Hexagonal boron nitride is an indirect bandgap semiconductor

Abstract

Hexagonal boron nitride is a wide bandgap semiconductor with a very high thermal and chemical stability often used in devices operating under extreme conditions. The growth of high-purity crystals has recently revealed the potential of this material for deep ultraviolet emission, with an intense emission around 215 nm. In the last few years, hexagonal boron nitride has been raising even more attention with the emergence of two-dimensional atomic crystals and Van der Waals heterostructures, initiated with the discovery of graphene. Despite this growing interest and a seemingly simple structure, the basic questions of the bandgap nature and value are still controversial. Here, we resolve this long-debated issue by bringing the evidence for an indirect bandgap at 5.955 eV by means of optical spectroscopy. We demonstrate the existence of phonon-assisted optical transitions, and we measure an exciton binding energy of about 130 meV by two-photon spectroscopy.Comment: To appear in Nature Photonic

Overtones of interlayer shear modes in the phonon-assisted emission spectrum of hexagonal boron nitride

“…1, we display the PL spectrum of bulk hBN above 5.7 eV (below 218 nm), at 10 K. In this spectral region, the PL signal corresponds to the intrinsic phonon-assisted emission with five phonon replicas having maxima at 5.76, 5.79, 5.86, 5.89, and 5.93 eV. These lines display an energy spacing exactly matching the splitting of the different phonon branches in the middle of the Brillouin zone (around T points) since radiative recombination in hBN must be assisted by the emission of phonons of wave-vector MK in order to fulfill momentum conservation [4,6]. In Refs.…”

“…In Refs. [4,6], the five phonon replicas at 5.76, 5.79, 5.86, 5.89, and 5.93 eV had been labeled LO, TO, LA, TA, and ZA, respectively, following the study of the vibrational properties in hBN reported in Ref. [7].…”

“…In our experimental setup, the sample is held on the cold finger of a closed-cycle cryostat for temperature-dependent measurements from 10 K to room temperature. We perform two-photon excitation spectroscopy, and the excitation beam is provided by the second harmonic of a Ti:Sa oscillator, tuned at 408 nm in resonance with the sharp peak in the two-photon excitation spectrum [4]. The spot diameter is of the order of 100 μm, with a power of 50 mW.…”

“…Graphite is a gapless compound with conduction and valence bands having extrema at the K points of the Brillouin zone, whereas hBN is a wide band-gap material (∼6 eV [4]) with an indirect configuration of very peculiar nature since the conduction band minimum sits at the M point while the valence band maximum is around the K point of the Brillouin zone. This is a major difference with cubic semiconductors where the valence band maximum is located at the zone center ( point), as in diamond, silicon, germanium etc....…”

“…2) instead of phonons * guillaume.cassabois@umontpellier.fr from the same valley as for the conduction band minimum in indirect cubic semiconductors. This original phenomenology in hBN results in an optical response involving phonons with a finite group velocity, leading to phonon replicas displaying sharp resonances [4] instead of the standard broad bands expected from the mapping of the excitonic quasicontinuum in phonon-assisted optical processes [5].…”