Development of edge field emission cold cathodes based on low work function Cu–Li alloy coatings

Abstract

Vertical edge emitter arrays of thin walled (500–8000 Å) hollow cylinders have been fabricated by the deposition of Cu–20 at. % Li alloy (Cu4Li) thin films on arrays of Si posts, followed by selective etching of the Si core. What remains is a hollow cylinder of Cu4Li, where the thickness of the emitting edge is determined by the deposited film thickness. Additionally, lateral edge emitters were fabricated from arrays of holes in SiO2 or Si3N4 films on flat Si surfaces. In this case, the emitting edge is formed by selectively overetching the Si beneath the hole, creating an edge from the overhang of the SiO2 (Si3N4) thin film upon which the Cu4Li alloy is deposited. The Cu4Li alloy provides a surface covered with a monolayer of Li atoms formed via Gibbsian segregation. The formation of Li–Cu atom dipoles on the surface of the alloy yields a work function lower than that of either a pure Li or Cu surface, resulting in enhanced electron emission and very low threshold fields. The alloy coating maintains a segregated monolayer of Li atoms on the surface even under adverse operational conditions, including ion bombardment of the surface. Edge emitters offer many potential advantages over the standard microtip arrays including ease of fabrication, higher emission currents per emitter due to the larger emission area of the edge, and greater physical robustness. The Cu4Li edge emitters exhibit threshold fields as low as E0=5 V/μm, high current densities, and long-lived, stable emission.