Annealing of electron-, proton-, and ion-produced vacancies in Si

Positron lifetime and Doppler measurements were performed on float-zone-refined and variously doped Czochralski-grown Si. The samples were irradiated by various particles ($e^{-}$, $p$, Kr) with energies between $2\mathrm{MeV}$ and $245\mathrm{MeV}$. Electron or proton irradiation gave rise to divacancies, whereas the damage from ion implantation (Kr) was mainly in the form of four-vacancy clusters, with only a small fraction of vacancies in the form of divacancies. In the case of impurity-lean Si, detailed isothermal annealing at various temperatures between $125°\mathrm{C}$ and $500°\mathrm{C}$ showed that after an initial fast decrease in divacancy concentration, a much slower process of vacancy agglomeration took place. At $450°\mathrm{C}$ agglomeration steadily progressed even after $30\mathrm{h}$ of annealing at which point the average cluster size corresponded to ten monovacancies. In impurity-rich Si, containing oxygen and dopants, there is nearly no initial decrease in vacancy concentration, and isothermal and isochronal annealings showed that vacancy agglomeration was also nearly absent. Doppler data showed that vacancy-dopant complexes slowly acquired oxygen as annealing progressed, and these complexes survived annealing at $500°\mathrm{C}$ for many hours. Measurements between $8\mathrm{K}$ and $530\mathrm{K}$ on samples containing divacancies, or larger clusters, showed a temperature dependence of the positron trapping rate that cannot be explained by the clusters being negatively charged, but can be explained if neutral clusters have a weakly bound positron state which at low temperatures makes trapping more efficient. Generally, the present positron experiments have given an indication for the type of defects that survive annealing at temperatures where electron paramagnetic resonance and infrared spectroscopy yield little useful information.