Significance The sunlight with a wavelength shorter than 280 nm cannot penetrate the atmosphere and reach the surface of the earth, for which the 200--280 nm waveband is typically referred to as solar-blind region. In recent years, solar-blind ultraviolet photodetectors are widely used in military and civil fields such as missile guidance, space secure communications, ozone hole detection, and flame monitoring due to the advantages of low background noise and high sensitivity.

In recent years, the studies of solar-blind ultraviolet photodetectors have mainly focused on wide-bandgap semiconductor materials such as Al_xGa_1-xN, Mg_xZn_1-xO, and Ga₂O₃. The bandgap of Al_xGa_1-xN and Mg_xZn_1-xO is located in the solar-blind region mainly by adjusting the relative composition of Al and Mg. Owning to the relatively high adhesion coefficient and low surface mobility of Al atoms, Al_xGa_1-xN with high Al content will lead to a larger dislocation density of the epitaxial layer, while phase segregation phenomena are often present for Mg_xZn_1-xO with high Mg contents. These factors will lead to poor device performance. Ga₂O₃ is a representative ultra-wide bandgap semiconductor material, with a typical band ranging from 4.2 to 5.3 eV that almost occupies the entire solar-blind region of the solar spectrum. The relatively large bandgap renders it as an ideal candidate for solar-blind ultraviolet detection application. Thanks to the rapid advances in materials synthesis technique, we has witnessed a significant progress in solar-blind ultraviolet photodetectors based on Ga₂O₃ in the past decade. By this token, it is necessary to summarize the recent advances, which may be beneficial for bringing out new high-performance devices with new geometries.

Progress In the past few years, various fabrication technologies have been widely adopted to synthesize Ga₂O₃ materials with different crystal structures. For example, in 2004, the β-Ga₂O₃ single crystals of 1 inch (1 inch=2.54 cm) in diameter have been successfully grown by floating zone for the first time in Japan. Meanwhile, the domestic research on the growth of β-Ga₂O₃ single crystals has achieved remarkable results. In 2006, Shanghai Institute of Optics and Fine Mechanics employed the same floating zone to grow the β-Ga₂O₃ single crystal. Meanwhile, Tianjin Institute of Electronic Materials has achieved the growth of 2-inch β-Ga₂O₃ single crystals by using a new edge-defined film-fed growth method in 2018.

What is more, various solar-blind ultraviolet photodetectors composed of different forms of Ga₂O₃ with a variety of device geometries have been investigated. Some effective device optimization approaches such as controlled doping and light manipulation have been introduced to boost the photoresponse of the Ga₂O₃ based devices. For instance, Sooyeoun et al. from Korea University fabricated the photodetector based on the back-gated field-effect transistor structure using exfoliated β-Ga₂O₃ flakes, whose maximum responsivity of the device exceeds 10⁴ A/W (Fig. 4). Tao's research group from Shandong University has fabricated the Ga₂O₃ field-effect-transistor-based solar-blind photodetector, its responsivity is up to 10⁵ A/W, and the external quantum efficiency exceeds 10⁶%. Besides, our group has reported a catalyst-free vapor-solid growth technique for synthesizing β-Ga₂O₃ nanowires with single-crystalline quality. The photodetector based on β-Ga₂O₃ nanowires can operate properly at a large applied bias of 200 V with the responsivity being enhanced to as high as 10³ A/W, and the external quantum efficiency can reach 10⁵%. Fang's research group from Fudan University has fabricated a high-performance solar-blind avalanche photodetector based on highly crystallized ZnO-Ga₂O₃ core-shell heterojunction. The responsivity can reach up to 10³ A/W under -10 V bias, and the corresponding external quantum efficiency is as high as 10⁶% [Figs.13(b)--(d)]. Moreover, the solar-blind photodetectors based on Ga₂O₃ heterojunction, p-n junction, and Schottky junction with typical self-powered behavior can work normally without external power supply, which might find potential application in special environments.

Conclusions and Prospects In summary, we has witnessed extensive progress in Ga₂O₃ materials and solar-blind ultraviolet photodetectors in the past decade. In respect of material synthesis, the Ga₂O₃ single crystals can be grown at present, and the process route for depositing high-quality and large-area Ga₂O₃ thin films by MOCVD, MBE, magnetron sputtering, and other technologies is mature. Solar-blind photodetectors based on different forms of Ga₂O₃ have shown superior device performance including high responsivity and external quantum efficiency. What is more, the dark current of the device is as low as 1 pA, and the response speed is in the order of μs. It is believed that the study of Ga₂O₃ device will mainly focus on the following aspects. First, We need accurately control the size and morphology of Ga₂O₃ nanomaterials and develop new approach such as doping to reduce the surface state and control internal defects, which is essential for the development of high-performance devices. Second, the effect of defects on crystal quality in single crystals growth, which is an important factor to grow high quality Ga₂O₃ single crystal, should be extensively studied. Third, the p-type conductivity of Ga₂O₃ material remains an unsolved problem, which severely limits its application in optoelectronics and power devices. Finally, relevant integration technologies should be developed to solve the problems of device arrangement and assembly. Large area array detectors are to be developed such as focal plane solar-blind imaging systems with the advantages of low power consumption, small size, good compatibility with CMOS readout circuits, and high integration. Additionally, the development of integrated circuit technology that is suitable for Ga₂O₃ related focal plane array is also very important. It is believed that through the continuous efforts in this field, the Ga₂O₃ based solar-blind ultraviolet photodetectors will be able to be applied to national defense military and civilian fields as soon as possible.