Materials Science Forum Vols. 615-617

Abstract: We proposed a kinetic model for SiC oxidation, named ‘silicon and carbon (Si-C) emission model’, taking into account the emission of Si and C atoms from the SiC–oxide interface, which suppresses the oxidation rate at the interface. Based on the model, we calculated oxide growth rates for SiC (0001) Si- and (000–1) C-face and found that the calculated values exhibit good fits to the measured ones in the entire oxide thickness range for both faces. We also calculated depth profiles of Si and C interstitials and oxidants, and discussed the oxidation mechanism of SiC as well as the difference in the oxidation process of Si-face and C-face.

Abstract: We explain the phenomenon of sodium enhanced oxidation (SEO) using computational techniques, and analyze a set of probable hypotheses, which can elucidate the observation of high inversion channel mobility achieved with SEO. The ability of the Na to screen the interface traps, reduce carbon cluster type of defect formation, and enhance the oxidation rate can be explained using these calculations. We observe an increased availability of electrons near the interface in the presence of Na. The sodium atom also helps in breaking the intramolecular oxygen bond. The electronic and atomic structure of the interface cluster, and electric field computations showed that the carbon cluster formed at the oxide side of the interface could be screened by the Na ion.

Abstract: The purpose of this work is to compare the density of shallow interface states (Dit) at the interface of SiO2/SiC MOS capacitors as deducted by the conductance spectroscopy (CS) and thermally dielectric relaxation current (TDRC) techniques. Both capacitors of 4H- and 6H-SiC (n-type) are investigated, and both ordinary dry oxidation and an improved industrial procedure have been employed. The two techniques are found to give rather good agreement for interface states located ≥0.3 eV below the conduction band edge (Ec) while for more shallow states vastly different distributions of Dit are obtained. Different reasons for these contradictory results are discussed, such as strong temperature and energy dependence of the capture cross section of the shallow interface states.

Abstract: Using impedance spectroscopy (IS) for the characterization of SiO2/4H-SiC (MOS) structures, insight on the capacitive and resistive contributions in different physical regions of the MOS structures is obtained. Changing the DC bias conditions, semiconductor, interface as well as oxide traps can be detected. The MOS capacitance, as extracted from IS data, is different from the one obtained using capacitance voltage (CV) measurements, due to the possibility of distinguishing different charge transfer processes using IS. For instance, in the investigated capacitors, a clear contribution is revealed from ionic conduction processes at bias voltages close to zero.

Abstract: We have characterized 4H-SiC–oxide interfaces fabricated by thermal oxidation of SiC using spectroscopic ellipsometry in the wide spectral range from visible to deep UV region. It was found that there exists an interface layer, around 1 nm in thickness, regardless of the oxide thickness from 15 nm to 40 nm. The optical constants of the interface layer have similar spectral dependence to those of SiC, though the absolute value of the refractive indices is 0.5–1 larger than that of SiC. We have discussed the structure of the interface layer based on the oxidation mechanism of SiC, like the Si-emission model.

Abstract: Thermal oxidation process of silicon carbide in ultra-thin oxide regime has been studied by performing in-situ and real time spectroscopic ellipsometry. We found the thermal oxidation at 700°C forms no or extremely thin interface layers between SiC and oxide layers. In contrast, the oxidation at 850°C forms an interface layer of around 1 nm in thickness, having similar thickness and optical constants of the interface layers formed by the oxidation at higher temperature than 1000°C. To make clear the conditions no interface layer is formed, i.e., whether low temperature growth or thin oxide thickness is crucial, we have performed the oxidation at 850°C in the reduced oxygen pressure. Based on the results of these experiments, we discussed the origin of the formation of interface layers as well as the oxidation mechanism of SiC.

Abstract: The SiO2/SiC interface is characterized for carbon accumulation using the carbon isotope 13C as a marker layer combined with secondary ion mass spectroscopy (SIMS). SiC was epitaxially grown using an isotopically enriched propane source and subsequently oxidized to a thickness required to consume the entire 13C layer. Mass specific depth profiles through the oxide film yield residual carbon concentrations at or below 3x1011 cm-2. The depth resolution of SIMS and natural abundance of 13C in the bulk SiC film limit sensitivity but allow us to set a limit of 2.5x1014 cm-2 carbon build up at or near the interface.

Abstract: Single event transient currents induced in 6H-SiC MOS capacitors are measured by using oxygen ions. Charges collected from the samples are calculated by the transient currents. Applying the drift-diffusion model to the charges, the diffusion length of electron is estimated. Transient currents induced in the gamma ray irradiated MOS capacitors are also investigated. No significant change in the transient currents is observed after gamma ray irradiation.

Abstract: In this work, the electrical characteristics and the reliability of 80nm thick deposited oxides annealed in NO and N2O on the 4H-SiC Si-face for gate oxide application in MOS devices is analyzed by C-V, I-V measurements and by constant current stress. Compared to thermally grown oxides, the deposited oxides annealed in N2O or NO showed improved electrical properties. Dit-values lower than 1011cm-2eV-1 have been achieved for the NO sample. The intrinsic QBD-values of deposited and annealed oxides are one order of magnitudes higher than the highest values reported for thermally grown oxides. Also MOSFETS were fabricated with a channel mobility of 20.05 cm2/Vs for the NO annealed deposited oxide. Furthermore annealing in NO is preferred to annealing in N2O regarding µFE- and QBD-values.

Abstract: We propose a treatment combining nitrogen plasma exposure and forming gas annealing (FGA) to improve the electrical properties of SiO2/SiC interfaces. Although conventional FGA at 450°C alone is not effective for reducing interface traps and fixed charges, our combination treatment effectively reduces both even at moderate temperatures. We achieved further improvement by applying our treatment at higher (over 900°C) FGA temperatures, including lower interface state density (Dit) values for both deep and shallow energy levels (1 - 4 x 1011 cm-2eV-1). Considering that nitrogen incorporation promotes hydrogen passivation of interface defects, a possible mechanism for the improved electrical properties is that interface nitridation eliminates carbon clusters or Si-O-C bonds, which leads to the formation of simple Si and C dangling bonds that can be readily terminated by hydrogen. We therefore believe that our treatment is a promising method for improving the performance of SiC-based MOS devices.