Thanks to the low false alarm rate and high signal-to-noise ratio, deep ultraviolet (DUV) photodetector (PD) shows great application potential in ozone hole detection, high-voltage electric fire alarm, stealth bomber, and missile alarm. Gallium oxide (Ga₂O₃) is one of the most ideal materials for DUV PDs due to its suitable and tunable bandgap (4.5-5.3 eV), simple preparation process, and high stability. Nowadays, many studies focused on crystalline Ga₂O₃ film DUV PDs, but the lattice mismatch and strict growth parameters during the preparation put forward higher requirements for substrate materials and growth equipment. Compared with crystalline Ga₂O₃, amorphous Ga₂O₃ films have low preparation requirements and are easier to generate larger photocurrents due to the promotion of carrier separation by intrinsic defects. However, amorphous Ga₂O₃ is prone to higher dark current due to more defects. It is necessary to introduce noble metal Ag nanoparticles (Ag-NPs) to improve the photodetection performance of amorphous Ga₂O₃. On one hand, the formation of Schottky barriers between Ag-NPs and Ga₂O₃ films helps reduce the dark current of amorphous Ga₂O₃. On the other hand, the surface plasmon vibration of Ag-NPs can enhance the absorption of Ga₂O₃to UV light. Additionally, Ag-NPs can generate a large number of hot carriers under UV light to allow hot electrons with sufficient energy to overcome the Schottky barriers. We provide a feasible approach to realize DUV PDs with low dark current and high photo-to-dark current ratio (PDCR).

Amorphous Ga₂O₃ films are grown on sapphire substrates by the facile radio frequency (RF) magnetron sputtering technology. The sputtering is carried out at room temperature for 70 min with chamber pressure and Argon flow rate of 1 Pa and 20 sccm respectively. The obtained Ga₂O₃ films are cut into four parts, and three of them are continuously sputtered with Ag-NPs on the Ga₂O₃surface by direct-current (DC) magnetron sputtering. The sputtering time is 5, 10, and 20 s respectively. The obtained samples are labeled as 5 s Ag-NPs/Ga₂O₃, 10 s Ag-NPs/Ga₂O₃, and 20 s Ag-NPs/Ga₂O₃. Finally, the four samples obtained previously are annealed in a tube furnace for 2 h at an annealing temperature of 200 °C. The crystal structure of the sample is characterized by X-ray diffractometer (XRD), the cross-section morphology of the sample is by scanning electron microscope (SEM), and the surface morphology of the sample is by atomic force microscope (AFM). Additionally, the absorption spectrum features Q6 ultraviolet-visible (UV-Vis) spectrophotometer, and the low-pressure mercury lamps with wavelengths of 254 nm and 365 nm are employed as the ultraviolet light source. A pair of cylindrical metal indium (In) with a diameter of 1 mm and a spacing of 1 mm is pressed on the surface of samples as electrodes, and the current-voltage (I-V) characteristics and transient response curve (I-t) of the PDs are measured by a B1505A power device analyzer.

AFM results confirm the introduction of Ag-NPs on the surface of amorphous Ga₂O₃ films, and the surface mean square root (RMS) roughness of the Ag-NPs/Ga₂O₃ film after sputtering Ag nanoparticles for 20 s is significantly increased from 0.218 to 6.390 nm [Figs. 1 (c) and 1(d)]. Meanwhile, the absorption of Ga₂O₃ to UV light also increases obviously after sputtering Ag-NPs [Fig. 1 (e)]. 20 s Ag-NPs/Ga₂O₃ presents a lower dark current than amorphous Ga₂O₃ due to the Schottky junction formed between Ag-NPs and Ga₂O₃ films, which further forms a potential barrier and reduces the dark current [Fig. 2 (a)]. Under the irradiation of 254 nm UV light, Ag-NPs/Ga₂O₃ films exhibit a higher photocurrent than amorphous Ga₂O₃. In particular, the photocurrent of 20 s Ag-NPs/Ga₂O₃ at 5 V is 18.8 times higher than that of amorphous Ga₂O₃[Fig. 2 (b)]. This may be due to the enhanced scattering of UV light by the plasmonic vibrations of the Ag-NPs on the surface of Ga₂O₃ films, thus leading to enhanced absorption of UV light by Ga₂O₃ and an increase in the photocurrent of the Ag-NPs/Ga₂O₃ PD. Additionally, the Ag-NPs vibration may generate a large number of hot carriers to have enough energy to cross the Schottky barriers between Ag-NPs and Ga₂O₃ films, which leads to an increase in the photocurrent of the PD. At this point, the PDCR is as high as 5.9×10⁵, the rejection ratio (254 nm/365 nm) is 1.6×10⁴[Fig. 2 (c)], and the responsivity is 36.1 mA/W [Fig. 3 (c)], with the detectivity of 2×10¹⁴ Jones and external quantum efficiency of 17.7% [Fig. 3 (d)]. Meanwhile, both the amorphous Ga₂O₃ detector and the Ag-NPs/Ga₂O₃ detector have short response time (Fig. 4).

In summary, the Ag-NP composite amorphous Ga₂O₃ film DUV PD is prepared by a room temperature magnetron sputtering technology. The PD exhibits an excellent photodetection performance. Under 5 V voltage, the dark current of the detector is as low as 94 fA and the PDCR is as high as 5.9×10⁵, and the rejection ratio (254 nm/365 nm) is 1.6×10⁴, with the responsivity of 36.1 mA/W, detectivity of 2×10¹⁴ Jones, and external quantum efficiency of 17.7%. This is not only attributed to the plasmonic vibration of Ag-NPs under UV light, which scatters more incident light into the Ga₂O₃ film layer to enhance the UV light absorption of the Ga₂O₃ films, but also to the generation of a large number of hot carriers by Ag-NPs under UV light. These hot carriers enable the hot electrons to overcome the Schottky barriers formed by Ag-NPs and Ga₂O₃ films, which brings about a significant increase in the PD photocurrent. In addition, the formation of the Schottky barriers between Ag-NPs and Ga₂O₃ films helps reduce the dark current in the amorphous Ga₂O₃. This study implies that the introduction of noble metal nanoparticles provides a viable solution to DUV PDs with low cost, dark current, and high PDCR.