Pairing mechanism in interaction of atomic hydrogen with epitaxial erbium silicide

Hydrogen desorption from two-dimensional (2D) and bulk-like (3D) erbium sil icides on Si(lll) has been investigated by means of thermal desorption spec troscopy (TDS) and high resolution electron energy loss spectroscopy (HREEL S). By TDS, hydrogen is found to desorb as a single feature with a maximum in temperature located at 620 K on 2D silicide, whereas two desorption peak s at 460 K and 615 K, respectively, are measured on 3D silicide. These spec tra are analyzed in the frame of the formulation developed by Redhead, and compared with hydrogen desorption from a monohydride Si(lll) surface. On er bium silicide films, first-order kinetics is always observed in sharp contr ast with the second-order kinetics measured for hydrogen desorption from th e Si(lll) surface. This remarkable behavior can be understood in terms of a pairing mechanism in which, for stability reasons, the H chemisorption sit es appear in the form of pairs that contain either 0 (empty pair) or 2 (fil led pair) H atoms. Using HREELS, two kinds of hydrogen chemisorption sites were identified prior to desorption. It is deduced that the features at 460 K on 3D and at 620 K on 2D erbium silicides are clearly related to concomi tant hydrogen chemisorbed on the top layer silicon dangling bonds and in an interstitial site of the Er layer beneath. In contrast, the desorption pea k at 615 K on bulk-like silicide is assigned to hydrogen absorption in bulk sites of the subsurface region. (C) 1999 Elsevier Science B.V. All rights reserved.