543-Bongiorno-PhysRevLett-8-2004-086102-1

543-Bongiorno-PhysRevLett-8-2004-086102-1 - VOLUME 93, N...

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Reaction of the Oxygen Molecule at the Si ± 100 ² - SiO 2 Interface During Silicon Oxidation Angelo Bongiorno 1,2, * and Alfredo Pasquarello 1,2 1 Institut de The ´orie des Phe ´nome `nes Physiques (ITP), Ecole Polytechnique Fe ´de ´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland 2 Institut Romand de Recherche Nume ´rique en Physique des Mate ´riaux (IRRMA), CH-1015 Lausanne, Switzerland (Received 5 March 2004; published 16 August 2004) Using constrained ab initio molecular dynamics, we investigate the reaction of the O 2 molecule at the Si ± 100 ² - SiO 2 interface during Si oxidation. The reaction proceeds sequentially through the incorpo- ration of the O 2 molecule in a Si-Si bond and the dissociation of the resulting network O 2 species. The oxidation reaction occurs nearly spontaneously and is exothermic, irrespective of the O 2 spin state or of the amount of excess negative charge available at the interface. The reaction evolves through the generation of network coordination defects associated with charge transfers. Our investigation suggests that the Si oxidation process is fully governed by diffusion. DOI: 10.1103/PhysRevLett.93.086102 PACS numbers: 81.65.Mq, 68.43.Bc, 71.15.Pd, 81.15.Aa The oxide thickness in current Si-based electronic devices has dropped below 20 A ˚ [1]. In this regime, optimal device performance requires controlling the ox- ide growth at the atomic scale. Further progress therefore requires a detailed understanding of the fundamental processes responsible for silicon oxidation [2,3]. Since the work of Deal and Grove [4], the silicon oxidation process is generally assumed to occur sequentially through (i) the O 2 diffusion through the oxide network toward the Si - SiO 2 interface, and (ii) an activated O 2 reaction with the Si substrate at the interface. The diffu- sion process has been extensively characterized both ex- perimentally [3,5] and theoretically [6], and its description nowadays meets a large consensus. At vari- ance, the oxidation reaction has remained far less under- stood. This is related to the failure of the Deal-Grove model in describing the oxidation kinetics for thin ±lms, the regime dominated by the oxidation reaction at the interface [4,7]. Kinetics models accounting for the thin- ±lm regime separate into those preserving the activated oxidation reaction [8,9] and those relying uniquely on spatially varying diffusion properties [8,10]. However, no direct observations can at present distinguish among these modelling schemes. Furthermore, such models re- main limited at reproducing the kinetics of the silicon oxidation process. A deeper understanding of the atomic- scale processes at the Si ± 100 ² - SiO 2 interface appears at present only accessible within density-functional inves- tigations [11–13]. However, dif±culties in modeling the structure of the Si ± 100 ² - SiO 2 interface have hitherto pre- vented the study of the oxidation reaction directly at the interface.
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543-Bongiorno-PhysRevLett-8-2004-086102-1 - VOLUME 93, N...

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