Electron-phonon coupling and surface-state polarons on Si(111)2×1

Electrons in surface states are usually treated decoupled from lattice motion. In reality, that coupling is at least as important as in the bulk, and possibly much more so, for surface bands are generally narrower. We present here a theory of polaron effects on the dangling-bond states of the Si(111)2×1 reconstructed surface, probably the best-studied crystal surface in every other respect. In view of the current debate over the nature of the reconstruction, we consider two alternative situations, ionic buckling (Haneman model) and covalent -bonded chains (Pandey model). With parameters chosen to approximate as closely as possible the known experimental facts, the buckled surface is found to be strong coupling with small surface-state polarons, while the -bonded surface is a relatively weak-coupling case. Among other things, the temperature-dependent red shifts of optical-absorption lines predicted for the two situations are different by almost 1 order of magnitude. This study provides a first theoretical illustration of large- and small-polaron effects on surface states. It could also specifically help discriminating between 2×1 reconstruction models of the Si(111) surface, when its temperature-dependent spectroscopy becomes available. We finally discuss the general relevance of polaron effects in experimental surface-state spectroscopic studies, including optical absorption and luminescene, as well as photoemission and scanning tunneling spectroscopy.