Patent Publication Number: US-2018030602-A1

Title: Fabrication method of strontium niobium oxynitride film having small carrier density and its use

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a fabrication method of a strontium niobium oxynitride film having small carrier density and its use. 
     2. Description of the Related Art 
     NPL1 discloses that strontium niobium oxynitride represented by the chemical formula SrNbO 2 N absorbs light having a wavelength of not more than 700 nanometers. Strontium niobium oxynitride is one kind of a perovskite niobium oxynitride. Furthermore, NPL1 discloses a semiconductor photoelectrode fabrication method in which SrNbO 2 N particles are deposited on a fluorine-doped tin oxide substrate by an electrophoretic deposition method. According to NPL1, the thus-fabricated semiconductor photoelectrode is irradiated with light to generate oxygen due to water splitting on the surface of the semiconductor photoelectrode. 
     NPL2 discloses a method for growing a SrNbO 3-x N x  film (0≦x≦1) by a pulse laser deposition method on a KTaO 3  single-crystal substrate having an orientation plane of a (100) plane. According to NPL2, the thus-grown SrNbO 3-x N x  film has carrier density of not less than 1×10 21  cm −3 . 
     NPL3 discloses a method for growing a SrTaO 3-x N x  film (0≦x≦1.2) by a pulse laser deposition method on a SrTiO 3  single-crystal substrate having an orientation plane of a (100) plane. NPL3 does not disclose the carrier density of the thus-grown SrTaO 3-x N x  film. 
     CITATION LIST 
     NPL1: Kazuhiko Maeda et al, “SrNbO 2 N as a Water-Splitting Photoanode with a Wide Visible-Light Absorption Band”, Journal of the American Chemical Society, vol. 133, pp. 12334-12337 (2011) 
     NPL2: Daichi Oka et. al., ‘Electric Transport Properties of Nb-based perovskite oxynitride epitaxial thin films“, Proceedings of The 61st Japan Society of Applied Physics Spring Meeting, 2014, 18p-E8-13, 06-149. 
     NPL3: Daichi Oka et. al., “Possible ferroelectricity in perovskite oxynitride SrTaO 2 N epitaxial thin films”, Scientific Reports, Vol. 4, pp 4987 (2014) 
     SUMMARY 
     An object of the present invention is to provide a fabrication method of a strontium niobium oxynitride film having small carrier density and its use. 
     The present invention provides a method for growing a strontium niobium oxynitride film, the method comprising: 
     (a) growing, on a strontium titanate substrate, by a sputtering method, the strontium niobium oxynitride film having carrier density of not more than 1×10 18  cm −3 . 
     The spirit of the present invention includes: 
     (I) strontium niobium oxynitride having carrier density not more than 1×10 18  cm −3 , and 
     (II) a strontium niobium oxynitride film having carrier density not more than 1×10 18  cm −3 . 
     The spirit of the present invention further includes: 
     (III) a photosemiconductor substrate comprising the strontium niobium oxynitride film, 
     (IV) a hydrogen generation device comprising the photosemiconductor substrate, and 
     (V) a hydrogen generation method using the photosemiconductor substrate. 
     The present invention provides a fabrication method of a strontium niobium oxynitride film having small carrier density and its use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a semiconductor photoelectrode  100 . 
         FIG. 2  is a graph showing an X-ray diffraction measurement result in the inventive example 1. 
         FIG. 3  is a graph showing an X-ray diffraction measurement result in the inventive example 2. 
         FIG. 4  shows a cross-sectional view of a hydrogen generation device comprising the semiconductor photoelectrode  100 . 
         FIG. 5  shows a cross-sectional view of the semiconductor photoelectrode  100 . 
         FIG. 6  is a graph showing an X-ray diffraction measurement result in the inventive example 3. 
         FIG. 7  is a graph showing an X-ray diffraction measurement result in the inventive example 4. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, the embodiment of the present invention will be described with reference to the drawings. 
