Defects in Irradiated Silicon: Electron Paramagnetic Resonance of the Divacancy

Two electron paramagnetic resonance spectra produced in silicon by 1.5-MeV electron irradiation are described. Labeled Si-G6 and Si-G7, they are identified as arising from the singly positive and singly negative charged states of the divacancy, respectively. The observed hyperfine interactions with neighboring ${\mathrm{Si}}^{29}$ nuclei and $g$ tensors are discussed in terms of a simple molecular-orbital treatment of the defect by the method of linear combination of atomic orbitals. In addition to the anisotropy associated with the vacancy-vacancy direction in the lattice, an additional distortion occurs which is identified as a manifestation of the Jahn-Teller effect. Thermally activated reorientation from one Jahn-Teller distortion direction to another causes motional broadening and narrowing effects upon both spectra in the temperature region 40-110°K. The motion is also studied by stress-induced alignment at lower temperatures, and the activation energy for the process is found to be ∼0.06 eV for each charge state. Alignment of the vacancy-vacancy axis direction in the lattice is also achieved by stressing at elevated temperatures. The activation energy for this reorientation process is ∼1.3 eV. The magnitude and sense of the alignment in both kinds of stress experiments are consistent with the microscopic model of the defect. It is pointed out that when the divacancy reorients its vacancy-vacancy axis, it is also diffusing through the lattice. The 1.3 eV is therefore also its activation energy for diffusion. Analysis of higher temperature annealing studies allow a lower limit estimate for the binding energy of the two vacancies as $\gtrsim 1.6$ eV. The electrical level structure is deduced and it is concluded that the divacancy introduces one donor and two acceptor levels in the forbidden gap.