Thickness Dependence and Strain Effects in Ferroelectric Bi₂FeCrO₆ Thin Films

We report on the ferroelectric properties of epitaxially grown Bi2FeCrO6 (BFCO) films with thicknesses of 7.5, 49, and 98 nm obtained by pulsed laser deposition. Because of the strains induced by the Nb-doped SrTiO3(001) substrate, the films exhibit a variable Fe–Cr order along the growth axis, with a disordered phase located near the interface and an increased order at the top of the films. This is first evidenced by X-ray diffraction and UV–vis–NIR absorption measurements as the ordered/disordered phases show different lattice parameters and band gaps. The strain effects which depend on the film thickness are found to strongly impact the ferroelectric properties. For the 49 nm thick film, piezoresponse force microscopy (PFM) reveals an out-of-plane intrinsic polarization orientation, effect which is absent for 98 nm thick films. The polarization anisotropy increases when reducing the thickness to 7.5 nm. The intrinsic polarization of the as-deposited 49 nm films induces a significant shift of the current–voltage characteristics, demonstrating an I(V) hysteresis loop strongly weighted toward positive voltage values. The strain effects impact therefore on ferroelectric domains polarization. Conductive atomic force microscopy (C-AFM) performed on poled areas demonstrates a significant asymmetric current–voltage characteristics and an open-circuit voltage up to 4.3 V for the 49 nm thick sample which decreases to 1.2 V for the 98 nm thick one. These values are larger than or at least comparable to the band gap of the ordered phase Bi2FeCrO6 phase (1.5 eV). Local current densities up to 20 A/cm2 were measured by C-AFM under weak illumination, thus confirming the potential of Bi2FeCrO6 for ferroelectric solar cells.