Ferroelectric Switching in Two-dimensional β-Ga2O3 Down to Sub-Nanometer

Ferroelectric materials engineered into atomically thin films hold significant promise for nanoscale sensors and actuators, as well as high-density integrated logic and memory devices. Maintaining ferroelectric properties at these reduced dimensions is crucial for advancing next-generation electronics, particularly in applications requiring device miniaturization without compromising, and potentially enhancing, functionality. However, the size effect in ferroelectricity, especially the depolarization phenomenon at ultrathin regimes, presents significant challenges. This depolarization diminishes the performance and stability of ferroelectric materials, limiting their practical potential. Additionally, pronounced interface and surface effects at these scales further complicate the material's behavior, making optimal functionality difficult to achieve. Therefore, designing suitable low-dimensional films to overcome these challenges is imperative.<br/>In this study, we demonstrate ferroelectricity in two-dimensional β-Ga₂O₃ crystalline thin films down to an unprecedented sub-nanometer thickness. Similar to the van der Waals surfaces of two-dimensional (2D) materials, the β-Ga₂O₃ (100) surface exhibits weak surface interactions, with surface free energy comparable to that of graphene. These properties facilitate the easy exfoliation of β-Ga₂O₃ epitaxial layers along the (100) surface.[1,2] We have successfully exfoliated epitaxial β-Ga₂O₃ films down to a half-unit cell and transferred it to arbitrary substrates, akin to two-dimensional materials.[3,4] We find that ultrathin epitaxial β-Ga₂O₃ undergoes a non-ferroelectric to ferroelectric phase transition under biaxial strain at reduced thickness, as predicted by theoretical calculations. Ferroelectricity in Ga2O3 is preserved down to 0.6 nm, corresponding to half-unit-cell thickness. Utilizing ferroelectric Ga₂O₃, we further demonstrate ferroelectric tunnel junction (FTJ) devices that operate at room temperature with a large on/off ratio. This work opens new avenues for exploring ferroelectric materials at the sub-nanometer scale, facilitating the development of low-power applications that can be hetero-integrated with silicon technology via the back end of line (BEOL) process.<br/><br/>Reference:<br/>[1] Kim, H., Chang, C. S., Lee, S., Jiang, J., Jeong, J., Park, M., ... & Kim, J. (2022). Remote epitaxy. Nature Reviews Methods Primers, 2(1), 40.<br/>[2] Qiao, K., Liu, Y., Kim, C., Molnar, R. J., Osadchy, T., Li, W., ... & Kim, J. (2021). Graphene buffer layer on SiC as a release layer for high-quality freestanding semiconductor membranes. Nano letters, 21(9), 4013-4020.<br/>[3] Kim, H., Liu, Y., Lu, K., Chang, C. S., Sung, D., Akl, M., ... & Kim, J. (2023). High-throughput manufacturing of epitaxial membranes from a single wafer by 2D materials-based layer transfer process. Nature nanotechnology, 18(5), 464-470.<br/>[4] Kim, H., Liu, Y., Lu, K., Chang, C. S., Sung, D., Akl, M., ... & Kim, J. (2023). High-throughput manufacturing of epitaxial membranes from a single wafer by 2D materials-based layer transfer process. Nature nanotechnology, 18(5), 464-470.