4H-SiC has attractive properties for power devices of high voltage applications, however, the performance of MOSFETs is often severely limited by the poor SiO₂/SiC interface quality. One of the common techniques to passivate the interface defects is the nitrogen introduction by NO annealing [1] while introduction of wide variety of impurity elements have also been studied [2]. Recently we have demonstrated the benefit of post-oxidation annealing in wet ambient at low-temperature as an alternative way to improve the SiO₂/4H-SiC (0001) interface quality, which effectively works with only < 1nm additional growth of oxide at the interface [3]. In this study we investigated the validity of the combined passivation process of the NO annealing and the low-temperature H₂O-annealing after the dry oxidation of 4H-SiC, to take the advantages of both the nitrogen passivation and the good quality interface formation by H₂O-oxidation [4].

After HF cleaning of 4º off-axis 4H-SiC (0001) wafers covered with 5 μm-thick n-type epitaxial layers (N_D ~ 1×10¹⁶ cm^-3), ~30 nm-thick SiO₂ layers were thermally-grown in dry oxygen at 1300ºC. As the post-oxidation annealing techniques, the annealing in NO:N₂ =1:2 ambient at 1150ºC for 2hrs (NO-POA) and the additional annealing in H₂O:O₂=9:1 ambient at 800ºC (LT-H₂O-POA) for various duration time, were sequentially conducted. Finally Au electrode was deposited to form a MOS capacitor. For the NO-POA process of SiC after thermal growth of SiO₂, it has been clarified that nitrogen atoms are introduced mostly in the first few monolayers of SiC, and remains even after removal of SiO₂ by chemical etching completely [5]. We evaluated the effects of our NO-POA and LT-H₂O-POA on the amount of nitrogen introduced at the interface, from the relative change of core level XPS intensity ratio of N1s to the substrate component of Si2p, after the complete removal of SiO₂ layer. It was found that the long-time LT-H₂O-POA results in a significant reduction of nitrogen density at the interface after the growth of several MLs of SiO₂, due to the consumption of nitrogen-introduced SiC surface layers for the oxidation by H₂O. However, it would be noteworthy that most of the nitrogen still remains at the interface after a short-time LT-H₂O-POA to cause the growth of limited amount of SiO₂ around 1 ML.

Next we evaluated the interface state density (D_it) of SiO₂/SiC MOS capacitors fabricated with different duration time of LT-H₂O-POA after NO-POA, by using conductance method. We found D_it was significantly reduced by short-time LT-H₂O-POA, and minimized when only 1 ML of SiO₂ was grown during the H₂O-POA. The D_it value at the energy level at 0.2 eV below the conduction band edge of SiC was reduced below 5×10¹⁰ cm^-2eV^-1, which is less than half of the value observed just after the NO-POA (without H₂O-POA). This would be the minimum D_it ever reported for 4H-SiC (0001) MOS capacitor at this energy level. On the other hand, LT-H₂O-POA with excess duration time resulted in an increase of D_it. After the long-time LT-H₂O-POA to cause ~1 nm SiO₂ growth, the D_it value became even larger than the one just after the NO-POA. This deterioration would be caused by the partial lost of passivating nitrogen at the interface by the growth of new oxide layers, as indicated by the XPS analysis. One of the methods to evaluate the density of such slow traps in SiC MOS capacitors is the characterization of the hysteresis of CV curve observed for a voltage sweep just after the ejection of trapped charges by UV irradiation [6]. We observed that the slow trap density also significantly decreased by short-time LT-H₂O-POA when the additional growth of SiO₂ was limited to ~1 ML, but gradually increased by a long-time one, which is a quite similar behavior as D_it change by H₂O-POA.

These results show that the roles of nitrogen introduction and oxidation by H₂O on SiO₂/SiC interface defect annihilation are different from each other, and the interface with minimized D_it and less near-interface oxide traps is achievable by the sequential combination of those two POA processes, when the amount of additional oxidation by H₂O is limited to ~ 1ML of SiO₂.

References

[1] J. Rozen et al., IEEE Trans. Electron Dev. 58, 3808 (2011).

[2] D. J. Lichtenwalner et al., Appl. Phys. Lett. 105, 182107 (2014).

[3] H. Hirai and K. Kita, ICSCRM 2017, Washington DC.

[4] H. Hirai and K. Kita, Appl. Phys. Lett. 110, 152104 (2017).

[5] R. Kosugi et al., Appl. Phys. Lett. 99, 182111 (2011).

[6] H. Yano et al., IEEE Trans. Electron Dev. 46, 504 (1999).