Active oxidation: Silicon etching and oxide decomposition basic mechanisms using density functional theory A. Hemeryck a,b, * , N. Richard b , A. Este `ve a , M. Djafari Rouhani a a Laboratoire d’Analyse et d’Architecture des Syste `mes/CNRS, 7, Avenue du Colonel Roche, 31077 Toulouse, France b CEA-DIF, BP 12, 91680 Bruye ` res-le-Cha ˆtel, France Received 12 January 2007; accepted for publication 7 March 2007 Available online 13 March 2007 Abstract The etching of silicon atom from the Si(1 0 0)-p(2 · 2) surface, i.e. the desorption of SiO molecules from this surface, either clean or pre-oxidized, is investigated at the density functional theory level. The reaction paths for desorption are given as a function of the initial oxidation state of the extracted silicon atom. The associated activation energies and the atomic configurations are discussed. Particularly, it is shown that desorption of SiO molecules takes place during conventional thermal oxide growth (2 eV activation) via non-oxidized silicon atoms. Further SiO extraction mechanisms of higher silicon oxidation states required higher temperatures. In particular, doubly oxidized silicon atoms (Si 2+ ) are able to decompose with an activation of 4 eV which corresponds to the actual temperature where decomposition of oxides is observed. This confirms the statement that decomposition of oxide layer nucleates at the interface with silicon where Si 2+ has been detected thanks to XPS experiment. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Density functional calculations; Etching; Desorption; Oxidation; Semiconducting surfaces 1. Introduction Over the past decades, the silicon oxidation has at- tracted outstanding attention due to its technological importance for the microelectronic silicon-based device industry [1]. Nevertheless, a deep atomic scale understand- ing of it is still lacking. Many experimental [2–5] and theo- retical [6,7] studies have been performed establishing that two reaction products may form during silicon oxidation depending on the substrate temperature and oxygen pres- sure: a SiO 2 film can be grown and/or the etching of the sil- icon surface with desorption of SiO can occur. The formation of a layer of SiO 2 on the silicon surface can be referred to as the ‘‘passive’’ oxidation process for the high substrate temperatures and low oxygen pressures. The adsorption of oxygen molecules on silicon surfaces un- der these conditions yielded several theoretical studies [8– 13]. The chemisorption of the oxygen molecule, dissociative or not, leads mainly to three structures: once the molecule is dissociated, each of the oxygen atoms can be directly incorporated into (i) the dimer bond, (ii) the Si–Si back- bond of a dimer unit and (iii) on top of a dimer atom. This position has also been referred to as ‘‘strand positioning’’ in the literature [8]. Uchiyama et al. [12] have previously named the above three structures as on dimer (OD), back- bond (BB) and on top (T), respectively. While these struc- tures contribute to the oxide film growth, they also express the SiO molecules tendency to desorb from the surface, when looking carefully at the dissociation mechanisms of the oxygen molecule on the silicon surface and at the huge amount of charge redistribution over the covalent bonds during this process [8,13]. For higher oxygen pressures, the ‘‘active’’ oxidation oc- curs and contributes to the formation of the SiO volatile molecules emanating from the general reaction: O+ Si(s) ! SiO(g). In this equation, the oxygen atom is the 0039-6028/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.03.008 * Corresponding author. Address: CEA-DIF, BP 12, 91680 Bruye `res- le-Cha ˆtel, France. Tel.: +33 1 69 26 40 58; fax: +33 1 69 26 70 53. E-mail address: ahemeryc@laas.fr (A. Hemeryck). www.elsevier.com/locate/susc Surface Science 601 (2007) 2082–2088