Hexagonal boron nitride (hBN) is a wide indirect bandgap semiconductor which was synthesized already in the 19^th century, but only recently high quality, millimeter-size, single crystals were produced [1] leading to the realization of a light-emitting device operating in the deep UV [2]. This achievement paved the way for applications of hBN to advanced optoelectronics, making it to be considered a challenger for aluminum nitride [3]. Furthermore, Bourrelier et al. reported in 2016 single photon emission of color centers emitting at 4.1 eV [4]. However, the question of nature of defects giving rise to this behavior is still under debate.

To find out the origin of such emission we performed high hydrostatic pressure studies of the low-temperature photoluminescence of bulk h-BN crystals. We explored the  3.3–4 eV spectral range using  diamond anvil cell technique. Our results show that emission energy decreases with the pressure less sensitively than the bandgap [5]. It is a distinct behavior from the shift of the bandgap which is typical for deep traps. Interestingly, hydrostatic pressure measurements revealed also the existence of energy levels varying differently under pressure than 4.1 eV line. We observed  a smaller decrease of those emission energy levels compared to the rest of the levels in this spectral range or even an increase of it.  Calculations of pressure dependencies of various defect levels in hBN demonstrated that some of the observed UV lines are associated with carbon-related defects, and their pressure behavior depends strongly on hBN polytype i.e., different layer stacking sequences. Our results demonstrate that tuning the stacking sequence in different polytypes of a given crystal provides unique “fingerprints” contributing to the identification of defects in 2D materials.

[1] K. Watanabe et al., Nat. Mater., 3, 404 (2004).

[2] K. Watanabe et al., Int. J. Appl. Ceram. Technol., 8, 977 (2011).

[3] H.X. Jiang et al., ECS J. Solid State Sci. Technol., 6, Q3012 (2017).

[4] R. Bourrellier et al., Nano Lett,. 16 4317 (2016).

[5] K. Koronski et al., Superlattices and Microstructures, 131, 1 (2019).