WO2024116357A1

WO2024116357A1 - Semiconductor photoelectrode

Info

Publication number: WO2024116357A1
Application number: PCT/JP2022/044284
Authority: WO
Inventors: 裕也渦巻; 紗弓里; 晃洋鴻野; 浩伸蓑輪; 淳荒武
Original assignee: 日本電信電話株式会社
Priority date: 2022-11-30
Filing date: 2022-11-30
Publication date: 2024-06-06

Abstract

This semiconductor photoelectrode comprises a first semiconductor thin film 3 including a first thin film 3-1 and a second thin film 3-2 arranged alternately, wherein the first thin film 3-1 comprises a group III-V compound semiconductor, and the second thin film 3-2 comprises a group III-V compound semiconductor having a band gap larger than that of the first thin film.

Description

Semiconductor Photoelectrodes

This disclosure relates to semiconductor photoelectrodes.

The device that produces hydrogen through a water splitting reaction using a semiconductor photoelectrode has an oxidation tank and a reduction tank that are connected via a proton exchange membrane. An aqueous solution and an oxidation electrode are placed in the oxidation tank, and an aqueous solution and a reduction electrode are placed in the reduction tank. The oxidation electrode and reduction electrode are electrically connected by a conductor.

The water splitting reaction using photocatalysts consists of a water oxidation reaction and a proton reduction reaction. When light is irradiated onto an n-type photocatalyst material, electrons and holes are generated and separated in the photocatalyst. The holes move to the surface of the photocatalyst material and contribute to the water oxidation reaction. Meanwhile, the electrons move to the reduction electrode and contribute to the proton reduction reaction. Ideally, this type of oxidation-reduction reaction progresses, resulting in a water splitting reaction.

Oxidation reaction: _2H2O + 4h ⁺ → _O2 + 4H ⁺
Reduction reaction: 4H ⁺ + 4e ^- → 2H ₂

A thin semiconductor film is used for the oxidation electrode. When light is irradiated in an aqueous solution, the target water oxidation reaction takes place on the surface of the thin semiconductor film. For example, thin semiconductor films with a narrow band gap, such as indium gallium nitride, are expected to improve the efficiency of solar energy conversion because they can absorb a wider range of wavelengths.

However, semiconductor thin films with narrow band gaps have poor crystallinity due to the difficulty of crystal growth, and as a result, even if the wavelengths that can be absorbed are expanded, the rate of electron-hole recombination is high, meaning that the expected efficiency cannot be achieved.

In addition, on the surface of the semiconductor thin film, an etching reaction occurs as a side reaction, starting from dislocations on the surface of the semiconductor thin film. The etching reaction for gallium nitride is as follows:

Etching reaction: 2GaN + _3H2O + _6h ⁺ → _N2 + _Ga2O3 + 6H ⁺
The more dislocations (lattice defects) there are on the surface of the semiconductor thin film (poor crystallinity), the more the etching reaction progresses, and the fewer reaction sites there are for the target reaction to proceed, resulting in a problem that the light energy conversion efficiency decreases within a few hours. For example, indium gallium nitride thin films have more lattice defects than gallium nitride thin films due to the difficulty of crystal growth, and the etching reaction progresses more easily, which is a problem.

This disclosure has been made in light of the above, and aims to improve the light energy conversion efficiency of semiconductor photoelectrodes.

The semiconductor photoelectrode of one embodiment of the present disclosure comprises a first semiconductor thin film in which a first thin film made of a III-V compound semiconductor and a second thin film made of a III-V compound semiconductor having a larger band gap than the first thin film are arranged alternately.

According to this disclosure, it is possible to improve the light energy conversion efficiency of semiconductor photoelectrodes.

FIG. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode of this embodiment. FIG. 2 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode of Example 8. FIG. 3 is a diagram showing an outline of an apparatus for carrying out an oxidation-reduction reaction test.

Below, an embodiment of the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to the embodiment described below, and modifications may be made without departing from the spirit of the present disclosure.

