Direct SiO2/β-SiC(100)3×2 interface formation from 25°C to 500°C

Article abstract of DOI:10.1063/1.115612

We investigate the β-SiC(100)3×2 surface oxidation by core level and valence band photoemission spectroscopies using synchrotron radiation. Low molecular O2 exposures on the (3×2) surface reconstruction leads to direct SiO2/β-SiC(100)3×2 interface formati

Full text of DOI:10.1063/1.115612

Direct SiO2/βSiC(100)3×2 interface formation from 25°C to 500°C
F. Semond, L. Douillard, P. Soukiassian, D. Dunham, F. Amy, and S. Rivillon
Citation: Applied Physics Letters 68, 2144 (1996); doi: 10.1063/1.115612
View online: http://dx.doi.org/10.1063/1.115612
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Direct SiO2 /␤-SiC(100)3؋2 interface formation from 25 °C to 500 °C
F. Semond, L. Douillard, and P. Soukiassiana)
`
ˆ
Commissariat a l’Energie Atomique, DSM-DRECAM-SRSIM, Batiment 462, Centre d’Etudes de Saclay,
´
´
91191 Gif sur Yvette Cedex, France, and Departement de Physique, Universite de Paris-Sud,
91405 Orsay Cedex, France
D. Dunham, F. Amy,b) and S. Rivillonb)
Department of Physics, Northern Illinois University, DeKalb, Illinois 60115-2854
͑Received 27 November 1995; accepted for publication 1 February 1996͒
We investigate the ␤-SiC͑100͒3ϫ2 surface oxidation by core level and valence band photoemission
spectroscopies using synchrotron radiation. Low molecular O2 exposures on the ͑3ϫ2͒ surface
reconstruction leads to direct SiO2 /␤-SiC͑100͒3ϫ2 interface formation already at room
temperature ͑RT͒. To our best knowledge, this is the first example of RT oxidation leading directly
to dominant silicon dioxide growth by O2 chemisorption only. The amount of SiO2 is enhanced
when the surface temperature is raised by few hundred degrees only ͑Ͻ500 °C͒ during O2 exposures
leading to ‘‘bulk oxide’’ formation already at small thicknesses. These findings are also relevant in
low-temperature semiconductor oxide processing. © 1996 American Institute of Physics.
͓S0003-6951͑96͒04214-0͔
Silicon carbide ͑SiC͒ is a promising IV-IV compound
semiconductor having a strong technological interest.1 In
fact, this ‘‘refractory’’ semiconductor could be very useful in
its ability to provide devices able to operate at rather el-
evated temperatures ͑Ϸ600 °C͒, especially when compared
to conventional semiconductors as silicon ͑Ͻ150 °C͒. How-
ever, unlike many other elemental or compound semiconduc-
tors, SiC has so far received much less attention in surface
science. One of the key issues in successful device applica-
tions is surface oxidation. Despite its major importance, this
was not very much investigated in surface/interface science,
with few studies only on direct and promoted oxidation of
SiC surfaces.2–5 These investigations were performed using
conventional techniques,2–5 with, unlike Si surfaces, no such
SiC oxidation studies using synchrotron radiation.
In this letter, we present the first investigation of SiC
oxidation using synchrotron radiation. We show that ‘‘bulk-
like’’ SiO2 could be grown already at small thicknesses on
the ␤-SiC͑100͒3ϫ2 surface ͑Si-rich͒ at low surface tempera-
tures and low O2 exposures. SiO2 is found to be the domi-
nant oxide product already at room temperature ͑RT͒. To our
best knowledge, these findings, which bring very new in-
sights in understanding low-temperature oxide growth and in
SiC surface oxidation, have never been observed previously
even on Si surfaces. This shows the central importance of
surface structure in the successful growth of SiO2.
