Chemical Vapor Deposition
When it comes to making nanoscale structures for
nanotechnology applications, nothing beats Chemical Vapor Deposition (CVD) in
cost, reproducibility, and flexibility.
This simple technique has been used for many years in the semiconductor
industry to perform thin-film deposition (epitaxy) on substrates.
The basic operation of the device involves a gaseous
precursor (or source) material that enters a sealed chamber. By maintaining a cooler temperature at the
other end of the chamber, the gas will naturally move in that direction. Once there, it will chemically bond to an
inert substrate and form a thin film.
Many dangerous byproducts are left behind and must be properly collected
and disposed of. Most CVD machines have
never failed spectacularly due to their lack of moving parts. It's really just a series of pressure valves
and hatches with heating coils.
In the traditional sense, CVD is used to create layers of
semiconductors, insulators, or conductors in a sort of 'sandwich' of
layers. Most lasers and quantum wells can
be fashioned in this sense.
For nanotechnology applications, the CVD fabrication process
has proven to be extremely flexible. In
most instances, the target substrate need only be coated with a catalytic layer
(usually gold) before it is placed in the chamber. The catalyst will then melt and form clusters (or balls) on the surface
of the substrate. It is from these clusters
that important structures like wires, tubes, and whiskers can be created on the
nanoscale. It can also be used under some
circumstances to create nanocrystalline layers on top of the substrate in the traditional
mode of epitaxial growth.
One form of CVD, known as MOCVD (metal-organic chemical vapor
deposition), offers various advantages over the traditional process. For one thing, the use of organometallic
compounds means that lower temperatures can be used for the deposition
process. This saves time and costs.
A key product of an MOCVD process is the all-important
quantum dot. We'll talk more about
quantum dots in a later section.
Suffice to say that MOCVD processes are one of the only ways to make
large numbers of high-quality quantum dots at a good price.
The family of CVD techniques is the most flexible method to
create nanostructures today. You're
probably interested to know exactly how the aforementioned structures are grown
with these techniques. Don't worry, I
will get to them shortly.
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