JP4783686B2 - III-V group nitride semiconductor, photocatalytic semiconductor element, photocatalytic oxidation-reduction reaction apparatus, and photoelectrochemical reaction execution method - Google Patents

Description

  The present invention relates to a group III-V nitride semiconductor, a photocatalytic semiconductor element, a photocatalytic oxidation-reduction reaction apparatus, and a photoelectrochemical reaction execution method that receive light to develop a catalytic action of a redox reaction.

In recent years, using photocatalysts, in the energy field, for example, water is electrolyzed to obtain energy such as hydrogen gas by light irradiation, and in the environmental field, harmful substances and organic substances are widely decomposed by light irradiation. It has been broken.
As a photocatalyst, for example, a group III-V nitride semiconductor compound such as gallium nitride (GaN) (hereinafter also referred to as “III-V group compound”) has high durability against heat, gas resistance, and solvent resistance. Thus, it can be suitably used in, for example, a photocatalytic reaction in a high-temperature operating environment.

As the III-V group compound, for example, a compound in which it is used as a semiconductor light emitting device is disclosed (for example, see Patent Document 1 and Patent Document 2).
When used as a semiconductor light emitting device, it is generally performed by using a plurality of layers having different semiconductor characteristics for the purpose of operating the device (see, for example, Patent Documents 3 and 4). ).

  However, these stacked semiconductors are used as semiconductor light-emitting devices, and attempts to improve energy conversion efficiency such as photocatalytic efficiency from the crystal characteristics due to the interaction between stacked semiconductor crystals are now underway. Never before.

  Moreover, when using it for an oxidation reduction reaction as a photocatalyst semiconductor element which comprises the electrolysis apparatus by a photocatalyst, it is performed using several layers from which semiconductor characteristics mutually differ (for example, refer patent documents 5-8). .)

  However, a laminated semiconductor used in such a photocatalytic electrolysis apparatus is not a structure in which single crystals are continuously laminated, but is formed by laminating amorphous or polycrystalline materials. There has never been an attempt to improve energy conversion efficiency such as photocatalytic efficiency from a crystal structure in a semiconductor element used in an electrolysis apparatus.

JP 2001-135575 A JP 2003-63898 A JP 2003-047859 A Japanese Patent Publication No. 06-028737 JP 2005-133174 A US Patent Application Publication No. 2004/0195564 US Patent Application Publication No. 2005/0156205 Japanese Patent Laying-Open No. 2005-028225

The present invention has been made in view of the above circumstances, and its object is to make a group III-V nitride for photocatalytic oxidation-reduction reaction that causes oxidation-reduction reaction with high light conversion efficiency by light irradiation. An object is to provide a semiconductor and a photocatalytic semiconductor element having the same.
Another object of the present invention is to provide a photocatalytic oxidation-reduction reaction apparatus capable of causing an oxidation-reduction reaction with high light conversion efficiency by light irradiation, and a photoelectrochemical reaction execution method using this apparatus. .

The group III-V nitride semiconductor of the present invention is for photocatalytic oxidation-reduction reaction,
Having a base layer having different semiconductor characteristics and a surface layer laminated on the base layer,
By the surface layer and the base layer surface to contact is formed, at least the surface layer have a carrier transfer promoting effect,
The base layer and the surface layer have different conductivity types,
The surface layer has a thickness of 10 to 18 nm .

  Further, the band gap energy EgL of the material forming the surface layer may be smaller than the band gap energy EgH of the material forming the base layer.

The photocatalytic semiconductor element of the present invention is for a photocatalytic oxidation-reduction reaction,
The group III-V nitride semiconductor is laminated on a substrate.

The photocatalytic redox reaction apparatus of the present invention is for a photocatalytic redox reaction,
One of the pair of electrodes for electrolysis that are in contact with the electrolytic solution and electrically connected to each other is made of the above-mentioned group III-V nitride semiconductor,
It is characterized in that an oxidation reaction or a reduction reaction occurs on the catalytic reaction surface by irradiating light to the catalytic reaction surface constituting the group III-V nitride semiconductor.

  In this photocatalytic oxidation-reduction reaction device, light is applied to the catalytic reaction surface of the III-V nitride semiconductor and a voltage is applied between a pair of electrodes for electrolysis, whereby the oxidation reaction or reduction reaction is performed. It is preferable that the structure be generated on the catalytic reaction surface.

  The photoelectrochemical reaction execution method of the present invention uses the above III-V nitride semiconductor, irradiates the catalytic reaction surface in the III-V nitride semiconductor with excitation light, and performs oxidation reaction or It is characterized by causing a reduction reaction.

  According to the group III-V nitride semiconductor and the photocatalytic semiconductor element of the present invention, the carriers generated by light irradiation are successively laminated so that the base layer and the surface layer having different semiconductor characteristics have an interface. As a result of the influence of new semiconductor characteristics obtained by the action, specifically the new band edge potential and carrier mobility, the carriers generated by the light irradiation can be efficiently converted into redox reactions on the catalytic reaction surface. In the end, it is possible to cause an oxidation-reduction reaction with high light conversion efficiency by light irradiation.

  According to the photocatalytic oxidation-reduction reaction apparatus and the photoelectrochemical reaction execution method of the present invention, since the photocatalytic semiconductor element is provided, an oxidation-reduction reaction can be caused with high photoconversion efficiency by light irradiation on the catalytic reaction surface. it can.

  Hereinafter, the present invention will be specifically described.

  FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a photocatalytic semiconductor element having a III-V group nitride semiconductor of the present invention in a state in which a current collecting member is provided, and FIG. It is explanatory drawing which shows the structure of a group V nitride semiconductor typically.

<III-V group nitride semiconductor>
The group III-V nitride semiconductor 20 of the present invention is for a photocatalytic oxidation-reduction reaction, and has a base layer 20α having different semiconductor characteristics in a multilayer form and a surface layer 20β laminated on the base layer 20α. By forming an interface where 20α and the surface layer 20β are in contact with each other, at least the surface layer 20β has an effect of promoting carrier movement. Hereinafter, the surface layer is also referred to as a carrier movement promoting layer.

Examples of the III-V group compound constituting the base layer 20α and the carrier transfer promoting layer 20β of the III-V group nitride semiconductor 20 include those represented by the following general formula (1).
Formula _{(1): Al y Ga 1} -xy In x N 1-mn P m As n
[In the general formula (1), 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1, 0 ≦ m ≦ 1, 0 ≦ n ≦ 1, 0 ≦ m + n ≦ 1. ]
Of the III-V group compounds represented by the above general formula (1) as the III-V group compounds constituting the base layer 20α and the carrier transfer promoting layer 20β of the III-V group nitride semiconductor 20, m = n = In the case of using a compound that is 0 and contains only nitrogen as a group V atom, as described in Japanese Patent Application No. 2005-025831, xy ≦ 0.45, 0 ≦ x ≦ 1, 0 ≦ y ≦ By using the compound which is 1, hydrogen oxidation-reduction reaction can be performed with high light conversion efficiency, and hydrogen gas can be generated.

