Papers by Author: M. Bockstedte

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: The existence of point defects is one of the key problems in SiC technology. Combined experimental and theoretical investigations can be successful in identification of point defects. We report the identification of a basic intrinsic defect in p-type SiC. In addition, we predict the existence of interstitial-related electrically active defects which may be detected by experimental tools.

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: The negative carbon vacancy antisite complex is analysed by ab initio theory in view of the SI5 EPR-center. The complex occurs in a Jahn-Teller distorted ground state and a meta stable state. This and the calculated hyperfine structure agree nicely with the temperature dependent EPR spectra of SI5. An interpretation of the photo-EPR experiments is proposed.

Abstract: Electron paramagnetic resonance (EPR) studies of the P6/P7 centers in 4H- and 6H-SiC are reported. The obtained principal values of the hyperfine tensors of C and Si neighbors are in good agreement with the values of the neutral divacancy (VCVSi 0) calculated by ab initio supercell calculations. The results suggest that the P6/P7 centers, which were previously assigned to the photo-excited triplet states of the carbon vacancy-carbon antisite pairs in the double positive charge state (VCCSi 2+), are related to the triplet ground states of the C3v/C1h configurations of VCVSi 0.

Abstract: Only recently the well-resolved hyperfine structure of the P6/P7 EPR center has been experimentally observed. Based on the calculated hyperfine tensors we assign the P6/P7 center to the high spin state neutral divacancy, which is the ground state in agreement with the experiment. We propose a mechanism to explain the loss of divacancy signal at high tem- perature annealing in semi-insulating SiC samples. We discuss the possible correlation between the divacancy and some photoluminescence centers.

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.