Molecular beam epitaxy of high-quality CuI thin films on a low temperature grown buffer layer

We show a growth of high-quality thin films of a wide bandgap semiconductor copper iodide (CuI) on Al2O3 substrates by molecular beam epitaxy. Employing a thin buffer layer deposited at a lower temperature (160 °C) prior to the main growth, the maximum growth temperature is elevated up to 240 °C, resulting in a significant improvement in the crystallinity as verified by sharp x-ray diffraction peaks as well as a step-and-terrace structure observed by atomic force microscopy. Optimum films show more intense free exciton emission in photoluminescence spectra than others, implying the suppression of defects. These results indicate that the fabrication process developed in this study is quite effective at realizing high-quality CuI thin films.