␤-SiC͑100͒ thin films ͑Ϸ1 ␮m͒ were prepared at LETI by
chemical vapor deposition on a Si͑100͒2ϫ1 wafer miscut by
4°. Native oxides removal is achieved by annealing at
1150 °C resulting in clean and carbon rich surfaces. Subse-
quent Si deposition, using a carefully outgassed Si source,
followed by thermal annealings at 950 °C for 10 min pro-
duce clean single-domain ͑3ϫ2͒ surfaces, as checked by
sharp low-energy electron diffraction ͑LEED͒ patterns. To
avoid photochemical effects, the beam line was closed dur-
ing oxygen exposures. We have decomposed the Si 2p spec-
tra using conventional combination of Lorentzian and Gauss-
ian functions to take into account both Si 2p life time and
instrumental broadening. We used a branching ratio of 0.5, a
0.6 eV spin-orbit splitting and a 0.1 eV Lorentzian width.6,7
Other experimental details could be found elsewhere.5,8
We first look at the clean ␤-SiC͑100͒3ϫ2 surface using
the Si 2p core level ͑Fig. 1͒ recorded in both bulk- and
surface-sensitive modes ͑at photon energies h␯ of 115 and
150 eV͒. The Si 2p spectra are decomposed into three com-
ponents A, B, and C at binding energies of 99.2, 100.2, and
99.5 eV, respectively. The 99.2 eV component ͑A͒, located
at same binding energy than pure Si, which could result from
Si ad-atoms, is also assigned to the presence of Si clusters
resulting from Si deposition during the 3ϫ2 surface prepa-
ration. In fact, for ␤-SiC͑100͒3ϫ2 surfaces prepared with
longer Si deposition time, we clearly observe the same three
peaks ͑A, B, C,͒ by a simple visual inspection.9 Peak A
visually disappears after thermal annealings but could still be
identified after peak decomposition ͑Fig. 1, two bottom spec-
tra͒, thereby supporting further its assignment to Si clusters.
Peak A is also visible on the two top spectra ͑Fig. 1͒ after a
10ϩ4 Langmuir ͑1 Langmuirϭ1 Lϭ10Ϫ6 Torr-s) oxygen
exposure at 300 and 500 °C. Component B is related to Si–C
bonds corresponding to bulk SiC and peak C to the top Si
surface layer in the ͑3ϫ2͒ structure.6,7
The photoemission experiments were performed at the
Synchrotron Radiation Center, University of Wisconsin-
Madison using the 1 GeV Aladdin storage ring and ‘‘grass-
hopper’’ Mark II or Mark V monochromators. The photo-
electron energy was measured by a cylindrical mirror
analyzer. The overall energy resolution of ϳ0.4 eV at Si 2p
and Ϸ1.1 eV at C 1s core levels was established from a gold
film also providing Fermi level position. Single-domain
We now turn to the oxidation study using the Si 2p core
level ͑Fig. 1͒ for an oxygen exposure of 10ϩ4 L at tempera-
tures ranging from 25 to 500 °C. In order to make compari-
son with silicon oxidation easier, the spectra are presented
a͒
Also at: Department of Physics, Northern Illinois University, DeKalb, Il-
linois 60115-2854. Electronic mail: patrick@gcad.doit.wisc.edu
b͒
ˆ
´
Permanent address: Maitrise de Physique et Applications, Departement de
´
Physique, Universite de Paris-Sud, 91405 Orsay Cedex, France.
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FIG. 2. Valence band and O 2s core level for ␤-SiC͑100͒3ϫ2 surfaces
exposed to 10ϩ4 L of O2 at various temperatures from 25 to 500 °C. For
comparison, the valence band of a SiO2 /Si͑111͒7ϫ7 interface is also pre-
sented. The photon energy was 150 eV.
FIG. 1. Si 2p core level for clean ␤-SiC͑100͒3ϫ2 surface and exposed to
10ϩ4 L of O2 at various temperatures ranging from 25 to 500 °C. The photon
energy was 150 and 115 eV for surface and for bulk sensitive modes, re-
spectively. For comparison, bars ͑0 to IV͒ indicates various Si oxidation
states (Si 0, Siϩ, Si2ϩ, Si 3ϩ, and Si4ϩ) bonding energies in the case of
very thin silicon oxides ͑see. e.g., Ref. 10͒. The two bottom spectra show the
Si 2p core level decompositions, fits, background subtraction, and residual
background for the clean ␤-SiC͑100͒3ϫ2 surface in bulk- ͑bottom͒ and
surface-sensitive ͑second from bottom͒ modes.
