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Guillaume Cassabois - CNRS, Montpellier University

Two-photon spectroscopy in hexagonal boron nitride


Hexagonal boron nitride (hBN) is a wide bandgap semiconductor with a large range of basic applications relying on its low dielectric constant, high thermal conductivity, and chemical inertness. The growth of high-quality crystals in 2004 has revealed that hBN is also a promising material for light-emitting devices in the deep ultraviolet domain, as illustrated by the demonstration of lasing at 215 nm by accelerated electron excitation [1], and also the operation of field emitter display-type devices in the deep ultraviolet [2]. With a honeycomb structure similar to graphene, bulk hBN has recently gained tremendous attention as an exceptional substrate for graphene with an atomically smooth surface, and more generally, as a fundamental building block of Van der Waals heterostructures [3].

In spite of this rising interest for hBN and the large number of studies devoted to this material of seemingly simple crystal structure, the very basic question of the bandgap nature is still controversial. There is a strong contrast in the literature between ab initio band structure calculations predicting an indirect bandgap crystal and optical measurements concluding to a direct one [1].

I will discuss our recent experiments by two-photon spectroscopy demonstrating that hBN is an indirect bandgap material. I will show that the optical properties of hBN are profoundly determined by phonon-assisted transitions involving either virtual or real excitonic states. I will present our measurements of the exciton binding energy by two-photon excitation, leading to the estimation of 6.08 eV for the single-particle bandgap in hBN.

REFERENCES :

[1] K. Watanabe, T. Taniguchi, and H. Kanda, Nat Mater 3, 404-409 (2004).

[2] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, and M. Taniguchi, Nat Photon 3, 591 (2009).

[3] A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).

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