Chemistry Reference
In-Depth Information
5.3.1
Chemical Cleaning
The preparation of the substrates prior to the deposition is critical for the result of
the dewetting process. Since the catalyst has to form an alloy with the silicon of
the substrate to transform to liquid phase, the presence of an interlayer between the
metallic layer and the substrate may hinder the process. To avoid that, the substrate
is prepared by removing impurities and clean the surface.
The first step is an ultrasonic bath in deionized water, then in acetone to dissolve
organic molecules present on the surface.
Later RCA SC-1 and RCA SC-2 processes (Kern and Puotinen
1970
) are per-
formed to further remove organic and ionic contaminants. HF etchings are done
between RCA SC-1 and RCA SC-2 and at the end to remove the silicon oxide layer
formed by the chemical processes and exposition to air.
5.3.2
Catalyst Deposition Methods
Catalyst distribution stage has a pivotal role aiming to uniform and controlled growth.
The essential step prior to VLS growth is to have uniform catalyst particles coverage
of the substrate.
In order to obtain that, in our laboratory we followed two different approaches:
it is possible to spread directly nanoparticles on a substrate or to deposit a thin film
and then induce the so-called “dewetting” process.
For the growth of
-SiC nanowires in a vapour phase epitaxy reactor the deposition
of the catalyst on the silicon substrate was carried out by two physical method
(electron beam and sputtering) for nickel, iron and gold, by electrophoretic deposition
for iron and by drop casting of a magnetite nanoparticles solution.
For the growth of core-shell SiC-SiO
2
nanowires the best and most straightforward
method was found to be the drop casting of an ethanol solution of metal nitrates
(Ni(NO
3
)
2
or Fe(NO
3
)
3
).
In order to acquire a uniform distribution we went through different routes: two
physical methods and two chemical methods allowed us to obtain films of different
metals with controlled thickness.
β
Physical Methods and Electrophoretic Deposition
With e-beam evaporation de-
position and radiofrequency sputtering, a 4 nm thick film was found to be the best
candidate for SiC nanowires growth.
Electrophoretic deposition (EPD) (Van der Biest and Vandeperre
1999
) proved to
be an efficient method to obtain films for our aim, since it has high yield and it is
easy to perform, but its drawbacks are correlated to the difficulty of reaching a good
control over film thickness and uniformity.
The metal deposition rate can be controlled in a certain range by varying the
electrical potential difference and deposition duration. For the present study iron
was deposited starting from a ferrous sulphate (FeSO
4
) solution in water with a low
electric field (potential used was 5 V) for a 30 min deposition time.
