Local vibrational modes in Mg-doped gallium nitride

Four local vibrational modes are reported for Mg-doped wurtzite GaN, which as grown possesses high concentrations of hydrogen. The modes, studied by Raman and infrared absorption spectroscopy appear to form two pairs. Based on the observed selection rules, one pair, with room-temperature frequencies of 2168 and 2219 ${\mathrm{cm}}^{\mathrm{-}1}$ is assigned to inequivalent Mg-H complexes in the c plane and parallel to the c axis, respectively. The origin of the second pair of modes at 2151 and 2185 ${\mathrm{cm}}^{\mathrm{-}1}$, which are IR inactive, is speculatively linked to the presence of diatomic molecules such as ${\mathrm{N}}_2$ or ${\mathrm{H}}_2$.