5. Energy band structure of SiO2/4H-SiC interfaces and its modulation induced by intrinsic and extrinsic interface charge transfer

The energy band structure and interface quality of SiO2/4H-SiC fabricated on (0001) Si-face and cCCCwDdzw"!z/1/0.0!/z3/z(/+z%\*2!/0%#0! z5z)!\*/z+"z/5\*\$.+0.+\*z^z\$!.)(z+4% ¥ tion was conducted using a conventional furnace at temperatures ranging from 1000 to 1100ºC. Thin and thick oxide layers of about 3 and 40 nm were prepared by choosing the oxidation 0!),!.01.!z\* z0%)!z,,.+,.%0!(5z"+.z%w"!z\* zw"!z/1/0.0!/^z+.z0\$!z0\$%'z+4% !z/)¥ ples, the oxide layers were thinned to about 3 nm thick by a diluted HF solution. To determine band structures of SiO2/SiC, we examined the band gap of the thermal oxides and valence band +""/!0z0z0\$!z%\*0!."!^z\$!z!\*!.#5z\* z#,z+"z0\$!/!z+4% !/z#.+3\*z+\*z%w"!z\* zw"!z/1¥ /0.0!/z3/z"%./0z!/0%)0! z".+)zzD/z!\*!.#5z(+//z/,!0.^z-!1/!z0\$!z,\$+0+!(!0.+\*/z#!\*!.0¥ ed in oxides suffer energy losses originating from plasmon and electron-hole excitations, the energy band gap can be determined by the threshold energy of an energy loss spectrum for an intense O 1s signal [23]. As shown in Fig. 7, O 1s energy loss spectra for thermal oxides on the Si-face and C-face substrates clearly indicate that the energy band gap of the oxides is identical regardless of substrate orientation (8.7 eV) [24]. Considering the high oxidation temperatures +2!.zDCCC~z\* z(+3z+\*!\*0.0%+\*z+"z.!/% 1(z.+\*z%),1.%0%!/z3%0\$%\*z0\$!.)((5z#.+3\*z+4¥ ides on SiC [17], these results seem to be quite reasonable.

interstitials forming local C-C dimers located on the SiC bulk side as a possible origin of the electrical defects [22]. Therefore, it is concluded that, for improving the performance of SiC- /! z
z !2%!/\_z3!z/\$+1( z"+1/z+1.z00!\*0%+\*z+\*z 0\$!z0+)%z+\* %\*#z"!01.!z\* z.¥ bon impurities within the channel region rather than the thick transition layer near the

Figure 7. O 1s energy loss spectra for thermal oxides on (0001) Si-face and (000-1) C-face 4H-SiC substrates. The onset G>L@==P;AL9LAGF >JGEL@=N9D=F;=LG;GF<M;LAGF:9F<K :9F<?9H;9F:=<=L=JEAF=< >JGEL@==F=J?QDGKK0@=N9s lence band maximum of SiC substrates and the oxides was determined by the valence spectra taken from SiO2/SiC structures and a reference SiC surface [24]. Figure 8 represents measured and deconvoluted valence spectra obtained after 3-nm oxidation of the Si-face and C-face substrates, in which the valence band maximum of the thermal oxides was estimated by subtracting the reference SiC spectra ( ) from the measured SiO2/SiC spectra ( ) both for the Si- and

5. Energy band structure of SiO2/4H-SiC interfaces and its modulation

The energy band structure and interface quality of SiO2/4H-SiC fabricated on (0001) Si-face and cCCCwDdzw"!z/1/0.0!/z3/z(/+z%\*2!/0%#0! z5z)!\*/z+"z/5\*\$.+0.+\*z^z\$!.)(z+4% ¥ tion was conducted using a conventional furnace at temperatures ranging from 1000 to 1100ºC. Thin and thick oxide layers of about 3 and 40 nm were prepared by choosing the oxidation 0!),!.01.!z\* z0%)!z,,.+,.%0!(5z"+.z%w"!z\* zw"!z/1/0.0!/^z+.z0\$!z0\$%'z+4% !z/)¥ ples, the oxide layers were thinned to about 3 nm thick by a diluted HF solution. To determine band structures of SiO2/SiC, we examined the band gap of the thermal oxides and valence band +""/!0z0z0\$!z%\*0!."!^z\$!z!\*!.#5z\* z#,z+"z0\$!/!z+4% !/z#.+3\*z+\*z%w"!z\* zw"!z/1¥

