Ordering of As impurities in a Si dislocation core A. Maiti, a) T. Kaplan, M. Mostoller, M. F. Chisholm, and S. J. Pennycook Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 S. T. Pantelides Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 ~Received 5 July 1996; accepted for publication 13 November 1996! We demonstrate by ab initio calculations that segregation of As in a dislocation core in Si occurs in the form of an ordered chain of As atoms running along the dislocation pipe. All As atoms in the chain achieve threefold coordination and the segregation energy is close to 1 eV per As atom. © 1997 American Institute of Physics. @S0003-6951~97!01003-6# Plastically deformed materials release excess strain by creating extended defects such as dislocations. In semicon- ductor devices, dislocations can severely affect the behavior of dopant impurities because: ~1! the dislocation cores may provide a fast diffusion pathway to impurities, which se- verely influences the dopant profile in an uncontrolled way, and ~2! impurities may get trapped in the core regions lead- ing to preferential segregation and electrical deactivation of the impurity. It has generally been believed that impurities get trapped at sites with imperfect bond order in dislocation cores. How- ever, experimental 1–3 and theoretical 4–12 work has estab- lished that dislocations and grain boundaries in Si and Ge actually reconstruct in such a manner that all atoms are four- fold coordinated. Therefore, a single impurity atom occupy- ing a substitutional site in the core does not experience any appreciable difference from its environment in the bulk ma- terial away from the core, resulting in small segregation en- ergies. For example, in the case of As segregation in a sym- metric tilt boundary in Ge, the segregation energy was found to be only of order 0.1 eV. 12 However, in a very recent ab initio study 13 we found that As impurities form ordered chains of As dimers ~a periodically repeated array of two substitutional As atoms at neighboring sites!, or in some cases, perfectly ordered chains of As atoms along the grain boundary cores, with segregation energies as high as 0.5 eV per As atom. In such chain configurations, each As atom relaxes away from its partner atom in the dimer through re- pulsion, and attains its preferred threefold coordination, thereby lowering energy. It was also found that isolated As dimers lead to threefold coordination, but the excess binding energy over single substitutional As atoms is small, ;0.1 eV per atom, and sometimes there is no energy gain at all. It follows that the energy gain from attaining threefold coordi- nation is largely cancelled by the elastic energy cost of dis- torting the Si backbonds. However, a significant additional energy gain occurs through chain formation. In this letter, we show that As segregation at an isolated dislocation core in Si occurs in the same manner as in grain boundaries, namely in the form of ordered chains of threefold-coordinated As atoms. In fact, the strains associ- ated with isolated dislocation cores are in general much larger than in grain boundaries. These strains lead to larger segregation energies in a dislocation core than in the grain boundary, as high as ;0.9 eV per As atom. In earlier work using cluster calculations, Jones et al. 7 found that As and P dimers in a dislocation core gain substantial energy by achieving threefold coordination. However, chains of threefold-coordinated impurities were not considered. Before delving into computational details, a brief intro- duction to dislocation terminology is in order. The major operative slip system in diamond cubic structures involves the glide of $111% planes along ^110& directions. Well sepa- rated perfect glide dislocations lie primarily along ^110& di- rections, and are either screw or 60° dislocations, the latter deriving its name from the fact that the angle between the dislocation line direction and the Burgers vector b is 60°. In materials with low stacking fault energy, the 60° dislocation is often found to be dissociated into two Shockley partial dislocations separated by a stacking fault. These two Shock- ley partials have their Burgers vectors ( b5 a 6 ^ 112& ) aligned, respectively, at 30° and 90° to their ^110& line direction, and are commonly referred to as the 30° and 90° partials. 14 In this work, we have chosen the 90° partial dislocation core in Si as a concrete system to perform our segregation study. As in our previous work, 13 the total energy calculations and structural relaxations were carried out using density functional theory with the exchange and correlation energy treated in the local density approximation. We used a non- cubic periodic supercell with two oppositely oriented 90° partial cores separated by a distance of 13 Å ~Fig. 1!. An energy cutoff of 150 eV was used, and the Brillouin zone integration was performed using two special k points, chosen according to the Monkhorst–Pack scheme. 15 Atoms were re- a! Electronic mail: nnd@ornl.gov FIG. 1. View along the dislocation line of a periodic supercell containing two oppositely oriented 90° partials with the asymmetric reconstruction; the supercell for the symmetric reconstruction appears almost identical in the same view. The supercell contains 64 atoms and the cores are separated by 13 Å. 336 Appl. Phys. Lett. 70 (3), 20 January 1997 0003-6951/97/70(3)/336/3/$10.00 © 1997 American Institute of Physics