APPLIED PHYSICS LETTERS 99, 142105 (2011)

Behavior of nitrogen atoms in SiC-SiO2 interfaces studied by electricallydetected magnetic resonance
T. Umeda,1,a) K. Esaki,1 R. Kosugi,2 K. Fukuda,2 T. Ohshima,3 N. Morishita,3 and J. Isoya4
1

Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan
3
Japan Atomic Energy Agency (JAEA), Takasaki 370-1292, Japan
4
Graduate School of Library, Information, and Media Studies, University of Tsukuba, Tsukuba 305-8550, Japan
2(Received 8 June 2011; accepted 6 September 2011; published online 4 October 2011)
The microscopic behavior of nitrogen atoms in the SiO2-SiC interface regions of n-channel lateral
4 H-SiC metal-oxide-semiconductor field effect transistors (MOSFETs) was studied using
low-temperature electrically detected magnetic resonance spectroscopy and other techniques. The
results show that nitrogen atoms eliminated shallow interface states observable at 20 K and further
diffused into the channel region of the MOSFETs as shallow donors. These two behaviors enable
C 2011 American Institute ofnitrogen atoms to change the channel conductivity of SiC MOSFETs. V
Physics. [doi:10.1063/1.3644156]
Nitrogen and hydrogen are well known as useful elements for controlling SiC-SiO2 interfaces.1 Determining how
to make better use of these atoms is crucial for SiC device
technologies, especially in terms of improving the channel
mobility of SiC metal-oxide-semiconductor (MOS) field
effect transistors (MOSFETs).2,3 Similarly to the wellstudied Si-SiO2 interfaces,4 dangling-bond (DB) defects
were found at SiC-SiO2 interfaces (on carbon atoms)(Ref. 5) and their reaction with hydrogen atoms wasobserved in oxidized porous-SiC.6 Also, it was demonstrated
that nitrogen atoms eliminated the g (g value) ¼ 2.003 center7 originating from silicon-vacancy defects in the interface
region.7–9 Such elimination is the key to modifying the
MOSFETs performance.9 However, microscopic behaviors
of nitrogen at the interface may be more complicated. For
example, the incorporation of nitrogen into the interface
affects on not only the channel mobility, but also the threshold voltage (Vth).10 Furthermore, the low channel mobility
may originate from both the high interface-state density andother scattering mechanisms.11 We wonder if there might bestill unknown behaviors of nitrogen at the interface.
In this letter, we applied low-temperature electricallydetected magnetic resonance (EDMR) spectroscopy to SiCSiO2 interfaces in fully processed 4H-SiC MOSFETs. The
EDMR technique has been already applied to this interface
system in real MOSFET devices and has succeeded to detect
electron spin resonance (ESR) signals in the interface
region.7–9 These studies were demonstrated at room temperature (RT). On the contrary, our EDMR studies were performed at low temperatures (down to 4 K). Since many ESR
signals, related to shallow energy levels or associated withthermal activation process, become observable at low temperatures, our observation expectedly detects shallow interface states that are located close to the conduction band edge
(EC). In fact, we found that nitrogen atoms that have incorporated into the interface can remove some of the shallow
interface defects that interact with the channel current andeven diffuse into the channel region as shallow donors.
a)Electronic mail: umeda@bk.tsukuba.ac.jp.

