Domain epitaxy of p-type transparent conducting γ-CuI films on MgO (111) substrates via physical vapor transport

γ-CuI is an intrinsically p-type semiconductor with considerable potential for transparent optoelectronic devices owing to its wide bandgap (∼3.1 eV) and high carrier mobility [>40 cm2/(V s) in bulk]. However, the epitaxial growth of high-quality γ-CuI films is hindered by the large lattice mismatch with conventional transparent single-crystal substrates. In this study, we demonstrate the direct domain epitaxy of γ-CuI films on MgO (111) substrates via physical vapor transport (PVT), despite a conventional lattice mismatch of 43.4%. The films exhibit a preferred out-of-plane (111) orientation and triangular crystalline domains with distinct terrace-and-step features. X-ray diffraction φ-scan and electron backscatter diffraction analyses reveal two in-plane domains rotated by 60° relative to each other. Combined with transmission electron microscopy results, these analyses establish the epitaxial relationship between the γ-CuI film and the MgO substrate as γ-CuI ⟨112¯⟩ (111)//MgO [01¯1] (111) and identify an interfacial 4/5 lattice-plane matching relationship, which results in a residual mismatch of approximately 0.57%. The films exhibit an average visible transmittance of more than 85% and an optical bandgap of approximately 3.01 eV. Hall measurements reveal p-type conductivity, with a hole mobility of 14.93 cm2/(V s) and an electrical conductivity of 0.70 S/cm. These results demonstrate that the direct domain epitaxy of γ-CuI on a highly lattice-mismatched MgO (111) substrate is achievable through interfacial multiplane matching using the PVT method.