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Nanoelectronics research at Ames encompasses
topics in molecular devices and miniaturization of
conventional semiconductor devices. The objective is
to acquire the knowledge necessary to build future
generations of computing devices and sensors to
fulfill NASA's challenges in aerospace transport and
space missions. There were three significant accomplishments
in FY99. First, we modeled electron
transport in capped carbon nanotubes and gleaned
the effect of caps and defects on electron emission,
which is important in the use of the nanotubes as
probe tips and wires. Second, through modeling and
analysis we related conductance to mechanical
deformation of carbon nanotubes, which is important
in the use of nanotubes as sensors. Third, we developed
a simulator for quantum mechanical transport
in semiconductor devices, which provides important
capability to study future generations of ultrasmall
devices. A brief description of each follows.
The large length-to-diameter ratio of carbon
nanotubes makes them good candidates for molecular
wires and field emitters, and for use in probe-tip
applications where electron emission from the tip of
the capped tube is important. The results show that
transmission probability mimics the behavior of the
electronic density of states at all energies except the
localized energy levels of a polyhedral cap (figure 1).
The close proximity of a substrate causes hybridization
of the localized state. As a result, subtle quantum
interference between various transmission paths gives
rise to antiresonances in the transmission probability,
at energies of the localized states (figure 1). Our
observations indicate that by appropriately engineering
the location of defects, these antiresonances can
be transformed to huge transmission resonances. This
is especially useful because these resonances offer a
way to obtain a large current density in a narrow
energy window around the localized energy level.
A potential application of carbon nanotubes as
sensors is exploiting the relationship between
mechanical deformation and electronic properties of
the tubes. Our work provides fundamental insights
into this relationship by providing detailed answers
for the band-gap variation with tensile and torsional
strain as a function of nanotube chirality, diameter,
and magnitude of strain. The electronic properties of
a nanotube in equilibrium are determined by indices
(n, m), which define the chirality and diameter. The
significant results are that (1) the magnitude of slope
of band gap versus strain has an almost universal
behavior that depends only on the chiral angle, and
(2) the sign of slope depends only on the value of
(n - m) mod 3. Figure 2 demonstrates these results for
the case of tensile strain. For example, (6,5) and (6,4)
nanotubes have chiral angles close to each other but
the slope of band gap versus strain has opposite
signs.
As devices continue to be miniaturized, modeling
tools based on quantum physics become increasingly
important. The difficulty in building such a
simulator lies in developing a set of physical approximations
that enable solutions on available
supercomputers. We have developed such a two-dimensional
device simulator which solves the
nonequilibrium Green's function equations and
Poisson's equation self-consistently on a nonuniform
spatial grid. Figure 3 shows the self-consistently
calculated charge density for one such case. The
simulation predicts the expected small electron
density close to the gate (x = 0 nm) at the large
potential barrier created by the gate oxide.
Point of Contact: M. Anantram
(650) 604-1852
anant@nas.nasa.gov
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Fig. 1. (a) Density of states (DOS) versus energy in the
cap region of a (10,10) nanotube with a polyhedral
cap. The peak in DOS corresponds to localized
energy levels in the cap. (b) The transmission
antiresonances correspond to the DOS peaks in (a).
The inset shows an expanded region of one antireso-nance.
In the presence of appropriate defects these
transmission antiresonances are converted to reso-nances
capable of carrying large currents when
compared to the background energies.
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Fig. 2. Band gap versus tensile strain: for semicon-ducting
tubes, the sign of slope of band gap versus
strain depends only on the value of (n - m) mod 3
values of 1, -1 and 0, respectively.
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Fig. 3. Self-consistently calculated charge density
when the gate and drain bias is equal to 0.5 V. The
grid spacing is about an Angstrom near the gate
(x = 0 nm), and 10 times larger near the substrate
(x = 120 nm). The x-axis is perpendicular to the gate
(from gate to substrate) and the y-axis is along the
transport direction (from source to drain). X and Y are
in units of nm, and density is in units of cm -3.
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