Carbon Diffusion through SiO₂ from a Hydrogenated Amorphous Carbon Layer and Accumulation at the SiO₂/Si Interface

Abstract

Carbon diffusion in a SiO 2 /Si system was investigated. The source was provided by chemical vapor deposition of a hydrogenated amorphous carbon layer onto the oxide at low temperature. From layers with low oxygen content, no carbon outdiffusion was detected up to 1190 • C. If the O content was high, the diffusion would start suddenly at 1140 • C, and carbon accumulation would be found on the Si side of the SiO 2 /Si interface in the form of SiC precipitates. These results are interpreted by assuming oxygen-assisted dissociation of carbon atoms from the carbon layer in form of CO molecules, fast CO diffusion through SiO 2 and an exothermic reaction of CO with Si. No carbon segregation was found in SiO 2 . Consequences of carbon island formation during SiC oxidation are pointed out.

Theoretical investigation of carbon defects and diffusion in α-quartz

“…Our findings regarding the CO extraction and diffusion are in good qualitative agreement with the results of experiments Krafcsik et al 2,31 which indicate that carbon diffusion does not start through high-quality SiO 2 up to temperatures in excess of 900°C, 32 but then it proceeds very fast. They also show that at the temperature used for the oxidation of SiC (Ϸ1200°C), the removal of carbon from the interface should not be limited by the diffusion of C through the oxide.…”

“…The highest barrier in this diffusion process is 2.7 eV. These results show that carbon atoms can be removed from a continous network of SiO 2 with an activation energy between 2.7 and 3.1 eV. On the other hand, CO can diffuse with a very low ͑0.4 eV͒ barrier in silica.…”

“…Especially in the carboxyl configuration the coordination and the chemical bonding of the atoms are ideal. Experimentally detectable concentrations of these defects could be present in SiO 2 The last defect we calculated is a CO molecule adjacent to an oxygen vacancy, CO i ϩV O . This configuration could also be involved in the removal of built-in carbon from the SiO 2 lattice.…”

“…Furthermore, there are signs of interface features unknown for Si, related to incomplete removal of carbon. Spectroscopic studies 1 suggest the presence of C nanoclusters at the SiC/SiO 2 interface, giving rise to defect states resembling defects in amorphous hydrogenated carbon, a-C:H. In recent experiments 2 it has been shown that excess oxygen is needed to extract carbon atoms of such layers, and the resulting C-O species diffuses fast in high-quality SiO 2 .…”

Implantation conditions for diamond nanocrystal formation in amorphous silica

“…It was demonstrated that diffusing carbon in a fused quartz substrate is often bonded to oxygen by C-O complexes. 24 Therefore, oxygen concentration should be considered in further work.…”