Direct observation of the lowest indirect exciton state in the bulk of hexagonal boron nitride

Abstract

We combine electron energy-loss spectroscopy and first-principles calculations based on densityfunctional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of ∼ 100 meV and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the ΓK-direction of the hexagonal Brillouin zone.Our data support the recent report by Cassabois et al. [1] who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor. 1 arXiv:1706.04806v1 [cond-mat.mes-hall] 15 Jun 2017Hexagonal Boron Nitride (h-BN) is the binary analog of graphite. Like its carbon counterpart, it consists of sp 2 -hybridized hexagonal layers that are stacked along the crystallographic c-axis. This leads to many similarities between these two materials like the applicability as dry lubricants or the possibility to wrap the hexagonal sheets into nanotubes [2,3].More recently, h-BN came into focus in the field of 2D semiconductors, either as a substrate to boost the performance of graphene devices [4] or as an important ingredient in so called van der Waals heterostructures [5]. These overall similarities between the carbon and BN-case notwithstanding, the electronegativity difference between the boron-and nitrogen-atoms has profound implications which show up most prominently in the electronic structure, in particular the fundamental band gap. Though its apparent simplicity in terms of structure and electronic properties, there has been an ongoing argument about the nature and size of the band gap. For a long time, there has been a strong controversy about whether the fundamental gap is direct or indirect and reported values on the size of the gap E G range from below 4 eV to more than 7 eV, depending on the sample quality and the employed method (see e.g. [6,7] for extended compilations of available data on E G ).With time, in particular in the theoretical community, consensus emerged that the gap is indirect [8-10] but still, calculated values differed substantially between 3.9 eV and 5.95 eV.These predictions were, however, strongly challenged by the remarkable observation of a strong photoluminescence (PL) signal in h-BN reported in 2004 [11] which promised great potential for lasing in the deep ultraviolet. Recently, Cassabois et al. [1] took an important step forward in resolving this long-standing debate. They inferred -although only indirectly -the existence of the lowest indirect exciton (iX in their nomenclature) at an energy of 5.955 eV from a careful analysis of detailed PL data thereby establishing h-BN as an indirect semiconductor also from an experimental perspective. They also could show that the presence of sharp excitonic absorption (and/or emission) in a material with an indirect band gap -which appears to contradict the traditional understanding [12] -occurs as a conseq...

Direct and indirect excitons in boron nitride polymorphs: A story of atomic configuration and electronic correlation

“…A second interesting aspect is the peculiar dispersion of the lowest conduction band along the KM direction, which is the most relevant direction for the optical properties of this material. 27,32,55 The qualitative behaviours of the AA' and the ABC phases are similar: away from K the lowest conduction band disperses almost linearly and has a minimum at M , while in the case of the AB stacking the dispersion is flatter and has a concave shape away from K. Still, beyond a local maximum between K and M it also attains its minimum at M . The dispersion of the bottom conduction is 0.75 eV in the AA', 0.51 eV in the ABC and only 0.10 eV in the AB.…”

Direct band-gap crossover in epitaxial monolayer boron nitride

“…An isolated monolayer of hexagonal boron nitride (mBN) is predicted theoretically to be a direct-gap semiconductor with a band-gap of around 6 eV 8,9 and with indirect–direct crossover similar to that of molybdenum disulphide 3 . It has been demonstrated that bulk hBN is an indirect-gap semiconductor with a band-gap of 5.95 eV 10–15 . The investigation of the direct-gap properties of mBN is an important issue, not only from a fundamental point of view but also for applications in DUV optoelectronics, with the exciting prospect of it becoming an active layer with highly efficient light–matter coupling in the DUV.…”

Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN

“…To elucidate this controversy, the ab initio Bethe-Salpeter equation (BSE) has been solved to obtain the exciton dispersion E exc (Q), with Q being the exciton momentum. Quasiparticle energies have been computed within the perturbative GW method and further blue shifted by 0.47 eV on the basis of recent electron energy loss spectroscopy (EELS) experiments [32]. The statedependent corrections of GW ensure an accurate dispersion at the single-particle level.…”