Materials Science Forum Vols. 717-720 (2012) pp 427-432
Online available since 2012/May/14 at www.scientific.net© (2012) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.717-720.427

Electrically Detected Magnetic Resonance (EDMR) Studies of
SiC-SiO2 Interfaces
T. Umeda1,a,*, R. Kosugi2,b, K. Fukuda2,c, N. Morishita3,d,T. Oshima3,e, K. Esaki4, and J. Isoya4,g1

Institute of Applied Physics, University of Tsukuba, Tsukuba, 305-8573, Japan

2

National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8568, Japan
3

Japan Atomic Energy Agency, Takasaki, 370-1292, Japan
4

University of Tsukuba, Tsukuba 305-8550, Japanaumeda@bk.tsukuba.ac.jp, br-kosugi@aist.go.jp, ck-fukuda@aist.go.jp, dmorishita.norio@jaea.go.jp,eohshima.takeshi20@jaea.go.jp, gisoya.junichi.fw@u.tsukuba.ac.jp, *corresponding author

Keywords: EDMR, ESR, SiC-SiO2 interface, interface states, MOSFET, hydrogen, nitrogen,shallow donors, channel mobility.

Abstract. We discuss the results of electrically detected magnetic resonance (EDMR) spectroscopyon SiC-SiO2 interfaces interacting with hydrogen and nitrogen. Three types of Si-face 4H-SiC
metal-oxide-semiconductor field effect transistors (MOSFETs) were examined by EDMR in thetemperature range of 4–300 K. These samples revealed several different characteristics from those of
the earlier electron spin resonance (ESR) and EDMR studies on SiC-SiO2 interfaces. The mostsignificant finding was the high-density doping of nitrogen shallow donors into the channel region
after the post nitridation anneal. The incorporated nitrogen donors were observed as an “Nh” EDMR
signal at 4–20 K. The roles of these nitrogen donors are discussed in connection with the electricalproperties of SiC MOSFETs.
ESR and EDMR Studies on SiC-SiO2 Interface Systems
ESR spectroscopy on metal-oxide-semiconductor (MOS) systems was first demonstrated in 1971 [1].
It successfully detected three types of ESR centers, Pa, Pb, and Pc in Si-SiO2 interfaces [1], andcurrently, we are familiar with the Pb centers [Si dangling bonds (DBs)] which are dominant andintrinsic defects in this interface system. Many other ESR centers in this MOS system have since beenreported [2]. In contrast, ESR might not be so successful for studying SiC-SiO2 interfaces, despite
them being similar to the Si-SiO2 interface and having a much higher interface-state density. There isa clear difference between Si and SiC; ESR measurements on Si always require a special
pre-treatment of the sample surface, owing to undesirable ESR signals from the surface/interface
region. In contrast, such a pre-treatment is not necessary for SiC samples, even if their surfaces areoxidized.
Why doesn’t ESR work the same way for SiC-SiO2 as it does for Si-SiO2 interfaces? There is as
yet no answer, but it may be related to several differences between the two MOS systems.
The first notable difference is the wider band gap of SiC (e.g., 3.24 eV for 4H-SiC) [3]. In principle,
this wider band gap makes it possible to involve novel types of interface states in SiC MOS systems
that are different from the Pb-like centers in Si. The other notable difference is the presence of carbon
atoms in the SiC-SiO2 interface. Thus, carbon-related defects have been proposed as the origin of the
SiC MOS interface states [3-6]. For example, ESR studies revealed an amorphous carbon signal of g
(g value) ≈ 2.003 in some oxidized 3C/4H/6H-SiC wafers [3,4] (in particular, this signal was strong in
p-type 3C MOS samples). In addition, theoretical studies predicted a high density of carbon dimers (apair of threefold coordinated carbons) in the interface [5,6]. Although such carbon dimers have notbeen experimentally detected so far, similar defects (the HEI5/6 centers) have been identified byconducting ESR on bulk samples [7]. Therefore, ESR may have the potential for detecting the key
defects arising from carbon atoms.
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There are several important reports on carbon-related interface defects detected by ESR andelectrically detected ESR [we shall call this “electrically detected magnetic resonance (EDMR)”].
