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Hexagonal Boron Nitride, direct bandgap or indirect bandgap?

Feng Wu

February 27, 2017 - Posted in Discussion
Hexagonal Boron Nitride, direct bandgap or indirect bandgap

Hexagonal boron nitride has attracted a lot of attentions because of its unique properties, such as a wide bandgap, low dielectric constant, high thermal conductivity, and chemical inertness. In spite of this rising interest in hBN and the large number of studies devoted to this material, with its seemingly simple crystal structure, the very basic question of the nature of its bandgap remains controversial, that is whether it is a direct bandgap or indirect bandgap.

Recently, there was a newly paper published in Nature Photonics demonstrating that the hexagonal Boron Nitride is an indirect bandgap material. The first evidence for recombination assisted by phonon emission in hBN arises from the observation of a thermal distribution of excitons in the high-energy tail of the different emission lines, as shown in Fig. 1(b).
 Feng Fig 1.png
Figure 1 a, Photoluminescence spectrum of hBN at 10 K for one-photon excitation at 6.3 eV. b, Photoluminescence spectrum of hBN for two-photon excitation at 3.03 eV as a function of temperature
Another evidence is the observation of different phonon replicas of the free exciton, which are in good agreement with the calculation results, as shown in Fig 2.b.
Feng Fig 2.png 
Figure 2 b Identification of the phonon modes involved in the phonon-assisted recombination lines in hBN. The scattering path in the first Brillouin zone is indicated by the green arrow, corresponding to the phonon wavevector.
Although the authors demonstrated the indirect bandgap nature of hBN, there are a lot of questions which need to be explained and understood more deeply and clearly. One of them is the strong emission line at 5.76 eV, at which even stimulated emission has been observed, this seems infeasible if the material has an indirect bandgap. Much more investigations need to be done to verify the optical transition of hBN and the mechanism of the phonon-assisted emission if hBN is an indirect material.
G. Cassabois, P. Valvin and B. Gil, “Hexagonal boron nitride is an indirect bandgap semiconductor”, Nature Photonics, 10 262 (2016).
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