Optical spectra and microscopic structure of the oxidized Si(100) surface: Combined in situ optical experiments and first principles calculations Katalin Gaál-Nagy, Andrei Incze, and Giovanni Onida Dipartimento di Fisica, Università di Milano, European Theoretical Spectroscopy Facility (ETSF), and CNISM-CNR-INFM, via Celoria 16, I-20133 Milano, Italy Yves Borensztein, Nadine Witkowski, and Olivier Pluchery Institut des Nanosciences de Paris, CNRS and University Pierre and Marie Curie, Paris 6, 140 rue de Lourmel, 75015 Paris, France Frank Fuchs and Friedhelm Bechstedt Institut für Festkörpertheorie und Optik, Friedrich-Schiller-Universität, Max-Wein-Platz 1, D-07743 Jena, Germany Rodolfo Del Sole Dipartimento di Fisica, Università di Roma “Tor Vergata,” European Theoretical Spectroscopy Facility (ETSF), and CNR-INFM-SMC, via della Ricerca Scientifica, I-00133 Roma, Italy Received 31 July 2008; revised manuscript received 22 November 2008; published 21 January 2009 We have investigated the first stages of the room-temperature oxidation of the Si100 surface combining experimental surface optical spectra with the results of ab initio calculations. High-resolution reflectance anisotropy spectra RAS and surface differential reflectance spectra SDRS have been measured for the clean surfaces and various exposures up to 183 L, which have been compared with calculated RAS and SDRS in the independent-particle approximation. Our results, yielding a consistent description of both RAS and SDRS, suggest the coexistence of different structural domains, whose weight changes smoothly with the oxygen exposure. The main oxidation mechanisms together with their occurrence versus coverage are discussed. DOI: 10.1103/PhysRevB.79.045312 PACS numbers: 78.68.+m, 73.20.-r, 78.40.-q I. INTRODUCTION The oxidation process of silicon surfaces, and particularly of the Si100 surface, is of strong technological interest, driven by the downscaling of metal-oxide semiconductor de- vices. The latter requires nowadays gate insulator oxide lay- ers with a thickness of less than 2 nm. 1 Even if new high- dielectric materials are explored, 2 Si oxidation continues to play a key role through the SiO 2 -Si100 interface formation. However, our understanding of the Si100 oxidation process is still incomplete, particularly about its initial stages, which correspond to low-oxygen exposure and small coverages. Adsorption sites, surface structural changes, and oxygen re- action paths are still under debate. 3 From the experimental point of view, optical reflectance anisotropy RA spectroscopy and surface differential reflec- tance SDR spectroscopies are two techniques which can be successfully used to investigate the oxidation process in real time. 4 RA and SDR spectroscopies are fast nondamaging techniques and can be easily performed “in situ” in a wide range of pressure and temperature. Besides other more direct techniques, optical measurements can be used to obtain structural information about surface reconstructions. How- ever, this requires reliable theoretical predictions of the op- tical spectra for various surface geometries and stoichiom- etries. Such calculations can be performed within the ab initio density-functional theory Kohn-Sham DFT-KSRefs. 5 and 6 scheme even for quite large and complex surface unit cells. For this reason, ab initio calculations associated with surface-sensitive linear optical techniques such as RA and SDR spectroscopies have become extensively used in the last years. The clean Si100 surface is a paradigmatic example for surface reconstructions. It reconstructs by dimerization of Si-Si pairs at the topmost surface layer by forming “dimer rows” in the direction perpendicular to the dimer axis. Adja- cent rows of dimers are separated from each other by “val- leys” which are about 2.67 Å wide. Due to the fact that the dimers are slightly buckled and that the buckling direction can alternate along one or both Cartesian directions in the surface plane, the surface periodicity can yield different reconstructions: besides the 2  1, also a p2  2 and a c4  2 reconstruction are observed. At the clean Si100 surface the Si-Si dimers, and the surface states related to them, have been recognized to be responsible for some spectral features in reflectance aniso- tropy spectra RAS and surface differential reflectance spec- tra SDRS. 7 Optical techniques have been successfully em- ployed also to study the adsorption of several atomic and molecular species including H, N 2 O, C 6 H 6 , and O 2 . 8–14 In the case of O 2 , features in the SDRS of Si100 at oxygen exposures of a few langmuirs L have been described con- sidering a dissociative process and the breaking of surface dimers. 10,14 However, the way that oxidation influences the optical response is still not completely clarified. On the other hand, the mechanism of the first stages of room-temperature oxidation of Si100 has been studied in recent years with many different experimental techniques. Scanning reflection electron microscopy SREM combined with Auger-electron spectroscopy AES and core-level x-ray photoemission spectroscopy XPS have shown that Si100 oxidation proceeds layer by layer 15 and that the first silicon layer is oxidized by molecular oxygen without an energy PHYSICAL REVIEW B 79, 045312 2009 1098-0121/2009/794/04531210 ©2009 The American Physical Society 045312-1