As a feedstock for the group III atom in the group III-V compound constituting the group III-V nitride semiconductor 20, trimethylgallium ((CH ₃ ) ₃ Ga) (hereinafter also referred to as “TMGa”), triethylgallium ( Trialkylgalliums such as (C ₂ H ₅ ) ₃ Ga); trialkylaluminums such as trimethylaluminum ((CH ₃ ) ₃ Al) and triethylaluminum ((C ₂ H ₅ ) ₃ Al); trimethylindium (( And trialkylindiums such as CH ₃ ) ₃ In) and triethylindium ((C ₂ H ₅ ) ₃ In). These can be used alone or in combination of two or more.

  As the feedstock for the group V atoms of the group III-V nitride semiconductor 20, ammonia, hydrazine, methyl hydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine, t-butylamine, ethylenediamine, and the like can be used. . These can be used alone or in admixture of two or more. Of these feedstocks, it is preferable to use ammonia for ease of handling.

Further, the group III-V compound constituting the group III-V nitride semiconductor 20 is doped, for example, with an impurity such as a silicon atom (Si), which is an impurity having a higher valence than the group III atom. be able to. Thus, by doping an impurity having a higher valence than the group III atom, the group III-V nitride semiconductor 20 becomes an n-type. Silane (SiH ₄ ), disilane (Si ₂ H ₆ ), monomethylsilane (Si (CH ₃ ) H ₃ ), or the like can be used as a feedstock for such silicon atoms (Si).
On the other hand, a p-type group III-V nitride semiconductor can be obtained by doping, for example, magnesium (Mg), which is an impurity having a lower valence than group III atoms. Cyclopentadienyl magnesium ((C ₂ H ₅ ) ₂ Mg) or the like can be used as such a feedstock for magnesium atoms (Mg).

The group III-V compound constituting such a group III-V nitride semiconductor 20 preferably has high single crystallinity. Since the group III-V compound has high single crystallinity, the group III-V nitride semiconductor 20 has a reduced crystal defect density, and recombination of carriers generated by light irradiation is suppressed. Therefore, the oxidation-reduction reaction can be caused with high light conversion efficiency.
Here, “high single crystallinity” means that the degree of existence of a grain boundary separating one single crystal grain from another single crystal grain is low, and that dislocations, precipitates, and point defects exist in the crystal. It shows that there are few crystal defects such as.

  The measurement of semiconductor characteristics such as p-type characteristics, n-type characteristics, and carrier concentration of the III-V group compound forming the base layer 20α and the carrier transfer promoting layer 20β of the III-V nitride semiconductor 20 is not particularly limited. In the present specification, by the van der Pauw method, the current and voltage when the magnetic field is not applied is set to a linear current value, and a magnetic field of 4000 gauss is applied at 25 ° C. It is. When it is difficult to directly measure each layer, such as when it has a multilayer structure, p-type characteristics, n for a single layer separately prepared from the same compound as the III-V group compound forming the target layer By measuring the mold characteristics and the carrier concentration, the semiconductor characteristics of each layer can be analogized.

In addition, the group III atom composition ratio and the group V atom composition ratio of the group III-V compound forming the base layer 20α and the carrier transfer promoting layer 20β can be measured by an ordinary method for measuring the composition ratio. If it is difficult to directly measure each layer, such as when it has a multilayer structure, the group III atomic composition of a single layer separately prepared from the same compound as the group III-V compound that forms the target layer By measuring the ratio and the V group atomic composition ratio, the composition ratio of each layer can be analogized.
In this specification, the group III atom composition ratio and the group V atom composition ratio of each layer are measured by measurement of lattice constants by X-rays and conventionally known photoluminescence measurement at room temperature. As a method for measuring the lattice constant by X-rays, an appropriate known method can be cited. For example, the incident angle and the reflection angle are the same with respect to the direction perpendicular to the (0001) plane which is the catalytic reaction surface R. It can be obtained by measuring the reflection angle on the (0002) plane.

The group III-V nitride semiconductor 20 has a photocatalyst carrier generation portion M that is continuous with the catalyst reaction surface R. The photocatalyst carrier generation portion M is formed on the catalyst reaction surface R or the opposite surface with excitation light L ( 3), a portion capable of causing light absorption, carriers are generated in the photocatalyst carrier generation portion M due to the light absorption, and the generated carriers are subjected to catalytic reaction. By being transported to the surface R, an oxidation reaction or a reduction reaction occurs on the catalytic reaction surface R.
The photocatalyst carrier generation portion M differs depending on the kind of substances constituting the base layer 20α and the carrier movement promotion layer 20β, the thickness of the carrier movement promotion layer 20β, or the intensity of irradiated light. Part, all of the carrier movement promotion layer, all of the carrier movement promotion layer and a part of the base layer, or all of the carrier movement promotion layer and the base layer.
FIG. 2 shows a case where the photocatalyst carrier generation portion M is composed of the entire carrier movement promoting layer 20β and a part of the base layer 20α.
The photocatalyst carrier generation portion M is located on the side of the catalyst reaction surface R from the position where effective light absorption is obtained when the excitation light L is incident from the catalyst reaction surface R (for example, the position at the depth t in FIG. 2). It is said that part.

  Examples of the combination of the base layer and the carrier migration promoting layer that constitute the III-V nitride semiconductor 20 and that have different semiconductor characteristics can include the following [1] to [3].

[1] np structure The III-V group nitride semiconductor 20 having the np structure in the first example is such that the base layer 20α and the carrier transfer promoting layer 20β have different conductivity types. is there.
For example, as will be described in detail later, when the photocatalytic semiconductor element 10 (see FIG. 3) using the group III-V nitride semiconductor 20 is used as an anode, the base layer 20α is an n-type group III-V compound. And the carrier transfer promoting layer 20β is formed of a p-type III-V group compound.
Here, the “conductivity type” refers to n-type and p-type classification, a semiconductor whose electric carrier is an electron (e ⁻ ) is called an n-type semiconductor, and an electric carrier is a hole (h ⁺ ). This semiconductor is called p-type.
In this example, the thickness of the carrier movement promoting layer 20β is such that the III-V group compound constituting the carrier movement promoting layer has a large thickness within a range where the behavior as a p-type semiconductor is not exhibited. Preferably, the thickness of the carrier movement promoting layer 20β differs depending on the III-V group compound constituting the carrier movement promoting layer 20β, but does not behave as a p-type semiconductor, for example, 18 nm or less, more preferably 10 ~ 18nm. When the thickness of the carrier movement promotion layer 20β is larger than 18 nm, the III-V group compound constituting the carrier movement promotion layer behaves as a p-type semiconductor.
By adopting such an np structure, the band edge potential of the conduction band is shifted in the negative direction as compared with a photocatalytic semiconductor element using a group III-V nitride semiconductor composed only of an n-type semiconductor layer. Therefore, the potential difference from the hydrogen generation potential can be increased, and the water can be electrolyzed with high light conversion efficiency.

