Nanocrystalline diamond films: Transmission electron microscopy and Raman spectroscopy characterization

The structure of fine-grained diamond films with crystallite size as small as 10-50 nm has been studied with cross-section transmission electron microscopy and Raman spectroscopy. The nanocrystalline films of 1 to 2 mu m thickness were grown on Si substrates by d.c. plasma chemical vapor deposition in Ar/CH4/H-2 gas mixtures, with methane concentration CH4/(H-2+CH4) varied from 3 to 100%. The substrates were seeded with 5 nm diamond powder to enhance the nucleation density. Submicron thick nanocrystalline films were also grown as a first layer for successive growth of large-grain films. The films demonstrate a kind of columnar growth even for the case of grains with a size of a few tens of nanometers. The crystallites showed many imperfections, twins on (111) planes being the dominant defect type. The twin density is very high, but often they are only a few atomic layers wide. A buffer layer of beta-SiC with a thickness up to 400 nm can be observed at the diamond/silicon interface. In addition to the diamond phase, Raman spectroscopy revealed disordered sp(3) and sp(2) carbon phases, presumably located at grain boundaries. High intrinsic stresses in the film prevent the observation of the size-induced low-frequency shift of the fundamental diamond peak at 1332 cm(-1). The appearance of fingerprints of C-N vibrations and distorted diamond in Raman spectra is discussed.