Band structures in orientation-controlled CuI thin films under epitaxial strain

Cuprous iodide (CuI) is a wide bandgap (similar to 3.1 eV) semiconductor of great recent interest because of its high transparency, high hole conductivity, large exciton binding energy, and sizable spin-orbit interaction. However, studies on single-crystalline thin films have been scarcely reported. Here, we report on the epitaxial growth of single-crystalline CuI films and their optical characterization to elucidate the modification of band structure caused by the anisotropic strain. Thin films were grown by molecular beam epitaxy on nearly lattice-matched (only 0.1%-mismatched) InAs substrates with (001), (110), and (111) crystal orientation, yielding pseudomor-phic structures with high lattice coherence and atomic-level flatness. Both reflectance and photoluminescence spectra exhibit sharp exciton profiles over a wide temperature range. We assign the exciton transition energies to deduce the band structure modifications associated with the epitaxial strain varying with temperature and growth orientation. Furthermore, we determine the deformation potentials which relate the strain to the band structure modification. The systematic studies of the strain effect for CuI thin films on various substrate planes will pave the way for the future optoelectronic application of CuI-based heterostructures.