Further, for example, when the photocatalytic semiconductor element 10 using the III-V group nitride semiconductor 20 is used as a cathode, the base layer 20α is formed of a p-type III-V compound and the carrier movement promoting layer 20β is an n-type. Of the III-V group.
In this example, the thickness of the carrier movement promoting layer 20β is such that the III-V group compound constituting the carrier movement promoting layer has a large thickness within a range where the behavior as an n-type semiconductor is not exhibited. preferable.
By adopting such a structure, the band edge potential of the valence band shifts in the positive direction as compared with the photocatalytic semiconductor element using a group III-V nitride semiconductor composed only of the p-type semiconductor layer. Therefore, the potential difference from the oxygen generation potential can be increased, and water can be electrolyzed with high light conversion efficiency.

  In addition, in the III-V group nitride semiconductor 20 having such an np structure, the carrier transfer promotion layer 20β has a conductivity type different from that of the III-V group compound constituting the base layer 20α. The group −V nitride semiconductor 20 as a whole is considered to exhibit the conductivity type of the group III-V compound constituting the base layer 20α.

[2] n ⁺ -n ^- structure n ⁺ -n of the second embodiment ^- III -V nitride semiconductor 20 of the structure includes a base layer 20α and carrier movement promoting layer 20β is, the conductivity types are the same The carrier concentration of the base layer 20α is higher than the carrier concentration of the carrier movement promoting layer 20β.
For example, when the photocatalytic semiconductor element 10 using the III-V group nitride semiconductor 20 is used as an anode, the base layer 20α and the carrier transfer promoting layer 20β are each formed of an n-type III-V group compound, and The carrier concentration of the base layer 20α is set higher than, for example, 8.0 × 10 ¹⁷ cm ^{−3 and} the carrier concentration of the carrier movement promoting layer 20β is set to 8.0 × 10 ¹⁷ cm ⁻³ or less. The carrier concentration of the base layer 20α is more preferably 1.0 × 10 ¹⁸ cm ⁻³ or more, and particularly preferably 2.0 × 10 ¹⁸ cm ⁻³ or more. The carrier concentration of the carrier movement promoting layer 20β is more preferably 5.0 × 10 ¹⁷ cm ⁻³ or less, and particularly preferably 1.0 × 10 ¹⁷ cm ⁻³ or less.
The difference in carrier concentration between the base layer 20α and the carrier movement promoting layer 20β is preferably 5.0 × 10 ¹⁷ cm ⁻³ or more in order to obtain effective carrier mobility.
With such an n ⁺ -n ⁻ structure, a concentration gradient is formed in the photocatalyst carrier generation portion M, and a large carrier mobility can be obtained. As a result, high photoconversion efficiency can be obtained.

  Further, for example, when the photocatalytic semiconductor element 10 using the III-V group nitride semiconductor 20 is used as a cathode, the base layer 20α and the carrier transfer promoting layer 20β are each formed of a p-type III-V group compound, and The carrier concentration of the base layer 20α may be higher than the carrier concentration of the carrier movement promotion layer 20β.

[3] EgH-EgL Structure The III-V group nitride semiconductor 20 having the EgH-EgL structure of the third example has a bandgap energy EgL of a III-V group compound forming the carrier transfer promoting layer 20β. It is supposed to be smaller than the band gap energy EgH of the III-V group compound to be formed.
For example, when the photocatalytic semiconductor element 10 using the III-V group nitride semiconductor 20 is used as an anode, the base layer 20α and the carrier transfer promoting layer 20β are each formed of an n-type III-V group compound, and The difference between the band gap energy EgH of the base layer 20α and the band gap energy EgL of the carrier transfer promoting layer 20β is set to 0.1 eV or more by adjusting the group III atom composition. The difference between the band gap energy EgH of the base layer 20α and the band gap energy EgL of the carrier transfer promotion layer 20β is preferably 0.1 eV or more and 1.5 eV or less, more preferably 0.2 eV or more and 1.2 eV or less. Particularly preferably, it is 0.3 eV or more and 1.0 eV or less.
Usually, as the band gap energy Eg is reduced, the band edge potential of the conduction band is strongly influenced by the characteristics of the layer made of InGaN, and thus shifts in the positive direction, and the potential difference from the hydrogen generation potential is reduced. Although it is located on the plus side from the position of the hydrogen generation potential, the band edge potential of the conduction band is reduced by making the layer thickness small as a carrier movement promoting layer having a band gap energy Eg smaller than that of the base layer. Shifting in the positive direction is suppressed.
The thickness of the carrier movement promoting layer 20β constituting the III-V nitride semiconductor 20 of this example is preferably 300 nm or less, more preferably, although it varies depending on the group III atom composition and the group V atom composition. It is 200 nm or less, and particularly preferably 50 nm or less.
With such an EgH-EgL structure, the thickness is regulated, and the carrier movement promoting layer 20β is configured to have a band gap energy Eg smaller than that of the base layer 20α. High utilization efficiency of light can be obtained in a state in which the potential difference is secured.

Further, for example, when the photocatalytic semiconductor element 10 using the III-V group nitride semiconductor 20 is used as a cathode, the base layer 20α and the carrier transfer promoting layer 20β are each formed of a p-type III-V group compound, and The band gap energy EgL of the carrier transfer promoting layer 20β is made smaller than the band gap energy EgH of the base layer 20α by adjusting the group III atom composition.
Usually, as the band gap energy Eg is decreased, the band edge potential of the valence band shifts in the negative direction, the potential difference from the oxygen generation potential is decreased, and further, the potential difference is positioned on the negative side from the position of the oxygen generation potential. However, as the carrier movement promoting layer having a band gap energy Eg smaller than that of the base layer, the band edge potential of the valence potential band is prevented from shifting in the negative direction by making the layer thickness small.
With such an EgH-EgL structure, the thickness is regulated, and the carrier transfer promoting layer 20β is configured to have a band gap energy Eg smaller than that of the base layer 20α. High light utilization efficiency can be obtained in a state where a potential difference from the potential is secured.

In the base layer 20α and the carrier transfer promotion layer 20β of the group III-V nitride semiconductor 20 as described above, a laminated structure in which two or more of the np structure, the n ⁺ -n ^- structure, and the EgH-EgL structure are combined. May be formed. For example, the base layer 20α is made of an n-type III-V group compound, the carrier transfer promotion layer 20β is made of a p-type III-V group compound, and the carrier concentration of the base layer 20α is the carrier concentration of the carrier transfer promotion layer 20β. It can be assumed that it is lower.
In addition, a stacked structure having two or more interfaces formed by the np structure, the n ⁺ -n ⁻ structure, and the EgH—EgL structure may be formed regardless of the same kind.

  Further, the group III-V nitride semiconductor 20 is not limited to the two-layer configuration of the base layer 20α and the carrier transfer promotion layer 20β, and one or more semiconductor layers may be further formed as the lower layer of the base layer 20α. As these semiconductor layers, for example, a contact layer for improving conductivity with a metal having a current collecting action, or a solar cell layer usually composed of a plurality of layers for further improving light utilization efficiency And so on.

