Adsorbate−Surface Phonon Interactions in Deuterium-Passivated Si(111)-(1 × 1)

Abstract

The vibrational spectra of atomically flat hydrogen- and deuterium-passivated Si(111)-(1 × 1) surfaces are\ninvestigated using theoretical and experimental techniques. An unexpected isotopic shift is observed for the\nSi−H bending mode when hydrogen is replaced with deuterium, with δ(Si−H) = 626 cm^-1 and δ(Si−D) =\n537 cm^-1. The Si−H stretching mode, in contrast, behaves as expected with ν(Si−H) = 2083 cm^-1 and\nν(Si−D) = 1516 cm^-1. Density functional studies reveal that the mode observed at 537 cm^-1 is mostly a\nphonon mode, which results from the coupling of near-surface phonons with the lower frequency bending\nmode of the deuterated surface. This coupling causes a shift in the ordering of the expected vibrational modes\nwhen hydrogen is replaced by deuterium. We also suggest that such a mode around 535 cm^-1 should be seen\nfor many other surfaces and is a general feature of many monovalently terminated Si(111) surfaces.