Carbon Nanotube Electronics

parkagendaElectronics - Devices

Nov 2, 2013 (3 years and 10 months ago)

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Carbon Nanotube Electronics


Phaedon Avouris

IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598, USA


There is currently a wide interest in understanding material properties and phenomena on the
nanometer scale. Part of this in
terest is due to the general trend of miniaturization observed in many
technologies, particularly in electronics. For the main part, however, it is due to the fact that material
properties and physical processes can change drastically in this scale where t
he quantum nature of
matter becomes clearly evident. I will start my talk with a brief survey of these expected changes, and
then focus on nanoelectronics. The scaling behavior of silicon transistors will be discussed and
problems that will be encountered
in the not too distant future will be presented. Molecular electronics
is one of the proposed successors of silicon technology. Among the different systems considered so far,
carbon nanotubes appear to be the most promising.


Carbon nanotubes (CNTs) are a
new class of macromolecules with unique mechanical, thermal
and electrical properties. In particular, depending on their chirality, the CNTs can be metals with a
conductivity better than copper that can withstand current densities 100
-
1000


higher that nor
mal
metals, or semiconductors like silicon but with a diameter
-
dependent band
-
gap. Because of the long
length (many micrometers) and small diameter (~ 1 nanometer) they approach an ideal one
-
dimensional (1D) physical system. This reduced dimensionality str
ongly affects their electrical
properties leading to a new mode of electrical transport: ballistic transport and other interesting
properties.

I will first discuss the electronic structure of CNTs and its influence and that of the reduced
dimensionality on

their electrical conductance. Then I will focus on field
-
effect transistors (CNTFETs)
that use a single semiconducting single
-
walled CNT as their channel. Hole conducting (
p
-
type) CNT
transistors will be discussed, followed by ambipolar and electron
-
based

(
n
-
type) nanotube transistors. I
will show that all CNTFETs fabricated from as
-
produced CNTs are
p
-
type.
n
-
type CNTFETs, however,
can be made by: (a) doping
p
-
type devices or (b) simply annealing in vacuum
p
-
type devices to remove
adsorbed oxygen. The lat
ter process is reversible; exposure of the
n
-
CNTFET to oxygen reconverts it to
a
p
-
CNTFET. I will show that the two methods of
p
-
to
-
n

conversion proceed by different mechanisms.
From the effects of the ambient gases along with more detailed electrical meas
urements we have
concluded that the CNTFETs, unlike the conventional silicon transistors, are “Schottky
-
barrier
transistors;” thus the gate field induces switching by modulating the contact resistance (the junction
barriers). Oxygen adsorption at the junct
ions modifies the barriers (i.e. the local band
-
bending of the
CNT) and affects the injection of carriers (holes or electrons). Model calculations that support these
conclusions will be presented. Despite the fact that our CNTFETs are still far from being
fully
optimized, comparison with the state
-
of
-
the
-
art silicon MOSFETs with our top
-
gated CNTFETs shows
that the latter have superior performance characteristics.

Having both
p
-
type and
n
-
type CNTFETs we are able to fabricate the first nanotube integrated
c
ircuits: complementary logic gates. Two types of “NOT” gates (voltage inverters) are presented: an
inter
-
molecular and an intra
-
molecular gate. The latter involves the encoding of the logic function
along the length of a single nanotube.


Finally, I will d
iscuss the fabrication of CNTFETs using as
-
produced CNTs bundles containing
both metallic and semiconducting CNTs. We accomplish this by using the process of “constructive
destruction” which is based on controlled current
-
induced breakdown. This process ef
fectively
sidesteps the need to first remove the metallic nanotubes from the bundles, or to selectively synthesize
the semiconducting tubes. Using this simple new approach, arrays of CNTFETs can easily be
fabricated.