Patent Publication Number: US-2021166941-A1

Title: Method for manufacturing silicon carbide epitaxial substrate and method for manufacturing silicon carbide semiconductor device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing a silicon carbide epitaxial substrate and a method for manufacturing a silicon carbide semiconductor device. The present application claims priority to Japanese Patent Application No. 2018-154412 filed on Aug. 21, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     WO 2017/056691 (PTL 1) discloses a method for epitaxially growing a silicon carbide layer on a silicon carbide single-crystal substrate. 
     CITATION LIST 
     Patent Literature 
     PTL 1: WO 2017/056691 
     SUMMARY OF INVENTION 
     A method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure includes the following steps. A susceptor having a substrate placement surface, a silicon carbide single-crystal substrate having a first main surface and a second main surface opposite to the first main surface, and a reaction chamber in which the susceptor is disposed are prepared. The silicon carbide single-crystal substrate is placed on the substrate placement surface such that the second main surface faces the substrate placement surface. A silicon carbide layer is formed on the first main surface by supplying a mixed gas including silane, ammonia and hydrogen to the reaction chamber. The first main surface is a (000-1) plane or a plane inclined by an angle of less than or equal to 8° relative to the (000-1) plane. The substrate placement surface has an area of more than or equal to 697 cm 2  and less than or equal to 1161 cm 2 . In the forming of the silicon carbide layer, when an X axis indicates a first value representing, in percentage, a value obtained by dividing a flow rate of the silane by a flow rate of the hydrogen, and a Y axis indicates a second value representing a flow rate of the ammonia in scan, the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates, where the first coordinates are (0.038. 0.0019), the second coordinates are (0.069, 0.0028), the third coordinates are (0.177, 0.0032), the fourth coordinates are (0.038, 0.0573), the fifth coordinates are (0.069, 0.0849), and the sixth coordinates are (0.177, 0.0964). After the forming of the silicon carbide layer, an average value of carrier concentration in the silicon carbide layer is more than or equal to 1×10 15  cm −3  and less than or equal to 3×10 16  cm −3 . 
     A method for manufacturing a silicon carbide epitaxial substrate according to the present disclosure includes the following steps. A susceptor having a substrate placement surface, a silicon carbide single-crystal substrate having a first main surface and a second main surface opposite to the first main surface, and a reaction chamber in which the susceptor is disposed are prepared. The silicon carbide single-crystal substrate is placed on the substrate placement surface such that the second main surface faces the substrate placement surface. A silicon carbide layer is formed on the first main surface by supplying a mixed gas including silane, ammonia and hydrogen to the reaction chamber. The first main surface is a (000-1) plane or a plane inclined by an angle of less than or equal to 8° relative to the (000-1) plane. The substrate placement surface has an area of more than or equal to 697 cm 2  and less than or equal to 1161 cm 2 . In the forming of the silicon carbide layer, when an X axis indicates a first value representing, in cm −2 , a value obtained by dividing a value, which is obtained by dividing a flow rate of the silane by a flow rate of the hydrogen, by the area, and a Y axis indicates a second value representing a flow rate of the ammonia in sccm, the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates, where the first coordinates are (4.10×10 −7 , 0.0019), the second coordinates are (7.44×10 −7 , 0.0028), the third coordinates are (1.91×10 −6 , 0.0032), the fourth coordinates are (4.10×10 −7 , 0.0573), the fifth coordinates are (7.44×10 −7 , 0.0849), and the sixth coordinates are (1.91×10 −6 , 0.0964). After the forming of the silicon carbide layer, an average value of carrier concentration in the silicon carbide layer is more than or equal to 1×10 15  cm −3  and less than or equal to 3×10 16  cm −3 . 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial schematic cross-sectional view showing a configuration of a manufacturing apparatus for a silicon carbide epitaxial substrate according to a first embodiment. 
         FIG. 2  is a schematic cross-sectional view along line II-II in  FIG. 1 . 
         FIG. 3  is a schematic plan view showing a configuration of a susceptor in the manufacturing apparatus for the silicon carbide epitaxial substrate according to the first embodiment. 
         FIG. 4  is a flowchart schematically showing a method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment. 
         FIG. 5  is a schematic cross-sectional view showing a first step of the method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment. 
         FIG. 6  shows a relation between a SiH 4  flow rate/H 2  flow rate and a NH 3  flow rate in the method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment. 
         FIG. 7  shows a relation between a value obtained by dividing the (SiH 4  flow rate/H 2  flow rate) by an area of a substrate placement surface and the NH 3  flow rate in the method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment. 
         FIG. 8  is a schematic cross-sectional view showing a second step of the method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment. 
         FIG. 9  is a schematic plan view showing a configuration of a susceptor in a manufacturing apparatus for a silicon carbide epitaxial substrate according to a first variation. 
         FIG. 10  is a schematic plan view showing a configuration of a susceptor in a manufacturing apparatus for a silicon carbide epitaxial substrate according to a second variation. 
         FIG. 11  is a partial schematic cross-sectional view showing a configuration of a manufacturing apparatus for a silicon carbide epitaxial substrate according to a second embodiment. 
         FIG. 12  is a partial schematic cross-sectional view showing a configuration of a manufacturing apparatus for a silicon carbide epitaxial substrate according to a third embodiment. 
         FIG. 13  is a flowchart schematically showing a method for manufacturing a silicon carbide semiconductor device according to the present embodiment. 
         FIG. 14  is a schematic cross-sectional view showing a first step of the method for manufacturing the silicon carbide semiconductor device according to the present embodiment. 
         FIG. 15  is a schematic cross-sectional view showing a second step of the method for manufacturing the silicon carbide semiconductor device according to the present embodiment. 
         FIG. 16  is a schematic cross-sectional view showing a third step of the method for manufacturing the silicon carbide semiconductor device according to the present embodiment. 
         FIG. 17  shows a relation between a SiH 4  flow rate/H 2  flow rate and a NH 3  flow rate in a method for manufacturing a silicon carbide epitaxial substrate according to each evaluation sample. 
         FIG. 18  shows a relation between a value obtained by dividing the (SiH 4  flow rate/H 2  flow rate) by an area of a substrate placement surface and the NH 3  flow rate in the method for manufacturing the silicon carbide epitaxial substrate according to each evaluation sample. 
     
    
    
