Isotope Effects on the Raman Spectrum of SiC

Substituting C-12 by the C-13 isotope in SiC changes the Raman frequencies. We have used isotopic substitution in several polytypes of SiC (4H, 6H, 15R) to study the change in the phonon dispersion relation of SIC and compare the results with calculations based on a linear-chain model. In a second set of experiments we use isotopic substitution to study the incorporation of carbon from the crucible during the growth of SiC by the modified Lely technique. To this end, a stoichiometric mixture of silicon and pure C-13 was used as the charge. The incorporation of C-13 in the grown crystal as monitored by spatially resolved Raman spectroscopy yields a gradient of C-13 that increases beginning at the seed crystal and reaches a maximum of similar to 40 % within 300 mum; the equilibrium concentration of C-13 is a mere 27 % after 1.5 mm growth.