Patent Publication Number: US-2023162978-A1

Title: Semiconductor film and method for manufacturing same

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor film and a method for manufacturing the same. 
     BACKGROUND ART 
     A technique for growing a β-Ga 2 O 3 -based single crystal film by the HVPE (Halide Vapor Phase Epitaxy) method is known (see, e.g., Patent Literature 1). In the technique described in Patent Literature 1, a β-Ga 2 O 3 -based single crystal film is epitaxially grown on a Ga 2 O 3 -based substrate by flowing a gallium source gas, an oxygen source gas, and a dopant source gas into a region of a reaction chamber of a vapor phase growth apparatus in which the Ga 2 O 3 -based substrate is placed. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: Japanese Patent No. 5984069 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In growing a β-Ga 2 O 3 -based single crystal film by the HVPE method, however, a problem may arise that killer defects, which are critical defects causing significant degradation in device characteristics, are more likely to occur than in growing a single crystal film of another nitride semiconductor such as GaN by the HVPE method, and the causes of killer defects and methods for reducing killer defects have not been disclosed in the known techniques. 
     Thus, it is an object of the invention to provide a semiconductor film formed of a β-Ga 2 O 3 -based single crystal with few killer defects, and a method for manufacturing the semiconductor film. 
     Solution to Problem 
     To achieve the object, an aspect of the present invention provides a method for manufacturing a semiconductor film defined in (1) to (4) below, and a semiconductor film defined in (5) below. 
     (1) A method for manufacturing a semiconductor film, comprising: 
     placing a semiconductor substrate comprising a β-Ga 2 O 3 -based single crystal in a reaction chamber of an HVPE apparatus so that a growth base surface of the semiconductor substrate faces upward or downward; and 
     epitaxially growing a semiconductor film comprising a β-Ga 2 O 3 -based single crystal on the growth base surface of the semiconductor substrate by flowing a Ga chloride gas, an oxygen-including gas and a dopant-including gas into a space in the reaction chamber where the semiconductor substrate is placed, 
     wherein when the semiconductor substrate is placed so that the growth base surface faces upward, an inlet for the dopant-including gas into the space is positioned higher than an inlet for the oxygen-including gas into the space and an inlet for the Ga chloride gas into the space is positioned higher than the inlet for the dopant-including gas into the space, and 
     wherein when the semiconductor substrate is placed so that the growth base surface faces downward, the inlet for the dopant-including gas into the space is positioned higher than the inlet for the Ga chloride gas into the space and the inlet for the oxygen-including gas into the space is positioned higher than the inlet for the dopant-including gas into the space. 
     (2) The method for manufacturing a semiconductor film defined in (1), wherein the Ga chloride gas comprises a GaCl gas, wherein the oxygen-including gas comprises an O 2  gas, and wherein the dopant-including gas comprises a SiCl 4  gas.
 
(3) The method for manufacturing a semiconductor film defined in (1) or (2), wherein in the placing the semiconductor substrate, the semiconductor substrate is placed in the reactor chamber so that the growth base surface faces downward.
 
(4) The method for manufacturing a semiconductor film defined in (3), wherein in the epitaxially growing the semiconductor film, the Ga chloride gas, the oxygen-including gas and the dopant-including gas are flowed into the space at a flow velocity of not less than 110 cm/s.
 
(5) A semiconductor film, comprising:
 
     a β-Ga 2 O 3 -based single crystal comprising Cl, 
     wherein an in-plane density of defects continuing from a front surface to a back surface in a thickness direction is not more than 10 defects/cm 2 . 
     Advantageous Effects of Invention 
     According to the invention, it is possible to provide a semiconductor film formed of a β-Ga 2 O 3 -based single crystal with few killer defects, and a method for manufacturing the semiconductor film. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a vertical cross-sectional view showing a crystal laminated structure in the first embodiment of the present invention. 
         FIG.  2 A  is a schematic diagram illustrating a state of a placed semiconductor substrate  10  and positions of introducing source gases in the first embodiment of the invention. 
         FIG.  2 B  is a schematic diagram illustrating a state of the placed semiconductor substrate  10  and positions of introducing the source gases in Comparative Example. 
         FIG.  3    is a schematic diagram illustrating a state of the placed semiconductor substrate  10  and positions of introducing the source gases in the second embodiment of the invention. 
         FIG.  4 A  is an emission microscope observation image showing a semiconductor film formed by a method according to the second embodiment. 
         FIG.  4 B  is an emission microscope observation image showing a semiconductor film as Comparative Example formed by a known method. 
