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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: sungho.choi@vanderbilt.edu (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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