     DETAILED DESCRIPTION 
     Problems to be Solved by the Present Disclosure 
     An object of the present disclosure is to achieve rapid growth of a silicon carbide layer, while improving the flatness of a surface of the silicon carbide layer and in-plane uniformity of carrier concentration. 
     Effects of the Present Disclosure 
     According to the present disclosure, rapid growth of a silicon carbide layer can be achieved while the flatness of a surface of the silicon carbide layer and in-plane uniformity of carrier concentration are improved. 
     Overview of Embodiments of the Present Disclosure 
     An overview of embodiments of the present disclosure is described first. Regarding crystallographic denotation herein, an individual orientation is represented by [ ], a group orientation is represented by &lt; &gt;, an individual plane is represented by ( ), and a group plane is represented by { }. A crystallographically negative index is normally expressed by putting “-” (bar) above a numeral, however, the crystallographically negative index is expressed herein by putting a negative sign before the numeral. 
     (1) A method for manufacturing a silicon carbide epitaxial substrate  100  according to the present disclosure includes the following steps. A susceptor  210  having a substrate placement surface  211 , a silicon carbide single-crystal substrate  10  having a first main surface  41  and a second main surface  42  opposite to first main surface  41 , and a reaction chamber in which susceptor  210  is disposed are prepared. Silicon carbide single-crystal substrate  10  is placed on substrate placement surface  211  such that second main surface  42  faces the substrate placement surface. A silicon carbide layer  20  is formed on first main surface  41  by supplying a mixed gas including silane, ammonia and hydrogen to reaction chamber  201 . First main surface  41  is a (000-1) plane or a plane inclined by an angle of less than or equal to 8° relative to the (000-1) plane. Substrate placement surface  211  has an area of more than or equal to 697 cm 2  and less than or equal to 1161 cm 2 . In the forming of silicon carbide layer  20 , when an X axis indicates a first value representing, in percentage, a value obtained by dividing a flow rate of the silane by a flow rate of the hydrogen, and a Y axis indicates a second value representing a flow rate of the ammonia in sccm, the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates, where the first coordinates are (0.038, 0.0019), the second coordinates are (0.069, 0.0028), the third coordinates are (0.177, 0.0032.), the fourth coordinates are (0.038, 0.0573), the fifth coordinates are (0.069, 0.0849), and the sixth coordinates are (0.177, 0.0964). After the forming of silicon carbide layer  20 , an average value of carrier concentration in silicon carbide layer  20  is more than or equal to 1×10 15  cm −3  and less than or equal to 3×10 16  cm −3 . Note that the unit “sccm (standard cc/minute)” of the flow rate indicates a flow rate “cc/minute” under standard conditions (0° C., 101.3 kPa). 
     (2) A method for manufacturing a silicon carbide epitaxial substrate  100  according to the present disclosure includes the following steps. A susceptor  210  having a substrate placement surface  211 , a silicon carbide single-crystal substrate  10  having a first main surface  41  and a second main surface  42  opposite to first main surface  41 , and a reaction chamber  201  in which susceptor  210  is disposed are prepared. Silicon carbide single-crystal substrate  10  is placed on substrate placement surface  211  such that second main surface  42  faces the substrate placement surface. A silicon carbide layer  20  is formed on first main surface  41  by supplying a mixed gas including silane, ammonia and hydrogen to reaction chamber  201 . First main surface  41  is a (000-1) plane or a plane inclined by an angle of less than or equal to 8° relative to the (000-1) plane. Substrate placement surface  211  has an area of more than or equal to 697 cm 2  and less than or equal to 1161 cm 2 . In the forming of silicon carbide layer  20 , when an X axis indicates a first value representing, in cm −2 , a value obtained by dividing a value, which is obtained by dividing a flow rate of the silane by a flow rate of the hydrogen, by the area, and a Y axis indicates a second value representing a flow rate of the ammonia in sccm, the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates, where the first coordinates are (4.10×10 −7 , 0.0019), the second coordinates are (7.44×10 −7 , 0.0028), the third coordinates are (1.91×10 −6 , 0.0032), the fourth coordinates are (4.10×10 −7 , 0.0573), the fifth coordinates are (7.44×10 −7 , 0.0849), and the sixth coordinates are (1.91×10 −6 , 0.0964). After the forming of silicon carbide layer  20 , an average value of carrier concentration in silicon carbide layer  20  is more than or equal to 1×10 15  cm −3  and less than or equal to 3×10 16  cm −3 . 
     (3) A method for manufacturing a silicon carbide semiconductor device according to the present disclosure includes the following steps. Silicon carbide epitaxial substrate  100  manufactured with the method according to (1) or (2) is prepared. The silicon carbide epitaxial substrate is processed. 
     Details of Embodiments of the Present Disclosure 
     The following describes one embodiment (hereinafter also referred to as “the present embodiment”) of the present disclosure. However, the present embodiment is not limited as such. In the following description, the same or corresponding elements are designated by the same characters and the same description thereof will not be repeated. 
     First Embodiment 
     A configuration of a manufacturing apparatus  200  for a silicon carbide epitaxial substrate  100  according to a first embodiment is initially described. 
     As shown in  FIG. 1 , manufacturing apparatus  200  is a hot wall type lateral CVD (Chemical Vapor Deposition) apparatus, for example. Manufacturing apparatus  200  mainly includes a reaction chamber  201 , a gas supplier  235 , a controller  245 , a heating element  203 , a quartz tube  204 , a heat insulator (not shown), and an induction heating coil (not shown). 
     Heating element  203  has a cylindrical shape, for example, and forms reaction chamber  201  inside. Healing element  203  is made of graphite, for example. The heat insulator surrounds an outer circumference of heating element  203 . The heat insulator is provided inside quartz tube  204  in contact with an inner circumferential surface of quartz tube  204 . The induction heating coil is wound along an outer circumferential surface of quartz tube  204 , for example. The induction heating coil is configured to receive alternating current by an external power supply (not shown). Heating element  203  is thus inductively heated. As a result, reaction chamber  201  is heated by heating element  203 . 
     Reaction chamber  201  is a space formed by being surrounded by an inner wall surface  205  of heating element  203 . In reaction chamber  201 , a silicon carbide single-crystal substrate  10  is disposed. Reaction chamber  201  is configured to heat silicon carbide single-crystal substrate  10 . Silicon carbide single-crystal substrate  10  has a maximum diameter of more than or equal to 100 mm. Reaction chamber  201  is provided with a susceptor  210  that holds silicon carbide single-crystal substrate  10 . Susceptor  210  is disposed on a stage  202 . Stage  202  is configured to rotate by a rotation shaft  209 . Rotation of stage  202  allows rotation of susceptor  210 . 
     Manufacturing apparatus  200  further includes a gas inlet  207  and a gas outlet  208 . Gas outlet  208  is connected to an exhaust pump (not shown). An arrow in  FIG. 1  indicates a flow of gas. Gas is introduced through gas inlet  207  into reaction chamber  201 , and exhausted through gas outlet  208 . A pressure in reaction chamber  201  is adjusted in accordance with a balance between an amount of the supplied gas and an amount of the exhausted gas. 
     Gas supplier  235  is configured to supply a mixed gas including silane, ammonia, and gas including hydrogen and carbon atoms to reaction chamber  201 . Specifically, gas supplier  235  may include a first gas supplier  231 , a second gas supplier  232 , a third gas supplier  233 , and a carrier gas supplier  234 . 
     First gas supplier  231  is configured to supply a first gas including carbon atoms. First gas supplier  231  is a gas cylinder tilled with the first gas, for example. The first gas is propane (C 3 H 8 ) gas, for example. The first gas may be methane (CH 4 ) gas, ethane (C 2 H 6 ) gas, acetylene (C 2 H 2 ) gas, or the like, for example. 
     Second gas supplier  232  is configured to supply a second gas including silane gas. Second gas supplier  232  is a gas cylinder filled with the second gas, for example. The second gas is silane (SiH 4 ) gas, for example. The second gas may be a mixed gas of silane gas and gas other than silane. 
     Third gas supplier  233  is configured to supply a third gas including ammonia gas. Third gas supplier  233  is a gas cylinder filled with the third gas, for example. The third gas is a doping gas including N (nitrogen atoms). The ammonia gas is more likely to be thermally decomposed than nitrogen gas having a triple bond. By using the ammonia gas, in-plane uniformity of carder concentration can be expected to be improved. 