One famous study reported a carbon DB center (labeled “PbC”, their g values are g|| = 2.0023 and g⊥ =2.0032) in oxidized porous-SiC [8,9]. This is a really Pb-like center at the SiC-SiO2 interface, and it
exhibited a chemical reaction with hydrogen [9], similarly to the Pb center [2]. Unfortunately, thesame ESR signal has not been detected from single-crystal wafer samples, so the areal density of PbC
at the interface is not yet known. Accordingly, we cannot conclude if the PbC center accounts for the
major part of the high-density interface states.
Another type of carbon-related interface center was found in 4H/6H-SiC MOSFETs by taking
advantage of the EDMR technique [10-12]. This center had g = 2.0027 and the same signatures as thewell-studied silicon vacancy in bulk (the VSi– center [13]). Accordingly, the g = 2.0027 center has
been identified as a silicon-vacancy center in the interface [12]. Basically, a silicon vacancy has four
carbon DBs which generate several energy levels in the mid gap of SiC [13]. The important featuresof this center are that (1) it was present in some 4H/6H-SiC MOSFETs because it has been detectedby EDMR on these devices, (2) its signal intensity was extremely high, suggesting a highconcentration of this defect, and (3) it could be annihilated when the MOS interface was nitrided [12].
From these results, this center was proposed as a dominant interface defect [12]. Moreover, after thenitridation of the interface, another type of Pb-like defect was observed by EDMR [11,12]. This was
taken to be a sort of DB perpendicular to the interface, with new g values of g|| = 2.0026 and g⊥ =2.0010.
The above ESR/EDMR studies indicate that it is possible to correlate the origin of the SiC-SiO2
interface states with carbon-related interface defects. Furthermore, their reactions with hydrogen andnitrogen at the interface were also observed. We should, however, point out that these findings are
only the beginning, and it will be necessary to continue ESR and EDMR studies on a wider variety of
SiC-SiO2 interfaces. The reason is that we have obtained different EDMR results using different
samples.
Low-temperature EDMR Study
We employed the same EDMR technique [10-12] as used in SiC MOSFETs. However, contrary to
the earlier ESR/EDMR studies [4,8-12] that were done at room temperature, our EDMR
measurements were mainly done at low temperatures (down to 4 K). It is generally known that many
ESR centers, related to shallow energy levels or associated with thermal activation processes, become
detectable when the temperature goes down (e.g., carbon dimers in the bulk were clearly detectedunder such conditions [7]). As expected, our study revealed such low-temperature signals.
For the EDMR measurements, we prepared three different MOSFET samples. They are n-channel
lateral 4H-SiC MOSFETs (gate length = 100 µm, gate width = 150 µm) fabricated on n or p epitaxial
layers of Cree 8°-off 4H-SiC(0001) Si-face wafers (for the n epitaxial layer, [N] ≈ 5×1015 /cm3 and Al
implantation was performed with [Al] ≈ 1×1020 /cm3). Source/drain regions were formed byhigh-dose phosphorus implantation ([P] ≈ 1×1020 /cm3) and were connected to Ni contacts. Gate
oxides were grown by dry oxidation (dry SiO2 thickness = 50 or 60 nm) and were subjected topost-nitridation annealing (NO-N2O ambient at 1250°C) or post-hydrogen annealing (H2 ambient at
650°C). We used poly-Si gate electrodes. Finally, three 4H-SiC MOSFET samples, labeled“Nitrogen,” “Hydrogen,” and “Dry” (dry oxidation only), were prepared. Their field-effect channel
mobilities (µFE) varied from 2.5 cm2/V·s (the Dry sample) to 19.2 cm2/V·s (the Nitrogen sample).
Some of the MOSFETs were subjected to γ-ray irradiation (average energy = 1.2 MeV, dose = 2.3
Mrad) at room temperature.
The EDMR spectra were measured using lateral channel currents (Ids) between the source anddrain of each MOSFET in the dark. Only ESR centers interacting with the channel current could be
detected in this regime. In principle, the inversion layer of the channel region allows paramagnetic
states only in the vicinity of the conduction band edge (EC). Thus, ESR centers in such a shallow
region will be detectable. The currents Ids were 1–100 nA under drain-source biases (Vds) of 1–5 V

