C4 defect and its precursors in Si: First-principles theory

The irradiation of c-Si produces self-interstitials (Siᵢ), which interact with substitutional carbon to produce interstitial carbon (Cᵢ). The latter is mobile at room temperature and interacts with interstitial oxygen (Oᵢ) to form the C3 center (CᵢOᵢ). If enough Siᵢ's are provided, C3 traps one more Siᵢ and forms the C4 center (SiᵢCᵢOᵢ). The first two defects, i.e., Cᵢ and CᵢOᵢ, have been abundantly studied by experimentalists and theorists alike. Their electrical, optical, and magnetic properties are very well understood. On the other hand, the C4 defect is incompletely characterized experimentally and lacks precise theoretical description. We present here the results of first-principles calculations of the configurations, binding energies, vibrational spectra, and estimated gap levels of these defects, with emphasis on the C4 center. There are three configurations of C4, labeled (a), (b), and (c). All of them exhibit vibrational modes consistent with the existing data. However, the origin of the new IR line (~760 cm^-1) associated with C4 differs: it is O related in C4(a) and C4(c) but C related in C4(b). Further, the three configurations have distinct gap levels.