Doping Dependence of Thermal Oxidation on n-Type 4H-SiC

The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range of 1000 °C to 1200 °C for samples with nitrogen doping in the range of 6.5 × 10¹⁵ to 9.3 × 10¹⁸/cm³, showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates. The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in the linear-rate-constant preexponential factor from 10^-6 to 10^-2 m/s and the parabolic-rate-constant preexponential factor from 10^-9 to 10^-6 m²/s. The increase in the linear rate constant was attributed to defects from doping-induced lattice mismatch, which tend to be more reactive than bulk crystal regions. The increase in the diffusion-limited parabolic rate constant was attributed to the degradation in the oxide quality originating from the doping-induced lattice mismatch. This degradation was confirmed by the observation of a decrease in the optical density of the grown oxide films from 1.4 to 1.24. The linear activation energy varied from 1.6 to 2.8 eV, while the parabolic activation energy varied from 2.7 to 3.3 eV, increasing with doping concentration. These increased activation energies were attributed to the higher nitrogen content, leading to an increase in the effective bond energy stemming from the difference in C-Si (2.82 eV) and Si-N (4.26 eV) binding energies. This paper provides crucial information in the engineering of SiO₂ dielectrics for SiC metal-oxide-semiconductor structures, which typically involve regions of very different doping concentrations, and suggests that thermal oxidation at high doping concentrations in SiC may be defect mediated.