WO2020116151A1 - 窒化物半導体光電極の製造方法 - Google Patents

窒化物半導体光電極の製造方法 Download PDF

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WO2020116151A1
WO2020116151A1 PCT/JP2019/045259 JP2019045259W WO2020116151A1 WO 2020116151 A1 WO2020116151 A1 WO 2020116151A1 JP 2019045259 W JP2019045259 W JP 2019045259W WO 2020116151 A1 WO2020116151 A1 WO 2020116151A1
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Prior art keywords
nitride semiconductor
heat treatment
electrode
layer
gallium nitride
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PCT/JP2019/045259
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English (en)
French (fr)
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裕也 渦巻
紗弓 里
陽子 小野
武志 小松
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日本電信電話株式会社
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Priority to US17/292,361 priority Critical patent/US20220002886A1/en
Publication of WO2020116151A1 publication Critical patent/WO2020116151A1/ja

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    • H01L21/02617—Deposition types
    • H01L21/0262—Reduction or decomposition of gaseous compounds, e.g. CVD

Definitions

  • the present invention relates to a method for producing a nitride semiconductor photoelectrode having a photocatalytic function that exhibits a catalytic function by light irradiation to cause a chemical reaction of an oxidation target material or a reduction target material.
  • Fig. 3 shows a water decomposing device using semiconductor photoelectrodes.
  • the water decomposition reaction using a photocatalyst consists of an oxidation reaction of water and a reduction reaction of protons, and they are as follows.
  • the n-type photocatalyst material is irradiated with light, electrons and holes are generated and separated in the photocatalyst.
  • the holes move to the surface of the photocatalytic material and contribute to the oxidation reaction of water.
  • the electrons move to the reduction electrode and contribute to the reduction reaction of protons.
  • such a redox reaction proceeds and a water splitting reaction occurs.
  • the oxidation tank 110 includes an aqueous solution 111 and an oxidation electrode 112.
  • the oxidation electrode 112 is in contact with the aqueous solution 111.
  • the aqueous solution 111 is, for example, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or hydrochloric acid.
  • the oxidation electrode 112 is a nitride semiconductor, titanium oxide, or amorphous silicon.
  • the reduction tank 120 includes an aqueous solution 121 and a reduction electrode 122.
  • the reduction electrode 122 is in contact with the aqueous solution 121.
  • the aqueous solution 121 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution.
  • the reduction electrode 122 is a metal or a metal compound, and is, for example, nickel, iron, gold, platinum, silver, copper, indium, or titanium.
  • a proton film 130 is sandwiched between the oxidation tank 110 and the reduction tank 120, and the protons generated in the oxidation tank 110 diffuse into the reduction tank 120 via the proton film 130.
  • the proton film 130 is, for example, Nafion (registered trademark), which is a perfluorocarbon material composed of a hydrophobic Teflon skeleton made of carbon-fluorine and a perfluoro side chain having a sulfonic acid group.
  • the oxidation electrode 112 and the reduction electrode 122 are electrically connected by a conducting wire 132, and electrons are transferred from the oxidation electrode 112 to the reduction electrode 122.
  • the light source 140 is, for example, a xenon lamp, a mercury lamp, a halogen lamp, a pseudo solar light source, sunlight, or a combination thereof.
  • Light having a wavelength that can be absorbed by the material forming the oxidation electrode 112 is irradiated. For example, in an electrode made of gallium nitride, the wavelength that can be absorbed is 365 nm or less.
  • FIG. 4 shows the configuration (cross section) of the conventional oxidation electrode 112.
  • the oxidation electrode 112 is a semiconductor thin film, and for example, the gallium nitride thin film 12 grown on the sapphire substrate 11 is used.
  • nickel oxide 13 is formed on the gallium nitride thin film 12 as a co-catalyst for generating oxygen.
  • a nickel thin film (about 1 nm) is vapor-deposited on the gallium nitride thin film 12 and heat-treated in the air for about 1 hour on a hot plate set at about 300° C. to form the nickel oxide 13.
  • the oxide electrode 112 as shown in FIG. 4 When the oxide electrode 112 as shown in FIG. 4 is used, among the electrons and holes generated in the gallium nitride thin film 12, holes move from the gallium nitride thin film 12 to the nickel oxide 13 and water on the surface of the nickel oxide 13. Oxidation reaction proceeds. In order for holes to move smoothly, the valence band of the gallium nitride semiconductor needs to be at a level lower than the valence band of nickel oxide. However, for example, in the case of a visible-responsive semiconductor photocatalyst thin film such as indium gallium nitride that can be expected to improve the light absorption rate, the level of the valence band becomes higher as the band gap becomes narrower.
  • the valence band of nickel oxide 13 produced by the conventional method is located at a level lower than the valence band of the visible-responsive semiconductor photocatalyst thin film, and a barrier that prevents holes from moving is generated. Even if the absorptance is improved, holes cannot move due to the generated barrier, and there is a problem that the light energy conversion efficiency is lowered.