0003-6951/2011/99(14)/142105/3/$30.00

These donors have an important role for changing the channel conductivity of SiC MOSFETs.
The samples we studied were n-channel lateral 4 H-SiC
MOSFETs [gate length (L): 100 lm, gate width (W):
150 lm] fabricated on epitaxial layers of Cree 8 -off
4 H-SiC(0001) Si-face wafers. Aluminum implantation (Alconcentration  1  1020 cm3) was carried out over thechannel region of the n-type epitaxial layer (residual nitrogenconcentration  5  1015 cm3). Source/drain regions wereformed by high-dose phosphorus implantation (P concentration  1  1020 cm3) and connected to nickel ohmic contacts. Gate oxides (60 nm in thickness) were grown by dry
oxidation and were subjected to post-nitridation annealing(NO-N2O at 1250  C). Gate electrodes were formed by polySi. Finally, two 4 H-SiC MOSFET samples, named“Nitrogen” and “Dry” (dry oxidation only, oxide
thickness ¼ 50 nm), were prepared for the EDMR measurements. Their field-effect channel mobility (lFE) is evaluatedin Fig. 1(a).
Figure 2 shows a summary of EDMR measurements for
our MOSFET samples. EDMR spectra were measured using
lateral channel currents (Ids) between the source and drain ofeach MOSFET in the dark. The observed EDMR centersshould be interacting with the channel currents. The currents
Ids were 1–100 nA under drain-source biases (Vds) of 1–5 V
and gate-substrate biases (Vgs) of 1–30 V at 4–295 K [seeFig. 1(b)]. The Vgs over 30 V was not available because oflarge gate leakage currents. To excite ESR transition, weused a Bruker Bio-Spin TE011 microwave cavity and a
9.4-GHz microwave at a power of 32–200 mW. Then, we
measured ESR-induced changes in Ids (expressed in ppm)
using a lock-in amplifier which synchronized with magneticfield modulation at 1.56 kHz.
As shown in Figs. 2(a) and 2(b), RT EDMR signals suchas interface signals of the silicon-vacancy and the danglingbond centers7–9 were not observed in our MOSFETs. In
Fig. 1(c), EDMR signal intensities (peak-to-peak intensities inthe EDMR spectra) are plotted for each experiment. At 295 K,
any defect signals were below the noise level. As decreasing
the temperature, we could detect a low-temperature EDMR

99, 142105-1

C 2011 American Institute of Physics
V

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FIG. 1. (Color online) Electrical properties and EDMR signal intensities of
Nitrogen and Dry 4 H-SiC MOSFET samples. (a) Field-effect mobility (leftaxis) of MOSFETs derived from room-temperature Ids-Vgs curves, and simulation (right axis) for carrier concentrations (Qch) of the channel, whereCox ¼ 7.0  108 F/cm2 and Vth ¼ 1.8 V for the Dry sample andCox ¼ 6.9  108 F/cm2 and Vth ¼ 0.2 V for the Nitrogen sample. These Cox
values were determined by MOS capacitors on the same wafer as the MOSFET samples. (b) Ids-Vgs curves measured for Vds ¼ 0.1 V at 295 K and 1 V at
4 K. (c) EDMR signal intensities (peak-to-peak heights in EDMR spectra) of
the Nitrogen (circles) and Dry (squares) samples. “þ” symbols represent noiselevels of each measurement at which no EDMR signals were detected.

signal at 20 K in the Dry sample [Figs. 1(c)]. This signal was
similar to the PH0 signal,12 but its identification is not decisive.
In addition, a broad “unidentified” signal seemed to appear in
the Dry sample. This signal may involve a powder patternowing to randomly orientated defects. In the Nitrogen sample,
these signals were not found [Figs. 1(c), 2(d) and 2(g)]. As
similarly to the earlier EDMR results at RT,7–9 nitrogen atoms
also annihilated low-temperature EDMR signals. This behavior
is an important role of nitrogen, as already discussed in the earlier work.9 Furthermore, even after c-ray irradiation [Fig. 2(e)],the Nitrogen samples did not show any EDMR signals. Thisbehavior is in contrast to the case of hydrogen, in which hydrogen removal was observed by c-ray irradiation.12 Therefore,
nitrogen atoms formed stable binding states at the interface.
We found another significant behavior of nitrogen in theinterface region. In the Nitrogen sample, a very strong EDMR
signal was observed at 4 K [Figs. 2(f) and 2(h)], which welabel “Nh.” The angular dependence of the Nh signal, shownin Fig. 3, reveals a c-axial (C3v) symmetry with gjj ¼ 2.0047
and g\ ¼ 2.0008, which is similar to the case of nitrogenshallow donors at the carbon h site of 4 H-SiC [NC(h); C3v
symmetry, gjj ¼ 2.0055, and g\ ¼ 2.0010].13 Although triplet
hyperfine splitting (hfs) of 14N nucleus (I ¼ 1, naturalabundance ¼ 100%) was not resolved, this behavior is frequently seen for NC(h), because of small 14N hfs (0.2 mT,13
see Fig. 3). Judging from the signal width of Nh (0.4–0.5 mT,
slightly angular-dependent, cf. Fig. 3), 14N hfs is at least
much smaller than 0.4 mT for the Nh center. This range isAppl. Phys. Lett. 99, 142105 (2011)

