【International Papers】Demonstration of Bixbyite-Structured δ-Ga₂O₃ Thin Films Using β-Fe₂O₃ Buffer Layers by Mist Chemical Vapor Deposition

      Gallium oxide (Ga₂O₃) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga₂O₃. Here, we demonstrate the epitaxial growth of metastable δ-Ga₂O₃ thin films by mist chemical vapor deposition using β-Fe₂O₃ buffer layers. X-ray diffraction (XRD) 2θ–ω scan pattern revealed that (004) κ-Ga₂O₃ grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In₂O₃ buffer layer, whereas (222) δ-Ga₂O₃ grew on (222) β-Fe₂O₃. The β-Fe₂O₃ buffer layer led to the epitaxial growth of the δ-Ga₂O₃ thin film. The lattice mismatch between the equivalent crystal structures of β-Fe₂O₃ and δ-Ga₂O₃ triggered this growth. XRD analysis shows that δ-Ga₂O₃ grew epitaxially on the β-Fe₂O₃ buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga₂O₃ grown on β-Fe₂O₃ was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga₂O₃ exhibited a cubic bixbyite structure. The optical band gap of δ-Ga₂O₃ was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga₂O₃ epitaxial thin film for the first time.

Paper Link：https://pubs.acs.org/doi/10.1021/acsaelm.2c01750