Papers by Author: Takashi Nakamura

Abstract: The mechanism of flatband voltage shift in SiC metal-oxide-semiconductor (MOS) capacitors with stacked gate dielectrics consisting of aluminum oxynitride (AlON) layers and SiO₂ underlayers was investigated by varying the AlON and SiO₂ thicknesses. The flatband voltages of the fabricated capacitors with fixed SiO₂ underlayer thicknesses were almost independent of the AlON thickness, indicating the negligible charges in AlON layer. On the other hand, when varying SiO₂ underlayer thickness, the flatband voltage decreased with an increase in capacitance equivalent thickness (CET), and the slope of their linear fit was comparable to that for SiC MOS capacitors without AlON layer. These observations can be well explained by assuming interface charges at AlON/SiO₂ interface with an amount comparable, but a polarity opposite to, those at SiO₂/SiC interface.

Abstract: Dynamic and static characteristics of SiC power MOSFETs at high temperature up to 380°C were investigated. Investigated devices have exhibited a behavior as a normally-off MOSFET even at such high temperature as 380°C. Temperature dependence of the MOSFET characteristics are reported in this paper, such as threshold voltage (V_TH), on-resistance, internal gate resistance, and turn-on and turn-off losses (E_ON, E_OFF). E_ON decreases and E_OFF increases with increased temperature. Temperature dependence of switching losses is affected by transfer time of V_DS, which was mainly determined from V_TH.

Abstract: The radiative defect centers in thermally-grown SiO₂/4H-SiC structures with high-temperature post-oxidation annealing (POA) in various ambient gas, i.e. Ar, H₂, and NO_x, were examined using cathodoluminescence (CL) measurement. It was found that radiative centers with an extremely high luminescent efficiency were remained at the SiO₂/SiC interfaces after Ar-POA and FGA. Thus, these defect centers are very stable against high-temperature annealing and reducing ambient. In contrast, NO_x-POA significantly reduced amounts of the radiative defects that might be related to channel mobility improvement in SiC-MOSFETs.

Abstract: In order to take full advantage of the high power density available by the packaging of SiC devices, this group has been exploring die attachment methods based on the sintering of silver particles between the device and Silver-plated Cu substrates with void free results. We have also extended this capability from not only attaching single chips assembled in TO-247 packages but also to multiple chips attached at once on a copper substrate. This multi-chip processing capability will aid in further development of SiC power modules.

Abstract: The impact of a sacrificial oxidation treatment on subsequent gate oxide formation on 4H-SiC(0001) substrates was investigated. Although x-ray photoelectron spectroscopy (XPS) analysis revealed that the SiC surface after removing a 40-nm-thick sacrificial oxide by diluted HF solution was almost identical to that of an as-grown epilayer, the subsequent dry O₂ oxidation resulted in a thinner SiO₂ layer for the sample with the sacrificial oxidation in the ultrathin film regime (~3 nm). The metal-oxide-semiconductor (MOS) capacitor with sacrificial oxidation also exhibited a larger frequency dispersion in capacitance-voltage (C-V) characteristics, indicating that interface property had been degraded. However, when the oxide thickness reached about 10 nm, there was no difference in frequency dispersion with and without sacrificial oxidation. This means that the SiO₂ growth in the initial stage of oxidation was significantly affected by the sacrificial oxidation treatment.

Abstract: The impact of mobile ions intrinsically generated in thermally grown SiO₂ by high-temperature forming gas annealing (FGA) on the SiO₂/4H-SiC interface properties was studied by means of electrical characterization of SiC metal-oxide-semiconductor (MOS) capacitors. Unlike Si devices, mobile ions located at the interfaces were found to cause a remarkable stretch-out of capacitance-voltage (C-V) curve near the accumulation condition, and the degree of stretch-out was more pronounced with increasing probe frequency. This suggests that the interface states with a long emission time constant are formed near the conduction band edge due to the mobile ions. To clarify this unusual phenomenon, several characterization techniques to evaluate interface state densities (D_it), including Terman, conductance, and C-ψ_s methods, were employed. The D_it values estimated for SiO₂/SiC interfaces with mobile ions were a few times as large as those without mobile ions.

Abstract: We focused on the inability of the common high-low method to detect very fast interface states, and developed methods to evaluate such states (Cψ_S method). We have investigated correlation between the interface state density (D_IT) evaluated by the Cψ_S method and MOSFET performance, and found that the D_IT(Cψ_S) was well reflected in MOSFET performance. Very fast interface states which are generated by nitridation restricted the improvement of subthreshold slope and field-effect mobility.

Abstract: The bulk properties of thermally grown SiO₂ on 4H-SiC(0001) substrates were thoroughly investigated by capacitance-voltage (C-V) measurement, atomic force microscopy (AFM), spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). The equivalent oxide thickness (EOT) extracted from the capacitance-voltage (C-V) characteristics of TiN/SiO₂ capacitors was proportional to the physical thickness (T_phys), but the slope of the linear fit was found to be 1.11, indicating that the permittivity of SiO₂ on 4H-SiC formed by thermal oxidation is only about 3.5, which is lower than the commonly accepted value of 3.9. Since XPS analysis revealed that the oxide of SiC was stoichiometric and the atomic concentration of residual carbons in the oxide measured by SIMS was sufficiently low (10¹⁷ cm^-3), the low permittivity of thermal oxides of 4H-SiC may originate from the reduced bulk density, which can be predicted by the Clausius-Mossotti relation.

Abstract: We report on the harmful impact of ultraviolet (UV) light irradiation on thermally grown SiO2/4H-SiC(0001) structures and its use in subsequent thermal annealing for improving electrical properties of SiC-MOS devices. As we previously reported [1], significant UV-induced damage, such as positive flatband voltage shift and hysteresis in capacitance-voltage curves as well as increased interface state density, was observed for SiC-MOS devices with thermally grown oxides. Interestingly, the subsequent annealing of damaged SiO2/SiC samples resulted in superior electrical properties to those for untreated (fresh) devices. These findings imply that UV irradiation of the SiO2/SiC structure is effective for eliciting pre-existing carbon-related defects and transforming them into a simple configuration that can be easily passivated by thermal treatment.

Abstract: The impact of ultraviolet (UV) light irradiation on thermally grown SiO₂/4H-SiC structures was investigated by characterizing the 4H-SiC metal-oxide-semiconductor (MOS) capacitors fabricated with and without UV irradiation onto the oxide layers. The UV irradiation was found to significantly increase a hysteresis in capacitance-voltage (C-V) characteristics and cause a positive flatband voltage (V_FB) shift, suggesting the generation of oxide charges and traps. Since the values of C-V hysteresis and V_FB shift depend on the UV irradiation time, the electrical defects were considered to be induced during UV irradiation. In contrast, UV irradiation caused no marked change for the reference Si-MOS capacitors, indicating that the generation of UV-induced electrical defects was an intrinsic property of thermally grown SiO₂/SiC structures. A detailed characterization of SiC-MOS capacitors with terraced SiO₂ layers revealed that the UV-induced defects were located near the SiO₂/SiC interface. The interfacial fixed charge density (Q_OX) was estimated to be 1.7×10¹² cm^-2 for the sample with UV irradiation, while that of the sample without UV irradiation was 1.0×10¹² cm^-2. Also, a slight increase was found in interface state density (D_it) due to UV irradiation. These results imply that the UV-induced defect generation correlates with residual carbon impurities at the SiO₂/SiC interface.