one-tenth smaller than the case of k-site shallow nitrogen
donors, NC(k) (3.6 mT,13 see Fig. 3), implying that both theNC(h) and Nh centers are associated with shallower donor
states. Also note that observable temperature (<20 K) are inagreement between NC(h) and Nh. Therefore, we concluded
that the Nh signal originates from the h-site nitrogen shallow
donors in the channel region. The Nh signal exhibited astrong lifetime broadening and disappeared above 20 K [cf.
Fig. 2(h)]. Thermal activation energy for this lifetime broadening (DE), which was estimated using a signal-width analysis,14 was 3 meV for the Nh center [Fig. 2(i)]. This value isabout half of the reported value (7.6 meV) for NC(h). Thus,
shallow donors appear to be quantitatively different in thechannel regions and bulk regions. Such a difference is also
seen in their g values. Interestingly, we could not observe
k-site nitrogen donors in the channel currents, despite their
ESR signal being more easily detectable over a wide temperature range (80 K). The reason for this is unclear but may berelated to their deeper energy level (EC  0.09 eV) comparedto that of NC(h) (EC  0.05 eV).
The above result indicates that nitrogen atoms can diffuse
not only into the oxide layer, but also into the SiC substrate after post-nitridation annealing. In fact, the Nitrogen sample
showed an enhanced channel current at low temperatures, as
shown in Fig. 1(b). At 4 K, it could activate a channel current
at Vgs  21 V, while the Dry sample could not at the same
temperature. This enhanced channel current easily accounts
for the “nitrogen doping” in the channel region. Moreover, theNh signal intensity was so intense [1000 ppm, see Fig. 1(c)]as compared to the defect signals at 20 K in the Dry sample(10 ppm), which suggests a substantial level of nitrogen doping. Actually, we have measured the “doped” nitrogen atoms
by means of x-ray photoemission spectroscopy (XPS), andalthough those results will be presented in our separate paper,15 we would like to reiterate a key point here. In this study,
we prepared “Nitrogen” and “Dry” wafer samples similarly tothe 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 both the Nitrogen and Dry wafer samples, oxygen
peaks due to the oxide layer were eliminated after the etching.
However, in the Nitrogen wafer sample, nitrogen peaks werestill clearly detectable, in addition to silicon and carbon peaksof the substrate. On the basis of this observation, we estimatedthe nitrogen concentration in the substrate to be in the order of1014 cm2.15 These nitrogen atoms partly changed into shallow donors by high-temperature annealing during the nitridation process, finally appearing as the Nh center.
We evaluated the effect of such high-density nitrogen
donors in the channel region on the electrical properties of
MOSFETs. In principle, nitrogen donors should increase thecarrier concentration (Qch) of the channel and simultaneously decrease Vth of a MOSFET via the compensation of
acceptor concentration (NA) in the channel (cf. Vth ! NA1/2(Ref. 16)). In fact, a decrease was observed from 1.8 V to
0.2 V [Fig. 1(b)]. Such a Vth drop is a common phenomenon
for the case of nitrogen,10 and our findings gives a reason forthis phenomenon.
The nitrogen donors also influence on the channel conductivity, which depends on both Qch and lFE. In general, lFE

