Stability of large vacancy clusters in silicon

Abstract

Using a density-functional-based tight-binding method we investigate the stability of various vacancy clusters up to a size of 17 vacancies. Additionally, we compute the positron lifetimes for the most stable structures to compare them to experimental data. A simple bond-counting model is extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for V 6 to V 14 . The structures, where the vacancies form closed rings, such as V 6 and V 10 , are especially stable against dissociation. For these structures, the calculated dissociation energies are in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes are consistent with measurements.

Stable Fourfold Configurations for Small Vacancy Clusters in Silicon fromab initioCalculations

“…This can be explained using simple bond counting arguments: the crystal can reconstruct almost perfectly around a hexavacancy, making all atoms remain fourfold. For smaller clusters, the same calculations [15,16,17] conclude that the most stable configurations occur when atoms are removed sequentially from the hexagonal ring.The ring hexavacancy is known to be a good trap for various impurities, such as carbon, oxygen, and copper atoms [16]. It is reasonable to expect, therefore, that it may also be an efficient trap for self-interstitials.…”

“…This can be explained using simple bond counting arguments: the crystal can reconstruct almost perfectly around a hexavacancy, making all atoms remain fourfold. For smaller clusters, the same calculations [15,16,17] conclude that the most stable configurations occur when atoms are removed sequentially from the hexagonal ring.…”

“…However, vacancy clusters can also be present in as-grown crystals [14]. The presence of defects in crystalline semiconductors determines, to a large extent, their electrical and optical properties, making their study of great importance.Calculations performed using density-functionaltheory (DFT) molecular dynamics [15,16], the Hartree-Fock method [15,16], and the DFT tightbinding method [17], among others, predict that the ring hexavacancy should be significantly more stable than other types of vacancy clusters. This can be explained using simple bond counting arguments: the crystal can reconstruct almost perfectly around a hexavacancy, making all atoms remain fourfold.…”

Internally consistent approach for modeling solid-state aggregation. I. Atomistic calculations of vacancy clustering in silicon

“…The results in ref. [36] also were poorly represented by a simple dangling bond model, and an extended model based on new bond formation due to atomic relaxation was proposed.…”

“…Despite these limitations, notable progress in this area recently has been made. Two series of tight-binding calculations 35,36 recently have shown that the binding energies of vacancy clusters are a non-monotonic function of size, and cannot be expressed simply in terms of nearest-neighbor interactions 32,37 . In ref.…”