Search for new transparent conductors: Effect of Ge doping on the conductivity of Ga2O3, In2O3 and Ga1.4In0.6O3

Only a small amount (≤3.5 mol%) of Ge can be doped in Ga2O3, Ga1.4In0.6O3 and In2O3 by means of solid state reactions at 1400 °C. All these samples are optically transparent in the visible range, but Ge-doped Ga2O3 and Ga1.4In0.6O3 are insulating. Only Ge-doped In2O3 exhibits a significant decrease in resistivity, the resistivity decreasing further on thermal quenching and H2 reduction. The resistivity of 2.7% Ge-doped In2O3 after H2 reduction shows a metallic behavior, and a resistivity of ∼1 mΩ cm at room temperature, comparable to that of Sn-doped In2O3.