Comparison of low-voltage field emission from TaC and tungsten fiber arrays
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I. INTRODUCTION As a result of modern fabrication techniques, field emission cathodes have been developed that are capable of producing relatively large current densities (5 A/cm 2 cw and 30 A/cm 2 pulsed) at low voltages (100-200 V). 1 " 3 The device reported here has been fabricated using thin-film technology and is based on an in situ eutectic composite containing arrays of conducting fibers with large spatial packing densities.4 This low voltage field emitter (LVFE), shown schematically in Fig. 1, consists of an array of free-standing metal fibers, each surrounded by a concentric metal electrode (extractor) in close proximity to the fiber tip. Due to the small interelectrode spacing (typically less than 1 ^ m ) and the field enhancement provided by the pointed emitting tip, a relatively small potential applied to the extractor produces an electric field at the pin tip sufficient to cause emission of electrons in vacuo. Simultaneous, parallel operation of an array of these emitters is possible because they are electrostatically isolated from each other by the integral extractor electrode. High fiber packing densities provided by the eutectic composite structure give these devices the potential for significant increases in current density relative to the thermionic cathodes in use today. Details of the fabrication process for the LVFE, including growth of the eutectic composite and formation of the emitter structure, are available in the literature.1'3'5 In theory this emitter structure can be fabricated on any substrate with free-standing fibers of a suitable emitter material. In addition to tungsten (the most frequently used emitter material), arrays of Mo and Si have also been investigated as field emission sources using similarly constructed thin-film geometries.6'7 In a J. Mater. Res. 3 (1), Jan/Feb 1988
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previous article8 the effects of emitter tip geometry on emission current were examined for field emitter arrays fabricated from W fiber-ZrO2/Y2O3 matrix (ZYW) composites. These cathodes had low leakage current because of the insulating oxide matrix, and they provided a maximum dc emission current of 5.0 A/cm 2 at 160 V. Arrays of transition-metal carbide fibers have been emission tested in exposed fiber geometries, using large planar anodes, at interelectrode spacings that required
Mo EXTRACTOR
FIG. 1. Schematic diagram of a single LVFE electron gun based on a TaC-NiCr eutectic composite.
0003-6951 /88/010067-08$01.75
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© 1988 Materials Research Society
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Cochran, Lee, and Hill: Comparison of field emitter array devices
kilovolt potentials to achieve the required electric fields for emission.9 Several eutectic composites, including VC, NbC, and TaC, all in a NiCr matrix, were tested in this manner, and produced emission current densities as high as 5.8 A/cm 2 . In addition to being available in the form of eutectic composites, an attractive feature of the carbide fibers is their low thermionic work function,10 approxim
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