         FIG.  5 A  is an optical microscope observation image showing a surface of a semiconductor film formed by the method according to the second embodiment on which etch pits appeared. 
         FIG.  5 B  is an optical microscope observation image showing a surface of a semiconductor film as Comparative Example formed by the known method on which etch pits appeared. 
         FIG.  6    is a vertical cross-sectional view showing a Schottky barrier diode used to evaluate Schottky barrier diode characteristics described later. 
         FIG.  7 A  is a graph showing reverse leakage characteristics of a Schottky barrier diode which includes a semiconductor film formed by a method according to the first embodiment. 
         FIG.  7 B  is a graph showing reverse leakage characteristics of a Schottky barrier diode which includes a semiconductor film formed by the method according to the second embodiment. 
         FIG.  8    is a graph showing reverse leakage characteristics of a Schottky barrier diode which includes a semiconductor film as Comparative Example formed by the known method. 
         FIG.  9 A  is a diagram illustrating distribution of breakdown voltage characteristics of Schottky barrier diodes on a 2-inch wafer before dicing into individual Schottky barrier diodes provided with a semiconductor film formed by the method according to the second embodiment. 
         FIG.  9 B  is a diagram illustrating distribution of breakdown voltage characteristics of Schottky barrier diodes on a 2-inch wafer before dicing into individual Schottky barrier diodes provided with a semiconductor film formed by the method according to the second embodiment. 
         FIG.  9 C  is a diagram illustrating distribution of breakdown voltage characteristics of Schottky barrier diodes on a 2-inch wafer before dicing into individual Schottky barrier diodes provided with a semiconductor film formed by the method according to the second embodiment. 
         FIG.  10 A  is a graph showing reverse characteristics of a Schottky barrier diode which includes a semiconductor film formed by the method according to the second embodiment and a 2.3-mm square anode electrode. 
         FIG.  10 B  is a graph showing forward characteristics of the Schottky barrier diode which includes the semiconductor film formed by the method according to the second embodiment and the 2.3-mm square anode electrode. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To investigate the cause of killer defects generated in β-Ga 2 O 3 -based single crystal films grown by the HVPE method, the present inventors made plural Schottky barrier diodes of different element sizes which have a β-Ga 2 O 3  single crystal film grown by a known common method using the HVPE method, and then evaluated their reverse breakdown voltage characteristics. 
     The result was as follows: elements having near-ideal characteristics with suppressed reverse leakage current were obtained with a yield of about 80% when the element size (the size of the circular anode electrode) was not more than 400 μm in diameter, but the number of elements with large reverse leakage current increased as the element size increased, and the yield of elements with a diameter of 1 mm was substantially 0%. 
     From the fact that the element characteristics depend on the element size as described above, it was presumed that killer defects were present in crystal films. In addition, from the yield and the anode electrode area, it was estimated that the density of killer defects in the β-Ga 2 O 3  crystal film grown by the known method was about 200 defects/cm 2 . The killer defect density directly affected the manufacturable element size, i.e., the maximum current rating of devices, and the β-Ga 2 O 3  crystal film made by the known method allowed to manufacture only Schottky barrier diodes with a rated current (a current value when a forward voltage of about 1.5 to 2.5 V is applied) of not more than 1A. 
     Then, as a result of intensive study, the inventors found that the main cause of killer defects in β-Ga 2 O 3 -based crystal films is grains of Ga oxide formed in a vapor phase in a reaction chamber of an HVPE apparatus. In growth of β-Ga 2 O 3 -based single crystal films by the HVPE method, the reaction rate of oxygen with Ga chloride gas is so fast that the reaction occurs in the vapor phase and grains of Ga oxide are formed and deposited on Ga 2 O 3 -based substrate. The grains of Ga oxide, when incorporated into an epitaxially growing β-Ga 2 O 3 -based single crystal film, causes disturbance of crystal periodicity, resulting in occurrence of many killer defects that continues from a front surface to a back surface in a thickness direction of the film and could act as leakage paths. 
     Meanwhile, it has been confirmed that generation of many killer defects as in the case of growing β-Ga 2 O 3 -based crystal films does not occur when growing GaN-based crystal films by the HVPE method. It is considered that this is because the reaction rate of ammonia gas with Ga chloride gas is relatively slow and formation of grains of GaN-based compound in the vapor phase hardly occur. 
