Diamond nucleation density as a function of ion-bombardment energy in electron cyclotron resonance plasma

A low-pressure study on diamond nucleation on mirror-polished Si(100) wafers using defined ion-bombardment energy is presented. The substrate was negatively biased to several tens of V in an electron cyclotron resonance methane-hydrogen plasma at 1×10^-3 Torr for nucleation, and then exposed to a typical hot-filament system at 40 Torr for subsequent growth. The nucleation density counted after the growth was enhanced up to ∼10⁸ cm⁻² for a narrow bias-voltage range of −20–−50 V in the initial nucleation treatment. The threshold and optimum ion energies for the nucleation enhancement were found to be 20–30 eV and around 50 eV, respectively, just above the threshold for shallow ion implantation. Cross-sectional transmission electron microscopy and selected-area electron diffraction for the deposits after the nucleation treatment revealed that diamond crystallites with sizes smaller than a few tens of nm were embedded in a matrix of amorphous carbon. The nucleation density as a function of ion energy was compared with the fractional increase in carbon sp−³ bonding caused by subplantation. The results confirm the nucleation pathway through the ion-induced densification beneath a surface, which is largely different from the conventional condensation of adsorbed species on a surface.