(Invited) Understanding of Growth Kinetics of Thermal Oxides on 4H-SiC (0001) for Control of MOS Characteristics

1. Introduction

Characteristics of SiO₂/4H-SiC MOS interface have been extensively investigated since the improvement of MOS channel mobility is crucial for the reduction of on-resistance of SiC power-MOSFETs. The recent drastic improvement of the crystal quality of SiC epitaxial layers has provided us an opportunity to improve the MOS characteristics by the control of oxidation processes. In this study we investigate the growth kinetics of thermal oxides on 4H-SiC in nanometer-thick region, to understand the guidelines to control the MOS characteristics.

2. Growth Kinetics of Thermal Oxides on 4H-SiC

The oxidation kinetics of SiC surface has been studied by modifying the Deal-Grove model, taking account of the out-diffusion of byproduct CO through the oxide [1], while the kinetics for the thinner film region has not been clarified. Some reports have suggested that the oxidation in thin film region does not follow the linear relationship [2].                                                       
    To clarify the growth kinetics in the thin film region, we oxidized 4H-SiC (0001) wafers at 1100~1300¢ªC in dry O₂ with RTA furnace. The observed oxide film thicknesses at 1300¢ªC are shown in Fig. 1 (a), as a function of oxidation time. Linear relationships were clearly observed at all examined temperatures, suggesting that the oxidation rate is limited by the interface reaction. From the temperature dependence of the growth rate, the activation energy of the interface reaction was estimated to ~3.8 eV, as shown in Fig. 1 (b). This value seems coincident with some of the calculated energies required for CO detachment from the substrate [3], rather than the experimentally obtained ones in early studies [1, 4]. From the O₂pressure-dependence of the growth rate, we found that the reaction order of the rate-limiting step would be 0.4~0.6, which also suggests that a reaction consuming a single oxygen atom, for example CO detachment through the formation of carbonyl complex at the interface, would limit the interface reaction rate.                       

    We also found an offset by extrapolating the slope to y-axis in Fig. 1 (a), which clearly shows the existence of an abnormally fast growth limited in very early stage of the growth (< 2nm). We speculate that the sudden growth rate change will correspond to the change of energetic barrier of the interface reaction, probably the change of CO ejection pathways. One of the possible explanations for this transition would be the thickness dependent oxide microscopic structures in ultrathin region, which might affect the kinetics of oxygen penetration or CO ejection processes. Actually we have clarified by FTIR-ATR studies, that the microscopic structural deformation of the near-interface oxides due to the strain or substoichiometry is induced by the substrate only for < 2 nm region [5].

3. 4H-SiC MOS Characteristics

The C-V characteristics of the MOS capacitor with 13.5-nm-oxide grown by the combination of 1300°C oxidation and 800°C annealing in dry O₂ are shown in Fig. 2. Thanks to the oxide grown at high-temperature in the interface-reaction-limited mode, significantly improved MOS characteristics where D_it was estimated to be 2~4×10¹¹ cm^-2eV^-1for 0.2~0.4 eV below conduction band, was attained only by dry oxidation process without any passivation processes.

4. Conclusions

A linear regime of thermal oxidation of 4H-SiC (0001) was clearly observed for nanometer-thick region. High-temperature dry oxidation with interface-reaction-limited mode should be beneficial to improve the MOS characteristics.

References

[1] Y. Song et al., J.Appl.Phys.95, 4953 (2004).

[2] K. Kouda et al., J.Appl.Phys.112, 024502 (2012).

[3] A. Gavrikov et al., J.Appl.Phys.104, 093508 (2008).

[4] T. Yamamoto et al., Jpn.J.Appl.Phys.,47, 7803 (2008).

[5] H. Hirai et al., Appl. Phys. Lett. 103, 132106 (2013).