     This invention relates to a method for manufacturing a semiconductor film formed of a β-Ga 2 O 3 -based single crystal, which can reduce the amount of Ga oxide grains formed in a vapor phase and deposited on a Ga 2 O 3 -based substrate in a reaction chamber of an HVPE apparatus and reduce killer defects, and a semiconductor film which is manufactured by the manufacturing method, is formed of a β-Ga 2 O 3 -based single crystal and includes few killer defects. 
     First Embodiment 
     (Configuration of a Crystal Laminated Structure) 
       FIG.  1    is a vertical cross-sectional view showing a crystal laminated structure  1  in the first embodiment of the invention. The crystal laminated structure  1  has a semiconductor substrate  10  formed of a β-Ga 2 O 3 -based single crystal, and a semiconductor film  12  formed of a β-Ga 2 O 3 -based single crystal and epitaxially grown on a growth base surface  11  of the semiconductor substrate  10 . 
     The β-Ga 2 O 3 -based single crystal here is a β-Ga 2 O 3  single crystal which is a Ga 2 O 3  single crystal having a β-crystal structure, or a β-Ga 2 O 3  single crystal doped with elements such as Al and In, and may be, e.g., a (Ga x Al y In (1-x-y) ) 2 O 3  (0&lt;x≤1, 0≤y≤1, 0&lt;x+y≤1) single crystal which is a β-Ga 2 O 3  single crystal doped with Al and In. The band gap is widened by adding Al and is narrowed by adding In. The semiconductor substrate  10  may also include a dopant such as Si. 
     A plane orientation of the growth base surface  11  of the semiconductor substrate  10  is, e.g., (001), (010), (100), (011), (−201), or (101). 
     To form the semiconductor substrate  10 , e.g., a bulk crystal of a Ga 2 O 3 -based single crystal grown by a melt-growth technique such as the FZ (Floating Zone) method or the EFG (Edge Defined Film Fed Growth) method is sliced and the surface thereof is then polished. 
     The semiconductor film  12  includes Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, B, Al, In, Tl, C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi, S, Se, Te, etc., as a dopant doped while growing crystal. 
     A concentration of the dopant included in the semiconductor film  12  is, e.g., not less than 1×10 13  atoms/cm 3  and not more than 5×10 20  atoms/cm 3 , preferably, not less than 6.5×10 15  atoms/cm 3  and not more than 2.1×10 20  atoms/cm 3 . Meanwhile, a density of carrier generated by the doping of the dopant is, e.g., not less than 1×10 15  cm −3  and not more than 1×10 20  cm 3 . 
     The semiconductor film  12  also includes Cl at a concentration of not more than 5×10 16  atoms/cm 3 . This results from that the semiconductor film  12  is formed by the HVPE method using Cl-including gas. Generally, Cl-including gas is not used when forming a β-Ga 2 O 3 -based single crystal film by a method other than the HVPE method, and the (3-Ga 2 O 3 -based single crystal film does not include Cl, or at least does not include not less than 1×10 16  cm 3  of Cl. 
     The semiconductor film  12  is formed by the HVPE (Halide Vapor Phase Epitaxy) method with a high crystal growth rate, and thus can be formed thick, e.g., can be formed to have a thickness of not less than 1000 nm. In general, a growth rate of the β-Ga 2 O 3 -based single crystal film by industrial HVPE is 200 μm/h, and in this case, a film of up to 1000 μm in thickness can be formed in a realistic time. In other words, it is possible to form the semiconductor film  12  having a thickness of not less than 1000 nm and not more than 1000 μm. Meanwhile, use of the MBE method is not realistic in actual production since a crystal growth rate of the β-Ga 2 O 3 -based single crystal film is about 120 nm/h and it requires not less than 8 hours to form a film of not less than 1000 nm in thickness. 
     Meanwhile, an in-plane density of killer defects continuing from a front surface to a back surface in a thickness direction of the semiconductor film  12  can be not more than 10 defects/cm 2  when using a method for manufacturing the semiconductor film  12  described later. The in-plane density of killer defects included in the semiconductor film  12  can be measured by observation with an emission microscope at an anode bias of −200V. 
     (Structure of an HVPE Apparatus) 
     Next, an example structure of an HVPE apparatus used to grow the semiconductor film  12  in the first embodiment of the invention will be described. 
     An HVPE apparatus  2  whose vertical cross section is shown in  FIGS.  2 A and  2 B  is used in the method for manufacturing a semiconductor film in the first embodiment of the invention. The HVPE apparatus  2  is a vapor phase growth apparatus for the HVPE method, and has a reaction chamber  20  having a space  24  for placement of the semiconductor substrate  10 , and a first gas introducing port  21 , a second gas introducing port  22  and a third gas introducing port  23  to introduce source gases for growth of the semiconductor film  12  into the space  24 . The reaction chamber  20  is formed of, e.g., quartz glass. 
