Electron Confinement Due to Stacking Control of Atomic Layers in SiC Polytypes: Roles of Floating States and Spontaneous Polarization

摘要

We report on first-principles total-energy electronic-structure calculations that clarify the stability and electronic structures of heterocrystalline superlattices consisting of SiC polytypes. The calculated local density of states unequivocally reveals substantial effects of spontaneous polarization in hexagonal polytypes. The polarization in the hexagonal region renders the band lineup slanted in real space along the stacking direction in the superlattice; furthermore, the counterpolarization in the cubic region makes it slanted in the reverse direction. We find that electrons are confined near the interface in the cubic region and that holes are under a negligible band offset. We also find that the slanted band lineup causes a downward (upward) shift of the conduction (valence) band edge and the band gap becomes narrower than that in the bulk polytype, offsetting the band gap increase due to the quantum confinement. The calculated Kohn–Sham orbitals of the conduction band bottoms distribute not at atomic sites but over interstitial channels in the 3C region, thus showing the floating nature common to sp~3-bonded materials. It is found that the penetration of the floating states into the hexagonal region further modifies the band gap.