Silicon carbide metal-oxide-semiconductor field-effect transistors (SiC-MOSFETs) are promising for next-generation power transistors. In general, SiC-MOSFETs are fabricated using a 4H-SiC(0001) (“Si face”) surface to ensure the stability in their threshold voltages (V_th). In contrast, a 4H-SiC(000-1) (“C face”) surface exhibits a wider range of the V_th instability, although C-face MOSFETs can achieve a much higher field-effect mobility (μ = 60-100 cm²/V·s) than the standard Si-face MOSFETs (μ = 20-30 cm²/V·s) [1]. If the C-face MOSFETs overcome their V_th instability, they become excellent high-performance SiC-MOSFETs. Furthermore, since the C face resemble to other high-μ surfaces such as “m face” and “a face,” understanding of the C-face MOS interfaces will be useful for the other important interfaces.

The development of a number of C-face MOSFETs has revealed that there are opposite types of C-face MOSFETs with quite different V_th instabilities [2]. We classified them into “good type” and “bad type,” depending on their V_th instabilities. The “bad-type” MOSFETs commonly showed a large V_th shift; the drain-current (I_d) versus gate-voltage (V_g) curve was horizontally shifted toward the negative direction after applying a negative V_g stress (-30 V) for 10³ sec. On the contrary, in the “good-type” MOSFETs, the negative V_th shift is drastically reduced even against a much longer stress time (10⁴ sec). Curiously, the same epi-wafer or the same oxidation process caused both the types of C-face MOSFETs [2]. Therefore, from a view point of fabrication processes, it was difficult to understand the cause of the V_th instability. To clarify a microscopic mechanism of the V_th instability, we performed an electrically-detected-magnetic-resonance (EDMR) study on interface defects related to the V_thinstability of C-face MOSFETs. EDMR enables us to detect electron-spin-resonance (ESR) centers in small-sized electronic devices [1].

We prepared lateral n-channel C-face 4H-SiC MOSFETs on 4º-off 4H-SiC(000-1) epi-wafers. A 50-nm-thick gate oxide was grown by wet oxidation at 1000ºC and was subjected to H₂ POA at 1100ºC for 30 min. This process ensured a high μ over 60 cm²/V·s. Some of the MOSFETs were subjected to gamma-ray irradiation in order to de-passivate hydrogen-terminated interface states. After the irradiation, we observed an increase in the V_thshift [2]. The dose was set to 0.5 to 40 Mrad. EDMR measurements were carried out at room temperature with a 1.5-kHz magnetic-field modulation.

EDMR spectra of both the “bad-type” and “good-type” MOSFETs were dominated by the same defect, that we named “C-face defects” [1,2]. A primary difference among the two types was found in their signal intensities; i.e., the “bad-type” samples revealed much larger EDMR signal intensities than the “good-type” ones. The C-face defects were only detectable under a negative V_g, indicating that they have doubly-occupied levels (ESR-inactive states) in V_g ≥ 0V. Furthermore, we assigned these levels to be neutral donor levels, because they should be electrically-inactive in V_g ≥ 0V.

EDMR signal intensities increased with increasing a negative V_g, due to a conversion from doubly-occupied states (ESR-inactive, charge = 0) to singly-occupied states (ESR-active, charge = +1). Further increasing a negative V_g, the signal intensity reached a peak at a certain V_g (we define it as V_peak) and turned into a decrease. This behavior indicates that the formation of empty states (ESR-inactive, charge = +2) has started. From the V_g dependence, we can estimate the density of the C-face defects (N_levels), because V_peak should be dependent on N_levels. We performed device simulations on our C-face MOSFETs to estimate a relationship between V_peak and N_levels. Finally, N_levels of various C-face MOSFETs were estimated over a wide range from 4×10¹² cm^-2 to 13×10¹² cm^-2.

We also found that N_levels strongly correlated with the negative V_th shifts in the C-face MOSFETs, meaning that the C-face defects are related to the negative V_th shifts or positive fixed charges in the oxide layer. However, EDMR did not focus on the oxide layer. After removing a negative V_g, the EDMR signals disappeared immediately, clearly indicating that EDMR observed hole traps at the interface. The strong correlation between interfacial hole traps (C-face defects) and the positive fixed charges (hole traps in the oxide layer) suggests that the C-face defects are also formed in the oxide layer. They may be partly incorporated into the oxide layer during the oxidation. By reducing the C-face defects as small as possible, we can obtain high-performance SiC-MOSFETs with both high channel mobility and high reliability in V_th.

[1] T. Umeda et al., ECS Transactions vol. 58, 7 (2013). [2] G. W. Kim et al., Mater. Sci. Forum vol. 858, 591 (2016).