Experimental Study on Microwave Assisted CVD Growth of Carbon Nanotubes on Silicon Wafer using Cobalt as a Catalyst

Abstract

Controlled growth of carbon nanotubes is an important step in the realization of practical nanoscale devices for applications in nanoelectronics, sensors, field emission displays, and microelectro mechanical systems (MEMS), among others. In this investigation microwave assisted chemical vapor deposition (CVD) technique has been used to synthesize carbon nanotubes on a silicon wafer substrate. Since a transition metal catalyst, is needed for growth of nanotubes, a thin film of cobalt catalyst (2 ~ 5 nm) is deposited on the silicon wafer substrate using pulsed laser deposition (PLD) technique using an excimer laser (248nm). The CVD process conditions, including, growth time, plasma pretreatment time, process gases and flow rate of carbon source gas (methane) are studied towards obtaining controlled growth of nanotubes. Further, patterned catalyst film is deposited by the PLD technique to synthesize vertically aligned nanotubes on patterned catalyst film. The carbon nanotubes are characterized using SEM, TEM, AFM and -Raman Spectroscopy. Carbon nanotubes were grown on a cobalt catalyst deposited silicon wafer using microwave plasma enhanced CVD. A growth time of 5 ~10 minutes is found to be optimal for growth of nanotubes with fairly uniform diameters. A plasma pretreatment time of 5 ~ 15 minutes is found to be a favorable condition for well defined growth of nanotubes. Nitrogen gas is found to be essential for the growth of CNT and hence is included as one of the important process gases. Presence of hydrogen gas is found to be important to synthesize nanotubes with fewer amorphous particles. Methane flow rate of 10 ~ 15 sccm is found to be an optimum for synthesis of vertically aligned nanotubes. Finally patterned growth of nanotubes, where individual tubes are vertically aligned normal to the substrate, was obtained by patterning the catalyst film. Diameters of the tubes were determined using TEM and AFM and are found to vary from 20 ~ 150 nm. -Raman spectroscopy indicates that the nanotubes formed are of the multi-walled type.