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and gate-substrate biases (Vgs) of 1–30 V at 4–300 K. We were not able to measure Vgs over 30 V
because of large gate leakage currents. At 4 K, only the Nitrogen samples could activate the channel
current, and only for these samples could we measure the 4 K EDMR spectra. The reason for thisexception will be explained later. To excite the ESR transition, we used a Bruker Bio-Spin TE011
microwave cavity and 9.4-GHz microwaves at a power of 32–200 mW. Small changes in Ids(expressed in ppm) due to ESR were monitored by a lock-in amplifier synchronized with amagnetic-field modulation at 1.56 kHz. The readers may consult our papers (Refs. [14] and [15]) for
more details on the experimental parameters.
In the following sections, we separately present our EDMR results for the Hydrogen sample andthe Nitrogen/Dry samples, from which we will discuss the behaviors of shallow interface defects,hydrogen, and nitrogen in the SiC-SiO2 interfaces.
Hydrogen-related Interface Defects

Current change (ppm)

EDMR results for the Hydrogen sample were already presented in Ref. [14]. Here, we shall review
the important points of those results. (1) In the as-prepared MOSFETs, no EDMR signals wereobserved at 20–300 K. We emphasize that no interface signals of PbC [8,9], silicon vacancy [10,12],or new DB center [11,12] were observed in our samples. (2) After the γ-ray irradiation, two EDMR
signals were detected at 20 K, labeled “PH0” and “PH1” (Fig. 1). The role of γ-ray irradiation is most
likely to remove the effect of hydrogen passivation in the Si MOS system [2]. (3) The PH1 signal
exhibited an isotropic doublet hyperfine splitting (hfs) of 5.4 mT, which is most probably due to ahydrogen nucleus (1H, nuclear spin I = 1/2, natural abundance = 99.99%) nearby this defect. In
summary, the observed defects should be related to hydrogen.
The PH0/PH1 signals disappeared in the temperature range between 20 and 50 K, most probably due
to the lifetime broadening. This feature was not seen in the previous interface signals [3,4,8-12]. The
g values of PH0/PH1 (g = 2.003–2.004, slightly anisotropic) were close to those of the silicon-vacancy
signal (g = 2.0027) [10,12,13] or amorphous carbon signal (g = 2.0030) [3,4]. The signal width was
found to be fairly anisotropic (peak-to-peak width = 1.3 mT and 0.8 mT for the [0001] and [11-20]
directions, respectively), and this indicated an anisotropic structure for the PH0/PH1 centers.
Our provisional assignment for the PH0/PH1 centers is that they are a family of carbon DBs
with/without a nearest hydrogen atom [14]. We imagined that in as-prepared samples, they werepassivated by hydrogen atoms and became visible after breaking C-H bonds by γ-ray irradiation. A
single 1H hfs (5.4 mT) of the PH1 center suggests that a passivated hydrogen is immediately captured
at the neighboring site of a carbon DB. In contrast, in the case of the PH0 center, a hydrogen atom may
diffuse far away from a defect site. These centers may be consistent with the predicted carbon-dimer
defects [5,6]. The carbon dimers hold DB-like orbitals on threefold coordinated carbons, which caninteract with hydrogen atoms [5].

30
20
10

4H-SiC MOSFET
channel current
in the dark
20 K, 8°PH1

PH05.4 mT

0
-10
-20
320

330

340

350

360

Magnetic field (mT)Fig. 1. PH0/PH1 EDMR signals in the Hydrogen sample [14]. These signals were observable at 20 K
and after γ-ray irradiation.

