Noise and emission characteristics of NbC/Nb field emitters

Several applications require single field emitters with adequate long-term stability and life in a moderate vacuum. Our previous studies have shown that the refractory carbides of transition metals are capable of higher currents and are more environmentally tolerant than common field emitter materials such as W, Mo, or Si. We undertook a systematic study of the emission characteristics and stability of individual NbC/Nb microtips at low currents and at residual pressures ranging from 10(-9) to 10(-5) Torr. We reported preliminary results one year ago [J. Vac. Sci. Technol. B 22, 1643 (2004)]. We now present additional data and interpretations. These experimental data were obtained by two types of measurement: (a) continuous recording of the field emitted current at a fixed voltage and (b) recording of the emitted current with a saw-tooth applied voltage, which yields a large number of Fowler-Nordheim plots which are analyzed by the slop-intercept method. Our experimental results show greater instability than we expected. We conclude that for NbC/Nb emitters fabricated by physical vapor deposition, emission is dominated by a very small number of extremely sharp but unstable nanoprotrusions, with an effective emitting area (from the slope-intercept charts) of the order of a few nm(2). Consequently, we lack the stability and control of the beam directional intensity which are required for most applications. Prior to stable operation, single emitters should be conditioned to eliminate sharp protrusions, enhance paraxial emission, and increase the effective emitter area. We are now studying effective conditioning techniques for microtips. (c) 2005 American Vacuum Society.