Chemistry Reference
In-Depth Information
Figure 3.15. Schematic representation of a horizontal hot-wall CVD apparatus.
film exhibits the properties of (TMTSF)
2
ClO
4
alone, since its resistivity decreases
towards a sharp superconducting transition with an onset critical temperature of
T
c
4 K. This method is thus restricted to the use of single crystals as templates
and to the replacement of anions, but has proved its validity. The overlayer films
can also be prepared electrochemically.
1
.
Dry preparation methods
Chemical vapour deposition
The chemical vapour deposition (CVD) technique can be regarded as an extension of
the sublimation method used for the preparation of single crystals discussed before,
but adapted to thin film growth implying deposition on substrates. A schematic
diagram of a horizontal hot-wall CVD apparatus is shown in Fig. 3.15.
After introducing the substrates into the reactor, usually made out of glass,
and charging the vaporization crucibles with the selected precursor materials, the
entire experimental set-up is pumped down for several hours (typically overnight)
while the mixing zone and the reactor vessel are baked out at the final chosen
temperatures. The experiment starts by vaporizing the precursors at low pressures
and transporting them by a carrier gas (typically helium or argon) through heated
lines to the mixing zone and then to the deposition zone. The mixing zone is heated
to a higher temperature than the vaporization zone in order to avoid condensation
of the starting compounds. Obviously, all temperatures have to be set below the
thermal decomposition temperatures of the precursors.
What differentiates CVD from other evaporation techniques is that growth units,
the fundamental bricks essential for growth, are chemically generated at the mix-
ing zone. The formation of growth units implies either chemical reaction of the
