Bulk and surface thermal stability of ultra nanocrystalline diamond films with 10–30 nm grain size prepared by chemical vapor deposition

The thermal stability of nanocrystalline diamond films with 10–30 nm grain size deposited by microwave enhanced chemical vapor deposition on silicon substrate was investigated as a function of annealing temperature up to $1200°C$⁠. The thermal stability of the surface-upper atomic layers was studied with near edge x-ray absorption fine structure (NEXAFS) spectroscopy recorded in the partial electron yield mode. This technique indicated substantial thermally induced graphitization of the film within a close proximity to the surface. While in the bulk region of the film no graphitization was observed with either Raman spectroscopy or NEXAFS spectroscopy recorded in total electron yield mode, even after annealing to $1200°C$⁠. Raman spectroscopy did detect the complete disappearance of transpolyacetylene (t-PA)-like $ν1$ and $ν3$ modes following annealing at $1000°C$⁠. Secondary ion mass spectroscopy, applied to investigate this relative decrease in hydrogen atom concentration detected only a $∼30%$ decrease in the bulk content of hydrogen atoms. This enhanced stability of $sp3$ hybridized atoms within the bulk region with respect to graphitization is discussed in terms of carbon bond rearrangement due to the thermal decomposition of t-PA-like fragments.