Fixed nitrogen atoms in the SiO2/SiC interface region and their direct
relationship to interface trap density
R. Kosugi, T. Umeda, and Y. Sakuma
Citation: Appl. Phys. Lett. 99, 182111 (2011); doi: 10.1063/1.3659689
View online: http://dx.doi.org/10.1063/1.3659689
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i18
Published by the American Institute of Physics.

Related Articles
InGaN epilayer characterization by microfocused x-ray reciprocal space mappingAppl. Phys. Lett. 99, 181909 (2011)
Polarity replication across m-plane GaN/ZnO interfaces
Appl. Phys. Lett. 99, 181910 (2011)
Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor
deposition grown GaN
J. Appl. Phys. 110, 093503 (2011)
Investigation of Ge1-xSnx/Ge with high Sn composition grown at low-temperature
AIP Advances 1, 042118 (2011)
Mechanisms of suppressing secondary nucleation for low-power and low-temperature microwave plasma selfbias-enhanced growth of diamond films in argon diluted methane
AIP Advances 1, 042117 (2011)

Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors

Downloaded 07 Nov 2011 to 130.158.131.239. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 99, 182111 (2011)

Fixed nitrogen atoms in the SiO2/SiC interface region and their direct
relationship to interface trap density
R. Kosugi,1,a) T. Umeda,2 and Y. Sakuma1
1

Advanced Power Electronics Research Center (ADPERC), National Institute of Advanced Industrial Scienceand Technology (AIST), Tsukuba 305-8568, Japan
2
Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan(Received 29 June 2011; accepted 20 October 2011; published online 4 November 2011)
Nitrogen atoms fixed in the SiO2/SiC interface region were studied by x-ray photoelectron
spectroscopy (XPS) and capacitance-voltage (C-V) measurements. A thin oxide film (<5 Å)formed during annealing in an NO atmosphere on a (0001) 4H-SiC surface, incorporating nitrogenatoms into the interface region. Even after complete removal of the oxide layer by etching inhydrofluoric acid, XPS spectra clearly showed a strong N 1 s peak, revealing the presence of fixed
nitrogen atoms with an areal density of 1014 cm2 in the interface region. To evaluate theirinfluence on interface traps, metal-oxide-semiconductor capacitors were formed by deposition of agate oxide layer. The fixed nitrogen atoms decrease the interface trap density after post-annealing
C 2011 American Institute of Physics. [doi:10.1063/1.3659689]
at high temperature. VThe low channel mobility of SiC metal oxide semiconductor field effect transistors (MOSFETs) is a critical issue
for SiC power MOSFETs. Nitridation can be used toimprove the channel mobility (lch) in (0001) 4H-SiC MOSFETs.1,2 Improvement of lch is accompanied by a reduced
interface trap density (Dit). This is because passivation and/or extinction of interfacial defects by nitridation decreases
carrier trapping and coulomb scattering in an inversion layer.
However, nitridation has two problems that affect the threshold voltage (Vth) of nitrided MOSFETs. One is the universal
relationship between Vth and lch: Vth decreases as lch
increases.3 The other is the shift of Vth caused by applicationof a high electrical field to the gate oxide.4 To overcome
such a trade-off relationship between Vth and lch and obtainreliable switching operations, greater insight into the role ofnitrogen at nitrided SiO2/SiC interfaces and the mechanismof lch improvement by nitridation is required.
In this letter, x-ray photoelectron spectroscopy (XPS)
measurements are used to determine the behavior of nitrogenatoms incorporated into the SiO2/SiC interface region by NOannealing. A significant amount of nitrogen atoms remain in
the SiO2/SiC interface even after oxide removal by sustained
etching in hydrofluoric acid (HF). To clarify their effect on
SiO2/SiC interface traps, metal oxide semiconductor (MOS)
capacitors including such nitrogen atoms were examinedusing simultaneous high- and low-frequency capacitancevoltage (CV) measurements.
N-type (0001) 4H-SiC substrates were purchased from
Nippon Steel Company, and epitaxial layers were grownby Showa Denko Inc. The thickness and effective
doping density (Nd  Na) of the epilayer were 10 lm and1.0  1016 cm3, respectively. Nitridation was performed at800–1467  C for 5 min under a 20% NO (balance Ar) atmosphere. A cold wall furnace5 was used for high temperaturenitridation above 1300  C. The XPS apparatus consisted of a
monochromatic Al ka light source (hm ¼ 1486.6 eV) and ana)

Electronic mail: r-kosugi@aist.go.jp.

