ARXPS studies of SiO 2 -SiC interfaces and oxidation of 6H SiC single crystal Si-(001) and C-(001) surfaces
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H-J. Michel and J. Halbritter Kernforschungszentrum Karlsruhe IMF I, Postfach 3640, 76021 Karlsruhe, Germany (Received 22 March 1994; accepted 29 August 1994)
The main puzzle in oxidation of hexagonal SiC is the slower rate of the Si-terminated surface as compared to the C-termmated surface, which is blamed on an unknown interface compound. ARXPS is a unique method to identify minor amounts of interface compounds, especially for smooth surfaces. Our ARXPS analysis of oxidized Si-(OOl) and C-(OOl) surfaces of 6H SiC reveals the interface oxide Si 4 C 4 _ x O 2 (x < 2), likely a reaction product of a peroxidic O2-bond to a SiC double layer. Si4C4_xO2 occurs in_ larger thickness (—1 nm) at the slowly oxidizing Si-(OOl) surface, whereas the C-(OOl) surface shows smaller amounts, diminishing fast with oxidation above 1000 K. Evidence is presented that with increasing amount of Si 4 C 4 ^O2 the oxidation of SiC to S1O2 is reduced. ARXPS is consistent with a layer of S1O2 containing less than 3% S14C4O4 being an oxidation product of Si4C4_xO2. At the surface of SiO2, graphite and some Si 4 C 4 O 4 exist, aside from standard adsorbates.
I. INTRODUCTION In recent years, methods have been developed to grow reproducibly high-quality single crystals of silicon carbide.1'2 The most common modifications of SiC 3C (cubic), 4H, and 6H (hexagonal) are largebandgap semiconductors that can be doped p- or rc-type. The large bandgap makes it possible for use in high-temperature and high power electronic applications. In addition, chemical inertness, oxidation resistance,3-4 and extreme hardness of SiC (9.2 on Mohs scale) make it an excellent structural ceramic for use in harsh environments. For SiC to fulfill its potential as an electronic material, methods must be developed to produce high quality, insulating oxide layers on SiC surfaces in a reproducible fashion. Away from the SiC surface the oxides consist mainly of amorphous SiO 2 , as for Si-SiO 2 , where optimal electronic properties have been found. Thus, the interface SiC-SiO 2 and impurities in SiO 2 , occurring in SiC-oxidation only, are of crucial importance. Therefore, the study of the surface oxidation process should take into account the crystallographic polarity of SiC, since both, surface symmetry and atomic composition, depend on the orientation of the surface examined. For 6H SiC the ideal Si-(OOl) basal surface is terminated by silicon atoms and the C-(001) surface is terminated by carbon atoms shown in Fig. I.3"8 Studies of the high temperature oxidation of these two basal surfaces have shown that the two surfaces have significantly different oxidation rates.3"5 As summarized 3088
J. Mater. Res., Vol. 9, No. 12, Dec 1994
by Tressler,3-4 the C-(001) SiC surface oxidizes more slowly than silicon but much faster than the Si-(OOl) surface of SiC. While these studies provided valuable information on the macroscopic oxidation of 6H SiC, they are unable to address the question of SiC-SiC>2 interface composition and structure during initial and intermediate stages of ox
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