High-Temperature Raman Spectroscopy of Nano-Crystalline Carbon in Silicon Oxycarbide

The microstructure of segregated carbon in silicon oxycarbide (SiOC), hot-pressed at T = 1600 °C and p = 50 MPa, has been investigated by VIS Raman spectroscopy (λ = 514 nm) within the temperature range 25-1000 °C in air. The occurrence of the G, D' and D bands at 1590, 1620 and 1350 cm^-1, together with a lateral crystal size L < 10 nm and an average distance between lattice defects L ≈ 8 nm, provides evidence that carbon exists as nano-crystalline phase in SiOC containing 11 and 17 vol % carbon. Both samples show a linear red shift of the G band up to the highest temperature applied, which is in agreement with the description of the anharmonic contribution to the lattice potential by the modified Tersoff potential. The temperature coefficient χ_G = -0.024 ± 0.001 cm^-1/°C is close to that of disordered carbon, e.g., carbon nanowalls or commercial activated graphite. The line width of the G band is independent of temperature with FWHM-values of 35 cm^-1 (C-11) and 45 cm^-1 (C-17), suggesting that scattering with defects and impurities outweighs the phonon-phonon and phonon-electron interactions. Analysis of the Raman line intensities indicates vacancies as dominating defects.