  Boron nitride exhibits diverse crystal structures, predominantly a layered
arrangement with strong intraplanar covalent bonds and weak interplanar van der
Waals bonds. While commonly referred to as hexagonal BN (hBN), the
sp$^2$-bonded BN atomic planes can also arrange in other configurations like
Bernal (bBN) or rhombohedral (rBN) stacking orders. Variations in the
orientation and translation of successive atomic layers lead to changes in
crystal symmetry, potentially resulting in piezoelectric, pyroelectric or
ferroelectric effects. However, distinguishing between different polytypes
using conventional methods like X-ray diffraction or Raman spectroscopy
presents a significant challenge. In this work, we demonstrate that the optical
response of the 4.1 eV defect can serve as an indicator of the polytype. To
this end, we study BN samples grown by metalorganic vapor phase epitaxy
(MOVPE), which contain different polytypes. The identification of the polytypes
was achieved by X-ray diffraction and transmission electron microscopy.
Photoluminescence and cathodoluminescence measurements with a high spatial
resolution allowed for the deconvolution of the signal into two components from
which we can extract a zero-phonon line (ZPL) at 4.096 eV (302.6 nm) for hBN
and 4.143 eV (299.2 nm) for rBN. We performed calculations that enable us to
identify the defect as a carbon dimer CBCN (C2) and show that the ZPL shift
reflects differences in the crystal environment for different polytypes.
Furthermore, we demonstrate that different polytypic composition ratios of hBN
and rBN can be achieved by MOVPE, which could pave the way for future
applications in large-area van der Waals heterostructures.
