Breaking the Responsivity–Speed Trade‐Off in Ga ₂ O ₃ Solar‐Blind Photodetectors via Oxygen‐Vacancy Stratification for Underwater Optical Communication and Sensing

Underwater optical sensing and communication place stringent requirements on photodetectors, which need to simultaneously deliver high responsivity and rapid temporal response. Gallium oxide (Ga 2 O 3 ) is an attractive semiconductor for underwater solar‐blind ultraviolet photodetection owing to its ultrawide bandgap and chemical stability; however, their performance remains strongly constrained by oxygen‐vacancy‐mediated carrier dynamics in Ga 2 O 3 , which underlie the responsivity–speed (RS) dilemma. Here, we report a pioneering oxygen‐vacancy stratification strategy that vertically integrates vacancy‐rich and vacancy‐poor Ga 2 O 3 layers to establish a built‐in electric field. This field accelerates carrier separation and transport while simultaneously achieving high photogain and rapid response, thereby overcoming the RS trade‐off. Under 255 nm illumination at zero bias, the device achieves a responsivity of 93.53 mA/W with rise and decay times of 8.7 and 2.8 ms, respectively, delivering the best combined responsivity–speed performance among reported Ga 2 O 3 ‐based self‐powered underwater photodetectors. Leveraging this performance, the device enables high‐speed underwater optical communication and, when integrated with a support vector machine classifier, supports simultaneous detection and identification of heavy‐metal ions, including Pb 2+ , Cu 2+ , and Fe 2+ . These results establish oxygen‐vacancy stratification as a general route to engineer carrier dynamics in wide‐bandgap oxides and advance high‐performance underwater optoelectronic sensing and communication systems.