Formation of Amorphous Carbon and Graphite in CVD Diamond upon Annealing: A HREM, EELS, Raman and Optical Study

Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350–1450 °C. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.