[Configuration of semiconductor photoelectrode]
Fig. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode of this embodiment. The semiconductor photoelectrode shown in Fig. 1 includes an insulating or conductive substrate 1, a semiconductor thin film 2 (second semiconductor thin film) made of a III-V compound semiconductor arranged on the substrate 1, a semiconductor thin film 3 (first semiconductor thin film) made of a III-V compound semiconductor arranged on the semiconductor thin film 2, and an oxygen generating catalyst layer 4 arranged on the semiconductor thin film 3.

Substrate 1 is an insulating or conductive substrate such as a gallium nitride substrate, a sapphire substrate, or a silicon-based substrate.

The semiconductor thin film 2 is made of a III-V compound semiconductor such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), or indium gallium nitride (InGaN). The semiconductor thin film 2 is an n-type semiconductor.

The semiconductor thin film 3 of this embodiment is a semiconductor thin film in which a first thin film 3-1 (semiconductor thin film) made of a III-V compound semiconductor and a second thin film 3-2 (semiconductor thin film) made of a III-V compound semiconductor with a larger band gap than the first thin film 3-1 are arranged alternately. In other words, the semiconductor thin film 3 forms a quantum well structure in which a second thin film 3-2 (several nm thick) with a larger band gap than the first thin film 3-1 is regularly sandwiched between two first thin films 3-1 as an intermediate layer.

This promotes the separation of electron-hole pairs generated in the semiconductor thin film 3 (semiconductor photocatalyst), improving the light energy conversion efficiency. This is because, if the second thin film 3-2 is sufficiently thin, the electron-hole pairs generated in the semiconductor thin film 3 by light energy flow at high speed between the minibands formed by the interaction between quantum wells, promoting their separation.

In addition, the structure in which the second thin film 3-2 is sandwiched between the first thin films 3-1 has the effect of alleviating the strain energy that occurs when the semiconductor thin film 3 is grown, improving the crystallinity of the semiconductor thin film 3 and enabling the solar energy conversion efficiency to be maintained for a long period of time.

In the embodiment described later, indium gallium nitride (InGaN) having a narrow band gap is used for the first thin film 3-1, and gallium nitride (GaN) having a larger band gap than indium gallium nitride is used for the second thin film 3-2, but the present invention is not limited to this. For example, pairs of InGaN and AlGaN, GaN and AlGaN, _InxGa1 _-xN and _InyGa1 _- yN (x>y), _AlxGa1 _-xN and _AlyGa1 _-yN (x<y), etc. may be used as the pair of the first thin film 3-1 and the second thin film 3-2.

The band gap of the first thin film 3-1 is preferably about 6.0 eV or less, and more preferably about 3.0 eV or less. The film thickness of the first thin film 3-1 is preferably 10 nm to 70 nm, and more preferably 20 nm to 50 nm. In the examples described below, indium gallium nitride with an indium composition of 10% was used for the first thin film 3-1, but the indium composition is preferably 1% to 40%.

The second thin film 3-2 is preferably made of a III-V compound semiconductor with fewer lattice defects than the first thin film 3-1. The second thin film 3-2 reduces the contact points between the aqueous solution and the surface dislocations (lattice defects) that are the starting points of the etching reaction, and aims to extend the life of the amount of hydrogen and oxygen produced by the water splitting reaction (light energy conversion efficiency). The thickness of the second thin film 3-2 is preferably 1 nm to 2 nm.

The deposition methods for the first thin film 3-1 and the second thin film 3-2 include physical vapor deposition methods such as vacuum deposition and sputtering, chemical vapor deposition methods such as metal-organic vapor deposition, and liquid phase deposition.

The oxygen generating catalyst layer 4 can be obtained by forming a metal on the semiconductor thin film 3 and then oxidizing it. Alternatively, an oxide can be formed directly on the semiconductor thin film 3. Methods for forming metal films include physical vapor deposition methods such as vacuum deposition and sputtering, chemical vapor deposition methods such as metal-organic vapor deposition, and liquid phase deposition.