  • the present invention has been made in view of the above-mentioned conventional technique, and an object thereof is to provide a method for manufacturing a nitride semiconductor photoelectrode capable of improving the light energy conversion efficiency.
  • the invention according to a first aspect is a method for manufacturing a nitride semiconductor photoelectrode, which comprises a first step of forming an n-type gallium nitride layer on an insulating or conductive substrate, A second step of forming an indium gallium nitride layer on the n-type gallium nitride layer, a third step of forming a nickel layer on the indium gallium nitride layer, and a fourth step of heat-treating the nickel layer in an oxygen atmosphere.
  • the point is to include and.
  • the gist of the invention according to the second aspect is that in the invention according to the first aspect, metal organic chemical vapor deposition (MOCVD) is used in the first step and the second step.
  • MOCVD metal organic chemical vapor deposition
  • the gist of the invention according to the third aspect is that in the invention according to the first or second aspect, an electron beam (EB) vapor deposition method is used in the third step.
  • EB electron beam
  • the invention according to a fourth aspect is the invention according to any one of the first to third aspects, wherein the fourth step is a temperature of 250° C. or higher and 400° C. or lower, and a holding time is 30 minutes or longer and 2 hours.
  • the main points are as follows.
  • the invention according to a fifth aspect is the invention according to any one of the first to fourth aspects, wherein the nickel layer after the fourth step is an oxygen-excessive nickel oxide layer, and is used as a p-type semiconductor. The point is to show the characteristics.
  • FIG. 6 is a cross-sectional view of a nitride semiconductor photoelectrode in an example of the present invention. It is a block diagram of the water decomposition apparatus by a semiconductor photoelectrode. It is sectional drawing of the conventional oxidation electrode.
  • the oxygen generating co-catalyst is heat-treated in an oxygen atmosphere to obtain oxygen.
  • a nitride semiconductor photoelectrode having an interface in which the valence band of the oxygen generating co-catalyst is higher than the valence band of the nitride semiconductor thin film is obtained by using an excess oxide material to form a p-type semiconductor. By using it, the efficiency of light energy conversion is improved.
  • FIG. 1 shows a manufacturing process of the nitride semiconductor photoelectrode in Example 1.
  • a sapphire substrate was used as the substrate.
  • a silicon-doped n-GaN semiconductor thin film was epitaxially grown by MOCVD on a 2-inch sapphire substrate (first step).
  • Ammonia gas and trimethylgallium were used as the growth raw materials, silane gas was used as the n-type impurity source, and hydrogen was used as the carrier gas to be fed into the growth furnace.
  • the film thickness of n-GaN was set to 2 ⁇ m, which is sufficient to absorb light.
  • the carrier density was 3 ⁇ 10 18 cm ⁇ 3 .
  • indium gallium nitride InGaN with an indium composition ratio of 5% was grown (second step).
  • Ammonia gas, trimethylgallium, and trimethylindium were used as growth raw materials, and hydrogen was used as a carrier gas to be fed into the growth furnace.
  • the film thickness was 100 nm, which was sufficient to absorb light sufficiently.
  • a 2-inch semiconductor thin film was cleaved into four equal parts, and one of them was used for electrode production.
  • EB was vapor-deposited with Ni having a thickness of about 1 nm on the InGaN surface (third step). Then, this semiconductor thin film was put into an electric furnace and heat-treated at 200° C. for 15 minutes in an oxygen atmosphere (fourth step).
  • NiO As a result of elemental analysis of the cross section, it was found that NiO was formed because Ni and O were detected. From the TEM observation result of the cross section, the film thickness of NiO was about 2 nm. Further, it was found by titrating the formed NiO that O was excessively contained with respect to Ni.
  • FIG. 2 is a cross-sectional view showing the structure of the nitride semiconductor photoelectrode in Example 1.
  • the nitride semiconductor photoelectrode in Example 1 has an insulating or conductive substrate (sapphire substrate) 1 and an n-type gallium nitride (n-GaN) layer 2 arranged on the substrate 1.
  • n-GaN n-type gallium nitride
  • InGaN indium gallium nitride
  • NiO oxygen-rich nickel oxide
  • ⁇ Nickel oxide which is a co-catalyst for oxygen generation, exhibits characteristics as a p-type semiconductor when it has an oxygen-rich composition.
  • the aqueous solution 111 was a 1 mol/l sodium hydroxide aqueous solution.
  • the aqueous solution 121 was a 0.5 mol/l potassium hydrogen carbonate aqueous solution.
  • Platinum manufactured by Niraco
  • Nafion registered trademark
  • each reaction tank 110/120 was sampled at an arbitrary time during the light irradiation, and the reaction product was analyzed by 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.
  • Example 2 a nitride semiconductor electrode was produced in which the heat treatment step (fourth step) of Example 1 was performed at 200° C. for 30 minutes.
  • the other points are similar to those of the first embodiment.
  • Example 3 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 200° C. for 1 hour. The other points are similar to those of the first embodiment.
