Engineering Photoresponse in Epitaxial BiFe0.5Cr0.5O3 Thin Films through Structural Distortion

Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in the epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like phase to a rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of similar to 150 meV in the bandgap. Moreover, wavelength- resolved photocurrent measurements reveal that the absorption onset is red-shifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films. The crystallographic symmetry is shown to be a decisive factor in modifying the valence band maximum and conduction band minimum characteristics in the perovskite oxides, revealing an emerging type of Mott multiferroic in the BFCO system. This work provides a practical strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.