JP6470190B2 - Semiconductor photocatalyst - Google Patents

Description

  The present invention relates to a semiconductor photocatalyst composed of a nitride semiconductor and having a photocatalytic function.

  Many technologies using photocatalysts have been developed. For example, there is a technique using an oxidation electrode and a reduction electrode (see Non-Patent Document 1). For example, as shown in FIG. 4, an oxidation tank 401, a reduction tank 402, an acid or alkali aqueous solution 403 stored in the oxidation tank 401, an aqueous salt solution 404 stored in the reduction tank 402, an aqueous solution 403, and an aqueous solution. A proton membrane 405 that moves protons between 404, an oxidation electrode 406 immersed in an aqueous solution 403, a reduction electrode 407 immersed in an aqueous solution 404, and a conductive wire 408 connecting the oxidation electrode 406 and the reduction electrode 407 are used. Electrolysis.

The aqueous solution 403 is, for example, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, and hydrochloric acid, and the aqueous solution 404 is, for example, an aqueous potassium hydrogen carbonate solution, an aqueous sodium hydrogen carbonate solution, an aqueous potassium chloride solution, and an aqueous sodium chloride solution. Oxidizing electrode 406 is, for example, nitrogen compound semiconductor is composed of titanium oxide or amorphous silicon, the reduction electrode 407, for example, nickel, iron, gold, platinum, silver, copper, indium, titanium, or an alloy or a metal, It is composed of compounds.

  A proton membrane 405 is disposed between the oxidation tank 401 and the reduction tank 402, and protons generated in the aqueous solution 403 diffuse into the aqueous solution 404 through the proton membrane 405. The proton membrane 405 is, for example, Nafion (registered trademark). Nafion (registered trademark) is a perfluorocarbon material composed of a hydrophobic tetrafluoroethylene skeleton composed of carbon-fluorine and a perfluoro side chain having a sulfonic acid group.

  The oxidation electrode 406 and the reduction electrode 407 are electrically connected by a conductive wire 408, and electrons can move from the oxidation electrode 406 to the reduction electrode 407. The light source 410 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 constituting the oxidation electrode 406 is irradiated. For example, the oxidation electrode 406 made of gallium nitride can absorb light having a wavelength of 365 nm or less.

S. Yotsuhashi et al., "CO2 Conversion with Light and Water by GaN Photoelectrode", Japanese Journal of Applied Physics, vol.51, 02BP07, 2012. S. Y. Reece et al., "Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts", Science, vol.334, pp.645-648, 2011.

  However, in the above-described technique, since there are many components and the reaction system is complicated, a simpler reaction system and miniaturization are problems. On the other hand, as a technique that can be reduced in size with a simpler configuration, there is a technique that includes a single photocatalyst without requiring a wiring structure (see Non-Patent Document 2).

  For example, as shown in FIG. 5, an aqueous solution 502 such as an acid or an alkali accommodated in a photocatalyst tank 501 and a photocatalyst 503 immersed in the aqueous solution 502 are provided. The photocatalyst 503 is made of a nitride semiconductor, titanium oxide, amorphous silicon, or the like. Note that, on the surface of the photocatalyst 503, for example, a metal promoter that promotes the decomposition reaction of water is supported. When light from the light source 510 is irradiated onto the photocatalyst 503, hydrogen and oxygen are generated.

  This configuration is simpler than the above-described photocatalytic reaction system, and is expected in that the system can be reduced in cost and size. However, this reaction system has a problem that the light energy conversion efficiency is low.

  The present invention has been made to solve the above-described problems, and an object thereof is to further improve the efficiency of the photocatalytic reaction with a simple configuration.

A semiconductor photocatalyst according to the present invention includes a first semiconductor layer formed on a main surface of a substrate, made of an n-type nitride semiconductor containing Ga, and having a main surface of an N- polar (0001) plane , and a first semiconductor is formed on and in contact with the main surface of the layer, the result main surface of a nitride semiconductor containing Ga and second semiconductor layer which is a (0001) plane of the group III polarity, on the main surface of the second semiconductor layer A Schottky metal layer formed by Schottky bonding.

In the semiconductor photocatalyst, the second semiconductor layer, the lattice constant of the substrate planar direction than the first semiconductor layer is smaller state.

  In the semiconductor photocatalyst, the first semiconductor layer may be n-type.

  In the semiconductor photocatalyst, the second semiconductor layer and the contact region between the second semiconductor layer and the Schottky metal layer include exposed portions.

  The semiconductor photocatalyst may be provided with a plurality of island-shaped Schottky metal layers.

