A novel vacuum electron source based on ballistic electron emission

A novel vacuum electron source is proposed, consisting of a sharp metal tip that is positioned within tunnelling range of a conducting thin foil. A tunnelling bias potential is applied across the gap such that electrons are injected into the foil. The electron beam emerging from the opposing surface of the foil has a narrow energy spread due to the energy filtering effect of the potential step at the surface. Calculations for the electron emission are presented that include the tunnel junction, ballistic electron transport through the foil, and subsequent emission from the foil surface. It is expected that the emission energy spread is 100 meV for an emission current ratio of 1%. For a typical tunnel current of 10 nA, this implies an emission current of 0.1 nA. The corresponding reduced brightness is 6.6 x 10(7) A m(-2) sr(-1) V-1. For the emission to be sufficient for a practical electron source, it is necessary that the foil thickness does not exceed the mean free path for electron scattering which is approximately 5 nm at the excitation energy of interest. Polycrystalline platinum foils of 5 nm thickness have been fabricated that are mechanically stable and withstand the fords exerted by a scanning tunnelling microscope tip under ambient conditions. Further experimental effort is currently undertaken.