tures during oxidation ͑Fig. 1͒. The behavior of the major
peak between 1 and 2 eV is of special interest. It is likely to
result from different contributions. Peak C ͑related to top Si
atoms͒ collapses during oxidation while peak B ͑Ϸ1 eV͒ is
still present at lower intensity. Also, the formation, at the
interface between SiO2 and ␤-SiC͑100͒3ϫ2 surface, of a
complex oxide involving both Si and C atoms as SiOC is
likely since it would be consistent with the peak observed
around 2 eV. A detailed peak decomposition showing the
various oxidation states at the interface will be provided
elsewhere.9 Another interesting feature is the presence ͑at
very low intensity͒ of peak A at 99.2 eV which could be seen
by simple visual inspection for the surfaces oxidized at 300
and 500 °C. It indicates the presence of pure Si clusters
which have not been oxidized under the present conditions.
Further valuable insights about SiO2 /␤-SiC͑100͒3ϫ2
interface formation could be found by looking at the valence
band. Figure 2 displays the valence band and the O 2s core
level of this system following the same oxidation sequence
described above. For comparison, we also show in Fig. 2 a
similar spectrum for a SiO2 /Si interface. Upon oxygen expo-
sures from 25 to 500 °C, the ␤-SiC͑100͒3ϫ2 valence band
exhibits the growth of O 2p electronic levels at binding en-
ergies of 7.6, 10.4, and 14.3 eV, characteristic of SiO2
formation.10 In fact, as can be seen from the SiO2 /Si valence
band, the O 2p spectral features are the same. In addition,
one can notice that the O 2s core level binding energy ͑25.5
eV͒ and shape for the SiO2 /␤-SiC͑100͒3ϫ2 and
SiO2 /Si͑111͒7ϫ7 interfaces are identical which further
stresses that the oxide products are the same in both cases.
Interestingly, the O 2s core level does not apparently exhibit
a second component.
with a relative binding energy scale having its origin at the Si
2p3/2 component of pure silicon. Upon RT O2 exposure, the
Si 2p core level, recorded in the surface sensitive mode ͑h␯
ϭ150 eV͒ exhibits a new chemical component ͑Peak M͒
shifted by 3.9 eV to higher binding energy at 103.1 eV. This
indicates the formation, already at room temperature, of sili-
con oxides having very high oxidation states as SiO2. It
shows that the ␤-SiC͑100͒3ϫ2 surface behaves very differ-
ently from the silicon ones for which RT O21e0xposures result
in lower oxidation states, primarily as Si3ϩ
.
We study next the effect of surface temperature on the
␤-SiC͑100͒3ϫ2 surface oxidation. When the 10ϩ4 L of O2
exposure is performed at a surface temperature of 300 °C,
peak M intensity is significantly increased while its energy
position is shifted by 4.1 eV at 103.3 eV. This shows that the
amount of SiO2 increases significantly when the surface tem-
perature is raised by few hundred degrees. Also, peak M
higher binding energy ͑by 0.2 eV from its previous energy
position at RT͒ indicates that SiO2 oxide becomes more
‘‘bulk-like’’.11 To get a better insight on the oxide thickness,
we recorded the Si 2p core level at a photon energy of 115
eV ͑Fig. 1͒ which provides a more bulk-sensitive informa-
tion about the growth of the SiO2 /␤-SiC͑100͒3ϫ2 interface.
As can be seen, peak M remains intense at lower photoelec-
tron kinetic energies. A surface temperature of 500 °C during
O2 exposure leads to further growth of SiO2 as evident from
peak M additional intensity increase with an even larger core
level shift (⌬ESi2pϭ4.5 eV) at 103.7 eV.
