Wurtzite semiconductor compounds have two silent modes, B1 l and B1 h. A silent mode is a vibrational mode that carries neither a dipole moment nor Raman polarizability. Thus, they are forbidden in both infrared reflectivity and Raman spectroscopy. Astonishingly, we detected the B1 l mode in high-quality, ultra-narrow GaN nanowires using resonant Raman scattering, although the B1 h was not observed, and there is no immediate explanation for this asymmetric finding. The Raman experiments were performed using several laser lines from 647 to 325 nm; the latter is a wavelength in which Raman becomes resonant. Actually, we observed the B1 l mode only in resonance, indicating that the appearance of this mode is related to Fröhlich electron-phonon interactions; i.e., a dipole moment emerging in the B1 l silent mode may not be present in the B1 h mode. To shed light onto the physical origin of these observations, we performed density functional theory calculations of the lattice dynamics in GaN. We performed a careful analysis of the different physical mechanisms that allow the forbidden mode to appear to explain the physics underlying the nonzero dipole moment in the B1 l mode, and the reason why this dipole moment is not present in the B1 h mode.
Keywords: Extremely narrow nanowires; Fröhlich electron−phonon interaction; Raman spectroscopy; crystalline perfection; isotopic composition; silent modes.