A plasma-enhanced chemical vapor deposition (PECVD) growth technique has been developed where the choice of starting substrate was found to influence the electrical characteristics of the resulting carbon nanofiber (CNF) tubes. It has been determined that, if the tubes are grown on refractory metallic nitride substrates, then the resulting tubes formed with dc PECVD are also electrically conducting.
Individual CNFs were formed by first patterning Ni catalyst islands using e-beam evaporation and liftoff. The CNFs were then synthesized using dc PECVD with C2H2:NH3 = [1:4] at 5 Torr and 700 °C, and ≈200-W plasma power. Tubes were grown directly on degenerately doped silicon substrates with resistivity ρ≈1–5 mΩ-cm, as well as NbTiN. The ≈200-nm thick refractory NbTiN deposited using magnetron sputtering had ρ≈113 μΩ-cm and was also chemically compatible with CNF synthesis. The sample was then mounted on a 45° beveled Al holder, and placed inside a SEM (scanning electron microscope). A nanomanipulator probe stage was placed inside the SEM equipped with an electrical feed-through, where tungsten probes were used to make two-terminal electrical measurements with an HP 4156C parameter analyzer.
The positive terminal nanoprobe was mechanically manipulated to physically contact an individual CNF grown directly on NbTiN as shown by the SEM image in the inset of figure (a), while the negative terminal was grounded to the substrate. This revealed the tube was electrically conductive, although measureable currents could not be detected until ≈6 V, after which point current increased sharply until compliance (≈50 nA) was reached at ≈9.5 V. A native oxide on the tungsten probe tips may contribute to a tunnel barrier, which could be the reason for the suppressed transport at low biases. Currents up to ≈100 nA could be cycled, which are likely to propagate via the tube surface, or sidewalls, rather than the body, which is shown by the I-V in figure (a).
Electrical conduction via the sidewalls is a necessity for dc NEMS (nanoelectro-mechanical system) applications, more so than for the field emission applications of such tubes. During the tests, high conductivity was expected, because both probes were shorted to the substrate, as shown by curve 1 in the I-V characteristic in figure (b). When a tube grown on NbTiN was probed, the response was similar to the ≈100 nA and
is represented by curve 2 in figure (b), which could be cycled and propagated via the tube surface or the sidewalls. However, no measureable currents for the tube grown directly on Si were observed as shown by curve 3 in figure (b), even after testing over a range of samples. This could arise from a dielectric coating on the sidewalls for tubes on Si. As a result of the directional nature of ion bombardment during dc PECVD, Si from the substrate is likely re-sputtered and possibly coats the sidewalls.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Refer to NPO-47157, volume and number of this NASA Tech Briefs issue, and the page number.