Non-adiabatic and ultrafast dynamics of hydrogen adsorbed on silicon

Light stimulated reactions, and the closely related electron stimulated reactions, form the basis of all lithographic techniques currently used and under commercial development for the production of integrated, circuits (ICs) and microelectromechanical systems (MEMS). A long-standing goal is to understand how radiation is converted into excited electrons and how these electrons exchange their energy with substrate atoms and atoms bound to the surface of the substrate. The lifetime of such excited states is of order 1 x 10(-15) s (1 fs). The electrons exchange energy with the lattice of the solid in the'sub-picosecond (< 1 x 10(-12) s) range. Therefore, to probe these excitations, one is compelled to use pulsed lasers with a pulse-duration in the femtosecond range. Such lasers are capable of extremely high peak powers and non-linear phenomena can feature prominently in the photochemistry initiated by them. This, in turn, leads to reaction dynamics that differ markedly from the dynamics that rule linear photochemistry. Great strides have been made in the ability of theory to model and explain surface chemistry on metals. The theoretical description of semiconductor surfaces remains challenging, though progress is steady. An even greater challenge is the calculation of excited states and the associated non-adiabatic processes, though such calculations are becoming an increasingly activearea of research. This review treats ultrafast energy relaxation and the dynamics of desorption induced by non-adiabatic excitations. Photon and electron stimulated as well as scanning probed tip induced processes are discussed. (c) 2004 Elsevier Ltd. All rights reserved.