2. Initial oxidation of 4H-SiC(0001)

Synchrotron XPS analysis was performed using photon energy of 686.5 eV at BL23SU in the SPring-8 [16]. The starting substrate was as-grown 48-off-angled 4H-SiC(0001) wafer with an n-type epitaxially grown layer. After RCA cleaning and subsequent native oxide removal with a diluted hydrofluoric acid (HF) solution, thermal oxidation was conducted in dry oxygen ambient using a conventional tube furnace at 1100℃. To remove surface contamination due to air exposure, some of the samples were annealed in situ in an analysis chamber under an ultra-high vacuum condition.

Figure 1(a) represents changes in Si 2p core-level spectra as dry oxidation progresses on the 4H-SiC(0001) surface at 1100℃ [17]. Peak intensity was normalized with the bulk signal. Oxide growth on the SiC surfaces was contirmed with an increase in the chemical shift component in the Si 2p core-level spectra at around 104.5 eV. Capacitance-voltage (C-V) measurement of the corresponding Al/SiO%SiC capacitors also revealed that oxidation for 10 and 30 min yielded roughly 3.5 and 5.7-nm-thick oxides, respectively. To investigate atomic bonding feature at SiO2/SiC interfaces, Si 2p signals were analyzed by taking into account spin-orbit splitting. Figure 1(b) shows typical deconvoluted Si 2032 and 2212 peak compo nents obtained with the manner adopted in the previous research on SiO2/Si interfaces [3, 4]. Then, the Si 2p3/2 spectra taken from SiO2/SiC were deconvoluted into five components originating from bulk SiC and SiO2 portions together with intermediate oxide states (Si+) 5i2+, Si24). High-resolution XPS analysis allows us to detect small amount of intermediate states from an atomically abrupt oxide/substrate interface, and, in addition, these intermediate components can be a good indicator of structural imperfection at SiO /SiC interfaces. As shown in Fig. 1(c), we obtained a reasonable curve fitting with these components and confirmed that the total amount of the intermediate states is sufficiently small compared with that of thin thermal oxides. From these results, it is concluded that the physical thickness of the transition layer is as thin as a tew atomic layers, which corresponding to areal density of Si-O bonds in the range of a few times 1015 cm². This indicates formation of a near-perfect SiO2/SiC interface and coincides well with a recent report based on high-resolution medium energy ion scattering [9].

Figure 1. Synchron XPS spectra taken from the cleaned and oxidized 4H-SiC(0001) surfaces; (a) change in Si 2p core-level spectra as dy oxidation progresses, (b) peak deconvolution with 2px;2 components, (c) result of curve fitting of Si 2px2 core-level with bulk SiC and SiO2 signals and intermediate oxide states for the SiO2/SiC sample prepared by 10-min oxidation.

sensitive conditions (at small take-off angle (e)). In addition, it was found that the chemical shift component originating from carbon-oxides was totally removed by vacuum annealing at 500ºC (see Fig. 2(b)). Since stable chemical bonds existing at the SiO2/SiC interface are \$. z 0+z !+),+/!z1\* !.zz)+ !.0!z\*\*!(%\*#z 0!),!.01.!\_z3!z00.%10! z 0\$!z.+\*w+4¥ ide signal to surface contamination. This clearly demonstrates that atomic bonding at the thermally grown SiO2u%cCCCDdz%\*0!."!z%/z +)%\*0! z5z%wz+\* /z \* z 0\$0z.+\*z%)¥ ,1.%05z3%0\$z%0/z+4% !z"+.)z(+0! z\*!.z0\$!z%\*0!."!z%/z!(+3z0\$!z !0!0%+\*z(%)%0z+"zz\*(¥

Fundamental Aspects of Silicon Carbide Oxidation

http://dx.doi.org/10.5772/51514

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Figure 3 shows the change in the total amount of intermediate oxide states in Si 2p3/2 spectra (Si1+, Si2+, Si3+) [17]. Although a thick transition layer at SiO2/SiC interface was ruled out, we observed a slight increase in the intermediate oxide states with an increase of the oxide thickness, unlike in the SiO2u%z%\*0!."!\_z3\$%\$z!4\$%%0/zz,!."!0z%\*0!."!z.!#. (!//z+"z+4¥

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.!#. ¥

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\$!.¥

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!/,!¥

3. Interface structures beneath thick thermal oxides grown on 4H-

SiC(0001) Si-face and (000-1) C-face substrates

less of the substrate orientation and oxide thickness.

mal oxidation of the SiC(0001) surface.

ysis (about sub-1 atomic percent in general).

ide thickness [4].

Figure 2. C 1s core-level spectra taken from the oxidized 4H-SiC(0001) surface using synchrotron radiation; (a) angleresolved XPS analysis, (b) results of in situ vacuum annealing at 500ºC.

Figure 3. Change in the total amount of intermediate oxide states in Si 2p3/2KH=;LJ9 AFO@A;@L@=AFL=FKALQJ9LAG:=s 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 sensitive conditions (at small take-off angle (e)). In addition, it was found that the chemical shift component originating from carbon-oxides was totally removed by vacuum annealing at 500ºC (see Fig. 2(b)). Since stable chemical bonds existing at the SiO2/SiC interface are \$. z 0+z !+),+/!z1\* !.zz)+ !.0!z\*\*!(%\*#z 0!),!.01.!\_z3!z00.%10! z 0\$!z.+\*w+4¥ ide signal to surface contamination. This clearly demonstrates that atomic bonding at the thermally grown SiO2u%cCCCDdz%\*0!."!z%/z +)%\*0! z5z%wz+\* /z \* z 0\$0z.+\*z%)¥ ,1.%05z3%0\$z%0/z+4% !z"+.)z(+0! z\*!.z0\$!z%\*0!."!z%/z!(+3z0\$!z !0!0%+\*z(%)%0z+"zz\*(¥ ysis (about sub-1 atomic percent in general).

Figure 3 shows the change in the total amount of intermediate oxide states in Si 2p3/2 spectra (Si1+, Si2+, Si3+) [17]. Although a thick transition layer at SiO2/SiC interface was ruled out, we observed a slight increase in the intermediate oxide states with an increase of the oxide thickness, unlike in the SiO2u%z%\*0!."!\_z3\$%\$z!4\$%%0/zz,!."!0z%\*0!."!z.!#. (!//z+"z+4¥ ide thickness [4].
