Binding and diffusion of hydroxyl radicals on Si(100): A first-principles study

We present a local density functional investigation of the adsorption geometry and the surface diffusion activation energies of hydroxyl (OH) radicals resulting from dissociative water adsorption on Si(100)-2×{}1. Similarly to atomic hydrogen, OH prefers to bind to a single surface silicon atom. Due to both dative interactions with surface dangling bonds, and to adsorbate-adsorbate hydrogen-bond-like interactions, the O-H bonds tend to be oriented perpendicularly to the dimer direction, in agreement with electron stimulated desorption ion angular distribution data. The energetics of OH diffusion, investigated both on a clean and on a saturated surface, is rather similar to that of hydrogen, with slightly lower barriers. In particular, the intradimer barrier is found to be {~}0.2 eV lower, which implies that room-temperature intradimer adsorbate oscillations should occur {~}10³ times faster for OH. The absolute value of this barrier (0.9 eV) is in agreement with experimental scanning tunneling microscopy observations.