Engineering Boundary Conditions to Stabilize Improper Ferroelectricity in Hexagonal LuFeO₃ Films Down to the Monolayer Limit

Ultrathin ferroelectric films with out-of-plane polarization and high Curie temperatures are key to miniaturizing electronic devices, including low-power non-volatile memories. Most ferroelectrics employed in devices are proper ferroelectrics, where spontaneous polarization is the primary order parameter. Unfortunately, the Curie temperature of proper ferroelectrics is drastically reduced as the ferroelectric becomes thin; nearly all proper ferroelectrics need to be thicker than several unit cells. The absence of an ultrathin limit has been predicted, but not verified for improper ferroelectrics. These are ferroelectrics where the polarization emerges secondary to the primary order parameter, such as a structural distortion. The prime issue in thin films of improper ferroelectrics has been that they clamp to substrates that lack a structural distortion; such clamping thwarts the needed structural distortion and thus ferroelectricity. In this talk I will describe the use of an insulating substrate covered by a conducting electrode and followed by a monolayer transition layer that are not improper ferroelectrics themselves, but that do possess a structural distortion akin to that of the improper ferroelectric deposited upon them by molecule-beam epitaxy. The result is ferroelectricity with an undiminished Curie temperature in a formula-unit-thick (0.5-unit-cell) improper ferroelectric hexagonal LuFeO₃(<i>h</i>-LuFeO₃) film grown on a SrCo₂Ru₄O₁₁ bottom electrode with a carefully engineered monolayer transition layer. Our results* demonstrate the absence of a critical thickness for improper ferroelectricity and provide a methodology for creating ultrathin improper ferroelectrics by stabilizing its primary order parameter.<br/>* This work was performed in collaboration with the following coauthors: Yilin Evan Li, Rachel A. Steinhardt, Megan E. Holtz, Kunhikrishnan Premakumari Harikrishnan, Rustem Ozgur, Zhuyun Xiao, Evan Krysko, Adriana LaVopa, Petrucio Barrozo da Silva, Charles M. Brooks, Mario Brützam, Hai Li, Tanay A. Gosavi, Chia-ching Lin, Dmitri E. Nikonov, Ian A. Young, Dmitri A. Tenne, Rob N. Candler, Padraic Shafer, Elke Arenholz, Julia A. Mundy, Craig J. Fennie, Ramamoorthy Ramesh, David A. Muller, Robert J. Cava, and Christo Guguschev.