Experimental Studies and Thermodynamic Modeling of the Interaction of O 2 with SiC
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Experimental Studies and Thermodynamic Modeling of the Interaction of O2 with SiC Y. Song1 and F. W. Smith Department of Physics The City College of the City University of New York New York, NY 10031, U.S.A. 1 Physics Department Vanderbilt University Nashville, TN, 37235, U.S.A. ABSTRACT We report on experimental studies and thermodynamic modeling of the reaction of O2 with the 4H- and 6H-SiC surfaces at high temperatures. This reaction leads to the growth of passivating SiO2 layers at high O2 pressures, etching of the surfaces at lower pressures, and enhancements of the surface segregation of carbon at still lower pressures. A pressuretemperature phase diagram for the oxidation of SiC is presented. Evidence for the thermal decomposition of the SiO2 layer on SiC is also presented. INTRODUCTION While it is well known that the thermal oxidation of SiC can lead to the passivation of its surface, the growth of SiO2 on SiC is slower than on Si and is also anisotropic. In addition, the oxidation of SiC is chemically much more complex than that of Si and the presence of carbon can lead to undesirable effects. These include defects at the SiC/SiO2 interface1 associated with the presence of C and also the surface segregation of a layer of graphitic C during heating of SiC at high temperatures2-5. Efforts to develop SiC-based devices continue to be hampered by the poor quality of the SiC/SiO2 interface. Studies of the reaction of oxygen with SiC are also interesting from a fundamental surface science perspective since, as reported here, three distinct regions have been observed in the phase diagram describing the SiC + O2 interaction. We report here the results of our studies of the reaction of oxygen with SiC at high temperatures T. This reaction can lead to the growth of an SiO2 layer at high O2 pressures P(O2) (passive oxidation), etching of the surface at lower P(O2) (active oxidation), and can also play an important role in the surface segregation of carbon. These results have allowed us to present here a unified P(O2)-T phase diagram for the SiC + O2 reaction. The corresponding phase diagram6 for the simpler Si + O2 reaction consists of only two regions, corresponding to passive oxidation and active oxidation. A thermodynamic model predicting these three distinct regions is outlined. Finally, the differences observed for the interaction of O2 with the 4H and 6H-SiC polytypes and with their Si- and C-terminated surfaces, (0001) and (0001 ), respectively, are discussed.
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Temperature( C) 1600
1400
1200
1000
800
1
Region 4
Region 3
SiC+O2 SiO2(s)+C(s)
0.1
Region 2 O2 Pressure (Torr)
0.01
2SiC+3O2 2SiO2(g)+2CO(g) 1E-3
SiC+O2 SiO(g)+CO(g) 1E-4
Region 1 1E-5
1E-6 0.5
SiC+2CO SiO2(g)+3C(s) SiC+2SiO23SiO(g)+CO(g) SiC+SiO CO(g)+2Si(g)
(2+x)SiC+O2 (2+x)Si(g) +2CO(g)+xC(s)
0.6
0 .7
0 .8
0 .9
-1
1000/T (K )
Figure 1. Predicted phase diagram for the SiC + O2 reaction; see text. THERMODYNAMIC MODEL The model employed7 for the prediction of the vapor phase products - Si(g), SiO(g), CO(g), CO2
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