  • Example 4 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 200° C. for 2 hours. The other points are similar to those of the first embodiment.
  • Example 5 a nitride semiconductor electrode was produced by setting the heat treatment step of Example 1 to 200° C. for 3 hours.
  • the other points are similar to those of the first embodiment.
  • Example 6 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 200° C. for 4 hours. The other points are similar to those of the first embodiment.
  • Example 7 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 250° C. for 15 minutes. The other points are similar to those of the first embodiment.
  • Example 8 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 250° C. for 30 minutes. The other points are similar to those of the first embodiment.
  • Example 9 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 250° C. for 1 hour.
  • the other points are similar to those of the first embodiment.
  • Example 10 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 250° C. for 2 hours. The other points are similar to those of the first embodiment.
  • Example 11 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 250° C. for 3 hours.
  • the other points are similar to those of the first embodiment.
  • Example 12 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 250° C. for 4 hours. The other points are similar to those of the first embodiment.
  • Example 13 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 300° C. for 15 minutes. The other points are similar to those of the first embodiment.
  • Example 14 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 300° C. for 30 minutes. The other points are similar to those of the first embodiment.
  • Example 15 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 300° C. for 1 hour. The other points are similar to those of the first embodiment.
  • Example 16 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 300° C. for 2 hours. The other points are similar to those of the first embodiment.
  • Example 17 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 300° C. for 3 hours. The other points are similar to those of the first embodiment.
  • Example 18 a nitride semiconductor electrode was produced by setting the heat treatment step of Example 1 to 300° C. for 4 hours.
  • the other points are similar to those of the first embodiment.
  • Example 19 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was 400° C. for 15 minutes. The other points are similar to those of the first embodiment.
  • Example 20 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was 400° C. for 30 minutes. The other points are similar to those of the first embodiment.
  • Example 21 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was 400° C. for 1 hour.
  • the other points are similar to those of the first embodiment.
  • Example 22 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was 400° C. for 2 hours. The other points are similar to those of the first embodiment.
  • Example 23 a nitride semiconductor electrode was produced by setting the heat treatment step of Example 1 to 400° C. for 3 hours.
  • the other points are similar to those of the first embodiment.
  • Example 24 a nitride semiconductor electrode was produced by setting the heat treatment step of Example 1 to 400° C. for 4 hours.
  • the other points are similar to those of the first embodiment.
  • Example 25 a nitride semiconductor electrode was produced in which the heat treatment step of Example 1 was performed at 500° C. for 15 minutes. The other points are similar to those of the first embodiment.
  • Example 26 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 500° C. for 30 minutes. The other points are similar to those of the first embodiment.
  • Example 27 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 500° C. for 1 hour. The other points are similar to those of the first embodiment.
  • Example 28 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 500° C. for 2 hours. The other points are similar to those of the first embodiment.
  • Example 29 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 500° C. for 3 hours. The other points are similar to those of the first embodiment.
  • Example 30 a nitride semiconductor electrode was produced in which the heat treatment process of Example 1 was performed at 500° C. for 4 hours. The other points are similar to those of the first embodiment.
  • Comparative Example 1 a nitride semiconductor electrode was produced by using the heat treatment process of Example 8 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 2 a nitride semiconductor electrode was produced by using the heat treatment process of Example 9 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 3 a nitride semiconductor electrode was produced by using the heat treatment process of Example 10 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 4 a nitride semiconductor electrode was produced by using the heat treatment process of Example 14 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 5 a nitride semiconductor electrode was produced by using the heat treatment process of Example 15 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 6 a nitride semiconductor electrode was produced by using the heat treatment process of Example 16 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 7 a nitride semiconductor electrode was produced by using the heat treatment process of Example 20 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 8 a nitride semiconductor electrode was produced by using the heat treatment process of Example 21 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Comparative Example 9 a nitride semiconductor electrode was produced by using the heat treatment process of Example 22 in an air atmosphere. The other points are similar to those of the first embodiment.
  • Table 1 shows the amounts of oxygen/hydrogen gas produced 10 hours after the light irradiation in Examples and Comparative Examples. The amount of each gas produced was normalized by the surface area of the semiconductor photoelectrode. It was found that oxygen and hydrogen were generated upon light irradiation in all cases.
  • the amount of hydrogen/oxygen produced was low at any heat treatment holding time.
  • the heat treatment temperature is 500° C. or higher, Ni is rapidly oxidized and thermally expanded into NiO by the high temperature heat treatment, and voids are generated at the interface with the photocatalyst thin film, so that the electrode performance deterioration starts from the voids. It is considered that, after 10 hours, the catalyst was roughly deactivated.
  • the heat treatment conditions for NiO formation were a temperature of 250°C or higher and 400°C or lower and a holding time of 30 minutes or longer and 2 hours or shorter.
