Changes in the Raman spectrum of type-Ib diamonds were studied as a function of the concentration of N impurities, neutron irradiation fluence and 13C isotope content. It was found that the N had no appreciable effect upon the Raman spectrum for concentrations ranging up to 330ppm, apart from a small broadening. At neutron fluences of more than 7 x 1017/cm2, a low-energy shift of the Raman peak and broadening occurred simultaneously. Thermal recovery of these changes began at annealing temperatures below 300C, and was almost complete at about 1000C. This implied that phonon scattering was greatly enhanced by atoms that were displaced slightly away from normal sites, as well as by interstitials, but was less affected by vacancies. The Raman intensity decreased suddenly at high neutron fluences, and was partially restored by annealing at 1700C. A lack of full recovery of the Raman intensity during annealing was attributed to absorption of the exciting light, due to the formation of color centers. In the case of 13C substitution, a deviation of the Raman energy was found with respect to that for a virtual crystal model with a mean reduced mass. This reflected the contribution which isotope substitution made to the self-energy. Maxima in the energy deviation, and broadening, occurred at about 70% of 13C.
H.Hanzawa, N.Umemura, Y.Nisida, H.Kanda, M.Okada, M.Kobayashi: Physical Review B, 1996, 54, 3793-9