The thickness of the photocatalytic carrier generation portion M in the III-V nitride semiconductor 20, that is, the depth t from the catalytic reaction surface R capable of causing light absorption is 0.01 to 0.5 μm, preferably 0.01. It is -0.2 micrometer, More preferably, it is 0.01-0.05 micrometer.
Here, “the depth t from the catalytic reaction surface capable of causing light absorption” means that the depth at which normal light absorption can be caused is that the intensity of light decreases exponentially due to light absorption. However, in the present invention, it means the depth at which the light absorption amount is 99%.
The thickness (depth t) of the photocatalyst carrier generation portion M can be calculated by the equation (2): I / I ₀ = exp (−x / Q). In the above formula (2), Q is the absorption coefficient of the III-V group compound constituting the photocatalytic carrier generation portion M, I is the light intensity at the depth x, and I ₀ is incident on the catalytic reaction surface R. It is the intensity of light.
When the III-V group compound constituting the photocatalytic carrier generation portion M is Al _y Ga _1-xy In _x N _1-mn P _m As _n such as GaN, the absorption coefficient Q is about 1 × 10 ⁵ cm ⁻¹ . Therefore, the depth at which the light absorption amount is 99% is 0.46 μm.

In the group III-V nitride semiconductor 20 described above, various changes can be made in the semiconductor layer constituting the lower layer than the base layer 20α.
For example, the semiconductor compound constituting the semiconductor layer is not limited to n-type and p-type, but may be a pn junction type or a laminated structure having a carrier concentration step or Alternatively, it may be of a multilayer structure such as a laminated structure of a plurality of layers having different single crystallinity, or a laminated structure of a plurality of layers made of III-V group compounds having different compositions. Further, for example, silicon (Si), germanium (Ge), other III-V group compounds, II-IV group compound semiconductors, or oxides such as TiO ₂ may be used.

<Photocatalytic semiconductor element>
The photocatalytic semiconductor element 10 of the present invention is for a photocatalytic oxidation-reduction reaction. For example, a buffer layer 21 is provided on one surface of a substrate 25 made of sapphire, GaN, Si, SiC, LiAlO ₃ , LiGaO ₃ or the like. The group III-V nitride semiconductor 20 is laminated.
In FIG. 1, reference numeral 27 denotes a current collecting member formed by laminating a titanium layer 27b and a gold layer 27a in this order on the catalytic reaction surface R of the photocatalytic semiconductor element 10, and 29 is a solder 28 or the like. A conductive wire bonded to a current collecting member. The material of the current collecting member 27 and the like is not particularly limited as long as the purpose can be achieved.

  The thickness of the group III-V nitride semiconductor 20 in the photocatalytic semiconductor element 10 is preferably 1.5 to 4.0 μm, for example. When the thickness of the group III-V nitride semiconductor 20 is in the above range, carriers are generated by light irradiation and the generated carriers are reliably conducted to the catalytic reaction surface R. Also, depending on the conductivity of the III-V group compound constituting the III-V nitride semiconductor 20, there is no practical problem even if a thinner layer or a thicker layer is used.

In the photocatalytic semiconductor device 10, the base layer 20α and the carrier transfer promotion layer 20β of the group III-V nitride semiconductor 20 are, for example, an atmospheric pressure crystal growth method or a reduced pressure crystal growth method by a metal organic chemical vapor deposition method (MOVPE method). The known crystal growth method can be used.
Specifically, a group V atom feedstock, a group III atom feedstock, and an impurity feedstock to be doped as necessary together with a carrier gas such as hydrogen gas or nitrogen gas on the substrate 25 in a gas state. By supplying under heating, a thermochemical reaction occurs on the substrate 25, so that these feedstocks are decomposed into constituent elements and the constituent elements react with each other, and the target III-V compound is formed into the substrate. 25 is grown on top.

The heating temperature (hereinafter also referred to as “growth temperature”) for growing the III-V group compound constituting the buffer layer 21 is preferably 400 to 700 ° C., more preferably 500 to 600 ° C. If the growth temperature is too low, it may be difficult to grow the III-V group compound that constitutes the buffer layer. On the other hand, if the growth temperature is too high, the resulting buffer layer may have its surface. When a group III-V nitride semiconductor is grown thereon, there is a possibility that the effect of increasing the single crystallinity of the group III-V nitride semiconductor cannot be exhibited.
The growth temperature of the III-V group compound constituting the base layer 20α and the carrier transfer promoting layer 20β of the III-V nitride semiconductor 20 is higher than the growth temperature of the buffer layer 21. For example, the III-V group compound is In Is not included, for example, 900 to 1400 ° C, preferably 950 to 1150 ° C. When the III-V group compound contains In, the temperature is set to, for example, 700 to 800 ° C. from the viewpoint of the temperature suitable for the incorporation of In atoms.
Here, the growth temperature of the buffer layer 21 and the group III-V nitride semiconductor 20 is the temperature of the substrate in each step.

  According to the group III-V nitride semiconductor 20 and the photocatalytic semiconductor element 10 described above, carriers generated by light irradiation are successively laminated by the base layer 20α and the carrier movement promoting layer 20β having different semiconductor characteristics. As a result of new semiconductor characteristics obtained by the interaction of the light, specifically, the influence of the band edge potential and carrier mobility, the carriers generated by the light irradiation can be efficiently converted into the redox reaction on the catalytic reaction surface. In the end, it is possible to cause an oxidation-reduction reaction with high light conversion efficiency by light irradiation.

  Thus, the multi-layer stacked structure having different semiconductor properties can provide photoelectrochemical properties such as band edge potential and carrier mobility as a whole, so that the group III-V nitridation for photocatalytic oxidation-reduction reaction can be obtained. Physical semiconductors and photocatalytic semiconductor elements can be designed with a large degree of freedom.

<Photocatalytic oxidation-reduction reactor>
FIG. 3 is a schematic cross-sectional view showing an example of the configuration of the photocatalytic oxidation-reduction reaction apparatus of the present invention.
In this photocatalytic oxidation-reduction reaction apparatus 11, one of a pair of electrodes for electrolysis that are brought into contact with an electrolytic solution and electrically connected to each other has the above-described group III-V nitride semiconductor 20 in contact therewith. It consists of the photocatalytic semiconductor element 10 which has. In the photocatalytic semiconductor element 10, only the catalytic reaction surface R of the III-V nitride semiconductor 20 is in contact with the electrolytic solution.
In the following description, the III-V group nitride semiconductor 20 is assumed to be n-type.

  In this photocatalytic oxidation-reduction reaction apparatus 11, the other electrode corresponding to the photocatalytic semiconductor element 10 is configured by a metal electrode 18 made of a metal such as platinum, and thereby the photocatalytic semiconductor element 10 that is one electrode. Is an anode that undergoes an oxidation reaction on the catalytic reaction surface R of the group III-V nitride semiconductor 20 constituting this, and the metal electrode 18 that is the other electrode is the catalytic reaction surface R of one electrode on the surface thereof. The cathode corresponding to the electrochemical reaction in FIG.

