NRL Inventions Bring Carbon Nanotubes Closer to Electronic Device Application


4/22/2002 - 25-02r
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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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