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Applied Surface Science 253 (2007) 5411–5414
Nitridation of the SiO2/4H–SiC interface studied by
surface-enhanced Raman spectroscopy
S.H. Choi a,*, D. Wang b, J.R. Williams c, M. Park c, W. Lu d, S. Dhar a, L.C. Feldman a
a Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USAb Department of Physics, Furman University, Greenville, SC 29613, USA
c Department of Physics, Auburn University, Auburn, AL 36849, USAd Department of Chemistry, Center for Physics and Chemistry of Materials, Fisk University, Nashville, TN 37208, USA
Received 4 April 2006; received in revised form 22 November 2006; accepted 12 December 2006
Available online 17 December 2006
Abstract
We employ surface-enhanced Raman spectroscopy (SERS) to investigate the effect of nitridation on interfacial carbon at the SiO2/4H–SiC
interface. These results demonstrate that the interfacial carbon clusters are strongly modified by post-nitridation process and the nitrogen take-up
correlates with the reduction in the interface state density.
# 2006 Elsevier B.V. All rights reserved.
Keywords: SERS; 4H–SiC
Silicon carbide (SiC) is currently being explored as a
material for the next generation of high-power and high-
temperature electronics. The growth of high-quality SiO2 on
SiC is a crucial step to realize these applications. The fact that
SiC can be oxidized in a conventional way, similar to Si, is a
great advantage over other wide band-gap semiconductors.
However, the development of SiC metal-oxide-semiconductor
field effect transistors (MOSFETs) has been impeded by the
low carrier mobility in the channel region. Although this oxide
is similar to SiO2 grown on Si, the SiO2/SiC interface results in
much poorer electronic properties than the SiO2/Si interface.
These interface properties are directly related to interface
defects that form following high-temperature oxidation
process.
The likely source of defects, suggested by many authors, is
the accumulation of carbon clusters at the interface [1–5]. Ion
scattering probes and other techniques have set a limit on this
total carbon excess of well below 1015 cm�2 [6]. It is extremely
difficult to detect such small excesses by physical/chemical
means at the buried interface. A number of authors have
* Corresponding author at: 6301 Stevenson Center Station B, Nashville, TN
37235, USA. Tel.: +1 615 936 5967; fax: +1 615 343 7263.
E-mail address: [email protected] (S.H. Choi).
0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2006.12.021
reported possible detection [1–3]. Using electron energy loss
spectroscopy (EELS), Chang et al. reported that the carbon
concentration at the SiO2/SiC (0 0 0 1) interface may be higher
than in the oxide or in the bulk SiC [1]. MacFarlane and Zvanut
show that carbon-related defects at the SiO2/SiC interface are
detected by electron paramagnetic resonance (EPR) [2] and Lu
et al. have used surface-enhanced Raman spectroscopy (SERS)
to show the existence of carbon clusters at the C-face of SiC [3].
An important recent advance is the significant reduction of
interface defect densities, Dit, following a nitric oxide (NO)
post-oxidation [4–6,16]. It is of critical importance to further
identify these interfacial defects and understand the beneficial
effects of the NO treatment. In this paper we report new results,
using SERS, that demonstrates the modification of interfacial
carbon clusters following the NO process.
Raman spectroscopy is an effective technique to investigate
carbon structures, although with limited sensitivity at low
concentration [7]. Surface-enhanced Raman scattering strongly
enhances sensitivity for surface characterization [8]. SERS is
associated with photochemically roughened metallic surfaces
(usually Ag, Au noble metals) and yields a 105–106 times
enhanced signal over that of non-metallized surfaces [9].
Although the mechanism of enhanced Raman scattering is still
a matter of discussion and not quantitatively understood, it is
generally agreed that it involves excitation of surface plasmons
Fig. 1. (a) The SERS spectra of the C-face 4H–SiC samples; (i) after oxidation
and (ii) after oxidation followed by NO anneal. (The peak at 1428 cm�1 is a
laser plasma line that corresponds to the 471.3 nm emission line of He.) The
inset shows schematic diagram of sample preparations for the SERS experi-
ment. (b) Overlapped SERS spectra, showing the difference between these two
graphs in the range of D and G position. *Only for NO post-annealed sample.
S.H. Choi et al. / Applied Surface Science 253 (2007) 5411–54145412
confined to metal layers. We have recently reported the
application of SERS to the interface characterization of SiO2/
4H–SiC, using the Ag overlayer method, and detecting
interfacial carbon by comparison of the oxidized and non-
oxidized SiC surface [3]. The findings indicated that carbon
clusters were in much greater abundance on the C-face than the
Si-face, and the nature of the clusters was graphitic-like. In the
present paper, we report SERS measurements of the carbon on
the different faces of SiC surface following NO post-annealing
and further characterize the structural nature of the carbon
compounds.