The semiconductor photocatalyst may include an ohmic metal layer electrically connected to the first semiconductor layer. In this case, the ohmic metal layer may be formed in ohmic contact with the main surface of the second semiconductor layer and electrically connected to the first semiconductor layer. For example, a plurality of island-like ohmic metal layers may be formed on the main surface of the second semiconductor layer.

  As described above, according to the present invention, on the main surface of the first semiconductor layer made of a nitride semiconductor and having a main surface with group V polarity, the second surface made of a nitride semiconductor and having the main surface with group III polarity. Since the semiconductor layer is formed, it is possible to obtain an excellent effect that the efficiency of the photocatalytic reaction is further improved with a simple configuration.

FIG. 1A is a cross-sectional view showing a configuration of a semiconductor photocatalyst according to Embodiment 1 of the present invention. FIG. 1B is a plan view showing a partial configuration of the semiconductor photocatalyst according to Embodiment 1 of the present invention. FIG. 2 is a configuration diagram showing the configuration of an experimental apparatus in which a semiconductor photoelectrode is used. FIG. 3A is a cross-sectional view showing the configuration of the semiconductor photocatalyst according to Embodiment 2 of the present invention. FIG. 3B is a plan view showing a partial configuration of the semiconductor photocatalyst according to Embodiment 2 of the present invention. FIG. 4 is a configuration diagram showing a configuration of a photocatalytic technique using an oxidation electrode and a reduction electrode. FIG. 5 is a configuration diagram showing a configuration of a photocatalytic technique using one photocatalyst.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIGS. 1A and 1B. FIG. 1A is a cross-sectional view showing a configuration of a semiconductor photocatalyst according to Embodiment 1 of the present invention. FIG. 1B is a plan view showing the configuration of the semiconductor photocatalyst according to Embodiment 1 of the present invention.

  The semiconductor photocatalyst includes a first semiconductor layer 101 and a second semiconductor layer 102 formed on the main surface of the first semiconductor layer 101. The first semiconductor layer 101 is made of a nitride semiconductor and has a main surface with group V polarity (N polarity). The second semiconductor layer 102 is made of a nitride semiconductor and has a main surface with group III polarity (Ga polarity). For example, the first semiconductor layer 101 may be made of GaN, and the second semiconductor layer 102 may be made of AlGaN.

  For example, the first semiconductor layer 101 and the second semiconductor layer 102 may be formed by crystal growth of GaN and AlGaN on a substrate 111 made of sapphire having a (0001) plane as the main surface. In addition, the first semiconductor layer and the second semiconductor layer are formed on different substrates in a state where each growth surface has a group V polarity (growth in the −C axis direction), and both the group V polarity surfaces are bonded together. Thereafter, the substrate may be removed from the second semiconductor layer. The main surface of the second semiconductor layer obtained by removing the substrate has Group III polarity.

  The semiconductor photocatalyst includes a Schottky metal layer 103 formed on the main surface of the second semiconductor layer 102 by Schottky junction (contact). In the first embodiment, the ohmic metal layer 104 electrically connected to the first semiconductor layer 103 is provided. For example, the ohmic metal layer 104 is formed in ohmic contact (contact) with the first semiconductor layer 101. For example, the ohmic metal layer 104 may be in ohmic contact with the side portion of the first semiconductor layer 101. The ohmic metal layer 104 may be in ohmic contact with the second semiconductor layer 102. Note that the ohmic metal layer 104 may be omitted.

  Here, it is important that the second semiconductor layer 102, the Schottky metal layer 103, and the contact region between the second semiconductor layer 102 and the Schottky metal layer 103 have exposed portions.

  According to the semiconductor photocatalyst in the above-described embodiment, the first semiconductor layer 101 has a main surface having group V polarity, and the second semiconductor layer 102 has a main surface having group III polarity. Thus, the holes generated by receiving the light move to the main surface side of the second semiconductor layer 102 and reach the Schottky metal layer 103 that is in Schottky junction to be collected. Further, electrons generated by receiving light move to the interface side between the first semiconductor layer 101 and the second semiconductor layer 102, reach the ohmic metal layer 104, and are collected.

  In the state described above, as shown in FIG. 2, if the semiconductor photocatalyst 100 according to the first embodiment is placed (immersed) in the aqueous solution 202 of the electrolyte accommodated in the photocatalyst tank 201, holes are collected. In addition, an oxidation reaction occurs in the Schottky metal layer 103, and a reduction reaction occurs in the ohmic metal layer 104 in which electrons are collected. As a result, water can be electrolyzed (oxygen generation by water oxidation reaction, hydrogen generation by reduction of protons generated by water oxidation).

