Ga 2 O 3 -based solar-blind photodetectors have been extensively investigated for a wide range of applications. However, to date, a lot of research has focused on optimizing the epitaxial technique or constructing a heterojunction, and studies concerning surface passivation, a key technique in electronic and optoelectronic devices, are severely lacking. Here, we report an ultrasensitive metal−semiconductor− metal photodetector employing a β-Ga 2 O 3 homojunction structure realized by lowenergy surface fluorine plasma treatment, in which an ultrathin fluorine-doped layer served for surface passivation. Without inserting/capping a foreign layer, this strategy utilized fluorine dopants to both passivate local oxygen vacancies and suppress surface chemisorption. The dual effects have opposite impacts on device current magnitude (by suppressing metal/semiconductor junction leakage and inhibiting surfacechemisorption-induced carrier consumption) but dominate in dark and under illumination, respectively. By means of such unique mechanisms, the simultaneous improvement on dark and photo current characteristics was achieved, leading to the sensitivity enhanced by nearly 1 order of magnitude. Accordingly, the 15 min treated sample exhibited striking competitiveness in terms of comprehensive properties, including a dark current as low as 6 pA, a responsivity of 18.43 A/W, an external quantum efficiency approaching 1 × 10 4 %, a specific detectivity of 2.48 × 10 14 Jones, and a solar-blind/UV rejection ratio close to 1 × 10 5 . Furthermore, the response speed was effectively accelerated because of the reduction on metal/semiconductor interface trap states. Our findings provide a facile, economical, and contamination-free surface passivation technique, which unlocks the potential for comprehensively improving the performance of β-Ga 2 O 3 solar−blind metal− semiconductor−metal photodetectors.

β‐Ga2O3 solar‐blind photodetectors (PDs) are attracting great attention for broad applications. However, their detection sensitivities are still lower than expected after tremendous efforts. The phenomenon of localized surface plasmon resonance (LSPR) offers another approach beyond conventional techniques to engineer the photodetection performance, but extending the plasmonic properties into the deep‐ultraviolet region faces severe challenges, among which strictly controlling the nanostructures structural properties is extremely prominent besides material selection. Herein, well‐defined Al@Al2O3 core−shell nanostructure arrays are fabricated with sub‐50 nm feature sizes, narrow dimension distributions, periodic graphene‐like patterns, and extremely high densities (up to 152.7 counts µm−2). The decorated β‐Ga2O3 PDs exhibit significantly enhanced sensitivities without response spectra broadening. Particularly, the sample with a 42‐nm nanostructure array possesses an ultrahigh specific detectivity (4.22 × 1015 Jones), being one of the top among film‐type gallium oxide PDs, and an excellent responsivity of 216.0 A W−1 peaked at 235 nm. Moreover, the passivation effect of self‐terminating native oxide shell is confirmed. The finite‐difference time‐domain simulations based on isolated, dimer, and arrayed models not only demonstrate the presence of LSPR, but also reveal the critical contribution of nanostructure density. The findings provide an alternative platform to break the bottleneck and develop ultra‐sensitive, truly solar‐blind PDs for advanced optoelectronic systems.