Raman scattering from as-grown or annealed C δ-doped superlattices revealed lines at 1752 and 1856/cm. The second was of lower intensity, but exhibited a resonant enhancement for incident light with an energy of 3eV. These lines were comparable to those attributed to the vibrational modes of 2 directly bonded di-carbon centres in GaAs. First-principles calculations were used to determine the structure and vibrational modes of C-C defects which were located at various substitutional and interstitial sites in AlAs and GaAs. It was noted that the frequency of the C-C stretching mode was sensitive to the charge state and orientation. The di-carbon complex, when centered on an As site, was a deep donor and, in its positively charged state, had axes aligned close to <110> or <111> directions. The calculated frequencies and energies for the 2 orientations were essentially the same, and so these 2 structures offered an explanation for the 2 observed di-carbon Raman modes. The alternative attribution of one of the 2 modes to a defect such as an interstitial complex or neutral substitutional dimer, was ruled out as being incompatible with the experimental observations.

Raman Scattering Observations and ab initio Models of Dicarbon Complexes in AlAs B.R.Davidson, R.C.Newman, C.D.Latham, R.Jones, J.Wagner, C.C.Button, P.R.Briddon: Physical Review B, 1999, 60[8], 5447-55