  • the hydrogen/oxygen production amounts of Examples 8, 9, 10, 14, 15, 16, 20, 21, and 22 are higher than those of Comparative Examples 1, 3, 4, 5, 6, 7, 8, and 9. All were 10 times, and it was found that the atmosphere during the heat treatment had to be an oxygen atmosphere. When the atmosphere is air, it is considered that NiO does not become oxygen-rich NiO and holes cannot move through the barrier at the interface between NiO and InGaN.
  • the fourth step was performed in an electric furnace in the examples, it may be heat treatment on a hot plate installed in a groove box in an oxygen atmosphere.
  • the aqueous solution 111 used for the redox reaction test may be an aqueous solution of potassium hydroxide or hydrochloric acid other than sodium hydroxide.
  • the aqueous solution 121 may be a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution other than potassium hydrogen carbonate.
  • hydrogen was used as the target product in the examples, carbon dioxide can be obtained by changing the reducing electrode 122 (eg, Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, Ru) or the atmosphere in the cell. It is also possible to generate a carbon compound by the reduction reaction of and an ammonia by the reduction reaction of nitrogen.
  • the method for manufacturing a nitride semiconductor photoelectrode includes the first step of forming the n-type gallium nitride layer 2 on the insulating or conductive substrate 1 and the n-type gallium nitride.
  • a second step of forming the indium gallium nitride layer 3 on the layer 2 includes a third step of forming the nickel layer 4 on the indium gallium nitride layer 3, and a fourth step of heat-treating the nickel layer 4 in an oxygen atmosphere. Including.
  • MOCVD metal organic chemical vapor deposition
  • EB electron beam
  • the fourth step is performed at a temperature of 250° C. or higher and 400° C. or lower and a holding time of 30 minutes or longer and 2 hours or shorter. As a result, it is possible to realize a nitride semiconductor photoelectrode that can maintain the light energy conversion efficiency with high efficiency for a long time.
  • the nickel layer 4 after the fourth step becomes the oxygen-excessive nickel oxide layer 4 and exhibits characteristics as a p-type semiconductor.
  • holes generated in the indium gallium nitride semiconductor thin film by light irradiation can move to nickel oxide.

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Abstract

光エネルギー変換効率の向上を図ることが可能な窒化物半導体光電極の製造方法を提供する。窒化物半導体光電極の製造方法は、絶縁性または導電性の基板１上にｎ型窒化ガリウム層２を形成する第１工程と、ｎ型窒化ガリウム層２上に窒化インジウムガリウム層３を形成する第２工程と、窒化インジウムガリウム層３上にニッケル層４を形成する第３工程と、酸素雰囲気中においてニッケル層４を熱処理する第４工程とを含む。

