Enhanced mobility of p-type transparent semiconductor CuI thin films grown on thermal expansion matched substrates

Cuprous iodide (CuI) is a promising p-type transparent conductor with high hole mobility and excellent optical transparency. However, its practical application is hindered by the significantly lower mobility observed in thin films compared to bulk crystals, which is primarily attributed to defects formed during the growth process. In thin-film systems, strain is another critical yet often overlooked factor influencing carrier transport. In this study, we examine the impact of strain arising from thermal expansion mismatch in epitaxial CuI films grown by molecular beam epitaxy on CaF2 and Al2O3 substrates. Films on CaF2, whose thermal expansion coefficient closely matches that of CuI, exhibit metallic conduction and a record-high mobility of 33.9 cm2 V−1 s−1 at 2 K. In contrast, films on Al2O3, with a much larger thermal mismatch, show lower mobility and insulating behavior. Exciton spectroscopy reveals that the thermal mismatch introduces structural disorder during the cooling process, which degrades carrier transport. These results highlight the crucial importance of strain engineering, particularly thermal expansion matching, for improving the performance of CuI thin films.