Photoconductivity Studies of Radiation-Induced Defects in Silicon

The annealing behavior and the uniaxial-stress responses of the radiation-induced defect causing the $E_c-0.75\mathrm{}$-, $E_c-0.54\mathrm{}$-, $E_c-0.42\mathrm{}$-, and $E_c-0.18\mathrm{}$-eV photoconductivity energy levels in $n$-type silicon were studied after 1.5-MeV electron and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ $\gamma$-ray irradiation at 300 °K. The results suggest that the $E_c-0.75\mathrm{}$-eV level arises from the electronic transition of the neutral charge state of the divacancy to the conduction band. This level also occurs in heavily irradiated $p$-type silicon when the Fermi level is too high to observe the divacancy-associated 0.32-eV photoconductivity band. The $E_c-0.75\mathrm{}$-eV level is found to be correlated with the 0.32-eV band, indicating that they are due to two different charge states of the same defect. The $E_c-0.18\mathrm{}$-eV level arises from the $A$-center defect which exhibits only one kind of stress response, i.e., the atomic redistribution among the allowable orientations. No electronic response was observed for the $A$ center in our photoconductivity measurements. Our data fit very well with the $A$-center model derived from electron-paramagnetic-resonance studies. The $E_c-0.42\mathrm{}$-eV level was quite complicated. Besides the radiation-induced divacancy defect located near this level, we present evidence that some additional trap center inherent as an in-grown defect in the sample also gives rise to this level. The $E_c-0.54\mathrm{}$-eV level anneals out around 150 °C. Whether it really arises from the singly charged state of the divacancy or not is still a question.