Very high temperature annealing of Cu2O obtained by the bifacial oxidation of free-standing Cu foils

Abstract

Cu₂O has been obtained via the thermal oxidation of free-standing Cu foils with a thickness of 127 μm under Ar and O₂ at 1020 °C followed by controlled cooldown at −5 °C at low pressure under an inert flow of Ar. We obtain single crystal grains of Cu₂O with sizes of ∼500 × 500 μm² which have a cubic crystal structure, but these extend only halfway through the bulk down to a layer of Kirkendall voids due to the bifacial oxidation of the Cu. The voids are nearly eliminated by annealing between 1120 °C and 1160 °C under Ar which also leads to grain growth. However, the out diffusion of the voids through the single crystal Cu₂O grains is accompanied by the formation of holes at the surface. We show that the layer of voids can be removed by polishing the Cu₂O down to ∼10 μm in order to preserve the single crystal nature of the grains obtained at 1020 °C while keeping the thermal budget to a minimum and discuss the limitations in exploiting the Cu₂O obtained in this way for the fabrication of devices.