Samples used for the experiment were n-type (0 0 0 1̄) C-face
and (0 0 0 1) Si-face (both 88off axis) doped to a concentration of
8 � 1015 cm�3 and 4.8 � 1015 cm�3, respectively, supplied by
CREE Inc. The wafers were cleaned by the RCA process before
oxidation. The Si-face sample was oxidized at 1150 8C in oxygen
for 3 h to yield an oxide thickness of about 90 nm. The C-face
sample was similarly pre-cleaned and oxidized at 1150 8C in
oxygen for 1 h to yield about 60 nm thickness. Nitridation was
performed at 1175 8C for 2 h with 0.5 l/min flow rate of NO at
atmospheric pressure. Earlier measurements have shown that this
process results in about 1 � 1015 � 0.1 cm�2 nitrogen on the
carbon face and about a factor of �3 lower on the silicon face
[18]. It has also been shown that the nitrogen is confined to a very
thin layer,�1.5 nm, at the interface [19]. SERS requires that the
noble metal film, such as Au and Ag, should be in direct contact
with the species to be detected, so the oxide layer is entirely
etched by 5% hydrofluoric acid (HF) solution for 12 min (Fig. 1
inset) Since the etching rate of SiO2 in 5% HF is about 0.25 nm/s,
this etching condition is sufficient to remove all of oxide layer.
Using nuclear reaction analysis (NRA), we have shown that the
nitrogen layer is not removed after HF dip. Ag films were
deposited on the oxide-etched samples by pulsed laser deposition
(PLD) from 99.99% pure silver target in a background pressure
of�9 � 10�6 Torr. The KrF ablation laser (l = 248 nm) was set
to a fluence of 330 mJ at 25 Hz. Optical transmission of the Ag
films deposited on glass substrate was used to establish the Ag
deposition condition that will maximize the surface-enhanced
Raman scattering effect by matching the frequency of the
excitation laser with that of the surface plasmon resonance. The
morphological structure of the grown Ag films was examined by
a scanning electron microscopy (SEM) without any pretreat-
ment, showing a homogeneous island type Ag film with cluster
diameters of �20 nm. Micro-Raman spectroscopy was carried
out at room temperature using an optical microscope coupled
with a spectrometer (Jobin-Yvon). The 441.6 nm line from a He-
Cd laser (Kimmon Electric) was used for excitation. The
backscattering geometry was used for the Raman measurement.
The polarization states of the incident and the scattered light were
not analyzed. The laser beam (nominal power of 80 mW) was
focused onto a spot �5 mm in diameter on the sample surface.
Stokes’ SERS spectra were collected using the spectrometer with
a thermoelectrically cooled charge coupled device (CCD)
detector.
Fig. 1 (a) shows the normalized Raman spectra of C-face
samples oxidized and followed by NO oxidation. The main
features in the Raman spectra of disordered graphite are the D
and G bands, which lie at around 1350 and 1580 cm�1,
respectively. The G band, at 1580 cm�1, has E2g symmetry
corresponding to an in-plane bond-stretching motion. The D
band around 1350 cm�1 is a breathing mode of A1g symmetry
involving, phonons near the K zone boundary [7]. 4H–SC
exhibits several second-order Raman bands in the region of
1450–1800 cm�1 and these bands strongly overlap with the
Raman scattering from carbon [10]. Comparing the oxidized
with the NO post-annealed case, Fig. 1 (b), the intensity of the
carbon related band in the G peak region (1575 cm�1) has been
reduced; the change in D band region is less obvious. The
Raman spectrum of a clean SiC surface (no oxidation) is
identical to the nitrided one, shown in (ii).
In order to further elucidate the carbon cluster production
mechanism, we have obtained SERS spectra in each of the
different crystal faces by subtracting the Raman spectrum of
NO annealed sample from that of the oxidized one (Fig. 2).
Fig. 2. The carbon cluster related SERS spectra of the each SiC face. (a) The spectrum on C-face was obtained by subtracting the spectrum of the NO annealed
samples from that of the oxidized one (subtract from (1) to (2)); (b) same on Si-face (from (3) to (4)); (c) comparison between C-face and Si-face (from (1) to (3)). The
2 and 4 Gaussian fitted curves are also shown and solid bold arrows indicate subtraction direction.