In the examples described below, NiO (nickel oxide) is used for the oxygen generating catalyst layer 4, but this is not limited to this. The oxygen generating catalyst layer 4 may be at least one metal selected from the group consisting of Ni, Co, Cu, W, Ta, Pd, Ru, Fe, Zn, and Nb, or an oxide of said metal. The thickness of the oxygen generating catalyst layer 4 is preferably 1 nm to 10 nm, and more preferably 1 nm to 3 nm, which allows sufficient light transmission.

[Preparation of Examples and Comparative Examples]
Examples 1 to 11 in which the semiconductor photoelectrode of this embodiment was fabricated will be described below. Comparative Examples 1 and 2 in which the second thin film 3-2 was not provided will also be described.

Example 1
In the semiconductor photoelectrode of Example 1, an n-type GaN substrate was used as the substrate 1 .

A silicon-doped n-GaN semiconductor thin film was epitaxially grown by metalorganic chemical vapor deposition on a 2-inch n-GaN substrate 1 to form the semiconductor thin film 2. The n-GaN semiconductor thin film had a thickness of 5 μm and a carrier density of 3×10 ¹⁸ cm ^-3 .

Next, as the first thin film 3-1, an indium gallium nitride ( _In0.1Ga0.9N : band gap approximately 3.0eV) alloy semiconductor with an indium composition ratio of 10% was epitaxially grown on the n _{-GaN semiconductor thin film by metal organic chemical vapor deposition. The thickness of the In0.1Ga0.9N} _{semiconductor} _thin film was set to 20nm, which is sufficient to maintain the crystal quality.

Next, as the second thin film 3-2, GaN (band gap approximately 3.4 eV) was epitaxially grown by metalorganic chemical vapor deposition on the In _0.1 Ga _0.9 N semiconductor thin film. The thickness of the GaN semiconductor thin film was set to 1 nm, which is sufficient to promote the transport of electrons and holes due to the quantum well structure.

Then, the first thin film 3-1 and the second thin film 3-2 were grown alternately to form the semiconductor thin film 3. The entire semiconductor thin film 3 had a thickness of about 500 nm, which is sufficient to absorb light. In Example 1, 22 pairs of the first thin film 3-1 and the second thin film 3-2 were arranged.

Here, the first thin film 3-1 and the second thin film 3-2 were grown alternately, and then the first thin film 3-1 was grown last. That is, in the semiconductor thin film 3 of Example 1, the first thin film 3-1 was formed on the semiconductor thin film 2, and the first thin film 3-1 was also formed on the outermost surface on the oxygen generating catalyst layer 4 side (directly below the oxygen generating catalyst layer 4). Moreover, for all the first thin films 3-1 of Example 1, the same In _0.1 Ga _0.9 N semiconductor thin films (band gap approximately 3.0 eV) were used with the same film thickness.

Next, Ni was vacuum-deposited to a thickness of about 1 nm on the first thin film 3-1 ( _In0.1Ga0.9N _{semiconductor} thin film) which was the outermost surface layer (top layer) of the semiconductor thin film 3. This semiconductor electrode was then heat-treated in air at 300°C for 1 hour to form NiO. The cross section of the sample was observed with a TEM and the thickness of the NiO film was found to be 2 nm. In this way, the semiconductor photoelectrode of Example 1 was obtained.

Example 2
Example 2 is a semiconductor photoelectrode in which the film thickness of the second thin film 3-2 of the semiconductor thin film 3 is set to 2 nm.

Example 3
Example 3 is a semiconductor photoelectrode in which the first thin film 3-1 of the semiconductor thin film 3 is made of In _0.2 Ga _0.8 N (band gap: about 2.63 eV).

Example 4
Example 4 is a semiconductor photoelectrode in which the thickness of the first thin film 3-1 of the semiconductor thin film 3 is 50 nm.

