Direct observation of the band structure in bulk hexagonal boron nitride

Abstract

A promising route towards nanodevice applications relies on the association of graphene and transition metal dichalcogenides with hexagonal boron nitride ($h\text{\ensuremath{-}}\mathrm{BN}$). Due to its insulating nature, $h\text{\ensuremath{-}}\mathrm{BN}$ has emerged as a natural substrate and gate dielectric for graphene-based electronic devices. However, some fundamental properties of bulk $h\text{\ensuremath{-}}\mathrm{BN}$ remain obscure. For example, the band structure and the position of the Fermi level have not been experimentally resolved. Here, we report a direct observation of parabolic dispersions of $h\text{\ensuremath{-}}\mathrm{BN}$ crystals using high-resolution angle-resolved photoemission spectroscopy (ARPES). We find that $h\text{\ensuremath{-}}\mathrm{BN}$ exfoliation on epitaxial graphene enables overcoming the technical difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at constant energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual $p$-type character of typical $h\text{\ensuremath{-}}\mathrm{BN}$.