     Carrier gas supplier  234  is configured to supply a carrier gas such as hydrogen. Carrier gas supplier  234  is a gas cylinder filled with hydrogen, for example. 
     Controller  245  is configured to control a flow rate of the mixed gas supplied from gas supplier  235  to reaction chamber  201 . Specifically, controller  245  may include a first gas flow rate controller  241 , a second gas flow rate controller  242 , a third gas flow rate controller  243 , and a carder gas flow rate controller  244 . Each of the controllers may be a MFC (Mass Flow Controller), for example. Controller  245  is disposed between gas supplier  235  and gas inlet  207 . In other words, controller  245  is disposed in a flow path that connects between gas supplier  235  and gas inlet  207 . 
     In the axial direction of reaction chamber  201 , a density of windings of the induction heating coil may be changed. The density of windings [the number of windings/m] is the number of windings of the coil per unit length in the axial direction of the apparatus. For example, in order to thermally decompose ammonia effectively at the upstream side, the density of windings of the induction heating coil at the upstream side may be higher than the density of windings of the induction heating coil at the downstream side. 
       FIG. 2  is a schematic cross-sectional view along line II-II in  FIG. 1 . As shown in  FIG. 2 , a region surrounded by inner wall surface  205  of heating element  203  is substantially rectangular in shape, for example. A width of the region surrounded by inner wall surface  205  of heating element  203  in a direction along the radial direction of silicon carbide single-crystal substrate  10  may be smaller than a width of the region surrounded by inner wall surface  205  of heating element  203  in a direction perpendicular to the radial direction of silicon carbide single-crystal substrate  10 . Manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the present embodiment has large reaction chamber  201 . Specifically, reaction chamber  201  has a cross-sectional area of 176 cm 2 , for example, in a plane perpendicular to the direction of movement of the mixed gas. The cross-sectional area of reaction chamber  201  may be more than or equal to 132 cm 2 , or more than or equal to 150 cm 2 . The cross-sectional area of reaction chamber  201  may be less than or equal to 220 cm 2 , or less than or equal to 200 cm 2 . Note that the cross-sectional area of reaction chamber  201  is the area of the region surrounded by inner wall surface  205  of heating element  203  (see  FIG. 2 ). 
     As shown in  FIG. 3 , susceptor  210  can have a plurality of substrates disposed thereon. From a different viewpoint, susceptor  210  is of batch type. Susceptor  210  has a substrate placement surface  211 , a bottom surface  212 , and a side surface  215 . Substrate placement surface  211  is a surface on which the substrates are placed. Bottom surface  212  is a surface opposite to substrate placement surface  211 . Bottom surface  212  is a surface mounted on stage  202 . Side surface  215  is continuous to each of substrate placement surface  211  and bottom surface  212 . Substrate placement surface  211  has a plurality of substrate placement portions  213 , a top surface  214 , and a center  216 . As seen in a direction perpendicular to top surface  214 , center  216  is disposed at a position coinciding with rotation shaft  209 . Each of the plurality of substrate placement portions  213  is a recess. Silicon carbide single-crystal substrate  10  is disposed in each of the plurality of substrate placement portions  213 . There are three substrate placement portions  213 , for example. The plurality of substrate placement portions  213  are rotationally symmetrically positioned with respect to center  216 . Specifically, substrate placement portions  213  are positioned at 0°, 120° and 240°, as seen from center  216 . As seen in the direction perpendicular to top surface  214 , each of the plurality of substrate placement portions  213  is substantially circular in shape. 
     Manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the present embodiment has large susceptor  210 . Specifically, substrate placement surface  211  of susceptor  210  has an area of 929 cm 2 , for example. In this case, one-half (radius) of a diameter  250  of substrate placement surface  211  is 17.2 cm, for example. The area of substrate placement surface  211  of susceptor  210  may be more than or equal to 697 cm 2 , or more than or equal to 750 cm 2 , for example. The area of substrate placement surface  211  of susceptor  210  may be less than or equal to 1161 cm 2 , or less than or equal to 950 cm 2 . 
     Next, a method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment is described. 
     First, a step of preparing silicon carbide single-crystal substrate  10  (S 1 :  FIG. 4 ) is performed. A silicon carbide single crystal having a polytype of 4H is fabricated by sublimation, for example. Then, the silicon carbide single crystal is sliced by a wire saw, for example, whereby silicon carbide single-crystal substrate  10  is prepared (see  FIG. 4 ). Silicon carbide single-crystal substrate  10  has a first main surface  41  and a second main surface  42 . Second main surface  42  is a surface opposite to first main surface  41 . The silicon carbide single crystal has a polytype of 4H—SiC, for example. The 4H—SiC has better electron mobility, dielectric strength, and the like than other polytypes. Silicon carbide single-crystal substrate  10  includes an n type impurity such as nitrogen. The conductivity type of silicon carbide single-crystal substrate  10  is n type, for example. 
     First main surface  41  is a (000-1) plane or a plane inclined by an angle of less than or equal to 8° relative to the (000-1) plane. When first main surface  41  is inclined relative to the (000-1) plane, the inclination direction (off direction) is a &lt;11-20&gt; direction, for example. The inclination angle (off angle) relative to the (000-1) plane may be more than or equal to 1°, or more than or equal to 2°. The off angle may be less than or equal to 7°, less than or equal to 6°, or less than or equal to 4°. Second main surface  42  is a (0001) plane or a plane inclined by an angle of less than or equal to 8° relative to the (0001) plane. 
     First main surface  41  of silicon carbide single-crystal substrate  10  has a maximum diameter (diameter) of more than or equal to 100 mm. The diameter may be more than or equal to 150 mm, more than or equal to 200 mm, or more than or equal to 250 mm. Although the upper limit of the diameter is not particularly limited, the upper limit of the diameter may be 300 mm, for example. 
     Next, a step of placing the silicon carbide single-crystal substrate on a substrate placement surface of a susceptor (S 2 :  4 ) is performed. Specifically, susceptor  210  shown in  FIG. 3  is prepared. Silicon carbide single-crystal substrate  10  is disposed in substrate placement portion  213  of susceptor  210 . As shown in  FIG. 3 , when susceptor  210  has three substrate placement portions  213 , three silicon carbide single-crystal substrates  10  are disposed in these substrate placement portions  213 , respectively. 
     As shown in  FIG. 5 , silicon carbide single-crystal substrate  10  is placed on substrate placement surface  211  such that second main surface  42  of silicon carbide single-crystal substrate  10  is in contact with substrate placement portion  213  of susceptor  210 . Susceptor  210  with silicon carbide single-crystal substrate  10  placed on substrate placement surface  211  is disposed in reaction chamber  201 . Silicon carbide single-crystal substrate  10  may be placed on substrate placement surface  211  inside reaction chamber  201 , or may be placed on substrate placement surface  211  outside reaction chamber  201  and then disposed in reaction chamber  201 . 
     Next, a step of forming a silicon carbide layer on the silicon carbide single-crystal substrate (S 3 :  FIG. 4 ) is performed. Specifically, manufacturing apparatus  200  described above is used to form a silicon carbide layer  20  on silicon carbide single-crystal substrate  10  by epitaxial growth. For example, after the pressure in reaction chamber  201  is reduced from atmospheric pressure to about 1×10 −6  Pa, the temperature of silicon carbide single-crystal substrate  10  is started to be increased. During the temperature increase, hydrogen (H 2 ) gas serving as the carrier gas is introduced from carrier gas supplier  234  into reaction chamber  201 . A flow rate of the hydrogen gas is adjusted by carrier gas flow rate controller  244 . 
     After the temperature of silicon carbide single-crystal substrate  10  reaches, for example, about 1600° C., source material gas, dopant gas and carrier gas are supplied to reaction chamber  201 . Specifically, a mixed gas including silane, ammonia, hydrogen and propane is supplied to reaction chamber  201 , whereby the gases are thermally decomposed to form silicon carbide layer  20  on silicon carbide single-crystal substrate  10 . A C/Si ratio of the mixed gas may be 1.0, for example. 
