The local structure of CAs acceptors was investigated by studying the non-degenerate localized vibrational modes of H-CAs pairs with A1 symmetry, rather than those of isolated CAs impurities. Infra-red absorption and Raman scattering measurements were performed on 12C-doped epilayers that had been exposed to a radio-frequency H or D plasma, or which contained H which had been incorporated during growth. It was argued that the H or D atoms occupied a bond-centered site between CAs and Ga atoms rather than between CAs and Al atoms, at low temperatures. An  ab initio  local density functional calculation indicated that the energy was thereby lowered by 0.24eV. The analysis led to the attribution of 5 anti-symmetrical stretch modes and 5 symmetrical modes to H-CAs pairs at sites where the C atom that was originally unpaired had 0, 1, 2, 3 or 4 Al nearest neighbors.

R.E.Pritchard, R.C.Newman, J.Wagner, F.Fuchs, R.Jones, S.Oberg: Physical Review B, 1994, 50[15], 10628-36