On the manifestation of phosphorus-vacancy complexes in epitaxial Si:P films

In situ doped epitaxial Si:P films with P concentrations >1 × 10²¹ at./cm³ are suitable for source-drain stressors of n-FinFETs. These films combine the advantages of high conductivity derived from the high P doping with the creation of tensile strain in the Si channel. It has been suggested that the tensile strain developed in the Si:P films is due to the presence of local Si₃P₄ clusters, which however do not contribute to the electrical conductivity. During laser annealing, the Si₃P₄ clusters are expected to disperse resulting in an increased conductivity while the strain reduces slightly. However, the existence of Si₃P₄ is not proven. Based on first-principles simulations, we demonstrate that the formation of vacancy centered Si₃P₄ clusters, in the form of four P atoms bonded to a Si vacancy, is thermodynamically favorable at such high P concentrations. We suggest that during post epi-growth annealing, a fraction of the P atoms from these clusters are activated, while the remaining part goes into interstitial sites, thereby reducing strain. We corroborate our conjecture experimentally using positron annihilation spectroscopy, electron spin resonance, and Rutherford backscattering ion channeling studies.