The nature of the bonding of atomic Al to Si(111): is there a specific site-bond relationship?

Abstract

Structural parameters for chemisorption of atomic Al above the atop (T 1), threefold eclipsed (T 4) and threefold open (H 3) sites of Si(111), have been obtained through the ab initio cluster model method using cluster models of increasing size, different kinds of embedding hydrogen atoms and ab initio self consistent field (SCF) and configuration interaction (CI) wavefunctions. An analysis of the interaction of atomic Al above each site has been carried out from both SCF and CI wavefunctions. After including electronic correlation effects, the T 4 site is the preferred one in agreement with experiment. Interaction above this site (and also above H 3) is strongly covalent while the use of different techniques of analysis univocally shows that interaction above the T 1 site is strongly ionic. Consequently, it is shown that the same adsorbate exhibits two different ways of bonding to the same surface depending on the active site considered.