Nature of exciton transitions in hexagonal boron nitride

Abstract

In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride (h-BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. Here, the free exciton transition (FX) in h-BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h-BN, we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h-BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h-BN.

Bandgap and exciton binding energies of hexagonal boron nitride probed by photocurrent excitation spectroscopy

“…4(a) and 4(c), the binding energy of the impurity bound exciton can also be obtained: E bx ¼ 5.69 eV À 5.5 eV % 0.2 eV. This value again agrees well with the energy difference between the free and impurity bound exciton emission lines observed in h-BN bulk crystals 34 and epilayers. 15 Based on these results, we have constructed in Fig.…”

“…3. In addition to the direct observation of the band-to-band transition reported here, all previous experimental results, including (a) the PL emission intensity of h-BN being two orders of magnitude larger than that of AlN, 4,14 (b) huge optical absorption coefficient in the order of 7.5 Â 10 5 /cm for above bandgap photons, 12,13 (c) lasing of h-BN under e-beam pumping 3 and (d) recent identification of only one direct intrinsic free exciton transition with all of its phonon modes being associated with the center of the Brillouin zone in h-BN bulk crystals 34 all support the view that h-BN is a direct bandgap semiconductor. An interesting remaining question is concerning the observation of exciton dissociation at an applied voltage as low as 20 V (corresponding to an electric field $2.2 Â 10 4 V/ cm), considering the very large exciton binding energy in h-BN.…”

Temperature effects in 3.9 eV photoluminescence of hexagonal boron nitride under band-to-band and subband excitation within 7–1100 K range

“…37 Later, a lot of articles have also reported on luminescence bands near the edge of the bandgap in h-BN involving impurity-bound (or trapped) excitons. 15,25,[38][39][40][41] Let us look into the process of selective excitation of the (ONCN)-complex using the model proposed and describe, step by step, the transition of e between the C and O impurity levels.…”

QED theory of electron beam-induced electronic excitation and its effect on sputtering cross sections in 2D crystals

“…Fitting our cross section curves to the data of Cretu et al yields an excellent agreement if the predicted excitation lifetime is set to τ ∼ 240 fs. This predicted lifetime is much shorter than the reported excitation lifetime of ∼0.75 ns in pristine hBN [36], indicating that the sputtering process can significantly reduce the excitation lifetimes of hBN. We suspect that the atomic motion gives rise to non-radiative relaxation pathways that are not explicitly accounted for here.…”