Toward a Low-Temperature Route for Epitaxial Integration of BiFeO₃ on Si

Epitaxial thin-film growth enables novel functionalities, particularly if significant barriers to integration with existing technologies, scalability and excessive temperature of films, can be addressed. Here, we demonstrate a step toward addressing both challenges by combining hybrid molecular beam epitaxy and atomic layer deposition to epitaxially integrate BiFeO₃ on Si wafers via a SrTiO₃ metamorphic buffer layer. The solid–solid transformation of atomic-layer-deposited amorphous Bi–Fe–O films into epitaxial BiFeO₃ thin films is investigated by in situ annealing utilizing transmission electron microscopy. The amorphous Bi–Fe–O layer undergoes a very complex crystallization process, encompassing phenomena such as reorientation, recrystallization, and grain growth. Our in situ transmission electron microscopy study revealed that a growth front of epitaxial crystallites emerged from the interface with the (001)-oriented SrTiO₃ as temperature increased, whereas randomly oriented BiFeO₃ crystallites formed simultaneously away from the interface. Structural rearrangement and recrystallization of crystallites took place at temperatures below 400 °C. At the final stage, above 400 °C, epitaxial crystallites larger than 60 nm merged into a single crystalline film. Our results demonstrate that this approach permits high-quality epitaxial integration of BiFeO₃ thin films at back-end-of-line-compatible temperatures below 500 °C on metamorphic SrTiO₃ buffer layers on Si.