Introduction

Medium Energy Ion Scattering (MEIS) is possibly one of the lesser known techniques of surface science. It is closely related to Low Energy Ion Scattering (LEIS) and High Energy Ion Scattering (HEIS), but operates with ions (usually H⁺ or He⁺) at energies of around 100 keV. The understanding of MEIS can be achieved on a somewhat more intuitive level than a technique such as LEED, or even STM. A brief description follows, whilst more information is available from York SPG.

Software

The software used to analyse MEIS data by the Daresbury community is based on Igor Pro for Mac and Windows. In the course of my work I have written a few macros, mainly to investigate different r-factors (well, the macros are actually adapted from those written and supplied by Paul Bailey at Daresbury). These macros are available for download, along with notes on some of the bugs I encountered. I also have a a description for producing graphics from MEIS crystals.

The Technique

MEIS is, as the name suggests, an ion scattering technique. The principle of MEIS is relatively easily understood. The sample of interest is mounted on a precision manipulator in vacuum and light ions (usually H⁺ or He⁺ with energy of 100 – 400 keV) are scattered from it before being detected over a range of angles. These scattered ions are also detected over a range of energies and so a two dimensional spectra of scattered ion intensity as a function of scattering angle and energy can be produced. The Daresbury facility detects scattered ions in a 27° window over an energy range of about 1.5 keV. Several of these spectra are usually tiled together to produce a spectrum as shown in figure 1.

Surface sensitivity is gained by aligning the ion beam along a major crystallographic direction. The top few atomic rows then shadow the deeper crystal from the impinging ion beam. This means that the detected scattered ions give information have been scattered mainly from the first few surface layers. By aligning the detector to detect scattered ions around another major direction (so called “double alignment”) surface structural information can be gained. Scattered ions are blocked by atoms in the crystal closer to the surface, which causes a reduction in scattered ion intensity at a characteristic angle. Any surface relaxation or reconstruction is shown by a shift in the angular position of these blocking dips, the appearance of additional dips, etc. See figure 2.

MEIS has the additional advantage that the energy scale represents both a mass and depth scale. The deeper an ion penetrates the crystal, the more energy it will loose, so scattered ions with lower energy were scattered from deeper in the crystal. However there is also a dependency on the mass of the scatterer: Ions scattered from lighter atoms will have less energy than those scattered from heavy ions. This has the advantage that the signal from different elements can be analysed independently if they have sufficiently different masses. This has been used to great affect in the analysis of 2D silicides by our group at York.

At York MEIS is used primarily as a structural determination technique. As such the position of blocking curves are important. To analyse data, cross-sections are first taken to give a plot of scattering yield as a function of angle for each independently resolvable element. A similar cross-section through the data from bulk substrate scattering can then be compared with known blocking dip positions to give an angular calibration for each data set. Further corrections for (Rutherford scattering cross section, etc) are then performed. A trial structure is “guessed” (either from geometrical considerations of the positions of the blocking dips in various entry/exit direction configurations or from other evidence) and this is simulated using Monte Carlo methods embodied in the VEGAS code. The simulated blocking curves are then compared to the experimental curves and the trial structure refined until a solution is converged upon.