Additional insights about the atomic bonding configura-
tion could be obtained by looking in Fig. 3 at the C 1s core
level for the clean and 104 L of O2 exposed ␤-SiC͑100͒3ϫ2
We now look at the other Si 2p core level spectral fea-
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Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Semond et al. 2145
130.18.123.11 On: Sun, 21 Dec 2014 10:57:23

much larger amounts of SiO2. Even at 500 °C, the surface
temperature remains much lower than temperatures com-
monly used to achieve SiO2 /Si interface formation. In addi-
tion, the SiO2 oxide layer is carbon free as previously sug-
gested for other SiC surfaces.2–5 Any direct C oxidation
would result in the formation of CO or CO2 which will de-
sorb from the surface into the vacuum as evident from the
lack of any related spectral feature.2–5 The changes observed
at the C 1s and Si 2p core levels would be more consistent
with the presence of a carbon interlayer bonded to SiO2 and
located at the SiO2 /SiC interface. Most significantly, the ox-
ide formed at 500 °C (⌬ESi 2pϭ4.5 eV) corresponds ex-
actly to the binding energy achieved for much thicker oxides
͑with a 144° Si–O–Si bond angle͒,11 despite the fact that
here, the oxide thickness is below 10 Å.
In conclusion, we have investigated the direct
SiO2 /␤-SiC͑100͒3ϫ2 interface formation by low molecular
O2 exposures using photoemission spectroscopy with syn-
chrotron radiation. Our results indicate that SiO2(⌬ESi 2p
ϭ3.9 eV) is already the dominant oxidation product at RT.
It shows that the nature of surface reconstruction plays a
major role in successful oxidation. Raising the surface tem-
perature by only few hundred degrees during O2 exposure
leads to ‘‘bulk-like’’ carbon free SiO2 oxide formation
(⌬ESi 2pϭ4.5 eV) already at small thicknesses. These find-
ings bring new and important insights in SiC surface passi-
vation as well as in low temperature oxide formation.
This work was supported by U. S. National Science
Foundation ͑NSF͒ under contract No. DMR 92-23710 and by
Northern Illinois University Graduate School Fund. It is
based upon research conducted at the Synchrotron Radiation
Center ͑SRC͒, University of Wisconsin-Madison which is
supported by NSF under contract No. DMR 92-12658. The
authors are grateful to C. Jaussaud and L. di Ciccio at LETI
FIG. 3. C 1s core level for clean and exposed to 10ϩ4 L of O2
␤-SiC͑100͒3ϫ2 surfaces at room temperature. The photon energy was
340 eV.
surfaces. The C 1s core level has been recorded in surface
sensitive mode at a photon energy of 340 eV. As can be seen
in Fig. 3, the C 1s core level is shifted by 0.6 eV to higher
binding energy upon exposure with a slight broadening only,
suggesting that the C atoms are somewhat involved in the
oxidation process. This cannot be related to graphite forma-
tion which would result in core level shifts larger than 2
eV.5,8 Also, it does not seem to indicate presence of direct
C–O bonds on the surface, in agreement with the picture
obtained from the O 2s core level showing only a single
component as for SiO2 /Si interface ͑Fig. 2͒. In fact, in this
case, much larger core level shifts would be expected.12
However, it is consistent with the formation of a complex
oxide as SiOC as suggested above from the Si 2p results.
Our above results support the picture of direct
SiO2 /␤-SiC͑100͒3ϫ2 interface formation already at RT
which, to our best knowledge, is the first observation of this
kind. Even on Si surfaces, direct dominant SiO2 formation
could be achieved at much higher temperatures only ͑above
900 °C͒. Interestingly, we do not observe a similar behavior
with other ␤-SiC͑100͒ surface reconstructions like ͑2ϫ1͒
and ͑1ϫ1͒ which are much more difficult to oxidize than Si.9
This stresses the crucial importance of surface structure in
the oxidation process. In fact, the ␤-SiC͑100͒3ϫ2 surface is
a much more open surface ͑when compared, e.g., to
Si͑100͒2ϫ1 or to Si͑111͒7ϫ7͒ which would allow O atoms
to interact more easily with the Si atoms. Furthermore, recent
theoretical calculations for Si-rich ␤-SiC͑100͒ surfaces pre-
´
͑CEA-Technologies Avancees͒ for providing SiC͑100͒
samples. We thank the SRC staff for helpful assistance.
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2146 Appl. Phys. Lett., Vol. 68, No. 15, 8 April 1996 Semond et al.
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