Optical absorption and redox kinetics of YBa2Cu3O7 − δ thin films studied by optical in-situ spectroscopy

Abstract

Optical absorption and redox kinetics of YBa2Cu3O7−δ thin films in oxidizing (O2) and reducing (Ar/H2) atmospheres were studied at temperatures from 200°C to 500°C by means of in situ UV–vis-NIR optical spectroscopy. The optical spectra in oxidizing atmospheres are characterized by optical absorption of oxygen holes, O− (OI′), e. g., at about 450nm at 200°C, whereas those in reducing atmospheres are dominated by a band at about 600nm due to electron hopping between Cu-ions. The fast redox processes of oxygen incorporation into and oxygen release from YBCO thin films induced by sudden changes in the ambient atmosphere between O2 and Ar/H2 are found to be controlled by surface exchange reaction. The oxygen surface exchange coefficients, kδ, determined from optical absorption relaxation experiments are about 4.96×10−7m/s for the oxidation process and about 4.85×10−9m/s for the reduction process at 500°C. The temperature dependence of kδ yields an activation energy of about 0.3eV for both oxidation and reduction processes in the studied temperature range. In addition, the rapid oxidation processes can be explained in terms of a high concentration of electrons in the reduced state of YBCO thin films, facilitating electron-transfer steps at the surface of YBCO film for oxygen exchange.