  Since the Schottky metal layer 103, the second semiconductor layer 102, the ohmic metal layer 104, and the contact region between the second semiconductor layer 102 and the Schottky metal layer 103 have exposed portions, they contact the aqueous solution 202. As a result, the above-described reaction occurs. Note that light may be emitted from a light source 204 disposed outside the photocatalyst tank 201 through which light passes.

  In addition, by changing the metal species of the Schottky metal layer 103 and the atmosphere inside the photocatalyst tank 201, the generation of hydrocarbons such as carbon monoxide, ginsan, methanol, methane by the reduction reaction of carbon dioxide, or nitrogen It is also possible to produce ammonia by a reduction reaction.

  According to the embodiment, by stacking the first semiconductor layer 101 having the group V polarity and the second semiconductor layer 102 having the group III polarity, the electrons and holes generated by the photoreaction are separated and shot. Since the holes separated by the Schottky metal layer 103 to be key-joined are collected and the electrons are collected by the ohmic metal layer 104 to be ohmic-joined, the above-described catalytic reaction efficiency can be further improved. In addition, according to the embodiment, since it can be configured with one solution and one semiconductor photocatalyst, it is not necessary to have a complicated configuration.

  In addition, the Schottky metal layer 103 is preferably formed on the second semiconductor layer 102 by including a plurality of island portions 131, for example. For example, as shown in the plan view of FIG. 1B, a plurality of islands 131 having a diameter of 10 μm in plan view may be arranged in a square array with a distance between centers of 110 μm. With this configuration, the second semiconductor layer 102, the Schottky metal layer 103, and the contact region between the second semiconductor layer 102 and the Schottky metal layer 103 can be exposed in a wider area. In addition, by arranging the islands 131 regularly, it is possible to suppress an unintended reaction that deteriorates the material, for example, due to local collection of holes.

  Further, as described above, the first semiconductor layer 101 is composed of n-type GaN, the second semiconductor layer 102 is composed of AlGaN, and the second semiconductor layer 102 has a smaller lattice constant than the first semiconductor layer 101. Then, the above-described movement of electrons and holes occurs more efficiently by piezoelectrolysis generated by the difference in lattice constant between these materials. In addition, by making the first semiconductor layer 101 n-type, the above-described movement of electrons and holes can be caused more efficiently.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described with reference to FIGS. 3A and 3B. FIG. 3A is a cross-sectional view showing the configuration of the semiconductor photocatalyst according to Embodiment 1 of the present invention. FIG. 3B is a plan view showing the configuration of the semiconductor photocatalyst according to Embodiment 1 of the present invention.

  The semiconductor photocatalyst includes a first semiconductor layer 301 and a second semiconductor layer 302 formed on the main surface of the first semiconductor layer 301. The first semiconductor layer 301 is made of a nitride semiconductor and has a main surface with group V polarity (N polarity). The second semiconductor layer 302 is made of a nitride semiconductor and has a main surface with group III polarity (Ga polarity). For example, the first semiconductor layer 301 may be made of GaN, and the second semiconductor layer 302 may be made of AlGaN. For example, the first semiconductor layer 301 and the second semiconductor layer 302 may be formed by crystal growth of GaN and AlGaN on a substrate 311 made of sapphire whose main surface has a (0001) plane orientation.

  In addition, the semiconductor photocatalyst has an ohmic junction (contact) with the Schottky metal layer 303 formed on the main surface of the second semiconductor layer 302 and a Schottky metal layer 303 formed on the main surface of the second semiconductor layer 302. And an ohmic metal layer 304 that is electrically connected to the first semiconductor layer 301. For example, the ohmic metal layer 304 is electrically connected to the semiconductor layer 101 by an alloy region 341 formed by diffusing the metal constituting the ohmic metal layer 304. The Schottky metal layer 303 and the ohmic metal layer 304 are electrically separated. Note that the ohmic metal layer 304 may be omitted. Even in this case, it is important that the second semiconductor layer 302, the Schottky metal layer 303, and the contact region between the second semiconductor layer 302 and the Schottky metal layer 303 have exposed portions.

  For example, a plurality of island-shaped Schottky metal layers 303 are formed on the main surface of the second semiconductor layer 302. In other words, the Schottky metal layer may be composed of a plurality of islands. Similarly, a plurality of ohmic metal layers 304 having an island shape are formed on the main surface of the second semiconductor layer 302. For example, as shown in the plan view of FIG. 3B, a plurality of Schottky metal layers 303 having a diameter of 10 μm in a plan view and a plurality of ohmic metal layers 304 having a diameter of 10 μm in a plan view, respectively. It suffices to make every other square array. The distance between the centers of adjacent island portions may be 110 μm.

