C-face interface defects in 4H-SiC MOSFETs studied byelectrically detected magnetic resonance
T. Umeda1,a *, M. Okamoto2,b, R. Arai1, Y. Satoh1, R. Kosugi2,c, S. Harada2,d,H. Okumura2,e, T. Makino3,f, and T. Ohshima3,g1
2

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

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568,
Japan
3aJapan Atomic Energy Agency (JAEA), Takasaki 370-1292, Japanbcumeda@bk.tsukuba.ac.jp, mitsuo-okamoto@aist.go.jp, r-kosugi@aist.go.jp,
ds-harada@aist.go.jp, eh-okumura@aist.go.jp, ftakafumi.makino@jaea.go.jp,
gohsima.takeshi28@jaea.go.jp

Keywords: 4H-SiC MOSFET, C face interface, EDMR, interface defect, threshold voltage, deep
interface-state density.

Abstract. This
paper reports an EDMR (electrically detected magnetic resonance) observation on_4H-SiC(0001) “C face” MOSFETs. We found a new strong EDMR signal in wet-oxidized C-face
4H-SiC MOSFETs, which originates from intrinsic interface defects on C-face SiC-SiO2 structures.

SiC-MOS interfaces: “C face” versus “Si face”
The capability of metal-oxide-semiconductor (MOS) structures is a great advantage of SiC ascompared with other wide-gap semiconductors. Especially, 4H-SiC MOSFETs (MOS Field EffectTransistors) will have a wide range of impacts and applications in power electronics. The SiC-SiO2interface has structural similarities to the famous Si-SiO2 interface; namely, they form abrupt
interfaces consisting of Si-O bonds. However, electronic properties of the two MOS interfaces arequite different. The interface-state density (Dit) is typically much higher on 4H-SiC (1013 ~ 1011
cm-2eV-1) than on Si (1011 ~ 1010 cm-2eV-1). These high-density interface states are believed to linkwith a serious degradation of the channel mobility (field-effect mobility, μFE) and an instability of thethreshold voltage (Vth) of MOSFETs. Therefore, the identification of their microscopic origins is a big
issue for SiC MOSFETs.
Regarding with this issue, we should remember that μFE and Vth are strongly
dependent on wafer_orientations. Figure 1 shows current-voltage characteristics of 4H-SiC(0001) “C face” MOSFETs and4H-SiC(0001) “Si face” MOSFETs, revealing their quite different behaviors. The Si-face MOSFETs
exhibit low μFE [1~20 cm2/Vs, see Fig. 1(c)] in both as-oxidized (dry oxidation) and improved(nitrided oxide) samples. On the contrary, the two C-face MOSFETs show a more dramatic change.
The dry-oxide MOSFET on C face could not activate any channel currents, while the wet-oxide
MOSFET was drastically improved and achieved a very high μFE up to ~ 100 cm2/Vs. In addition tothe wet oxidation [1], H2 annealing processes are also effective on C face, indicating that hydrogen isthe key atom to change this interface [1].
Such a strong sensitivity to hydrogen is similar to the case of silicon. In Si-MOS structures,hydrogen atoms passivate the Pb centers (interfacial Si dangling bonds) [2], and their de-passivation atelevated temperatures and under a bias stress causes the Vth shift, e.g., known as the “NBTI”(negative-bias temperature instability) problem [2]. In SiC, the Vth shift after a negative bias stress islarger on C face than on Si face, as is shown in Fig. 1. By analogy to the case of silicon, hydrogenmight be related to the larger Vth shift on C face.

Fig. 1. Current-voltage characteristics of (a) our n-channel lateral C-face 4H-SiC MOSFETs and (b)Si-face 4H-SiC MOSFETs, where Id is a drain current, Vg is a gate voltage. Drain voltage (Vd) was set
to 0.1V, and source and well voltages were kept to 0V. Solid and dashed curves were measured before
and after a negative Vg stress (-15V, duration = 400 s), respectively. (c) Field-effect mobility (µFE) for
the three MOSFETs shown in (a) and (b). Oxide thicknesses are 50~60 nm for “Dry” (dry oxidation),“Wet” (wet oxidation), and “Nitrided” (dry oxidation + nitridation anneal at 1250 ºC). Gate size(length/width) was 100/150 μm.
EDMR observation on “C-face” MOS interfaces
To reveal microscopic information about the interface states on C face and their interaction withhydrogen, and their correlation with the Vth shift, we examined C-face 4H-SiC MOSFETs by meansof electrically detected magnetic resonance (EDMR) spectroscopy. Figure 2(a) shows our EDMR
setup, which is based on an electron spin resonance (ESR) spectrometer and an ultra-high-sensitivity
current meter. EDMR has already succeeded to reveal several types of interface defects in Si-face
MOSFETs [3-7]. Actually, we have observed the PH0 center at Si-face MOS interfaces whichcorresponds to shallow interface states on Si face [3-5]. In those measurements, we monitored thechannel current under a positive gate bias exceeding Vth [see Fig. 2(b)]. As is shown in the figure, thiscondition focuses on shallow interface states near the conduction band edge (EC), because the Fermilevel (EF) at the MOS interface is moved to EC, and singly-occupied states, which respond to ESR andEDMR, should be generated at shallow levels near EF. On the other hand, EDMR measurements inthis study were carried out under an opposite condition [Fig. 2(c)]. We examined deep interface states
under a negative gate bias, in which they can transform from doubly-occupied states tosingly-occupied states by lowering EF at the interface. Since the Vth shift is generally associated withrelatively deep electronic states, we tried to detect such states. It should be noted that the existence ofhigh-density deep interface states on C face was strongly suggested by previous capacitance-voltage
analyses [1].
To activate the current under a negative Vg, we used a forward current flowing between drain andwell regions. A part of this current can interact with the interface states. We found that relatively small
currents (~10 nA) were suitable for detecting the interface signals. For EDMR measurements, weprepared C-face 4H-SiC MOSFETs on an optimum 4º-off p-type epi-layer with wet oxidation (52-nmthick) and post H2 anneal (800 ºC). They showed a high channel mobility as shown in Fig. 1(c),ensuring a good quality of our C-face MOS interfaces.

