Silver whiskers and epitaxial layers on tungsten tips as fem specimens; Their growth and behaviour on annealing and oxidation

In order to investigate whether field-emission microscopy (FEM) can be applied to the examination of adsorption on low-melting metals, a study has been made of the preparation and the stabilization of silver surfaces that are suitable as field electron emitters. Single silver whiskers, showing negligibly distorted FEM patterns, could be grown at 873 K by in-situ deposition of silver vapour onto the apex of thermally cleaned blunt tungsten tips. Single-crystal silver layers could be grown at 296 K by the same technique on field-evaporated sharp tungsten tips. The nucleation and growth of the layers take place more uniformly on these crystallographically perfect surfaces than on thermally cleaned tips. At elevated temperatures (673 K) considerable diffusion of silver from the apex of the tip to the shank was noticed when the latter had been thoroughly cleaned by previous heating. The occurrence of this effect was avoided by applying the field-evaporation step in the tip-preparation procedure after the tip had been strongly oxidized ; thus, the oxidized tungsten bordering the cleaned apex acts as a silver diffusion barrier. The silver surfaces obtained have a high degree of cleanliness. Adsorbed oxygen can easily be removed by heating to 673 K ; hardly any blunting results from this treatment. However, when a silver layer has been strongly oxidized, complete field desorption of silver reveals oxidation also of the tungsten at the tungsten-silver interface. Application of positive voltages to a crystalline silver layer on a tungsten tip at 77 K in ultra-high vacuum deforms the layer extensively before actual field evaporation of silver is achieved.