Dielectric Properties of Ultrathin CaF2 Ionic Crystals
Chao Wen,1 Alexander G. Banshchikov,2 Yury Y. Illarionov,2,3 Werner Frammelsberger,4 Theresia Knobloch,3 Fei Hui,1 Nikolai S. Sokolov,2 Tibor Grasser3 and Mario Lanza,1,*
1Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nanoscience and Technology, Soochow University, 199 Ren-AiRoad, Suzhou 215123, China.
2Ioffe Institute, Polytechnicheskaya 26, Saint Petersburg 194021, Russia.
3Institute for Microelectronics (TU Wien), Gusshausstrasse 27-29, Vienna 1040, Austria.
4Department of Mechanical Engineering and Mechatronics, Deggendorf Institute of Technology, Dieter-Görlitz-Platz 1, Deggendorf 94469, Germany.
Mechanically exfoliated 2D hexagonal boron nitride (h-BN) is currently the preferred dielectric material to interact with graphene and 2D transition metal dichalcogenides in nanoelectronic devices, as they form a clean van der Waals interface. However, h-BN has a low dielectric constant (≈3.9), which in ultrascaled devices results in high leakage current and premature dielectric breakdown. Furthermore, the synthesis of h-BN using scalable methods, such as chemical vapor deposition, requires very high temperatures (>900°C), and the resulting h-BN stacks contain abundant few-atoms-wide amorphous regions that decrease its homogeneity and dielectric strength. Here it is shown that ultrathin calcium fluoride (CaF2) ionic crystals could be an excellent solution to mitigate these problems. By applying >3000 ramped voltage stresses and several current maps at different locations of the samples via conductive atomic force microscopy, it is statistically demonstrated that ultrathin CaF2 shows much better dielectric performance (i.e., homogeneity, leakage current, and dielectric strength) than SiO2, TiO2, and h-BN. The main reason behind this behavior is that the cubic crystalline structure of CaF2 is continuous and free of defects over large regions, which prevents the formation of electrically weak spots.