Fast states at SiO 2 /SiC interfaces annealed in NO at 1150-1350 C have been investigated. The response frequency of the interface states was measured by the conductance method with a maximum frequency of 100 MHz. The interface state density was evaluated based on the difference between quasi-static and theoretical capacitances (CÀw S method). Very fast states, which are not observed in as-oxidized samples, were generated by NO annealing, while states existing at an as-oxidized interface decreased by approximately 90%. The response frequency of the very fast states was higher than 1 MHz and increased when the energy level approaches the conduction band edge. For example, the response frequency (time) was 100 MHz (5 ns) at E C ÀE T ¼ 0.4 eV and room temperature. The SiO 2 /SiC interface annealed in NO at 1250 C showed the lowest interface state density, and NO annealing at a temperature higher than 1250 C is not effective because of the increase in the very fast states. V

We propose a method to accurately determine the surface potential (w S) based on depletion capacitance, and the interface state density (D IT) was evaluated based on the difference between quasi-static and theoretical capacitances in SiC metal-oxide-semiconductor capacitors (CÀw S method). We determined that this method gives accurate values for w S and D IT. From the frequency dependence of the capacitance measured at up to 100 MHz, a significant fast-interface-state response exists at 1 MHz, which results in the overestimation of w S if it is determined based on the flatband capacitance at 1 MHz. The overestimation of w S directly affects the accuracy of the energy level. D IT at a specific energy level is underestimated by the overestimation of w S. Furthermore, the fast interface states that respond at 1 MHz cannot be detected by the conventional high(1 MHz)-low method. The CÀw S method can accurately determine the interface state density including the fast states without high-frequency measurements. V

We investigated the effects of the interface state density (DIT) at the interfaces between SiO2 and the Si-, C-, and a-faces of 4H-SiC in n-channel metal-oxide-semiconductor field-effect transistors that were subjected to dry/nitridation and pyrogenic/hydrotreatment processes. The interface state density over a very shallow range from the conduction band edge (0.00 eV < EC − ET) was evaluated on the basis of the subthreshold slope deterioration at low temperatures (11 K < T). The interface state density continued to increase toward EC, and DIT at EC was significantly higher than the value at the conventionally evaluated energies (EC − ET = 0.1–0.3 eV). The peak field-effect mobility at 300 K was clearly inversely proportional to DIT at 0.00 eV, regardless of the crystal faces and the oxidation/annealing processes.

We have investigated the unique interface states (NI) generated by NO annealing at the SiO2/SiC interfaces by low-temperature conductance measurements, which is more suitable for characterization of very fast interface states than high-frequency conductance measurements at room temperature. Although only a part of the NI states can be evaluated by measurements at room temperature, the whole picture of the NI states, especially near the conduction band edge (0.07 eV ≤ EC−ET), has been revealed by the low temperature measurements. The NI peak was present at the interface even without NO annealing. The NI density increased with NO annealing temperature. The NI density at the energy levels shallower than 0.2 eV exceeded 1012 cm−2eV−1 after NO annealing. The capture cross section of the NI states is uniquely larger than that of conventional interface states.

Using electrically-detected-magnetic-resonance spectroscopy and a device simulation, we studied dominant interface defects, named "C-face defects," in C-face 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). The C-face defects act as hole traps via their donor levels, when they are not passivated by hydrogen atoms. The densities of unpassivated C-face defects were estimated to be from 4×10 12 cm -2 to 13×10 12 cm -2 in various C-face MOSFETs, which correlated with negative thresholdvoltage (V th ) shifts. We explained influences of the C-face defects on the V th instability and the channel mobility of C-face MOSFETs. C-face 4H-SiC MOSFETsSilicon carbide metal-oxide-semiconductor field-effect transistors (SiC-MOSFETs) are promising for low-energy-loss and high-power-density power transistors. SiC-MOSFETs are usually fabricated using a 4H-SiC(0001) ("Si face") surface to ensure a stability in their threshold voltages (V th ) (1,2). In contrast, a 4H-SiC(0001 _ ) ("C face") surface exhibits a wider range of the V th instability (2,3), although C-face MOSFETs can achieve a much higher field-effect mobility (µ FE = 60-100 cm 2 V -1 s -1 ) than the standard Si-face MOSFETs (µ FE = 20-30 cm 2 V -1 s -1 ) (1-4). Figure 1 shows a typical example of the V th instability observed in C-face MOSFETs. In MOSFET (a) which we call "bad type," the drain-current (I d ) versus gate-voltage (V g ) curve is horizontally shifted toward the negative direction after applying a negative V g stress (-30 V) for 1800 sec. On the contrary, in MOSFET (b) or "good type," the negative V th shift is drastically reduced even against a much longer stress time (20480 sec). However, the microscopic origin of such V th instability as well as microscopic differences between C-face and Si-face MOS interfaces is still unclear.In this paper, we present an electrically-detected-magnetic-resonance (EDMR) and device-simulation study on C-face MOSFETs. EDMR spectroscopy detected a large number of hole traps at the C-face MOS interface, which we have named the "C-face defects" (2,5). They were only observed on oxidized C face, and deeply connect with the characteristic properties of C-face MOSFETs. For instance, a primary difference in the samples shown in Figs. 1(a) and (b) was found in the densities of the C-face defects. We also found that their densities correlated with the negative V th shifts in C-face MOSFETs. Accordingly, we propose a microscopic model for the V th instability that the C-face