Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures

Low-dimensional self-organized surface structures, induced by (sub)monolayer metal adsorbates on semiconductor surfaces may give rise not only to a variety of emergent electronic properties, but also to a multitude of specific localized vibronic features. The focus of this review is on the analysis of these novel surface vibration eigenmodes. The application of in situ surface Raman spectroscopy under UHV conditions on clean semiconductor surfaces and those with self-ordered adsorbates, in close conjunction with the calculations of Raman spectra, based on the first-principles determination of the structural, electronic and vibronic properties, allows a consistent determination of the vibration eigenfrequencies, symmetry properties, and elongation patterns of the systems of interest. The localized nature of the surface eigenmodes determines the surface sensitivity, independent of the large penetration depth of light. The surface contribution can be selectively enhanced by employing resonance conditions to surface electronic transitions. Moreover, surface and bulk contributions can be separated by taking difference spectra between various stages of surface preparation. The relevant surfaces are Ge and especially Si with different orientations ((111) and vicinal (hhk)), on which the adsorption of various metals (Au, Sn, Pb, or In) gives rise to two- and quasi-one-dimensional structures (e.g. Au-(5 × 2)/Si(111)) with a variety of vibration modes. The Raman analysis of these modes not only enables the distinction between different proposed structural models (e.g. for Au-($\sqrt{3}\times \sqrt{3}$)/Si(111)), but also gives access to the role of electron-phonon coupling in structural phase transitions (e.g. for In-(8 × 2)–(4 × 1)/Si(111)).