Example 5
Example 5 is a semiconductor photoelectrode in which the bottom layer of the semiconductor thin film 3 is the second thin film 3-2. Specifically, as the semiconductor thin film 3, the second thin film 3-2 is formed on the semiconductor thin film 2, the first thin film 3-1 is formed thereon, and the first thin film 3-1 and the second thin film 3-2 are alternately formed, and then the top surface layer is the first thin film 3-1. The oxygen generating catalyst layer 4 is formed on the top surface layer, the first thin film 3-1. Other points are the same as those of Example 1.

Example 6
Example 6 is a semiconductor photoelectrode in which the bottom layer and the outermost layer (uppermost layer) of the semiconductor thin film 3 are the second thin film 3-2. Specifically, as the semiconductor thin film 3, the second thin film 3-2 is formed on the semiconductor thin film 2, the first thin film 3-1 is formed thereon, the first thin film 3-1 and the second thin film 3-2 are alternately formed, and then the outermost layer is the second thin film 3-2. The oxygen generating catalyst layer 4 is formed on the outermost layer, the second thin film 3-2. Other points are the same as those of Example 1.

Example 7
Example 7 is a semiconductor photoelectrode in which the outermost surface layer of the semiconductor thin film 3 is a second thin film 3-2. Specifically, as the semiconductor thin film 3, a first thin film 3-1 is formed on the semiconductor thin film 2, a second thin film 3-2 is formed thereon, and the first thin film 3-1 and the second thin film 3-2 are alternately formed, and then the outermost surface layer is the second thin film 3-2. An oxygen generating catalyst layer 4 is formed on the outermost surface layer, the second thin film 3-2. Other points are the same as those of Example 1.

Example 8
Fig. 2 is a cross-sectional view showing the configuration of the semiconductor photoelectrode of Example 8. In the semiconductor photoelectrode shown in Fig. 2, first thin films 5-1-1 to 5-1-N of the semiconductor thin film 5 formed on the semiconductor thin film 2 are different from the first thin film 3-1 of the semiconductor thin film 3 of Example 1 shown in Fig. 1, but the rest is similar to the semiconductor photoelectrode of Example 1. The second thin film 5-2 is similar to the second thin film 3-2 of Example 1.

The semiconductor thin film 5 shown in the figure has N (N>1) first thin films 5-1-1 to 5-1-N. The first thin films 5-1-1 to 5-1-N may also be referred to as first thin films 5-1-n. n (n>1) indicates the order in which the first thin films are formed.

Each first thin film 5-1-n in Example 8 is formed by growing indium gallium nitride with a composition gradient of indium from m1% to m2% (m1 < m2) from the substrate 1 toward the oxygen generating catalyst layer 4. That is, the first thin film 5-1-n of the semiconductor photoelectrode 3 has an indium composition ratio (concentration) that is gradually changed (increased) in the stacking direction. m1 and m2 are set for each first thin film 5-1-n. That is, as n increases, m1 and m2 are also set to larger values. As a result, each first thin film 5-1-n is arranged so that the band gap narrows in the direction from the second semiconductor thin film 2 toward the oxygen generating catalyst layer 4. Here, m2 = m1 + 2.

In addition, in Example 8, m2 of the nth (n>1) first thin film 5-1-n is equal to m1 of the n+1th first thin film 5-1-n+1. Therefore, the bandgap at the top of the oxygen generating catalyst layer 4 side of the nth arranged (formed) first thin film 5-1-n is equal to the bandgap at the bottom of the second semiconductor thin film 2 side of the next n+1th arranged first thin film 5-1-n+1.

The manufacturing method for the semiconductor photoelectrode of Example 8 is described below.

The semiconductor photoelectrode of Example 8 used an n-type GaN substrate for substrate 1.

Next, as the first thin film 5-1-1, indium gallium nitride ( _{In0.02Ga0.98N: band gap approximately 3.32eV} ) with an indium composition ratio gradient of 0→2.0% from the substrate 1 toward the oxygen evolution catalyst layer 4 was grown on the n _- _GaN semiconductor thin film. The thickness of the _{In0.02Ga0.98N} semiconductor thin film was set to 50 nm, which is sufficient to maintain the crystal quality.

