Thermodynamic and Kinetic Stability of p-type Transparent Conducting Oxide

Abstract

Stability of p-type transparent conducting oxides, CuAlO₂, CuGaO₂ and SrCu₂O₂, at high temperature has been studied by thermogravimetry, X-ray diffraction and thermodynamic calculation. It has been revealed that they decompose at high temperature in air according to the following chemical reaction formula; 4CuMO₂＋O₂ → 2CuO ＋2CuM₂O₄ (M = Al, Ga) and 28SrCu₂O₂＋17O₂ → 2Sr₁₄Cu₂₄O₄₁＋8CuO. Assuming the first order reaction, kinetics of the chemical reactions and their activation energies have been evaluated from weight variation by time at the constant temperature measured by thermogravimetry. It was concluded that the order of the kinetic stability was CuAlO₂, CuGaO₂ and SrCu₂O₂. Ellingham diagram of the chemical reactions prepared in this study showed that thermodynamic stability of CuAlO₂ was higher than that of CuGaO₂ and SrCu₂O₂. It was also revealed that CuAlO₂, CuGaO₂ and SrCu₂O₂ are not thermodynamically stable in air below 800 ℃, 1200 ℃ and 1140 ℃, respectively, showing agreement with the weight increase due to the chemical reactions at constant temperatures in air. It can be expected that the p-type transparent conducting oxides would be kinetically stable with decreasing oxygen partial pressure, since temperatures where CuAlO₂, CuGaO₂ and SrCu₂O₂ decomposed decreased in the Ellingham diagram. However, decomposition of SrCu₂O₂ was observed even at 400 ℃ under oxygen partial pressure of 10 ^－3 atm, which could be attributed to fast kinetics of the reaction even at 400 ℃ under such a low oxygen partial pressure.