Effects of Wet Ambient on Dry Oxidation Processes at 4H-SiC/SiO2 Interface: An Ab Initio Study
Authors: Shimizu Tsunashi , Akiyama Toru , Nakamura Kohji , Ito Tomonori , Kageshima Hiroyuki , Uematsu Masashi , Shiraishi Kenji 
Published: December 2020

4H-SiC is a key material for applying in power electronics devices because of its superior physical properties. Furthermore, SiO2 oxide films which are crucial for manufacturing electronic devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs) are easily obtained in 4H-SiC. However, a large number of defects are generated at the interface between 4H-SiC and SiO2 oxide film and result in degradation of the performance in SiC-MOSFETs such as channel mobility. It has been known that the oxidation processes strongly depend on the substrate orientation and oxidizing ambient such as dry and wet oxidations. Indeed, recent experimental researches have revealed that the mixture of wet and dry oxidants improves the MOS electrical properties and effects of the coexistence of H2O and O2 on the oxidation processes have been explored.[1,2] However, the reaction mechanisms during the oxidation with coexisting oxidants at the interface have never been investigated from theoretical viewpoints. In our previous study, the reaction mechanisms of dry and wet oxidation at Si- and C-face interfaces have been investigated based on ab initio calculations.[3,4] We have also investigated the structural changes during the reaction of O2 molecule coexisting with wet oxidants at Si- and C-face 4H-SiC/SiO2 interfaces.[5] We have revealed that H2O molecule and single OH group hardly contribute to the reaction and CO molecule is formed from O2 molecule at the Si-face interface. On the other hand, CO2 molecule is formed after the reaction of both O2 and H2O molecules at the C-face interface. We also find that the reaction is more thermally activate than dry and wet oxidation processes and there are no localized electronic states at the interface indicating the formation of interfacial defects in MOSFETs. In this study, we extend our theoretical approach to explore the energy barrier and reaction process at the interface including both dry and wet oxidants. The physical origins of the oxidation-rate difference caused the coexistence of O2 and H2O molecules are discussed on the basis of calculated energy barriers of the reaction.
We examine several reaction pathways for the interface where O2 molecule coexists with wet oxidants (H2O molecule and OH groups) using the nudged elastic band (NEB) method. During the reaction at the C-face interface where O2 molecule coexisting with H2O molecule, we find a plausible reaction pathway which has the lowest energy barrier. The O2 and H2O molecules are stable located in SiO2 region, such that it is reasonable to consider their structure as the initial state of the reaction. The NEB calculations demonstrate that O2 molecule moves toward the interface through the transition state, in which Si-O-CO bond is formed from the reaction of O2 molecule at the interface. The metastable structure consisting of Si-O-Si bond and CO molecule is formed after the transition state. The energy of this configuration is lower than that of the initial state by 4.85 eV and the energy barrier is 0.8 eV. This corresponds to the structural change during dry oxidation [3] and H2O molecule hardly contribute to O2 molecule at the interface during the process of CO molecule formation. However, the stable state with the formation of CO2 molecule, whose energy is lower than the initial state by 7.68 eV, is newly found as the final state. Therefore, the H2O molecule easily reacts with CO molecule in SiO2 region.[5] It should be noted that the formation of HCOOH from CO and H2O molecules and its dissociation occur during the formation of CO2 molecule. These results suggest that more stable interface stucture is realized by the presense of H2O molecule during the oxidation processes of O2 molecule at the C-face interface. Furthermore, other reaction processes including OH groups as wet oxidant as well as those at the Si-face are discussed.
[1] K. Kita, H. Hirai, and K. Ishinoda, ECS Trans. 80, 123 (2017).
[2] K. Kita, H. Hirai, H. Kajifusa, K. Kuroyama, and K. Ishinoda, Microelectron. Eng. 178, 186 (2017).
[3] T. Akiyama, A. Ito, K. Nakamura, T. Ito, H. Kageshima, M. Uematsu, and Shiraishi, Surf. Sci. 641, 174 (2015).
[4] T. Akiyama, S. Hori, K. Nakamura, T. Ito, H. Kageshima, M. Uematsu, and K. Shiraishi, Jpn. J. Appl. Phys. 57, 04FR08 (2018).
[5] T.Shimizu, T. Akiyama, A.-M. Pradipto, K. Nakamura, T. Ito, H. Kageshima, M. Uematsu, and K. Shiraishi, Jpn. J. Appl. Phys. 59 SMMD01(2020).

Figure 1