Oxygen-diffusion-induced phase boundary migration in copper oxide thin films

The transition of copper oxide thin films from CuO to ${\mathrm{Cu}}_2$O during vacuum annealing has been studied by transmission electron microscopy and Rutherford backscattering spectroscopy. Dark field images show that isolated and large ${\mathrm{Cu}}_2$O grains emerge from the small CuO grain matrix. The abrupt change in oxygen concentration across the phase boundary between CuO and ${\mathrm{Cu}}_2$O in the reaction is different from the continuous change of $\delta$ in oxidation and reduction of the superconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ oxide. The growth of ${\mathrm{Cu}}_2$O grains is linear with time and has an activation energy of 1.1 eV. We propose that the discontinuous morphology of grain growth of ${\mathrm{Cu}}_2$O is due to the migration of the ${\mathrm{Cu}}_2$O-CuO phase boundary induced by oxygen out-diffusion along the moving phase boundary.