     (Embodiment) 
       FIG. 1  shows a cross-sectional view of a semiconductor photoelectrode  100  according to the embodiment. The semiconductor photoelectrode  100  comprises a strontium titanate substrate  110  (hereinafter, referred to as “substrate  110 ”) and a strontium niobium oxynitride film  120 . The substrate  110  may include another layer, as far as the surface of the substrate  110  is formed of strontium titanate. Desirably, the substrate  110  is single-crystal. The strontium titanate is represented by the chemical formula SrTiO 3 . The strontium niobium oxynitride may be represented by the chemical formula SrNbO 3-x N x  (where x is more than 0 and not more than 3, desirably, x=1). The strontium niobium oxynitride is one kind of n-type semiconductors. 
     The strontium niobium oxynitride film  120  is formed on the surface of the substrate  110 . Desirably, the strontium niobium oxynitride film  120  has an orientation plane. More preferably, the strontium niobium oxynitride film  120  has an orientation plane of a (001) plane. 
     (Fabrication Method) 
     A fabrication method according to the present embodiment will be described below. 
     While the temperature of the substrate  110  is maintained at not less than 500 degrees Celsius and not more than 750 degrees Celsius, the strontium niobium oxynitride film  120  is grown on the substrate  110 . The substrate  110  is formed of strontium titanate, as described above. It is desirable that the substrate  110  is formed of single-crystal strontium titanate. It is desirable that the grown strontium niobium oxynitride film  120  has an orientation plane. 
     It is desirable that the substrate  110  has a principal surface of a (001) plane, a (110) plane, or a (111) plane. In other words, it is desirable that the surface of the substrate  110  formed of strontium titanate is oriented in a [001] direction, a [110] direction, or a [111] direction. It is more desirable that the substrate  110  comprises strontium titanate having only an (001) orientation plane, only an (110) orientation plane, or only an (111) orientation plane on the surface thereof. 
     The present inventors found that a strontium niobium oxynitride film grown by a sputtering method has significantly lower carrier density than a strontium niobium oxynitride film grown by a pulse laser deposition method. Specifically, the strontium niobium oxynitride film grown by a laser deposition method has high carrier density of not less than 1×10 21  cm −3  (See NPL2), whereas the strontium niobium oxynitride film grown by a sputtering method has low carrier density of not more than 1×10 18  cm −3 , as demonstrated in the inventive examples which will be described later. In one embodiment, the strontium niobium oxynitride film grown by a sputtering method has low carrier density of not more than 1×10 17  cm −3 . As will be described later, the low carrier density improves hydrogen generation efficiency. 
     In the sputtering method, it is desirable that a target formed of strontium niobate represented by the chemical formula Sr 2 Nb 2 O 7  is used. Sputtering is carried out in an atmosphere of a mixture of argon and nitrogen. It is desirable that the atmosphere further contains oxygen. In this way, the strontium niobium oxynitride film  120  is grown on the substrate  110 . 
     When the substrate  110  has an orientation plane of a (001) plane, the strontium niobium oxynitride film  120  also has an orientation plane of a (001) plane. It is more desirable that the strontium niobium oxynitride film  120  has a (001) orientation only. Likewise, when the substrate  110  has an orientation plane of a (110) plane, the strontium niobium oxynitride film  120  also has an orientation plane of a (110) plane. In this case, it is more desirable that the strontium niobium oxynitride film  120  has a (110) orientation only. When the substrate  110  has an orientation plane of a (111) plane, the strontium niobium oxynitride film  120  also has an orientation plane of a (111) plane. In this case, it is more desirable that the strontium niobium oxynitride film  120  has a (111) orientation only. 