In this photocatalytic oxidation-reduction reaction apparatus 11, the catalytic reaction surface R of the III-V nitride semiconductor 20 is irradiated with light, whereby an oxidation reaction occurs in the photocatalytic semiconductor element 10 and a reduction reaction occurs in the metal electrode 18. Arise.
Here, in addition to irradiating the catalytic reaction surface R of the III-V nitride semiconductor 20 as described above with light, an appropriate voltage is applied between the photocatalytic semiconductor element 10 and the metal electrode 18. As a result, the oxidation reaction in the photocatalytic semiconductor element 10 and the reduction reaction in the metal electrode 18 are promoted. Here, the magnitude of the voltage applied between the photocatalytic semiconductor element 10 and the metal electrode 18 is set to be not more than the magnitude of the voltage substantially necessary for the photocatalytic oxidation-reduction reaction. For example, when the photocatalytic oxidation-reduction reaction is performed using water as the electrolytic solution, the magnitude of the applied voltage is substantially 2 V or less in consideration of the overvoltage, preferably the difference in electrode potential of water decomposition. It is preferable to set it to 23 V or less.

Specifically, in the photocatalytic oxidation-reduction reaction apparatus 11, the oxidation tank 12A and the reduction tank 12B are communicated with each other through the connection tube 12C at the lower part, and are communicated with these at the upper part of each of the oxidation tank 12A and the reduction tank 12B. Gas collecting pipes 12E and 12F extending upward in the state are provided, and thereby, an electrolytic solution tank 12 filled with the electrolytic solution is configured. The electrolyte can be, for example, water (H ₂ O), hydrochloric acid / potassium hydroxide / potassium chloride aqueous solution (HCl / KOH / KCl), or the like. In addition, you may form electrolyte solution using substances other than water as a solvent.
Further, in the oxidation tank 12A, the photocatalytic semiconductor element 10 is in a state where the opening 15 formed in the peripheral wall of the oxidation tank 12A is watertightly closed so that the catalytic reaction surface R of the photocatalytic semiconductor element 10 is in contact with the electrolytic solution. In the reduction tank 12B, a metal electrode 18 is inserted so as to extend through the bottom of the reduction tank 12B. The photocatalytic semiconductor element 10 and the metal electrode 18 are inserted into the reduction tank 12B. Is electrically connected through a potentio stud 19. Furthermore, if necessary, a reference electrode 17 made of, for example, a silver / silver chloride / sodium chloride electrode for measuring the potential of the catalytic reaction surface R is provided so as to extend upward through the bottom of the oxidation tank 12A. Has been inserted.
A light transmitting window 12D for irradiating the catalytic reaction surface R with light L from a light source (not shown) is formed in a portion of the peripheral wall of the oxidation tank 12A facing the opening 15. Yes.
In FIG. 3, reference numeral 19 a denotes a voltage application power source used when a voltage is applied between the photocatalytic semiconductor element 10 and the metal electrode 18.

The light source for irradiating the catalytic reaction surface R with the light L is not particularly limited as long as it emits light having energy larger than the band gap of the III-V group compound constituting the III-V group nitride semiconductor 20. Sun, mercury lamp, xenon lamp, incandescent lamp, fluorescent lamp, LED, laser, and the like can be used.
For example, when the group III-V compound composing the group III-V nitride semiconductor 20 is gallium nitride (GaN), the band gap is 3.4 eV. .

In such a photocatalytic oxidation-reduction reaction apparatus 11, a photoelectrochemical reaction is performed as follows. That is, for example, when the electrolytic solution is water (H ₂ O), first, the photocatalytic carrier is obtained by irradiating the catalytic reaction surface R of the photocatalytic semiconductor element 10 with the light L from the light source through the light transmitting window 12D. electrons in generating moiety M (e ^-) and holes (h ⁺⁾ are generated, the hydroxide ions (OH in the electrolyte solution by a hole (h ⁺⁾ in the areas in contact with the electrolytic solution of the catalyst reaction surface R ⁻ ) Or water (H ₂ O) is oxidized and an electron (e) moved from the photocatalyst carrier generation portion M via the potentiostat 19 in a region in contact with the electrolytic solution on the surface of the metal electrode 18 ^- ) Causes a reduction reaction in which hydrogen ions (H ⁺ ) or water (H ₂ 0) in the electrolyte is reduced.
As a result, oxygen gas is generated in the photocatalytic semiconductor element 10 in the oxidation tank 12A, and hydrogen gas is generated in the metal electrode 18 in the reduction tank 12B. These oxygen gas and hydrogen gas are collected in the gas collection pipes 12E and 12F, respectively. The

  According to the photocatalytic oxidation-reduction reaction apparatus and the photoelectrochemical reaction execution method of the present invention, the base layer 20α and the carrier transfer promoting layer 20β having different semiconductor characteristics are stacked in succession on the carriers generated by light irradiation. As a result of new semiconductor characteristics obtained by the interaction of the light, specifically, the influence of the band edge potential and carrier mobility, the carriers generated by the light irradiation can be efficiently converted into the redox reaction on the catalytic reaction surface. In the end, it is possible to cause an oxidation-reduction reaction with high light conversion efficiency by light irradiation.

  In addition, the multi-layer stacked structure having different semiconductor characteristics can obtain photoelectrochemical characteristics such as separate band edge potential and carrier mobility as a whole. Can be designed with.

Various changes can be made in the photocatalytic oxidation-reduction reaction apparatus 11 described above.
For example, the present invention is not limited to using the photocatalytic semiconductor element 10 having the n-type III-V group nitride semiconductor 20 and the metal electrode 18 as a pair of electrodes for electrolysis, but having a p-type group III-V nitride semiconductor. A photocatalytic semiconductor element (hereinafter also referred to as “p-type photocatalytic semiconductor element”) and a metal electrode can be used. In this case, the p-type photocatalytic semiconductor element functions as a cathode and the metal electrode functions as an anode. Specifically, by irradiating light on the catalytic reaction surface of the p-type photocatalytic semiconductor element, electrons (e ⁻ ) and holes (h ⁺ ) are generated in the photocatalytic carrier generation portion, and electrons are generated on the catalytic reaction surface. (E ⁻ ) causes a reduction reaction in which hydrogen ions or water in the electrolytic solution is reduced, and hydroxylation in the electrolytic solution is caused by holes (h ⁺ ) transferred through the potentiostat 19 on the surface of the metal electrode. Oxidation reaction occurs in which product ions or water is oxidized.

  For example, a photocatalytic semiconductor element having an n-type III-V nitride semiconductor (hereinafter also referred to as “n-type photocatalytic semiconductor element”) and a p-type photocatalytic semiconductor element can be used as the pair of electrodes for electrolysis. . In this case, the n-type photocatalytic semiconductor element functions as an anode to cause an oxidation reaction, and the p-type photocatalytic semiconductor element functions as a cathode to cause a reduction reaction.