  With this configuration, the second semiconductor layer 302, the Schottky metal layer 303, and the contact region between the second semiconductor layer 302 and the Schottky metal layer 303 can be exposed in a wider area. In addition, by regularly arranging the island-shaped portions (ohmic metal layer 304), it is possible to suppress unintended reactions that cause, for example, material degradation due to local hole collection.

  According to the semiconductor photocatalyst in the above-described embodiment, the first semiconductor layer 301 has a main surface with group V polarity, and the second semiconductor layer 302 has a main surface with group III polarity. Thus, the holes generated by receiving the light move to the main surface side of the second semiconductor layer 302, reach the Schottky metal layer 303 that is in Schottky junction, and are collected. Further, electrons generated by receiving light move to the interface side between the first semiconductor layer 301 and the second semiconductor layer 302 and reach the ohmic metal layer 304 that is electrically connected to the first semiconductor layer 301. To be collected.

  Also in the second embodiment described above, as in the first embodiment described above, if the semiconductor photocatalyst in the second embodiment is disposed (immersed) in an aqueous electrolyte solution, a shot in which holes are collected. An oxidation reaction occurs in the key metal layer 303, and a reduction reaction occurs in the ohmic metal layer 304 in which electrons are collected. As a result, water can be electrolyzed (oxygen generation by water oxidation reaction, hydrogen generation by reduction of protons generated by water oxidation).

  Since the contact region between the Schottky metal layer 303, the second semiconductor layer 302, the ohmic metal layer 304, and the contact region between the second semiconductor layer 302 and the Schottky metal layer 303 includes an exposed portion, these contact with the aqueous solution. As a result, the above-described reaction occurs. In addition, what is necessary is just to irradiate light from the light source arrange | positioned outside the photocatalyst tank which accommodates aqueous solution.

  In addition, by changing the metal species of the Schottky metal layer 303 and the atmosphere of the aqueous solution, it is possible to generate hydrocarbons such as carbon monoxide, mist, methanol, and methane by the reduction reaction of carbon dioxide, or by the reduction reaction of nitrogen. Ammonia can also be produced.

  According to the second embodiment, by stacking the first semiconductor layer 301 having the group V polarity and the second semiconductor layer 302 having the group III polarity, the electrons and holes generated by the photoreaction are separated, and the shot is performed. The holes separated by the Schottky metal layer 303 to be key-joined are collected. Electrons are collected by the ohmic metal layer 304 that is electrically connected to the first semiconductor layer 301. As a result, the above-described catalytic reaction efficiency can be further improved. Moreover, according to Embodiment 2, since it can be comprised with one solution and one semiconductor photocatalyst, it is not necessary to set it as a complicated structure.

  Further, as described above, when the first semiconductor layer 301 is made of n-type GaN and the second semiconductor layer 302 is made of AlGaN, the above-described electrons and electrons are generated by piezoelectrolysis generated by a lattice constant difference between these materials. Hole movement will occur more efficiently. In addition, by making the first semiconductor layer 301 n-type, the above-described movement of electrons and holes can be caused more efficiently.

  Next, it demonstrates in detail using an Example.

[Example 1]
First, production of a semiconductor photocatalyst in Example 1 will be described. The semiconductor photocatalyst of Example 1 has the configuration described in Embodiment 1. First, a sapphire substrate having a main surface of (0001) plane is prepared. Next, on the sapphire substrate, by well-known metal organic vapor phase epitaxy, GaN which is doped with silicon and made n-type is crystal-grown in a state of group V polarity to form a first semiconductor layer. Next, Al _0.05 Ga _0.95 N is epitaxially grown in a group III polarity state to form a second semiconductor layer.

As described above, a V group polarity first semiconductor layer made of n-GaN having a thickness of 2 μm is formed on the sapphire substrate, and Al _0.05 Ga _0.95 N having a thickness of 100 nm is formed on the first semiconductor layer. A Group III polar second semiconductor layer was formed. The first semiconductor layer had a carrier density of 3 × 10 ¹⁸ cm ⁻³ . The layer thickness of 100 nm of the second semiconductor layer is approximately the same as the penetration length of light having a wavelength absorbed by the second semiconductor layer from the main surface direction in the second semiconductor layer. The thickness of the first semiconductor layer is set to 2 μm so that light having a wavelength that is not absorbed by the second semiconductor layer is efficiently absorbed by the first semiconductor layer.

Note that the first semiconductor layer made of n-GaN has a lattice constant of 0.3189 nm in the substrate plane direction. The second semiconductor layer made of Al _0.05 Ga _0.95 N has a lattice constant of 0.3185 nm in the substrate plane direction. In this case, the second semiconductor layer has a smaller lattice constant than the first semiconductor layer.

