Method for producing semiconductor photoelectrode

Abstract

A method for manufacturing a semiconductor optical electrode, said method including: a step for forming n-type gallium nitride 12 on a substrate 11; a step for forming indium gallium nitride 13 on the n-type gallium nitride 12; and a step for forming p-type nickel oxide 14 on the indium gallium nitride 13. In the step for forming the p-type nickel oxide, nickel oxide is sputtered in an atmosphere in which oxygen is mixed with a sputtering gas.

Description

Method for manufacturing semiconductor photoelectrodes

[0001] The present disclosure relates to a method for manufacturing a semiconductor photoelectrode.

[0002] There is a device that generates hydrogen through a water splitting reaction using a semiconductor photoelectrode (Non-Patent Document 1). This device has an oxidation tank and a reduction tank 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 the reduction electrode are electrically connected by a conductor.

[0003] The water splitting reaction using a photocatalyst consists of a water oxidation reaction and a proton reduction reaction. When light is irradiated onto an n-type photocatalytic material, electrons and holes are generated and separated within the photocatalyst. The holes move to the surface of the photocatalytic 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 oxidation-reduction reaction proceeds, resulting in the water splitting reaction.

[0004] Oxidation reaction: 2H2O + 4h + → O2 + 4H + Reduction reaction: 4H + + 4e - → 2H2

[0005] S. Yotsuhashi, et al., “CO2Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, The Japan Society of Applied Physics, 2012, Volume 51, pp. 02BP07-1-02BP07-3

[0006] When a gallium nitride thin film (semiconductor thin film), for example, is used as the oxidation electrode, holes generated and separated under light irradiation are consumed in the etching reaction of the gallium nitride itself as well as in the oxidation reaction of water, resulting in degradation of the gallium nitride thin film and a decrease in light energy conversion efficiency with increasing light irradiation time.

[0007] Etching reaction: 2GaN + 3H2O + 6h + → N2 + Ga2O3 + 6H+ To prevent this type of degradation, research is being conducted into forming a protective layer of a promoter (nickel oxide) for oxygen generation to improve the device's lifespan. Nickel oxide is formed by depositing a thin nickel film on a thin gallium nitride film and then heat-treating it in air. Of the electrons and holes generated in the thin gallium nitride film, the holes move from the thin gallium nitride film to the nickel oxide, and the oxidation reaction of water proceeds on the surface of the nickel oxide. For the holes to move smoothly, the valence band of the thin gallium nitride film must be at a lower level than the valence band of the nickel oxide.

[0008] However, in the case of a visible-light-responsive semiconductor photocatalytic thin film, such as indium gallium nitride, which is expected to improve light absorption, the valence band level becomes higher as the band gap narrows. As a result, the valence band of nickel oxide is positioned at a lower level than the valence band of the visible-light-responsive semiconductor photocatalytic thin film, creating a barrier that prevents holes from moving. Therefore, even if the light absorption rate is improved using a visible-light-responsive semiconductor photocatalytic thin film, the created barrier prevents holes from moving, and nickel oxide cannot function as a promoter protective layer.

[0009] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a technology for forming a protective layer that prevents the generation of a barrier that prevents holes from moving, even in a semiconductor thin film with high light absorption.

[0010] A method for manufacturing a semiconductor photoelectrode according to one aspect of the present disclosure includes the steps of forming n-type gallium nitride on a substrate, forming indium gallium nitride on the n-type gallium nitride, and forming p-type nickel oxide on the indium gallium nitride, wherein the step of forming the p-type nickel oxide involves sputtering nickel oxide in an atmosphere in which oxygen is mixed into a sputtering gas.

[0011] According to the present disclosure, it is possible to provide a technique for forming a protective layer that prevents the formation of a barrier that prevents holes from moving, even in a semiconductor thin film with high light absorption.

[0012] Fig. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode according to the present disclosure. Fig. 2 is a flowchart showing an example of a method for manufacturing a semiconductor photoelectrode according to the present disclosure. Fig. 3 is a diagram showing an overview of an apparatus for performing an oxidation-reduction reaction test.

[0013] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Note that the present disclosure is not limited to the embodiments described below, and modifications may be made without departing from the spirit and scope of the present disclosure.