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However, we do not consider that the PH0/PH1 centers are dominant defects at the interface. Their
EDMR signal intensities were smaller than 30 ppm in Ids; they were too small, e.g., compared with thecase of the interface silicon-vacancy center (102–103 ppm) [12]. Furthermore, it should be mentionedthat some of the Hydrogen samples did not exhibit these signals even after γ-ray irradiation. Thus, it isdifficult to correlate the PH0/PH1 centers with the major part of the SiC-SiO2 interface states. Thisconclusion seems consistent with the general agreement about SiC-SiO2 interfaces such that thehydrogen annealing is often ineffective for improving their quality [3,5], and it is strikingly incontrast to the case of Si-SiO2 interfaces [2].
Nitrogen Shallow Donors in the Channel
We compared the Nitrogen and Dry samples fabricated with almost the same process. EDMR results
for these samples will appear in a separate paper in detail [15], and we pick up again notable points of
those results. (1) In both the Nitrogen and Dry samples, the room-temperature signals of the PbC
center, the interface silicon vacancy, and the new DB center [8-12] were below the detection limit.
Therefore, in some types of MOSFETs like ours, these interface defects may not be dominant. (2) On
the other hand, at 20 K, EDMR signals became detectable in the Dry sample [Figs. 2(a) and (c)]. One
signal was similar to the PH0 signal [Fig. 2(c)]. There might also be a very broad resonance in higher
magnetic fields [Fig. 2(a)]. If this broad resonance was a powder-pattern signal, its g values wereroughly estimated in the range between 1.0 and 1.4. Since this range was out of the normal range of
ESR centers in SiC, we can draw no conclusion about this signal at present. However, its signal
intensity would be 100 times larger than that of the PH0-like signal because of its large signal width.
Thus, this signal might be a candidate for the origin of the interface states. (3) The above EDMR
signals were absent in the Nitrogen sample [Figs. 2(b) and (c)]. This behavior is similar to thosereported for the room-temperature EDMR signals [10,12]. In summary, nitrogen atoms are effective
for removing some interface defects detectable at room temperature and at 20 K as well. (4) Even
after γ-ray irradiation, the Nitrogen samples did not show any EDMR signals at 20 K. This behavior
was in contrast to the case of hydrogen. Therefore, nitrogen atoms should form stable binding statesin the interface region. (5) At 4 K, a very strong sharp signal was observed in the Nitrogen samples(Fig. 3). This signal was unchanged after the γ-ray irradiation. We labeled it “Nh”.
The Nh signal was strongly anisotropic. The angular dependence of this signal showed a c-axial(C3v) symmetry with g|| = 2.0047 and g⊥ = 2.0008, which is similar to the case of nitrogen shallow
donors at the carbon h site of 4H-SiC (NC(h)) [16]. Table 1 summarizes the ESR signatures of the Nhcenter and two shallow donors, NC(h) and NC(k). Nh and NC(h) showed similar hyperfine splitting of
the 14N nucleus (I = 1, natural abundance = 100%) and observable temperatures. Therefore, we
concluded that the Nh signal originates from the h-site nitrogen shallow donors in the channel region.
The Nh signal exhibited strong lifetime broadening (Fig. 3) and eventually disappeared above 20 K.

Current change (ppm)

100
10
0
-10
-20

4H-SiC MOSFETs
channel currents
in the dark
20 K; B||c

80
60
40

332

20

(c)

Dry
Nitrogen
334

336

338

340

342

344

(a) Dry0-20-40

broad resonance?

PH0-like(b) Nitrogen ×10

-60
0

100

200

300

400

500

600

700

800

Magnetic field (mT)Fig. 2. EDMR signals of shallow interface defects in the Dry sample [15]. These signals became
observable at 20 K.

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Fig. 3. Nh EDMR signal in the Nitrogen sample, observable below 20 K [15].
Table 1. ESR parameters of nitrogen shallow donors in 4H-SiC MOSFET and bulk 4H-SiC.

