NOVELTY - The method comprises forming a buffer layer (2) comprising a group-III nitride semiconductor on a substrate (1) using a sputtering method, and forming a group-III nitride semiconductor layer (50, 51) and electrodes on the buffer layer. The group-III nitride semiconductor layer is formed on the buffer layer by the sputtering method, a metal-organic chemical vapor deposition method, a molecular beam epitaxy method, a chemical beam epitaxy method, and a maximum likelihood method. The electrodes are formed on the group-III nitride semiconductor layer.    USE - The method is useful for manufacturing a thin film solar cell, which is useful for power source for a satellite, and household purposes.    ADVANTAGE - The method is capable of economically and rapidly manufacturing the thin film and flexible solar cell with improved productivity and reproducibility, excellent characteristics and enhanced photoelectric conversion efficiency and without threading dislocation.    DETAILED DESCRIPTION - The method comprises forming a buffer layer (2) comprising a group-III nitride semiconductor on a substrate (1) using a sputtering method, and forming a group-III nitride semiconductor layer (50, 51) and electrodes on the buffer layer. The group-III nitride semiconductor layer is formed on the buffer layer by the sputtering method, a metal-organic chemical vapor deposition method, a molecular beam epitaxy method, a chemical beam epitaxy method, and a maximum likelihood method. The electrodes are formed on the group-III nitride semiconductor layer. One of group-III nitride semiconductor layers (p-type layer/n-type layer) has a p-n junction. The sputtering method comprises reacting nitrogen in a plasma, radical, or atomic state with a group-III element for forming the buffer layer. The method further alternately repeating a process of supplying a dopant element and simultaneously supplying a compound comprising group-III element and a nitrogen raw material, and performing a heat treatment at a temperature of 300-1200 degrees C on the layer formed by supplying the dopant element. The heat treatment is performed in an atmosphere that does not contain a gas of a compound comprising hydrogen atom or hydrogen gas.    DESCRIPTION OF DRAWING(S) - The diagram shows a cross-sectional view of a solar cell element.    Substrate (1)    Buffer layer (2)    Underlying layer (3)    Group-III nitride semiconductor layers (50, 51)    Ohmic electrode (60)    Protective film. (61)