Stoichiometric CuFeO2 Thin Films Synthesized by Pulsed-Laser Deposition Using an Oxidized Target with Cu(II) Ions

Abstract

The delafossite-type CuFeO2 is a promising p-type semiconductor for photocatalytic water splitting under sunlight due to its suitable bandgap and earth-abundant constituent elements. However, the synthesis of single-phase CuFeO2 thin films is often hindered by Fe-rich secondary phases such as γ-Fe2O3 and CuFe2O4, resulting in poor crystallinity. We obtain stoichiometric and high-quality CuFeO2 epitaxial films by using pulsed-laser deposition with an intentionally oxidized target containing Cu(II) ions, a mixture of CuFe2O4 and CuO. In contrast, the use of a pure CuFeO2 target with Cu(I) ions results in the formation of Fe-rich thin films, including CuFe2O4 secondary phase due to surface reduction of the Cu(I) target induced by laser ablation. Structural, optical, and spectroscopic characterizations combined with density-functional-theory simulations reveal that Cu 3dz2–O 2p states dominate the valence-band maximum, while Fe 3d states are located at the conduction-band minimum, with a bandgap of 1.44 eV in the stoichiometric CuFeO2 films. We provide a successful route for synthesizing high-quality delafossite-type CuFeO2 films, which is crucial for designing their semiconducting properties via chemical doping and forming artificial heterostructures for photocatalytic applications.