Theory of boron-vacancy complexes in silicon

Abstract

The substitutional boron-vacancy ${\mathrm{B}}_{s}\mathrm{V}$ complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in $p$-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. ${\mathrm{B}}_{s}\mathrm{V}$ is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with ${C}_{1}$ symmetry is almost degenerate with the second nearest neighbor configuration that has ${C}_{1h}$ symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of ${\mathrm{B}}_{s}\mathrm{V}$ has the lowest energy in the negative charge state. An assignment of the three energy levels associated with ${\mathrm{B}}_{s}\mathrm{V}$ is made. The experimentally observed ${E}_{v}+0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${E}_{v}+0.37\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ levels are related to the donor levels of second nearest neighbor ${\mathrm{B}}_{s}\mathrm{V}$ with ${C}_{1}$ and ${C}_{1h}$ symmetry respectively. The observed ${E}_{v}+0.11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex ${\mathrm{B}}_{s}{\mathrm{V}}_{2}$ is also studied and is found to be stable with a binding energy between ${\mathrm{V}}_{2}$ and ${\mathrm{B}}_{s}$ of around $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Its energy levels lie close to those of the ${\mathrm{V}}_{2}$. However, the defect is likely to be an important defect only in heavily doped material.