Trap-Assisted Photoconductive Gain and Wavelength-Dependent Response of Ti 3 C 2 F MXene/GaN van der Waals Heterojunction Ultraviolet Detectors

Ultraviolet photodetection is essential for secure optical communication, environmental monitoring and early warning systems, yet conventional GaN-based Schottky detectors rely on evaporated metal electrodes that introduce interfacial defects and limited UV transmittance. Here we report a solution-processed Ti 3 C 2 F MXene/GaN van der Waals Schottky heterojunction UV photodetector formed by low-temperature drop-coating. X-ray photoelectron spectroscopy and Raman analysis confirm the surface termination and structural integrity of the transferred MXene, and density-functional-theory calculations rationalize the tunable Schottky barrier. The device exhibits a low dark current of ~74 nA near zero bias, about 30% lower than that of conventional Ti/Au Schottky GaN detectors. Wavelength-resolved measurements at 300, 310 and 320 nm show a photocurrent ordering of 320 nm > 310 nm > 300 nm, governed by the interplay between photon penetration depth and near-surface recombination. Under 320 nm illumination at 1.91 W/cm 2 and -4 V, a responsivity of 0.58 A/W, an external quantum efficiency (EQE) of 212% and a specific detectivity of 3.35 × 10 11 Jones are obtained; the above-unity EQE indicates trap-mediated internal photoconductive gain in GaN. Periodic UV switching yields rise/fall times of 1.22/0.80 s with an on/off ratio of ~2.2 and <5% peak-to-peak variation over four cycles. These results establish MXene/GaN van der Waals heterojunctions as a low-cost, solution-processable platform for wavelength-resolved UV detection.