Self‐Powered ZnSnN ₂ /GaN Photodiodes via Fine Stoichiometry Control and Photon Trapping Micropatterned Heterojunctions Under Low‐Light Irradiation

Recent advances in energy‐autonomous optoelectronic devices have attracted significant attention for next‐generation applications. However, developing compound semiconductor‐based self‐powered photodiodes remains challenging due to difficulties in precise band alignment control and limited light absorption efficiency. Here, we demonstrate a self‐powered photodiode based on a ZnSnN ₂ (ZTN)/GaN heterostructure, featuring an enhanced built‐in electric field via fine stoichiometry control and light‐trapping micropatterned heterojunctions. Through stoichiometric engineering, the ZTN thin‐film exhibited an optimized carrier concentration of 3.34 × 10 ¹⁹  cm ⁻³ and a bandgap of 2.27 eV. Consequently, the heterostructure achieved a strong built‐in electric field of 88 kV cm ⁻¹ due to the degenerate n‐type properties of ZTN. To further reinforce light absorption, we introduced periodic microhole patterns, and the resulting micropatterned heterojunction exhibited a substantial carrier lifetime of 6.2 ns, representing a 1.8‐fold enhancement over the thin‐film structure. Finally, the device demonstrated robust power‐saving operation under zero‐bias conditions, successfully driving a commercial temperature/humidity sensor. Moreover, the device exhibited a linear dynamic range of 15.1 dB and stable linearity ( θ  ≈ 0.27) even under low‐light conditions, ensuring reliable operation in varying illumination environments. These results suggest that our dual approach of stoichiometric and structural engineering offers a scalable pathway for next‐generation self‐powered optoelectronic systems.