ed in oxides suffer energy losses originating from plasmon and electron-hole excitations, the energy band gap can be determined by the threshold energy of an energy loss spectrum for an intense O 1s signal [23]. As shown in Fig. 7, O 1s energy loss spectra for thermal oxides on the Si-face and C-face substrates clearly indicate that the energy band gap of the oxides is identical

!1/!z0\$!z,\$+0+!(!0.+\*/z#!\*!.0¥

induced by intrinsic and extrinsic interface charge transfer

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SiO2/SiC interface.

242 Physics and Technology of Silicon Carbide Devices

C-face substrates (see

/,!0.z - d^z\$!/!z.!/1(0/z !)+\*/0.0! z0\$0z0\$!z2(!\*!z\* z+""/!0z+"z0\$!z%2/SiC for the w"!z/1/0.0!z3/z+10zC^Gz!z(.#!.z0\$\*z0\$0z"+.z0\$!z%w"!^z \*z %0%+\*\_z3!z+\* 10! /%)%(.z/5\*\$.+0.+\*zz\*(5/%/z"+.z0\$%'z+4% !z/),(!/z0+z!4)%\*!z0\$!z!""!0/z+"z%\*0!."! !"!0/z+\*z!\*!.#5z\* z)+ 1(0%+\*z/z0\$!z+4% !z0\$%'\*!//z%\*.!/! ^

Figure 8. Measured and deconvoluted valence band spectra for SiO2/SiC structures formed on (0001) Si-face and (000-1) C-face 4H-SiC substrates.

z !2%!/z.1%((5z !,!\* /z+\*z 0\$!z/1/0.0!z+.%!\*00%+\*z\* z+4% !z 0\$%'\*!//^z%\*! 0\$!z 0\$%\*z+4% !z+\*z 0\$!zw"!z/1/0.0!/z!4\$%%0/z/)((!.z+\* 10%+\*z\* z+""/!0/z 0\$\*z 0\$+/! +\*z0\$!z%w"!z/1/0.0!/\_z3!z+\*(1 !z0\$0z0\$!z !#. ! z.!(%%(%05z+"z%w
z !2%!/z"¥ ricated on the C-face surface is an intrinsic problem, which is probably due to the difference in the electronegativity between Si and C atoms bonded with O atoms at the interface.

Furthermore, considering the accumulation of negative fixed charges at the SiO%SiC interface, the increase in the conduction band offset for thick oxides both on the Si-face and Cface substrates can be explained by an extrinsic energy band modulation due to the interface defects. This enlarged band offset for the thick MOS devices is preferable from the viewpoint of reducing gate leakage, but electrical defects should negatively impact on the device performance and reliability. Therefore, fundamental tactics, such as applying deposited gate oxides and band engineering by utilizing stacked structures, are indispensable to take advantage of C-face SiC-MOS devices [25-28].

Figure 9. Energy band diagrams of SiO2/4H-SiC(0001) structures obtained by synchrotron XPS analysis. The measured values of the valence band offsets for SiQ₂/SiC interfaces formed under various conditions are indicated.

We also evaluated the modulation of energy band alignment of SiO%HH-SiC(0001) structures due to the interface defect passivation [29]. It was found that, although the hydrogen incorporation into the SiO%SiC interface is effective in improving the interface property, both XPS analysis and electrical measurements revealed that interface detect passivation induces a reduction of conduction band offset. This indicates that the larger conduction band offset at the as-oxidized SiO%SiC interface is attributed to the high density of interfacial carbon related defects.