Next, as the second thin film 5-2, GaN (band gap approximately 3.4 eV) was grown on the In _0.02 Ga _0.98 N semiconductor thin film. The thickness of the GaN semiconductor thin film was set to 1 nm, which can promote the transport of electrons and holes due to the quantum well structure.

Next, as the first thin film 5-1-2, indium gallium nitride ( _{In0.04Ga0.96N} : band gap approximately 3.24 eV) was grown on the GaN semiconductor thin film, with the indium composition ratio being graded from 2.0 to 4.0% from the substrate 1 toward the oxygen evolution catalyst layer 4. Next, as the second thin film _5-2 , GaN (band gap approximately 3.4 _{eV) was grown on the In0.04Ga0.96N} _{semiconductor} thin film.

In this manner, the first thin film 5-1-n and the second thin film 5-2 were grown _{alternately until the first thin film 5-1-N, the outermost layer of the semiconductor thin film 5, became In0.2Ga0.8N} ₍ band gap approximately 2.63 eV), to form the semiconductor thin film 5. The entire semiconductor thin film 5 had a thickness of approximately 500 nm, which is sufficient to sufficiently absorb light.

Here, the first thin film 5-1-n and the second thin film 5-2 are grown alternately, and then the first thin film 5-1-N is grown last. That is, in the semiconductor thin film 5 of Example 8, the first thin film 5-1-1 is formed on the semiconductor thin film 2, and the first thin film 5-1-N is also formed on the outermost surface layer on the oxygen generating catalyst layer 4 side (directly below the oxygen generating catalyst layer 4).

Next, Ni was vacuum-deposited to a thickness of about 1 nm on the surface of the first thin film 5-1-N ( _In0.2Ga0.8N _{semiconductor} thin film) which was the outermost layer of the semiconductor thin film 5. This semiconductor electrode was then heat-treated in air at 300°C for 1 hour to form NiO. The cross section of the sample was observed with a TEM and the thickness of the NiO film was found to be 2 nm. In this way, the semiconductor photoelectrode of Example 8 was obtained.

In Example 8, a pair of InGaN and GaN is used for the first thin film 5-1-n and the second thin film 5-2, but this is not limited to this. For example, the pair of the first thin film 5-1-n and the second thin film 5-2 may be InGaN and AlGaN, GaN and AlGaN, etc.

<Example 9>
Example 9 is a semiconductor photoelectrode in which the bottom layer of the semiconductor thin film 5 is the second thin film 5-2. Specifically, as the semiconductor thin film 5, the second thin film 5-2 is formed on the semiconductor thin film 2, the first thin film 5-1-1 is formed thereon, the first thin film 5-1-n and the second thin film 5-2 are formed alternately, and then the top surface layer is the first thin film 5-1-N. The oxygen generating catalyst layer 4 is formed on the top surface layer, the first thin film 5-1-N. Other points are the same as those of Example 8.

Example 10
Example 6 is a semiconductor photoelectrode in which the bottom layer and the outermost layer (uppermost layer) of the semiconductor thin film 5 are the second thin film 5-2. Specifically, as the semiconductor thin film 5, the second thin film 5-2 is formed on the semiconductor thin film 2, the first thin film 5-1-1 is formed thereon, the first thin films 5-1-n and the second thin films 5-2 are formed alternately, and then the outermost layer is the second thin film 5-2. The oxygen generating catalyst layer 4 is formed on the outermost layer, the second thin film 5-2. Other points are the same as those of Example 8.

Example 11
Example 11 is a semiconductor photoelectrode in which the outermost surface layer of the semiconductor thin film 5 is the second thin film 5-2. Specifically, as the semiconductor thin film 5, a first thin film 5-1-1 is formed on the semiconductor thin film 2, a second thin film 5-2 is formed thereon, and the first thin films 5-1-n and the second thin films 5-2 are alternately formed, and then the outermost surface layer is the second thin film 5-2. The oxygen generating catalyst layer 4 is formed on the outermost surface layer, the second thin film 5-2. Other points are the same as those of Example 8.

