Atomically Sharp Field Emitters and Oxygen Caused Thermal Faceting of W111 Tip

摘要

Ultrasharp field emitters produced on the basis of the W [111] tip have been investigated in the last decade for their unique properties as sources of highly collimated and coherent electron beams, emission stability and easy emitter regeneration. Several methods of sharp microtip preparation had been developed, including those based on prolonged annealing of the tip in oxygen atmosphere (with O{sup}2 pressure in the range 10{sup}(-4)-10{sup}(-5) Torr) [1-3], where mechanisms of oxygen induced massive corrosion of tungsten and desorption of tungsten oxides were utilised to reduce the tip radius of curvature. But it is the phenomenon of faceting which causes the very apex of the W[111] emitter to attain its desirable shape. It is known that adsorption of oxygen on tungsten causes high surface energy anisotropy, what leads to faceting. Oxygen induced faceting of W[111] plane has been reported to start at temperatures as low as 800 K [4] and could be observed after annealing at temperatures as high as 1800 K [5]. This phenomenon was observed both on planar crystals and on curved surfaces of field emitter tips, for low O{sub}2 exposures (0.5 L, 1020 K [6]; 0.6 L, 1200 K [7]) as well as for high exposures (up to 60 L [8], 1000 L [6]). Unfortunately, there is few data concerning oxygen promoted faceting of W[111] for intermediate O{sup}2 exposures 0.5-several L.