     As shown in  FIG. 6 , the X axis indicates a value (first value) representing, in percentage, a value obtained by dividing a flow rate of the silane by a flow rate of the hydrogen, whereas the Y axis indicates a value (second value) representing a flow rate of the ammonia in sccm. The first value and the second value fall within a hexagonal region (hatched region in  FIG. 6 ) surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates. The first coordinates are (0.038, 0.0019), the second coordinates are (0.069, 0.0028), the third coordinates are (0.177, 0.0032), the fourth coordinates are (0.038, 0.0573), the fifth coordinates are (0.069, 0.0849), and the sixth coordinates are (0.177, 0.0964). 
     For example, the flow rate of the carrier gas (hydrogen) supplied to reaction chamber  201  is adjusted to be 100 slm using carrier gas flow rate controller  244 . The flow rate of the second gas (silane gas) supplied to reaction chamber  201  is adjusted to be  104  scan using second gas flow rate controller  242 . In this case, the value representing, in percentage, the value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen is 0.038%. The flow rate of the third gas (ammonia gas) is adjusted to be 0.0019 sccm using third gas flow rate controller  243 . In this case, the value (first value) representing, in percentage, the value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen and the value (second value) representing the flow rate of the ammonia in scan fall within the hatched region in FIG.  6 . 
     As described above, the method for manufacturing the silicon carbide epitaxial substrate according to the present embodiment uses large susceptor  210 . Considering the area of substrate placement surface  211  of susceptor  210 , the flow rate of the silane, the flow rate of the hydrogen, and the flow rate of the ammonia are controlled as follows. Specifically, as shown in  FIG. 7 , when the X axis indicates a value (first value) representing, in cm −2 , a value obtained by dividing the value, which is obtained by dividing the flow rate of the silane by the flow rate of the hydrogen, by the area of substrate placement surface  211 , and the Y axis indicates a value (second value) representing the flow rate of the ammonia in seem, the first value and the second value fall within a hexagonal region (hatched region in  FIG. 7 ) surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates. The first coordinates are (4.10×10 −7 , 0.0019), the second coordinates are (7.44×10 −7 , 0.0028), the third coordinates are (1.91×10 −6 , 0.0032), the fourth coordinates are (4.10×10 −7 , 0.0573), the fifth coordinates are (7.44×10 −7 , 0.0849), and the sixth coordinates are (1.91×10 −6 , 0.0964). 
     For example, the flow rate of the carrier gas (hydrogen) supplied to reaction chamber  201  is adjusted to be 100 slm using carrier gas flow rate controller  244 . The flow rate of the second gas (silane gas) supplied to reaction chamber  201  is adjusted to be 38 sccm using second gas flow rate controller  242 . Substrate placement surface  211  has an area of 929 cm 2 , for example. In this case, the value (first value) representing, in cm −2 , the value obtained by dividing the value, which is obtained by dividing the flow rate of the silane by the flow rate of the hydrogen, by the area of substrate placement surface  211  is 4.10×10 −7 . The flow rate of the third gas (ammonia gas) is adjusted to be 0.0019 sccm using third gas flow rate controller  243 . In this case, the value (first value) representing, in cm −2 , the value obtained by dividing the value, which is obtained by dividing the flow rate of the silane by the flow rate of the hydrogen, by the area of substrate placement surface  211 , and the value (second value) representing the flow rate of the ammonia in sccm fall within the hatched region in FIG.  7 . 
     The flow rate (sccm) of the ammonia may be more than or equal to 0.0019, more than or equal to 0.0028, or more than or equal to 0.0032. The flow rate (sccm) of the ammonia may be, for example, less than or equal to 0.0964, less than or equal to 0.0849, or less than or equal to 0.0573. The value (%) representing, in percentage, the value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen may be more than or equal to 0.038, or more than or equal to 0.069. The value (%) representing, in percentage, the value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen may be less than or equal to 0.177, for example. The flow rate of the silane is more than or equal to 20 sccm and less than or equal to 300 sccm, for example. The flow rate of the hydrogen is more than or equal to 80 slm and less than or equal to 150 slm, for example. 
     The growth rate of silicon carbide layer  20  may be more than or equal to 3 μm/h, more than or equal to 15 μm/h, more than or equal to 25 μm/h, or more than or equal to 33 μm/h. The growth rate of silicon carbide layer  20  may be less than or equal to 50 μm/h. The growth rate of silicon carbide layer  20  may be determined by a ratio of the flow rate of the hydrogen to the flow rate of the silane. Since the silane is a source material gas, the growth rate of silicon carbide layer  20  increases with an increase in the flow rate of the silane. On the other hand, since the hydrogen has a characteristic to etch silicon carbide, the growth rate of silicon carbide layer  20  decreases with an increase in the flow rate of the hydrogen. In the present embodiment, silicon carbide layer  20  can be grown rapidly. Specifically, the value representing, in percentage, the value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen is more than or equal to 0.038%. 
     As described above, the mixed gas of silane, propane, ammonia and hydrogen is supplied to reaction chamber  201 , whereby silicon carbide layer  20  is formed on silicon carbide single-crystal substrate  10 . Silicon carbide layer  20  has a thickness of more than or equal to 10 μm, for example. Note that methane (CH 4 ), ethane (C 2 H 6 ), acetylene (C 2 H 2 ), or the like may be used instead of propane. During the supply of the mixed gas to reaction chamber  201 , silicon carbide single-crystal substrate  10  may be rotated around rotation shaft  209 . Silicon carbide epitaxial substrate  100  including silicon carbide single-crystal substrate  10  and silicon carbide layer  20  (see  FIG. 8 ) is manufactured in this manner. Silicon carbide layer  20  has a fourth main surface  44  in contact with silicon carbide single-crystal substrate  10 , and a third main surface  43  opposite to fourth main surface  44 . 
     According to silicon carbide epitaxial substrate  100  manufactured with the method described above, in-plane uniformity of carrier concentration in silicon carbide layer  20  can be improved while an average value of the carrier concentration in silicon carbide layer  20  is maintained to fall within a certain concentration range. 
     Specifically, after the step of forming silicon carbide layer  20 , the average value of the carrier concentration in silicon carbide layer  20  is more than or equal to 1×10 15  cm −3  and less than or equal to 3×10 16  cm −3 . The in-plane uniformity of the carrier concentration is less than or equal to 10%, for example. The in-plane uniformity of the carrier concentration is a representation, in percentage, of a value obtained by dividing the standard deviation of the carrier concentration by the average value of the carrier concentration. The carrier concentration may be measured by a mercury probe type C (capacitance)-V (voltage) measuring device, for example. Specifically, one probe is placed on third main surface  43  of silicon carbide layer  20  and another probe is placed on second main surface  42  of silicon carbide single-crystal substrate  10 . The one probe has an area of 0.01 cm 2 , for example. Voltage is applied between the one probe and the other probe, and a capacitance between the one probe and the other probe is measured. When the vertical axis indicates 1/C 2  (reciprocal of the square of the capacitance) and the horizontal axis indicates V (voltage), the carrier concentration is determined based on the inclination of a straight line of measurement data. A depth of measurement for the carrier concentration is adjusted in accordance with applied voltage. In the present embodiment, the carrier concentration is measured in a region of silicon carbide layer  20  extending by at most about 10 μm from third main surface  43  toward second main surface  42 . 
     When a plurality of silicon carbide epitaxial substrates  100  are simultaneously manufactured in a batch manner, variation in the carrier concentration can be reduced between each of the plurality of silicon carbide epitaxial substrates  100 . Specifically, the difference in average value of the carrier concentration is less than or equal to 3%, for example, between each of the plurality of silicon carbide epitaxial substrates  100 . 
     Third main surface  43  has a root-mean-square deviation (Sq) of less than or equal to 0.4 nm, for example. The root-mean-square deviation (Sq) is a parameter obtained by extending root-mean-square roughness (Rq) to three dimensions. The root-mean-square deviation (Sq) can be measured by a white-light interference microscope, for example. A region for which the root-mean-square deviation (Sq) is measured can be a square region having each side of 250 μm. 
     (First Variation) 
     As shown in  FIG. 9 , susceptor  210  may be able to have four silicon carbide single-crystal substrates  10  disposed thereon. From a different viewpoint, substrate placement surface  211  of susceptor  210  has four substrate placement portions  213 , top surface  214 , and center  216 . Four substrate placement portions  213  are rotationally symmetrically positioned with respect to center  216 . Specifically, substrate placement portions  213  are positioned at 0°, 90°, 180° and 270°, as seen from center  216 . 
