The p-n junction ultraviolet (UV) photodetectors based on wide bandgap semiconductor materials demonstrate advantages including low energy consumption, rapid response, and high responsivity, enabling applications in fire warning, ozone detection, and missile tracking. Gallium oxide (Ga₂O₃), a next-generation semiconductor material, exhibits excellent thermal stability and an optical bandgap ranging from 4.4 to 5.2 eV, enabling strong UV light absorption. Its inherent oxygen defects provide n-type conductivity properties, although achieving stable p-type conductivity remains a technical challenge. Gallium oxide (GaN), another wide bandgap material, exhibits high carrier mobility and superior thermal stability, with well-established p-type doping technology. The combination of Ga₂O₃ and GaN forms a heterojunction interface characterized by minimal lattice mismatch and low conduction band offset. These properties establish Ga₂O₃/GaN heterojunction as an optimal material for UV photodetector fabrication. While significant advances have been achieved in aspects of the preparation and UV detection performance of porous Ga₂O₃/GaN heterojunctions, the oxidation mechanism of various porous GaN films and their influence on heterojunction detection performance remain unclear, which is essential for developing high-performance photodetectors. This research initially employs UV-assisted electrochemical etching to fabricate porous GaN films in different electrolytes, followed by high-temperature oxidation to produce porous Ga₂O₃/GaN heterojunctions. The study compares and analyzes the oxidation mechanism of different porous GaN films and the detection performance of Ga₂O₃/GaN heterojunctions.

The p-GaN epitaxial wafer is initially cut into 10 mm×5 mm segments. These segments undergo sequential ultrasonic cleaning in isopropanol, acetone, and deionized water for 15 minutes each, followed by nitrogen gas drying. The cleaned p-GaN film serves as the anode, with a platinum plate as the cathode. Under UV light irradiation, porous GaN films are prepared through 10-minute etching at 10 V using NaCl, NaNO₃, and NaOH solutions as electrolytes. The three porous GaN films undergo thermal oxidation in a quartz tube furnace for 120 minutes at 900 ℃, maintaining an oxygen flow rate of 1.5 L/min. Following oxidation and furnace cooling, the porous Ga₂O₃/GaN heterojunctions are completed. A portion of the Ga₂O₃ film is removed using hot phosphoric acid to expose the underlying GaN layer, and circular Ag/In contact electrodes are deposited on the Ga₂O₃ and GaN film areas via DC magnetron sputtering, completing the porous Ga₂O₃/GaN heterojunction detector (Fig. 1). Scanning electron microscopy characterizes the microstructures of the porous GaN films and porous Ga₂O₃/GaN heterojunctions. X-ray diffractometer analysis examines the crystalline structures of the porous Ga₂O₃/GaN heterojunctions. Room-temperature Raman spectral measurements utilize a Raman spectrometer. An Ultraviolet-Visible-Near Infrared Spectrophotometer tests the optical properties, while a semiconductor parameter analyzer collects and analyzes the detector’s electrical signals.

The Ga₂O₃ films formed through thermal oxidation of porous GaN films maintain a three-dimensional porous structure, with an irregular interface between the Ga₂O₃ and GaN films due to lattice structure differences. The porous Ga₂O₃/GaN heterojunctions prepared via etching in NaCl, NaNO₃, and NaOH solutions followed by oxidation exhibit average pore sizes of 28.6, 36.7, and 41.3 nm, respectively, with corresponding Ga₂O₃ layer thicknesses of 269, 327, and 502 nm. The enhanced thickness of the Ga₂O₃ film correlates with the increasing pH value of the etching solution, where holes and OH? ions jointly facilitate the GaN film oxidation process, resulting in porous GaN films with larger pore sizes and higher pore densities. This configuration provides additional oxidation sites, yielding a thicker Ga₂O₃ layer (Fig. 2). XRD and room-temperature Raman spectra reveal characteristic peaks corresponding to β-Ga₂O₃ films, confirming their formation during thermal oxidation. The GaN (001) plane predominantly transforms into the β-Ga₂O₃ (-201) plane during this process. Peak intensity variations reflect the thickness changes of the Ga₂O₃ layer in the Ga₂O₃/GaN heterojunction (Fig. 3). The porous Ga₂O₃/GaN heterojunction prepared through NaOH solution etching and subsequent oxidation demonstrates enhanced UV light absorption capacity, attributed to its larger pore size and complex microcavity structure, which effectively restrict photon escape and extend the light transmission path (Fig. 4). This heterojunction exhibits superior light absorption capability and increased Ga₂O₃ layer thickness, resulting in enhanced photocurrent while maintaining low dark current. Under 0 V bias, the heterojunction maintains a dark current of 0.22 nA. The photo-to-dark current ratio under 254 nm UV illumination achieves 10520, with a responsivity of 108.4 mA/W, an external quantum efficiency of 52.9%, a detectivity of 1.36×10¹² Jones, and a response time of 0.35 s/0.13 s. The device exhibits consistent stability during continuous on-off light cycling (Figs. 6 and 7).

In summary, porous Ga₂O₃/GaN heterojunctions were successfully fabricated on porous GaN films through etching with NaCl, NaNO₃, and NaOH solutions utilizing thermal oxidation methodology. The heterojunction prepared via NaOH solution etching and subsequent oxidation demonstrated optimal detection performance. Under 0 V bias, the device achieved a dark current of 0.22 nA, a photo-to-dark current ratio exceeding 10⁴ under 254 nm UV illumination, a responsivity of 108.4 mA/W, an external quantum efficiency of 52.9%, a detectivity of 1.36×10¹² Jones, and a response time of 0.35 s/0.13 s. The exceptional detection performance stems from the large pore size and complex microcavity structure of the heterojunction, which effectively restrict photon escape and extend the light transmission path, enhancing UV light absorption. Furthermore, the porous GaN film etched in NaOH solution exhibits higher average pore size and pore density, providing additional oxidation sites and yielding a thicker Ga₂O₃ layer post oxidation. The increased intrinsic resistance of the thicker Ga₂O₃ layer reduces dark current, enhancing overall device performance. This research contributes significantly to the advancement and application of high-performance Ga₂O₃/GaN heterojunction photodetectors.