  Next, an ohmic metal layer was formed on the side surface of the stacked structure of the first semiconductor layer and the second semiconductor layer. For example, first a 25 nm thick titanium layer is deposited, followed by a 50 nm thick aluminum layer, followed by a 25 nm thick titanium layer, followed by a 100 nm thick platinum layer, An ohmic metal layer was formed by these laminated structures. The outermost surface of the ohmic metal layer is a platinum layer. Each layer may be formed (deposited) by, for example, a vacuum evaporation method. Note that gold, silver, copper, nickel, tungsten, tantalum, palladium, or ruthenium may be used instead of platinum.

As described above , after forming the ohmic metal layer on the side surface of the stacked structure of the first semiconductor layer and the second semiconductor layer, heat treatment is performed in a nitrogen atmosphere under conditions of 800 ° C. and 30 seconds, The ohmic junction was formed between the first semiconductor layer and the second semiconductor layer. Note that this heat treatment may be performed in an atmosphere such as air, inert gas, oxygen gas, hydrogen gas, or vacuum under reduced pressure as long as the composition of each semiconductor layer does not change.

  Next, a plurality of island parts made of platinum were formed on the main surface of the second semiconductor layer to form a Schottky metal layer. For example, a resist pattern having an opening on the island is formed by a known photolithography technique, and platinum is deposited on the resist pattern by vacuum evaporation. It is formed to a thickness of about 100 nm. Thereafter, by removing (lifting off) the resist pattern, the platinum deposited in the opening remains and becomes an island. Each island portion has a circular shape in plan view with a diameter of 10 μm, and the distance between the centers of the island portions in the substrate plane direction is 110 μm. The plurality of islands were arranged in a square pattern. The formed island portion is in a Schottky junction with the second semiconductor layer.

Next, the semiconductor photocatalyst chip (sample 1) was formed by cutting the laminated structure produced as described above. The chip was formed in such a size that the effective area where the photocatalytic function is expressed is 1 cm ² .

  Comparative sample 1 for comparison with sample 1 in Example 1 described above was prepared. In Comparative Sample 1, both the first semiconductor layer and the second semiconductor layer were formed with Group III polarity. Other configurations were the same as those of Sample 1.

[Example 2]
Next, production of a semiconductor photocatalyst in Example 2 will be described. The semiconductor photocatalyst of Example 2 has the configuration described in Embodiment 2. First, a sapphire substrate having a main surface of (0001) plane is prepared. Next, on the sapphire substrate, by well-known metal organic vapor phase epitaxy, GaN which is doped with silicon and made n-type is crystal-grown in a state of group V polarity to form a first semiconductor layer. Next, Al _0.05 Ga _0.95 N is epitaxially grown in a group III polarity state to form a second semiconductor layer.

As described above, a V group polarity first semiconductor layer made of n-GaN having a thickness of 2 μm is formed on the sapphire substrate, and Al _0.05 Ga _0.95 N having a thickness of 100 nm is formed on the first semiconductor layer. A Group III polar second semiconductor layer was formed. The first semiconductor layer had a carrier density of 3 × 10 ¹⁸ cm ⁻³ . The layer thickness of 100 nm of the second semiconductor layer is approximately the same as the penetration length of light having a wavelength absorbed by the second semiconductor layer from the main surface direction in the second semiconductor layer. The thickness of the first semiconductor layer is set to 2 μm so that light having a wavelength that is not absorbed by the second semiconductor layer is efficiently absorbed by the first semiconductor layer.

Note that the first semiconductor layer made of n-GaN has a lattice constant of 0.3189 nm in the substrate plane direction. The second semiconductor layer made of Al _0.05 Ga _0.95 N has a lattice constant of 0.3185 nm in the substrate plane direction. In this case, the second semiconductor layer has a smaller lattice constant than the first semiconductor layer.

  Next, an ohmic metal layer was formed on the main surface of the second semiconductor layer. For example, a resist pattern having an opening on the island is formed by a known photolithography technique, and the following metals are deposited on the resist pattern by vacuum deposition. For example, a 25 nm thick titanium layer was first deposited, followed by a 50 nm thick aluminum layer, followed by a 25 nm thick titanium layer, followed by a 100 nm thick platinum layer. The outermost surface is a platinum layer. Note that gold, silver, copper, nickel, tungsten, tantalum, palladium, or ruthenium may be used instead of platinum.

  Thereafter, the resist pattern is lifted off, so that platinum deposited in the opening remains and becomes an island-shaped ohmic metal layer. Each ohmic metal layer was circular in plan view with a diameter of 10 μm, and the distance between the centers of each ohmic metal layer in the substrate plane direction was 220 μm. Further, the plurality of ohmic metal layers were arranged in a square.

