【Member Papers】Professor Tang Weihua's research team at Nanjing University of Posts and Telecommunications achieved Tunable Ga₂O₃ solar-blind photosensing performance via thermal reorder engineering and energy-band modulation,

      Gallium oxide (Ga₂O₃) has become a popular material for the new generation of power electronics and optoelectronic devices due to its ultra-wide bandgap and high critical breakdown electric field, which can meet the current development needs of high power and miniaturization of devices. At present, solar-blind ultraviolet photodetection is an important application of Ga₂O₃, and improving its photodetection performance is one of the hottest topics in Ga₂O₃ research. Since the surface is the main part of the device for carrier transport and signal capturing, the modulation of the surface will change the performance of the device to a large extent. However, as a very thin active layer, the properties of the active channel at surface are very hardly to achieve stable control. Many surface modulation methods have been explored and used to improve the performance of photodetectors, among which the introduction of localized surface plasmon resonance and localized surface Schottky junctions is one of the most common methods, but this method is relatively costly and complicated to operate. In this work, a simple and inexpensive thermal reorder engineering for modulating the surface electronic structure of Ga₂O₃ thin films is proposed to further enable the modulation of their photoelectric properties.

      In this work, four Ga₂O₃ films were grown by a low-cost and fast plasma enhanced chemical vapour deposition (PECVD) method, and three of them were annealed in vacuum, oxygen and oxygen plasma with a high temperature of 1000 °C for 1 h, respectively. By fitting the bandgap and analyzing X-ray photoelectron spectroscopy (XPS) of the four Ga₂O₃ films (shown in Figure 1), it was found that thermal reorder caused the energy band structure (including the bandgap width and valence band positions) of the surface of the Ga₂O₃ films to be changed to different degrees. Figure 2 shows the results of the curve fitting of O 1s for four films, where O_I denotes the lattice oxygen and O_II denotes the oxygen vacancies in the films, from which it can be seen that the oxygen-rich annealing environment reduces the concentration of oxygen vacancies in the films, while the oxygen-deficient annealing environment increases the oxygen vacancies in them. In addition, the bandgap of Ga₂O₃ films increases with the increase of oxygen vacancy concentration as calculated by the first principle, which is in good agreement with the experimental data, and the energy band structures of the four Ga₂O₃ films derived from the calculation results and experiments are shown in Figure 3.

      In addition, metal-semiconductor-metal (MSM) photodetectors were prepared based on these four Ga₂O₃ thin films, and indium metal was used as the electrodes, and it was observed that the dark current of the photodetectors could be reduced from 154.63 pA to 269 fA after thermal treatment, with a 100-fold increase in the photo-to-dark current ratio (PCDR) and a 10-fold increase in the detectability (D*), as shown in Figure 4. The high-temperature treatment led to crystal reorganization, which largely improved the crystalline quality of the films and hence the performance of the photodetectors. In addition, oxygen vacancies are a major defect in metal oxide crystals, as shown in Figure 5, which trap photogenerated carriers during photodetector operation, thus enhancing the self-trapping effect of the device and leading to a decrease in photodetection capability. After thermal annealing in an oxygen-rich atmosphere, the suppression of the oxygen vacancy concentration on the film surface also improves the photosensing performance of the film to some extent.

      In this work, surface modification of Ga₂O₃ films grown by PECVD is achieved by a low-cost and easy-to-operate thermal annealing method, and the specific effect of this thermal reorder engineering on the performance of Ga₂O₃-based MSM photodetectors is experimentally explored, concerning the blending of surface physics and photo physics for wide band gap semiconductor Ga₂O₃, translating to an achievable surface modification progress, based on the energy-band modulation theory.