     (Second Variation) 
     As shown in  FIG. 10 , susceptor  210  may be able to have eight silicon carbide single-crystal substrates  10  disposed thereon. From a different viewpoint, substrate placement surface  211  of susceptor  210  has eight substrate placement portions  213 , top surface  214 , and center  216 . Eight substrate placement portions  213  are rotationally symmetrically positioned with respect to center  216 . Specifically, substrate placement portions  213  are positioned at 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°, as seen from center  216 . When eight silicon carbide single-crystal substrates  10  each having a diameter of 150 mm are placed on substrate placement surface  211 , diameter  250  of substrate placement surface  211  is 650 mm, for example. In this case, substrate placement surface  211  has an area of 3318 cm 2 . 
     Second Embodiment 
     Next, a configuration of manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to a second embodiment is described. Manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the second embodiment is mainly different from manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment in the positions of gas inlet  207  and gas outlet  208 , and is otherwise similar in configuration to manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment. The configuration different from that of manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment is principally described below. 
     As shown in  FIG. 11 , manufacturing apparatus  200  mainly includes reaction chamber  201 , gas supplier  235 , controller  245 , heating element  203 , quartz tube  204 , the heat insulator (not shown), the induction heating coil (not shown), gas inlet  207 , and gas outlets  208 . 
     As shown in  FIG. 11 , heating element  203  is provided with a gas supply hole  206 . Gas inlet  207  is connected to gas supply hole  206 . Gas outlets  208  are provided at one end and the other end of quartz tube  204 . Gas is introduced through gas supply hole  206  into reaction chamber  201  along the direction perpendicular to first main surface  41  of silicon carbide single-crystal substrate  10 . Arrows in  FIG. 11  indicate flows of gas. After being introduced into reaction chamber  201 , the gas splits to flow toward the one end and the other end of quartz tube  204 , and is exhausted through each of gas outlets  208  provided at the opposite sides of quartz tube  204 . 
     Third Embodiment 
     Next, a configuration of manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to a third embodiment is described. Manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the third embodiment is mainly different from manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment in being a vertical CVD apparatus, and is otherwise similar in configuration to manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment. The configuration different from that of manufacturing apparatus  200  for silicon carbide epitaxial substrate  100  according to the first embodiment is principally described below. 
     As shown in  FIG. 12 , manufacturing apparatus  200  may be a vertical CVD apparatus. Manufacturing apparatus  200  mainly includes reaction chamber  201 , gas supplier  235 , controller  245 , heating element  203 , quartz tube  204 , the heat insulator (not shown), the induction heating coil (not shown), gas inlet  207 , and gas outlet  208 . 
     As shown in  FIG. 12 , heating element  203  is provided to surround side surface  215  of susceptor  210 . Gas inlet  207  is disposed above susceptor  210  in the vertical direction. Gas outlet  208  is disposed below susceptor  210  in the vertical direction. Gas is introduced through gas supply hole  206  into reaction chamber  201  along the direction perpendicular to first main surface  41  of silicon carbide single-crystal substrate  10 . An arrow in  FIG. 12  indicates a flow of gas. After being introduced into reaction chamber  201 , the gas flows toward gas outlet  208  along the direction perpendicular to first main surface  41  of silicon carbide single-crystal substrate  10 . 
     Next, functions and effects of the method for manufacturing silicon carbide epitaxial substrate  100  according to the present embodiment are described. 
     Silicon carbide epitaxial substrate  100  used to manufacture a silicon carbide semiconductor device is required to achieve excellent in-plane uniformity of carrier concentration and excellent flatness of the surface of silicon carbide layer  20  while maintaining an average carrier concentration to fall within a certain range required in silicon carbide semiconductor devices. In recent years, in addition to achieving the characteristics described above, it has been required to grow silicon carbide layer  20  at a higher rate. 
     However, if the growth rate of silicon carbide layer  20  is simply increased, the flatness of the surface of silicon carbide layer  20  may be deteriorated. Moreover, for maintaining the excellent flatness of the surface of silicon carbide layer  20 , the average carrier concentration in silicon carbide layer  20  may fall out of the range required in power devices. In other words, it has been very difficult to achieve the rapid growth of silicon carbide layer  20 , the excellent flatness of the surface of silicon carbide layer  20 , and the excellent in-plane uniformity of the carrier concentration, while maintaining the average carrier concentration in silicon carbide layer  20  to fall within the certain range required in silicon carbide semiconductor devices. 
     If silicon carbide epitaxial substrate  100  is manufactured, using large manufacturing apparatus  200 , under the same conditions as those for conventional small manufacturing devices, it may not be possible to achieve the excellent in-plane uniformity of the carrier concentration and the excellent flatness of the surface of silicon carbide layer  20 , while maintaining the average carrier concentration to fall within the certain range required in silicon carbide semiconductor devices. Note that large manufacturing apparatus  200  refers to, for example, manufacturing apparatus  200  having reaction chamber  201  capable of accommodating susceptor  210  having substrate placement surface  211  with an area of more than or equal to 697 cm 2 . 
     The present inventors conducted extensive research on a method for manufacturing silicon carbide epitaxial substrate  100  satisfying the above-described requirements using large manufacturing apparatus  200 . Consequently, they made the following findings and conceived of one embodiment of the present disclosure. Specifically, it was found that silicon carbide epitaxial substrate  100  satisfying the above-described requirements can be realized by using silane as a source material gas, using hydrogen as a carrier gas, and using ammonia as a dopant gas, and controlling a silane flow rate, a hydrogen flow rate and an ammonia flow rate to fall within a certain range. Specifically, when the X axis indicates a first value representing, in percentage, a value obtained by dividing the flow rate of the silane by the flow rate of the hydrogen, and the Y axis indicates a second value representing the flow rate of the ammonia in sccm, the flow rate of the silane, the flow rate of the hydrogen, and the flow rate of the ammonia are controlled such that the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates. In this case, the first coordinates are (0.038, 0.0019), the second coordinates are (0.069, 0.0028), the third coordinates are (0.177, 0.0032), the fourth coordinates are (0.038, 0.0573), the fifth coordinates are (0.069, 0.0849), and the sixth coordinates are (0.177, 0.0964). 
     From a different viewpoint, when the X axis indicates a first value representing, in cm −2 , a value obtained by dividing the value, which is obtained by dividing the flow rate of the silane by the flow rate of the hydrogen, by the area of substrate placement surface  211 , and the Y axis indicates a second value representing the flow rate of the ammonia in sccm, the flow rate of the silane, the flow rate of the hydrogen, and the flow rate of the ammonia are controlled such that the first value and the second value fall within a hexagonal region surrounded by first coordinates, second coordinates, third coordinates, fourth coordinates, fifth coordinates and sixth coordinates in XY plane coordinates. In this case, the first coordinates are (4.10×10 −7 , 0.0019), the second coordinates are (7.44×10 −7 , 0.0028), the third coordinates are (1.91×10 −6 , 0.0032), the fourth coordinates are (4.10×10 −7 , 0.0573), the fifth coordinates are (7.44×10 −7 , 0.0849), and the sixth coordinates are (1.91×10 −6 , 0.0964). 
     By controlling the flow rate of the silane, the flow rate of the hydrogen, and the flow rate of the ammonia as described above, when large manufacturing apparatus  200  is used, the rapid growth of silicon carbide layer  20  can be achieved while the flatness of the surface of silicon carbide layer  20  and the in-plane uniformity of the carrier concentration are improved. In addition, when silicon carbide layers  20  are simultaneously grown on a plurality of silicon carbide single-crystal substrates  10  (from a different viewpoint, when batch processing is performed), variation in the in-plane uniformity of the carrier concentration between each of the plurality of silicon carbide epitaxial substrates  100  can be reduced. 
