(Invited) First-Principles Study on Electron Conduction at 4H-SiC(0001)/SiO2 Interface
Authors: Ono Tomoya , Kirkham Christopher , Iwase Shigeru 
Published: February 2020

Silicon carbide (SiC) is attracted much attention due to its excellent physical properties, such as a high thermal conductivity, high breakdown strength, and large band gap. However, unlike Si metal-oxide-semiconductor (MOS) field-effect transistors (FETs), SiC-MOSFETs suffer from unacceptably low carrier mobility. Large amount of defects at SiC/SiO2 interface, which are generated in oxidation process, is expected to be one of the origins for the low carrier mobility. We carried out first-principles calculations based on density functional theory to reveal the relationship between the electronic structure and the interface defects appearing in the oxidation process.
SiC has many different polytypes, determined based on their stacking along the [0001] direction. Stacking ordered ABCAB… is called hexagonal while that ordered ABABA… is called cubic. Among various polytypes, most widely used polytype for SiC-MOSFETs is 4H-SiC, in which Si and C atoms can occupy one of three positions along the [1-100] direction, normally labelled ABCBAB.... It was reported that interlayer states, whose charge density accumulates in the interlayer region, rather than near atomic sites, lie near the conduction band edge (CBE) in elemental semiconductors. Previous work looking at bulk SiC found that the interlayer states appear between subsequent SiC bilayers with cubic stacking, and lie at the CBE [1]. In the present study [2], we found by the electronic structure calculation for a 4H-SiC(0001)/SiO2 interface that the location of the interlayer states along the SiC CBE changes depending on the two possible interface models: the interface model where the top of the interface is the cubic face (cubic model) has interlayer states at the first bilayer of the interface, whereas one does not appear until the second bilayer for the interface model with the top of the interface being the hexagonal face (hexagonal model). When O atoms, which are expected to be introduced in the oxidation process, are sequentially inserted between Si and C atoms at the interface, the electronic structure of the valence band edge changes in the same manner whether the cubic or hexagonal face is the top layer of the interface. On the other hand, the insertion of O atoms removes the interlayer states of the CBE at the first SiC bilayer in the case of the cubic model, while there are no effects in the case of the hexagonal model. The interlayer states appear due to the difference in the electrostatic potentials between the Si-surrounded tetrahedral interstitial site and C-surrounded one. The inserted O atoms raise the electrostatic potential of the Si-surrounded site due to the strong electronegativity of O, removing the interlayer states from the interface of the cubic model. On the other hand, changes at the CBE are negligible in the case of hexagonal model since there are no states to remove at the first bilayer.
The effect of the interlayer states at the CBE on scattering property was examined by electron-transport calculations [3]. Electrons at the CBE of the cubic model are significantly scattered by the defects because of the absence of the interlayer states while those at the hexagonal model is not. We also investigated the scattering property of the possible defects at the interface, e.g., C interstitial and carbonyl interstitial, and found that the scattering due to the absence of the interlayer states is larger than that of those interstitials. Since recent SiC-MOSFETs mainly use the conduction band as a channel, these results imply that the behavior of the interlayer states at the CBE might play an important role in the performance of SiC-MOSFETs.
[1] Y. Matsushita, S. Furuya, and A. Oshiyama, Phys. Rev. Lett. 108, 246404 (2012). [2] C. Kirkham and T. Ono, J. Phys. Soc. Jpn. 85, 024701 (2016). [3] S. Iwase, C. Kirkham and T. Ono, in preparation.