The major theme of this research is to understand and control atomic structure and chemical reactivity of semiconductor surfaces, kinetics of reactions involved in etching, surface preparation and epitaxial deposition of crystalline semiconductors. The experimental aspects of the work will emphasize investigation of the structure and structural changes of silicon surfaces exposed to various reactive chemical environments (e.g., adsorption site geometries, reconstruction and stabilization of the silicon 100 surface exposed to halogen atoms). Techniques used in this portion of the work include: low energy electron diffraction impact collision ion scattering spectroscopy. The second category of experiments will focus on the thermal reactions that occur on semiconductor surfaces, measuring the rates and energetics of the reactions. The influence of surface structure on chemical reactions will be studied using temperature programmed desorption.