IntroductionUltra-wide bandgap (UWBG) semiconductor materials have attracted extensive attention in the fields of optoelectronics, sensing systems, and high-power devices due to their high critical electric field, high-temperature resistance, and excellent photoelectric properties. In this paper, high-quality Sn⁴⁺-doped ZnGa₂O₄ crystals with a volume of approximately 10 cm³ were successfully obtained by the VGF method. The crystal structure of the grown Sn⁴⁺: ZnGa₂O₄ crystals was detected by single crystal X-ray diffraction (XRD), and their crystal quality was studied by high-resolution X-ray diffraction (HRXRD). In addition, the optical and electrical properties of the grown Sn⁴⁺: ZnGa₂O₄ crystals were investigated by UV-Vis spectroscopy, XPS, and Hall measurements. The results show that the optical properties are affected by the Sn⁴⁺ concentration, and it is demonstrated for the first time that the electrical properties of ZnGa₂O₄ single crystals can be controlled by the concentration of doped Sn⁴⁺. The carrier concentration of the grown Sn⁴⁺-doped ZnGa₂O₄ single crystal at room temperature is higher than that of previously studied ZnGa₂O₄ single crystals (1.39 × 10¹⁹ cm^–3) and Sn⁴⁺-doped β-Ga₂O₃ (3 × 10¹⁸ – 3 ×10¹⁹ cm^–3). The improved photoelectric properties of ZnGa₂O₄ crystals after Sn⁴⁺ doping are expected to find applications in high-power devices and optoelectronic devices.MethodsZnGa₂O₄ bulk single crystals doped with Sn⁴⁺ were grown by the VGF method. High-purity (99.999%) Ga₂O₃, ZnO and SnO₂ powders were used as starting materials. The three powders were mixed for 80 h at a speed of 40 revolutions per minute, and then the mixed powders were cold-pressed into blocks at a pressure of 2000 bar and calcined in air at 600 ℃ in an alumina crucible for 16 h. Subsequently, ZnGa₂O₄ bulk single crystals doped with Sn⁴⁺ were grown in a crucible with an inner diameter of 60 mm and a height of 60 mm. The furnace was heated to 1900 ℃at a rate of 250 ℃/h in an atmosphere of argon and 12% oxygen, and maintained at this temperature for about 2 h. Then the furnace was cooled slowly at a rate of 2–3 ℃ per minute. Sn⁴⁺ doped ZnGa₂O₄ bulk single crystals were obtained.Results and discussionXRD analysis indicates that the Sn⁴⁺-doped ZnGa₂O₄ crystal maintains a pure spinel phase without any impurity phases, and the lattice constant slightly increases due to the introduction of Sn⁴⁺. The high-resolution X-ray diffraction (HRXRD) results show that the full width at half maximum (FWHM) of the rocking curve of the crystal is only 82 arcsec, indicating excellent crystal quality. X-ray photoelectron spectroscopy (XPS) analysis confirms that Sn⁴⁺ mainly enters the octahedral sites of the lattice by replacing Ga³⁺. X-ray fluorescence spectroscopy (XRF) analysis indicates that Sn⁴⁺ is successfully doped into the crystal, but the actual doping concentration is lower than the theoretical value due to the volatility of SnO₂. Optical performance tests show that Sn⁴⁺ doping slightly reduces the optical band gap of ZnGa₂O₄ crystals and increases the absorption in the near-infrared band, which is related to the increase in carrier concentration. Hall measurement results show that Sn⁴⁺ doping significantly increases the carrier concentration of the crystal (up to 3.32×10¹⁹ cm^–3) and reduces the resistivity (0.006 Ω?cm), outperforming undoped ZnGa₂O₄ and Sn⁴⁺-doped β-Ga₂O₃ crystals.ConclusionsIn this study, high-quality Sn⁴⁺-doped ZnGa₂O₄ single crystals with a volume of 10 cm³ were successfully grown by the vertical gradient freezing (VGF) method. The obtained Sn⁴⁺-doped ZnGa₂O₄ crystals showed no significant shift in diffraction peaks and a full width at half maximum (FWHM) of the rocking curve as low as 82 arc seconds, confirming the extremely high quality of the single crystals. X-ray fluorescence spectroscopy (XRF) and X-ray photoelectron spectroscopy (XPS) analyses respectively confirmed the contents of Zn, Ga, O, and Sn elements, as well as the fact that Sn⁴⁺ were mainly incorporated into the ZnGa₂O₄ crystal by replacing Ga³⁺ at octahedral sites in the spinel structure. With the increase of Sn⁴⁺ doping concentration, the optical band gap of ZnGa₂O₄ crystals decreased to 4.58 eV and 4.56 eV, respectively. The introduction of Sn⁴⁺ did not cause significant changes in the cutoff absorption edge at about 275 nm, but increased absorption in the near-infrared wavelength range, which was related to the increase in carrier concentration. Hall measurements verified the effect of Sn⁴⁺ doping on the electrical properties of ZnGa₂O₄ single crystals. The results indicated that the introduction of Sn⁴⁺ significantly increased the carrier concentration of ZnGa₂O₄ crystals (2.65×10¹⁹–3.32×10¹⁹ cm^–3) and decreased the resistivity (0.006–0.008 Ω·cm). These properties were superior to undoped ZnGa₂O₄ single crystals and Sn⁴⁺-doped β-Ga₂O₃ single crystals. Overall, the Sn⁴⁺-doped ZnGa₂O₄ crystals grown in this study demonstrated significant application potential in power devices and solar-blind detection, and are expected to play a key role in the development of future high-power and high-frequency electronic and optoelectronic devices.