  Diamond has attracted attention as a next-generation semiconductor because of
its various exceptional properties such as a wide bandgap and high breakdown
electric field. Diamond field effect transistors, for example, have been
extensively investigated for high-power and high-frequency electronic
applications. The quality of their charge transport (i.e., mobility), however,
has been limited due to charged impurities near the diamond surface. Here, we
fabricate diamond field effect transistors by using a monocrystalline hexagonal
boron nitride as a gate dielectric. The resulting high mobility of charge
carriers allows us to observe quantum oscillations in both the longitudinal and
Hall resistivities. The oscillations provide important information on the
fundamental properties of the charge carriers, such as effective mass,
lifetime, and dimensionality. Our results indicate the presence of a
high-quality two-dimensional hole gas at the diamond surface and thus pave the
way for studies of quantum transport in diamond and the development of low-loss
and high-speed devices.
