1) growing a contact layer thin film on a substrate using an electron beam conventionally, and annealing the grown contact layer thin film in a nitrogen-oxygen atmosphere at 400° C. to 600° C. through a rapid thermal annealing furnace; wherein the contact layer thin film is made of ITO or Ni, and if the contact layer thin film is made of the ITO, the ITO has a growth thickness of 10 nm to 20 nm; and if the contact layer thin film is made of the Ni, the Ni has a growth thickness of 1 nm to 4 nm;

2) growing a first Ga₂O₃ thin film by sputtering through magnetron sputtering under argon conditions, and controlling a thickness of the first Ga₂O₃ thin film to be 10 nm to 20 nm; wherein the contact layer thin film is disposed between the first Ga₂O₃ thin film and the substrate;

3) growing a doped thin film by sputtering through magnetron sputtering under argon conditions, the doped thin film being an Ag, Al or Ti thin film, and controlling a thickness of the doped thin film to be 3 nm to 7 nm;

4) growing a second Ga₂O₃ thin film by sputtering through magnetron sputtering under argon conditions, and controlling a thickness of the second Ga₂O₃ thin film to be 10 nm to 20 nm; and

5) annealing the grown thin films integrally in a nitrogen-oxygen atmosphere at 500° C. to 600° C. through the rapid thermal annealing furnace, so that permeation, diffusion and fusion occur between thin film materials to form a metal-doped Ga₂O₃ thin film,

wherein the metal-doped Ga₂O₃ thin film in the step 5) is formed by fusing the contact layer thin film, the first Ga₂O₃ thin film, the doped thin film, and the second Ga₂O₃ thin film; and the metal-doped Ga₂O₃ thin film has a thickness of 24 nm to 67 nm.