Model for the current plateau observed on the (310) plane of a room temperature W field electron source

An unexpected pause is observed in the usual exponential current decay when residual gas adsorption occurs on the (310) plane of a room temperature W field emitter (CFE). For most ultrahigh vacuum environments currently achievable, the primary residual gas is H2. High index crystal planes are known to consist of terraces separated by step walls. For a (310) orientation, the bcc crystal terraces and step walls consist of (100) and (110) orientations, respectively. Evidence is presented that the step wall adsorption sites are the preferential sites for initial H2 adsorption causing the work function (ϕ) to decrease. According to Fowler–Nordheim (FN) theory, which relates the CFE emission current and ϕ, the current should increase. Instead, the decrease in ϕ is accompanied by a concomitant decrease in the FN equation pre-exponential factor, which leads to a relatively constant value of the CFE current until these step-wall sites are saturated, after which the ϕ increases with further H2 adsorption. The latter leads to an exponential decrease in the emission current. This fortuitous balance between the ϕ and FN equation pre-exponential factor upon initial H2 adsorption appears to be unique to the (310) crystal plane of a W CFE.