Monday, June 19, 2006

"CHIPS: Record-low gate voltage for nanotube field emitters"

Scientists at the U.S. Naval Research Laboratory in Washington D.C., 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. Its low voltage-high current operation also yields the high transconductance--the rate of increase in current with increase in voltage--that is necessary for many electronic device applications. 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 acetylene 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. The proximity of the gates yielded the high local electric fields from the relatively low voltage input. The high-density field emitter arrays packed 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 rapidly as high-voltage field emitters do. The low gate voltages avoid the damage typical in high-voltage devices, such as dielectric breakdown and residual ion aputtering, 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 that the 1.2 amps per square centimeter current density is more than adequate for applications such as spacecraft propulsion systems (ion thrusters and tethers), miniature x-ray sources, cathodoluminescent devices (flat-panel displays) and mass spectrometers. Another factor of ten to twenty higher current density is needed for high frequency electronics.