Effect of substrate bias on growth and properties of carbon nanotubes deposited under no hydrogen introduction by MPCVD

Abstract

In order to examine the consequence of replacing hydrogen gas with high substrate bias, processes were successfully demonstrated to fabricate various carbon catalyst-assisted nanostructures, including carbon nanotubes (CNTs), by microwave plasma chemical vapor deposition (MPCVD), under the pure CH4 as the only source gas and without additional H-2 gas introduction. The Si wafers were first deposited with catalyst by physical vapor deposition (PVD), and then followed by H-plasma pretreatment to obtain the well-distributed catalyst particles on wafer. The pretreated specimens were then deposited in MPCVD to manipulate the various carbon nanostructures. The main process parameters include catalyst material (Co, Ni and Fe), the substrate bias (0 to - 320 V), and deposition time (1 to 20 min). The as-deposited nanostructures were characterized by SEM, HRTEM, Raman spectroscopy, and field emission I-V measurement. The results indicate that the various nanostructures on Si wafers can be manipulated under sufficient high negative substrate bias (>-200 V) without the additional hydrogen introduction. These results also improve our understanding on growth mechanisms of CNTs or other nanostrucrures, where the most of the proposed mechanisms in the literature were too much emphasis on the role of hydrogen. On important process parameters, except the substrate bias, it is interesting to note that effect of deposition time on nanostructures is essentially to extend the more CNTs into the plasma zone, which can result in different purification effect on CNTs, vary the alignment and sizes of CNTs, and expose the more nanostructures near the substrate surface. Under the present deposition 2 conditions, the best nanostructures with the best field emission properties (Turn-on field = 5 V/mu m, maximum current density > 35 mA/cm(2)) are the well-aligned CNTs with proper tube number densities, purities and sizes. (c) 2005 Elsevier B.V. All rights reserved.