Control of 4H-SiC (0001) Thermal Oxidation Process for Reduction of Interface State Density
Thermally-grown oxides on SiC have been considered to induce significant amount of interface defects and near-interface traps, which limit the electron mobility of SiC-MOSFETs. Since the carbon residues are the most possible origin of those defects , it is crucial to employ preferred oxidation conditions for the smooth CO desorption. In this study we demonstrate the formation of SiC MOS interface with reduced interface state density (Dit) <1011 cm-2eV-1, simply by the control of thermal oxidation conditions.
2. Reactions of SiC-O2 System
If we assume a reaction with non-equilibrium state where the products are immediately removed away, the reaction with the most negative free energy change would be thermodynamically preferred among the possible reactions. The ideal reaction, SiC+3/2O2→SiO2+CO where the SiO2 formation is accompanied with the direct ejection of CO molecule from SiC, is predicted to be dominant only in the limited range of temperature for a given O2 pressure. This is because carbon precipitation (SiC+O2→SiO2+C) will dominate for low temperature region, whereas oxygen vacancy formation (SiC+O2→SiO+CO) will be significant for high temperature region . The temperature window for the ideal reaction in 1-atm-O2 is estimated to be around 1100oC – 1300oC, if we take account of the solubility of O2 in SiO2, ~ 2.5×1016 cm-3. Actually, we have observed a high activation energy of the growth rate on 4H-SiC (0001) for these conditions , which is in good agreement with the calculated energy barrier for the direct CO ejection from the interface .
3. Nearly-ideal MOS Characteristics on SiC
4H-SiC (0001) wafers, with ~1×1016 cm-3 n-doped epitaxial layers, were oxidized at 1100 and 1300oC in 1-atm O2 ambient to grow ~14 nm SiO2. A ramp furnace with a short rise/fall time was employed, to demonstrate the oxidation only within the preferred temperature range, by minimizing the unwanted additional oxidation at low temperature. The C-V characteristics of the MOS capacitor are shown in Fig. 1, together with an ideal C-V curve given by Poisson’s equation. The good agreement indicates a successful formation of the interface with very low interface defect density. Especially, the nearly-ideal behavior in accumulation region indicates a suppression of border-trap formation in near-interface oxide .
The Dit values estimated by the conductance method at 150 – 300 K were around 1011 cm-2eV-1 or less, for the energy range 0.1 – 0.4 eV below the conduction band, as shown in Fig. 2. Both 1100oC and 1300oC oxidations result in similar Dit, however, it should be noted that the best results were demonstrated by applying the low-temperature post-oxidation annealing at 800oC in 1-atm O2 where Dit was reduced to less than 1011 cm-2eV-1 even 0.1 eV below the conduction band edge. This temperature is sufficiently low to neglect the additional oxidation of SiC, but expected to annihilate the oxygen vacancies  induced by high-temperature oxidation. These Dit values are even less than the best reported ones with P-passivated interface .
We demonstrated nearly-ideal MOS characteristics on 4H-SiC (0001) with low interface state density <1011cm-2eV-1simply by the control of thermal oxidation conditions, without applying any additional passivation techniques.
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