     The HVPE apparatus  2  also has a heating unit (not shown) placed around the reaction chamber  20  to heat the space  24  in the reaction chamber  20 . This heating unit is, e.g., a resistive or radiation heating device. 
     The source gases for growth of the semiconductor film  12  are introduced into the space  24  of the reaction chamber  20  through the first gas introduction port  21 , the second gas introduction port  22 , and the third gas introduction port  23 . 
     An inlet  21   a  of the first gas introducing port  21  to the space  24 , an inlet  22   a  of the second gas introducing port  22  to the space  24 , and an inlet  23   a  of the third gas introducing port  23  to the space  24  have different heights (different positions in a direction perpendicular to the growth base surface  11  of the placed semiconductor substrate  10 ), in such a manner that the inlet  22   a  is located above the inlet  23   a  and the inlet  21   a  is located above the inlet  22   a . In addition, to prevent variation in condition of the semiconductor film  12  in the in-plane direction of the growth base surface  11 , the positions of the inlets  21   a ,  22   a  and  23   a  in a horizontal direction orthogonal to the flowing direction of the source gas (positions in a direction perpendicular to the paper surface of  FIGS.  2 A and  2 B ) are substantially the same. 
     The source gases for growth of the semiconductor film  12  include a gallium source gas which is a Ga chloride gas such as GaCl gas, GaCl 2  gas, GaCl 3  gas or (GaCl 3 ) 2  gas, an oxygen source gas which is an oxygen-including gas such as O 2  gas or H 2 O gas, and a dopant source gas which is a dopant-including gas such as SiCl 4  gas, GeCl 4  gas, SnCl 4  gas or PbCl 2  gas. 
     Each source gas is flowed into the reaction chamber  20  using an inert gas such as Ar gas or N 2  gas as a carrier gas. 
     (Method for Manufacturing a Semiconductor Film) 
     The method for manufacturing the semiconductor film  12  in the first embodiment of the invention includes a step of placing the semiconductor substrate  10  formed of a β-Ga 2 O 3 -based single crystal in the reaction chamber  20  of the HVPE apparatus  2 , and a step of epitaxially growing the semiconductor film  12  formed of a β-Ga 2 O 3 -based single crystal on the growth base surface  11  of the semiconductor substrate  10  by flowing the Ga chloride gas, the oxygen-including gas and the dopant-including gas into the space  24  in the reaction chamber  20  where the semiconductor substrate  10  is placed. 
     In the first embodiment, the semiconductor substrate  10  is placed on a bottom side of the space  24  in the reaction chamber  20  so that the growth base surface  11  faces upward. Here, the growth base surface  11  facing upward means a state in which the growth base surface  11  faces a side opposite to a ground surface and an angle formed by the growth base surface  11  and a horizontal plane is in a range of −5 to +5°. 
     The Ga chloride gas, the oxygen-including gas and the dopant-including gas flowed in through the inlets  21   a ,  22   a  and  23   a  are mixed in the space  24 . Then, the growth base surface  11  of the semiconductor substrate  10  is exposed to the gas mixture and the semiconductor film  12  is epitaxially grown. 
     In the step of epitaxially growing the semiconductor film  12 , pressure in the space  24  is maintained at, e.g., 1 atm. Meanwhile, a growth temperature of not less than 900° C. is required to grow the semiconductor film  12 . Single crystal may not be obtained at less than 900° C. 
     Here, it is preferable to use GaCl gas as the Ga chloride gas which is the gallium source gas. The temperature at which a driving force for growth of Ga 2 O 3  crystal is maintained is the highest with the GaCl gas among the Ga chloride gases. Growth at a high temperature is effective to obtain a high-quality Ga 2 O 3  crystal with high purity, hence, it is preferable to use a GaCl gas having a high driving force for growth at high temperature. 
     Meanwhile, as the dopant-including gas which is the dopant source gas, it is preferable to use a chloride-based gas to suppress unintentional inclusion of other impurities, and when, e.g., Si, Ge, Sn or Pb, which are Group 14 elements among the dopants listed above, is used as a dopant, a chloride-based gas used is respectively SiCl 4 , GeCl 4 , SnCl 4  or PbCl 2 , etc. In addition, the chloride-based gas is not limited to those compounded with only chlorine, and, e.g., a silane-based gas such as SiHCl 3  may be used. 
     The dopant such as Si is doped while growing the β-Ga 2 O 3 -based single crystal. 