     The strontium niobium oxynitride film  120  grown in this way has low carrier density of not more than 1.0×10 13  cm −3 , as described above. 
       FIG. 4  shows a cross-sectional view of a hydrogen generation device  600  comprising the semiconductor photoelectrode  100 . In the present embodiment, the semiconductor photoelectrode  100  comprises the strontium niobium oxynitride film  120 . The strontium niobium oxynitride is a photosemiconductor and can be used as a photocatalyst. The hydrogen generation device shown in  FIG. 4  comprises the semiconductor photoelectrode  100 , a counter electrode  630 , a liquid  640 , and a container  610 . As described above, the semiconductor photoelectrode  100  comprises the substrate  110  and the strontium niobium oxynitride film  120  grown on the substrate  110 . 
       FIG. 5  shows a cross-sectional view of the semiconductor photoelectrode  100 . The strontium titanate substrate  110  doped with niobium or lanthanum may be used. The strontium titanate substrate  110  doped with niobium or lanthanum is electrically conductive. As shown in  FIG. 5 , an ohmic electrode  111  may be formed on the conductive strontium titanate substrate  110 . The ohmic electrode  111  is electrically connected to a conducting wire  650 . The substrate  110  is a perovskite (e.g., a perovskite oxide). 
     It is desirable that the counter electrode  630  is formed of a material having a small overvoltage on the hydrogen generation reaction. Alternatively, it is desirable that the counter electrode  630  may be formed of a semiconductor photoelectrode capable of generating hydrogen. In particular, an example of the material of the counter electrode  630  is platinum, gold, silver, nickel, ruthenium oxide represented by the chemical formula RuO 2 , iridium oxide represented by the chemical formula IrO 2 , or a p-type semiconductor. Two or more materials may be used for the counter electrode  630 . 
     The liquid  640  is water or an electrolyte aqueous solution. The electrolyte aqueous solution is acidic or alkaline. An example of the electrolyte aqueous solution is a sulfuric acid aqueous solution, a sodium sulfate aqueous solution, a sodium carbonate aqueous solution, a phosphate buffer solution, or a borate buffer solution. The liquid  640  may be constantly stored in the container  610  or may be supplied only in use. 
     The container  610  contains the semiconductor photoelectrode  100 , the counter electrode  630 , and the liquid  640 . It is desirable that the container  610  is transparent. In particular, it is desirable that at least a part of the container  610  is transparent so that light can travel from the outside of the container  610  to the inside of the container  610 . A user of the hydrogen generation device  600  prepares such a hydrogen generation device  600 . 
     When the strontium niobium oxynitride film  120  is irradiated with light, oxygen is generated on the surface of the strontium niobium oxynitride film  120 . Light such as sunlight travels through the container  610  and reaches the strontium niobium oxynitride film  120 . Electrons and holes are generated respectively in the conduction band and valence band of the part of the strontium niobium oxynitride film  120  in which the light has been absorbed. Since the strontium niobium oxynitride film  120  is an n-type semiconductor, the holes migrate to the surface of the strontium niobium oxynitride film  120 . 
     Water is split on the surface of the strontium niobium oxynitride film  120  as shown in the following reaction formula (1) to generate oxygen. On the other hand, electrons migrate from the strontium niobium oxynitride film  120  to the counter electrode  630  through the conducting wire  650 . Hydrogen is generated as shown in the following reaction formula (2) on the surface of the counter electrode  630 . 
       4h + +2H 2 O→O 2 ↑+4H +   (1)
         (h +  represents a hole)       

