| In this thesis we theoretically study chemical reactions on silicon surfaces using first-principles calculations based on Density Functional Theory (DFT).; To elucidate the reaction mechanisms, the String method recently introduced by E et al. (Phys. Rev. B, 66, 052301 (2002)) was combined with first-principles molecular dynamics in the context of DFT. This approach enables us to accurately determine the chemical reaction pathways and transition states on the potential energy surface given by DFT. The strengths and limitations of the present approach are analyzed with several reactions, and the implications are discussed.; The chemisorption of a hydrogen molecule on the reconstructed Si(001) 2x1 surface is re-examined first. Adsorption experiments indicate the existence of an adsorption barrier whereas desorption experiments do not observe such barrier. The "barrier puzzle" has received much attention in the last decade and only recently a consistent explanation has emerged in the literature. The surface reconstruction is found to play a prominent role in understanding the reaction and its mechanism.; The surface radical chain reaction on the hydrogen-terminated Si(111) surface is then studied to obtain insights into experimental observations concerning the organic functionalization of the surface. The chain reaction approach is considered to be a particularly promising way of functionalizing silicon surfaces as the basis of organic-inorganic hybrid devices. The feasibility of the approach is experimentally observed to depend on the type of molecule. Our work shows that the molecular conjugation greatly influences the viability of the reaction by stabilizing the reaction intermediate state. We also show that the consequence of such an effect for the reaction depends on the type of interface that the adsorbed organic molecule forms with the silicon surface. In the same context, we studied an optically activated functionalization that was recently demonstrated experimentally. Of the two possible reaction mechanisms, the one which leads to the surface radical chain reaction was theoretically determined to be a more favorable mechanism, in agreement with experimental observations. |
