【Member News】Great Progress has Been Made in Highly Sensitive, Self-Powered Gallium Oxide Solar Blind Ultraviolet Optical Detectors by the Research Team at Xiamen University

      Due to being absorbed by the atmosphere, there is almost no UV light between 200 and 280nm on the earth's surface. The light in this band is called solar blind ultraviolet, and the detection to this band is called helioblind UV detection. It is not affected by sunlight background, so solar blind ultraviolet detection has the advantages of high sensitivity and anti-interference energy. Solar blind ultraviolet photovoltaic detector has important applications in missile warning, power grid corona detection, flame detection, ozone layer sunlight ultraviolet monitoring and other fields.

Figure 1. Application of solar blind deep ultraviolet optical detector and ultra-wide band gap gallium oxide material

      Gallium oxide (Ga₂O₃) is a new wide band gap semiconductor (with band gap width 4.9 eV), which has the advantages of good thermal stability, large band gap width, large ultraviolet absorption coefficient, and easy to be processed, etc. It is an ideal semiconductor material for solar blind ultraviolet detection. Many reports have been made on solar blind UV photoelectric detectors based on Ga₂O₃, among which, pn-junction photodetectors has fast response and self-powered characteristics. The advantages of pn-junction photodetectors are closely related to their internal depletion regions. However, most of the reports claims that pn-junction photodetectors have a narrow width in the depletion regions due to heavy doping in the p-region or n-region, which undoubtedly limits the performance of the photodetectors.

      In order to solve the above problems, the research group of Zhang Hongliang and Cheng Qijin of Xiamen University developed a pin (p-GaN / i-Ga₂O₃ / n-Ga₂O₃) heterojunction self-powered solar blind ultraviolet photovoltaic detector with fully depleted active region based on Ga₂O₃. The pin heterojunction photodetector has superior solar-blind detection capability, and the device shows high responsiveness (72 mA / W), high detection rate (3.22×10¹² Jones), and high light-to-dark current ratio (1.88×10⁴) without external power supply. The device has a fast response speed (rise time of 7 ms, drop time of 19 ms) and good dynamic response characteristics (drop time of 185 μs under pulsed laser excitation of 248 nm); and the device has good stability, and the response characteristics of the device barely change after many switches or placement.

Figure 2. Design and band structure of pin-system (p-GaN / i-Ga₂O₃ / n-Ga₂O₃) in fully depleted active region; the device achieves self-drive and fast response speed at zero bias.

      On the other hand, the author also made a detailed study of the electronic structure of Ga₂O₃ / GaN heterojunction interface, such as band offset and built-in potential, and analyzed the source of superior device performance in photodetectors from the perspective of electronic structure and band. According to the research, the i-Ga₂O₃ layer is completely depleted as the main light absorption region (active region) in the Ga₂O₃ / GaN pin heterojunction photodetector by strictly regulating the width of the depletion regions, which significantly improves the performance of the photodetector. The main reasons are as follows: First, in most pn-junction photodetectors, due to the narrow depletion region, most of the photogenerated carriers are outside of the depletion region, and are prone to be compound during diffusion. Only a small fraction of the photogenerated carriers can enter the depletion region and contribute to the photocurrent generation. That is why photodetector responsiveness is low. While the photodetector prepared in this work with a wide depletion region allows for more efficient separation of photoborne carriers, so as to reduce the chance of phophophotophoto recombination, improve the light absorption efficiency, and increase the responsiveness of the photodetector; Second, since the photodetector prepared in this work has a fully depleted active region, when the light shines onto the photodetector, photoborne carriers can directly do high speed drift motion under the action of the built-in electric field in the depletion zone, avoiding the long diffusion process, which significantly improves the response speed of the photodetector; Third, the i-Ga₂O₃ layer with low carrier concentration suppresses the dark current of the photodetector, which improves the detection rate of the photodetectors.

      The related work was published in the famous international journal Advanced Optical Materials, titled "A Fast Self-Powered Solar-Blind Ultraviolet Photodetector Realized by Ga₂O₃ / GaN PIN Heterojunction with a Fully Depleted Active Region", Chen Wencheng, a graduate student from Xiamen University, is the first author of the paper, Associate Professor Cheng Qijin and Professor Zhang Hongliang are the corresponding authors. This work provides an important foundation for the development of self-powered, rapidly responsive high-performance solar-blind UV photodetectors.