Investigations of van der Waals Epitaxial Growth of Aurivillius Phase Ferroelectrics and Multiferroics

Ferroelectrics, materials characterized by spontaneous switchable polarization that retains its orientation in the absence of an electric field, have garnered significant global academic interest due to their applications in non-volatile memory devices, neuromorphic synaptic junctions, and other advanced technologies. Notably, materials exhibiting coupled ferroelectric and magnetic order parameters, termed magnetoelectric multiferroics, can enable multi-level memory and energy-efficient electric fields for the reading and writing of data. One prominent example of such a system is the manganesecontaining Aurivillius phase Bi₆Ti_xFe_yMn_zO₁₈ (x = 2.80 to 3.04, y = 1.32 to 1.52, z = 0.54 to 0.64), which features layered crystal structures with interleaved fluorite and (Bi₂O₂)²⁺ layers sandwiched between metal perovskite units (A_n−1B_nO_3n+1)^2- , where A and B represent 12- and 6-coordinate transition metal cations, respectively. Despite having enormous potential, the development of layered perovskite ferroelectric and multiferroic tunnel junctions faces several challenges. These include the high cost of single crystal epitaxial substrates and the lack of consistent, high throughput processes for producing higher ‘n’ layer thin films. Addressing these issues is critical for advancing the field and realizing the full potential of these materials in practical applications. In response to these challenges, this study explores the use of fluorophlogopite mica as a substrate allowing van der Waals epitaxy with the Aurivillius phases. These substrates offer a cost-effective alternative to single crystal substrates and do not require lattice matching, facilitating largely unstrained growth. Our findings indicate that the ferroelectric domain sizes in thin films grown on these substrates are significantly larger. Moreover, this research investigates the role of the van der Waals epitaxy in promoting the growth of higher ‘n’ layer Aurivillius phase thin films. The thin films synthesized in this study were produced using Direct Liquid Injection Chemical Vapor Deposition (DLI-CVD), a high-throughput, scalable, industry compatible vapor-phase CVD process. In conclusion, our study demonstrates that utilizing van der Waal’s epitaxial substrates can address key challenges in the production of ferroelectric and multiferroic thin films, paving the way for future technological advancements and scalable manufacturing processes in this field.