Transition metal carbide field emitters for field-emitter array devices and high current applications

We report on the use of transition metal carbides, primarily zirconium carb ide, for field emission cathodes. This work encompasses three areas first, the use of carbide films as overcoatings on molybdenum field-emitter cathod es, second, the deposition of thick carbide films to form field-emitter con es directly in field-emitter arrays, and third, the use of single emitters formed from macroscopoic, single crystal carbide for use at high currents a nd in adverse vacuum conditions. Recent work with ZrC film overcoatings on Mo field emitters has focused on the processing steps required to obtain st able, low work function surfaces. Clean substrates are found to be essentia l. Emission degradation sometimes noted after exposure to air has been succ essfully reversed through operation in 10(-5) Ton pressure of hydrogen. The most promising results applicable for use with arrays have been achieved w ith the direct deposition of zirconium carbide cones in blank field-emitter arrays. Thick films were deposited by physical vapor deposition from refin ed, crystalline carbide sources. Operation of single and multitip arrays ha s resulted in high currents (>1 mu A) per tip and relatively low turn-on vo ltages given the gate geometry. These improvements are primarily attributed to the lower work function associated-with these carbide materials. We als o report on the use of etched transition metal carbides as single emitters for applications including high-current, small-spot sources for accelerator s and free electron lasers, and cold cathodes for operation in poor vacuum environments. Stable, single emitter currents in the mA range have been dem onstrated. A brief discussion is given regarding the ability to extract inf ormation from I(V) data using the Fowler-Nordheim equation and log(I/V-2) v s 1/V plots. From this we propose a method to determine if nanoprotrusions account for a large fraction of the emission current measured. (C) 1999 Ame rican Vacuum Society. [S0734-211X(99)04102-5].