On the basis of first-principles calculations and experimental secondary ion mass spectroscopy, electrical transport, and Raman measurements performed on homoepitaxial diamond, we find that at high concentrations boron atoms tend to segregate in dimers. The study of the electronic, vibrational, and electron-phonon coupling properties, corroborated by Raman measurements, shows that boron dimers may be associated to the broad Raman peak around $500{\mathrm{cm}}^{-1}$ as well as to some of the gap states which have been reported in the literature, and that they are both electrically and vibrationally inactive with respect to the electron-phonon coupling driving the superconducting transition in metallic diamond. These results bear important consequences on the evolution of the critical temperature with the impurity concentration in B-doped diamond.

• Received 18 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.094509