Gallium oxide (Ga 2 O 3 ) 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-Ga 2 O 3 . Here, we demonstrate the epitaxial growth of metastable δ-Ga 2 O 3 thin films by mist chemical vapor deposition using β-Fe 2 O 3 buffer layers. X-ray diffraction (XRD) 2θ−ω scan pattern revealed that (004) κ-Ga 2 O 3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In 2 O 3 buffer layer, whereas (222) δ-Ga 2 O 3 grew on (222) β-Fe 2 O 3 . The β-Fe 2 O 3 buffer layer led to the epitaxial growth of the δ-Ga 2 O 3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe 2 O 3 and δ-Ga 2 O 3 triggered this growth. XRD analysis shows that δ-Ga 2 O 3 grew epitaxially on the β-Fe 2 O 3 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-Ga 2 O 3 grown on β-Fe 2 O 3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga 2 O 3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga 2 O 3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga 2 O 3 epitaxial thin film for the first time. KEYWORDS: δ-Ga 2 O 3 , β-Fe 2 O 3 , bixbyite structure, epitaxial growth, mist CVD

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