Introduction
4H-SiC metal-oxide-semiconductor field-effect transistors (4H-SiC MOSFETs) are promising key devices for future power electronics.  Their on-state performance can be significantly improved by using C-face [4H-SiC(0001) face] MOS interfaces, because they exhibit several times higher field-effect mobility ( μfe ) (60–100 cm²/V s) than the standard Si-face MOSFETs (20–40 cm 2 /V s).  However, C-face MOS interfaces sometimes increase the instability of the threshold voltages ( V h ) of MOSFETs [1-3], which may be one of the hardest barriers for the commercialization of C-face MOSFETs.  To understand the microscopic mechanism of the V th instability in C-face MOSFETs, we are investigating their MOS interfaces by means of electrically-detected-magnetic-resonance (EDMR) spectroscopy, which enables an electrical detection of electron spin resonance (ESR) of defects in semiconductor devices.  In earlier studies, we have reported that the C-face MOS interfaces have intrinsic defects, which we named "C-face defects" [3-5].  In this study, we examined the relationship between the C-face defects and the V th instability in C-face MOSFETs.
Vth instability in C-face 4H-SiC MOSFETs
We prepared two types of lateral n -channel C-face 4H-SiC MOSFETs.   Their gate length and width are 100 μm and 150 μm, respectively.  A 50-nm-thick gate oxide was grown by wet oxidation at 1000 °C, and was subjected to H 2 POA at 1100°C for 30 min.    The oxidation processes as well as epitaxial growth and ion-implantation processes were the same for the two types of MOSFETs [3], however, we found a significant difference in their V th instability.  Accordingly, hereafter we call them "bad type" MOSFET and "good-type" MOSFETs.  Fig. 1 shows an example of drain-current ( I d ) versus gate-voltage ( V a ) curves of the two types of MOSFETs.   In "bad-type" MOSFETs [Fig. 1(a)], Id-Vg curves were horizontally shifted toward the negative direction (i.e., a negative V th shift occurred) after applying negative V g stress or, at times, even without any stress.  The field-effect mobility of "bad-type" MOSFETs was estimated to be 55–70 cm²/V -s.  In contrast, "good-type" MOSFETs [Fig. 1(b)] did not show such large negative V th shifts, and their V th always remained positive values.   Furthermore, their field-effect mobility was increased to 85–100 cm²/V·s. We suspected an influence of gate electrode, because the "bad-type" MOSFET shown in Fig. 1(a) had an Al gate electrode, while a poly-Si electrode was used for the MOSFET shown in Fig. 1(b).   However, we obtained "good-type" MOSFETs with Al electrode. At present, there are no clear differences between the "good-type" and "bad-type" MOSFETs in terms of their fabrication processes.  It is found that the "good-type" MOSFETs are the major type, and the ‘‘bad-type’’ MOSFETs are the rare type. Using the “bad-type” MOSFETs, we studied the negative V th shifts as a function of stress time, stress temperature, and an irradiation dose of gamma ray (γ ray), which are summarized in Fig. 2. In these experiments, we applied a negative V g stress ( V stress ) to the test devices. The negative Vth 100 shifts showed a saturation behavior with a time Bad type constant of 10² sec (330, 370, and 560 sec for 80 Vstress = -10, -20, and -30 V, respectively) at o They were room temperature [Fig. 2(a)]. 60 + thermally activated, and they were no longer observed at 200 K and lower [Fig. 2(b)]. They 40 could be enhanced by γ-ray irradiation [Fig. Before irradiation 2(c)].  In the "good-type" MOSFETs, no large After irradiation 20 V th shifts were observed even after γ-ray irradiation [square symbols in Fig. 2(c)].  As 0 20 30 0 10 40 seen in Fig. 2(b), Id-Vg curves were simply horizontally shifted during the Vg stress, indicating that the channel mobility remained unchanged after the Vg stress.  Actually, after Fig. 3.    Field-effect mobility ( μfe ) estimated γ-ray irradiations, we found almost the same μ FE in the γ-ray irradiation experiments.  Values as observed before the irradiation (Fig. 3). In Fig. 2(c), the irradiation data may be classified into two groups such as 0 to 4 Mrad and 8 to 40 Mrad.   In the latter group, the Vth shifts seemed to be suppressed as opposed to those in the former group.  We speculate that the reason for this may be a recovery process with a long time constant, cancelling the V th shifts during a long-time irradiation (it took several weeks). Similar recovery phenomena were often observed in irradiation experiments on other semiconductor devices [6].
EDMR studies on C-face MOSFETs
Fig. 4(a) compares EDMR spectra of "bad-type" and "good-type" MOSFETs before γ-irradiation. The observed EDMR signals coincide with those of "C-face defects" [3-5].   The C-face defects were observed only in C-face MOSFETs [3-5], and we here focus on the relationship between their EDMR signals and the V th instability.   Obviously, the much smaller EDMR signal in "good-type" MOSFETs corresponds to the much smaller V th instability in this type of device [Fig. 2(c)]. Furthermore, the EDMR signal was drastically enhanced by the γ-irradiation in "bad-type" MOSFETs [1000 ppm → 2600 ppm after 4-Mrad irradiation, see Fig. 4(b)], as similarly to the case of the V th shifts.  The γ -irradiation is known to cause dissociation of hydrogen atoms from hydrogen-passivated defects [7].    Therefore, we concluded that the C-face defects were passivated by hydrogen atoms. In Fig. 4(b), we plotted the negative V th shifts as a function of the maximum EDMR intensities at the optimized bias conditions for three samples: an unirradiated and 4-Mrad-irradiated "bad-type" MOSFET and a 4-Mrad-irradiated "good-type" MOSFET.  The 4-Mrad irradiation caused the largest change in the V th shifts [Fig. 2(c)].    The linear line indicates a good correlation between the negative V th shifts and the EDMR intensities of C-face defects. The negative V th shifts can be caused by the generation of positive fixed charges in the oxide layer.  A typical candidate for the positive fixed charges is the famous E' center and their variants in SiO 2 [7], and some of them can be generated by the γ-irradiation [7].    Thus, such oxide defects are one of the possible candidates for the negative V th shifts observed in Fig. 2.    On the other hands, the C-face defects are interface defects, because they revealed anisotropic g factors in magnetic-field-rotation experiments with respect to the 4H-SiC crystalline substrate [5]. The result of Fig. 4(b) suggests that the interface defects also related with the formation of the fixed charges in the oxide layer.
Summary
We have studied relationships between the V th instability and the EDMR signals of C-face defects in C-face 4H-SiC MOSFETs.   The negative V th shifts correlated with the increase in the EDMR signals of C-face defects as well as with the γ-ray irradiation.