We investigated the interaction of As impurities with a 90º partial dislocation in Si. The calculations show that As atoms segregate to the dislocation core. The most stable site for the As atom is at the stacking fault side, which is favorable by 0.26 eV as compared to an As atom in a crystalline position. There is no charge transfer from the As impurity to the dislocation core when the impurity is far from it. The segregation of the impurity can be understood mainly due to structural effects. This result let us conclude that the experimentally observed negatively charged dislocations are due to impurities trapping at the core of the dislocation and not charge transfer from impurities away from the dislocation.

Bi-covered (<FONT FACE=Symbol>Ö</font>3× <FONT FACE=Symbol>Ö</font>3) reconstructed Si(111) surface has been studied by first principles calculations. Three different Bi coverages have been considered: 1 monolayer (ML), 1/3 ML and 2/3 ML, leading to the milkstool, T4 and the honeycomb structural models, respectively. Our total energy calculations show that the milkstool model is the energetically most stable structure for high Bi coverages followed by the T4 model for low Bi coverages, without going through a stable structure for the honeycomb model. We performed theoretical STM simulations for the three structures. For 1 ML coverage we observe the formation of Bi-trimers for occupied states, and a honeycomb image for empty states. It is suggested that the experimentally obtained STM image of a honeycomb structure does not correspond to a Bi-coverage of 2/3 ML, but it could represent a STM image of empty states localized in the T4 sites aside the Bi-trimers of the milkstool model.