Papers by Author: Sven Öberg

Abstract: Vacancies and interstitials in semiconductors play a fundamental role in both high temperature diffusion and low temperature radiation and implantation damage. In Ge, a seri- ous contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity mi- gration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density func- tional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium.

Abstract: In this paper we investigate the formation of interstitial nitrogen trimers N3 which have been suggested as a fast-diffusing species in silicon recently. Out-diffusion profiles of nitro- gen show the involvement of at least two independent nitrogen related defects in the diffusion process depending on the nitrogen concentration at different depths of the sample. When the nitrogen concentration is small it is proposed that nitrogen trimers are formed in a two step process. We present the structural properties of such a defect using density functional theory and examine the energetics of the two proposed reactions leading to the formation of N3.

Abstract: Recently, the interaction of copper with dislocations in p-type Si/SiGe/Si structures has been investigated experimentally and a new dislocation related DLTS-level at Ev +0.32 eV was detected after intentional contamination with copper. To determine the origin of this newly detected level, in this work we present first density functional calculations of substitutional copper at 90◦ and 30◦ partial dislocations in silicon. Defect–dislocation binding energies are determined and electrical gap levels are calculated and compared with the experimental data. As a result, the observed level at Ev + 0.32 eV is tentatively assigned to the single acceptor level of substitutional copper at the dislocation.

Abstract: The properties of point defects introduced by low temperature electron irradiation of germanium are investigated by first-principles modeling. Close Frenkel pairs, including the metastable fourfold coordinated defect, are modelled and their stability is discussed. It is found that damage evolution upon annealing below room temperature can be consistently explained with the formation of correlated interstitial-vacancy pairs if the charge-dependent properties of the vacancy and self-interstitial are taken into account. We propose that Frenkel pairs can trap up to two electrons and are responsible for conductivity loss in n-type Ge at low temperatures.

Abstract: First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We have shown that the Peierls barriers are strongly dependent on the dislocation core structures. Our results have revealed that the asymmetric reconstruction does not possess midgap states while the symmetric reconstructions, characterized by dangling bond on like atoms along the dislocation line, are always electrically active. We suggested that under forward bias, the free energies of the symmetric reconstructions are dynamically lowered by continuous electron-hole transitions between the respective deep levels and valence/conduction bands.

Abstract: First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We show that the stability of the dislocation cores and the Peierls barriers of the first kind are chargestate dependent. In intrinsic bulk the partials are stable in the neutral asymmetric reconstructions. These reconstructions have no deep states and are characterized by high Peierls barriers. In strongly doped regime the symmetric reconstructions can become more stable. These reconstructions are always electrically active with a half filled band across the band gap. In particular the symmetric reconstructions of the 30° partial have a lower Peierls barriers than the respective asymmetric ones and could be the cause of the 1.8 eV electroluminescence peak observed under carrier injection conditions.

Abstract: The donor and acceptor levels of defects in Ge as well as in Si are found using a local density functional method applied to large H-terminated defective clusters. The surfaces of the clusters are modified so that their band gaps are aligned with experimental values. It is shown that the resulting energies of the first donor and acceptor levels are within about 0.2 eV of the experimental values.

Abstract: The electronic properties and structure of a complex incorporating a self-interstitial (I) and two oxygen atoms are presented by a combination of deep level transient spectroscopy (DLTS), infrared absorption spectroscopy and ab-initio modeling studies. It is argued that the IO2 complex in Si can exist in four charge states (IO− 2 , IO02 , IO+ 2 , and IO++ 2 ). The first and the second donor levels of the IO2 complex show an inverted location order in the gap, leading to a E(0/ + +) occupancy level at Ev + 0.255 eV. Activation energies for hole emission, transformation barriers between different structures, and positions of LVM lines for different configurations and charge states have been determined. These observables were calculated by density-functional calculations, which show that they are accounted for if we consider at least two charge-dependent defect structures.