<Comparative example 1>
Comparative Example 1 is a comparative example of Examples 1 to 7. The semiconductor photoelectrode of Comparative Example 1 does not include a second thin film 3-2 (GaN layer), and uses a single layer of a first thin film 3-1 ( _In0.1Ga0.9N _: band gap approximately 3.0 eV) having a thickness of 500 nm as the semiconductor thin film 3. Other points are similar to those of Example 1.

<Comparative Example 2>
Comparative Example 2 is a comparative example of Examples 8 to 11. The semiconductor photoelectrode of Comparative Example 2 does not include a second thin film 5-2 (GaN layer), and uses a single layer (first thin film) of indium gallium nitride grown in the semiconductor thin film 5 with an indium composition gradient of 0 to 20% from the substrate 1 toward the oxygen evolution catalyst layer 4. The thickness of the semiconductor thin film 5 was 500 nm. Other points are the same as those of Example 8.

[Oxidation-reduction reaction test]
An oxidation-reduction reaction test was carried out for Examples 1 to 11 and Comparative Examples 1 and 2 using the device shown in FIG.

The apparatus in FIG. 3 comprises an oxidation tank 110 and a reduction tank 120. An aqueous solution 111 is placed in the oxidation tank 110, and an oxidation electrode 112 is placed in the aqueous solution 111. The oxidation electrode 112 is in contact with the aqueous solution 111. An aqueous solution 121 is placed in the reduction tank 120, and a reduction electrode 122 is placed in the aqueous solution 121. The reduction electrode 122 is in contact with the aqueous solution 121.

A 1 mol/l aqueous solution of sodium hydroxide was used as the aqueous solution 111 in the oxidation tank 110. In addition to sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous rubidium solution, an aqueous solution of cesium hydroxide, or hydrochloric acid may also be used as the aqueous solution 111.

The semiconductor photoelectrode under test was used as the oxidation electrode 112. Specifically, for each of Examples 1 to 11 and Comparative Examples 1 and 2, a part of the surface of the oxygen evolution catalyst layer 4 of the semiconductor photoelectrode was scratched off, and a lead was connected to a part of the exposed n-GaN surface, and soldered using indium (In). After that, the indium surface was covered with epoxy resin so as not to be exposed, and then installed as the oxidation electrode 112.

A 0.5 mol/l aqueous solution of potassium bicarbonate was used as the aqueous solution 121 in the reduction tank 120. As the aqueous solution 121, in addition to the aqueous solution of potassium bicarbonate, an aqueous solution of sodium bicarbonate, an aqueous solution of potassium chloride, or an aqueous solution of sodium chloride may also be used.

Platinum (manufactured by Nilaco) was used for the reduction electrode 122. The reduction electrode 122 may be a metal or a metal compound. For example, Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, Ru, etc. may be used as the reduction electrode 122.

The oxidation tank 110 and reduction tank 120 are connected via a proton membrane 130. Protons generated in the oxidation tank 110 diffuse into the reduction tank 120 via the proton membrane 130. Nafion (registered trademark) is used for the proton membrane 130. Nafion is a perfluorocarbon material that consists of a hydrophobic Teflon skeleton made of carbon and fluorine and perfluoro side chains with sulfonic acid groups.

The oxidation electrode 112 and reduction electrode 122 are electrically connected by a conductor 132, and electrons move from the oxidation electrode 112 to the reduction electrode 122.

A 300 W high-pressure xenon lamp (illuminance: 20 mW/cm ² (λ≦500 nm)) was used as the light source 140 , and the semiconductor photoelectrode installed as the oxidation electrode 112 was uniformly irradiated with light.

