【Member News】Zhang Hongliang's team from Xiamen University has made progress in the electronic structure of wide band semiconductor doping

      In recent years, gallium oxide (Ga₂O₃) semiconductor has been widely concerned by the scientific research and industry around the world. Gallium oxide has an ultra-wide band gap of 4.9 eV, which is higher than the third-generation semiconductor materials----3.2 eV of silicon carbide (SiC) and 3.39 eV of gallium nitride (GaN). A wider band gap width means that electrons need more energy to transition from the valence band to the conduction band, so gallium oxide has high voltage withstand (extremely strong critical field strength), high efficiency (lower conduction resistance), high power, radiation resistance and other characteristics. In terms of manufacturing process, gallium oxide crystals can be grown through mature melting method, which can greatly reduce the production cost compared with the third generation semiconductors SiC and GaN. Power electronic devices based on gallium oxide semiconductor hold potential applications in new energy vehicles, charging piles, rail transit, motor control and other fields. Deep ultraviolet photovoltaic detectors based on gallium oxide show important potential in missile early warning, high-voltage power grid corona detection, ozone hole monitoring and other fields. Therefore, gallium oxide and diamond, aluminum nitride (AlN) and other ultra-wide band gap semiconductors are considered the fourth generation semiconductor.

Figure 1. The development process of semiconductor materials and the application of semiconductor power electronic devices.

      The preparation of Ga₂O₃ semiconductor optoelectronic devices requires the precise regulation of the electrical properties of Ga₂O₃, and its essence is to regulate the electronic structure of Ga₂O₃ by introducing doping. In terms of n-type doping of Ga₂O₃, how to select efficient dopants is a key problem in the preparation of Ga₂O₃ epitaxial films. To solve the above problems, Zhang Hongliang team from Xiamen University explores the IV group dopant on the influence of Ga₂O₃ electronic structure, which is based on hard X-ray photoelectron spectrum (HAXPES) of a synchrotron radiation light source and hybrid density functional theory (Hybrid DFT) calculation. They put forward the theoretical model of Si resonance doping mechanism, clear and definite that Si is optimal dopant of Ga₂O₃ n-type doping.

      The research team used Si and Sn doping to regulate the carrier concentration of Ga₂O₃, and the highest carrier concentration of 2.6×10²⁰ cm^-3 and the highest conductivity of 2520 S / cm were achieved by 1% Si doping. The team studied the influence of doping on gallium oxide electronic structure by using a high-resolution hard X-ray photoelectron spectroscopy system based on a synchrotron radiation light source (Figure 2).

Figure 2. (A) Hard X-ray photoelectron energy spectrum of the synchrotron radiation light source.(B) Schematic diagram of the band structure with the band-gap renormalization effect.(c) Based on the parabolic and non-parabolic models carrier statistics, Burstein-Moss movement (∆BM), band gap renormalization effect (∆RN), and band gap renormalization effect as measured by HAXPES[∆RN (HAXPES)], the relationship between DFT calculation [∆RN (DFT)] and carrier concentration, .

      Further combined with DFT calculation, the team proposed the mechanism of resonance doping theory model of Si: it is proved that because Si 3s is located about 2 eV above the bottom of the conduction band, lack of orbital hybridization between the conduction band constituted by the Ga 4s state and the Si 3s-doped state, so the Ga₂O₃ conduction band bottom edge is almost unaffected by the dopant Si electronic state. Therefore, Ga₂O₃ can still maintain small effective quality, to ensure its high mobility at high carrier concentration. Comparatively, its corresponding Ge 4s and Sn 5s states, which are also the same group IV dopants, strongly hybridize with the Ga 4s states, resulting in a flat edge of the conduction band, increased electron effective mass and decreased electron mobility (Figure 3).

Figure 3. Density of states diagram of undoped and different IV groups (Si, Ge, Sn) doped Ga₂O₃, obtained from DFT calculation.

      In short, the team has achieved Ga₂O₃ doping with the highest current carrier concentration in the world through Si doping. Combined with high-resolution hard X-ray photoelectron spectroscopy of synchrotron radiation and DFT theoretical calculation, the team clarified that Si is the most preferred n-type dopant of Ga₂O₃, and proposed a theoretical model of Si resonance doping mechanism. The results provide an important theoretical and experimental basis for the preparation of conductive gallium oxide single crystal substrate and the regulation of doping electrical properties of gallium oxide films. The relevant work was published in Physical Review B, a well-known journal of condensed matter physics, under the title "Direct determination of band-gap renormalization in degenerately doped ultrawide band gap β-Ga₂O₃ semiconductor". Zhang Jiye from Xiamen University and Joe Willis from University College London are the co-first authors of this paper. This work was funded and supported by the National Key Research and Development Program and the National Natural Science Foundation of China.

Original link：https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.205305

Professor Zhang Hongliang's research group homepage：https://khlzhang.xmu.edu.cn/