【Domestic Papers】Research on nitrogen annealing of V-doped β-Ga₂O₃ single crystals grown by EFG

      Researchers from the Micius Laboratory and Shanghai Institute of Optics and Fine Mechanics have published a dissertation titled "Research on nitrogen annealing of V-doped β-Ga₂O₃ single crystals grown by EFG" in Journal of Alloys and Compounds.

Project Support

      This work is supported by the National Natural Science Foundation of China (Grant No. 52488301), and Shanghai Strategic Frontier Special Project (Grant No. 24DP1500100). The authors also thank Hangzhou Fujia Gallium Technology Co., Ltd. for helping in crystal growth and processing.

Background

      Ultra-wide bandgap semiconductors have progressed rapidly due to their ability to meet stringent requirements in extreme environments, including high voltage, high temperature, and strong corrosion―conditions where traditional silicon materials fall short. Emerging monoclinic beta-phase gallium oxide (β-Ga₂O₃) has garnered significant attention due to its exceptional properties: an ultra-wide bandgap (E_g ≃ 4.9 eV), high estimated critical electric field (Ec ≃ 8 MV cm^–1), and high Baliga’s figure of merit (BFOM ≃ 3400). The potential for large-scale commercial production of single-crystal substrates positions β-Ga₂O₃ as an ideal candidate material for high-power, high-efficiency electronic devices and solar-blind photodetectors. These applications demand exceptionally high standards across all aspects of the core single-crystal substrates.

Abstract

      The regulation of electrical and optical properties in ultra-wide bandgap beta-gallium oxide (β-Ga₂O₃) through the introduction of dopants and thermal annealing has been a focal point in semiconductor research. Herein, a high-quality V-doped β-Ga₂O₃ crystal was synthesized using the edge-defined film-fed growth (EFG) technique. A variety of characterization methods were employed to systematically examine changes in the crystal structural, optical, and electrical properties of the V-doped β-Ga₂O₃ before and after annealing in an N₂ atmosphere. Annealing treatment significantly improved crystal quality. Following N₂ annealing, carrier concentration reduced rather than enhanced, infrared region transmittance increased, and mid-to-high frequency Raman phonon peak intensities were altered, which were ascribed to the formation of Ga vacancies, the destruction of carbon DX centers, and the capture of electrons by interstitial Ga. Variations in the concentrations of interstitial Ga and Ga vacancy led to an enlargement in the area ratio of the blue luminescence peak. These findings provide deeper insight into the effects of N₂ annealing on the crystal structure and properties of V-doped β-Ga₂O₃.

Conclusions

      In summary, the 2-inch V-doped β-Ga₂O₃ single crystal with (100) plane was grown by the EFG technique. Rocking curves, Hall effect, XPS, optical transmittance, Raman spectroscopy, and PL properties were performed on samples before and after N₂ heat treatment. The FWHM values of 39.6″ and 28.8″ demonstrate that the crystalline quality of the annealed crystals was superior to that of the unannealed ones. The post-annealing decline in free electron concentration, enhancement in near-infrared transmittance, and variation in Raman peak intensity can be explained by the formation of V_Ga as the deep acceptor and the disruption of carbon DX centers as the shallow donor by introduced N atoms. Concurrently, Ga_i (Ga³⁺) trapped electrons and converted to Ga²⁺, further contributing to the reduction in carrier concentration after annealing. Room-temperature PL spectra reveal an increased proportion of blue emission peaks following N₂ annealing, attributable to heightened concentrations of Ga_i and V_Ga.

Fig. 1. (a) EFG apparatus and (b) tube furnace for β-Ga₂O₃ bulk single crystal growth and annealing.

Fig. 2. (a) High-resolution XRD patterns of undoped and V-doped β-Ga₂O₃ crystals. (b) Photograph of 2-inch V-doped β-Ga₂O₃ crystal.

Fig. 3. (a-c) Rocking curves, (d-f) 2D and (g-i) 3D AFM images of as-grown and N₂-annealed V-doped β-Ga₂O₃ crystals.

Fig. 4. (a) I-V characteristic of V-doped β-Ga₂O₃ crystal. (b) Monoclinic structure of the β-Ga₂O₃ unit cell.

Fig. 5. (a-c) C 1 s, (d-f) O 1 s, (g-i) Ga 3d XPS core level of as-grown and N₂-annealed V-doped β-Ga₂O₃ single crystals.

Fig. 6. (a) Transmission spectra and (b) optical band gaps of the undoped, as-grown, and N₂-annealed V-doped β-Ga₂O₃ crystals.

Fig. 7. Raman spectra of the undoped, as-grown, and N₂-annealed V-doped β-Ga₂O₃ crystals.

Fig. 8. RT-PL spectra of V-doped β-Ga₂O₃ single crystals: (a) as-grown, (b) N₂-annealed (20 h), (c) N₂-annealed (40 h), and N₂-annealed (over 40 h). (d) Relationship between annealing time and the proportion of each luminescence peak area in V-doped β-Ga₂O₃ samples.

 

DOI：

doi.org/10.1016/j.jallcom.2026.186909