Session Index

Hexagonal boron nitride (h-BN), an insulating two-dimensional layered material, has attracted great attention due to its fascinating properties and promising applications across the fields of photonics, quantum optics, and electronics. Given the layered structure of h-BN, various polytypes exist with high-symmetry stacking sequences where successive layers are rotated or translated, leading to a variety of physical properties. Similarly to silicon carbide, polytypism in sp2-bonded boron nitride is to lead to a wealth of variations in the opto-electronic properties. I will report on the optical properties of a variety BN polytypes including bulk crystals grown by precipitation out of molten metallic solutions and epilayers by produced by metal-organic chemical vapor deposition on sapphire substrates [1,2]. The sp2-bonded layered compound BN exists in more than a handful of different polytypes (i.e. different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype single crystals extremely tricky. Co-existence of polytypes in a similar crystal leads to formation of many interfaces and structural defects having deleterious influence on the internal quantum efficiency of the light- emission and on the mobility of carriers. Light emission, when recorded in standard experimental illumination conditions, is in general inhomogeneous and its yield is dominated to radiative recombination to localized stacking faults [3], even for crystals with state-of-the- art, high structural quality [4]. However, despite of this, lasing operation was reported at 215 nm [5], which has shifted the interest of sp2-bonded BN from the laboratories of basic sciences to the fields of optoelectronic and electrical device applications. Here I present X-ray diffraction, electron microscopy and Raman light scattering experiments that were performed on a large variety of samples grown using the complementary techniques alluded to above and that we interpret with the help of group theory arguments and calculations of the phonon dispersion relations (in the context of quantum espresso). They revealed the three coherently ordered AA’ (normal), AB (Bernal), and ABC or rBN (Rhombohedral) and AA stackings, and the disordered turbostratic (tBN) one. Characterization of the different structural polytypes are completed by micro- photoluminescence experiment measurements that furnish a one-by-one, unambiguous relationship between the sp2-stacking and its photoluminescence spectrum [6,7,8,9]. These, together with reflectance experiment disentangle the nature of the light-matter interaction processes in sp2-bonded BN. All polytypes studied here display an indirect configuration of their fundamental bandgap, some of them (the AA and AB two-layer stackings and the rBN three-layer one) offer efficient Second Harmonic Generation of light performances.
References:
[1] Jiahan Li et al. ACS Nano 15, 7032 (2021).
[2] M. Chubarov et al., Crystal Growth Design 12, 3215 (2012)
[3] R. Bourrellier et al. ACS Photonics 1, 857 (2014).
[4] G. Cassabois et al. Nature Photonics 10, 262, (2016).
[5] K. Watanabe et al. Nature Materials 3, 404 (2004).
[6] M. Rousseau et al. Physical Review Materials 5, 064602 (2021).
[7] M. Moret et al. Applied Physics Letters 119, 262102 (2021).
[8] S.Moon et al. Nature Materials (2025) DOI10.1038/s41563-025-02173-2
[9] W.Desrat et al submitted to Physical Review Materials (under review)

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A new approach for aluminum gallium nitride (AlGaN) growth is emerging, using materials made of two-dimensional unit layers, such as epitaxial graphene (EG). We present a new approach to the growth of AlGaN on EG on silicon carbide (SiC) substrate. Prior to the fabrication of EG, an ultra-thin AlN layer is deposited on SiC. Annealing the AlN / SiC heterostructure at high temperature leads to the fabrication of an EG layer at the interface, as confirmed by Raman spectroscopy. Next, AlGaN/GaN heterostructures are grown and their structural and optical properties assessed for the fabrication of quantum dot based UV LED.

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Plasma-assisted MBE growth of well-oriented device-relevant GaN nanowire arrays on Si or sapphire substrates with metallic ZrN buffer layers is reported. Kelvin probe and TEM measurements show perfect nitrogen-polarity of the nanowires without any sign of mixed polarity commonly observed for self-assembled GaN nanowires on Si. Electrical measurements show a low resistive ohmic electrical contact of ZrN to bottom parts of the nanowires, so the ZrN buffer can play a role of buried electrical contact allowing efficient controlling electrically-driven light emitters based on GaN nanowires. Moreover, a good rectifying characteristic of p-n junction inside a single nanowire was observed.

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Engineering the Fermi level position on the (Al)GaN surface is very important for
producing electrical contacts to (Al)GaN with desired characteristics, i.e. ohmic or Schottky
contact. In this paper it is shown that such contact engineering can be achieved by simply
depositing MXene on the (Al)GaN surface, and that the Fermi level position at the
MXene/(Al)GaN interface can be investigated using contactless electroreflectance.

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