Portland, Ore. -- Scientists at the U.S. Naval Research Laboratory in Arlington, Va., said they have fabricated arrays of high-current carbon nanotube field emitters with a record-low gate voltage of just 60 volts for emissions of up to 1.2 amps per square centimeter.
The arrays of carbon nanotube field emitters were grown by researchers David Hsu in the Chemistry Division and Jonathan Shaw of the Electronics Science and Technology Division, using chemical vapor deposition (CVD) on silicon substrates that had been prepatterned with 1-micron-diameter posts centered on 2.5-micron-diameter metal aperatures. Nickel or iron catalysts were deposited over the gated structures and then removed from the surfaces. The carbon nanotubes grew from the catalyst particles during CVD using ammonia and ethylene gas.
By applying a relatively low voltage to the gate apertures, a relatively high electric field was produced at the nanotube tips, thereby causing electrons to spew out by field emission, the researchers reported. None of the emitted electrons hit the gate aperatures where they could cause damage; rather, they were collected at the anode.
The proximity of the gates yielded the high local electric fields from the relatively low voltage input.
The high electric fields, in turn, yielded high-density field emitter arrays packing more than 75,000 emitters per square millimeter, according to the researchers.
The scientists said their low-voltage emitters do not degenerate during extended usage, as high-voltage field emitters do. The low gate voltages avoid the damage typical in high-voltage devices, such as gas ionization and residual ion sputtering, they said.
The field emitters were said to perform well at temperatures up to 700°C and in the presence of water vapor and other common gases.
The Navy researchers said the 1.2-amps-per cm2 current density is adequate for applications such as spacecraft propulsion systems (ion thrusters and tethers), miniature x-ray sources, flat-panel displays and mass spectrometers. Higher current density as high as a factor of 20 is also needed for high-frequency electronics.
For the future, the researchers are aiming to improve the uniformity of emission across the array, to boost the current density and to experiment with various open-aperture configurations.
The group has filed half a dozen patent applications pertaining to gated carbon nanotube field emitters.