The influence of boron doping on the structure and characteristics of diamond thin films

Diamond thin films doped with various boron concentrations were grown by hot-filament-assisted chemical vapor deposition. The charge carrier density in the films ranges from 3.4 × 10¹⁷ to 1.8 × 10₂₁ cm^-3. TEM studies show that all films are of cubic diamond structure with no impurity phase. As the boron concentration increases, the average grain size of the film decreases from 10 to 1 μn. A significant decrease in plane defect density and a slight increase in dislocation density were also observed as boron content increases. Penetration twin, caused by secondary nucleation, was found to be a common feature in films with high boron contents. It is considered that the small grain size in heavily boron doped samples is mainly caused by the formation of these twins. Significant differences were observed from the Raman spectra of the films with different boron content. As the boron concentration increases, the zone center optical phonon line at 1332 cm^-1 downshifts and weakens while broad peaks around 465 and 1220 cm^-1 appear and their intensities increases. This is considered to be the result of the breakdown of the k = 0 selection rule due to the high boron content. © 1997 Elsevier Science S.A.