3. Interface structures beneath thick thermal oxides grown on 4H-SiC(0001) Si-face and (000-1) C-face substrates

Figure 2. C 1s core-level spectra taken from the oxidized 4H-SiC(0001) surface using synchrotron radiation; (a) angle-

resolved XPS analysis, (b) results of in situ vacuum annealing at 500ºC.

238 Physics and Technology of Silicon Carbide Devices

Figure 3. Change in the total amount of intermediate oxide states in Si 2p3/2KH=;LJ9

tween the intermediate state and the bulk signal was plotted as a function of oxidation time.

/z,.!2%+1/(5z.!,+.0! \_z% !(z\$5 .+#!\*z,//%20%+\*z+"zz%z/1."!z3%0\$zz %(10! zz/+(1¥ 0%+\*z3/z.!(5z+0%\*! \_z\* z0\$!z%\*%0%(z/),(!z/1."!z"0!.z3!0z(!\*%\*#z3/z,.0%((5z+4%¥ dized and contaminated with adsorbates [18]. This implies that a chemical shift component of C 1s core-level spectra involves unavoidable signals due to surface contamination. Thus, we performed angle-resolved XPS and in situ vacuum annealing prior to XPS analysis in the analysis chamber. Figure 2 represents C 1s core-level spectra taken from the oxidized 4H-SiC(0001) surface [17]. As shown in Fig. 2(a), the chemical shift component originating from carbon-oxides (COx) increased with respect to the bulk signal (C-Si bond) under the surface

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Figures 4(a) and 4(b) represent typical deconvoluted Si 2p3/2 spectra obtained from the 40 nm-thick SiO2/SiC(0001) Si-face and (000-1) C-face substrates, respectively, in which the thick thermal oxides were thinned using a diluted HF solution prior to synchrotron XPS analysis [19]. Similar to the thin thermal oxides (see Fig. 1), the Si 2p3/2 spectra were fitted well with five components originating from bulk SiC and SiO2z,+.0%+\*/z0+#!0\$!.z3%0\$z%\*0!.¥ mediate oxide states. It's obvious that, for both cases, the total amount of the intermediate states was sufficiently small compared with that of the remaining oxides (about 3 nm thick). This implies that the physical thickness of the transition layer on the oxide side is as thin as a "!3z0+)%z(5!./z!2!\*z"+.z0\$!z0\$%'z0\$!.)(z+4% !/^z\$!/!z!4,!.%)!\*0(z.!/1(0/z(!.(5z%\* %¥ cate formation of a near-perfect SiO2u%z%\*0!."!z3%0\$z+\*2!\*0%+\*(z .5z+4% 0%+\*z.!#. ¥ less of the substrate orientation and oxide thickness.

Furthermore, for the thick thermal oxide on Si-face substrate, the composition of the bulk %z.!#%+\*z!\*!0\$z0\$!z+4% !z3/z!/0%)0! z".+)z0\$!z%\*0!\*/%05z.0%+zce%zE,fuezD/fd^z!z+¥ tained an identical intensity ratio to that of the initial as-grown SiC surface [17f^z\$!/!z!4¥ perimental results mean that, despite previous literature based on TEM observation [6-8], there exists no thick carbon-rich layer of a high atomic percentage at the SiO2/SiC interface \* z0\$0zz\*!.w,!."!0z%\*0!."!z +)%\*0! z5z%wz+\* /z%/z"+.)! z!2!\*z"+.z0\$!z0\$%'z0\$!.¥ mal oxidation of the SiC(0001) surface.

Figure 5 compares the change in the total amount of intermediate oxide states in Si 2p3/2 spectra obtained from SiO2/SiC interfaces. The intensity ratios between the intermediate states and the bulk signals for thin and thick thermal oxides grown on (0001) Si-face and (000-1) C-face substrates were plotted. Despite that the minimal intermediate oxide states #%\*z%),(5z.1,0z%\*0!."!\_z3!z+/!.2! zz/(%#\$0z%\*.!/!z%\*z0\$!z%\*0!.)! %0!z/00!/z!/,!¥ cially for thick thermal oxide interface on C-face substrates, which suggesting degradation of interface electrical properties of SiC-MOS devices formed on C-face substrates [20].

Figure 4. Si 2p22 core-level spectra obtained from the oxidized (a) 4H-SiC(0001) Si-face substrates. Before synchrotron XPS measurement, 40-nm-thick oxide layers were thinned using HF wet etching. The remaining oxide thickness was about 3 nm for both cases.

Figure 5. Change in the total amount of intermediate oxide states in Si 2px/> spectra taken from the oxide interfaces grown on SiC(0001) Si-face and C-face substrates.