As described above, after forming a plurality of ohmic metal layer and islands on the main surface of the second semiconductor layer, in a nitrogen atmosphere, it was carried out heat treatment under conditions of 800 ° C. · 30 seconds, ohmic metal layer An ohmic junction was formed between the first semiconductor layer and the second semiconductor layer. By the heat treatment, the metal constituting the ohmic metal layer diffuses into the semiconductor layer to form an alloy region. When the alloy region reaches the first semiconductor layer, the ohmic metal layer and the first semiconductor layer can be in an electrically connected state. Note that this heat treatment may be performed in an atmosphere such as air, inert gas, oxygen gas, hydrogen gas, or vacuum under reduced pressure as long as the composition of each semiconductor layer does not change.

  Next, a plurality of island-shaped Schottky metal layers made of platinum were formed on the main surface of the second semiconductor layer. For example, a resist pattern having an opening on the island is formed by a known photolithography technique, and platinum is deposited on the resist pattern by vacuum evaporation. It is formed to a thickness of about 100 nm. Thereafter, the resist pattern is lifted off, whereby platinum deposited in the opening remains and becomes a plurality of island-shaped Schottky metal layers. The Schottky metal layer is in a Schottky junction with the second semiconductor layer. The Schottky metal layer was circular in plan view with a diameter of 10 μm. The distance between the centers of the respective Schottky metal layers in the substrate plane direction was 220 μm. In addition, the plurality of island-shaped Schottky metal layers are arranged alternately with the plurality of island-shaped ohmic metal layers.

Next, the semiconductor photocatalyst chip (sample 2) was formed by cutting the laminated structure produced as described above. The chip was formed in such a size that the effective area where the photocatalytic function is expressed is 1 cm ² .

  Comparative sample 2 for comparison was prepared with respect to sample 2 in Example 2 described above. In Comparative Sample 2, both the first semiconductor layer and the second semiconductor layer were formed with Group III polarity. Other configurations were the same as those of Sample 2.

[Example 3]
Next, preparation of the semiconductor photocatalyst in Example 3 will be described. Example 3 has a configuration without an ohmic metal layer. First, a sapphire substrate having a main surface of (0001) plane is prepared. Next, on the sapphire substrate, by well-known metal organic vapor phase epitaxy, GaN which is doped with silicon and made n-type is crystal-grown in a state of group V polarity to form a first semiconductor layer. Next, Al _0.05 Ga _0.95 N is epitaxially grown in a group III polarity state to form a second semiconductor layer.

As described above, a V group polarity first semiconductor layer made of n-GaN having a thickness of 2 μm is formed on the sapphire substrate, and Al _0.05 Ga _0.95 N having a thickness of 100 nm is formed on the first semiconductor layer. A Group III polar second semiconductor layer was formed. The first semiconductor layer had a carrier density of 3 × 10 ¹⁸ cm ⁻³ . The layer thickness of 100 nm of the second semiconductor layer is approximately the same as the penetration length of light having a wavelength absorbed by the second semiconductor layer from the main surface direction in the second semiconductor layer. The thickness of the first semiconductor layer is set to 2 μm so that light having a wavelength that is not absorbed by the second semiconductor layer is efficiently absorbed by the first semiconductor layer.

Note that the first semiconductor layer made of n-GaN has a lattice constant of 0.3189 nm in the substrate plane direction. The second semiconductor layer made of Al _0.05 Ga _0.95 N has a lattice constant of 0.3185 nm in the substrate plane direction. In this case, the second semiconductor layer has a smaller lattice constant than the first semiconductor layer.

  Next, a plurality of island-shaped Schottky metal layers made of platinum were formed on the main surface of the second semiconductor layer. For example, a resist pattern having an opening on the island is formed by a known photolithography technique, and platinum is deposited on the resist pattern by vacuum evaporation. It is formed to a thickness of about 100 nm. Thereafter, the resist pattern is lifted off, whereby platinum deposited in the opening remains and becomes a plurality of island-shaped Schottky metal layers. The Schottky metal layer is in a Schottky junction with the second semiconductor layer. The Schottky metal layer was circular in plan view with a diameter of 10 μm. The distance between the centers of the respective Schottky metal layers in the substrate plane direction was 110 μm.

Next, the semiconductor photocatalyst chip (sample 3) was formed by cutting the laminated structure produced as described above. The chip was formed in such a size that the effective area where the photocatalytic function is expressed is 1 cm ² . Example 3 has a configuration in which the ohmic metal layer is removed from Example 1.