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142105-3

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Appl. Phys. Lett. 99, 142105 (2011)

FIG. 2. (Color online) EDMR spectra measured forthe channel currents of Nitrogen and Dry 4 H-SiC
MOSFET samples. (a) Dry sample for Ids ¼ 12 nA for
Vds ¼ 2 V, Vgs ¼ 2 V, and ma ¼ 1 mT, (b) Nitrogensample for Ids ¼ 41 nA for Vds ¼ 2 V, Vgs ¼ 0.3 V, andma ¼ 2 mT, (c) and (g) Dry sample for Ids ¼ 17–23 nA,
Vds ¼ 2 V, Vgs ¼ 11 V, and ma ¼ 1–2 mT, (d) and (g)Nitrogen sample for Ids ¼ 51 nA for Vds ¼ 2 V,
Vgs ¼ 10 V, and ma ¼ 1.9 mT, (e) Nitrogen sample after
c-ray irradiation (average energy ¼ 1.3 MeV,
dose ¼ 2.3 Mrad) for Ids ¼ 49 nA for Vds ¼ 2 V,
Vgs ¼ 10 V, and ma ¼ 2 mT, (f) and (h) Nitrogen sample for Ids ¼ 8–28 nA, Vds ¼ 2 V, Vgs ¼ 25–27 V, andma ¼ 0.14–0.25 mT, where ma is a magnetic-field
modulation amplitude, and all spectra were measured
for B (magnetic field) jj [0001]. (i) Lifetime broadening behavior of the Nh signal, where T2 is a spin relaxation time estimated from a lifetime broadening
analysis (Ref. 14). Activation energy DE generally corresponds to a valley-orbit splitting of shallow donor[DE ¼ 7.6 meV for NC(h) and 45.5 meV for NC(k)](Ref. 13).

is determined from the slope of a Ids-Vgs curve via an equation,
dIds/dVgs ¼ lFE  Cox  Vds/(L/W),17 in which Cox is an oxide
capacitance. Note that the slope dIds/dVgs is increased whenlFE or Cox or Qch increases, since Cox is linked with Qch via
Qch ¼ Cox  (VgsVth). Figure 1(a) (right axis) shows thesimulated Qch in the inversion layer of the MOSFETs using
the measured Cox and Vth values. As shown in this simulation,
Qch (1012 cm2) is much smaller than free carriers(1014 cm2) emitted from the “doped” nitrogen donors.
Therefore, it is quite possible that Qch is easily enhanced by afactor of tens due to the nitrogen doping, or alternatively, thelFE is overestimated by the same factor. Furthermore, theincreased Qch may increase the “Coulomb” mobility.11 These
provide an explanation as to how the channel conductivity
was improved by nitrogen. Actually, the enhancement by afactor of 7–64 was observed for Vgs  1.8 V in the lFE of our
Nitrogen and Dry samples [Fig. 1(a)], which is within the
expected range of our model. On the contrary, the doped nitro-

FIG. 3. (Color online) Angular maps of the Nh center (circles) and two
nitrogen shallow donors, NC(h) (solid lines) and NC(k) (dashed lines). Error
bars of Nh represent its peak-to-peak signal widths. Two angular maps were
measured with respect to the B rotation in the (1120) and (1100) planes,
where 0 angle represents that B jj [0001].

gen donors may act as a source of the surface roughness scattering11 via their surface potential fluctuation. Accordingly,lFE may balance with the opposite mechanisms of the nitrogen donors.
In summary, we studied the microscopic behavior of
nitrogen in the interface region of 4 H-SiC MOSFETs. Lowtemperature EDMR measurements revealed that the nitrogenatoms have two primary behaviors: (1) the removal of shallow interface states detectable at 20 K and (2) the nitrogen
doping to the channel region. Due to these behaviors, thechannel conductivity of SiC MOSFETs can be increased after post-nitridation annealing. The post-nitridation techniquefor SiC MOSFETs must be optimized to make better use of
these two behaviors of nitrogen.
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