Thickness-dependent microstructural properties of heteroepitaxial (00.1) CuFeO2 thin films on (00.1) sapphire by pulsed laser deposition

Abstract

Typical low-temperature frustrated triangular antiferromagnet CuFeO2 is attracting extensive interest due to its narrow-band-gap semiconductor properties. High-quality and impurity-free CuFeO2 epitaxial thin films would be preferable for fundamental studies on the physical and chemical properties. However, the heteroepitaxial growth of impurity-free CuFeO2 thin films has been a significant challenge due to its narrow formation window in the Cu–Fe–O system as well as the metastable nature of the Cu1+ cations. This work reports for the first time the fabrication and characterization of high-quality and impurity-free (00.1)-oriented CuFeO2 epitaxial thin films grown with relaxed interfaces on (00.1) sapphire substrates by pulsed laser deposition. Below the critical thickness of around 16 nm, the films exhibit a rhombohedral structure with relatively good crystalline quality where all Cu ions appear to be in the 1+ oxidation state, while the rocking curves display a narrow full width at half maximum of about 0.11°. Increasing the thickness, the (111)-oriented γ-Fe2O3 nanograins grow embedded in the CuFeO2 films. Here, an excess Fe3+-assisted growth mechanism is proposed to explain the iron oxide grain formation. This study provides insight into the heteroepitaxial growth of relaxed CuFeO2 thin films with high purity and crystalline quality as an ideal sample design to characterize the fundamental properties of this material in view of potential device applications.