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 h igher than that of Al an increasing electron yield was measured for increas ing thickness of the Cu layer until the yield of pure Cu was reached. For i mpinging He++ ions the measured yields can be fitted by a function of the l ayer thickness which contains an exponential term with a characteristic len gth 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 structure s. 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 el ectron cascades. The propagating electrons are traced until they possibly l eave 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 adjus ted 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 calc ulated 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. (C) 2 000 Elsevier Science B.V. All rights reserved.