A microscopic model of the Si(001) crystal surface was investigated by first principles calculations to clarify the behaviour of intrinsic point defects during crystal growth and thermal annealing. A c(4 x 2) structure model was used to describe the crystal surface in contact with vacuum. The calculations showed that a vacancy in the first or second atomic layer had about a 2.0eV lower formation energy than deeper inside the bulk and that there was a diffusion barrier to penetrate into the deeper crystal region. Furthermore, a vacancy in the first or second atomic layer was stabilized by the fact that Si atoms with dangling bonds attracted each other due to ionic and/or covalent bonding. There is, however, no barrier for the diffusion of a vacancy from the first layer to the second one. The tetrahedral (T)-site and dumb-bell site, in which a Si atom was captured from the surface and forms a self-interstitial, were found as stable sites near the third atomic layer. The T-site had a barrier of 0.48eV, whereas the dumb-bell site had no barrier for the interstitial to penetrate into the crystal from the vacuum. Self-interstitials in both the T- and dumb-bell sites in the third atomic layer had a 1.7 to 2.8eV lower formation energy than deeper in the bulk and there was a diffusion barrier to penetrate into the deeper crystal region; 32 sites were found as stable sub-surface vacancy positions, whereas only 8 sites were found to be stable self-interstitial positions. Using these results, a mechanism for the elimination of crystal-originated pits by thermal annealing was proposed. It was shown that the microscopic model was consistent with and allowed to fine-tune existing macroscopic models that were used to calculate the intrinsic point defects behaviour during crystal growth from a melt.

Ab initio Study of Vacancy and Self-Interstitial Properties Near Single Crystal Silicon Surfaces. E.Kamiyama, K.Sueoka, J.Vanhellemont: Journal of Applied Physics, 2012, 111[8], 083507