NONLINEAR-OPTICAL INVESTIGATIONS OF THE DYNAMICS OF HYDROGEN INTERACTION WITH SILICON SURFACES

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resul ting high sensitivity to hydrogen adsorption combined with unique feat ures of SHG as an optical probe has been exploited to study various ki netical and dynamical aspects of the adsorption system H-2/Si. Studies of surface diffusion of H/Si(111)7x7 and recombinative desorption of hydrogen from Si(111)7x7 and Si(100)2x1 revealed that the covalent nat ure of hydrogen bonding on silicon surfaces leads to high diffusion ba rriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2x1 and Si(111)7x7 could be observed for the first time by heat ing the surfaces to temperatures between 550 K and 1050 K and monitori ng the SH response during exposure to a high flux of H-2 or D-2. The m easured initial sticking coefficients for a gas temperature of 300 K r ange from 10(-9) to 10(-5) and strongly increase as a function of surf ace temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semicond uctor surfaces.