The light source 140 may irradiate light of a wavelength that can be absorbed by the material that constitutes the semiconductor photoelectrode installed as the oxidation electrode 112. For example, if the oxidation electrode 112 is made of gallium nitride, the wavelength that can be absorbed by the oxidation electrode 112 is 365 nm or less. The light source 140 may be a xenon lamp, a mercury lamp, a halogen lamp, a pseudo-sun light source, or sunlight, or a combination of these light sources.

In the redox reaction test, nitrogen gas was flowed in each reaction tank at 10 ml/min, the light irradiation area of the sample was 1 ^cm2 , and the

aqueous solutions

111 and 121 were stirred at the center of the bottom of each reaction tank using a stirrer and a stirrer at a rotation speed of 250 rpm.

After the atmosphere in the reaction vessel was sufficiently replaced with nitrogen gas, the light source 140 was fixed so as to face the surface of the semiconductor photoelectrode (oxidation electrode 112) prepared by the above-mentioned procedure on which the oxygen generating catalyst layer 4 was formed, and the semiconductor photoelectrode was uniformly irradiated with light.

Gas samples were taken from each reaction tank at any time during light irradiation, and the reaction products were analyzed using a gas chromatograph. As a result, it was confirmed that oxygen was produced in the oxidation tank 110, and hydrogen was produced in the reduction tank 120.

In the embodiment, the target product is hydrogen, but by changing the metal of the reduction electrode 122 (e.g., Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, Ru) or the atmosphere in the cell, it is also possible to produce carbon compounds through the reduction reaction of carbon dioxide, or ammonia through the reduction reaction of nitrogen.

[Test results]
Table 1 shows the amounts of oxygen and hydrogen gas generated immediately after light irradiation and 100 hours after light irradiation in Examples 1 to 7 and Comparative Example 1. The amounts of each gas generated were normalized by the surface area of the semiconductor photoelectrode. It was found that oxygen and hydrogen were generated during light irradiation in all examples.

Comparing the amounts of oxygen and hydrogen produced in Example 1 and Comparative Example 1, it was found that Example 1 produced about 1.5 times more than Comparative Example 1 immediately after light irradiation. In addition, differences in the amounts produced were observed as time passed from the start of light irradiation. In Comparative Example 1, the amounts of oxygen and hydrogen produced were each reduced by about 15% 100 hours after light irradiation, while in Example 1, the amounts of oxygen and hydrogen produced were maintained at a reduction of about 10% 100 hours after light irradiation.

Even if the thickness of the second thin film 3-2 (GaN layer) as an intermediate layer was increased by about 2 nm as in Example 2, or the thickness of the first thin film 3-1 (InGaN layer) was increased by about 50 nm as in Example 4, the amount of generation did not decrease significantly compared to Example 1, and it was possible to confirm the performance improvement due to the alternating growth of the first thin film 3-1 and the second thin film 3-2. Also, even if the bottom and/or top layer of the semiconductor thin film 3 was the second thin film 3-2 as in Examples 5 to 7, the amount of generation did not decrease significantly compared to Example 1, and it was possible to confirm the performance improvement due to the alternating growth of the first thin film 3-1 and the second thin film 3-2.

In addition, when the In composition was improved as in Example 3, it was confirmed that the amount of generated light was improved due to the broadening of the wavelength range that could be absorbed.

Table 2 shows the amounts of oxygen and hydrogen gas produced immediately after light irradiation and 100 hours after light irradiation in Examples 7 to 11 and Comparative Example 2. The amounts of each gas produced are normalized by the surface area of the semiconductor photoelectrode. It was found that oxygen and hydrogen were produced during light irradiation in all examples.

Comparing Example 8 and Comparative Example 2, the results were similar to those of Example 1 and Comparative Example 1. Specifically, when comparing the amounts of oxygen and hydrogen produced in Example 8 and Comparative Example 2, it was found that Example 8 produced about 1.3 times more oxygen than Comparative Example 2 immediately after light irradiation. In addition, differences in the amounts produced were observed as time passed from the start of light irradiation. In Comparative Example 2, the amounts of oxygen and hydrogen produced were each reduced by about 15% 100 hours after light irradiation, while in Example 8, the amounts of oxygen and hydrogen produced were maintained at a reduction of about 10% 100 hours after light irradiation.