[0014] [Configuration of Semiconductor Photoelectrode] Fig. 1 is a cross-sectional view showing an example of the configuration of a semiconductor photoelectrode (nitride semiconductor photoelectrode) of this embodiment. The semiconductor photoelectrode shown in Fig. 1 includes an insulating or conductive substrate 11, a first semiconductor thin film 12 disposed on the substrate 11, a second semiconductor thin film 13 disposed on the first semiconductor thin film 12, and a protective layer 14 disposed on the second semiconductor thin film 13.

[0015] The substrate 11 may be an insulating or conductive substrate such as a gallium nitride substrate, a sapphire substrate, or a silicon-based substrate. In this embodiment, the substrate 11 is a sapphire substrate, but is not limited to this.

[0016] The first semiconductor thin film 12 uses n-type gallium nitride (n-GaN), and the second semiconductor thin film 13 uses indium gallium nitride (InGaN), but these are not limited to these. The band gap of the semiconductor thin film used for the second semiconductor thin film 13 is preferably about 6.0 eV or less, and more preferably about 3.0 eV or less. In the examples described below, indium gallium nitride with an indium composition of 10% was used, but the indium composition is preferably 1% to 40%.

[0017] The protective layer 14 is a promoter protective layer for oxygen generation. In this embodiment, the protective layer 14 is made of p-type nickel oxide (p-NiO), a p-type semiconductor. This reduces the barrier that occurs in the valence band when the charge generated in the second semiconductor thin film 13 (indium gallium nitride) is transferred to the protective layer 14. Specifically, by having an interface where the valence band of the protective layer 14 is positioned at a higher level than the valence bands of the nitride semiconductor thin films (first semiconductor thin film, second semiconductor thin film), the solar light energy conversion efficiency of the semiconductor photoelectrode can be improved, and the life of the semiconductor photoelectrode can be extended.

[0018] Specifically, nickel oxide (NiO), which is a promoter for oxygen generation, exhibits the properties of a p-type semiconductor when it has an oxygen-excess composition. By manufacturing a semiconductor photoelectrode in which p-type nickel oxide is formed on an indium gallium nitride second semiconductor thin film 13, holes generated in the indium gallium nitride second semiconductor thin film 13 by light irradiation can migrate to the p-type nickel oxide. In this way, in this embodiment, by forming the oxygen generation protective layer 14 that can promote charge separation (generation and separation of electrons and holes) in the semiconductor photoelectrode, the efficiency and lifespan can be improved even in semiconductor photoelectrodes with high light absorption.

[0019] [Method for Manufacturing a Semiconductor Photoelectrode] FIG. 2 is a flowchart showing a method for manufacturing a semiconductor photoelectrode of this embodiment.

[0020] The method for manufacturing a semiconductor photoelectrode of the present embodiment includes a step (S1) of forming n-type gallium nitride 12 on a substrate 11, a step (S2) of forming indium gallium nitride 13 on the n-type gallium nitride 12, and a step (S3) of forming p-type nickel oxide 14 on the indium gallium nitride 13, and in the step (S3) of forming p-type nickel oxide, nickel oxide is sputtered in an atmosphere in which oxygen is mixed into a sputtering gas.

[0021] In S2, indium gallium nitride 13 is grown on the surface of the n-type gallium nitride 12 formed in S1. The step (S1) of forming the n-type gallium nitride 12 and the step (S2) of forming the indium gallium nitride 13 can be performed by metal organic chemical vapor deposition (MOCVD).

[0022] As the indium gallium nitride 13 grows, a p-type nickel oxide 14 serving as a protective layer is formed on the surface of the indium gallium nitride 13 (S3). The step of forming the p-type nickel oxide 14 (S3) can be performed by sputtering. Specifically, nickel oxide, which is an oxygen-generating promoter, is sputtered in an oxygen-mixed atmosphere. By converting nickel oxide into an oxygen-excess oxide material in this way, it becomes a p-type semiconductor, and an interface is provided in which the valence band of the p-type nickel oxide 14 is positioned at a higher level than the valence band of the nitride semiconductor thin film. This reduces the barrier that occurs in the valence band when transferring charges generated in the n-type gallium nitride 12 to the p-type nickel oxide 14 (protective layer).

