Hexagonal Layered YOCl as a Crystalline Oxyhalide Dielectric for Two‐Dimensional Electronics

ABSTRACT High‐κ gate dielectrics with low leakage and clean van der Waals interfaces are essential for the continued scaling of 2D electronics. Beyond conventional amorphous oxides, layered rare‐earth oxyhalides offer an emerging crystalline dielectric platform because they combine strong lattice‐mediated polarization, wide‐gap insulating characteristics, and non‐covalent integration capability with 2D semiconductors. However, controllable synthesis and device validation of hexagonal/trigonal rare‐earth oxychloride dielectrics remain largely unexplored. Here, we report the phase‐selective growth of hexagonal layered yttrium oxychloride, h‐YOCl, as a high‐κ dielectric for 2D transistors. First‐principles calculations reveal that h‐YOCl possesses a pronounced field‐induced polarization response arising from its mixed Y‐O/Y‐Cl bonding framework, favorable band offsets with MoS 2 , and a clean van der Waals interface. Experimentally, h‐YOCl nanosheets exhibit a high effective dielectric constant of 16.1, a wide bandgap of 5.54 eV, and a breakdown field up to 14.8 MV cm − 1 . When integrated as a top‐gate dielectric in MoS 2 field‐effect transistors, h‐YOCl enables efficient electrostatic control, yielding a subthreshold swing down to 88.7 mV dec − 1 , an on/off ratio of ∼5 × 10 7 , and reversible operation up to 423 K. These results establish hexagonal layered YOCl as a promising oxyhalide dielectric and highlight phase‐selective oxyhalide growth as a viable route toward low‐power 2D electronics.