15
previously reported as producing the smallest distribution of CNTs via this method
with a predominantly DWNT yield [37], so it was chosen as the preferred catalyst
thickness for the growth of these CNTs with a 10 nm barrier layer of aluminum
between it and the silicon wafer to prevent catalyst migration into the substrate.
These metals were all deposited on the silicon wafer via e-beam evaporation.
To efficiently control the flow of each gas through the tube furnace, mass
flow controllers (MFCs) were used; MFCs allow for tight, repeatable control over the
flow of gases as compared to analog "turn-dial" flow controllers. Three MKS MlOO
MFCs controlled by a MKS Type 247 Four-Channel Readout were banked together
(Figure 2.2) and used for this growth furnace to control the flow rates; these gases
include the hydrocarbon reaction gas (ethylene), the gas flowed through the bubbler
to pick up water vapor (15 vol% hydrogen, balance argon), and the gas which
carries the reaction gases through the furnace (15 vol% hydrogen, balance argon).
With the furnace setup fully assembled as seen in Figure 2.3, the next step
was to develop a consistent growth procedure. Based off of previous work [37] and
with considerations taken during preliminary growth attempts, the following
growth procedure was adopted:
1. Begin flowing the "carrier gas” (Qc) and heat furnace up to the "introduction
temperature” (Ti).
2. When stable, place substrate into the center of the growth zone and begin
heating to the desired set point for the "synthesis temperature” (Ts).