Carbon growth by thermal laser-assisted chemical vapour deposition

Abstract

The heated area in thermal laser-assisted chemical vapour deposition (LCVD) is much smaller than in large area CVD. This affects the growth conditions drastically. One important growth aspect is that CVD processes requiring homogeneous reactions in the vapour are not effective for thermal LCVD. In this work LCVD of carbon from a non-adsorbable precursor (CH 4) and an adsorbable precursor (C 2H 4) in combination with cold wall and hot wall reactors was investigated. The combination with the hot wall reactor also means that adsorbable carbon-carrying species can be generated homogeneously in the vapour before deposition. For laser CVD of carbon from CH 4 a high deposition temperature was needed. The threshold laser power density was 9.0 × 10 4W cm −2 (3500 K) for a hot wall reactor temperature of 1270 K. For carbon LCVD from C 2H 4 carbon lines could be deposited already under cold wall conditions at the same laser power density as was used in the CH 4 case. The thickness profile perpendicular to the grown lines was close to gaussian for deposition temperatures below 3600 K. Bimodal profiles, which were observed for higher temperatures, indicate evaporation or surface diffusion of carbon. The deposited lines were fine grained with a nodular appearance. Crystalline carbon was found to grow in a layer structure parallel to the substrate surface. The lines deposited from CH 4 were considerably wider than the laser spot diameter. This may be due to the thermal anisotropy of the material. The temperature profile and also the line will be broadened since the heat conduction is much better along the graphitic planes than perpendicular to them. The lateral growth direction of the lines as well as the orientation of the graphite layers indicate that the edge atoms of the graphitic basal planes are very active during the growth.