       4e − +4H + →2H 2 ↑  (2)
 
     There is a depletion layer having a band bending on a solid-liquid interface formed on the surface of the strontium niobium oxynitride film  120 . Theoretically, a depletion layer extends with a decrease in carrier density. Therefore, in a case where carrier density is low, electrons and holes generated in the conduction band and the valence band respectively are easily separated due to the internal electric field of the depletion layer. Since the semiconductor photoelectrode  100  according to the embodiment has low carrier density of less than 1.0×10 18  cm −3 , a hydrogen generation device comprising the semiconductor photoelectrode  100  according to the embodiment has high hydrogen generation efficiency. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail with reference to the following examples. 
     Inventive Example 1 
     In the inventive example 1, a semiconductor photoelectrode  100  shown in  FIG. 1  was fabricated as below. 
     First, a strontium niobium oxynitride film  120  having a thickness of 100 nanometers was grown by a reactive sputtering method on a perovskite strontium titanate substrate  110  having a (001) orientation only. In the reactive sputtering method, the temperature of the strontium titanate substrate  110  was maintained at 650 degrees Celsius. The material of the sputtering target was strontium niobate represented by the chemical formula Sr 2 Nb 2 O 7 . The sputtering was carried out in an atmosphere of a mixture of argon, oxygen, and nitrogen. The total pressure in the chamber used for the sputtering was 0.5 Pa. The flow rate of argon was 5 sccm. The flow rate of oxygen was 0.05 sccm. The flow rate of nitrogen was 10 sccm. In this way, the strontium niobium oxynitride film  120  was grown epitaxially. 
     Then, the carrier density of the strontium niobium oxynitride film  120  was calculated through the Hall effect measurement based on the Van der Pauw method. As a result, the strontium niobium oxynitride film  120  according to the inventive example 1 had carrier density of 5.5×10 15  cm −3 . 
     The semiconductor photoelectrode  100  was subjected to an X-ray diffraction analysis.  FIG. 2  shows the result. As is clear from  FIG. 2 , six peaks were observed. Among them, three peaks are derived from a (001) plane, a (002) plane, and a (003) plane of the SrTiO 3 . Other three peaks are derived from a (001) plane, a (002) plane, and a (003) plane of SrNbO 2 N. As just described, only peaks of (00h) planes of SrNbO 2 N were observed. This means that a strontium niobium oxynitride film having a (001) orientation only was formed on the strontium titanate substrate  110  having a (001) plane orientation. 
     Inventive Example 2 
     In the inventive example 2, the semiconductor photoelectrode  100  shown in  FIG. 1  was fabricated as below. The main difference from the inventive example 1 is that the atmosphere of the sputtering did not contain oxygen. 
     First, a strontium niobium oxynitride film  120  having a thickness of 100 nanometers was grown by a reactive sputtering method on a perovskite strontium titanate substrate  110  having a (001) orientation only. In the reactive sputtering method, the temperature of the strontium titanate substrate  110  was maintained at 650 degrees Celsius. The material of the sputtering target was strontium niobate represented by the chemical formula Sr 2 Nb 2 O 7 . The sputtering was carried out in an atmosphere of a mixture of argon and nitrogen. The total pressure in the chamber used for the sputtering was 0.5 Pa. The flow rate of argon was 5 sccm. The flow rate of nitrogen was 10 sccm. In this way, the strontium niobium oxynitride film  120  was grown. 
     Then, the carrier density of the strontium niobium oxynitride film  120  was calculated through the Hall effect measurement based on the Van der Pauw method. As a result, the strontium niobium oxynitride film  120  according to the inventive example 2 had carrier density of 1.7×10 17  cm −3 . 
     The semiconductor photoelectrode  100  was subjected to an X-ray diffraction analysis.  FIG. 3  shows the result. As is clear from  FIG. 3 , six peaks were observed. Among them, three peaks are derived from a (001) plane, a (002) plane, and a (003) plane of the SrTiO 3 . Other three peaks are derived from a (001) plane, a (002) plane, and a (003) plane of SrNbO 2 N. As just described, only peaks of (00h) planes of SrNbO 2 N were observed. This means that a strontium niobium oxynitride film having a (001) orientation only was formed on the strontium titanate substrate  110  having a (001) plane orientation. 
     The following Table 1 shows the results of the inventive examples 1-2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Inventive 
                 Inventive 
               
               
                   
                 example 1 
                 example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Substrate 
                 SrTiO 3  substrate 
                   
               
               
                   
                 having a (001) orientation only 
               
            
           
           
               
               
               
            
               
                 Growth temperature (Celsius) 
                 650 
                 650 
               
               
                 Film thickness (nanometer) 
                 100 
                 100 
               
               
                 Argon flow rate (sccm) 
                 5 
                 5 
               
               
                 Oxygen flow rate (sccm) 
                 0.05 
                 0 
               
               
                 Nitrogen flow rate (sccm) 
                 10 
                 10 
               
               
                 Orientation 
                 (001) only 
                 (001) only 
               
               
                 Carrier density (cm −3 ) 
                 5.5 × 10 15   
                 1.7 × 10 17   
               
