Di-carbon complexes in AlAs and GaAs

Heat treatment of heavily carbon doped AlAs and GaAs results in a loss of C -As shallow accepters. In Raman scattering experiments on annealed CBE grow n GaAs with C-12 and C-13 isotopes, and MOVPE grown AlAs it is found that t he loss of carriers is accompanied by the appearance of two high frequency lines. These lie near to the stretch mode of an isolated C-2 molecule (1855 cm(-1)). This is consistent with the formation of two types of di-carbon d efects in these materials where the C atoms are bonded together and one or both of which act as a donor. Using a local density functional method to in vestigate the structure and dynamics of several di-carbon defects, we find that the dimer at an As site is bistable and aligned approximately in a [10 0] direction in the neutral charge slate, and in a [110] direction when pos itively ionised. The calculated frequencies Lie within 10% of the measured values in both materials. Other defects are investigated too with a view of determining the structures giving rise to the modes.