Crop up of the B_1l mode in high quality GaN nanowires due to isotopic disorder

The B_1l and B_1h vibrational modes of GaN are silent, i. e. they cannot appear neither in Raman nor in infra-red spectroscopies. However, the B_1l mode appears with a small linewidth in the Raman scattering spectra on high quality ultra-narrow GaN nanowires, while the B_1h mode does not. The simulltaneous appearance of the A₁(LO) and B_1l in the ultraviolet region indicates: a) that the B_1l mode is related to a resonance effect, i. e. Fröhlich interaction plays a role and b) the mechanism allowing the appearance of the B_1l does not allow the observation of the B_1h. After performing density functional theory calculations and discarding several options we have concluded that the only difference between both modes is the isotopic composition [1]. While Ga, which contributes to the B_1l mode, has mainly two isotopes, ⁶⁹Ga and ⁷¹Ga, N, the main contributor to the high frequency mode, has basically one isotope, i.e. it is isotopically ordered.
[1] C. R-F et al., Nano Lett. 18, 5091 (2018).