Studies of doped nanocrystalline diamond films grown by parallel bias-enhanced CVD

The transformations induced by the application of a continuous bias voltage parallel to the growing surface during the sulfur-assisted hot-filament chemical vapor deposition (HFCVD) of nanocrystalline diamond (n-D) films were investigated by Raman spectroscopy (RS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The films were deposited on molybdenum substrates using CH₄, H₂ and H₂S. Bias voltages in the range of 0-800 V DC were applied parallel to the substrate surface continuously during deposition. The study revealed a significant improvement in the films´ density and a lowering in the defect density of the nanocrystalline diamond structure for parallel bias (PB) voltages above 400 V. These high PB conditions cause the preferential removal of electrons from the gaseous environment, thus leading to the net accumulation of positive species in the volume above the growing film, which enhances the secondary nucleation. The nanoscale carbon nuclei self-assemble into carbon nano-clusters with diameters in the range of tens of nanometers, which contain diamond (sp³-bonded C) in their cores and graphitic (sp²-bonded C) enclosures. Hence, the observed improvement in film density and in atomic arrangement appears to be connected to the enhanced presence of positively charged ionic species, consistent with models which propose that positively charged carbon species are the crucial precursors for CVD diamond film growth.