Untitled - IAP/TU Wien - Technische Universität Wien

Nanostructure Characterisation by Electron Beam Techniques
Energy loss functions and IMFPs derived by factor analysis of reflection
electron energy loss spectra
Hideki Yoshikawa * and Shigeo Tanuma
Advanced surface chemical analysis group, National Institute for Materials Science,
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
*YOSHIKAWA.Hideki@nims.go.jp
The energy loss function (ELF) given by optical constant is the basis of the fields of the estimation
of inelastic mean free path (IMFP) and background spectra for the quantitative surface chemical analysis by
electron spectroscopies. The optical constants are evaluated by optical measurements of
reflectivity/absorption coefficients and have been collected into handbooks and databases. But in high energy
vacuum-ultraviolet (VUV) region, optical experimental data are often missing because this requires
synchrotron radiation photon sources. Another approach to evaluate the ELFs is the analysis of electron
energy loss spectra (EELS), which can be easily measured in the VUV energy region with a standard
laboratory electron spectrometer. However, EELS spectra include complicated factors: surface energy loss,
multiple inelastic scattering, interference effects, momentum transfer and so on. Especially, the contribution
of the surface energy loss is inevitable for the reflection-type EELS (REELS) measurement. For determining
ELFs and optical constants, one has to extract the bulk energy-loss-structures from the REELS spectra by
subtracting the contribution of the surface energy-loss. Many previous works have been done for determining
ELFs from REELS measurements. They require stoichiometric clean surfaces to which theoretical formula
linking a relation between bulk and surface energy-loss-structures are applicable. It was not easy to
determine the ELFs of chemical compounds that easily become non-stoichiometric by surface ion-beam
cleaning. Therefore, stoichiometric clean surface is a tough prerequisite for previous methods.
In order to avoid the tough prerequisite for the analysis of chemical compounds, we successfully
obtained the bulk ELFs for semiconductors by applying the factor analysis method to a series of
primary-energy-dependent and emission-angle-dependent REELS spectra. This factor analysis method is
based on a multivariate analysis of REELS spectra and does not require a delicate theoretical evaluation of
the surface energy-loss-structure. Moreover, we have demonstrated that this method is useful to evaluate
bulk ELFs and optical constants for compound semiconductive-materials (GaAs, GaSb etc.) with damaged
surface-layer or contaminated surface whose surface energy-loss-structures can not be evaluated theoretically.
It means that this practical method does not require ideally-clean surfaces and eases the prerequisite for
various compound materials.
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