The First Principles Investigation of SiC/SiO2 Interfaces Obtained by Thermal Oxidation
At the beginning, we first explain the very important characteristics of the wave functions at SiC conduction band bottoms. According to the recent report by the first principles calculations, wave functions at SiC conduction bottoms are very unique characteristics . The amplitude of SiC conduction band wave function is not located on the atoms, but it distribute in the internal space. We call these CB state as an internal space state (ISS). It is also noted that the shape of internal space is sensitively depends on the SiC poly-types. In case of 3C-SiC, shape of internal space is straight and its length is infinite. Therefore, quantum confinement effect is essentially weak in 3C-SiC, leading to the relatively lower energy level of the ISS and a small band gap. In case of 4H-SiC, however, shape of internal space is zigzag-like. Quantum confinement effect for ISS located inside the zigzag-shaped internal space is off course very large which raises the energy level of ISS. As a result, 3C-SiC band gap is smaller than that of 4H-SiC by about 1eV. This is the reason why band gaps of SiC sensitively depend on the poly-types. The schematic illustration of poly-type dependent SiC band gaps. It is naturally expected that the above discussed SiC conduction band states which have ISS characteristics sensitively depend on the process induced strain, since strain modify their energy level positions as well as their wave functions.
In the calculations, we prepared a H-terminated 4H-SiC(0001) surface, and investigated the atomistic oxidation processes of this surface by the first principle calculation based on the density functional theory (DFT).
First, we inserted two O atoms between Si and H to make a SiO2-like circumstance. Next, one additional O atom is incorporated to a two-O-adsorbed-SiC(0001) surface. This O incorporation causes drastic structural changes on the surface. The schematic bond rearrangement before and after the additional O adsorption is given in Fig 3(c). As shown in this figure, oxidation induces large bond rearrangement. Adsorbed O atom forms two strong Si-O bonds and two Si-C bonds are broken. As a result, two C dangling bond are generated. Finally, two C atoms form a covalent bond to eliminate C dangling bonds. As a result, O adsorption induces large bond rearrangement and two Si-C bonds change into two Si-O bonds and one C-C bond. It is noted that C interstitials emitted during SiC oxidation are not necessary to form our proposed C-C defect. By comparing the typical bond formation energy of Si-O, Si-C and C-C, this bond rearrangement causes about 9 eV energy gain. This indicates that C-C bonds should be formed at the oxidation front. The existence of C-C bonds at the interface induces interface states near conduction band bottoms. Since a C-C bond length is much shorter than both Si-O and Si-C bond length, existence C-C bond causes local strain around it and ISS (SiC conduction band bottom) are strongly modified. As a result, local band gap near C-C bond becomes smaller by about 100meV. This local modulation of band gap induces electron trap states near the conduction band bottom which can degrade electron mobility of SiC/SiO2interfaces.
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