Characterization of heavily boron-doped diamond films

A series of boron-doped polycrystalline diamond films grown by hot-filament-assisted chemical vapor deposition was studied with transmission electron microscopy (TEM), secondary-electron microscopy, Raman spectroscopy, Hall-effect and current-voltage measurements. The grain size of the diamond films decreased as the resistivity of the films decreased. A drastic change of the Raman spectra with the film resistivity was observed. The zone-center optical phonon line at 1332 cm(-1) downnshifted and decreased as the film resistivity decreased, and even disappeared in the sample with the lowest resistivity. On the other hand, broad peaks around 465 cm(-1) and 1220 cm(-1) appeared and their intensity increased with decreasing resistivity. We attributed these phenomena to the breakdown of the k=0 selection rule of the Raman scattering owing to the heavy boron incorporation in the film. A TEM study showed that all the films were of the cubic diamond structure. No boron-car;bon compound was found in these films. This finding agreed well with the Raman result Increased mobility and non-valence-band conduction were observed. As to the defect study, twinning was found to be the dominant feature in these films, and an increase of dislocation density was also observed with decreasing resistivity of the films.