     Meanwhile, if hydrogen is included in an atmosphere during the growth of the semiconductor film  12 , surface flatness of the semiconductor film  12  and a driving force for crystal growth decrease. Therefore, it is preferable that an O 2  gas not including hydrogen be used as the oxygen-including gas which is the oxygen source gas. 
     The method for manufacturing the semiconductor film  12  in the first embodiment of the invention is characterized in a positional relationship between the semiconductor substrate  10  and the respective introducing ports for the gallium source gas, the oxygen source gas and the dopant source gas in the reaction chamber  20  of the HVPE apparatus  2 . 
       FIG.  2 A  is a schematic diagram illustrating a state of the placed semiconductor substrate  10  and the positions of introducing the source gases in the first embodiment of the invention.  FIG.  2 B  is a schematic diagram illustrating a state of the placed semiconductor substrate  10  and the positions of introducing the source gases in Comparative Example. 
     In the first embodiment of the invention, the Ga chloride gas and a carrier gas (GaCl gas and Ar gas are shown as an example) are flowed in through the first gas introducing port  21 , the dopant-including gas and a carrier gas (SiCl 4  gas and Ar gas are shown as an example) are flowed in through the second gas introducing port  22 , and the oxygen-including gas and a carrier gas (O 2  gas and Ar gas are shown as an example) are flowed in through the third gas introducing port  23 , as shown in  FIG.  2 A . 
     On the other hand, in Comparative Example shown in  FIG.  2 B , the oxygen-including gas and a carrier gas (O 2  gas and Ar gas are shown as an example) are flowed in through the first gas introducing port  21 , the dopant-including gas and a carrier gas (SiCl 4  gas and Ar gas are shown as an example) are flowed in through the second gas introducing port  22 , and the Ga chloride gas and a carrier gas (GaCl gas and Ar gas are shown as an example) are flowed in through the third gas introducing port  23 , in the same manner as a known method for growing a β-Ga 2 O 3 -based single crystal by the HVPE method. 
     As a result of intense study, the present inventors found that when the positions of introducing the source gases are set to the positions shown in  FIG.  2 A , the number of killer defects in the growing semiconductor film  12  is reduced as compared to when setting to the positions shown in  FIG.  2 B . It is considered that by making the oxygen-including gas reach the growth base surface  11  most easily and the Ga chloride gas reach the growth base surface  11  least easily, the amount of Ga oxide grains formed in the vapor phase and deposited on the growth base surface  11  is reduced for some reason. 
     In other words, when the semiconductor substrate  10  is placed with the growth base surface  11  facing upward, the number of killer defects in the growing semiconductor film  12  is reduced by configuring such that the inlet for the dopant-including gas into the space  24  is positioned higher than the inlet for the oxygen-including gas into the space  24  and the inlet for the Ga chloride gas into the space  24  is positioned higher than the inlet for the dopant-including gas into the space  24 . 
     Second Embodiment 
     The second embodiment of the invention is different from the first embodiment in the orientation of the semiconductor substrate  10  placed in the reaction chamber  20  of the HVPE apparatus  2 . The description for the same features as those in the first embodiment may be omitted or simplified. 
     (Method for Manufacturing a Semiconductor Film) 
     In the method for manufacturing the semiconductor film  12  in the second embodiment of the invention, the semiconductor substrate  10  is placed on an upper side of the space  24  in the reaction chamber  20  so that the growth base surface  11  faces downward. Here, the growth base surface  11  facing downward means a state in which the growth base surface  11  faces the ground surface side and the angle formed by the growth base surface  11  and the horizontal plane is in a range of −5 to +5°. 
       FIG.  3    is a schematic diagram illustrating a state of the placed semiconductor substrate  10  and positions of introducing the source gases in the second embodiment of the invention. 
     In the second embodiment of the invention, the semiconductor substrate  10  is placed with the growth base surface  11  facing downward, as shown in  FIG.  3   . Therefore, the oxygen-including gas and a carrier gas (O 2  gas and Ar gas are shown as an example) are flowed in through the first gas introducing port  21 , the dopant-including gas and a carrier gas (SiCl 4  gas and Ar gas are shown as an example) are flowed in through the second gas introducing port  22 , and the Ga chloride gas and a carrier gas (GaCl gas and Ar gas are shown as an example) are flowed in through the third gas introducing port  23 . 
