Optical properties of the neutral silicon split-vacancy center in diamond

The zero-phonon line (ZPL) at 1. 68 eV has been attributed to the negatively charged silicon split-vacancy center in diamond, (Si-V) ^-, and has been extensively characterized in the literature. Computational studies have predicted the existence of the neutral charge state of the center, (Si-V) ^0, and it has been experimentally observed using electron paramagnetic resonance (EPR). However, the optical spectrum associated with (Si-V) ^0 has not yet been conclusively identified. In this paper the 1. 31 eV band visible in luminescence and absorption is attributed to (Si-V) ^0 using an approach which combines optical absorption and EPR measurements. The intensities of both 1. 68 eV and 1. 31 eV bands are found to increase in deliberately Si-doped chemical vapor deposition (CVD) grown diamond, and also after electron irradiation and annealing, suggesting the involvement of both Si and a vacancy in the centers. The 1. 31 eV ZPL is unambiguously associated to Si by its shift to a lower energy when the dominant Si isotope is changed from ^28Si to ^29Si. Charge transfer between (Si-V) ^- and (Si-V) ^0 induced via ultraviolet photoexcitation or heating in the dark allows calibration factors relating the integrated absorption coefficient of their respective ZPLs to the defect concentration to be determined. Preferential orientation of (Si-V) ^0 centers in CVD diamond grown on 110-oriented diamond substrates is observed by EPR. The (Si-V) ^0 centers are shown to grow predominantly into CVD diamond as complete units, rather than by the migration of mobile vacancies to substitutional Si (Siₒ) atoms. Corrections for the preferential alignment of trigonal centers for quantitative analysis of optical spectra are proposed and the effect is used to reveal that the 1. 31 eV ZPL arises from a transition between the ^3-0. 16em{0ex}A₂₆ ground state and ^3-0. 16em{0ex}A₁ₔ excited state of (Si-V) ^0. A simple rate equation model explains the production of (Si-V) ^0 upon irradiation and annealing of Si-doped CVD diamond. In as-grown Si-doped diamond the (Si-V) defects only account for a fraction of the total silicon present; the majority being incorporated as Siₒ. The data show that both Siₒ and (Si-V) are effective traps for mobile vacancies.