Spatially resolved Raman studies of diamond films grown by chemical vapor deposition

The frequency and line shape of the diamond Raman line are examined in detail for a series of microwave-plasma-assisted chemical-vapor-deposition films grown on Si. The Raman lines in the films appear at higher frequency (shifts of up to 3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) than that of natural diamond and the observed lines are symmetric with broader linewidths than that of natural diamond, ranging from 5.7 to 17.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. In addition, the line frequencies and linewidths are correlated; the films with the highest vibrational frequencies have the largest linewidths. The data include single-point measurements on eight films grown under different conditions as well as 500 data points from different positions on a single film that were obtained in a spatially resolved Raman experiment. Several mechanisms for the frequency shift and the correlation of the linewidth with frequency are considered including phonon confinement, residual stress, and defect scattering. Contrary to the observations, Raman line shapes computed from the phonon-confinement model (which has been used successfully to model Raman scattering in microcrystalline Si and GaAs), using phonon-dispersion curves for diamond from the literature, are highly asymmetric at the linewidths observed. It is concluded that the observed shifts in the diamond Raman line do not arise from phonon confinement alone and arise primarily from compressive stress. The line broadening also is not produced by phonon confinement alone and may arise from decreasing phonon lifetime associated with scattering from defects or from an inhomogeneous stress distribution in the films. The observed correlation between Raman line frequency and width suggests that the degree of compressive stress may be associated with the density of microcrystalline defects.