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

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.