Two-dimensional excitons in three-dimensional hexagonal boron nitride

Abstract

The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54mo, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

Nature of exciton transitions in hexagonal boron nitride

“…Recently, much progress has been made in the growth of h-BN in the forms of bulk crystals by high temperature/high pressure (HT/HP) 4,5,15 as well as by solution techniques 3,13 and epilayers by metal organic chemical vapor deposition (MOCVD). [7][8][9][10] Epitaxial h-BN layers with high optical qualities can be achieved, 7,20 but these materials generally lack the intrinsic FX transitions above 5.7 eV due to the presence of native and point defects.…”

“…All these transition lines have been ascribed to the intrinsic or FX transitions and labeled as S-series lines. 13,15 One of the possibilities is that these emission lines correspond to the exciton transitions involving different splitting bands such as A, B, and C valance bands in AlN and GaN. 14,24 However, the relative emission intensities of the excitonic transitions involving different splitting bands are related to the density of states (DOS) of the respective bands, decay characteristics, and the statistical distributions of [exp(ÀDE/kT)], where DE is the energy difference between the two bands and T is the measurement temperature.…”

“…The experimental band gap was determined in the past only by optical absorption, transmission, and indirectly from photoluminescence (PL) lifetime measurements with values scattered around 6 eV at low temperatures. [11][12][13] The band-edge transitions in h-BN are distinctly different from that of wurtzite (w)-AlN having a comparable energy bandgap (Eg ¼ 6.1 eV at 10 K). 14 Fundamental optical transitions including the nature of the intrinsic or free exciton transitions in h-BN are not yet well understood.…”

“…21 However, the detailed features of the FX transitions in h-BN epilayers are distinctly different from those in h-BN bulk crystals, 21 which is what remains to be investigated and understood. Currently, bulk h-BN crystals are small (less than 1 mm by 1 mm), [3][4][5]13,15 but the basic properties measured from the bulk materials set the benchmarks for the further development of wafer scale epilayers. Here, we report the properties of the FX transition and strong exciton-phonon coupling in h-BN bulk crystals probed by time- PL spectroscopy.…”

“…A deep UV time-resolved PL system was utilized to probe the emission properties. 13,20 The system consists of a frequency quadrupled 100 fs Ti:sapphire laser providing an excitation photon energy of 6.28 eV and a monochromator (1.3 m) in conjunction with a single photon counting detection system providing a timeresolution of 20 ps and a streak camera system providing a time-resolution of 2 ps.…”