Structural transformation for BaBiO3-δ thin films grown on SrTiO3-buffered Si(001) induced by an in-situ molecular beam epitaxy cooldown process

Oxygen loss is a common defect type in perovskites which is caused by a low oxygen background pressure during growth. BaBiO_3-δ thin films are grown by molecular beam epitaxy on SrTiO₃-buffered Si(001) substrates. Although activated oxygen is supplied during growth, large amount of oxygen vacancies is created in the thin film depending on the cooldown process. Perovskite structure is obtained when the cooldown process includes an extended period during which activated oxygen is supplied. Another way for inducing the structural transformation is enabled via an ex-situ anneal at molecular oxygen. The transformation into BaBiO₃ is manifested as reconstructed octahedra based on transmission electron microscopy, Raman spectroscopy, and photoluminescence. Additionally, smaller out-of-plane lattice constant is observed for the perovskite phase supported by X-ray diffraction. Thermal mismatch and multivalency-facilitated tensile strain exerted on the layers by the underlying Si substrates are presented as the driving force behind the creation of oxygen vacancies.