     In other words, the inlet for the dopant-including gas into the space  24  is positioned higher than the inlet for the Ga chloride gas into the space  24  and the inlet for the oxygen-including gas into the space  24  is positioned higher than the inlet for the dopant-including gas into the space  24 . This makes the oxygen-including gas reach the growth base surface  11  most easily and the Ga chloride gas reach the growth base surface  11  least easily, and the number of killer defects included in the growing semiconductor film  12  is reduced. 
     Furthermore, since the grains of Ga oxide formed in the vapor phase move downward due to gravity, the grains of Ga oxide become further less likely to be deposited on the growth base surface  11  by placing the semiconductor substrate  10  with the growth base surface  11  facing downward. Therefore, the number of killer defects included in the semiconductor film  12  can be reduced more in the method for manufacturing a semiconductor film in the second embodiment than in the method for manufacturing a semiconductor film the first embodiment. 
     Meanwhile, when growing GaN-based crystal films by the HVPE method, formation of grains of GaN-based compound in the vapor phase hardly occur since the reaction rate of ammonia gas with Ga chloride gas is relatively slow, as mentioned above. Therefore, when growing GaN-based crystal films, generation of many killer defects as in the case of growing β-Ga 2 O 3 -based crystal films does not occur, and in addition, placing the substrate with the growth base surface facing downward has substantially no effect on reducing killer defects. A technique for growing a GaN-based crystal film by the HVPE method in a state in which the growth base surface of the substrate faces downward is known (Japanese Patent No. 3376809), but the purpose for this is to grow a uniform thin film by suppressing thermal convection of source gases around the substrate. 
     It has also been confirmed that the dopant incorporation rate is higher when placing the semiconductor substrate  10  with the growth base surface  11  facing downward than when placing the semiconductor substrate  10  with the growth base surface  11  facing upward. Since the donor concentration in the semiconductor film  12  becomes substantially the same as the charging amount of the dopant by placing the semiconductor substrate  10  with the growth base surface  11  facing downward, it is easy to control the donor concentration. 
     In addition, when increasing the flow velocity of the Ga chloride gas, the oxygen-including gas and the dopant-including gas, the grains of Ga oxide formed in the vapor phase are easily swept away by these source gases and the amount of Ga oxide grains deposited on the growth base surface  11  can be thereby reduced. 
     When growing the semiconductor film  12  on, e.g., a 2-inch diameter circular semiconductor substrate  10 , the number of killer defects included in the semiconductor film  12  can be significantly reduced by flowing the Ga chloride gas, the oxygen-including gas and the dopant-including gas into the space  24  at a flow velocity of not less than 110 cm/s, and can be further reduced by flowing the gases into the space  24  at a flow velocity of not less than 165 cm/s. 
     Effects of the Embodiments 
     According to the embodiments described above, it is possible to reduce the amount of the Ga oxide grains formed in the vapor phase and deposited on the semiconductor substrate  10  in the reaction chamber of the HVPE apparatus and reduce killer defects in the semiconductor film  12  formed of a β-Ga 2 O 3 -based single crystal. By using this semiconductor film  12  with few killer defects, semiconductor devices with excellent characteristics, e.g., Schottky barrier diodes with excellent forward current and reverse leakage current characteristics, can be produced with high yield. 
     Examples 
       FIG.  4 A  is an emission microscope observation image showing the semiconductor film  12  formed by the method according to the second embodiment described in reference to FIG.  3 .  FIG.  4 B  is an emission microscope observation image showing the semiconductor film as Comparative Example formed by the known method described in reference to  FIG.  2 B . 
     As for the emission microscope observation images in  FIGS.  4 A and  4 B , an anode electrode and a cathode electrode were respectively formed on the surface of the semiconductor film  12  and a surface of the semiconductor substrate  10  so as not to overlap each other, a region in which the anode electrode is formed was photographed by a CCD camera from the semiconductor substrate  10  side, and a pattern image of the 500 μm-diameter circular anode electrode photographed by irradiation with light in a state in which no voltage is applied between the two electrodes was superimposed onto an emission image photographed in a state in which a reverse voltage of 200V (positive voltage on the cathode electrode side and negative voltage on the anode electrode side) was applied between the two electrodes. 
     Black dots seen in the circle of  FIG.  4 B  are leakage paths illuminated due to reverse voltage application, and show locations of killer defects to be leakage paths. In the emission microscope observation image in  FIG.  4 A , luminous spots which are as noticeable as those in the emission microscope observation image in  FIG.  4 B  are not observed and the density of the luminous spots estimated using plural anode electrodes was not more than 10 spots/cm 2 . In other words, a density of killer defects in the semiconductor film  12  in the second embodiment was not more than 10 defects/cm 2 . 
