【Member News】Zhang Hongliang's team from Xiamen University has made progress in gallium oxide semiconductor bandgap engineering and solar-blind ultraviolet photodetectors

      In recent years, the third generation (wide band gap) semiconductor represented by gallium nitride (SiC) and silicon carbide (GaN) has gradually gained important applications in display, 5G communication, new energy vehicles, rail transit and other high-tech industries due to its significant advantages such as unique photoelectric properties, high breakdown field strength and electronic saturation rate. The technology and application with wide band gap semiconductor as the core have become a new strategic highland in the global semiconductor industry competition. China also proposed in the 14th Five-Year Plan to vigorously develop the third generation of wide-band gap semiconductor industry. Gallium oxide (Ga₂O₃) is an emerging wide band gap semiconductor after SiC and GaN, with a wider band gap width (4.9 eV) than GaN (3.3 eV), SiC (3.3 eV), and a higher breakdown electric field. It is a preferred material for new generation power electronic devices and deep ultraviolet solar-blind photodetectors (Figure 1). As a rising star in the semiconductor field, gallium oxide materials and devices have attracted great attention from the academic circles and industry of many countries.

      The gap width of gallium oxide can be further regulated by forming a (Al_xGa_1-x) ₂O₃ alloy semiconductor with Al₂O₃. On the one hand, larger band gap enables power devices to achieve higher breakdown field strength, and larger band gap also enables solar blind photodetectors to respond to wider spectral range and achieve full coverage detection. On the other hand, the heterojunction formed by (Al_xGa_1-x) ₂O₃ alloy semiconductor and Ga₂O₃ can form two-dimensional electron gas (2 DEG) at the interface, which can be applied to high electron mobility transistor (HEMT) devices.

Figure 1. Development of semiconductor materials (left) and solar-blind deep ultraviolet photodetection technology (right).

      Recently, Zhang Hongliang research group of Xiamen University, together with Qi Hongji and Chen Duanyang team of Shanghai Institute of Optical Machinery, adopted (Al_xGa_1-x) ₂O₃ alloying band project to realize a wide range of band width from 4.8 eV to 6 eV, and on this basis, completed the development of the solar-blind photodetector with full coverage in the solar-blind ultraviolet area (Figure 2). The solar-blind UV detector cutoff of this series can be adjusted from 263 nm to 236 nm, the response peak from 238 nm to 209 nm, with excellent specific detection rate (up to 10¹⁵ Jones), large UV visible inhibition ratio (10⁵), and good stability for long running. At the same time, the team developed a solar blind ultraviolet detector test module with digital interface to realize real-time sensitive monitoring of artificial arc (Figure 3). The whole solar-blind ultraviolet band detection technology can be widely used in various fields such as astronomy, geometeorology, science, materials science and environmental science. By drawing the spectral characteristics of the solar-blind light area with high precision, it provides more comprehensive and exact material structure and chemical composition information for the monitoring objects in these fields.

Figure 2. (a-b)Band gap evolution of (Al_xGa_1-x) ₂O₃ alloy film;  (c-d)effective coverage of solar-blind range by (Al_xGa_1-x) ₂O₃ alloy film photodetector.

Figure 3. Circuit integration of (AlxGa1-x) 2O3 alloy thin film photodetector, and sensitive monitoring of artificial arc signals.

      On the other hand, the author also made a deep study of the electronic structure of (Al_xGa_1-x) ₂O₃ through high-resolution X-ray photoelectron spectroscopy and density functional theory (DFT) calculation, and analyzed the source of the excellent performance of the devices in the photodetector from the perspective of electronic structure and band (Figure 4). It is found that the increase of the band gap of (Al_xGa_1-x) ₂O₃ alloy film is mainly due to the introduction of relatively high energy Al 3s state at the conduction band, and the hybrid of unoccupied Al 3s and Ga 4s leads to the edge of the conduction band moving up. However, the valence band edges vary less because they are mainly composed of occupied O 2p states with less hybridization with Ga 3 d. The calculation shows that the conduction band is still highly dispersion, which means that the electron effective quality is low and can maintain the high electron mobility of (Al_xGa_1-x) ₂O₃. This feature provides a solid theoretical basis for the high-performance solar-blind ultraviolet photoelectric detection application with full coverage.

Figure 4.  Electronic structure evolution of (Al_xGa_1-x)₂O₃ alloy thin film.

      This work provides an important foundation for the development of highly sensitive multispectral solar-blind ultraviolet photodetectors, and (Al_xGa_1-x) ₂O₃ band regulation and interfacial heterojunction 2 D electron gas. The related results was published in the international famous journal Advanced Optical Materials , titled “Tuning the Bandgaps of (Al_xGa_1-x) ₂O₃ Alloyed Thin Films for High-Performance Solar-Blind Ultraviolet Fully Covered Photodetectors”. Dr. Xiangyu Xu, a PhD student from Xiamen University, is the first author of the paper. This work has been funded and supported by the national Key R & D Program and the Shenzhen Municipal Basic Research Special Project.

Original links:https://onlinelibrary.wiley.com/doi/10.1002/adom.202300042

Homepage of Pro. Zhang Lianghong’s research group: https://khlzhang.xmu.edu.cn/