MEIS is a refinement of the more common technique of Rutherford backscattering spectrometry (RBS), but with enhanced depth and angle resolution. In a typical MEIS experiment a collimated beam of mono energetic (typically 100 keV) light ions (H+ or He+) impinges onto a target along a known direction. The energy and angle of the scattered ions are analysed simultaneously and allow MEIS to measure atomic mass, depth, and surface structure from the following physical principles;
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mass - ions scattered
from the surface of a material undergo energy loss by a 'billiard ball' type collision
with surface atoms. The scattered ion energy thus relates directly to the mass of the
scattering atom. This effect can be seen in Figure 1 where the
signal from O, Si and Ge are separated in energy |
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depth - ions scattered
from below the surface lose energy inelastically at a rate proportional to the ion's path
length in the target. This extra energy loss thus relates directly to the depth of the
scattering atom. In favourable cases MEIS can achieve a depth resolution of one atomic
layer. The example given in Figure 1 is a result from a Si quantum
well structure with a repeat distance of 5 nm. Three Ge rich layers approximately 5 nm
apart are clearly seen. The O signal is from the surface oxide formed when the sample has
been exposed to air. |
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surface structure - when the ion beam is aligned with a crystallographic axis the surface atoms shadow deeper atoms from the ion beam. This alignment therefore makes the technique surface specific and, for a particular crystal, certain ingoing directions can allow the ion beam to illuminate only the top one, two, or three layers according to choice. Ions scattered from the second layer will have their outward paths blocked at certain angles by first layer atoms. The variation in scattered ion intensity with angle thus relates to the geometrical arrangement of surface atoms. A complete solution of surface structure requires a comparison between experiment and simulation for several scattering geometries. Figure 2 shows schematically the scattering geometry and the position of surface and near-surface atoms. The surface atoms are slightly relaxed outwards from the bulk positions. Figure 3 shows the effect this surface relaxation has on the MEIS blocking spectrum where the shift of blocking features to higher scattering angles indicates an outward relaxation. The amount of relaxation can be found from the magnitude of the shift, either by geometrical calculation or computer simulation. By appropriate choice of scattering geometry, atomic displacements as small as 0.03 Å can be measured. A complete determination of the surface structure involves computer modelling of blocking spectra from several scattering geometries. |
Figure 1
Depth profile of Si/Ge quantum well structure with oxidised surface
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Figure 2
Schematic diagram of surface atoms and scattering geometry
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Figure 3
Shift in MEIS blocking features due to outward surface expansion
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