Direct Observation of the Lowest Indirect Exciton State in the Bulk of Hexagonal Boron Nitride

Abstract

We combine electron energy-loss spectroscopy and first-principles calculations based on density-functional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of $\sim 100\text{meV}$ and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the $\Gamma \text{K}$-direction of the hexagonal Brillouin zone. Our data support the recent report by Cassabois \emph{et al.} (Nat. Photon. \textbf{10}, 262) who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor.

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• Boron nitride substrates for high-quality graphene electronics
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• Momentum dependence of the excitons in pentacene