Domain epitaxial matching of γ-CuI film grown on Al2O3(001) substrate via physical vapor transport

The lattice mismatch between γ-CuI and Al2O3 (sapphire) is as large as 27.1% due to differences in crystal systems and lattice constants. To achieve epitaxial growth on the initial surface of the substrate, lattice mismatch can be minimized through domain matching. In this work, the γ-CuI film was epitaxially grown on the Al2O3(001) substrate by employing the physical vapor transport technology. The morphology, structure, and rotation domain matching of the films were investigated via SEM, XRD, electron backscatter diffraction, and STEM/TEM. The results revealed that the film grew preferentially along the (111) plane, and the full width at half maximum of the (111) diffraction peak in the rocking curve was 0.45°, which indicated the high degree of crystallinity of the film. The surface of the γ-CuI film exhibited two kinds of triangular crystal domains with a relative rotation angle of ∼60°. The interior of the crystal domains possessed a step-like structure. The two types of triangular crystal domains in the film were attributed to different rotation domains, and the interface was ascribed to the ⟨111⟩ 60° twin boundaries. The epitaxial matching mode was found to be CuI ⟨112̄⟩ (111)//Al2O3 [1̄1̄0] (001), resulting in ∼4.6% lattice mismatch between γ-CuI and Al2O3 along the Al2O3 ⟨1̄10⟩ direction, which is significantly lower than previously reported data.