       FIG.  5 A  is an optical microscope observation image showing a surface of the semiconductor film  12  formed by the method according to the second embodiment described in reference to  FIG.  3   , on which etch pits appeared.  FIG.  5 B  is an optical microscope observation image showing a surface of the semiconductor film as Comparative Example formed by the known method described in reference to  FIG.  2 B , on which etch pits appeared. 
     Etch pits are depressions formed due to an etching rate difference between defective portions and other portions during etching a crystal surface, and the locations and density of defects can be determined by observing the etch pits. The etch pits in  FIGS.  5 A and  5 B  were formed by immersing the semiconductor film  12  grown on the semiconductor substrate  10  in hot phosphoric acid for 1 hour. 
     In the observation image in  FIG.  5 B , the etch pit density of the semiconductor film in Comparative Example was about 10000/cm 2 . On the other hand, in the observation image in  FIG.  5 A , the etch pit density of the semiconductor film  12  was about 2500/cm 2  which is about ¼ of the etch pit density of the semiconductor film in Comparative Example 
       FIG.  6    is a vertical cross-sectional view showing a Schottky barrier diode  3  used to evaluate Schottky barrier diode characteristics described later. 
     The Schottky barrier diode  3  has the semiconductor substrate  10  formed of a β-Ga 2 O 3  single crystal and having an effective carrier concentration (a value obtained by subtracting an acceptor concentration N a  from a donor concentration N d ) of about 1×10 18 /cm 3  and a thickness of about 600 μm, the semiconductor film  12  formed of a β-Ga 2 O 3  single crystal and having an effective carrier concentration of about 1×10 16 /cm 3  and a thickness of about 6 μm, an anode electrode  31  having a Ni/Au laminated structure formed on a surface of the semiconductor film  12 , and a cathode electrode  32  having a Ti/Ni/Au laminated structure formed on the entire surface of the semiconductor substrate  10 . 
       FIG.  7 A  is a graph showing reverse leakage characteristics of the Schottky barrier diode  3  which includes the semiconductor film  12  formed by the method according to the first embodiment described in reference to  FIG.  2 A .  FIG.  7 B  is a graph showing reverse leakage characteristics of the Schottky barrier diode  3  which includes the semiconductor film  12  formed by the method according to the second embodiment described in reference to  FIG.  3   .  FIG.  8    is a graph showing reverse leakage characteristics of the Schottky barrier diode  3  in which a semiconductor film as Comparative Example formed by the known method described in reference to  FIG.  2 B  is provided in place of the semiconductor film  12 . 
     The anode electrodes  31  of the Schottky barrier diodes  3  of  FIGS.  7 A,  7 B and  8    are circular electrodes with a diameter of 500 μm. 
     In  FIGS.  7 A,  7 B and  8   , a leakage current of not more than 1×10 −4  A/cm 2  when applying a voltage of −200 V to the anode electrode is indicated as “Good” and more than 1×10 −4  A/cm 2  is indicated as “Bad”. 
     When the percentage of “Good” out of the total is defined as the yield, the yield of the Schottky barrier diode  3  with the semiconductor film  12  formed by the method according to the first embodiment described in reference to  FIG.  2 A  was 78%, the yield of the Schottky barrier diode  3  with the semiconductor film  12  formed by the method according to the second embodiment described in reference to  FIG.  3    was 89%, and the yield of the Schottky barrier diode  3  with the semiconductor film as Comparative Example formed by the known method described in reference to  FIG.  2 B  was 11%. 
       FIGS.  9 A to  9 C  are diagrams illustrating distribution of breakdown voltage characteristics of Schottky barrier diodes on a 2-inch wafer (the crystal laminated structure  1  with plural anode electrodes  31  and the cathode electrode  32 ) before dicing into individual Schottky barrier diodes  3  provided with the semiconductor film  12  formed by the method according to the second embodiment described in reference to  FIG.  3   . 
       FIGS.  9 A to  9 C  are diagrams when viewing the semiconductor film  12  from vertically above, and the numbers shown indicate breakdown voltage (voltage when a leakage current of 1 μA flows) of the Schottky barrier diodes  3  at the locations of the numbers, with negative values indicating reverse voltage. Meanwhile, since the measurement limit of the breakdown voltage is −200V, breakdown voltage of the Schottky barrier diodes  3  at locations indicated by “−200” in the drawings is not more than −200V. The Schottky barrier diodes  3  at the locations indicated by “−200” in the drawings can be judged to have sufficient breakdown voltage. 
