Oxidation Modelling for SiC
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etched trench [3, 4]. The lowest/highest oxidation rates are observed on the so-called silicon/carbon faces (the 0001/0001 planes respectively) with a corresponding increase in oxidation rate for planes between the two extremes. Dependency of oxidation rate on crystallographic plane is also observed in silicon where it is often explained by arguments based on the number of silicon-silicon bonds exposed to various crystal faces. As 0001/0001 planes have the same number of surface bonds but widely differing oxidation rates, such an explanation is not sufficient for SiC. This paper presents recent work in the development of a simple oxidation model for SiC proposed by the authors in an earlier paper [5]. The consequences of the model are explored using Monte Carlo based simulation techniques and new results on the oxidation of trench structures presented. THEORY Mechanistic Oxidation Model As full details of the proposed oxidation model have been presented in an earlier paper [5], we present only a brief discussion of the main issues here. The crystal structure and proposed bonding of 4H-SiC is shown schematically in Figure 1 (with the z-axis scale greatly exaggerated for clarity). From the point of view of the proposed oxidation model, the important structural characteristic of all the hexagonal SiC structures is the absence of 135 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
inversion symmetry along the z-axis of the chemical bonding between Si and C atoms. To explore the consequences of this on oxidation, consider oxygen molecules interacting with the crystal structure at the Si-face. The first stage of oxidation is oxygen atoms bonding to the Si atoms outside the crystal. The high electronegativity of the oxygen atoms will make the bond between the surface Si atoms and the back-surface C atoms (labelled ac in Figure 1) less ionic and hence lower the bond energy (see Figure 2) [6]. At the second stage of oxidation, the a-bond will be broken by the incoming oxygen resulting in a Si-O-C bond. The high electronegativity of the oxygen atom now in the a-bond position will increase the ionic nature of C-Si 3-bonds (see Figure 2) which will be consequently strengthened. C-Face Further oxidation proceeds via attack of these strengthened 13-bonds - a process which will be slower than the oxidation of the weakened oabond. Once the bonds around the carbon atom are fully oxidised, the release of a CO molecule can then occur - with subsequent re". "Si-Face ordering of the Si-O bonding structure. At the f S.."......... Si-face then, there are one weakened and three strengthened bonds resulting in an overall increase in the amount of energy that must be supplied by the oxidation process (i.e. the .......... ,,. average activation energy of the process is increased). Subsequent oxidation of other layers can proceed by the same mechanism with CO being released from the structure as .. . .. the oxidation of the bonds around each C atom is complete.
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At the C-face however, such a mechanism results in
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