Raman scattering observations and ab initio models of dicarbon complexes in AlAs

Raman scattering from an as-grown or annealed AlAs carbon δ-doping superlattice reveals lines at 1752 and 1856cm^-1: the latter line shows the weaker intensity but has a resonant enhancement for incident light with an energy of 3 eV. These lines are comparable with those assigned to vibrational modes of two directly bonded dicarbon centers in GaAs [J. Wagner et al., Phys. Rev. Lett. 78, 74 (1997)]. First principles calculations are carried out to determine the structure and vibrational modes of dicarbon C-C defects located at various substitutional and interstitial sites in both AlAs and GaAs. The frequency of the C-C stretch mode is sensitive to the charge state and orientation and errors are not expected to exceed 10%. The dicarbon complex centered at an arsenic site is a deep donor and in its positively charged state is found to have axes aligned close to either 〈110〉 or 〈111〉 directions. The calculated frequencies and energies for the two orientations are essentially the same, so that these two structures offer an explanation for the observation of the two dicarbon Raman modes. An alternative assignment of one of the two observed modes to a different defect, such as an interstitial complex or neutral substitutional dimers, are considered but are ruled out as being incompatible with the experimental observations.