               
                   
               
            
           
         
       
     
     Inventive Example 3 
     In the inventive example 3, an experiment similar to the inventive example 1 was conducted, except that the perovskite strontium titanate substrate  110  has not a (001) orientation only, but a (110) orientation only. 
     Inventive Example 4 
     In the inventive example 4, an experiment similar to the inventive example 1 was conducted, except that the perovskite strontium titanate substrate  110  has not a (001) orientation only, but a (111) orientation only. 
     The following Table 2 shows the results of the inventive examples 3-4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Inventive 
                 Inventive 
               
               
                   
                 example 3 
                 example 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Substrate 
                 SrTiO 3  substrate 
                 SrTiO 3  substrate 
               
               
                   
                 having a (110) 
                 having a (111) 
               
               
                   
                 orientation only 
                 orientation only 
               
               
                 Growth temperature (Celsius) 
                 650 
                 650 
               
               
                 Film thickness (nanometer) 
                 100 
                 100 
               
               
                 Argon flow rate (sccm) 
                 5 
                 5 
               
               
                 Oxygen flow rate (sccm) 
                 0.05 
                 0.05 
               
               
                 Nitrogen flow rate (sccm) 
                 10 
                 10 
               
               
                 Orientation 
                 (110) only 
                 (111) only 
               
               
                 Carrier density (cm −3 ) 
                 2.1 × 10 15   
                 1.8 × 10 15   
               
               
                   
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The strontium niobium oxynitride film according to the present invention can be used as a semiconductor photoelectrode used in a hydrogen generation device for generating hydrogen through light irradiation. 
     REFERENTIAL SIGNS LIST 
     
         
           100  Semiconductor photoelectrode 
           110  Strontium titanate substrate 
           111  Ohmic electrode 
           120  Strontium niobium oxynitride film 
           600  Hydrogen generation device 
           610  Container 
           630  Counter electrode 
           640  Liquid 
           650  Conducting wire 
       
    
     CONCLUSION 
     The inventions derived from the above disclosure will be listed below.
     1. A method for growing a strontium niobium oxynitride film, the method comprising:   

     (a) growing, on a strontium titanate substrate, by a sputtering method, the strontium niobium oxynitride film having carrier density of not more than 1×10 13  cm −3 .
     2. The method according to Item 1, wherein   

     a target used in the sputtering method is formed of strontium niobate; and 
     the strontium niobium oxynitride film is grown in an atmosphere containing nitrogen.
     3. The method according to Item 2, wherein   

     the strontium niobate is represented by the chemical formula Sr 2 Nb 2 O 7 .
     4. The method according to Item 2, wherein   

     the atmosphere further contains oxygen.
     5. The method according to Item 2, wherein   

     the atmosphere further contains argon.
     6. The method according to Item 4, wherein   

     the atmosphere further contains argon.
     7. The method according to Item 1, wherein   

     the strontium titanate substrate has a single orientation plane; and 
     the strontium niobium oxynitride film has a single orientation plane.
     8. The method according to Item 7, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (001) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (001) plane.
     9. The method according to Item 7, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (110) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (110) plane.
     10. The method according to Item 7, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (111) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (111) plane.
     11. The method according to Item 1, wherein   

     the strontium titanate substrate is doped with at least one selected from the group consisting of niobium and lanthanum.
     12. A strontium niobium oxynitride having carrier density of not more than 1×10 18  cm −3 .   13. The strontium niobium oxynitride according to Item 12, wherein   

     the carrier density is not more than 1×10 17  cm −3 .
     14. A strontium niobium oxynitride film having carrier density of not more than 1×10 18  cm −3 .   15. The strontium niobium oxynitride film according to Item 14, wherein   

     the carrier density is not more than 1×10 17  cm −3 .
     16. The strontium niobium oxynitride film according to Item 14, wherein   