0003-6951/2011/99(18)/182111/3/$30.00

energy analyzer with a two-dimensional multi-channel plate
detector. The take-off angle of the photoelectron was 37 .
MOS capacitors were fabricated as follows. The thin oxide
film that grew during pre-nitridation was removed by etching
in 5% HF for 2 min, and then a 50 nm thick oxide film wasdeposited by plasma-enhanced chemical vapor deposition(PECVD). When deposited oxide is used as a gate insulator,
post annealing is required to improve interface properties,6so the sample was annealed at 1200  C for 30 min following
oxide deposition. Both a gate electrode and back side substrate contact were formed by deposition of aluminum. Ditwas evaluated at room temperature using a simultaneous
high- and low-frequency CV technique. The sweep rate of
all quasi-static CV measurements was fixed at 0.1 V/s.
Figures 1(a)–1(c) show Si 2p and N 1s XPS spectra of
the as-grown epitaxial surface, and the same surface following nitridation at 1200  C for 5 min, and after HF etching.
The Si 2p spectrum in (a) shows a bulk SiC peak at 101.2 eVwith no oxide-related peaks at a higher binding energy.7 Nopeaks were observed in the N 1s spectrum, which indicatesthat the nitrogen concentration of the epitaxial layer(1016 cm3) is lower than the detection limit of the XPS. The
Si 2p spectrum in (b) shows a new component at 2.3 eVhigher binding energy than the bulk peak, which is attributed
to silicon dioxide (Siþ4).7 From the ratio of peak intensitiesof the oxide and bulk SiC components in the Si 2p spectrum,
the oxide thickness was estimated to be 4.3 Å. The N 1s
spectrum of the nitrided surface shows a clear peak at398.2 eV, indicating that a substantial amount of nitrogenhas been incorporated at the SiO2/SiC interface duringnitridation.8–10 Following these experiments, the thin oxide
layer was removed by etching in HF and then XPS spectra
were remeasured. The Si 2p spectrum in (c) does not show
any components originating from the oxide; only the firstepitaxial layer is present. Despite removal of the oxide, astrong N 1s peak is still observed. To confirm the absence ofthe silicon nitride layer on the substrate surface, etching in
50% HF for 10 min, which should remove the nitride,11 was99, 182111-1

C 2011 American Institute of Physics
VDownloaded 07 Nov 2011 to 130.158.131.239. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

182111-2

Kosugi, Umeda, and Sakuma

FIG. 1. Si 2p and N 1s XPS spectra of (a) as-grown epitaxial surface, andthe same surface (b) after nitridation at 1200 C for 5 min, and (c) after a subsequent HF etching.

also performed on the sample. However, the N 1s peak
was preserved. These results indicate that nitrogen atoms
are fixed in the SiO2/SiC interface region and remainunchanged by HF etching with different treatment times andconcentrations.
The amount of fixed nitrogen atoms was estimated using
the intensity ratio of Si 2p and N 1s signals. It was assumed
that nitrogen atoms were located within the first bilayer(0.62 Å). The mean free paths (k) of the Si 2p and N 1s photoelectrons in SiC were 14.5 Å and 12.9 Å, respectively,
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃas
calculated from the empirical relation, k ¼ 15 Ek=1482Å
(Ek: kinetic energy in eV). Consequently, the amount ofnitrogen was estimated to be 1.68  1014 cm2. Following
oxide removal by HF etching, the intensity of the N 1s peakincreased. If the most nitrogen atoms located on the oxideside are removed by HF etching, the intensity of the N 1s
peak should decrease significantly. Therefore, the amount of
fixed nitrogen is considered to be at least comparable to thatin the oxide layer. Previously, it was believed that incorporated nitrogen atoms of 1014 cm2 were distributed in theoxide side of a nitrided SiO2/SiC interface.12 However, ourfindings suggest a new model that nitrogen atoms are not
only embedded in the oxide layer but also fixed in the SiO2/
SiC interface region.
To confirm the effect of the fixed nitrogen atoms on theproperties of the SiO2/SiC interface, an n-type MOS capacitor was fabricated by oxide deposition on the nitrided surface