[Production Example 1a of Photocatalytic Semiconductor Element]
A photocatalytic semiconductor element was produced according to FIG. That is, first, a substrate made of sapphire at 100 kPa is annealed in hydrogen gas at 1100 ° C. for 10 minutes, and then the temperature of the substrate is lowered to 520 ° C. On the (0001) + c plane of this substrate, group V atoms and III Trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously supplied using hydrogen gas as a carrier gas so that the ratio to the group atoms (V / III ratio) is about 5000, and gallium nitride (GaN) for 10 minutes. A 50 nm thick buffer layer was grown.
Next, the temperature of the substrate is increased to 1040 ° C., and trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously used as hydrogen carrier gas on the surface of the buffer layer so that the V / III ratio is about 3800. 10 ppm of hydrogen-diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 5 sccm, and is made of gallium nitride (GaN) doped with silicon atoms (Si) for 60 minutes. An n-type group III-V compound layer (base layer) A made of the group V compound A was grown.
Furthermore, cyclopentadienylmagnesium (Cp ₂ Mg) is supplied, and p-type III − made of III-V compound B having a thickness of 10 nm made of gallium nitride (GaN) doped with magnesium atoms (Mg) for 20 seconds. A p-type carrier movement promoting layer B is laminated on the n-type base layer A by growing a group V compound layer (carrier movement promoting layer) B and then annealing in nitrogen at 700 ° C. for 20 minutes. In addition, a photocatalytic semiconductor element [1a] having an np structure was obtained.
The carrier concentration of each layer of the photocatalytic semiconductor element [1a] was measured by the van der Pauw method for a carrier measurement in which a single layer was separately grown. The carrier concentration of the base layer A doped with silane was 2. 0 a × 10 ¹⁸ cm ^-3, the carrier concentration of the carrier transfer-promoting layer B doped with magnesium was 1.2 × 10 ¹⁷ cm ^-3. Further, when the full width at half maximum of the X-ray rocking curve of the (0002) plane was measured, it was 280 arcsec. The surface of this photocatalytic semiconductor element [1a] was specular.

[Production Example 1b of Photocatalytic Semiconductor Element]
The photocatalytic semiconductor element [1b] is the same as the photocatalytic semiconductor element production example 1a except that the time for supplying the cyclopentadienyl magnesium is changed to 10 seconds so that the thickness of the carrier movement promoting layer B is 5 nm. Got. The photocatalytic semiconductor element [1b] has the same carrier concentration in the base layer A, the carrier concentration in the carrier movement promoting layer B, and the full width at half maximum of the (0002) plane X-ray rocking curve as in the photocatalytic semiconductor element [1a]. The surface of the element [1b] was specular.

[Production Example 2 of Photocatalytic Semiconductor Element]
A photocatalytic semiconductor element was produced according to FIG. That is, first, a substrate made of sapphire at 100 kPa is annealed in hydrogen gas at 1100 ° C. for 10 minutes, and then the temperature of the substrate is lowered to 520 ° C. On the (0001) + c plane of this substrate, group V atoms and III Trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously supplied using hydrogen gas as a carrier gas so that the ratio to the group atoms (V / III ratio) is about 5000, and gallium nitride (GaN) for 10 minutes. A 50 nm thick buffer layer was grown.
Next, the temperature of the substrate is increased to 1040 ° C., and trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously used as hydrogen carrier gas on the surface of the buffer layer so that the V / III ratio is about 3800. 10 ppm of hydrogen-diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 5 sccm, and is made of gallium nitride (GaN) doped with silicon atoms (Si) for 60 minutes. An n ⁺ -type III-V group compound layer (base layer) A made of a −V group compound A was grown.
Further, 10 ppm of hydrogen-diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 2 sccm, and the III-V group is made of gallium nitride (GaN) doped with silicon atoms (Si) for 12 minutes and having a thickness of 0.4 μm. By growing an n-type III-V group compound layer (carrier transfer promoting layer) C made of the compound C, an n ⁺ -n in which the n-type carrier transfer promoting layer C is laminated on the n ⁺ -type base layer A ^A photocatalytic semiconductor element [2] having a structure was obtained.
The carrier concentration of each layer of the photocatalytic semiconductor element [2] was measured by the van der Pauw method using a carrier measurement in which a single layer was separately grown. The carrier concentration of the carrier-promoting moving layer A was 2.0. × 10 ¹⁸ cm ^-3 , and the carrier concentration of the carrier migration promoting layer C was 1.2 × 10 ¹⁷ cm ^-3 . The full width at half maximum of the (0002) plane X-ray rocking curve was measured and found to be 330 arcsec. The surface of this photocatalytic semiconductor element [2] was specular.

[Production Example 3a of Photocatalytic Semiconductor Element]
A photocatalytic semiconductor element was produced according to FIG. That is, first, a substrate made of sapphire at 100 kPa is annealed in hydrogen gas at 1100 ° C. for 10 minutes, and then the temperature of the substrate is lowered to 520 ° C. On the (0001) + c plane of this substrate, group V atoms and III Trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously supplied using hydrogen gas as a carrier gas so that the ratio to the group atoms (V / III ratio) is about 5000, and gallium nitride (GaN) for 10 minutes. A 50 nm thick buffer layer was grown.
Next, the temperature of the substrate is increased to 1040 ° C., and trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously used as hydrogen carrier gas on the surface of the buffer layer so that the V / III ratio is about 3800. 10 ppm of hydrogen-diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 5 sccm, and is made of gallium nitride (GaN) doped with silicon atoms (Si) for 60 minutes. An n-type group III-V compound layer (base layer) A made of the group V compound A was grown.
Further, the substrate temperature is lowered to 750 ° C., and trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH ₃ ) are continuously supplied using hydrogen gas as a carrier gas so that the V / III ratio is about 10,000. Then, 10 ppm of hydrogen diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 2 sccm, and a silicon atom (Si) doped indium gallium nitride (InGaN) doped with silicon atom (Si) for 180 seconds has a thickness of 100 nm III-V group compound D By growing a III-V group compound layer (carrier transfer promoting layer) D, a photocatalytic semiconductor element [3a] having an EgH-EgL structure in which the carrier transfer promoting layer D was laminated on the base layer A was obtained.
The carrier concentration of each layer of this photocatalytic semiconductor element [3a] was measured by the van der Pauw method for a carrier measurement in which a single layer was grown separately. The base layer A was n-type and the carrier concentration was The carrier migration promoting layer D was 2.0 × 10 ¹⁸ cm ⁻³ , the In atom composition ratio x was 0.06, n-type, and the carrier concentration was 1.2 × 10 ¹⁷ cm ⁻³ . The full width at half maximum of the (0002) plane X-ray rocking curve was measured and found to be 320 arcsec. The surface of this photocatalytic semiconductor element [3a] was specular.
The band gap energy EgH calculated from the atomic composition ratio of the base layer A was 3.40 eV, and the band gap energy EgL calculated from the atomic composition ratio of the carrier migration promoting layer D was 3.15 eV.

[Production Examples 3b to 3d of Photocatalytic Semiconductor Element]
The photocatalyst except that the time for supplying TMGa, TMIn, NH ₃ and SiH ₄ was changed to 120 seconds, 360 seconds and 680 seconds, respectively, so that the thickness of the carrier movement promoting layer D was 67 nm, 201 nm and 378 nm, respectively. Photocatalytic semiconductor elements [3b] to [3d] were obtained in the same manner as in Production Example 3a of the semiconductor element. The carrier concentration of the base layer A, the atomic composition ratio and carrier concentration of the carrier movement promoting layer D, and the full width at half maximum of the X-ray rocking curve on the (0002) plane of the photocatalytic semiconductor elements [3b] to [3d] The surfaces of the photocatalytic semiconductor elements [3b] to [3d] were mirror-like.

