substrate 10 according to the present disclosure, when the carrier concentration of drift layer 22 is measured by a capacitance-voltage method, the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 is 1.3 or more, where N1 is the carrier concentration at a position 0.7 μm away from second main surface 2 toward first main surface 1, and N2 is the carrier concentration at a position 1.2 μm away from second main surface 2 toward first main surface 1. This allows the areal density of triangular defects to be reduced.

[0071] Furthermore, in silicon carbide epitaxial substrate 10 according to the present disclosure, the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 may be 4.5 or less. If the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 is too large, the breakdown voltage of a silicon carbide semiconductor device manufactured using silicon carbide epitaxial substrate 10 will deteriorate. By setting the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 to 4.5 or less, it is possible to suppress deterioration in the breakdown voltage of a silicon carbide semiconductor device manufactured using silicon carbide epitaxial substrate 10.

[0072] Furthermore, in silicon carbide epitaxial substrate 10 according to the present disclosure, when the carrier concentration at a position 1.1 μm away from second main surface 2 toward first main surface 1 is defined as third carrier concentration N3, the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 may be 1.1 or more. This allows for a further reduction in the areal density of triangular defects.

[0073] Furthermore, in silicon carbide epitaxial substrate 10 according to the present disclosure, the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 may be equal to or less than 2.5. If the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 is too large, the breakdown voltage of a silicon carbide semiconductor device manufactured using silicon carbide epitaxial substrate 10 will deteriorate. By setting the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 to be equal to or less than 2.5, it is possible to suppress deterioration in the breakdown voltage of a silicon carbide semiconductor device manufactured using silicon carbide epitaxial substrate 10. [Example]

[0074] (Sample preparation) Silicon carbide epitaxial substrates 10 according to Samples 1 to 4 were prepared. Silicon carbide epitaxial substrates 10 according to Samples 1 and 2 are examples. Silicon carbide epitaxial substrates 10 according to Samples 3 and 4 are comparative examples. The carrier concentrations of silicon carbide epitaxial substrates 10 were measured by the contact CV method described above.

[0075] In silicon carbide epitaxial substrate 10 according to Sample 1, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was set to 1.1 or more, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was set to 1.3 or more. Specifically, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was set to 1.17, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was set to 1.31.

[0076] In silicon carbide epitaxial substrate 10 according to Sample 2, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was set to less than 1.1, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was set to 1.3 or more. Specifically, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was set to 1.04, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was set to 1.32.

[0077] In silicon carbide epitaxial substrate 10 according to Sample 3, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was less than 1.1, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was less than 1.3. Specifically, the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 was 1.03, and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 was 1.12.

[0078] In silicon carbide epitaxial substrate 10 of Sample 4, the value obtained by dividing third carrier concentration N3 by first carrier concentration N1 was set to 1, and the value obtained by dividing second carrier concentration N2 by first carrier concentration N1 was set to 1.

[0079] (Evaluation method) The areal density of second triangular defects 40 on the surface (second main surface 2) of silicon carbide epitaxial substrate 10 of Samples 1 to 4 was measured. The areal density of second triangular defects 40 was measured using a surface defect inspection device (model number: SICA6X) manufactured by Lasertec Corporation. The definition of second triangular defects 40 is as described above.

[0080] (Evaluation results)

[0081] [Table 1]

[0082] Table 1 shows the surface densities of second triangular defects 40 on the surfaces (second main surfaces 2) of silicon carbide epitaxial substrates 10 according to Samples 1-4.

[0083] As shown in Table 1, the areal density of the second triangular defects 40 on the surface of the silicon carbide epitaxial substrate 10 according to Samples 1 and 2 was lower than the areal density of the second triangular defects 40 on the surface of the silicon carbide epitaxial substrate 10 according to Samples 3 and 4. The areal density of the second triangular defects 40 on the surface of the silicon carbide epitaxial substrate 10 according to Sample 1 was also lower than the areal density of the second triangular defects 40 on the surface of the silicon carbide epitaxial substrate 10 according to Sample 2.

[0084] From the above results, by setting the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 to 1.3 or more, 2 Triangular defect 4 It was also confirmed that the proportion of second triangular defects 40 can be further reduced by setting the value obtained by dividing the third carrier concentration N3 by the first carrier concentration N1 to 1.1 or more and the value obtained by dividing the second carrier concentration N2 by the first carrier concentration N1 to 1.3 or more.

[0085] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims rather than the above-described embodiments and examples, and it is intended to include any modifications within the scope of the claims that are equivalent to the claims. [Explanation of symbols]

[0086] 1 First main surface 2 Second main surface 3 Third principal surface 4 Fourth main surface 10 Silicon carbide epitaxial substrate 11 Silicon carbide substrate 12 Silicon carbide epitaxial layer 13 Orientation Flat 14 Arc-shaped portion 15 outer edge 21 Buffer layer 22 Drift Layer 30 First Triangular Defect 31 Part 1 32 Part 2 33 First starting point 40 Second Triangular Defect 41 Part 3 42 Part 4 43 Second starting point 101 1st direction 102 Second direction 110 Mercury Probe 111 Electrode 200 Manufacturing equipment 201 Chamber 203 Heating element 204 Quartz tube 205 Inner wall surface 206 Stages 207 Gas inlet 208 Gas exhaust port 209 Rotational Axis 210 Susceptor 231 First Gas Supply Unit 232 Second gas supply unit 233 Third Gas Supply Section 234 4th Gas Supply Section 235 Gas Supply Section 241 First gas flow control section 242 Second gas flow control section 243 Third gas flow control section 244 Fourth gas flow control section 245 Control Unit B1 1st flow rate B2 2nd flow rate B3 3rd flow rate C1 First time point C2 Second time point C3 Third time point C4 4th point C5 5th point L1 First length L2 Second length L3 Third length L4 Fourth length N1 First carrier concentration N2 Secondary carrier concentration N3 Third carrier concentration N4 fourth carrier concentration P1 1st position P2 2nd position P3 3rd position P4 4th position P5 5th position T1 First thickness T2 Second thickness T3 Third thickness T4 4th thickness T5 5th thickness W1 No. 1

Claims

1. a silicon carbide substrate; a silicon carbide epitaxial layer on the silicon carbide substrate; the silicon carbide epitaxial layer includes a buffer layer in contact with the silicon carbide substrate and a drift layer on the buffer layer; the drift layer has a thickness of 2 μm or more, the drift layer has a first main surface in contact with the buffer layer and a second main surface opposite to the first main surface, the second main surface is a surface of the silicon carbide epitaxial layer, When the carrier concentration of the drift layer was measured by a capacitance voltage method, a carrier concentration at a position 0.7 μm away from the second main surface toward the first main surface is defined as a first carrier concentration; When the carrier concentration at a position 1.2 μm away from the second main surface toward the first main surface is defined as a second carrier concentration, a value obtained by dividing the second carrier concentration by the first carrier concentration is 1.3 or more; The areal density of triangular defects originating inside the silicon carbide epitaxial layer is 0.001 defects / cm 2 0.08 pieces / cm or more 2 is as follows: A silicon carbide epitaxial substrate, wherein an areal density of triangular defects originating at a boundary between the silicon carbide substrate and the silicon carbide epitaxial layer is 0.005 defects / cm 2 or more and 0.3 defects / cm 2 or less.

2. 2 . The silicon carbide epitaxial substrate according to claim 1 , wherein a value obtained by dividing said second carrier concentration by said first carrier concentration is 4.5 or less.

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