[0023] The thickness of the p-type nickel oxide 14 is preferably 1 nm to 10 nm, and more preferably 1 nm to 3 nm, which allows sufficient light transmission.

[0024] [Apparatus for Redox Reaction Test] An apparatus used for the redox reaction test will be described with reference to Fig. 3. The apparatus includes a light source 101, an oxidation tank 102, a reduction tank 103, a proton exchange membrane 104, and a lead wire 106. The oxidation tank 102 contains an oxidation electrode 121 and an aqueous solution 122. The reduction tank 103 contains a reduction electrode 131 and an aqueous solution 132.

[0025] In the oxidation bath 102, the oxidation electrode 121 is in contact with an aqueous solution 122. The oxidation electrode 121 may be made of a nitride semiconductor, titanium oxide, amorphous silicon, or the like. The aqueous solution 122 is an aqueous solution in which an electrolyte that causes ion migration is dissolved, such as an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or hydrochloric acid.

[0026] In the reduction tank 103, the reduction electrode 131 is in contact with an aqueous solution 132. The reduction electrode 131 is a metal or a metal compound, such as nickel, iron, gold, platinum, silver, copper, indium, or titanium. The aqueous solution 132 is, for example, an aqueous solution of potassium hydrogen carbonate, sodium hydrogen carbonate, potassium chloride, or sodium chloride.

[0027] The proton exchange membrane 104 is sandwiched between the oxidation chamber 102 and the reduction chamber 103. Protons generated in the oxidation chamber 102 diffuse into the reduction chamber 103 through the proton exchange membrane 104. The proton exchange membrane 104 can be made of, for example, a perfluorocarbon material. The perfluorocarbon material is a material composed of a hydrophobic Teflon (registered trademark) skeleton made of carbon and fluorine and perfluoro side chains with sulfonic acid groups. The proton exchange membrane 104 may also be made of Nafion (registered trademark), a perfluorocarbon material.

[0028] The oxidation electrode 121 and the reduction electrode 131 are electrically connected by a conductor 106. Electrons move from the oxidation electrode 121 to the reduction electrode 131 via the conductor 106.

[0029] The light source 101 is, for example, a xenon lamp, a mercury lamp, a halogen lamp, a solar simulant, sunlight, or a combination thereof. The light source 101 emits light of a wavelength that can be absorbed by the material that constitutes the oxide electrode 121. For example, in the case of the oxide electrode 121 made of gallium nitride, the wavelength that can be absorbed by the oxide electrode 121 is 365 nm or shorter.

[0030] [Examples] Next, examples will be described. The aqueous solution 122 in the oxidation tank 102 used in the oxidation-reduction reaction test of the examples may be a potassium hydroxide aqueous solution or hydrochloric acid in addition to sodium hydroxide. The aqueous solution 132 in the reduction tank 103 may be a sodium bicarbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution in addition to potassium bicarbonate. While hydrogen is used as the target product in the examples, this is not limited to this. By changing the reduction electrode 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. The reduction electrode can be changed to, for example, Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, or Ru.

[0031] Example 1 (Fabrication of a Semiconductor Photoelectrode) In Example 1, a sapphire substrate was used as the substrate. A silicon-doped n-GaN semiconductor thin film was epitaxially grown on the sapphire substrate by MOCVD. The thickness of the n-GaN semiconductor thin film was set to a thickness (approximately 5 μm) sufficient to absorb light. The carrier density was 3×10 18 cm -3 To that extent.

[0032] Next, an InGaN alloy semiconductor (band gap approximately 3.0 eV) with a 10% indium composition ratio was epitaxially grown on the n-GaN semiconductor thin film by metalorganic chemical vapor deposition. The thickness of the InGaN semiconductor thin film was approximately 100 nm, which is sufficient to absorb light.