[Comparative Photocatalytic Semiconductor Element Production Examples 1 and 2]
A photocatalytic semiconductor element was produced according to FIG. That is, first, a substrate made of sapphire at 100 kPa is annealed in hydrogen gas at 1100 ° C. for 10 minutes, and then the temperature of the substrate is lowered to 520 ° C. On the (0001) + c plane of this substrate, group V atoms and III Trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously supplied using hydrogen gas as a carrier gas so that the ratio to the group atoms (V / III ratio) is about 5000, and gallium nitride (GaN) for 10 minutes. A 50 nm thick buffer layer was grown.
Next, the temperature of the substrate is increased to 1040 ° C., and trimethylgallium (TMGa) and ammonia (NH ₃ ) are continuously used as hydrogen carrier gas on the surface of the buffer layer so that the V / III ratio is about 3800. 10 ppm of hydrogen-diluted silane (SiH ₄ ) is continuously supplied at a flow rate of 5 sccm, and is made of gallium nitride (GaN) doped with silicon atoms (Si) for 60 minutes. By growing an n-type III-V group compound layer made of Group V compound A, a comparative photocatalytic semiconductor element [X] was obtained.
The carrier concentration of the base layer A of this comparative photocatalytic semiconductor element [X] was 2.0 × 10 ¹⁸ cm ^{−3 as} measured by the van der Pauw method. Further, when the full width at half maximum of the X-ray rocking curve of the (0002) plane was measured, it was 290 arcsec. The surface of this comparative photocatalytic semiconductor element [X] was specular.
A comparative photocatalytic semiconductor element [Y] was obtained in the same manner as in Comparative Photocatalytic Semiconductor Element Production Example 1 except that the carrier concentration was 1.2 × 10 ¹⁷ cm ⁻³ .

<Examples 1a, 1b, 2, 3a to 3d, Comparative Examples 1 and 2>
A photocatalytic oxidation-reduction reactor was produced according to FIG.
Specifically, the photocatalytic semiconductor elements [1a], [1b], [2], [3a] to [3d], the comparative photocatalytic semiconductor elements [X] and [Y] are used as anodes, and their catalytic reaction surfaces. Is provided with a current collecting member in which titanium and gold are laminated on the outer peripheral portion of the surface, and electrically connected to the cathode by a conductive wire, using a platinum electrode as the cathode, and further silver / silver chloride / sodium chloride The electrode is used as a reference electrode, and 1 mol / L HCl is used as an electrolytic solution. Photocatalytic oxidation-reduction reaction devices [1a], [1b], [2], [3a] to [3d], comparative photocatalytic oxidation-reduction reactions Apparatus [X] [Y] was manufactured.
A circular region having a diameter of 10 mm in the central portion of the surface constituting the catalytic reaction surface was irradiated with light from a light source of a 150 W xenon lamp.

[Flatband potential measurement test]
Photocatalytic oxidation-reduction reaction apparatus [1a], [1b], [2], [3a]-[3d] according to Examples 1a, 1b, 2, 3a-3d, and comparative photocatalytic oxidation-reduction reaction according to Comparative Example 1 The impedance was measured using the apparatus [X], and the flat band potential calculated from the Mott-Schottky plot was obtained. The results according to Examples 1a and 1b and Comparative Example 1 are shown in FIG. 4, and the results according to Examples 3a to 3d and Comparative Example 1 are shown in FIG. Further, the band edge potential (indicated by “■”) of the conduction band, the band edge potential of the valence band (indicated by “●”), and the hydrogen generation potential (H ⁺ ) according to Examples 1a and 1b and Comparative Example 1. / H ₂ ) and oxygen generation potential (O ₂ / H ₂ O) are shown in FIG. 6, and the band edge potential (indicated by “■”) and value of the conduction bands according to Examples 3a to 3d and Comparative Example 1. FIG. 7 shows the band edge potential (indicated by “●”), the hydrogen generation potential (H ⁺ / H ₂ ), and the oxygen generation potential (O ₂ / H ₂ O) of the electron band.

[Photo-induced current density measurement test]
Further, using each of the photocatalytic oxidation-reduction reaction apparatus [1a] according to Example 1a and the comparative photocatalytic oxidation-reduction reaction apparatus [X] according to Comparative Example 1, the hydrogen generation integrated amount and the photo-induction at the counter electrode at zero bias are used. The current density (mA / cm ² ) was measured. The results are shown in FIG. However, in FIG. 8, the thing related to the photocatalytic oxidation-reduction reaction apparatus [1a] is shown as (a), and the thing related to the comparative photocatalytic oxidation-reduction reaction apparatus [X] is shown as (a).

[Photoelectrochemical reaction test]
In addition, using the photocatalytic oxidation-reduction reaction apparatus [2] according to Example 2 and the comparative photocatalytic oxidation-reduction reaction apparatus [X] according to Comparative Example 1, a photo-induced current is applied in a state where no voltage is applied (zero bias). The density (mA / cm ² ) was measured. The results are shown in FIG. However, in FIG. 9, the result relating to the photocatalytic oxidation-reduction reactor [2] is indicated by “◯”, and the result relating to the comparative photocatalytic oxidation-reduction reactor [X] is indicated by “▲”.
Furthermore, the current density-voltage characteristics were measured using each of the photocatalytic oxidation-reduction reaction apparatus [3a] according to Example 3a and the comparative photocatalytic oxidation-reduction reaction apparatuses [X] and [Y] according to Comparative Examples 1 and 2. did. The results are shown in FIG. However, in FIG. 10, the results relating to the photocatalytic oxidation-reduction reactor [3a] are shown as (c), and the results relating to the comparative photocatalytic oxidation-reduction reactors [X], [Y] are shown as (e), (e). It was.

If a group III-V nitride semiconductor having n-type semiconductor characteristics is used, the flat band potential is almost the same as the conduction charge level. Therefore, if this flat band potential shifts to the negative side, The difference becomes larger and hydrogen generation becomes easier. On the other hand, if this flat band potential shifts to the plus side, the difference from the hydrogen generation potential becomes smaller and hydrogen generation becomes difficult. If it is shifted to the plus side, hydrogen generation cannot be obtained.
However, as is apparent from the results of FIGS. 4 and 6, in the photocatalytic oxidation-reduction reactors [1a] and [1b] according to Examples 1a and 1b, a p-type III-V group compound having a specific thickness is used. Since the carrier movement promoting layer is provided, the flat band potential is shifted to the negative side as compared with that according to Comparative Example 1, and the difference from the hydrogen generation potential is increased, resulting in high light conversion. It was confirmed that an oxidation-reduction reaction can be caused with efficiency.
On the other hand, when a p-type semiconductor layer having a thickness of 20 nm, 50 nm, and 100 nm was prepared as a carrier movement promoting layer, and a flat band potential measurement test was performed, the p-type semiconductor as the entire group III-V nitride semiconductor was obtained. The flat band potential could not be detected.
Further, as is clear from the results of FIG. 8, the np structure carrier transfer promoting layer according to Example 1a is provided as compared with the base layer-only III-V group nitride semiconductor according to Comparative Example 1. It was confirmed that the group III-V nitride semiconductor can obtain a large integrated hydrogen generation amount and photoinduced current density.

