Non-epitaxial integration of strain-tuned polycrystalline BiFeO3 thin films for silicon-based optoelectronics

Integrating thin bismuth ferrite films with silicon-based platforms offers a promising path for advanced optoelectronic devices. This work investigates how oxygen partial pressure during pulsed laser deposition governs the structure, microstructure, defect chemistry, and optical properties of BiFeO₃ films grown on Ti-buffered Si. Diffraction and microscopy confirm single-phase rhombohedral perovskite and indicate that the oxygen background tunes the lattice strain states and vacancy proxies. The lower-pressure film exhibits partial relaxed strain, finer grains, and a smaller oxygen-defect fraction, whereas the moderate-pressure film shows stronger tensile lattice strain, rougher grains, and a higher vacancy level. Spectroscopic ellipsometry results, analyzed with a multilayer model that includes buried Ti and TiO_x interlayers, reveals distinct differences in dielectric dispersion and absorption edges. The lower-pressure film displays direct and indirect bandgaps of about 2.61 and 2.25 eV, while the moderate-pressure film shows slightly larger values of about 2.68 and 2.32 eV. These shifts are consistent with coupled variations in strain and oxygen-related disorder that modulate Fe–O bond lengths and hybridization. Overall, the results demonstrate that pressure-controlled strain and defect engineering can tailor light–matter interaction in Si-integrated BiFeO₃ films for photonic and optoelectronic applications.