     (Method for Manufacturing Silicon Carbide Semiconductor Device) 
     Next, a method for manufacturing a silicon carbide semiconductor device  300  according to the present embodiment is described. 
     The method for manufacturing the silicon carbide semiconductor device according to the present embodiment mainly includes an epitaxial substrate preparing step (S 10 :  FIG. 13 ) and a substrate processing step (S 20 :  FIG. 13 ). 
     First, the epitaxial substrate preparing step (S 10 :  FIG. 13 ) is performed. Specifically, silicon carbide epitaxial substrate  100  is prepared with the above-described method for manufacturing silicon carbide epitaxial substrate  100  (see  FIG. 8 ). 
     Next, the substrate processing step (S 20 :  FIG. 13 ) is performed. Specifically, silicon carbide epitaxial substrate  100  is processed to manufacture the silicon carbide semiconductor device. The term “process” herein includes various types of processes such as ion implantation, heat treatment, etching, oxide film formation, electrode formation, and dicing. That is, the substrate processing step may include at least one process from the ion implantation, heat treatment, etching, oxide film formation, electrode formation, and dicing. 
     The following describes a method for manufacturing a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as an exemplary silicon carbide semiconductor device. The substrate processing step (S 20 :  FIG. 13 ) includes an ion implanting step (S 21 :  FIG. 13 ), an oxide film forming step (S 22 :  FIG. 13 ), an electrode forming step (S 23 :  FIG. 13 ), and a dicing step (S 24 :  FIG. 13 ). 
     First, the ion implanting step (S 21 :  FIG. 13 ) is performed. A p type impurity such as aluminum (Al) is implanted into third main surface  43  on which a mask (not shown) provided with an opening has been formed. A body region  132  having p type conductivity is thus formed. Next, an n type impurity such as phosphorus (P) is implanted into a predetermined position in body region  132 . A source region  133  having n type conductivity is thus formed. Next, a p type impurity such as aluminum is implanted into a predetermined position in source region  133 . A contact region  134  having p type conductivity is thus formed (see  FIG. 14 ). 
     In silicon carbide layer  20 , a portion other than body region  132 , source region  133  and contact region  134  serves as a drift region  131 . Source region  133  is separated from drift region  131  by body region  132 . The ion implantation may be performed with silicon carbide epitaxial substrate  100  being heated at more than or equal to about 300° C. and less than or equal to about 600° C. After the ion implantation, activation annealing is performed on silicon carbide epitaxial substrate  100 . The activation annealing activates the impurities implanted into silicon carbide layer  20 , to generate carriers in each region. The activation annealing may be performed in an argon (Ar) atmosphere, for example. The activation annealing may be performed at a temperature of about 1800° C., for example. The activation annealing may be performed for a period of about 30 minutes, for example. 
     Next, the oxide film forming step (S 22 :  FIG. 13 ) is performed. For example, silicon carbide epitaxial substrate  100  is heated in an atmosphere including oxygen, whereby oxide film  136  is formed on third main surface  43  (see  FIG. 15 ). Oxide film  136  is composed of silicon dioxide (SiO 2 ) or the like, for example. Oxide film  136  functions as a gate insulating film. The thermal oxidation process may be performed at a temperature of about 1300° C., for example. The thermal oxidation process may be performed for a period of about 30 minutes, for example. 
     After oxide film  136  has been formed, heat treatment may be further performed in a nitrogen atmosphere. For example, the heat treatment may be performed at about 1100° C. for about one hour in an atmosphere of nitrogen monoxide (NO), nitrous oxide (N 2 O), or the like. Further, heat treatment may be thereafter performed in an argon atmosphere. For example, the heat treatment may be performed at about 1100 to 1500° C. for about one hour in an argon atmosphere. 
     Next, the electrode forming step (S 23 :  FIG. 13 ) is performed. A first electrode  141  is formed on oxide film  136 . First electrode  141  functions as a gate electrode. First electrode  141  is formed by CVD, for example. First electrode  141  is composed of polysilicon or the like that contains an impurity and has conductivity, for example. First electrode  141  is formed at a position facing source region  133  and body region  132 . 
     Next, an interlayer insulating film  137  is formed to cover first electrode  141 . Interlayer insulating film  137  is formed by CVD, for example. Interlayer insulating film  137  is composed of silicon dioxide or the like, for example. Interlayer insulating film  137  is formed in contact with first electrode  141  and oxide film  136 . Next, oxide film  136  and interlayer insulating film  137  at a prescribed position are removed by etching. Source region  133  and contact region  134  are thus exposed at oxide film  136 . 
     A second electrode  142  is formed on the exposed portion by sputtering, for example. Second electrode  142  functions as a source electrode. Second electrode  142  is composed of titanium, aluminum, silicon and the like, for example. After second electrode  142  has been formed, second electrode  142  and silicon carbide epitaxial substrate  100  are heated at a temperature of about 900 to 1100° C., for example. Second electrode  142  and silicon carbide epitaxial substrate  100  are thus brought into ohmic contact with each other. Next, an interconnection layer  138  is formed in contact with second electrode  142 . Interconnection layer  138  is composed of a material including aluminum, for example. 
     Next, a third electrode  143  is formed on second main surface  42 . Third electrode  143  functions as a drain electrode, Third electrode  143  is composed of, for example an alloy including nickel and silicon (for example, NiSi or the like). 
     Next, the dicing step (S 24 :  FIG. 13 ) is performed. For example, silicon carbide epitaxial substrate  100  is diced along a dicing line, whereby silicon carbide epitaxial substrate  100  is divided into a plurality of semiconductor chips. Silicon carbide semiconductor device  300  is manufactured in this manner (see  FIG. 16 ). 
     Although the method for manufacturing the silicon carbide semiconductor device according to the present disclosure has been described above with reference to a MOSFET as an example, the manufacturing method according to the present disclosure is not limited as such. The manufacturing method according to the present disclosure is applicable to various types of silicon carbide semiconductor devices such as an IGBT (Insulated Gate Bipolar Transistor), a SBD (Schottky Barrier Diode), a thyristor, a GTO (Gate Turn Off thyristor), and a PiN diode. 
     (Evaluation) 
     (Preparation of Samples) 
     Manufacturing apparatus  200  of batch type shown in  FIGS. 1 to 3  was used to simultaneously manufacture three silicon carbide epitaxial substrates  100  according to each of samples 1 to 10. The area of substrate placement surface  211  of susceptor  210  was set to 929 cm 2 . The cross-sectional area of reaction chamber  201  was set to 176 cm 2 . Silicon carbide epitaxial substrates  100  according to samples 1 to 7 are samples of examples. Silicon carbide epitaxial substrates  100  according to samples 8 to 10 are samples of comparative examples. 
     As shown in Table 1, in the methods for manufacturing silicon carbide epitaxial substrates  100  according to samples 1 to 10, the H 2  flow rate was set to 134 slm, and the SiH 4  flow rate was changed to change the SiH 4  flow rate/H 2  flow rate. In the methods for manufacturing silicon carbide epitaxial substrates  100  according to samples 1 to 10, the SiH 4  flow rate was set to 51.0 sccm, 92.6 sccm, 237.4 sccm, 51.0 sccm, 92.6 sccm, 237.4 sccm, 92.6 sccm, 92.6 sccm, 35.7 sccm and 92.6 sccm, respectively. In the methods for manufacturing silicon carbide epitaxial substrates  100  according to samples 1 to 10, the NH 3  flow rate was set to 0.0019 sccm, 0.0028 sccm, 0.0032 sccm, 0.0573 sccm, 0.0849 sccm, 0.0964 sccm, 0.0283 sccm, 0.1415 sccm, 0.0107 sccm and 0.0020 sccm, respectively. 
     In the methods for manufacturing silicon carbide epitaxial substrates  100  according to samples 1 to 10, the SiH 4  flow rate/H 2  flow rate was set to 0.038%, 0.069%, 0.177%, 0.038%, 0.069%, 0.177%, 0.069%, 0.069%, 0.027% and 0.069%, respectively. In the methods for manufacturing silicon carbide epitaxial substrates  100  according to samples 1 to 10, the value obtained by dividing the SiH 4  flow rate/H 2  flow rate by the area of substrate placement surface  211  of susceptor  210  was set to 4.10×10 −7  cm −2 , 7.44×10 −7  cm −2 , 1.91×10 −6  cm −2 , 4.10×10 −7  cm −2 , 7.44×10 −7  cm −2 , 1.91×10 −6  cm −2 , 7.44×10 −7  cm −2 , 7.44×10 −7  cm −2 , 2.87×10 −7  cm −2  and 7.44×10 −7  cm −2 , respectively. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 11 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Reaction 
                 (SiH 4 /H 2 )/ 
                   
