【Domestic News】New Progress in Research on Gallium Oxide by the Research Team of Shandong University

Shandong University · Study on optical and electrical properties of Ni-doped β-Ga₂O₃ single crystals

      As one of the ultra-wide bandgap semiconductor materials, Ga₂O₃ has a bandgap of up to 4.8eV and a theoretical breakdown field strength of 8 MV/cm. It is the next generation of ultra-wide bandgap semiconductor materials after GaN and SiC, and has broad application prospects in high voltage power control, radio frequency communication, solar blind detection, harsh environment signal processing and so on. In recent years, breakthroughs have been made in the 4-6-inch Gallium Oxide single crystal growth technology, which has greatly promoted the research of Gallium Oxide related materials and devices, and is becoming an international research and industrial hot spot in the field of ultra-wide band gap semiconductors.

      The research group of Professor Jia Zhitai and Professor Tao Xutang of the State Key Laboratory of Crystal Materials, Shandong University published a paper entitled "Optical and electrical Properties of Ni-doped β-Ga₂O₃ Single Crystal" in the 8th issue of the Journal of Artificial Crystals, 2023 (First author: Chen Shaohua; Corresponding author: Mu Wenxiang, Jia Zhitai). In this paper, Ni-doped β-Ga₂O₃ single crystal was grown by EFG method. The crystal has high crystal quality, no obvious light absorption in near infrared band, semi-insulating electrical properties, optical band gap of about 4.74eV, ultraviolet cutoff is still in the solar blind band, and can be used to prepare high temperature, high pressure and high power devices.

      The Ni-doped β-Ga₂O₃ crystal sample obtained by EFG method is yellowish-brown as a whole (see Figure 1(a)). According to the ICP test results (see Table 1), it can be seen that the actual doped concentration of Ni is 0.00645%, and the doped color is relatively uniform. The results of PXRD test (see Figure 1(b)) show that all the grown crystals are β phase, and no other heterophase exists. Laue diffraction results at different points (see Figure 2) show that the crystals have good monocrystalline properties and no polycrystals exist in the interior.

Figure1: Ni-doped β-Ga₂O₃ single crystal photograph (a) and its PXRD pattern (b)

Table 1：ICP test results of Ni-doped β-Ga₂O₃ crystals

FIG. 2 Laue diffraction patterns at different positions of Ni-doped β-Ga₂O₃ single crystal (100) surface

The ultraviolet-visible transmission spectrum of 0.5mm Ni-doped β-Ga₂O₃3 crystal (see Figure 3) shows that its ultraviolet cutoff edge is 252.9 nm and its optical band gap is 4.74 eV, which has little change compared with the intrinsic GaO crystal (4.76 eV), and the ultraviolet cutoff edge is still in the solar blind band of 200~280 nm.

FIG. 3 UV-VIS spectra of Ni-doped β-Ga₂O₃ single crystal. (a) transmissivity spectra; And (b) Tauc diagrams of (α²hν) and hν

      The infrared transmittance spectra of Ni-doped β-Ga₂O₃ crystals (see Figure 4) show that they maintain high transmittance in both infrared and near-infrared bands. The cathodic fluorescence spectrum (see Figure 5) shows that the maximum peak intensity of the crystal at 240~600 nm is 394 nm, which is 24.1 nm redshifted compared with the intrinsic Ga₂O₃ crystal. There is an obvious peak at 560~800 nm, and the maximum peak intensity appears at 695.1 nm. It shows that Ni doping makes GaO₃ crystal have certain wideband NIR luminescence characteristics, which provides the possibility for β-Ga₂O₃ crystal to be used in wideband NIR luminescence devices.

FIG. 4 Infrared transmittance spectra of Ni-doped β-Ga₂O₃ single crystal

Figure 5 CL spectrum test results of Ni-doped β-Ga₂O₃ single crystal. (a) Ultraviolet-visible band; And (b) vision-near-infrared

CONCLUSION

      In this paper, high quality Ni-doped β-Ga₂O₃ single crystals have been grown by EFG. XRD pattern and Laue diffraction pattern show that the crystal quality is high and the crystal structure is not changed due to doping. There is no obvious light absorption in the near infrared band of the crystal, and it has semi-insulating electrical properties. Its optical band gap is about 4.74eV, and the ultraviolet cutoff edge is still in the sun-blind band. As a semi-insulating substrate, the crystal can be used to prepare high temperature, high pressure and high power devices. In this study, we found the broadband near-infrared luminescence characteristics of Ni-doped β-Ga₂O₃₃ single crystal in 600-800 nm band by CL spectrum, indicating that it has a high application prospect in the field of wideband near-infrared, and provides a reference for the rich and rapid development of β-GaO devices.

Analysis of stress field of β-Ga₂O₃ crystal sawing with diamond wire saw

Setting: Shandong Science and Technology Service Development and Promotion Center; School of Mechanical Engineering, Shandong University

Abstract: As a typical hard and brittle material, Gallium Oxide crystal (β-Ga₂O₃) is easy to crack during processing. Diamond wire saw is the main way to produce β-Ga₂O₃ wafers. Micro-crack damage layer will be produced on the surface of the wafers during slicing processing. Micro-cracks will expand under the action of stress, resulting in material breakage and fracture. In this paper, a finite element model of multi-wire cutting of β-Ga₂O₃(010) crystal surface with diamond wire saw was established. The distribution of mechanical stress, thermal stress and thermal coupling stress during sawing process was studied. The influence of different parameter combinations under sawing speed, feed rate and constant speed ratio on thermal coupling stress was analyzed. The results show that the thermal stress generated by sawing heat dominates the thermal coupling stress during sawing process, and the mechanical stress caused by sawing force accounts for a relatively small proportion, but the mechanical stress will affect the distribution of the thermal coupling stress, and the increase of the wire sawing speed and feed speed will cause the increase of the thermal coupling stress.

Sawing β-Ga₂O₃ crystal finite element model