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

Abstract

We report on the use of transition metal carbides, primarily zirconium carbide, for field emission cathodes. This work encompasses three areas first, the use of carbide films as overcoatings on molybdenum field-emitter cathodes, second, the deposition of thick carbide films to form field-emitter cones directly in field-emitter arrays, and third, the use of single emitters formed from macroscopoic, single crystal carbide for use at high currents and in adverse vacuum conditions. Recent work with ZrC film overcoatings on Mo field emitters has focused on the processing steps required to obtain stable, low work function surfaces. Clean substrates are found to be essential. Emission degradation sometimes noted after exposure to air has been successfully reversed through operation in 10−5 Torr pressure of hydrogen. The most promising results applicable for use with arrays have been achieved with the direct deposition of zirconium carbide cones in blank field-emitter arrays. Thick films were deposited by physical vapor deposition from refined, crystalline carbide sources. Operation of single and multitip arrays has resulted in high currents (>1 μA) per tip and relatively low turn-on voltages given the gate geometry. These improvements are primarily attributed to the lower work function associated with these carbide materials. We also report on the use of etched transition metal carbides as single emitters for applications including high-current, small-spot sources for accelerators and free electron lasers, and cold cathodes for operation in poor vacuum environments. Stable, single emitter currents in the mA range have been demonstrated. A brief discussion is given regarding the ability to extract information from I(V) data using the Fowler–Nordheim equation and log(I/V2) vs 1/V plots. From this we propose a method to determine if nanoprotrusions account for a large fraction of the emission current measured.