Surface physics with low-energy ion scattering

Abstract

Four versions of low-energy ion scattering (LEIS) measurements are briefly reviewed. In the first version discussed here the charged fraction of scattered noble-gas ions is detected with an electrostatic analyser. The composition of exclusively the outermost layer is probed in this way, even when the surface of the sample is not smooth. The second method probes the surface step density, and thereby growth oscillations, with grazing-angle specular scattering, whereby the scattered particles are measured without energy resolution. An example is given of the observation of oscillations during the growth of Ag on Ag (0 0 1) at several temperatures. The oscillations are compared with oscillations obtained from simulations of the growth process, combined with LEIS simulations. This second method is also sensitive to the structure of the outermost layer. There is no sensitivity to the composition of the surface. In the third method either the beam direction, or the direction of the detector, makes a grazing angle with the surface, and atoms scattered over a large (non-specular) angle are detected with time-of-flight (TOF) resolution. This provides sensitivity only to atoms protruding from the surface, like adatoms, or atoms at step edges. An example is presented whereby this method is used to measure adatom mobilities. Finally the conventional method of measuring with TOF resolution azimuth scans of scattered or recoiled particles in a shadowing or blocking geometry is described. The scans are compared with the results of computer simulations carried out for trial structures. The properties of the program MATCH that can be used efficiently for these simulations are summarised. As an example of the application of this method the determination of the surface structure of Fe 3O 4 (0 0 1) is presented. The features of the LEIS versions presented are compared with MEIS and HEIS features, with emphasis on the possibility to obtain information on the composition and structure, separately for the first few layers of a sample.