Aligned synthesis of carbon nanotubes by laser-assisted chemical vapor deposition

With the recent advances in the aligned and position-controlled growth of carbon nanotubes (CNTs), there have been intensive studies in optimizing growth process for fabrication of various CNT-based electronic devices. Using conventional thermal chemical vapor deposition (CVD) method, substrate as well as the chamber need to be globally heated to sufficiently-high reaction temperature. In this study, we report a new method to directly synthesize CNTs across pre-defined electrodes on silicon wafers using laser-assisted chemical vapor deposition (LCVD), which can induce local and instant heating at desired positions. A CW CO₂ laser (Synrad Inc, 10.6 μm) as well as a CW Nd:YAG laser (Quantronix Inc, 1064 nm) were used. The carbon feedstock was NH₃/C₂H₂ gas mixtures (volume ratio 10/1), with a process pressure of 80 Torr. Mo electrodes were patterned on silicon substrates by photolithography and focused ion beam. The catalyst islands, which consisted of Fe/Mo nanoporous structures [Fe(NO₃) ₃.9H₂O, MoO₂(acac)₂ and alumina nanoparticle mixtures] were then transferred to on the electrodes. The substrate temperature was around 700°C, monitored by a pyrometer. Laser irradiation lasted for 3 to 6 min. SWNTs bridging the electrodes were synthesized. Raman spectra indicated that the diameters of the synthesized SWNTs were from 1.2 - 1.4 nm. Electrical properties were characterized by measuring source-drain current against the gate and the source-drain voltage, i.e., I _ds-V_g and I_ds-V_ds respectively. I-V characterization showed that the SWNTs synthesized were mainly semiconducting.