Effect of impurities on Raman and photoluminescence spectra of AlN bulk crystals
ABSTRACTRaman scattering and photoluminescence (PL) spectroscopy with sub-bandgap excitation has been applied to explore tracing of common impurities (in particular of oxygen) in AlN. Bulk AlN crystals grown by the high temperature sublimation method were studied. PL bands have been observed at around 375 nm and at 560–660 nm and have been attributed to oxygen and to nitrogen vacancy/aluminium excess defects, respectively. The 375 nm UV PL band was found to shift with oxygen concentration. Micro-Raman spectra of the bulk AlN samples were measured in different polarisations. Besides normal Raman modes of AlN the presence of additional vibrational modes was detected. The modes were discussed …
Phonons of hexagonal BN under pressure: Effects of isotopic composition
Raman scattering experiments on isotopically enriched hexagonal boron nitride have been performed under pressure up to 11 GPa at room temperature. The sublinear increase of the Raman-active E2g mode frequencies has been characterized. The pressure behavior has been analyzed by means of a bond-stiffness–bond-length scaling parameter γ which takes into consideration the vast differences in a- and c-axis compressibilities. The interlayer shear mode exhibits a γ parameter similar to that of graphite, and the mode frequency in isotopically pure samples separates faster at low pressures as a result of van der Waals interactions. Because of the extremely low a-axis compressibility, the intralayer …
Pressure dependence of the interlayer and intralayer E2g Raman-active modes of hexagonal BN up to the wurtzite phase transition
We present a Raman-scattering study of the interlayer and intralayer ${E}_{2g}$ Raman-active modes of hexagonal boron nitride $(h\ensuremath{-}\mathrm{BN})$ under hydrostatic pressure for pressures up to the transition to the wurtzite phase (10.5 GPa). Pressure coefficients and Gr\"uneisen parameters are determined for both modes, and are compared to ab initio calculations based on density functional perturbation theory. The pressure coefficient of the low-energy interlayer mode is higher than that of the high-energy intralayer mode owing to the large compressibility of the $h\ensuremath{-}\mathrm{BN}$ crystal along the $c$ direction. Both modes exhibit a sublinear phonon frequency increase…