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 x 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 similar to 0.2 eV lower, which implies that room-temperature intradimer adsorbate oscillations should occur similar to 10(3) times faster for OH. The absolute value of this barrier (0.9 eV) is in agreement with experimental scanning tunneling microscopy observations.