Also, as in Examples 9 to 11, even when the bottom and/or top layer of the semiconductor thin film 5 was the second thin film 5-2, the amount of production did not decrease significantly compared to Example 8, and it was possible to confirm the performance improvement achieved by alternately growing the first thin film 5-1-n and the second thin film 5-2.

In addition, when comparing the amounts of oxygen and hydrogen generated in Example 8, in which the first thin film 5-1-n had an indium composition gradient, with Example 1, it was found that Example 8 generated approximately twice as much as Example 1.

In this way, in Examples 1 to 11, the semiconductor thin film 3 in which InGaN layers and GaN layers were grown alternately was used to increase the efficiency of the amount of hydrogen and oxygen produced by the water splitting reaction (light energy conversion efficiency) and extend the life of the device.

As described above, the semiconductor photoelectrode of this embodiment includes semiconductor

thin films

3 and 5 that alternately include first thin films 3-1, 5-1-n made of III-V group compound semiconductors and second thin films 3-2, 5-2 made of III-V group compound semiconductors with a larger band gap than the first thin films.

In this embodiment, a quantum well structure is formed in which multiple second thin films (intermediate layers) with a larger band gap than the first thin film are regularly sandwiched between the semiconductor thin film 3 (first thin film), promoting the separation of electron-hole pairs generated in the semiconductor thin film 3 (semiconductor photocatalyst) and improving the solar energy conversion efficiency.

In addition, the structure in which the second thin film is sandwiched has the effect of alleviating the strain energy that occurs when the semiconductor thin film 3 is grown, improving the crystallinity of the semiconductor thin film 3 and realizing a longer life for the solar energy conversion efficiency.

For example, in semiconductor thin films with a narrow band gap such as InGaN thin films, it is possible to reduce the rate of electron-hole recombination and improve the solar energy conversion efficiency. In addition, by reducing surface dislocations (lattice defects), which are the starting point of the etching reaction in GaN-based thin films, and improving crystallinity, it is possible to suppress the etching reaction and improve the stability of the light energy conversion efficiency.

1 Substrate 2 Semiconductor thin film (second semiconductor thin film)
3. Semiconductor thin film (first semiconductor thin film)
3-1 First thin film 3-2 Second thin film 4 Oxygen generating catalyst layer 5 Semiconductor thin film (first semiconductor thin film)
5-1-1 to 5-1-N First thin film 5-2 Second thin film

Claims

A semiconductor photoelectrode comprising a first semiconductor thin film, the first thin film being made of a III-V compound semiconductor and a second thin film being made of a III-V compound semiconductor having a larger band gap than that of the first thin film, and the second thin film being arranged alternately.
The semiconductor photoelectrode according to claim 1 , further comprising an oxygen evolution catalyst layer disposed on the first semiconductor thin film.
a second semiconductor thin film made of a III-V compound semiconductor disposed on the substrate;
the first semiconductor thin film is disposed on the second semiconductor thin film;
The semiconductor photoelectrode according to claim 1 , wherein the second semiconductor thin film is an n-type semiconductor.
the first thin film is indium gallium nitride;
The semiconductor photoelectrode according to claim 1 , wherein the second thin film is made of gallium nitride.
The semiconductor photoelectrode according to claim 2 , wherein each of the first semiconductor thin films is disposed so that a band gap thereof narrows in a direction toward the oxygen evolution catalyst layer.
a second semiconductor thin film made of a III-V compound semiconductor disposed on the substrate;
the first semiconductor thin film is disposed on the second semiconductor thin film;
3. The semiconductor photoelectrode according to claim 2, wherein a band gap at an uppermost portion of the first thin film arranged at an n-th position (n>1) of the first semiconductor thin films on the oxygen evolution catalyst layer side is equal to a band gap at a lowermost portion of the first thin film arranged at an n+1th position on the second semiconductor thin film side.