Bound substitutional impurity pairs in diamond: a density functional study

Substitutional nitrogen and boron are known to form nearest neighbour pairs in diamond, leading to donor and acceptor levels deep within the band-gap relative to the isolated counterparts. For n-type doping, even isolated nitrogen donors possess very deep levels, and thus larger impurities are used. With species such as phosphorus and sulphur elastic repulsion might be expected to prevent donor aggregation seen for the smaller dopant species. However, we show from density functional simulations that large impurities can form additional chemical interactions when in close proximity, rendering them energetically bound. These direct, localised interactions produce localised electrical levels deep in the band-gap, rendering them inactive for electrical conduction, and possibly optically active. As a consequence, pairing of P, S and other species in heavily doped and/or high-temperature annealed material would impact upon electrical activation of dopants.