The common hexagonal boron nitride (hBN) is regarded as an ideal wide-bandgap two-dimensional dielectric material due to its chemical stability, good thermal conductivity and atomic-level flatness without hanging bonds. While maintaining many excellent properties of hBN, rBN is a functional material with great application potential due to its non-center-symmetrical ABC stacking structure, intrinsic slip ferroelectric and nonlinear optical properties, which can provide new material solutions for transformative technology applications such as integrated storage and computing devices and deep ultraviolet light sources. However, compared with the common hBN crystals, rBN crystals belong to a metastable phase, and the preparation of multilayer rBN single crystals is challenging. The difficulty lies in the fact that the fast-growing first layer of hBN film has a shielding effect on the substrate catalysis, hindering the continuous growth of the subsequent layers, and the van der Waals effect of B and N atoms between the interfaces leads to the energy advantage of hBN crystals with AA'A stacking in the nucleation process. Therefore, the artificial fabrication of large-size rBN crystals has been a long-term problem, and it is also a direction to compete for tackle tough problems.
Figure 1. The principle and preparation process of multilayer rhobizoar boron nitride single crystal grown by epitaxial step of inclined plane
The Surface Physics Laboratory of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics has been deeply engaged in the surface preparation science of boron-carbon-nitrogen light element materials for 30 years since Academician Wang Enge founded the SF1 research group, and has achieved systematic innovation results. In recent years, team leaders researcher Bai Xuedong and associate researcher Wang Li have cooperated with Professor Liu Kaihui of Peking University to make breakthroughs in the preparation of boron nitride single crystals. In 2019, they developed a method for the fabrication of large-area single-layer hBN monocrystalline thin films using surface symmetry breaking substrate epitaxial non-center-symmetric two-dimensional single crystals (Nature 570, 91 (2019)). Recently, they further proposed an innovative mechanism based on surface symmetry to break the in-plane and out-of-plane co-regulation of the substrate, that is, by constructing a bevel high step composed of (100) and (110) crystal planes on the surface of single crystal nickel metal, the in-plane lattice orientation and out-of-plane slip vectors of the rBN lattice are matched and locked layer by layer in the nucleation stage of chemical vapor deposition, and then the formation of co-directional rBN domains is induced in a large area. Scanning transmission electron microscopy (STEM) observation showed that the rBN crystal domains with consistent orientation were seamlessly spliced layer by layer to form a crystal structure with precise ABC atom stacking, and rBN single crystals with an area of up to 4×4 square centimeters were successfully prepared.
Figure 2. Preparation and characterization of single crystal substrates and rhomboidic boron nitride domains
Through theoretical calculations, they found that the non-centrically symmetric stacking of rBN leads to the accumulation of interlayer polarization vectors in the out-of-plane direction, exhibiting ferroelectricity. Using the piezoelectric force scanning probe (PFM), the measurement verifies that rBN has high Curie temperature ferroelectricity, and realizes repeated erase and write operations in the ferroelectric domain region. The in-situ observations of transmission electron microscopy further confirm that the polarization flip of rBN originates from interlayer slip.
Figure 3. The aligned diamond ferrogonal boron nitride domains are seamlessly spliced layer by layer to form a uniform monocrystalline film
This achievement proposes a new method for the preparation of multilayer rhombocubic boron nitride single crystals with inclined step surfaces, innovates the surface epitaxial growth mode, artificially manufactures new crystals through the precise arrangement of three-dimensional space atoms, and endoives the previous boron nitride insulating medium with ferroelectric storage function, providing a new material strategy for the manufacture of integrated storage and computing devices, and helping the transformative development of chip technology in the era of artificial intelligence.
Figure 4. Rhombodic boron nitride slip ferroelectric characterization
The research paper was published online in the journal Nature on May 1, 2024 with the title of "Bevel-edge epitaxy of ferroelectric rhombohedral boron nitride single crystal". The paper was a collaborative effort by Chinese scholars. Associate researcher Wang Li of the Institute of Physics of the Chinese Academy of Sciences and doctoral student Qi Jiajie of Peking University, Dr. Wei Wenya of South China Normal University and doctoral student Wu Mengqi of Westlake University are the co-first authors, and researcher Bai Xuedong of the Institute of Physics of the Chinese Academy of Sciences, Professor Liu Kaihui of Peking University, researcher Zheng Xiaorui of Westlake University, Professor Ding Feng of Shenzhen Institute of Advanced Technology and associate researcher Wang Li of the Institute of Physics of the Chinese Academy of Sciences are the co-corresponding authors. The main collaborators are Academician Wang Enge of the Institute of Physics of the Chinese Academy of Sciences/Peking University, Professor Gao Peng of Peking University, Researcher Wang Wenlong of the Institute of Physics of the Chinese Academy of Sciences, Associate Chief Engineer and PhD student Sun Huacong of Li Xiaomin, Professor Wang Zhujun of ShanghaiTech University, Professor Xu Xiaozhi of South China Normal University, Professor Wu Menghao of Huazhong University of Science and Technology, Researchers Wu Muhong and Xu Zhi of Songshan Lake Materials Laboratory, etc.
This work was supported by the National Science Foundation of China, the Chinese Academy of Sciences, the Ministry of Science and Technology, and the Guangdong Provincial Project.
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