Phonon symmetries in hexagonal boron nitride probed by incoherent light emission

Abstract

Layered compounds are stacks of weakly bound two-dimensional atomic crystals, with a prototypal hexagonal structure in graphene, transition metal dichalcogenides and boron nitride. This crystalline anisotropy results in vibrational modes with specific symmetries depending on the in-plane or out-ofplane atomic displacements. We show that polarization-resolved photoluminescence measurements in hexagonal boron nitride reflect the phonon symmetries in this layered semiconductor. Experiments performed with a detection on the sample edge, perpendicular to the c-axis, reveal the strong polarization-dependence of the emission lines corresponding to the recombination assisted by the three acoustic phonon modes. We elucidate the dipole orientation of the fundamental indirect exciton. We demonstrate evidence of the so-far missing phonon replica due to the optical out-ofplane phonon mode.

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

“…1, the signal intensity of the X ZA/ZO 1 (T ) phonon replica is the smallest one because of the selection rules controlling radiative recombination assisted by phonon emission in hBN. Indeed, the ZA/ZO 1 phonon replica is forbidden by symmetry in our experimental geometry, where the emission wave vector is parallel to the c axis [4,9].…”

Exciton-phonon interaction in the strong-coupling regime in hexagonal boron nitride

“…The demonstration of the strong coupling regime in bulk hBN raises the interesting question of its link with the striking bright emission [28,29] despite the indirect bandgap [21,22]. Thanks to the efficient exciton-phonon interaction, we infer that the phonon-assisted recombination is fast enough to by-pass non-radiative relaxation, therefore leading to an intense emission in the deep ultraviolet.…”

Isotope engineering of van der Waals interactions in hexagonal boron nitride

“…In this wide bandgap semiconductor [19][20][21], the intrinsic optical response lies in the deep ultraviolet around 6 eV (200 nm), as seen in Fig.2(a) displaying the PL spectra in 10 BN, Na BN and 11 BN at 8K. Because hBN is an indirect bandgap semiconductor [22][23][24][25], phonon emission is required for momentum conservation with luminescence. As a matter of fact, the PL spectrum is composed of many emission lines, reflecting the various paths for recombination assisted by phonon emission.…”

“…The larger the phonon energy, the larger the energy detuning with the indirect bandgap at around 5.95 eV [22], and thus the lower the energy of the phonon replica. In Fig.2(a), the four peaks at 5.76, 5.79, 5.86, and 5.89 eV correspond to the recombination assisted by the emission of phonons of decreasing energy, more specifically one LO, TO, LA and TA phonon, respectively [22][23][24][25].…”