Electron emission yield from thin Al and insulating layers induced by 3 MeV He²⁺ and 3 keV electron impact

The kinetic electron yield was measured for the impact of 3 MeV He²⁺ ions and 3 keV electrons on thin layers of Al on Cu, of Al₂O₃ on Al, and of CeO₂ and CaF₂ on Si backings. The dependence of the yield on the layer thickness was determined. For Al on Cu a decreasing yield was observed for increasing Al layer thickness, since the yield of Cu is higher than that of Al. For insulating layers increasing yields were measured for increasing layer thickness. The observed yield dependencies were fitted by sums of exponential functions. The characteristic lengths of the exponential terms were interpreted as emission lengths. For Al and the metal oxides a small emission length in the range of 2-3 nm was found, for CeO₂ and electron impact on Al a second exponential term with a large emission length, 20-100 nm, was necessary to describe the measured yield dependence, which is probably due to backscattered projectile electrons and δ-electrons. The Al results are compared with computer simulations. The increasing yield of CaF₂ layers could be fitted by a function with only one exponential term and an emission length of 10.8 nm for He impact and 20.6 nm for electron impact.