Direct band-gap crossover in epitaxial monolayer boron nitride

Abstract

Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.

Optical absorption of single-layer hexagonal boron nitride in the ultraviolet

“…The computed gap with exchange corrections reads E src g = 7.6 eV. A value of η = 30 meV has been used, in agreement with the experimental measurements [30] at 10 K. The diffuse series (D−series) is due to excitons trapped in defects forming inside the gap; the sharp series (S−series) is due to (free) excitons.…”

“…with d the distance between hBN and HOPG substrate and K 0 (x) the modified Bessel function. We have verified numerically that for hBN at a distance of d = 3.5Å of the HOPG substrate (this distance corresponds to the measured step height (3.5Å) between HOPG and hBN [30,32] using AFM), the screened potential is well described by the approximation in Eq. (23), n = 1 E scr g (eV) r 0 (Å) d (Å) -1.546 eV 7.6( * ) 6.9( †) 3.5 Table 4.…”

“…For putting our theory to the test, we have compared the theoretical position of the excitonic energy levels of a hBN single-layer to photoluminescence (PL) of hBN single layer on highly oriented pyrolytic graphite (HOPG), published very recently [30]. In this paper, the authors were able to measure the PL of a single hBN layer in the frequency range λ ∈ [240, 200] nm, at a temperature of 10 K. They identified a broad deep-ultraviolet emission in the region λ ∈ [240, 210] nm.…”

“…( †) A self-consistent calculation using no approximations to the screened potential and with r 0 determined via a RPA calculation, gives r 0 = 7.2Å and E scr g ≈ 7.2 eV [41]. [30] and theoretical resonant absorption showing a peak at the energy level n = 1 given in Table 4. An effective mass m * = 0.6m 0 was used, in agreement with the estimations made in Sec.…”

Boron nitride for excitonics, nano photonics, and quantum technologies

“…Epitaxial hBN can be grown with different thicknesses from the monolayer (MLBN) to very thick stacking. We have shown that the band gap of MLBN is direct at K in reciprocal space [40] while the fundamental band gap of bulk hBN is indirect between M and K points of the first Brillouin zone [41,42]. The high internal quantum efficiency is correlated to the strong exciton-phonon coupling with an efficient and fast phonon emission that relaxes the k-dependent selection rule traditionally preventing indirect band gap semiconductors to strongly emit light via intrinsic processes [43][44][45][46][47][48].…”