Appl. Phys. Lett. 99, 182111 (2011)that was etched with HF. Figure 2 shows the Dit distributionof the MOS capacitor as a function of the energy of the conduction band edge (Ec). As a reference, a MOS capacitor (a)was also formed using deposited oxide on a substrate withoutnitridation. Both capacitors were post-annealed under thesame conditions after oxide deposition. Nitridation at 1467  C
for 5 min caused the number of interface traps to decrease
drastically. At Ec  E ¼ 0.2 eV, Dit was 20% of the reference
sample. Because the only difference between the capacitors
was nitridation, it is clear that the reduction in Dit was causedby the fixed nitrogen atoms in the SiO2/SiC interface regionof the nitrided sample.
The influence of nitridation temperature (Tnitridation) on
the amount of fixed nitrogen and MOS characteristics wasinvestigated, as shown in Figs. 3(a) and 3(b), respectively.
Figure 3(a) shows N 1s spectra for different Tnitridation of
800–1430  C for 5 min. The inset shows the relationshipbetween Tnitridation and the areal density of fixed nitrogen.
The straight line indicates a linear fitting. Interestingly, theareal density increased proportionally to Tnitridation. At themaximum Tnitridation (1430  C), the amount of fixed nitrogenatoms was 2.37  1014 cm2. Figure 3(b) shows the Dit distribution of the four MOS capacitors corresponding to thefour Tnitridation examined above. The inset shows the relationship between Tnitridation and the integral density of the interface traps (Nit), which was calculated from the integral of Dit
over Ec – E ¼ 0.2–0.5 eV. When Tnitridation  1100  C, a linear correlation between Tnitridation and Nit was observed. This
result indicates that the fixed nitrogen atoms are related tothe reduction in Dit and that Dit decreases as the amount ofnitrogen increases. In contrast, from 800 to 1100  C, thefixed nitrogen atoms did not affect Dit, even though theiramount almost doubled. This suggests that a threshold
amount of nitrogen is required for effective reduction of Dit.
Although the reason for such a threshold remains unclear atpresent, a similar trend has been reported in a previousstudy.12
Finally, the reason why nitrogen atoms incorporated
into the SiO2/SiC interface region are not removed by HF
etching is discussed. There are two possible mechanisms thatfix nitrogen atoms in the interface region. One is thatFIG. 2. Comparison of Dit for n-type MOS capacitors formed (a) withoutand (b) with nitridation at 1467  C for 5 min. A gate oxide layer was formedby PECVD. Both MOS capacitors were etched in HF just before oxide
deposition.

Downloaded 07 Nov 2011 to 130.158.131.239. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

182111-3

Kosugi, Umeda, and Sakuma

Appl. Phys. Lett. 99, 182111 (2011)

FIG. 3. (a) Dependence of the N 1s spectrum on Tnitridation.
The inset shows the relationship between Tnitridation andthe nitrogen areal density. The solid line indicates a linear fitting result. (b) Dit distribution for n-type MOS
capacitors formed at the corresponding Tnitridation in (a).
The inset shows the relationship between Tnitridation andNit, which is the integral of Dit at Ec  E ¼ 0.2–0.5 eV.

