Influence of the substrate temperature on the properties of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been deposited by microwave plasma chemical vapour deposition from CH4/N2 mixtures. They consist of diamond nanocrystals of 3–5 nm grain size, embedded in an amorphous matrix with a thickness of 1–1.5 nm. The matrix is mainly sp3 bonded, with about 20–30% sp2 material, and contains 10% hydrogen in the form of sp3 CHx units. The influence of the substrate temperature on the growth and the properties of the films was investigated in the range from 520 to 770 °C. The growth rate increases with increasing temperature, indicating a thermally activated process. The apparent activation energy of 0.38 ± 0.02 eV is lower than the values found for standard diamond deposition, thereby proving that differences between both processes exist. X-ray diffraction revealed that the crystalline properties of the NCD films (e.g. the grain size) are not affected by the temperature. From Raman and infrared measurements, it becomes evident, however, that the properties of the matrix are distinctively different at low and high temperatures, respectively. Possible reasons for these observations are discussed in some detail.