Influence of ambient gas pressure and carbon adsorption on dark current emission from a cathode

Electronic field emission current (dark current) from surfaces under vacuum at high field strengths can be reduced by the injection of gas into the ambient volume. A possible reversible mechanism responsible for this gas effect is proposed. The mechanism involves the formation of nanoscale emitter structures by polymerization of hydrocarbon contamination with low-flux ion bombardment at low pressure, and the destruction of these structures by high flux ion bombardment at sufficiently high pressure. Experimental evidence, in particular, x-ray photoelectron spectroscopy analysis of the electrode, is provided in support of this mechanism. Density functional theory calculations are presented to show that the morphology of the carbon layer, not its chemical composition, is the important parameter influencing dark current levels.