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

Abstract

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 $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\mathrm{Torr}$ 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 $\ensuremath{\sim}{10}^{8}{\mathrm{cm}}^{\ensuremath{-}2}$ for a narrow bias-voltage range of $\ensuremath{-}20--\ensuremath{-}50\mathrm{V}$ 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 ${\mathrm{sp}}^{3}$ 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.