The present results indicate that after post-nitridation annealing, nitrogen atoms can diffuse eveninto the channel region and act as shallow donors there. In fact, the Nitrogen sample showed anenhanced channel current at 4 K owing to this “nitrogen doping” effect, while the other MOSFET
samples could not activate the channel current at the same temperature. Moreover, the very intense
Nh signal (= 102–103 ppm) suggested a substantial level of nitrogen doping.
In fact, we could have measured the “doped” nitrogen atoms by means of x-ray photoemission
spectroscopy (XPS) [17]. For the XPS measurements, we prepared “Nitrogen” and “Dry” wafer
samples similarly to the corresponding Nitrogen and Dry MOSFET samples. Then, after completely
etching the oxide layer with a diluted hydrofluoric acid, we measured the XPS spectra of the two
wafer samples. In the Nitrogen and Dry wafer samples, the oxygen peaks due to the oxide layer werecompletely eliminated after the etching. However, in the Nitrogen wafer sample, nitrogen peaks werestill clearly detectable, in addition to silicon and carbon peaks of the substrate. On the basis of thisobservation, we estimated the nitrogen concentration in the substrate to be on the order of 1014 cm-2.
Some of the nitrogen atoms may change into shallow donors as a result of the high-temperatureannealing during the nitridation. The total nitrogen concentration measured by XPS increased linearlyas the nitridation temperature increased in the range between 800 and 1400 °C. Simultaneously, theinterface-state density was found to decrease from 2×1012 cm-2eV-1 to 5×1011 cm-2eV-1 at EC – 0.2 eV.
Effect of Nitrogen Donors on SiC MOSFET
We consider that nitrogen atoms play two significant roles in the Si-face SiC MOS system. One is toremove some interface defects, as evidenced in both the room-temperature EDMR studies [10-12]
and our low-temperature EDMR studies. Another is that they behave as a shallow donor in thechannel region. Below, we briefly discuss how the nitrogen shallow donors influence the electricalproperties of SiC MOSFETs.
First, nitrogen shallow donors increase the carrier concentration (Qch) of the channel region. This
increase may amount to up to 1014 cm-2 at maximum. Simultaneously, they cause a drop in thethreshold voltage (Vth) of the SiC MOSFET via compensation of the acceptor concentration (NA) inthe channel (c.f., Vth ∝ NA1/2). In fact, a drop in Vth has been observed in many nitride SiC MOSFETs[18], and in ours as well [15].

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Second, nitrogen shallow donors increase the “Coulomb mobility” [19] of electrons in the channel.
Since high-density free carriers suppress the Coulomb scattering mechanism [19], the channel
mobility (µFE) may be improved by the free carriers emitted from the “doped” nitrogen donors. In
addition, the increase in Qch causes a higher Ids current, which appears as a higher µFE value. Of course,
the reduction in the interface states caused by binding nitrogen atoms also contributes to increasingµFE.
In contrast, the high-density nitrogen donors increase the surface potential fluctuation, causing“surface roughness” scattering [19]. This mechanism degrades µFE. Consequently, the nitrogen
donors may have opposing effects on the channel conductivity, and hence, their presence may resultin an intermediate level of performance. This model offers one reason why the nitridation process
could not fully alleviate the channel mobility degradation of SiC MOSFETs.
Summary
We studied the SiC-SiO2 interface regions of three types (“Hydrogen”, “Nitrogen”, and “Dry”) of
4H-SiC MOSFETs by means of low-temperature EDMR measurements. The earlier ESR/EDMR
studies were done at room temperature, and our study revealed novel behaviors of this interface
system at low temperatures. We observed that hydrogen and nitrogen atoms at the interface could
passivate or annihilate some of the shallow interface defects. In the case of nitrogen, anothersignificant role was found: nitrogen doping of the channel region. We supposed that the high-density
nitrogen donors in the channel region greatly influence the channel conductivity of SiC MOSFETs.
We conducted that the post-nitridation technique for SiC MOSFETs must be optimized to makebetter use of the different behaviors of nitrogen atoms.
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Electrically Detected Magnetic Resonance (EDMR) Studies of SiC-SiO<sub>2</sub>Interfaces
10.4028/www.scientific.net/MSF.717-720.427