Thermal Conductivity of Ultrananocrystalline Diamond/Nonhydrogenated Amorphous Carbon Composite Films Prepared By Coaxial Arc Plasma Deposition

Diamond possesses the highest thermal conductivity (2 × 103 Wm-1K-1) in all bulk materials, owing to low phonon scattering in diamond. Since phonon scattering is drastically enhanced by grain boundaries, the thermal conductivity of polycrystalline diamond films is strongly dependent on the grain size. The thermal conductivity of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films comprising a large number of UNCD grains with diameters of less than 10 nm and an a-C:H matrix have been prepared mainly by chemical vapor deposition (CVD). The highest thermal conductivity reported so far is 12 Wm- 1K- 1, which is comparable with that of sp3-rich non-hydrogenated amorphous carbon (a-C). It has been reported for a-C:H and UNCD/a-C:H films that the thermal conductivity is degraded by the incorporation of H atoms in the films. Recently we have realized the formation of UNCD/a-C films comprising UNCD grains and an a-C matrix, by coaxial arc plasma deposition (CAPD). CAPD does not necessarily require a hydrogen atmosphere during the deposition for the formation of diamond grains. So far, the thermal conductivity of hydrogen-free UNCD/a-C films has never been studied because H atoms are unintentionally incorporated into the films from source gases in CVD. In this study, UNCD/a-C films were deposited under a base pressure, and the thermal conductivity of the films were measured for the first time to our knowledge. UNCD/a-C films were deposited on n-Si substrates at base pressures of less than 10-4 Pa and the substrate temperature of 550 °C using coaxial arc plasma gun equipped with a graphite target. Mo films with the thickness of 80 nm were deposited on the UNCD/a-C films by sputtering for the TDTR. The thermal conductivity of the UNCD/a-C films was measured by a time-domain thermoreflactance (TDTR) technique. The thermal conductivity of the UNCD/a-C films was estimated to be 23.7 Wm- 1K- 1 by TDTR. The thermal conductivity is evidently higher than that of UNCD/a-C:H prepared by CVD. Two reasons for it are supposed. One is that an a-C in UNCD/a-C is probably advantageous for thermal conduction as compared with an a-C:H in UNCD/a-C:H, since it has been reported for a-C:H that the thermal conductivity decreases with increasing hydrogen content in a-C:H. The other is that UNCD/a-C does not contain hydrogen atoms that prevent from phonon scattering, whereas the preferential existence of hydrogen atoms at grain boundaries between UNCD grains and those between UNCD grains and an a-C:H matrix enhances phonon scattering for UNCD/a-C:H films prepared by CVD, which results in a decrease in the thermal conductivity. Further details will be presented in the conference.