Papers by Author: Oleg Pankratov

Abstract: Defect signatures, such as deep level positions, hyperfine parameters, local vibrational modes and optical transitions characterize a defect and enable the identification of defect centers. This identification is a key to an understanding of complex phenomena like the defect kinetics. Albeit density functional theory enabled the identification of several defects and their kinetic properties, a new approach is needed to address the optical excitation of defect. Within a quasiparticle theory and taking into account excitonic effects we analyze the excited states of VC +.

Abstract: Employing density functional theory we investigate the model interface between 1 × 1-6H-SiC{0001} surfaces and graphene layers. We find that the first graphene layer is covalently bonded to the SiC substrate, opposing the earlier assumption of a weak van-der-Waals bonding. The interface at the Si-face is metallic, while on the C-face it remains semiconducting. Further graphene layers are then only weakly bound and the typical graphitic properties of the electronic structure appear.

Abstract: Using density functional theory, we investigate the 6H-SiC{0001} surfaces in the unreconstructed 1 × 1 and the H-passivated configuration. The strong correlation effects of the dangling bonds at the surface are treated by spin-polarised calculations including the Hubbard-U parameter. We find that the clean surfaces are semiconducting with surface states in good agreement with experimental data. The impact of the Hubbard-U is stronger on the C-terminated face. For the H-passivated surfaces we find resonances in the valence band. The antibonding C−H state is located in the upper part of the bandgap around the ¯􀀀-point.

Abstract: Nitrogen (N) donors in SiC are partially deactivated either by Si+-/N+-co-implantation or by irradiation with electrons of 200 keV energy and subsequent annealing at temperatures above 1450°C; simultaneously the compensation is decreased. The free electron concentration and the formation of energetically deep defects in the processed samples are determined by Hall effect and deep level transient spectroscopy. A detailed theoretical treatment based on the density functional theory is conducted; it takes into account the kinetic mechanisms for the formation of N interstitial clusters and (N-vacancy)-complexes. This analysis clearly indicates that the (NC)4-VSi complex, which is thermally stable up to high temperatures and which has no level in the band gap of 4HSiC, is responsible for the N donor deactivation.

Abstract: Kinetic mechanisms for the deactivation of nitrogen are investigated by ab initio theory. We find that the interaction of nitrogen with self-interstitials can lead to a deactivation of nitrogen, yet it cannot explain the experimentally observed nitrogen deactivation at high temperatures in silicon co-implanted samples. Our analysis suggests the aggregation of vacancies at high temperatures and the subsequent formation of passive nitrogen-vacancy complexes as a likely explanation.

Abstract: We observe new photoluminescence centers in electron-irradiated 6H-SiC with phonon replicas up to 250 meV and clear threefold isotope splitting of the highest energy mode. Based on ab initio calculations, we discuss the tri-carbon anti-site (C3)Si and the di-interstitial (C2)Hex as models for these centers.

Abstract: Using an ab initio method we analyze the mechanisms of the boron diffusion with emphasis on the role of the intrinsic interstitials. It is shown that the boron diffusion is dominated by a kick-out mechanism. The different effect of silicon and carbon interstitials gives rise to kinetic effects. A preference for a kick-in of the boron interstitial into the carbon lattice sites is found. Kinetic effects reported in co-implantation experiments and in-diffusion experiments are explained by our findings.

Abstract: We investigated the the interstitial configurations of the p-type dopants boron and aluminum and the n-type dopants nitrogen and phosphorus in 4H-SiC by an ab initio method. In particular, the energetics of these defects provides information on the dopant migration mechanisms. The transferability of the earlier results on the boron migration in 3C-SiC to the hexogonal polytype 4H-SiC is verified. Our calculations suggest that for the aluminum migration a kick-out mechanism prevails, whereas nitrogen and phosphorus diffuse via an interstitialcy mechanism.