Photoconductivity Studies of Defects in Silicon: Divacancy-Associated Energy Levels

A new technique using stress-induced reorientation of defect configuration in single crystals and measurement of the photoconductivity spectra with polarized light is developed and is applied to study defects in electron-irradiated silicon. The annealing behavior and the uniaxial stress response of the 1.5-MeV electron irradiation-induced defects causing the $E_c-\left(0.39\mathrm{} \mathrm{eV}\right)$ and the $E_c-\left(0.54\mathrm{} \mathrm{eV}\right)$ energy levels are studied. The results strongly indicate that these two levels arise from different charge states of the same defect having an atomic symmetry around a $〈111〉$ direction and a transition dipole along a $〈110〉$ direction. The activation energy for the annealing of the $E_c-\left(0.39\mathrm{} \mathrm{eV}\right)$ level is about 1.25 eV Correlating these results with those of previous electron-paramagnetic-resonance studies and infrared-absorption studies leads to the conclusion that the defect in question is the divacancy. Further evidence that the 0.32-eV (3.9-μ) photoconductivity band arises from the divacancy in silicon is given. This band is observed in high-resistivity (nominally undoped) $p$-type silicon, and it anneals in the same temperature region as the divacancy. The results are compared with linear combinations of atomic orbitals (LCAO) calculations. The dominant photoconductivity observed in 45-50-MeV electron-irradiated silicon is found to be an "energy band" that extends from the band edge down to energy of $\approx 0.3$ eV, whereas in 1.5-MeV electron-irradiated silicon it is found to be single levels and the energy band is much smaller in both magnitude and extent, extending only down to $\approx 0.8$ eV.