Drawing the geometry of 3d transition metal - boron pairs in silicon by means of electron emission channeling experiments

(1) IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Universidade do Porto, Portugal, and KU Leuven, Instituut voor Kern- en Stralingsfysica, Belgium
(2) Centro de Ciencias e Tecnologias Nucleares, Instituto Superior Tecnico, Universidade de Lisboa, 2686-953 Sacavem, Portugal
(3) KU Leuven, Instituut voor Kern- en Stralingsfysica, 3001 Leuven, Belgium
(4) Centro de Fisica Nuclear, Universidade de Lisboa, Lisboa 1649-003, Portugal
(5) IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Universidade do Porto, 4169-007 Porto, Portugal

The behavior of transition metals (TMs) in silicon is a subject that has been studied extensively during the last six decades. Their unintentional introduction during the Si production, crystal growth and device manufacturing have made them difficult contaminants to avoid. Once in silicon they easily form deep levels, either when in the isolated form, where they act as fast diffusers, or when forming precipitates. These effects usually reduce the efficiency of silicon-based devices, being dramatic, in particular, in photovoltaic applications. To mitigate these effects, the industry has relied on a class of procedures based on gettering techniques where TMs are immobilized away from the active region of devices, e.g. by using p-type layers rich in immobile boron dopants which easily pair with positively charged TM impurities through Coulomb interactions. Although some electrical properties of TM-boron pairs have been investigated experimentally, the information about their geometries is still scarce. In that respect, electron paramagnetic resonance [1] and Mossbauer spectroscopy [2] measurements have suggested a configuration where one of the most important TM in Si, Fe, is located on interstitial sites with tetrahedral symmetry next to substitutional boron acceptor dopants. Furthermore, density functional calculations have suggested the existence of a breathing mode relaxation around the boron atom within this arrangement [3]. A direct experimental confirmation of such a configuration is, however, still missing.

In this work we assess experimentally the lattice location of some TMs in silicon when pairing with boron acceptors.  One effective way is to use the electron emission channeling technique where the location of radioactive isotopes is deduced by the channeling effects of the β- particles emitted by the probe atoms. In the present investigation, we used the TM isotopes 56Mn, 59Fe, 61Co and 65Ni. We have observed that the lattice location of all TMs changes profoundly with doping. In particular, we have found that the majority of 56Mn, 59Fe and 61Co is located on sites near the tetrahedral interstitial site in p-type silicon, along the whole used annealing temperature range. On the contrary, in the case of 65Ni the interstitial fraction was much reduced. This confirms that the charge state of the TMs plays a major role in their immobilization, since Fe, Co and Mn are most positively charged in p-type silicon while Ni is assumed to be neutral. We have found that the detected displacement with respect to the ideal tetrahedral interstitial site is in accordance with the predicted displacements of transition metal-boron pairs. We discuss in detail the application of the emission channeling technique in this system in extracting information about the geometries of such pairs.

[1] W. Gehlhoff et al., Solid State Phen. 32 (1993) 219
[2] H. P. Gunnlaugsson et al., Hyperfine Interact. 169 (2006) 1315
[3] M. Sanati et al., Phys. Rev. B 76 (2007) 125204