Electronic structure of exfoliated and epitaxial hexagonal boron nitride

Hexagonal boron nitride (hBN) is an essential component in van der Waals heterostructures as it provides high-quality and weakly interacting interfaces that preserve the electronic properties of adjacent materials. While exfoliated flakes of hBN have been extensively studied using electron transport and optical probes, detailed experimental measurements of the energy- and momentum-dependent electronic excitation spectrum are lacking. Here, we directly determine the full valence-band (VB) electronic structure of micrometer-sized exfoliated flakes of hBN using angle-resolved photoemission spectroscopy with micrometer spatial resolution. We identify the pi- and sigma-band dispersions, the hBN stacking order, and determine a total VB bandwidth of 19.4 eV. We compare these results with electronic structure data for epitaxial hBN on graphene on silicon carbide grown in situ using a borazine precursor. The epitaxial growth and electronic properties are investigated using photoemission electron microscopy. Our measurements show that the fundamental electronic properties of hBN are highly dependent on the fabrication strategy.