Description

窒化物半導体光電極の製造方法
　本発明は、光照射により触媒機能を発揮して酸化ターゲット物質または還元ターゲット物質の化学反応を引き起こす光触媒機能を有する窒化物半導体光電極の製造方法に関する。
　従来例として、図３に半導体光電極による水の分解装置を示す。光触媒を用いた水の分解反応は、水の酸化反応とプロトンの還元反応からなり、それぞれ下記のとおりである。ｎ型の光触媒材料に光を照射した場合、光触媒中で電子と正孔が生成分離する。正孔は光触媒材料の表面に移動し、水の酸化反応に寄与する。一方、電子は還元電極に移動し、プロトンの還元反応に寄与する。理想的には、このような酸化還元反応が進行し、水分解反応が生じる。
　酸化反応：2H2O + 4h+→ O2 + 4H+ 　　　　　　　　　　　　　　　　 (1)
　還元反応：4H+ + 4e- → 2H2　　　　　　　　　　　　　　　　　　　 (2)
　具体的には、図３に示すように、酸化槽１１０は水溶液１１１と酸化電極１１２を具備している。酸化電極１１２は水溶液１１１に接している。水溶液１１１は例えば、水酸化ナトリウム水溶液、水酸化カリウム水溶液、塩酸である。酸化電極１１２は窒化物半導体や酸化チタン、アモルファスシリコンである。還元槽１２０は水溶液１２１と還元電極１２２を具備している。還元電極１２２は水溶液１２１に接している。水溶液１２１は例えば、炭酸水素カリウム水溶液、炭酸水素ナトリウム水溶液、塩化カリウム水溶液、塩化ナトリウム水溶液である。還元電極１２２は金属や金属化合物であり、例えば、ニッケル、鉄、金、白金、銀、銅、インジウム、チタンである。酸化槽１１０と還元槽１２０の間には、プロトン膜１３０が挟まれており、酸化槽１１０で生成したプロトンがプロトン膜１３０を介し、還元槽１２０へ拡散していく。プロトン膜１３０は例えば、ナフィオン（登録商標）であり、炭素－フッ素からなる疎水性テフロン骨格とスルホン酸基を持つパーフルオロ側鎖から構成されるパーフルオロカーボン材料である。酸化電極１１２と還元電極１２２は導線１３２で電気的に接続されており、酸化電極１１２から還元電極１２２へ電子の移動がなされている。光源１４０は例えば、キセノンランプ、水銀ランプ、ハロゲンランプ、疑似太陽光源、太陽光又はこれらの組み合わせである。酸化電極１１２を構成する材料が吸収可能な波長の光が照射される。例えば、窒化ガリウムで構成される電極では、吸収可能な波長は３６５ｎｍ以下の波長である。
　図４に従来の酸化電極１１２の構成（断面）を示す。図４のような従来例では、酸化電極１１２は半導体薄膜であり、例えば、サファイア基板１１上に成長した窒化ガリウム薄膜１２が用いられる。また、窒化ガリウム薄膜１２上には、酸素発生用の助触媒として酸化ニッケル１３が形成されている。従来は、窒化ガリウム薄膜１２上にニッケル薄膜（１ｎｍ程度）を蒸着し、約３００℃に設定されたホットプレート上で約１時間程度空気中で熱処理することで酸化ニッケル１３を形成している。
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 小野陽子ら、「窒化物半導体電極上に形成したNiO薄膜の光電流特性への効果」、2017年電気化学秋季大会、1L31．
　図４のような酸化電極１１２を用いた場合、窒化ガリウム薄膜１２で生じた電子・正孔のうち、正孔は窒化ガリウム薄膜１２中から酸化ニッケル１３へ移動し、酸化ニッケル１３の表面で水の酸化反応が進行する。正孔がスムーズに移動するためには、窒化ガリウム半導体の価電子帯が酸化ニッケルの価電子帯よりも低い準位にある必要がある。しかし、例えば、窒化インジウムガリウムのように光吸収率向上が期待できる可視応答化半導体光触媒薄膜の場合、バンドギャップが狭くなるに従い、価電子帯の準位が高くなる。従来の手法で作製された酸化ニッケル１３の価電子帯は、可視応答化半導体光触媒薄膜の価電子帯よりも低い準位に位置してしまい、正孔が移動できない障壁が生成されるため、光吸収率を向上しても、生成する障壁により正孔が移動できず、光エネルギー変換効率が低下する問題がある。
　本発明は、上述した従来の技術に鑑み、光エネルギー変換効率の向上を図ることが可能な窒化物半導体光電極の製造方法を提供することを目的とする。
　上記目的を達成するため、第１の態様に係る発明は、窒化物半導体光電極の製造方法であって、絶縁性または導電性の基板上にｎ型窒化ガリウム層を形成する第１工程と、前記ｎ型窒化ガリウム層上に窒化インジウムガリウム層を形成する第２工程と、前記窒化インジウムガリウム層上にニッケル層を形成する第３工程と、酸素雰囲気中において前記ニッケル層を熱処理する第４工程とを含むことを要旨とする。
　第２の態様に係る発明は、第１の態様に係る発明において、前記第１工程および前記第２工程で、有機金属気相成長法（ＭＯＣＶＤ）を用いることを要旨とする。
　第３の態様に係る発明は、第１または第２の態様に係る発明において、前記第３工程で、電子ビーム（ＥＢ）蒸着法を用いることを要旨とする。
　第４の態様に係る発明は、第１から第３のいずれか１つの態様に係る発明において、前記第４工程が、２５０℃以上４００℃以下の温度であり、保持時間が３０分以上２時間以下で行われることを要旨とする。
　第５の態様に係る発明は、第１から第４のいずれか１つの態様に係る発明において、前記第４工程後の前記ニッケル層が、酸素過剰の酸化ニッケル層となり、ｐ型の半導体としての特性を示すことを要旨とする。
　本発明によれば、光エネルギー変換効率の向上を図ることが可能な窒化物半導体光電極の製造方法を提供することが可能である。
本発明の実施例における窒化物半導体光電極の製造工程を示す図である。 本発明の実施例における窒化物半導体光電極の断面図である。 半導体光電極による水の分解装置の構成図である。 従来の酸化電極の断面図である。
　以下に、実施例によって本発明をより詳細に説明するが、本発明はこれら実施例に限定されるものではなく、本発明の趣旨を逸脱しない範囲内において変更を加えても構わない。
　＜概要＞
