Quantitative Low Energy Ion Scattering - accomplishments and challenges

Institut f. Experimentalphysik, Atom- und Oberflächenphysik, Johannes Kepler Universität Linz, Austria

Some fifty years ago the well-established ion beam techniques were applied at keV energies using noble gas ions and enhanced surface sensitivity was observed [1,2,3]. Very soon, Low Energy Ion Scattering (LEIS) developed to a widely used tool for analysis of structure and composition of solid surfaces [4,5] and references therein. Two features contribute to the great success of LEIS: first, its superb surface sensitivity and second, the fact that for most applications the yield of ions back scattered from one atomic species is independent of the other atoms present in the surface (absence of matrix effects), e.g. [6,7] and others.  

Quantitative surface composition analysis is based on accurate knowledge of the differential scattering cross section d/d and the fraction of ions amongst the backscattered particles. Due to the low ion energies involved, a realistic model is required for the influence of electronic screening on nuclear scattering. For this purpose, the universal potential has been shown to be a good choice as long as not too low ion energies are employed [8]. To apprehend why in general LEIS is not sensitive to band structure effects requires understanding of the prevailing charge exchange processes – mainly Auger neutralization and reionization in a close collision [5]. Recently, it has been demonstrated that due to distinct neutralization efficiencies of different allotropic forms of carbon [9] the concentration of organic carbon on graphene can be quantified when an optimized set-up is used [10].  

Very recently, interesting LEIS applications to ultrathin subsurface layers were reported. To gain quantitative information in this case one has to successfully handle additional processes: electronic stopping and multiple scattering related processes such as depth-dependent angular spread and increase in path length, loss of the unique relationship between final energy and scattering depth for a specific collision partner. It will be pointed out how to gain quantitative information on electronic stopping and how the influence of stopping and multiple scattering can be handled by use of Monte-Carlo simulations [11].  

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[10] S. Prusa, LEIS Users meeting 2014, May 22, 2014, Enschede, Netherlands.
[11] P. Bruner et al., J. Vac. Sci. Techn. A33, 01A122-1 – 01A122-7 (2015).