S.H. Choi et al. / Applied Surface Science 253 (2007) 5411–5414 5413
Clearly the difference in curve shows the largest intensity for
the carbon face, consistent with the Ref. [3]. The spectrum in
Fig. 2 (a) is fitted using four Gaussian peaks labeled as follows:
D band at 1382 cm�1, polyene at 1475 cm�1, G band around
1575 cm�1 and D0 at around 1620 cm�1[7,13–15]. The D0 band
corresponds to a maximum vibrational density of states in the
graphitic layer. In addition to the band indexing, other values
such as the relative intensity and full width at half maximum are
found to correlate with the graphite ordering. SERS was also
performed on Si-face samples and the results are shown in
Fig. 2(b) and (c). Previous results indicated a barely detectable
carbon layer at the Si interface via SERS [3]. As shown in
Fig. 2(b) there is only the barest hint of the D and G bands in the
difference curve and no definitive conclusions are forthcoming
for the Si-face other than to confirm that the Si-face contains
minimal carbon cluster content, as measured by SERS.
Ferrari et al. have presented a three-stage model, which
correlates the Raman spectra of disordered/amorphous carbon
with a degree of structural disorder and/or hybridization [7,12].
They have presented a so-called amorphization trajectory
which shows a systematic change of the G peak position and the
intensity ratio of the D to G peak, I(D)/I(G), with respect to
hybridization (sp2 and sp3) and disorder in bond-angle and
Table 1
D and G band characteristics obtained experimentally from SERS. The values wh
Measured value Ex
Sta
G-band position (cm�1) 1575 15
I(D)/I(G) 0.35 0 !
bond-length. From the fitted curves shown in Fig. 2(a), the G
peak position and I(D)/I(G) ratio are �1575 cm�1and �0.35,
respectively. So a rough prediction of the interfacial carbon
structure maybe obtained by comparison of our SERS data (for
the C-face) with their three-stage model, as shown in Table 1.
Comparison of these values with the amorphization trajectory
model suggests that the structure of the carbon clusters formed
at the SiO2/SiC interface is most likely graphitic. It should be
noted however, that this analysis is fairly crude considering the
potentially high level of complexity in C-bonding at this
interface condition.
The different behavior of the crystal faces is consistent with
observation of the presence of higher Dit value on the C-face.
Interfacial carbon–carbon bonds may act as defect trap sites at
the SiO2/4H–SiC interface. Our earlier results revealed that C-
face has the highest Dit after high-temperature oxidation [17],
consistent with the substantially greater carbon density for the
C-face than for the Si-face as revealed by our SERS
measurement. Although it is very difficult to obtain a
quantitative measure of the interfacial carbon content using
SERS, it is important to note that previous ion scattering studies
have set an upper limit of less than a monolayer (�1015 cm�2)
to the interfacial excess carbon [6]. This indeed highlights the
ich are presented from the three-stage model are shown for comparison
pected values [7]
ge 1 Stage 2 Stage 3
81 ! 1600 1600 ! 1510 1510 ! 1570
2.0 2.0 ! 0.2 �0
S.H. Choi et al. / Applied Surface Science 253 (2007) 5411–54145414
very high sensitivity of the SERS technique for the detection of
low concentration species on a complicated surface structure.
Finally, we comment on the possible bonding structures,
such as C–N and Si–N, formed with nitridation. We note that
energy positions of the Raman lines of C–N are similar to those
of nitrogen-free amorphous carbon using visible excitation
[12]. Assuming comparable Raman cross sections this
suggests that the reduction of the D, G peaks is not due to
the formation of a C–N compound, but imply either (i) the
removal of the carbon, perhaps due to enhanced oxidation in
NO or (ii) the creation of a SixNyCz layer, incorporating carbon
with in the dielectric [11]. This latter view is consistent with the
conclusions of Ref. [16], which infers a possible near interface
modification of the dielectric from the reduction of slow
interface states.
This SERS study provides definitive physical/chemical
identification of interfacial carbon clusters after high-
temperature oxidation and their strong modification after the
NO anneal. From a comparison of measured SERS results with
a phenomenological three-stage model, we suggest that the
nature of the carbon clusters formed at the interface is graphitic
structured carbon. As opposed to diamondlike structures with
s bonds, graphitic carbon containing p bonds are likely
interface defects [16]. Following nitridation evidence for the
carbon clusters is entirely removed at the SERS scale of
sensitivity.
Acknowledgements
This work has been supported by DARPA Contract and by IT
Scholarship Program supervised by Institute for Information
Technology Advancement (IITA) and Ministry of Information
and Communication (MIC), Republic of Korea. Portions of the
work performed at Auburn University were partially supported
by USDA.
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