  A comparative sample 3 for comparison was prepared with respect to the sample 3 in Example 3 described above. In Comparative Sample 3, both the first semiconductor layer and the second semiconductor layer were formed with Group III polarity. Other configurations were the same as those of Sample 3.

[Example 4]
Next, production of a semiconductor photocatalyst in Example 4 will be described. Example 4 has the same configuration as that of Example 3. The first semiconductor layer is composed of In _0.05 Ga _0.95 N with group V polarity, the second semiconductor layer is composed of GaN with group III polarity, Sample 4 was prepared in the same manner as in Example 3. The first semiconductor layer made of In _0.05 Ga _0.95 N has a lattice constant of 0.3207 nm in the substrate plane direction. The second semiconductor layer made of GaN has a lattice constant of 0.3189 nm in the substrate plane direction.

[Example 5]
Next, preparation of the semiconductor photocatalyst in Example 5 will be described. Example 5 has the same configuration as Example 3, and is composed of In _0.05 Ga _0.95 N with the first semiconductor layer having group V polarity, and Al _0.05 Ga _0.95 N with the second semiconductor layer having group III polarity. However, Sample 5 was produced in the same manner as Example 3 except for the other configurations. The second semiconductor layer made of Al _0.05 Ga _0.95 N has a lattice constant in the substrate plane direction of 0.3185 nm.

[Example 6]
Next, preparation of the semiconductor photocatalyst in Example 6 will be described. Example 6 has the same configuration as that of Example 3, and is composed of In _0.05 Ga _0.95 N with the first semiconductor layer having group V polarity, and In _0.01 Ga _0.99 N with the second semiconductor layer having group III polarity. However, the sample 6 was fabricated in the same manner as in Example 3 except for the other configurations. The second semiconductor layer made of In _0.01 Ga _0.99 N has a lattice constant of 0.3193 nm in the substrate plane direction.

[Example 7]
Next, preparation of the semiconductor photocatalyst in Example 7 will be described. Example 7 has the same configuration as Example 3, and is composed of Al _0.05 Ga _0.95 N with the first semiconductor layer having Group V polarity and Al _0.1 Ga _0.9 N with the second semiconductor layer having Group III polarity. However, Sample 7 was prepared in the same manner as Example 3 except for the other configuration. The second semiconductor layer made of Al _0.1 Ga _0.9 N has a lattice constant in the substrate plane direction of 0.3183 nm.

  In addition, Comparative Sample 4 and Comparative Sample 5 in which neither a Schottky metal layer nor an ohmic metal layer was formed were produced. Comparative sample 4 has a configuration in which the Schottky metal layer and the ohmic metal layer are removed from Example 1, and the second semiconductor layer having the Group III polarity is formed on the first semiconductor layer having the Group V polarity. . The comparative sample 5 has a configuration in which the Schottky metal layer and the ohmic metal layer are removed from the comparative example 1, and the second semiconductor layer having the III group polarity is formed on the first semiconductor layer having the III group polarity. .

[Redox reaction test]
Next, oxidation-reduction reaction test using sample 1, comparative sample 1, sample 2, comparative sample 2, sample 3, comparative sample 3, sample 4, sample 5, sample 6, sample 7, comparative sample 4 and comparative sample 5 The results will be described. In this test, the photocatalyst tank 201 described with reference to FIG. 2 was used as a cell, and a 1 mol / liter potassium hydroxide aqueous solution (125 ml) was used as the aqueous solution 202. As the semiconductor photocatalyst 100, Sample 1, Comparative Sample 1, Sample 2, Comparative Sample 2, Sample 3, Comparative Sample 3, Sample 4, Sample 5, Sample 6, Sample 7, Comparative Sample 4, and Comparative Sample 5 were used.

In the test, nitrogen gas was bubbled through the aqueous solution 202 at 200 ml / min for 30 minutes to defoam and replace, and then the photocatalyst tank 201 was sealed with a two-layered septum of silicone and fluororesin. The pressure in the photocatalyst tank 201 was atmospheric pressure. As the light source 204, a 300 W high-pressure xenon lamp (400 nm or more cut, illuminance 5 mW / cm ² ) was used. The light from the light source 204 was uniformly applied to the semiconductor photocatalyst 100 from the outside of the photocatalyst tank 201. The formation surface of the Schottky metal element and the ohmic metal layer was irradiated. The aqueous solution 202 was stirred by rotating the stirring bar 205 disposed at the center of the bottom of the photocatalyst tank 201 at a rotational speed of 250 rpm using a stirrer (not shown).

  When an arbitrary time passed, the gas in the photocatalyst tank 201 was collected from the septum portion with a syringe, and the reaction product was analyzed with a gas chromatograph mass spectrometer. This analysis confirmed the generation of hydrogen and oxygen.

