Anomalous diffusion of tin in silicon

The diffusion of Sn in Si has been studied using molecular-beam epitaxially grown Si layers with Sn and Sb distributions included during growth. The Sn and Sb profiles were measured by secondary-ion mass spectrometry. By comparing the diffusion of Sn and Sb in an inert N₂ ambient to that in a nitridating NH₃ ambient, in which case vacancies are injected, the fractional-vacancy contribution to the diffusion of Sn in Si was found to be equal to that of Sb which is known to be close to one. Thus, it is concluded that Sn diffuses predominantly by a vacancy-mediated mechanism in Si. The activation energy of diffusion, however, is found to be higher than expected for a vacancy-mediated diffusion mechanism; this is discussed considering Sn-vacancy configurations different from the configuration in which a vacancy is trapped next to a Sn atom.