EMISSION PROPERTIES OF ELECTRON POINT SOURCES

Emission properties and stability of ultrasharp [111]-oriented tungsten tips were studied in a combined field ion/electron microscope and with an electron energy analyzer. In particular the divergence angle phi of the emitted electrons is important for novel developments in electron microscopy. It is found that this divergence angle depends only weakly on the geometry of the ultimate tip (monomer, trimer) but shows a strong variation with overall tip radius, with dull tips providing smaller divergence angles. For emission currents up to 100 nA we found only a slight dependence of the divergence angle on current. At higher currents, however, we observed a distinct increase of this angle, which is attributed to Coulomb interactions between the electrons. The energy distribution of the field-emitted electrons showed only weak deviations from standard field-emission theory.