     The semiconductor film  12  of  FIG.  9 A  was grown by flowing the source gases (the Ga chloride gas, the oxygen-including gas and the dopant-including gas) into the space  24  at a flow velocity of 55 cm/s, the semiconductor film  12  of  FIG.  9 B  was grown by flowing the source gases into the space  24  at a flow velocity of 110 cm/s, and the semiconductor film  12  of  FIG.  9 C  was grown by flowing the source gases into the space  24  at a flow velocity of 165 cm/s. The direction of the arrow in the drawings indicates a direction in which the source gases flow. 
       FIGS.  9 A to  9 C  show that a region of the wafer, from which Schottky barrier diodes  3  with excellent breakdown voltage can be obtained, is larger when the flow velocity of the source gases is faster. It is considered that this is because the faster the flow velocity of the source gases, the easier it is to sweep away the grains of Ga oxide formed in the vapor phase, hence, the amount of Ga oxide grains deposited on the growth base surface  11  is reduced and killer defects in the semiconductor film  12  are reduced. 
     In addition, it is understood from  FIGS.  9 A to  9 C  that when the diameter of the wafer is 2 inches, the number of killer defects included in the semiconductor film  12  can be significantly reduced by flowing the source gases into the space  24  at a flow velocity of not less than 110 cm/s, and can be further reduced by flowing the gases into the space  24  at a flow velocity of not less than 165 cm/s. 
     However, the faster the flow velocity of the source gases, the more the amount of the source gases that flows away without contributing to the growth of the semiconductor film  12 , hence, efficiency of source gas usage decreases and consequently the manufacturing cost of the semiconductor film  12  increases. Therefore, it is preferable that the flow velocity just enough to obtain the Schottky barrier diodes  3  with excellent breakdown voltage from the whole region of the wafer be set as the upper limit. 
     In addition, when increasing the size of the wafer from which the Schottky barrier diodes  3  are cut out, the flow velocity of the source gases can be further increased to ensure that a region from which the Schottky barrier diode  3  with excellent breakdown voltage can be obtained is wide. When forming, e.g., a 4-inch diameter wafer, the flow velocity of the source gases should be twice as fast as when forming a 2-inch diameter wafer, and the number of killer defects included in the semiconductor film  12  can be significantly reduced by flowing the source gases into the space  24  at a flow velocity of not less than 220 cm/s, and can be further reduced by flowing the gases into the space  24  at a flow velocity of not less than 330 cm/s. 
       FIGS.  10 A and  10 B  are graphs respectively showing reverse characteristics and forward characteristics of the Schottky barrier diode  3  which includes the semiconductor film  12  formed by the method according to the second embodiment described in reference to  FIG.  3    and the 2.3-mm square anode electrode  31 . A dotted line in  FIG.  10 A  indicates the measurement limit. The forward characteristics shown in  FIG.  10 B  were measured by applying a pulse voltage with a pulse width of 1 ms. 
     In  FIG.  10 A , the magnitude of the leakage current is less than 1 μA even when the applied voltage is −460 V, which indicates that excellent reverse leakage characteristics are obtained. Meanwhile, in  FIG.  10 B , a current of not less than 10A flows when the applied voltage is not less than 2.1V, which indicates that excellent high-current characteristics are obtained. 
     Although the embodiments and Examples of the invention have been described, the invention is not intended to be limited to the embodiments and Examples, and the various kinds of modifications can be implemented without departing from the gist of the invention. In addition, the constituent elements in the embodiments and Examples can be arbitrarily combined without departing from the gist of the invention. 
     In addition, the invention according to claims is not to be limited to the embodiments and Examples described above. Further, it should be noted that not all combinations of the features described in the embodiments and Examples are necessary to solve the problem of the invention. 
     INDUSTRIAL APPLICABILITY 
     The invention provides a semiconductor film formed of a β-Ga 2 O 3 -based single crystal and including few killer defects, and a method for manufacturing the semiconductor film. 
     REFERENCE SIGNS LIST 
     
         
           1  CRYSTAL LAMINATED STRUCTURE 
           2  HVPE APPARATUS 
           3  SCHOTTKY BARRIER DIODE 
           10  SEMICONDUCTOR SUBSTRATE 
           11  GROWTH BASE SURFACE 
           12  SEMICONDUCTOR FILM 
           20  REACTION CHAMBER 
           21  FIRST GAS INTRODUCING PORT 
           22  SECOND GAS INTRODUCING PORT 
           23  THIRD GAS INTRODUCING PORT 
           24  SPACE 
           31  ANODE ELECTRODE 
           32  CATHODE ELECTRODE