(a) EDMR setup(b)magnetic field& 1.5kHz field Vg
modulation
VdGaten   Nshallow Dit(ESR active)

μ-waveEFS
CB junction current(c)~Acurrent change @1.5kHzVg > Vthdeep Dit(doubly occupied)ncurrent

CB channel current

deep Dit(ESR active)

EFVg < 0Fig. 2. (a) EDMR setup in this work. Microwave was 9.4 GHz at 200 mW. (b) Energy diagram at theMOS interface under flowing the channel current. This setup was used in previous low-temperature
EDMR studies [3-5]. In this mode, shallow interface states will become ESR-active singly-occupiedstates. (c) Energy diagram at the MOS interface in this work. For activating deep interface states, anegative gate bias (Vg) was applied.
Figure 3 shows a series of EDMR spectra of our C-face 4H-SiC MOSFET measured at room
temperature. As is seen in the figure, we have successfully detected EDMR signals on C face. Thesesignals were not observed in Si-face 4H-SiC MOSFETs, indicating that they are intrinsic defects on Cface. The EDMR signals were strongly observed under a large negative Vg, which is consistent with
our expectation explained above. Thus, the observed EDMR signals should be related to relatively
deep interface states. After applying a large negative Vg stress, a negative Vth shift as well as anincrease in the EDMR signal intensity was simultaneously observed. It supports a correlation between
the Vth shift and the observed EDMR signals. However, unfortunately, exact energy positions of thedeep interface states are not determined at present.
The EDMR spectra shown in Fig. 3 seem to include at least three signal peaks: i.e., one central
signal and two satellite signals with an equal intensity. The splitting width of the satellite signals wasestimated to be 1.1 mT. The room-temperature EDMR signals in SiC MOSFETs have been reportedby Prof. Lenahan’s group so far [6,7]. Their major signal was the Si-vacancy signal which wasbasically identical to the ESR signal of VSi− (negatively-charged Si vacancy at k site) in bulk
4H-/6H-SiC [8]. The VSi− signal has a doublet hyperfine structure of 29Si (nuclear spin = 1/2, naturalabundance = 4.7%). Likewise, the present spectrum also included the doublet structure, however, itssplitting width (1.1 mT) is clearly different from 0.3 mT of the VSi− center. Alternatively, we suggestthat the doublet structure originates from a hyperfine splitting of 1H (nuclear spin = 1/2, naturalabundance = 99.9%). The reasons why we judged so are that (1) the central signal and the satellite
signals were independent from each other, judging from their dependences on Vg, microwave power,and temperature, and (2) the presence of hydrogen atoms is quite reasonable on C face. Further
identification of the observed EDMR signals is running at present.

Current change (a.u.)

EDMR spectra
292Kµ-wave 9.4GHzId=15nA-17V-19.5V-22V-24.5V

330

-34V
-27V

-30.5V335

340345Magnetic field (mT)Fig. 3. EDMR observation on C-face interface defects in “Wet” C-face 4H-SiC MOSFET measured
under different gate biases. Magnetic-field-modulation width was 0.25 mT.

Summary_The 4H-SiC(0001) “C face” MOSFETs exhibit quite different behaviors as compared to the4H-SiC(0001) “Si face” MOSFETs, indicating a crucial difference in their MOS interfaces. We
suggested that the deep interface states on C face play the key role on the notable features of this face,such as a strong sensitivity to the hydrogen incorporation, a high channel mobility after the hydrogenincorporation, and a larger Vth shift after negative gate-bias stress. We performed a spectroscopic
observation on such deep states using EDMR. Consequently, new EDMR signals of the deep states
were detected at room temperature under specific bias conditions.

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