【Member Papers】Unable phase stabilization optimization for achieving single-phase ε-Ga₂O₃ epitaxy

      Researchers from the Xidian University have published a dissertation titled "Unable phase stabilization optimization for achieving single-phase ε-Ga₂O₃ epitaxy" in Journal of Alloys and Compounds.

Project Support

      The work was supported by the General Program of Natural Science Foundation of China (Grant 62274134); The Fundamental Research Funds for the Central Universities (Grant YJSJ24020); National Key Research and Development Program (Grant Nos. 2021YFA0716400 and 2023YFB3609900); The National Science Fund for Distinguished Young Scholars (Grant 61925404); Key R&D Project in Xi’an City (Grant 2023JH-ZCGJ-0013); Aerospace Institute 771 Innovation Fund (Grant 771CX2023007); Interdisciplinary Cultivation Program of Xidian Uni versity (Grant 21103240003).

Background

      The research focuses on ε-Ga₂O₃, an ultrawide-bandgap semiconductor characterized by spontaneous polarization and high lattice symmetry. Due to its excellent optical, electrical, and thermal properties, Ga₂O₃ has attracted extensive attention for applications in optoelectronic devices, high-power electronics, and RF resonators. Among its polymorphs (α, β, γ, δ, ε, and κ), ε-Ga₂O₃ stands out for its superior electron mobility and high breakdown voltage. However, obtaining high-quality ε-Ga₂O₃ films remains challenging, as current growth techniques often lead to α–ε mixed-phase films on c-sapphire substrates, hindering device performance improvement.

      To address this issue, this study employs a mist chemical vapor deposition (mist-CVD) approach and introduces a tunable phase stabilization optimization strategy. By simultaneously regulating two critical growth parameters—growth temperature and O₂ partial pressure—this method enables controlled transformation from α–ε mixed-phase to phase-pure ε-Ga₂O₃. The approach achieves interface-mediated thermodynamic and kinetic control, promoting ε-phase stabilization while suppressing secondary phase formation. As a result, high-quality, atomically smooth, single-phase ε-Ga₂O₃ films with coherent ε (002) orientation are obtained. This work provides new insights into phase stabilization mechanisms in Ga₂O₃ epitaxy.

Abstract

      ε-phase gallium oxide (ε-Ga₂O₃), with its inherent spontaneous polarization and high lattice symmetry, is a promising candidate for ultrawide bandgap electronics and piezoelectric devices. However, achieving phase-pure ε-Ga₂O₃ remains a significant challenge due to competing nucleation pathways and polycrystalline orientation competition, often leading to mixed-phase formation and structural defects. In this study, we report a tunable phase stabilization optimization tailored for mist chemical vapor deposition (mist-CVD), involving the simultaneous control of growth temperature and O₂ partial pressure to finely modulate growth thermodynamics and kinetics. This approach effectively suppresses disordered phase orientations and promotes a controlled transition from α–ε mixed phases to a high-purity single ε-phase. The resulting ε-Ga₂O₃ films on c-sapphire exhibit a full width at half maximum (FWHM) of 0.073° and a root-mean-square (RMS) roughness of 0.75 nm, indicating high crystallinity and surface smoothness. High-resolution transmission electron microscopy (HRTEM) reveals a clear polycrystalline-to-epitaxial transition layer and confirms a coherent ε (002) out-of-plane orientation. Importantly, the application of this approach to surface acoustic wave (SAW) devices yields a center frequency of 1.69 GHz and an effective coupling coefficient (keff²) of 0.328 %, demonstrating the functional viability of the obtained films. This work establishes a reproducible and interface-sensitive pathway for ε-Ga₂O₃ epitaxy and underscores the importance of interface engineering and phase control strategies in enabling the practical deployment of ultrawide bandgap oxide semiconductors.

Highlights

      ● Phase-pure ε-Ga₂O₃films grown on c-sapphire by mist-CVD via interface-mediated control.

      ● Growth temperature and O₂pressure optimization enables selective ε-phase formation.

      ● Polycrystalline orientation competition guides ε/α transition via interfacial matching.

      ● Record FWHM 0.073°, RMS 0.75 nm, and 1.69 GHz SAW device performance achieved.

      ● Phase stabilization optimization unlocks new potential in ultrawide bandgap oxides.

Conclusion

      In this study, we identified a distinct polycrystalline orientation competition layer at the early stage of ε-Ga₂O₃ film growth, whose evolution is strongly influenced by growth temperature and O₂ partial pressure. By systematically tuning these parameters, we developed a phase stabilization optimization mediated by the film/substrate inter face, which modulates both thermodynamic and kinetic factors governing phase formation. Specifically, the interface between c-sapphire and Ga₂O₃ introduces a lattice mismatch and interfacial energy landscape that initially leads to mixed-phase nucleation. However, as the growth temperature increases to 620 ℃, the release of interfacial strain energy and improved lattice matching facilitate a transition from the α–ε mixed-phase to a single ε-phase. Concurrently, a higher O₂ partial pressure slows down the growth rate, yielding thinner poly crystalline transition layers and promoting vertical grain alignment. Together, these interface-governed effects suppress random Ga atom migration, reduce phase disorder, and enable selective stabilization of ε-Ga₂O₃. As a result, the obtained ε-Ga₂O₃ films exhibited a significantly reduced full width at half maximum (FWHM) of 0.073◦, surpassing the MOCVD benchmark value of 0.09◦, along with a root-mean-square surface roughness of just 0.75nm. These improvements demonstrate the capability of our approach to simultaneously enhance phase purity and surface morphology. Furthermore, surface acoustic wave (SAW) devices were successfully fabricated using the ε-Ga₂O₃ films as piezoelectric layers, achieving a resonant frequency of 1.69GHz and an effective electromechanical coupling coefficient (keff²) of 0.328%. These results validate the potential of our interface engineering- based phase stabilization optimization for enabling high-quality ε-Ga₂O₃ epitaxy and demonstrate its relevance for next-generation ultrawide bandgap electronic and piezoelectric applications.