Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas

The transition from microcrystalline to nanocrystalline diamond films grown from ${\mathrm{Ar/H}}_{\mathrm{2}}{\mathrm{/CH}}_{\mathrm{4}}$ microwave plasmas has been investigated. Both the cross-section and plan-view micrographs of scanning electron microscopy reveal that the surface morphology, the grain size, and the growth mechanism of the diamond films depend strongly on the ratio of Ar to ${\mathrm{H}}_{\mathrm{2}}$ in the reactant gases. Microcrystalline grain size and columnar growth have been observed from films produced from ${\mathrm{Ar/H}}_{\mathrm{2}}{\mathrm{/CH}}_{\mathrm{4}}$ microwave discharges with low concentrations of Ar in the reactant gases. By contrast, the films grown from ${\mathrm{Ar/H}}_{\mathrm{2}}{\mathrm{/CH}}_{\mathrm{4}}$ microwave plasmas with a high concentration of Ar in the reactant gases consist of phase pure nanocrystalline diamond, which has been characterized by transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy. X-ray diffraction and Raman spectroscopy reveal that the width of the diffraction peaks and the Raman bands of the as-grown films depends on the ratio of Ar to ${\mathrm{H}}_{\mathrm{2}}$ in the plasmas and are attributed to the transition from micron to nanometer size crystallites. It has been demonstrated that the microstructure of diamond films deposited from ${\mathrm{Ar/H}}_{\mathrm{2}}{\mathrm{/CH}}_{\mathrm{4}}$ plasmas can be controlled by varying the ratio of Ar to ${\mathrm{H}}_{\mathrm{2}}$ in the reactant gas. The transition becomes pronounced at an ${\mathrm{Ar/H}}_{\mathrm{2}}$ volume ratio of 4, and the microcrystalline diamond films are totally transformed to nanocrystalline diamond at an ${\mathrm{Ar/H}}_{\mathrm{2}}$ volume ratio of 9. The transition in microstructure is presumably due to a change in growth mechanism from ${\mathrm{CH}}_{\mathrm{3}}\mathrm{\cdot }$ in high hydrogen content to ${\mathrm{C}}_{\mathrm{2}}$ as a growth species in low hydrogen content plasmas.