  As is clear from the results of FIGS. 5 and 7, in the photocatalytic oxidation-reduction reaction devices [3b] to [3d] according to Examples 3a to 3d, the flat band potential is shifted in the positive direction. However, when a carrier migration promoting layer having a specific thickness and a low band gap energy Eg is provided, the amount of light absorption of the carrier migration promoting layer is increased and compared with that according to Comparative Example 1. It was confirmed that the energy efficiency was high and a large photoinduced current density was obtained. In the case of the present example, the difference between the flat band potential and the hydrogen generation potential is, as is apparent from FIG. 7, that the carrier edge potential of the evangelistic band is determined when the carrier movement promoting layer has a thickness exceeding 400 nm. Shifts in the positive direction beyond the hydrogen generation potential, so that hydrogen cannot be generated.

As is clear from FIG. 9, in the photocatalytic oxidation-reduction reaction apparatus [2] according to Example 2, a larger photoinduced current density was obtained than in the photocatalytic oxidation-reduction reaction apparatus [X] according to Comparative Example 1. .
In FIG. 9, the solid line represents the photoinduced current density measured at zero bias for the same structure as that of the photocatalytic oxidation-reduction reactor [2], in which the carrier concentration of the carrier transfer promotion layer was changed, The dotted line shows the photo-induced current density measured with zero bias for the same configuration as that of the comparative photocatalytic oxidation-reduction reactor [X], in which the carrier concentration of the carrier migration promoting layer was changed.
Furthermore, the flat band potential of the n ⁺ -n ⁻ group III-V nitride semiconductor constituting the photocatalytic oxidation-reduction reactor [2] is III-V group nitride of only the n ⁺ layer according to Comparative Example 1. It was the same as the flat band potential of the semiconductor.
Further, as is apparent from FIG. 10, the group III-V nitride semiconductor having an n ⁺ -n ⁻ structure is composed of a group III-V nitride semiconductor having only an n ⁺ layer (Comparative Example 1) or only an n ⁻ layer. As compared with the group III-V nitride semiconductor (Comparative Example 2), it was confirmed that a large current density was obtained in the range of all applied voltages including the case of zero bias.

It is typical sectional drawing which shows an example of a structure of the photocatalytic semiconductor element which has a III-V group nitride semiconductor of this invention in the state in which the member for current collection was provided. FIG. 4 is an explanatory diagram schematically showing the structure of the group III-V nitride semiconductor of FIG. 1. It is typical sectional drawing which shows an example of a structure of the photocatalytic oxidation-reduction reaction apparatus provided with the photocatalytic semiconductor element which has a III-V group nitride semiconductor of this invention. 5 is a graph showing the relationship between the thickness of a carrier migration promoting layer made of p-GaN and a flat band potential when pH = 0 in a group III-V nitride semiconductor having an np structure. 5 is a graph showing the relationship between the thickness of a carrier transfer promotion layer made of InGaN and a flat band potential when pH = 0 in a III-V group nitride semiconductor having an EgH-EgL structure. In the III-V group nitride semiconductor having the np structure, the thickness of the carrier transfer promoting layer made of p-GaN, the band edge potential of the conduction band and the valence band, the hydrogen generation potential, It is a graph which shows the relationship with an oxygen generation potential. In the III-V group nitride semiconductor having the EgH-EgL structure, the thickness of the carrier transfer promoting layer made of InGaN, the band edge potential of the conduction band and the valence band, the hydrogen generation potential, and the oxygen generation when pH = 0 It is a graph which shows the relationship with an electric potential. 4 is a graph showing the time dependency of the hydrogen generation integrated amount and the photoinduced current density in a group III-V nitride semiconductor having an np structure. 5 is a graph showing the relationship between the carrier concentration and the photoinduced current density of a carrier movement promoting layer in a group III-V nitride semiconductor having an n ⁺ -n ⁻ structure. 5 is a graph showing a relationship between current density and voltage characteristics in a group III-V nitride semiconductor having an n ⁺ -n ⁻ structure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Photocatalyst semiconductor element 11 Photocatalytic oxidation-reduction reaction apparatus 12 Electrolyte tank 12A Oxidation tank 12B Reduction tank 12C Connection tube 12D Light transmission window 12E, 12F Gas collection pipe 15 Opening 16 O-ring 17 Reference electrode 18 Metal electrode 19 Potentio stud 19a Voltage application power source 20 III -V group nitride semiconductor 20α Base layer 20β Carrier movement promotion layer 21 Buffer layer 25 Substrate 27 Current collecting member 27a Gold layer 27b Titanium layer 28 Solder 29 Conductive wire L Light M Photocatalyst carrier generation part R Catalytic reaction surface

Claims (6)

A group III-V nitride semiconductor for photocatalytic oxidation-reduction reaction,
Having a base layer having different semiconductor characteristics and a surface layer laminated on the base layer,
By the surface layer and the base layer surface to contact is formed, at least the surface layer have a carrier transfer promoting effect,
The base layer and the surface layer have different conductivity types,
A III-V nitride semiconductor, wherein the surface layer has a thickness of 10 to 18 nm . 2. The group III-V nitride semiconductor according to claim 1 , wherein the band gap energy EgL of the material forming the surface layer is smaller than the band gap energy EgH of the material forming the base layer .   A photocatalytic semiconductor element for photocatalytic oxidation-reduction reaction,
  A photocatalytic semiconductor element, wherein the group III-V nitride semiconductor according to claim 1 or 2 is laminated on a substrate.   A photocatalytic redox reaction device for photocatalytic redox reaction,
  One electrode of the pair of electrodes for electrolysis that are brought into contact with the electrolytic solution and electrically connected to each other is made of the III-V group nitride semiconductor according to claim 1 or 2,
  A photocatalytic oxidation-reduction reaction apparatus characterized in that an oxidation reaction or a reduction reaction occurs on the catalytic reaction surface by irradiating light to the catalytic reaction surface constituting the III-V group nitride semiconductor.   The catalytic reaction surface in the group III-V nitride semiconductor is irradiated with light and a voltage is applied between the pair of electrodes for electrolysis, whereby an oxidation reaction or a reduction reaction occurs on the catalytic reaction surface. The photocatalytic oxidation-reduction reaction apparatus according to claim 4.   Using the group III-V nitride semiconductor according to claim 1 or 2, the catalyst reaction surface of the group III-V nitride semiconductor is irradiated with excitation light, and an oxidation reaction or a reduction reaction is performed on the catalyst reaction surface. A method for performing a photoelectrochemical reaction, characterized by comprising: