【Member Papers】Structural and optical properties of high crystalline quality orthorhombic к-Ga₂O₃ heteroepitaxial films grown by HVPE

      Researchers from the Tianjin University of Technology have published a dissertation titled "Structural and optical properties of high crystalline quality orthorhombic к-Ga₂O₃ heteroepitaxial films grown by HVPE" in Applied Physics A.

Project Support

      This work was supported by the Major Project of Tianjin Science and Technology (Grant No. 18ZXJMTG00230) and the Key Research and Development Program of Tianjin (Nos. 24YFXTHZ00200 and 24YFXTHZ00210).

Background

      Gallium oxide (Ga₂O₃) has become a pivotal material in next-generation power electronics and deep-ultraviolet (UV) optoelectronics because of its exceptional properties: an ultrawide bandgap (4.6–5.3 eV), a high breakdown electric field (>8 MV/cm), excellent optical transparency, and robust chemical stability. Ga₂O₃ exhibits multiple polymorphs, including the α-, β-, γ-, δ- and κ-Ga₂O₃ phases. Although the most thermodynamically stable monoclinic β-phase has been primarily researched, the orthorhombic κ-phase (space group Pna21) exhibits distinctive performance advantages, such as a high dielectric constant, spontaneous polarization, and piezoelectric properties. These exceptional characteristics enable its use in innovative devices, including high-mobility transistors and piezoelectric sensors.

Abstract

      Herein, high crystalline quality orthorhombic κ-Ga₂O₃ epitaxial films were grown on c-Al₂O₃ substrates via hydride vapor phase epitaxy. A pure-phase κ-Ga₂O₃ film deposited at 800 ℃ exhibited exceptional crystalline quality, with a symmetric rocking-curve full width at half maximum of 379″. The out-of-plane epitaxial relationship between the κ-Ga₂O₃ film and c-Al₂O₃ substrate was determined to be κ-Ga₂O₃ (001)//Al₂O₃ (0001), and the in-plane epitaxial relationships were κ-Ga₂O₃ [010]//Al₂O₃ [10_10] and κ-Ga₂O₃ [110]//Al₂O₃ [1_100]. The film deposited at 800 ℃ showed an average visible-light transmittance of 70%, an optical bandgap of 4.72 eV. Photoluminescence (PL) spectra show that the films exhibit strong ultraviolet (UV)-green emission in the wavelength range of 300 to 700 nm. The Hall mobility of this film was 25 cm² V⁻¹ s⁻¹, with a carrier concentration of 3.34 × 10¹⁸ cm⁻³.

Conclusion

      Herein, through the optimization of the growth-zone temperature, we prepared high crystalline quality orthorhombic κ-Ga₂O₃ epitaxial films on c-plane sapphire substrates via HVPE at 800 ℃ and investigated their surface morphology, crystal structure, crystalline quality, and optoelectronic properties. XRD analysis confirms that the sample prepared at 800℃ is a phase-pure κ-Ga₂O₃ epitaxial film, and it is observed that the epitaxial film undergoes a κ→β phase transition with increasing temperature. SEM characterization reveals that the κ-Ga₂O₃ epitaxial film exhibits excellent surface morphology. The out-of-plane epitaxial relationship was determined to be κ-Ga₂O₃ (001)//Al₂O₃ (0001), and the in-plane relationships were identified to be κ-Ga₂O₃ [010]//Al₂O₃ [10_10] and κ-Ga₂O₃ [110]//Al₂O₃ [1_100]. Notably, the prepared film showed a symmetric rocking-curve FWHM of 379″. The results of HRTEM and SAED confirmed an ordered atomic arrangement in the film. Furthermore, the film showed an average transmittance in the visible range of 70% and an optical bandgap of 4.72 eV. The Hall mobility of the film was 25 cm² V⁻¹ s⁻¹, with a carrier concentration of 3.34×10¹⁸ cm⁻³. The comprehensive characterization results provide critical insights into the structural, optical, and electrical properties of the pure-phase metastable κ-Ga₂O₃ films, laying a foundation for future device applications.