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[MUSIC]

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Hi.

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Welcome to lecture four of week two of
nanotechnology the basics.

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In this lecture we're going to talk about
some nano materials

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that may be useful eventually to replace
wires in, integrated circuits.

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The standard architecture in integrated
circuit today

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uses voltages to control currents in
transistors.

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So those voltages and currents need to be

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delivered to the transistors by wires, a
standard interconnect.

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The basic interconnect these days is
simple copper wire.

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It's an evaporated film of copper
patterned onto a semi-conductor substrate.

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There are two problems that arise when you

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try to shrink this down to nano
dimensions.

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The first problem has to do not merely
with how small the wire

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is, but how fast you're trying to switch
the voltages on that wire.

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We all know that our computers have gotten
faster as time goes on, so the cutting

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edge laptop computers these days have a
clock speed of

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several gigahertz, which means that there
is a wire in

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there where the voltage is being switched
from a high

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value, maybe 5 volts, to a low value, say
zero.

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Several billion times a second.

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So the faster you try to switch, a voltage
on a

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wire, the more energy loss there is as the
wire gets smaller.

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So smaller wires, it's harder to switch
the

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voltage then larger wires, and one, one
reason for

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that is, that the wires tend to radiate
more efficiently.

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And so the energy is radiated rather than
transported along the wire.

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A second issue has to do, again, not so
much with

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the size of the wire, but the proximity to
it's neighbor.

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So when we're shrinking the whole circuit

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down, we're moving wires closer to each
other.

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And if there's a voltage switching on one
wire,

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that can lead to switching on the wire
next door.

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So that parasitic coupling

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between wires also becomes a big challenge
in

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designing circuits to operate on the nano
scale.

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There are a couple of potential solutions
that

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nano materials can offer, and that is to

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through the route of different kinds of
materials,

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other than just conventional copper
evaporated on silicon.

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One candidate, is a carbon nanotube.
So if you take a single sheet of graphite,

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which makes this hexagonal array of carbon
atoms, and you roll it

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up, and make a tube, then what you got is
a carbon nanotube.

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It's got nothing but carbon, every atom is
a carbon atom, it has a diameter

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Of perhaps a nano meter, and it has a
length which can be well really long.

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It could be only ten nano meters or it
could

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be ten microns long or it could be
millimeters long.

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These things could be made with
ridiculously large aspect ratios,

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so they look like wires, and in fact
certain types of

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carbon nano tubes can conduct electricity
extremely well, better than copper even.

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So they could, in principal, be

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a replacement for lithographically
deposited metal wires.

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one of the challenges, well there's

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several challenges, associated with these
things.

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One of them is how do you, how do you grow
them?

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How do you make them?

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So if you want a characterize a, an
electronic circuit

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that contains the carbon nanotube.

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You need to be able to put down a nanotube
between two electrodes.

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So they are really three ways to do that,
one is

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simply put down the tube and then put
electrodes on top.

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The second way is to put electrodes down
on the surface and then put a tube on top.

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Andf the third is to put down electrodes

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on the surface and that actually grow the
tube.

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In situ between the electrodes.

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All three of those procedures have been
done and these pictures show you a couple

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of examples.

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This is really a single carbon nanotube

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bridging a gap between two metal
electrodes.

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This is a great way to measure the
properties

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of a single tube and to confirm that they

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really do have these remarkable properties
that are associated

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with the nano scale size and the molecular
perfection.

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The challenge here is, how do you scale
this up?

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What if I want to grow not just one tube,
but a billion of them.

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and is it really reproducable?

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And those are really big challenges,
because those are necessary steps

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in order to be able to use these things in
real circuits.

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So, there's another challenge, which is
the wiring problem.

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Right?

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We want to take advantage of the fact that
these structures are nanoscale in size.

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So if we build a single this stands for
single-walled nanotube.

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If we build a single wall nanotube
transistor.

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Which is a nanotube bridging the gap

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between two metal contacts.

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Which we can call the source and the drain
of a transistor.

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And then the nanotube acts as the channel
between the source and the drain.

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That's great, right?

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We have a a transistor with an active
region that's only one nanometer thick.

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The thickness of the tube, right.

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The diameter of the tube.

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The problem is that it's connected to

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these metal contacts, which are giant, by
comparison.

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And which extend out to electrodes which
are even more giant.

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Which are really enormous.
So are we really building a nanoscale

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circuit if we have all of this other stuff
surrounding our nanoscale element?

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Right, this is known as the wiring
problem, and it's really

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an issue not just for carbon nanotubes,
but for any nanocircuit element.

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If we have a single nano object.

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Connected to an extended large structure,
we're not doing nano science any more.

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Or to put it another way, we're doing nano
science, but we're

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not taking advantage of the fact that the
object is nano sized.

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So there are of course reproduce-ability
issues here, there are

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three of them that I'll mention associated
with nano tubes.

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Number one is the tube type, if you take
a, a sheet of graphite and

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roll it up there are many, many ways to do
that depending on the roll angle.

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Right?
The chirality of the, of the tube.

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And some of those are metals and some of
those are not.

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There are some of them are semiconducting.

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So if you want to make a wire between two

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points using a semiconducting tube
wouldn't be so useful.

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so, you need to be able to know what kind
of tube you have.

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And right now sorting tubes is difficult.

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It's a hard thing to do.

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Number two is how do you get the tube to
sit in the right place?

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Right?

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It's very difficult to place a one
nanometer object exactly

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where you want it, repeatedly over and
over and over.

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so that's a real challenge.

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And number three is the issue of contacts.

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Ultimately these tubes have to be
connected to some metal object.

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And so that means you're going to have a
connection between a carbon and

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a metal.

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And those connections are, are at this
point, not well understood.

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Theoretically not well characterized
experimentally and have large variability.