　本発明は、窒化物半導体薄膜で生成した電荷を酸素発生用助触媒に受け渡す際の価電子帯に生じる障壁を低減するため、酸素発生用助触媒を酸素雰囲気中で熱処理することにより、酸素過剰の酸化物材料とすることで、ｐ型半導体とし、酸素発生用助触媒の価電子帯が窒化物半導体薄膜の価電子帯よりも高い準位に位置する界面を持つ窒化物半導体光電極を用いることで、光エネルギー変換効率の向上を図る。
　＜実施例１＞
　（窒化物半導体光電極の作製）
　図１に、実施例１における窒化物半導体光電極の製造工程を示す。
　基板としてはサファイア基板を用いた。２インチのサファイア基板上に、シリコンをドープしたｎ－ＧａＮ半導体薄膜をＭＯＣＶＤ法によりエピタキシャル成長させた（第１工程）。成長原料には、アンモニアガス、トリメチルガリウムを用い、ｎ型不純物源にはシランガスを、成長炉内に送るキャリアガスには水素を用いた。ｎ－ＧａＮの膜厚は光を吸収するに十分足る２μｍとした。キャリア密度は３×１０１８ｃｍ－３であった。その後、インジウムの組成比を５％とした窒化インジウムガリウムＩｎＧａＮを成長した（第２工程）。成長原料には、アンモニアガス、トリメチルガリウム、トリメチルインジウムを、成長炉内に送るキャリアガスには水素を用いた。膜厚は光を十分に吸収するに足る１００ｎｍとした。その後、２インチの半導体薄膜を４等分にへき開し、そのうちの１枚を電極作製に使用した。次に、ＩｎＧａＮ表面に膜厚約１ｎｍのＮｉをＥＢ蒸着した（第３工程）。その後、この半導体薄膜を電気炉に入れ、酸素雰囲気中、２００℃、１５分間熱処理した（第４工程）。
　断面の元素分析を行った結果、ＮｉとＯが検出されたことから、ＮｉＯが形成されていることがわかった。断面のＴＥＭ観察結果から、ＮｉＯの膜厚は約２ｎｍであった。また、形成したＮｉＯを滴定することで、ＯがＮｉに対して過剰に含まれていることがわかった。
　（窒化物半導体光電極の構成）
　図２は、実施例１における窒化物半導体光電極の構成を示す断面図である。実施例１における窒化物半導体光電極は、図２に示すように、絶縁性または導電性の基板（サファイア基板）１と、基板１上に配置されたｎ型窒化ガリウム（ｎ－ＧａＮ）層２と、ｎ型窒化ガリウム層２上に配置された窒化インジウムガリウム（ＩｎＧａＮ）層３と、窒化インジウムガリウム層３上に配置された酸素過剰の酸化ニッケル（ＮｉＯ）層４とを備える。
　酸素発生用の助触媒である酸化ニッケルは酸素過剰の組成となった場合、ｐ型の半導体としての特性を示す。これにより、窒化インジウムガリウム半導体薄膜上に酸素過剰の酸化ニッケルを形成した窒化物半導体光電極を製造することで、光照射によって窒化インジウムガリウム半導体薄膜中で生じる正孔が酸化ニッケルへ移動できるようになる。このように、半導体電極での電荷分離（電子・正孔の生成と分離）を促進することによって光エネルギー変換効率の向上を図ることが可能となる。
　（酸化還元反応試験）
　次に、実施例１における酸化還元反応試験について説明する。ここでも、水の分解装置の構成図（図３）を用いるが、既に説明した点については詳しい説明を省略する。
　酸化還元反応試験では、ＩｎＧａＮ表面をけがき、ｎ－ＧａＮ表面を露出した。露出したｎ－ＧａＮ表面の一部に導線を接続し、Ｉｎを用いてはんだ付けした。その後、インジウム表面が露出しないようにエポキシ樹脂で被覆した。これを図３の酸化電極１１２として設置した。水溶液１１１は１ｍｏｌ／ｌの水酸化ナトリウム水溶液とした。ここで、水溶液１２１は０．５ｍｏｌ／ｌの炭酸水素カリウム水溶液とした。還元電極１２２は白金（ニラコ製）、プロトン膜１３０はナフィオン（登録商標）を用いた。各反応槽１１０・１２０において窒素ガスを１０ｍｌ／ｍｉｎ．で流し、サンプルの光照射面積を１ｃｍ２とし、水溶液１１１・１２１の攪拌は撹拌子とスターラーを用いて２５０ｒｐｍの回転速度で各反応槽１１０・１２０の底の中心位置で攪拌した。反応槽１１０・１２０内が窒素ガスに十分に置換された後、光源１４０を上述の手順で作製した半導体光電極のＮｉＯが形成されている面に向くように固定した。光源１４０には３００Ｗの高圧キセノンランプ（照度５ｍＷ／ｃｍ２）を用いて、半導体光電極に均一に光を照射した。光照射中任意の時間に、各反応槽１１０・１２０内のガスを採取し、ガスクロマトグラフにて反応生成物を分析した。その結果、酸化槽１１０では酸素が、還元槽１２０では水素が生成されていることを確認した。
　＜実施例２＞
　実施例２では、実施例１の熱処理工程（第４工程）を２００℃、３０分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例３＞
　実施例３では、実施例１の熱処理工程を２００℃、１時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例４＞
　実施例４では、実施例１の熱処理工程を２００℃、２時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例５＞
　実施例５では、実施例１の熱処理工程を２００℃、３時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例６＞
　実施例６では、実施例１の熱処理工程を２００℃、４時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例７＞
　実施例７では、実施例１の熱処理工程を２５０℃、１５分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例８＞
　実施例８では、実施例１の熱処理工程を２５０℃、３０分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例９＞
　実施例９では、実施例１の熱処理工程を２５０℃、１時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１０＞
　実施例１０では、実施例１の熱処理工程を２５０℃、２時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１１＞