[0033] Next, NiO was sputtered onto the surface of the InGaN semiconductor thin film to a thickness of approximately 2 nm. At this time, oxygen partial pressure was applied to the sputtering gas. In this example, argon (sputtering gas) was mixed with oxygen at a concentration of 5%. The oxygen concentration is preferably 1% to 20%. High-resolution cross-sectional TEM observation using a transmission electron microscope (TEM) confirmed that a 2-nm p-NiO layer had been formed.

[0034] (Redox Reaction Test) In the redox reaction test, a portion of the p-NiO surface of the semiconductor photoelectrode of this example was scribed to expose the n-GaN surface. A conducting wire was connected to a portion of the exposed n-GaN surface and soldered using indium (In). The exposed n-GaN surface was then covered with epoxy resin to prevent the indium surface from being exposed. This was installed as the oxidation electrode 121 in Figure 3. The aqueous solution 122 was a 1 mol / L aqueous solution of sodium hydroxide. The reduction electrode 131 was made of platinum, and Nafion (registered trademark) was used for the proton exchange membrane 104. The aqueous solution 132 was a 0.5 mol / L aqueous solution of potassium bicarbonate.

[0035] Nitrogen gas was flowed in each reaction vessel at a rate of 10 ml / min, and the light irradiation area of ​​the oxidation electrode 121 was set to 1 cm 2 The aqueous solution was stirred at the center of the bottom of each reaction vessel using a stirring bar and a stirrer at a rotation speed of 250 rpm. After the atmosphere inside the reaction vessel was sufficiently replaced with nitrogen gas, the light source 101 was fixed so as to face the surface of the semiconductor photoelectrode (oxidation electrode 121) of this example on which p-NiO was formed. The light source 101 was a 300 W high-pressure xenon lamp (illuminance 5 mW / cm 2 The semiconductor photoelectrode was uniformly irradiated with light using a ion beam splitter. Gas samples were collected from each reaction chamber at random times during the light irradiation, and the reaction products were analyzed using a gas chromatograph. As a result, oxygen was produced in the oxidation chamber 102, and hydrogen was produced in the reduction chamber 103.

[0036] As described above, the method for manufacturing a semiconductor photoelectrode of the present embodiment includes the steps of forming n-type gallium nitride on a substrate, forming indium gallium nitride on the n-type gallium nitride, and forming p-type nickel oxide on the indium gallium nitride, and in the step of forming the p-type nickel oxide, nickel oxide is sputtered in an atmosphere in which oxygen is mixed into a sputtering gas.

[0037] As described above, in this embodiment, by manufacturing a semiconductor photoelectrode in which p-type nickel oxide is formed as the protective layer 14, it is possible to improve the solar energy conversion efficiency and extend the lifespan.

[0038] Specifically, in this embodiment, in the step of forming p-type nickel oxide, nickel oxide is sputtered in an atmosphere in which oxygen is mixed into the sputtering gas. In this way, by manufacturing a semiconductor photoelectrode in which p-type nickel oxide (oxygen-excess nickel oxide) is formed as the protective layer 14 on an indium gallium nitride semiconductor thin film, it is possible to improve the solar energy conversion efficiency and extend the lifespan. In other words, by forming a protective layer that reduces the barrier to charge migration, it is possible to improve the efficiency and lifespan of a semiconductor photoelectrode with high light absorption.

[0039] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure.

[0040] REFERENCE SIGNS LIST 11 Substrate 12 n-type gallium nitride (first semiconductor thin film) 13 Indium gallium nitride (second semiconductor thin film) 14 p-type nickel oxide layer (protective layer) 101 Light source 102 Oxidation chamber 103 Reduction chamber 104 Proton exchange membrane 106 Conductor 121 Oxidation electrode 122, 132 Aqueous solution 131 Reduction electrode

Claims

1. A method for manufacturing a semiconductor photoelectrode, comprising: forming n-type gallium nitride on a substrate; forming indium gallium nitride on the n-type gallium nitride; and forming p-type nickel oxide on the indium gallium nitride, wherein the step of forming the p-type nickel oxide involves sputtering nickel oxide in an atmosphere in which oxygen is mixed into a sputtering gas.

2. The method for producing a semiconductor photoelectrode according to claim 1, wherein the step of forming the n-type gallium nitride and the step of forming the indium gallium nitride are carried out by metal organic chemical vapor deposition.

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