Entitled: an optical semiconductor, an optical semiconductor electrode and a photo-electrochemical cell using the same, and the energy system Technical field [0001] 　The present invention relates to a decomposing optical semiconductor water by irradiation of light, and an optical semiconductor electrode and a photo-electrochemical cell using the same, and an energy system using the photoelectrochemical cell. BACKGROUND [0002] 　Conventionally, by the light is irradiated to the semiconductor material that functions as an optical semiconductor, the optical semiconductor material is decomposed with water, hydrogen and oxygen are known to be taken (for example, see Patent Document 1 ). Patent Document 1, an n-type semiconductor electrode and the counter electrode are disposed in the electrolytic solution, the light is irradiated on the surface of the n-type semiconductor electrode, techniques which hydrogen and oxygen are taken from the surface of both electrodes is disclosed ing. Specifically, the n-type semiconductor electrode, TiO 2 has been described the use of electrodes or the like. [0003] 　However, TiO 2 band gap of (anatase) is 380nm. Therefore, semiconductor electrode disclosed in Patent Document 1 has a problem that only about 1% of the solar light can not be utilized. [0004] 　To solve this problem, Patent Document 2, GaN (band gap: 365nm, the crystal structure: wurtzite type) to, InN (crystal structure: wurtzite) single crystal In was a solid solution of z Ga 1- z I discloses a fabricated electrode (0 0.2, a phase separation occurs. Thus, In z Ga 1-z N is, by increasing the amount of In, it is difficult to reduce the bandgap further. [0005] 　As described above, In z Ga 1-z In N, it is difficult to realize a high In composition (composition In is included a higher ratio). On the other hand, in the case of an oxide semiconductor including Ga and In, it is possible to realize a high In composition. Patent Document 3, as the oxide semiconductors of high In composition, InGaO 3 (ZnO) M (M is an integer of 1 to 20) is disclosed. CITATION Patent literature [0006] Patent Document 1: JP 51-123779 JP Patent Document 2: Japanese Patent Publication No. 2003-24764 Patent Document 3: International Publication No. 2008/072486 pamphlet Summary of the invention Problems that the Invention is to Solve [0007] 　The present invention, by realizing a high In content in the single crystal is a solid solution with InN on GaN, an object of the band gap is provided lower than the semiconductor material, ie, an optical semiconductor that can be used with visible light. Furthermore, the invention is also an object to provide an optical semiconductor electrode and a photo-electrochemical cell using such an optical semiconductor. Furthermore, the invention is also an object to provide an energy system using the photoelectrochemical cell. Means for Solving the Problems [0008] 　The present inventors have found that the mobility of carriers is high, focused on an oxide of indium (In), gallium (Ga) and zinc (Zn) having excellent semiconductor characteristics. In particular, the inventors have an oxide semiconductor of an oxygen of high In composition disclosed in Patent Document 3 focuses on be replaced by nitrogen. That is, the present inventors, InN crystal phase is wurtzite type, by making a solid solution of GaN and ZnO, at least a portion of the crystal phase having a wurtzite stable, and the band gap is small We were focused on the possibility that the semiconductor material is obtained. [0009] 　As a result, the present inventors found that, In, Ga, Zn, an optical semiconductor containing O and N, the general formula: In 2x Ga 2 (1-x) O 3 (ZnO) y (x and y, 0.2 0.2, which by conventional In z Ga 1-z can be realized a narrow band gap which could not be realized in N. Thus, according to the present invention, the solid solution effect of ZnO is exhibited, it is possible to obtain a new optical semiconductor. [0023] 　In the optical semiconductor of the present invention, it is preferable that y in the general formula is 1 or more and 6 or less. [0024] 　If the optical semiconductor of the present invention has the composition (the formula: In 2x Ga 2 (1-x) O 3 (ZnO) y (x = 0.5 and, in 1 ≦ y ≦ 6), O-A If the part has a composition that is substituted with N), an optical semiconductor of the present invention can easily achieve a single phase, can have excellent optical semiconductor performance. [0025] 　Since a single phase of the optical semiconductor is easily achieved more, desirably y is an integer in the general formula. However, if the stoichiometry is satisfied, the value of y is not limited to an integer. For example, if y is 3/2, the optical semiconductor of the present invention In 2 Ga 2 O 6 (ZnO) 3 a. [0026] 　An optical semiconductor of the present invention, it is preferable that in the general formula y is 2 or 6. [0027] 　If the optical semiconductor according to the present invention has the composition (the formula: In 2x Ga 2 (1-x) O 3 (ZnO) y (x = 0.5 and,, y = 2 or 6 in), O-one If the part has a composition that is substituted with N), the optical semiconductor can be a stable wurtzite type crystal structure. Thus, In 2x Ga 2 (1-x) O 3 (ZnO) y i can be a part of the oxygen lattice is easily replaced by nitrogen. As a result, a single phase of the optical semiconductor is more easily obtained. [0028] 　An optical semiconductor of the present invention have the general formula: In 2x Ga 2 (1-x) O 3 (ZnO) y (x and y, 0.2