Nucleation and growth of helium bubbles in aluminum between 20 and 900 K

The nucleation and growth of helium bubbles have been monitored in ${}_{}{}^3{}_{}{}^{}\mathrm{implanted}$ aluminum by the positron-lifetime technique and transmission electron microscopy. Helium atoms are mobile during the implantation and are subsequently trapped by vacancies. The annealing stages I and III are observed at 40 and 200 K, respectively, in agreement with previous positron annihilation and electrical-resistivity data. The low implantation temperature of 20 K hinders bubble formation during implantation, but during isochronal annealing, around 425 K, helium starts to nucleate into bubbles via substitutional migration of the He-vacancy pairs. The migration energy for a He-vacancy pair is estimated to be 1.3±0.1 eV. Above 600 K the He bubbles grow through condensation of thermally produced vacancies, as well as bubble migration and coalescence. The created helium bubbles are extremely stable and survive the final 900-K annealing. The helium density, as a function of annealing temperature, was determined from the positron-lifetime data employing recent theoretical calculations. Also, the bubble size was estimated. The presence of helium bubbles was verified with transmission electron microscopy and they were found to be polygon shaped. The fraction of positrons annihilating in the helium bubbles was found to decrease when lowering the temperature below room temperature, in accord with results on empty voids in aluminum.