     the strontium niobium oxynitride film has a single orientation plane.
     17. The strontium niobium oxynitride film according to Item 16, wherein   

     the single orientation plane is an orientation plane of a (001) plane.
     18. The strontium niobium oxynitride film according to Item 16, wherein   

     the single orientation plane is an orientation plane of a (110) plane.
     19. The strontium niobium oxynitride film according to Item 16, wherein   

     the single orientation plane is an orientation plane of a (111) plane.
     20. A semiconductor photoelectrode comprising:   

     a strontium titanate substrate; and 
     a strontium niobium oxynitride film grown on the strontium titanate substrate, 
     wherein 
     the strontium niobium oxynitride film has carrier density of not more than 1×10 18  cm −3 .
     21. The semiconductor photoelectrode according to Item 20, wherein   

     the carrier density is not more than 1×10 17  cm −3 .
     22. The semiconductor photoelectrode according to Item 20, wherein   

     the strontium titanate substrate has a single orientation plane; and 
     the strontium niobium oxynitride film has a single orientation plane.
     23. The semiconductor photoelectrode according to Item 22, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (001) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (001) plane.
     24. The semiconductor photoelectrode according to Item 22, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (110) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (110) plane.
     25 The semiconductor photoelectrode according to Item 22, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (111) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (111) plane.
     26. The semiconductor photoelectrode according to Item 20, wherein   

     the strontium titanate substrate is doped with at least one selected from the group consisting of niobium and lanthanum.
     27. A hydrogen generation device, comprising:   

     a semiconductor photoelectrode according to Item 20; 
     a counter electrode electrically connected to the semiconductor photoelectrode; 
     a liquid in contact with the strontium niobium oxynitride film and the counter electrode; and 
     a container containing the semiconductor photoelectrode, the counter electrode, and the liquid, wherein 
     the liquid is water or an electrolyte aqueous solution; and 
     hydrogen is generated on a surface of the counter electrode when the strontium niobium oxynitride film is irradiated with light.
     28. The hydrogen generation device according to Item 27, wherein   

     the carrier density is not more than 1×10 17  cm −3 .
     29. The hydrogen generation device according to Item 27, wherein   

     the strontium titanate substrate has a single orientation plane; and 
     the strontium niobium oxynitride film has a single orientation plane.
     30. The hydrogen generation device according to Item 29, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (001) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (001) plane.
     31. The hydrogen generation device according to Item 29, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (110) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (110) plane.
     32. The hydrogen generation device according to Item 29, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (111) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (111) plane.
     33. The hydrogen generation device according to Item 27, wherein   

     the strontium titanate substrate is doped with at least one selected from the group consisting of niobium and lanthanum.
     34. A method for generating hydrogen, comprising:   

     (a) preparing a hydrogen generation device, comprising: 
     a semiconductor photoelectrode according to claim  16 ; 
     a counter electrode electrically connected to the semiconductor photoelectrode; 
     a liquid in contact with the strontium niobium oxynitride film and the counter electrode; and 
     a container containing the semiconductor photoelectrode, the counter electrode, and the liquid, 
     wherein 
     the liquid is water or an electrolyte aqueous solution; and 
     (b) irradiating the strontium niobium oxynitride film with light to generate hydrogen on a surface of the counter electrode.
     35. The method according to Item 34. wherein   

     the carrier density is not more than 1×10 17  cm −3 .
     36. The method according to Item 34, wherein   

     the strontium titanate substrate has a single orientation plane; and 
     the strontium niobium oxynitride film has a single orientation plane.
     37. The method according to Item 36, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (001) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (001) plane.
     38. The method according to Item 36, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (110) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (110) plane.
     39. The method according to Item 36, wherein   

     the single orientation plane of the strontium titanate substrate is an orientation plane of a (111) plane; and 
     the single orientation plane of the strontium niobium oxynitride film is an orientation plane of a (111) plane.
     40. The method according to Item 34, wherein   

     the strontium titanate substrate is doped with at least one selected from the group consisting of niobium and lanthanum.