Electron emission yield from thin Cu layers on Al induced by 3 MeV He⁺⁺ and 3 keV electron impact

For 3 MeV He⁺⁺ and 3 keV electron projectiles the kinetic electron emission yield for copper layers of different thickness on an aluminum backing was measured. Since the electron emission yield of Cu is almost a factor of 2 higher than that of Al an increasing electron yield was measured for increasing thickness of the Cu layer until the yield of pure Cu was reached. For impinging He⁺⁺ ions the measured yields can be fitted by a function of the layer thickness which contains an exponential term with a characteristic length of 2.6 nm. For 3 keV electron projectiles a much longer characteristic length of 4.8 nm was observed. To explain this dependence our Monte-Carlo simulation program for electron excitation and transport in metals was extended to handle layered structures. In this program the incoming projectiles generate primary electrons due to excitation or ionization of the target (electron gas and core electrons). The primary electrons propagate through the amorphous target and interact with the atomic cores and with the target electrons, thereby generating electron cascades. The propagating electrons are traced until they possibly leave the target or until their energy drops below the energy of the surface barrier. At the layer interfaces the energy levels of the metals are adjusted to have equal Fermi energy. Possible reflection of moving electrons at the interface is taken into account. Using this model the electron emission yield from the surface has been calculated as a function of layer thickness. The measured thickness dependence of the electron yield is well reproduced by the simulation which permits an interpretation on the basis of the underlying interaction processes.