nitrogen atoms form a special chemical bond so they become
resistant to HF etching. This kind of chemical bonding may
be formed during nitridation or HF etching.13 For example,
the formation of a perfect epitaxial monolayer of silicon oxynitride (SiON) on (0001) 6H-SiC surfaces after nitridation
has been reported.14 Because the proposed SiON structure
lacks dangling bonds and survived after exposure to air for
one month,14 it may be possible that this special layer has
chemical resistance to HF etching. However, the amount of
fixed nitrogen estimated in this study (0.1–0.4 monolayers)
suggests that a SiON layer may be produced in 10%–40% of
the interface area during NO annealing. Because the amountof fixed nitrogen atoms was unchanged after etching in 50%
HF, we can exclude the possibility that the low coverage of
the SiON layer is caused by gradual etching of a perfect
SiON monolayer.
Another possibility is that the fixed nitrogen atoms areincorporated into the SiC layer during nitridation. In anotherstudy where we applied electrically detected magnetic resonance to the channel region of (0001) 4H-SiC MOSFETs,15
we found that nitrogen shallow donors were doped in thechannel region after post-nitridation annealing. This result
can be explained by supposing that some part of the fixed
nitrogen atoms are incorporated into the SiC layer duringnitridation where they occupy substitutional sites (i.e., carbon sites), and are activated as shallow donors. This model
also provides a possible explanation for the universal relationship that an increase in lCH is associated with a reduction
in Vth. That is, the doped nitrogen donors produce additional
carriers that increase lCH as well as compensating foracceptors in the channel region, which reduces Vth.
In summary, investigation of nitrogen atomic profiles in
the SiO2/SiC interface by XPS measurements showed that asubstantial number of nitrogen atoms (areal density of1014 cm2) remain fixed even after complete removal of theoxide layer. This was directly confirmed by the N 1s XPS
spectra of a nitrided surface subjected to several subsequent
HF etching treatments. CV measurements of MOS capacitors
fabricated by oxide deposition on the nitrided surface
revealed a significant reduction in Dit compared with thecapacitor that was not subjected to nitridation. The amountof fixed nitrogen increases in direct proportion to Tnitridation,and Dit decreases as the amount of nitrogen increases. Thereare two possible causes for the presence of fixed nitrogenatoms in the SiO2/SiC interface region. The first is that nitrogen atoms form chemical bonds in the SiO2/SiC interface
region that are resistant to HF etching (for example, formation of a SiON layer at the interface). Another possibility isthat fixed nitrogen atoms are incorporated into the SiC layer
where they function as nitrogen shallow donors. We believethat better understanding of the fixed nitrogen atoms in SiO2/
SiC interfaces will allow further improvement and precise
control of the properties of these interfaces.
1G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, R. K. Chanana, R.
A. Weller, M. Di Ventra, S. T. Pantelides, L. C. Feldman, O. W. Holland,M. K. Das, and J. W. Palmour, IEEE Electron Device Lett. 22, 176 (2001).
2
J. R. Williams, G. Chung, C. C. Tin, K. MacDonald, D. Farmer, R. K.
Chanana, R. A. Weller, S. T. Pantelides, O. W. Holland, M. K. Das, and L.
C. Feldman, Mater. Res. Soc. Symp. Proc. 640 H3.5.1 (2001).
3
A. Agarwal, A. Burk, R. Callanan, C. Capell, M. Das, S. Haney, B. Hull,
C. Jonas, M. O’ Loughlin, M. O’Neil, J. Palmour, A. Powell, J. Richmond,
S. H. Ryu, R. Stahlbush, J. Sumakeris, and J. Zhang, Mater. Sci. Forum
600–603, 895 (2009).
4
A. J. Lelis, D. Habersat, G. Lopez, J. M. McGarrity, F. B. McLean, and N.
Goldsman, Mater. Sci. Forum 527–529, 1317 (2006).
5
R. Kosgui, K. Fukuda, and K. Arai, Matter Sci. Forum 483–485, 669(2005).
6
R. Kosugi, W. Cho, K. Fukuda, K. Arai, and S. Suzuki, J. Appl. Phys. 91,1314 (2002).
7C. Virojanadara and L. I. Johansson, Surf. Sci. Lett. 472, L145 (2001).
8K. Y. Cheong, S. Dimitrijev, J. Han, and B. Harrison, J. Appl. Phys. 93,5682 (2003).
9P. Jamet and S. Dimitrijev, Appl. Phys. Lett. 79, 323 (2001).
10K. McDonald, M. B. Huang, R. A. Weller, L. C. Feldman, J. R. Williams,
F. C. Stedile, I. J. R. Baumvol, and C. Radtke, Appl. Phys. Lett. 76, 568(2000).
11C.-E. Morosanu, Thin Sold Films 65, 171 (1980).
12K. McDonald, R. A. Weller, S. T. Pantelides, L. C. Feldman, G. Y. Chung,C. C. Tin, and J. R. Williams, J. Appl. Phys. 93, 2719 (2003).
13
S. Dhar, O. Seitz, M. D. Halls, S. Choi. Y. J. Chabal, and L. C. Feldman,J. Am. Chem. Soc. 131, 16808 (2009).
14
T. Shirasawa, K. Hayashi, S. Mizuno, S. Tanaka, K. Nakatsuji, F. Komori,and H. Tochihara1, Phys. Rev. Lett. 98, 136105 (2007).
15T. Umeda, K. Esaki, R. Kosugi, K. Fukuda, T. Ohshima, N. Morishita, andJ. Isoya, Appl. Phys. Lett. 99, 142105 (2011).

Downloaded 07 Nov 2011 to 130.158.131.239. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions