He Scattering from Compact Clusters and from Diffusion-Limited Aggregates on Surfaces: Observable Signatures of Structure

The angular intensity distribution of He beams scattered from compact clusters and from diffusion limited aggregates, epitaxially grown on metal surfaces, is investigated theoretically. The purpose is twofold: to distinguish compact cluster structures from diffusion limited aggregates, and to find observable {\em signatures} that can characterize the compact clusters at the atomic level of detail. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are large compared with momentum transfer due to surface corrugation. The diffusion limited aggregates on which the scattering calculations were done, were generated by kinetic Monte Carlo simulations. It is demonstrated, by focusing on the example of compact Pt Heptamers, that signatures of structure of compact clusters may indeed be extracted from the scattering distribution. These signatures enable both an experimental distinction between diffusion limited aggregates and compact clusters, and a determination of the cluster structure. The characteristics comprising the signatures are, to varying degrees, the Rainbow, Fraunhofer, specular and constructive interference peaks, all seen in the intensity distribution. It is also shown, how the distribution of adsorbate heights above the metal surface can be obtained by an analysis of the specular peak attenuation. The results contribute to establishing He scattering as a powerful tool in the investigation of surface disorder and epitaxial growth on surfaces, alongside with STM.Comment: 41 pages, 16 postscript figures. For more details see http://www.fh.huji.ac.il/~dan

Anisotropic optical response of the diamond (111)-2x1 surface

The optical properties of the 2$\times$1 reconstruction of the diamond (111) surface are investigated. The electronic structure and optical properties of the surface are studied using a microscopic tight-binding approach. We calculate the dielectric response describing the surface region and investigate the origin of the electronic transitions involving surface and bulk states. A large anisotropy in the surface dielectric response appears as a consequence of the asymmetric reconstruction on the surface plane, which gives rise to the zigzag Pandey chains. The results are presented in terms of the reflectance anisotropy and electron energy loss spectra. While our results are in good agreement with available experimental data, additional experiments are proposed in order to unambiguously determine the surface electronic structure of this interesting surface.Comment: REVTEX manuscript with 6 postscript figures, all included in uu file. Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.html Submitted to Phys. Rev.

Novel approach based on continuous trench modelling to predict focused ion beam prepared freeform surfaces

Focused Ion Beam (FIB) systems can be used to generate controlled micro- and nano-features on a solid surface. Prediction of surfaces fabricated in this way requires knowledge of the rate of removal of the target material in order to simulate the evolution of the surface. In this paper we present a continuous-time model of this process that allows us to predict the shape of trenches and freeform surfaces under the action of FIB. This type of model is appropriate when the points that describe the discrete trajectory of the beam (the line connecting consecutive dwell points) are close enough that the craters generated on the surface overlap. In the case of a straight beam path with constant dwell time, a trench of uniform depth is generated under these conditions. This simplified approach considerably reduces the computing time needed to simulate the process, without significantly reducing the accuracy of the predicted surface. Calibration of the model for a particular FIB setup and workpiece involves only a small number of experimental trials. The predictions of the model have been tested on a single crystalline Boron p-doped Si target material for several different beam paths. The depth and shape of the cross section of single and overlapping trenches with constant dwell time are predicted with high accuracy. The model was also used to predict three freeform surfaces, where the dwell time varies along each line of the beam path. Atomic force microscopy topography measurements show that the average relative error between the measured and simulated profiles is 210% depending on the complexity of the machined surface