Vibrational signature of the SiN defect in Si-doped GaN_xAs _1-x

Using first principles density functional theory in a supercell approach, we calculate the full spectrum of localized vibration modes associated with the SiN defect complex in GaAs. Two recently proposed structures are investigated in detail: (a) substitutional Si on a Ga site with substitutional N on an adjacent As site [ (Si_GaN_As)^q, where q is the charge state of the defect complex], and (b) Si and N atoms forming a split-interstitial defect on an As site [ (SiN)^q_As]. All the localized mode frequencies for both defect structures are calculated in the relevant charge states. We have also calculated the formation energy of the (Si_GaN_As)^q defect as a function of SiN separation, finding that the configuration with the Si on a second-nearest- neighbor Ga site is more favorable in the positive charge state than the configuration with the Si on a nearest-neighbor Ga site to the N atom.