Capillary Epitaxy of Wide‐Bandgap Copper Halide Single‐Crystalline Film for Narrowband UV Photodetector and Dual‐Mode Communication

Abstract Wide‐bandgap copper halide semiconductors hold significant potential for next‐generation ultraviolet optoelectronics due to their remarkable photophysical properties. However, defects in polycrystalline films, such as point/interfacial defects and grain boundaries, create pathways for moisture and ion diffusion, thereby compromising device performance and stability. Herein, the capillary epitaxy growth of CsCu 2 I 3 single‐crystal films (SCFs) is demonstrated with a bandgap of 3.68 eV on GaN, by leveraging capillary forces to guide the transport of the molten precursor and using lattice‐matching substrates to achieve crystallographic alignments. This strategy suppresses random nucleation and produces CsCu 2 I 3 SCFs with high crystallinity (rocking curve FWHM = 0.286°). The critical role of lattice matching is further underscored by contrasting the polycrystalline CsCu 2 I 3 or Cs 3 Cu 2 I 5 films grown on lattice‐mismatched substrates, as well as by the impressive open‐circuit voltage of 1.08 V achieved in the CsCu 2 I 3 SCF/GaN heterojunction. The CsCu 2 I 3 SCF/GaN ultraviolet photodetectors demonstrate nW‐level detection limit (≈2.81 nW) and self‐powered operation, combining the 24 nm narrowband bandwidth with dual‐mode (voltage/current) signal outputs for interference‐resistant detection and communication. The work not only establishes the first route for the epitaxial growth of copper halide SCFs but also paves the way for the hetero‐integration of perovskites onto mature semiconductor platforms for ultraviolet optoelectronics.