  In addition, hydrogen gas with respect to light irradiation time in sample 1, comparative sample 1, sample 2, comparative sample 2, sample 3, comparative sample 3, comparative sample 4, comparative sample 5, sample 4, sample 5, sample 6 and sample 7 Table 1 and Table 2 below show the production amount. The amount of hydrogen gas produced is shown normalized by the chip area.

  As shown in Tables 1 and 2, the amount of hydrogen in the cell increased with the lapse of light irradiation time in any sample. On the other hand, it was confirmed that Sample 1 in Example 1 has a larger amount of hydrogen generation than Comparative Sample 1. In addition, it was confirmed that Sample 2 in Example 2 has a larger amount of hydrogen generation than Comparative Sample 2. Sample 3 in Example 3 was confirmed to have a larger amount of hydrogen generation than Comparative Sample 3. In addition, it was confirmed that the comparative sample 4 has a larger amount of hydrogen generation than the comparative sample 5.

  As is clear from these comparisons, by stacking the group III polarity second semiconductor layer on the group V polarity first semiconductor layer, hydrogen is generated compared to the case where both have the same polarity. The amount was confirmed to increase significantly. Further, as shown in the results of Samples 4 to 7, it can be seen that the same effect can be obtained not only by the combination of nitride semiconductors of Sample 1 but also by combinations of other nitride semiconductors such as InGaN. For example, in the case of a ternary system, it can be seen that the same effect can be obtained even if the composition is different.

  As described above, according to the present invention, the main surface of the first semiconductor layer made of a nitride semiconductor and having the main surface of Group V polarity is formed on the main surface of the first semiconductor layer made of the nitride semiconductor and having the Group III polarity. Since the two semiconductor layers are formed, the efficiency of the photocatalytic reaction can be further improved with a simple configuration.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, in Embodiment 1, the Schottky metal element is composed of a plurality of island parts, but the present invention is not limited to this.

  For example, the Schottky metal element may be configured to include a plurality of through holes that penetrate to the second semiconductor layer. Also with this configuration, the contact region between the Schottky metal element and the ohmic metal layer can be exposed as a wider area. Also in this case, it is preferable to arrange the plurality of through holes at equal intervals, for example, in a square arrangement.

  Moreover, although the ohmic metal layer was formed in the side part of the 1st semiconductor layer, it does not restrict to this. For example, by forming the second semiconductor layer on the first semiconductor layer so as to have a smaller area in plan view than the first semiconductor layer, a part of the main surface of the first semiconductor layer is formed on the side of the second semiconductor layer. It can be in an exposed state. An ohmic metal layer may be formed in this region. An ohmic metal layer may be formed on the back surface of the first semiconductor layer. In this case, the ohmic metal layer may be composed of a plurality of islands, or a plurality of through holes may be formed in the ohmic metal layer.

  DESCRIPTION OF SYMBOLS 101 ... 1st semiconductor layer, 102 ... 2nd semiconductor layer, 103 ... 1st metal layer, 104 ... 2nd metal layer, 131 ... Island part.

Claims (7)

A first semiconductor layer formed on the main surface of the substrate and made of an n-type nitride semiconductor containing Ga, the main surface of which is an N- polar (0001) plane ;
A main surface of the first semiconductor layer is made of a nitride semiconductor containing Ga having a lattice constant smaller than that of the first semiconductor layer, the main surface of the first semiconductor layer being a group ( III ) polarity (0001) plane A second semiconductor layer formed;
And a Schottky metal layer formed by Schottky junction on the main surface of the second semiconductor layer. The semiconductor photocatalyst of claim 1 Symbol placement,
The semiconductor photocatalyst, wherein the first semiconductor layer is an n-type. The semiconductor photocatalyst according to claim 1 or 2 ,
The semiconductor photocatalyst comprising the exposed portion of the second semiconductor layer and the contact region between the second semiconductor layer and the Schottky metal layer. In the semiconductor photocatalyst according to any one of claims 1 to 3 ,
A semiconductor photocatalyst comprising a plurality of the Schottky metal layers in an island shape. In the semiconductor photocatalyst according to any one of claims 1 to 4 ,
A semiconductor photocatalyst comprising an ohmic metal layer electrically connected to the first semiconductor layer. The semiconductor photocatalyst according to claim 5 ,
The ohmic metal layer is formed on the main surface of the second semiconductor layer by ohmic junction and is electrically connected to the first semiconductor layer. The semiconductor photocatalyst according to claim 6 ,
A semiconductor photocatalyst characterized in that a plurality of island-shaped ohmic metal layers are formed on a main surface of the second semiconductor layer.