               
               
                   
                   
                   
                   
                   
                   
                 Chamber 
                 Reaction 
                   
               
               
                   
                   
                   
                   
                   
                 (SiH 4 /H 2 )/ 
                 Cross- 
                 Chamber 
                   
               
               
                   
                   
                   
                   
                 Susceptor 
                 Susceptor 
                 Sectional 
                 Cross- 
                   
               
               
                 Sample 
                 H 2   
                 SiH 4   
                 SiH 4 /H 2   
                 Area  
                 Area  
                 Area  
                 Sectional 
                 NH 3   
               
               
                 No. 
                 [slm] 
                 [sccm] 
                 [%] 
                 [cm 2 ] 
                 [cm −2 ] 
                 [cm 2 ] 
                 Area [cm −2 ] 
                 [sccm] 
               
               
                   
               
             
            
               
                 Sample 1 
                 134 
                  51.0 
                 0.038% 
                 929 
                 4.10 × 10 −7   
                 176 
                 2.16 × 10 −6   
                 0.0019 
               
               
                 Sample 2 
                 134 
                  92.6 
                 0.069% 
                 929 
                 7.44 × 10 −7   
                 176 
                 3.93 × 10 −6   
                 0.0028 
               
               
                 Sample 3 
                 134 
                 237.4 
                 0.177% 
                 929 
                 1.91 × 10 −6   
                 176 
                 1.01 × 10 −5   
                 0.0032 
               
               
                 Sample 4 
                 134 
                  51.0 
                 0.038% 
                 929 
                 4.10 × 10 −7   
                 176 
                 2.16 × 10 −6   
                 0.0573 
               
               
                 Sample 5 
                 134 
                  92.6 
                 0.069% 
                 929 
                 7.44 × 10 −7   
                 176 
                 3.93 × 10 −6   
                 0.0849 
               
               
                 Sample 6 
                 134 
                 237.4 
                 0.177% 
                 929 
                 1.91 × 10 −6   
                 176 
                 1.01 × 10 −5   
                 0.0964 
               
               
                 Sample 7 
                 134 
                  92.6 
                 0.069% 
                 929 
                 7.44 × 10 −7   
                 176 
                 3.93 × 10 −6   
                 0.0283 
               
               
                 Sample 8 
                 134 
                  92.6 
                 0.069% 
                 929 
                 7.44 × 10 −7   
                 176 
                 3.93 × 10 −6   
                 0.1415 
               
               
                 Sample 9 
                 134 
                  35.7 
                 0.027% 
                 929 
                 2.87 × 10 −7   
                 176 
                 1.51 × 10 −6   
                 0.0107 
               
               
                 Sample 10 
                 134 
                  92.6 
                 0.069% 
                 929 
                 7.44 × 10 −7   
                 176 
                 3.93 × 10 −6   
                 0.0020 
               
               
                   
               
            
           
         
       
     
     (Measurement) 
     The carrier concentration in silicon carbide layer  20  of silicon carbide epitaxial substrate  100  according to each of samples 1 to 10 was measured by a mercury probe type C-V measuring device. The carrier concentration was measured in a region extending by at most 60 mm in radius from the center of third main surface  43 . The carrier concentration was measured at a plurality of positions located at substantially regular intervals on a straight line passing through the center of third main surface  43  and parallel to the radial direction, and on a straight line perpendicular to this straight line. Specifically, the carrier concentration was measured at the center of third main surface  43 , and at positions spaced by 10 mm, 20 mm, 30 mm, 40 mm, 50 mm and 60 mm from the center in the radial direction. The carrier concentration was measured at a total of 25 locations. The average value of the carrier concentration is an arithmetic mean of measured values at these 25 locations. The in-plane uniformity of the carrier concentration is a representation, in percentage, of a value obtained by dividing the standard deviation of the carrier concentration by the average value of the carrier concentration. Note that the probe at the mercury side had an area of 0.01 cm 2 . 
     A root-mean-square deviation (Sq) in a central region of third main surface  43  was measured with a white-light interference microscope. A region for which the root-mean-square deviation was measured was a square region having each side of 250 μm. The root-mean-square deviation was measured at the center of third main surface  43 , and at a position spaced by 50 mm from the center in the radial direction. The root-mean-square deviation was measured at a total of two locations. The root-mean-square deviation (Sq) of third main surface  43  was used for morphology of third main surface  43 . 
     (Results) 
       FIGS. 17 and 18  show manufacturing conditions for silicon carbide epitaxial substrates  100  according to samples 1 to 10. Coordinates  101  to  110  correspond to the manufacturing conditions for silicon carbide epitaxial substrates  100  according to samples 1 to 10, respectively. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Substrate A 
                 Substrate B 
                 Substrate C 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Average 
                 Carrier Con- 
                 Mor- 
                 Average 
                 Carrier Con- 
                 Mor- 
                 Average 
                 Carrier Con- 
                 Mor- 
               
