Review of synthesis of low-work function Cu–Li alloy coatings and characterization of the field emission properties for application to field emission devices

Abstract

Alkali metals have extremely low-work functions and are therefore expected to result in significant enhancement of the electron emission if they are used as coatings on Mo or Si microtip field emission arrays (FEAs). However, the alkali metals are physically and chemically unstable in layers exceeding a few nanometers in thickness. Maximum enhancement of electron emission occurs for alkali metal layers 0.5–1 monolayer thick, but it is extremely difficult to fabricate and maintain such a thin alkali metal coating. We review here our work of the last three years focused on developing Cu–Li alloy coatings for application to field emitter Spindt-type tip arrays and fabrication of edge emitters with low-voltage high electron emission characteristics for field emission devices. The Cu–Li alloy coatings provide a chemically and thermally stable, self-replenishing, segregated lithium monolayer on the surface of the host Cu layer, resulting in a substantial reduction of the work function of the alloy, compared to that of a thick Li layer or a pure Cu surface. We present results that show a 13-fold reduction in the threshold voltage for electron emission, compared with uncoated Si FEAs. The Cu–Li alloy coatings yielded tip and edge emitters with threshold voltages of 2–10 V/μm, depending of the coating thickness and Li concentration in the alloy. New results are presented that show that we can now coat gated Si tip arrays with Cu–Li alloy films, which results in very low emission voltage.