　実施例１１では、実施例１の熱処理工程を２５０℃、３時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１２＞
　実施例１２では、実施例１の熱処理工程を２５０℃、４時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１３＞
　実施例１３では、実施例１の熱処理工程を３００℃、１５分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１４＞
　実施例１４では、実施例１の熱処理工程を３００℃、３０分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１５＞
　実施例１５では、実施例１の熱処理工程を３００℃、１時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１６＞
　実施例１６では、実施例１の熱処理工程を３００℃、２時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１７＞
　実施例１７では、実施例１の熱処理工程を３００℃、３時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１８＞
　実施例１８では、実施例１の熱処理工程を３００℃、４時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例１９＞
　実施例１９では、実施例１の熱処理工程を４００℃、１５分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２０＞
　実施例２０では、実施例１の熱処理工程を４００℃、３０分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２１＞
　実施例２１では、実施例１の熱処理工程を４００℃、１時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２２＞
　実施例２２では、実施例１の熱処理工程を４００℃、２時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２３＞
　実施例２３では、実施例１の熱処理工程を４００℃、３時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２４＞
　実施例２４では、実施例１の熱処理工程を４００℃、４時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２５＞
　実施例２５では、実施例１の熱処理工程を５００℃、１５分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２６＞
　実施例２６では、実施例１の熱処理工程を５００℃、３０分間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２７＞
　実施例２７では、実施例１の熱処理工程を５００℃、１時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２８＞
　実施例２８では、実施例１の熱処理工程を５００℃、２時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例２９＞
　実施例２９では、実施例１の熱処理工程を５００℃、３時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施例３０＞
　実施例３０では、実施例１の熱処理工程を５００℃、４時間とした窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例１＞
　比較対象事例１では、実施例８の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例２＞
　比較対象事例２では、実施例９の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例３＞
　比較対象事例３では、実施例１０の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例４＞
　比較対象事例４では、実施例１４の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例５＞
　比較対象事例５では、実施例１５の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例６＞
　比較対象事例６では、実施例１６の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例７＞
　比較対象事例７では、実施例２０の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例８＞
　比較対象事例８では、実施例２１の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜比較対象事例９＞
　比較対象事例９では、実施例２２の熱処理工程を空気雰囲気として窒化物半導体電極を作製した。その他の点においては実施例１と同様である。
　＜実施の効果＞
　実施例および比較対象例における、光照射から１０時間後の酸素・水素ガスの生成量を表１に示す。各ガスの生成量は、半導体光電極の表面積で規格化して示した。どの例でも光照射時に、酸素と水素が生成されていることがわかった。
　実施例８、９、１０、１４、１５、１６、２０、２１、２２の水素・酸素生成量は、その他の実施例に比べて、１０倍であることがわかった。
　実施例１、２、３、４、５、６では、いずれの熱処理保持時間でも水素・酸素の生成量が低かった。熱処理温度が２００℃以下の場合は、Ｎｉが十分に酸化されていないこと、あるいはＮｉＯが酸素過剰となっていないためと考えられる。
