Controlled electron
emission sources are critical to
a wide range of Navy and commercial
technologies, including
high power electronics, display technologies,
photon and x-ray
sources. NRL scientists have recently demonstrated
new electron
sources by growing carbon nanotubes directly inside
micro-fabricated structures on a silicon chip. Carbon nanotubes,
"re-discovered" in 1991, resemble tiny cylinders made
of a few atomic layers of graphite, having diameters ranging
from 2 to many tens of nanometer.
Dr. David S.Y. Hsu
of the Chemistry Division and Dr. Jonathan
Shaw of the
Electronics Science and Technology Division collaborated
on the
project, with Dr. Hsu doing the fabrication and Dr. Shaw
doing
the electrical measurements.
The new devices consist
of many small sets of carbon nanotubes,
with each set provided
with an individual metal aperture (called
a gate). Application
of a voltage to the gate apertures produces
a high electric
field at the nanotube tips, causing electrons
to be produced by
field emission. Dr. Hsu developed a means of
growing the
nanotubes inside arrays of the gated apertures. Only
few
reports on gated carbon nanotube emitters have been previously
published because of the difficulty of fabrication. According
to Dr. Hsu, NRL's gated structures are different from the others
and have outperformed them.
Dr. Hsu has received one
U.S. patent and has four US patent applications
on gated carbon
nanotube field emitter arrays.
The successful
fabrication and characterization of these pro-totypes
represent
an important advance that will bring carbon nanotubes
closer to
device application and provide significantly improved
field
emitter arrays. Potential Navy applications include high
voltage, high temperature, and high frequency electronics,
spacecraft
propulsion systems (ion thrusters and tethers),
miniature x-ray
sources, cathodoluminescent devices (flat panel
displays), and
miniature mass spectrometers.
The carbon nanotubes grown for the NRL field emitter had diameters
of 20 30 nanometers. Electrons move through the tubes with
unusually little resistance, so they are ideal for use as field
emitters, which carry very high current densities. Also important
for field emission are the graphite sheets' high mechanical strength
and low chemical reactivity. For most device application, the
nano-tubes need to be "gated" (i.e. having a gate
electrode
in close proximity to a group of them) to en-able low
voltage
operation as well as local small array
control.
Field emitter arrays (FEAs) are
micro-fabricated arrays of cold
field emission structures. The
conventional FEAs, known as the
"Spindt" tips,
usually consist of cone-shaped silicon
or metal tips centered
in small circular gate apertures. Sharp
emitter structures and
small gate apertures are necessary for
enabling field emission
at low gate voltages. The field emission
current increases very
rapidly with the field, so changing the
gate voltage by a small
fraction can produce a large change in
current. Ideally, none
of the emitted electrons strike the gate
and all are collected
at the anode, so FEA-based vacuum electron
devices can have
high current gain. Unlike glass "tubes"
with
thermionic sources, a packaged FEA-based device need not
be any
larger than a typical semiconductor package. Serious drawbacks
of conventional FEAs include emission degradation from trace
ambient gases and susceptibility to destruction by electrical
arcing that can occur between the gates and emitters. Previous
work by Dr. Shaw showed that in-sulating oxides formed on the
chemically active silicon or metal emitters can cause arcs. The
arcing problem is also compounded by the relatively high operating
voltages of current conventional FEAs (usually 70
120V).
The nanotube FEAs fabricated at NRL start to
produce measurable
current at gate potentials below 20V and
have produced as much
as 1mA at 41V. The low operating voltages
are mainly due to the
natural sharpness of the nanotubes. Drs.
Hsu and Shaw hope to
increase the maximum current. The devices
are more robust than
most previous types of FEAs; they operate
well in the presence
of water vapor and other common residual
gases, and at temperatures
up to 700°C (1200°F). Carbon
nanotube emitters do not
have an electrical arcing problem
because no insulating oxide
is formed on carbon. Because of the
low gate voltages required,
it is possible to avoid gas
ionization and damage due to residual
ion sputtering. The
nanotube FEAs have higher gate currents than
other FEAs, but we
have measured gate cur-rents as low as 2.5%
of the anode
current, the lowest reported to date.
Dr. Hsu grew
nanotubes on two types of gated structures; one
contained a
silicon post, the other did not. In both cases, nickel
or iron
catalysts are deposited over the entire gated structures,
then
removed from the oxide surfaces. The carbon nanotubes grow
from
the catalyst particles during chemical vapor deposition
using
ammonia and ethylene or acetylene gas. Figures 1 and 2
are SEM
images of the resulting cNT-decorated emitter cells.
Dr. Shaw used a special ultra high vacuum chamber equipped with
probe wires to contact the FEAs, measure emission current, and
analyze the energy of the emitted electrons. The sample temperature
and the pressure of various gases were varied during the
measurements.
The measurements showed that the current of
higher energy electrons
saturates, and that the effect changes
with the temperature and
when gases are present.
Work is planned to refine these gated carbon nanotube emitters,
especially with respect to obtaining even higher currents and
more reliable fabrication.
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