Carbon nanotubes grown by asymmetric bipolar pulsed-DC PECVD ☆

Asymmetric bipolar pulsed-DC plasmas have been recently applied as an industrial solution to the deposition of diamond-like carbon DLC films by plasma enhanced chemical vapour deposition (PECVD) and magnetron sputtering on large areas and with a high deposition rate. New applications of vertically aligned carbon nanotubes (VACNTs) require a suitable technology compatible with the industrial needs: large area and mass production. Among carbon nanotube production techniques, chemical vapour deposition (CVD) is one of the most widely used, specially for applications in which a high control over length, diameter and positioning of the CNTs is desired, as nanoelectronics, field emission displays (FEDs), gas sensors or functionalized CNTs arrays for biomedical and electrochemical applications. Particularly, plasma enhanced CVD (PECVD) favours a parallel growth of the CNTs to the electric field of the plasma sheath, which is of main interest in most of the applications mentioned above. In the present work, a comparative study between radiofrequency (RF) and pulsed-DC plasma sources for PECVD of CNTs is described. Pulsed-DC plasma conditions offer singular characteristics, over RF plasmas, such as: higher power density and electronic temperature inducing a higher ion density and enhancing ion bombardment on the substrate. Chemical species inside the plasma bulk were determined by optical emission spectroscopy (OES) during the growing process. Their concentration showed significant changes depending on the plasma excitation. CNTs were grown on c-Si (100) using Fe nanoislands (<50 nm) as the catalysts at 750 °C, gas flow ratio Φ NH3 :ΦC 2 H 2 =50:25 (in sccm) and pressure of 100 Pa. Effects of the plasma conditions on the CNTs structure and morphology (in the range of 5 to 20 μm long and 10 to 50 nm thick) were determined by scanning and transmission electron microscopy (SEM and TEM).

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