Following the oxidation of yttrium silicide epitaxially grown on Si(1 1 1) by core level photoemission spectroscopy C. Rogero a, * , S. Lizzit b , A. Goldoni b , J.A. Martı ´n-Gago a a Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain b ELETTRA, Sincrotrone Trieste, SS 14, km 163.5, Area Science Park, 34014 Basovizza-Trieste, Italy Received 7 June 2005; accepted for publication 4 December 2005 Available online 6 January 2006 Abstract We have identified, by means of synchrotron radiation X-ray photoemission spectroscopy, several core-level shifted components in the Si-2p photoemission core level peak from a thin yttrium silicide layer epitaxially grown on a Si(1 1 1) surface. We have unequivocally assigned these components to different environments of the Si atoms in the silicide structure. This information has been used to monitor a surface oxidation process promoted by room temperature oxygen adsorption, identifying the final product of this reaction as a silicate- type ternary compound. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Synchrotron radiation photoelectron spectroscopy; Epitaxy; Oxidation; Yttrium silicides; Metal–semiconductor interfaces 1. Introduction Rare earth (RE) silicides epitaxially grown on silicon substrates present appealing properties that make them interesting for technological applications: an extremely low value of the Schottky barrier height, an abrupt Si– metal interface, and the almost 0% mismatch of the silicide and the Si lattice parameters. The atomic structure of the inner silicide planes of a thick metallic RE silicide consists of graphite-like Si planes intercalated by RE planes. The Si planes contain vacancies, forming an ordered ( p 3 · p 3)R 30° network that leads to a RESi 1.7 stoichiometry [1,2]. Vacancies play an important role in the atomic and electronic structure of these silicides [3,4]. By dynamical low energy electron diffraction (LEED I–V) [5] we have recently found that the topmost plane is a buckled Si over- layer with no vacancies, showing a slightly vertical relaxa- tion of the Si down atom placed over the vacancy position in the last graphite-like Si plane. The atoms in this bilayer oc- cupy hexagonal positions, whereas the Si atoms in the inner planes are laterally relaxed towards the vacancies in order to homogenize the Si–Si bonds [5]. The structure is sche- matically presented in Fig. 1. In order to use these materials in real devices, a good knowledge of their behaviour under environmental condi- tions is crucial. In this sense, it is therefore appropriate to study the oxidation processes that gather knowledge about stability and protection against gaseous O 2 or H 2 O. Moreover, these studies could help to explore the possibility to built metal–insulator–semiconductor (MIS) elements under relatively mild oxidation conditions. Besides, this type of materials can be used as precursor for Si oxidation to form silicates. The oxidation of Si and Si–SiO 2 interfaces are of fundamental importance in the Si-based semiconductor technology. It has been established that Si oxidation for technological applications is a slow process at room temperature (RT). This fact has favoured the search of oxidation promoters as metallic thin layers of noble, alkali, transition, or RE elements, in order to find circumstances where good-quality dielectric layers can be grown [6,7]. Particularly, the use of RE deserves especial 0039-6028/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.12.011 * Corresponding author. Present address: School of Natural Sciences, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK. Tel.: +44 0191 222 6646; fax: +44 0191 222 6929. E-mail address: celia.rogero@ncl.ac.uk (C. Rogero). www.elsevier.com/locate/susc Surface Science 600 (2006) 841–846