Secondary electron yield enhancement by MgO capping layers

The yield of secondary electrons emitted from an epitaxial three monolayer (3 ML) NiO(1 0 0)/Ag(1 0 0) film excited by soft X-ray linearly polarized synchrotron radiation at the Ni L2,3 absorption threshold has been measured for different values of the thickness of a MgO(1 0 0) capping layer. Compared with the as grown 3 ML NiO(1 0 0)/Ag(1 0 0) film, we observe a significant enhancement by about a factor 1.2 of the secondary electron emission for the capped 8 ML MgO(1 0 0)/3 ML NiO(1 0 0)/Ag(1 0 0) sample. A further substantial yield enhancement by a factor 1.6 with respect to the uncapped NiO sample is observed after deposition of an additional 8 ML MgO(1 0 0) film, for a total capping layer thickness of 16 ML. The observed secondary electron yield enhancement is discussed in terms of modified electronic structure, surface work function changes, and characteristic electron propagation lengths. 2009 Elsevier B.V. All rights reserved.