Boron-assisted stabilization of low-resistivity mixed-valence Cu–O thin films prepared by reactive magnetron sputtering

This study systematically investigated the influence of boron incorporation in Cu-O thin films and the effect of oxygen partial pressure (pₒₓ) on the phase evolution, chemical bonding, and electrical characteristics of the prepared films. A phase transition from Cu₂O to Cu₂O/Cu₄O₃ to CuO was observed as oxygen partial pressure increased. Boron incorporation significantly broadened the stability window of the Cu₂O and Cu₄O₃ phases and delayed the transition to CuO at higher oxygen partial pressure. In the highly B-doped Cu-O films, Cu₄O₃ was stabilized even under oxygen-rich conditions along with the CuO phase, suggesting that boron significantly altered the oxidation pathway. The formation of B-O and possible B-O-Cu configurations altered the local oxygen chemistry and promoted mixed-valence copper oxide phases. Electrical measurements revealed that highly B-doped Cu-O films exhibited a delayed transition from a high-resistivity low-pₒₓ regime to a low-resistivity mixed-valence regime, ultimately reaching approximately 0.06 $Ω$ cm, among the lowest reported resistivities for a CuO-like material. These findings demonstrate that boron doping is an effective approach for tailoring the phase stability, defect chemistry, and electrical characteristics of Cu-O thin films for optoelectronic and photovoltaic applications.