Surface core-level shifts on Ge(100): c(4 x 2) to 2 x 1 and 1 x 1 phase transitions.

Abstract

By comparing, under identical experimental conditions, high-resolution synchrotron-radiation core-level photoemission spectra taken from both Ge(111) and Ge(100) samples, we establish that the decomposition of the Ge 3d lines from the clean Ge(100) 2\ifmmode\times\else\texttimes\fi{}1 surface at room temperature requires two surface components shifted by -0.23 and -0.60 eV relative to the bulk one. This deconvolution is fully consistent with the asymmetric-dimer reconstruction model of this surface. We further study the reversible phase transitions that occur on this surface: 2\ifmmode\times\else\texttimes\fi{}1\ensuremath{\leftrightarrows}c(4\ifmmode\times\else\texttimes\fi{}2) at low temperature; 2\ifmmode\times\else\texttimes\fi{}1\ensuremath{\leftrightarrows}1\ifmmode\times\else\texttimes\fi{}1 at high temperature. We show from both core-level and valence-band studies that the number of dimer bonds is essentially conserved in these transitions. We also suggest, by comparing a dimer with an Ising spin, that these transitions correspond, respectively, to an antiferromagnetic ordering at low temperatures and to a paramagnetic disordering at high temperatures.