　実施例２５、２６、２７、２８、２９、３０では、いずれの熱処理保持時間でも水素・酸素の生成量が低かった。熱処理温度が５００℃以上の場合は、Ｎｉが高温熱処理によって急激にＮｉＯへと酸化・熱膨張し、光触媒薄膜との界面に空隙が生成されたことで、空隙を起点とした電極性能劣化が進行し１０時間後にはおおよそ触媒として失活したためと考えられる。
　実施例７、１３、１９では、いずれの熱処理温度でも水素・酸素の生成量が低かった。熱処理保持時間が１５分以下の場合は、Ｎｉが十分に酸化されていないこと、あるいはＮｉＯが酸素過剰となっていないためと考えられる。
　実施例１１、１２、１７、１８、２３、２４では、いずれの熱処理温度でも水素・酸素の生成量が低かった。熱処理保持時間が３時間以上の場合は、Ｎｉが長時間の熱処理によってＮｉＯへと酸化・熱膨張した際、光触媒薄膜との界面に空隙が生成されたことで、空隙を起点とした電極性能劣化が進行し１０時間後にはおおよそ触媒として失活したためと考えられる。
　これら結果より、ＮｉＯ形成の熱処理条件は、温度が２５０℃以上４００℃以下、保持時間が３０分以上２時間以下ということを抽出した。また、実施例８、９、１０、１４、１５、１６、２０、２１、２２の水素・酸素生成量は、比較対象事例１、３、４、５、６、７、８、９に比べていずれも１０倍であり、熱処理の際の雰囲気が酸素雰囲気である必要が分かった。雰囲気が空気の場合、ＮｉＯが酸素過剰ＮｉＯとならず、ＮｉＯとＩｎＧａＮ界面の障壁を正孔が移動できないことが影響していると考えられる。
　以上から、熱処理工程を酸素雰囲気で２５０℃以上４００℃以下の温度、保持時間が３０分以上２時間以下とすることで、水分解反応による水素・酸素生成量の増加（光エネルギー変換効率の高効率化）を図ることができた。
Figure JPOXMLDOC01-appb-T000001
  
　＜変形例＞
　上記では幾つかの実施例について記載したが、開示の一部をなす論述および図面は例示的なものであり、限定するものであると理解すべきではない。この開示から当業者には様々な代替実施例および運用技術が明らかとなろう。
　例えば、第４工程は、実施例では電気炉中で実施したが、酸素雰囲気のグルーブボックス内に設置したホットプレート上での熱処理でも構わない。酸化還元反応試験に用いる水溶液１１１は水酸化ナトリウム以外に、水酸化カリウム水溶液、塩酸でも構わない。水溶液１２１は炭酸水素カリウム以外に炭酸水素ナトリウム水溶液、塩化カリウム水溶液、塩化ナトリウム水溶液でも構わない。実施例では目的生成物を水素としたが、還元電極１２２（例えば、Ｎｉ、Ｆｅ、Ａｕ、Ｐｔ、Ａｇ、Ｃｕ、Ｉｎ、Ｔｉ、Ｃｏ、Ｒｕ）やセル内の雰囲気を変えることで、二酸化炭素の還元反応による炭素化合物の生成や、窒素の還元反応によるアンモニアの生成も可能である。
　＜まとめ＞
　以上説明したように、本発明の実施例における窒化物半導体光電極の製造方法は、絶縁性または導電性の基板１上にｎ型窒化ガリウム層２を形成する第１工程と、ｎ型窒化ガリウム層２上に窒化インジウムガリウム層３を形成する第２工程と、窒化インジウムガリウム層３上にニッケル層４を形成する第３工程と、酸素雰囲気中においてニッケル層４を熱処理する第４工程とを含む。このように、窒化インジウムガリウム半導体薄膜上にニッケルを形成した後、酸素雰囲気で熱処理し、酸素過剰の酸化ニッケルを形成した窒化物半導体光電極を製造することで、光吸収率の高い半導体薄膜に対して、正孔が移動できない障壁を生成しない酸素発生用の助触媒を形成し、光エネルギー変換効率の向上を図ることが可能となる。
　また、第１工程および第２工程では、有機金属気相成長法（ＭＯＣＶＤ）を用いるのが望ましい。これにより、他の方法と比べて膜厚の偏差が少なく、高速成長が可能であるため、電荷の移動障壁を低減した半導体光電極を効率よく作製することが可能となる。
　また、第３工程では、電子ビーム（ＥＢ）蒸着法を用いるのが望ましい。これにより、他の方法と比べて高純度の薄膜を形成できるため、電荷の移動障壁を低減した半導体光電極を効率よく作製することが可能となる。
　また、第４工程は、２５０℃以上４００℃以下の温度であり、保持時間が３０分以上２時間以下で行われるのが望ましい。これにより、光エネルギー変換効率を高効率で長時間維持できる窒化物半導体光電極を実現することが可能となる。
　また、第４工程後のニッケル層４は、酸素過剰の酸化ニッケル層４となり、ｐ型の半導体としての特性を示す。これにより、光照射によって窒化インジウムガリウム半導体薄膜中で生じる正孔が酸化ニッケルへ移動できるようになる。
　１…基板
　２…ｎ型窒化ガリウム層
　３…窒化インジウムガリウム層
　４…酸化ニッケル層（ニッケル層）
　１１０…酸化槽
　１１１…水溶液
　１１２…酸化電極
　１２０…還元槽
　１２１…水溶液
　１２２…還元電極
　１３０…プロトン膜
　１３２…導線
　１４０…光源

Claims (5)

  1. 　絶縁性または導電性の基板上にｎ型窒化ガリウム層を形成する第１工程と、
    　前記ｎ型窒化ガリウム層上に窒化インジウムガリウム層を形成する第２工程と、
    　前記窒化インジウムガリウム層上にニッケル層を形成する第３工程と、
    　酸素雰囲気中において前記ニッケル層を熱処理する第４工程と
    　を含むことを特徴とする窒化物半導体光電極の製造方法。
  2. 　前記第１工程および前記第２工程では、有機金属気相成長法（ＭＯＣＶＤ）を用いることを特徴とする請求項１に記載の窒化物半導体光電極の製造方法。
  3. 　前記第３工程では、電子ビーム（ＥＢ）蒸着法を用いることを特徴とする請求項１または２に記載の窒化物半導体光電極の製造方法。
  4. 　前記第４工程は、２５０℃以上４００℃以下の温度であり、保持時間が３０分以上２時間以下で行われることを特徴とする請求項１から３のいずれか１項に記載の窒化物半導体光電極の製造方法。
  5. 　前記第４工程後の前記ニッケル層は、酸素過剰の酸化ニッケル層となり、ｐ型の半導体としての特性を示すことを特徴とする請求項１から４のいずれか１項に記載の窒化物半導体光電極の製造方法。
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