MEIS is a technique for the determination of the composition and geometrical structure of crystalline surfaces and shallowly buried interfaces. It involves energy analysis of scattered primary ions, typically H+ or He+ at incident energies in the 100-400 keV range, as a function of incidence and emission direction. It derives geometrical structure information through the use of elastic 'shadow cones' (in both the incident and backscattering parts of the ion trajectory) which are also used in lower (keV) ion energies, but because the shadow cones at high energies are much narrower, the precision of this structural information is potentially much higher. Narrower shadow cones also makes it possible to probe deeper into the subsurface that in low energy ion scattering. Combined with this improved structural sensitivity is an ability to obtain subsurface compositional information through the inelastic energy losses incurred as the ions penetrate further into the solid. The principle of this 'non-destructive depth profiling' is exactly the same as that in Rutherford backscattering typically conducted at energies above about 1 MeV, but by using somewhat lower energies and higher resolution detectors (based on the electrostatic deflection systems typically used in electron spectroscopy) much finer depth resolution can be achieved; in principle this can be single atomic layer resolution, although the stochastic nature of the loss mechanisms, rather than the instrumentation, provides the key constraint. The technique has been developed especially at the AMOLF Institute in Amsterdam by Saris, Van der Veen, Frenken and their colleagues (see. e.g. J F Van der Veen, Surface Science Reports 5 (1985) 199).
As a result of a grant application by Phil Woodruff together with Prof D Armour (Salford University) and staff at Daresbury Laboratory, the SERC agreed to finance a new UK MEIS facility to be sited at Daresbury Laboratory in the building of the Nuclear Structure Facility which was closed down by the Research Council. This facility was officially inaugurated on 1 April 1996 and has been operating as a user facility since September 1996. New funding for the use of this facility by Phil Woodruff's group was agreed in 1997, and since that time a major programme of work has been conducted from Warwick utilising this facility. The MEIS instrument incorporates parallel detection of a range of scattering angles and scattering energies, allowing rapid accumulation of blocking patterns used in the 'double-alignment' MEIS studies of surface crystallography. The figure below shows such a dataset from the Cu(100)c(2x2)-Au surface alloy phase (see ref 2 below), with the 'surface peaks' from Au and Cu scattering shown, as are some of the bulk blocking directions.
The scientific programme based in Phil Woodruff's group is relatively broad in that it seeks to explore the use of MEIS for a number of surface structural application areas in which it has either not been tested or its potential has not been fully exploited. We have also been developing a methodology appropriate to surface structure determination in these systems, and the first few publications (see below) describe these developments. Broadly the proposed areas of application fall into the following categories:
- Adsorbate structures and reconstructions, particularly of metal surfaces. The high sensitivity of MEIS to movements of substrate atoms parallel to the surface makes the technique largely complementary to LEED in the study of such systems. Preliminary studies of C- and N-induced reconstructions of Ni(100) have been extended to a range of other, mainly adsorbate-induced, reconstructions. Even in the case of incommensurate or long-range commensurate reconstructions MEIS proves effective, because while a detailed quantitative atomistic determination of such structures may not be viable, MEIS allows one to quantify the number of displaced atoms and the number of reconstructed layers.
- Structures of vicinal surfaces. Here the aim is to use the fine geometric probe to study relaxations at step-edges and the role of reconstructions on such non-singular surfaces.
- Alloy surfaces and epitaxial growth. This topic covers a range of subtopics; the formation and structure of surface alloy phases, even in immiscible systems; the depth variation of alloy components in the near surface region (especially oscillatory compositional variations in surface segregation); the study of interfacial structure and film perfection in metal heteroepitaxy; the growth modes - i.e. island, layer-by-layer, etc. In several of these problem areas systems of potential interest for their magnetic properties will be emphasised.
- Oxide surfaces. Some exploratory studies are envisaged to determine the potential of MEIS to study oxide-on-metal and metal-on-oxide growth and interfaces, a topic of great potential interest for heterogeneous catalysis, but one in which the current state of structural information is rudimentary.
Most of the early work concentrated on structural problems concerned with surface alloy phases, and revealed surprising patterns of behaviour in terms of adsorbate-induced stacking faults and low surface corrugation amplitudes. We have undertaken detailed density-functional theory (DFT) calculations which appear to confirm some simple ideas to explain these phenomena in terms of redistribution of surface valence electronic charge at the surface. More recently we have been applying the method to study metal surface (Ni(100) and Ni(111)) oxidation (for which the sub-surface penetration has providing wholly new information not provided by more conventional surface science techniques), to adsorbate-induced reconstructions (such as those produced on Cu(111) by atomic N and methanethiolate, CH3S-) and to the structure of oxide surfaces, notably of TiO2(110). In addition, we have been undertaking more fundamental studies of the inelastic energy losses experienced in MEIS through a collaboration with theoreticians in Berlin and in Porto Allegre, Brazil. Some of the publications listed below illustrate these results.
We have also applied MEIS to epitaxial layer structures such as quantum dots and quantum wells in the InAs-GaAs system, and showed for the first time how the energy spectrum contains enough information to derive composition profiles for discrete 3D nano-islands as well as laterally uniform layers.