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Meaning if you do it a hundred times you
don't always get the same answer, right?

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And so that turns out, those turn out to
be

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real big challenges in the fabrication of
nano scale circuitry.

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There are also some surprising or

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unintended effects in carbon nanotubes as
wires.

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These are really one dimensional
conductors.

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The electrons flow in just one direction
along these things.

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There's no way for them to flow

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in any other direction except the
longitude, right?

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And so if there's a defect in your tube,
if one atom is missing from your, from

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your perfect molecular lattice there, then
that's a

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defect and every electron is going to see
that defect.

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Now compare that to the case of a bulk
piece of silicon which might

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be a 100 nanometer cube, right, if I
remove one atom from the middle

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and, and leave a, a vacancy, a defect,
then

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most of the electrons flowing through this
cube won't

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see that defect at all, and so that defect

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will have a relatively small impact on the
conduction.

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But in a nanotube where the conduction is
one dimensional every electron

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sees every defect and so this really has
a, a, a big challenge.

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That means we have to be able to control
or eliminate defects and eliminating

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them is really hard so we have to at least
be able to control them.

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And the last thing I'll

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mention is this issue of adsorbates.

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What I'm talking about here is if you have
a tube lying

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in air then any molecule that falls on it
can stick to it.

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Water molecule floating around in the air,
or

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some organic, and when something sticks to
the outside.

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Then, it can impact the conduction.

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It can influence the electrons that are
flowing through

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the tube, and change the way that they
flow.

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So, these absorbates can strongly affect

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the electronic properties, and that means
either

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you have to package this thing really
well, to keep

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the absorbates off, to keep off all
molecules, basically have to

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put it in vacuum or you have to regard
this thing,

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not as a conductor of electricity, but as
a sensor, right?

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It can sense when molecules fall on it.

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So one person's packaging nightmare is
another person's sensor.

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That's the joke that people like to use.

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Okay.

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So, if you don't like carbon, then there
are other alternatives.

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We can also make nanowires out of
semi-conductors.

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So let's talk of it, a bit about
semi-conducting nanowires.

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There are methods for growing single

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crystal semi-conductor wires, that are
maybe.

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A few nanometers or 10 nanometers in
diameter, and again, many microns long.

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So they have very very large aspect ratio,
they look like wires, okay.

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And these structures have been
demonstrated

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in many different kinds of semi-conductor,

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including Silicon and lots of other things
too, okay.

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And i-, in addition to being able to grow

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these things in Silicon or Gallium
Arsenide or other materials.

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You can also dope them, so you can do
controlled doping distribution,

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either along the length of the wire, so
you could have a p-type

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length, and an n-type length, a p-type
length, and intrinsic length and so

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on, or from the inside out, so you can
make core shell structures,

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where the interior is, is, conducts
electrons and the exterior

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conducts holes, or something like that,
and so these complicated structures.

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which can be grown in controlled ways
offer

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a lot of very exciting possibilities for
example you

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might imagine building an, a pn junction
into the

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wire so your wire then becomes a
transistor itself.

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So there are, these are exciting
possibilities.

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there are a number of advantages, number
one, because these things are grown in

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a more controlled environment, the
reproducability is

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much improved relative to carbon nano
tubes.

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the they're grown as crystalline,
crystalline materials with facets, and

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so you can really know which facet you're
growing on.

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And so on.

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They're similar in their benefits, as far
as electron conduction to carbon

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nano tubes, it's still a 1 d conductor,
and so you still

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have a lot of the same benefits.

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there's a known chemistry for the contact,
so when these nano wires are contacting

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the metal, that junction is much more

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well characterized than a carbon metal
junction.

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this issue of surface states, is a little

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bit more well controlled, because you can
passivate

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the surface, and in principle, it should
be

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easier to interface these materials with
other materials.

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So a silicon nano wire contacting a
germanium nano wire is going to be

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a very well characterized contact.

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But of course there are also challenges,
right?

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You have the same wiring problem as you do
with nanotubes, if

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you have a single nanowire connected to a
macroscopic metal set of

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leads, then you really, are you really
doing nano science, are you

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really taking advantage of the nano scale
nature of your active area?

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many of these materials will oxidize in

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air, they will undergo oxidation chemistry
and

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that will change their properties, so one
has to be careful about that.

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You have the same susceptibility to single
defects, as you

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do in nano tubes, because these are still
1 d conductors.

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So, you'll have that same issues, and

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you'll have a similar vulnerability to
surface modifications.

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So, once again is it a sensor, or is it a
packaging nightmare?

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and if you are trying to build electronic
circuits,

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then packaging is going to be the answer
to your

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question, and nightmare is a word you'd
rather not have have to use.

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So, let's talk about what we've discussed
in this lecture.

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First of all, really, there's a key
challenge when shrinking down circuits.

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It's not simply the active components,
it's also the passive components.

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That is to say, just the wires that
connect things up.

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These can really be a big challenge.

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Number two, there are other materials
other than the conventional materials used

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by the semiconductor industry that offer
possible nano-solutions.

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That are very small.

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And can be really, really good conductors.
Even better than copper.

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Small does mean highly sensitive to the
environment.

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And so packaging becomes a real challenge
on the nanoscale.

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Probably a bigger challenge the smaller
you go.

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And lastly we still have not yet figured
out how to wire up a billion nano-devices.

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We can wire up one nano-device, but then
again, are you really doing nanoscience

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if you're not taking advantage of the fact
that your active area is small?

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Because you're sort of killing the
advantage by connecting it

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to a big thing, so we've got to figure out

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how to place a million or a billion nano
things

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and wire those up, and that's still a big
challenge.

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So, we'll talk more about nano

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materials as potential solutions for the
semi-conductor

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industry in the next lecture.
Thanks for listening.