               
                   
                 Carrier Con- 
                 centration 
                 phology 
                 Carrier Con- 
                 centration 
                 phology 
                 Carrier Con- 
                 centration 
                 phology 
               
               
                 Sample  
                 centration 
                 Uniformity 
                 &lt;Sq&gt; 
                 centration 
                 Uniformity 
                 &lt;Sq&gt; 
                 centration 
                 Uniformity 
                 &lt;Sq&gt; 
               
               
                 No. 
                 (cm −3 ) 
                 (%) 
                 (nm) 
                 (cm −3 ) 
                 (%) 
                 (nm) 
                 (cm −3 ) 
                 (%) 
                 (nm) 
               
               
                   
               
               
                 Sample 1 
                 1.1 × 10 15   
                  8.2 
                 0.1 
                 1.0 × 10 15   
                  8.6 
                 0.2 
                 1.0 × 10 15   
                  8.4 
                 0.3 
               
               
                 Sample 2 
                 1.0 × 10 15   
                  8.5 
                 0.2 
                 1.0 × 10 15   
                  8.2 
                 0.2 
                 1.1 × 10 15   
                  8.3 
                 0.2 
               
               
                 Sample 3 
                 1.1 × 10 15   
                  8.4 
                 0.3 
                 1.1 × 10 15   
                  8.2 
                 0.4 
                 1.0 × 10 15   
                  8.3 
                 0.3 
               
               
                 Sample 4 
                 3.0 × 10 15   
                  6.8 
                 0.3 
                 2.9 × 10 15   
                  6.8 
                 0.2 
                 3.0 × 10 15   
                  6.9 
                 0.4 
               
               
                 Sample 5 
                 3.0 × 10 15   
                  6.2 
                 0.3 
                 3.0 × 10 15   
                  6.2 
                 0.4 
                 3.0 × 10 15   
                  6.4 
                 0.4 
               
               
                 Sample 6 
                 1.1 × 10 15   
                  6.3 
                 0.2 
                 3.1 × 10 15   
                  6.4 
                 0.3 
                 3.0 × 10 15   
                  6.6 
                 0.2 
               
               
                 Sample 7 
                 1.0 × 10 16   
                  7.5 
                 0.3 
                 9.9 × 10 15   
                  7.6 
                 0.2 
                 1.0 × 10 16   
                  7.5 
                 0.3 
               
               
                 Sample 8 
                 5.1 × 10 16   
                 15.8 
                 0.2 
                 5.1 × 10 16   
                 15.5 
                 0.4 
                 5.1 × 10 16   
                 16.2 
                 0.2 
               
               
                 Sample 9 
                 8.0 × 10 15   
                  8.1 
                 0.7 
                 7.9 × 10 15   
                  8.2 
                 0.7 
                 8.0 × 10 15   
                  7.9 
                 0.6 
               
               
                 Sample 10 
                 7.0 × 10 14   
                 16.2 
                 0.4 
                 7.0 × 10 14   
                 16.4 
                 0.2 
                 7.0 × 10 14   
                 16.2 
                 0.2 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, the in-plane uniformities of the carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates A) according to samples 1 to 10 were 8.2%, 8.5%, 8.4%, 6.8%, 6.2%, 6.3%, 7.5%, 15.8%, 8.1% and 16.2%, respectively. The root-mean-square deviations (Sq) of the third surfaces of silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates A) according to samples 1 to 10 were 0.1 nm, 0.2 nm, 0.3 nm, 0.3 nm, 0.3 nm, 0.2 nm, 0.3 nm, 0.2 nm, 0.7 nm and 0.4 nm, respectively. Further, the average carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates A) according to samples 1 to 10 were 1.1×10 15  cm −3 , 1.0×10 15  cm −3 , 1.1×10 15  cm −3 , 3.0×10 15  cm −3 , 3.0×10 15  cm −3 , 3.1×10 15  cm −3 , 1.0×10 16  cm −3 , 5.1×10 16  cm −3 , 8.0×10 15  cm −3  and 7.0×10 14  cm −3 , respectively. 
     As shown in Table 2, the in-plane uniformities of the carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates B) according to samples 1 to 10 were 8.6%, 8.2%, 8.2%, 6.8%, 6.2%, 6.4%, 7.6%, 15.5%, 8.2% and 16.4%, respectively. The root-mean-square deviations (Sq) of the third surfaces of silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates B) according to samples 1 to 10 were 0.2 nm, 0.2 nm, 0.4 nm, 0.2 nm, 0.4 nm, 0.3 nm, 0.2 nm, 0.4 nm, 0.7 nm and 0.2 nm, respectively. Further, the average carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates B) according to samples 1 to 10 were 1.0×10 15  cm −3 , 1.0×10 15  cm −3 , 1.1×10 15  cm −3 , 7.9×10 15  cm −3 , 3.0×10 15  cm −3 , 3.1×10 15  cm −3 , 9.9×10 15  cm −3 , 5.1×10 16  cm −3 , 7.9×10 15  cm −3  and 7.0×10 14  cm −3 , respectively. 
     As shown in Table 2, the in-plane uniformities of the carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates C) according to samples 1 to 10 were 8.4%, 8.3%, 8.3%, 6.9%, 6.4%, 6.6%, 7.5%, 16.2%, 7.9% and 16.2%, respectively. The root-mean-square deviations (Sq) of the third surfaces of silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates C) according to samples 1 to 10 were 0.3 nm, 0.2 nm, 0.3 nm, 0.4 nm, 0.4 nm, 0.2 nm, 0.3 nm, 0.2 nm, 0.6 nm and 0.2 nm, respectively. Further, the average carrier concentrations in silicon carbide layers  20  of silicon carbide epitaxial substrates  100  (substrates C) according to samples 1 to 10 were 1.0×10 15  cm −3 , 1.1×10 15  cm −3 , 1.0×10 15  cm −3 , 3.0×10 15  cm −3 , 3.0×10 15  cm −3 , 3.0×10 15  cm −3 , 1.0×10 16  cm −3 , 5.1×10 16  cm −3 , 8.0×10 15  cm −3  and 7.0×10 14  cm −3 , respectively. 
     It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims rather than the embodiments described above, and is intended to include any modifications within the meaning and scope equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       10  silicon carbide single-crystal substrate;  20  silicon carbide layer;  41  first main surface;  42  second main surface;  43  third main surface;  44  fourth main surface;  100  silicon carbide epitaxial substrate;  101  to  110  coordinate;  131  drift region;  132  body region;  133  source region;  134  contact region;  136  oxide film;  137  interlayer insulating film;  138  interconnection layer;  141  first electrode;  142  second electrode;  143  third electrode;  200  manufacturing apparatus;  201  reaction chamber;  202  stage;  203  heating element;  204  quartz tube;  205  inner wall surface;  206  gas supply hole;  207  gas inlet;  208  gas outlet;  209  rotation shaft;  210  susceptor;  211  placement surface;  212  bottom surface;  213  placement portion;  214  top surface;  215  side surface;  216  center;  231  first gas supplier;  232  second gas supplier;  233  third gas supplier;  234  carrier gas supplier;  235  gas supplier;  241  first gas flow rate controller;  242  second gas flow rate controller;  243  third gas flow rate controller;  244  carrier gas flow rate controller;  245  controller;  250  diameter;  300  silicon carbide semiconductor device.