Highly Coherent Grain Boundaries Induced by Local Pseudomirror Symmetry in β-Ga<sub>2</sub>O<sub>3</sub>
Yuchao Yan, Yingying Liu, Ziyi Wang, Da Liu, Xu Gao, Yan Wang, Li Cheng, Keke Ma, Ningshao Xia, Jin Zhu, Tianqi Deng, Hui Zhang, Deren Yang
Abstract
Grain boundaries have an extensive influence on the performance of crystal materials. However, the atomic-scale structures and their relations to local and crystallographic symmetries remain elusive in low-symmetry crystals. Herein, we find that the local pseudomirror-symmetric atomic layer is the common physical origin of a series of highly coherent grain boundaries in the low-symmetry β-Ga 2 O 3 crystal. These include the (100) twin boundary and an emerging series of ( h -1′0′2) / ( h +1′0′2̅) coherent asymmetric grain boundaries (CAGBs). Owing to the local pseudomirror symmetry and the special geometric relation of the β-Ga 2 O 3 conventional cell, these CAGBs place 80% of the boundary atoms in pseudocoincident sites, exhibiting high coherence under the coincident-site lattice model. With a combination of density functional theory calculations, Czochralski growth experiment, and atomic-scale characterizations, the structure and stability of the (002) / (202̅)-A CAGB are confirmed, with an interface energy density as low as 0.36 J m –2 . This CAGB is responsible for the spontaneous formation of a twinned defect facet at the surface steps during the epitaxy growth of β-Ga 2 O 3, warranting a substrate orientation selection rule for β-Ga 2 O 3 . Through this study, we provide insights into the grain boundary physics in the low-symmetry β-Ga 2 O 3 crystal while emphasizing the importance of the local pseudosymmetries in the low-symmetry crystals.