Evolution of defect profiles in He-implanted silicon studied by slow positrons

He implanted in silicon forms clusters and then out-diffuses from the surface of the crystal when the implanted silicon samples are annealed from 200 to 700 degrees C, Evaporated He leaves empty voids in the Si crystal. The dimension of the voids and the dynamics of the void formation are strictly related to the He dose. A variable-energy positron beam has been used for monitoring the formation and the evolution of the void distribution in p-type (100) Si (1.7-2.4 Omega cm), created by implantation at 77 K of He ions ( 5x10(15) ions/cm(2)) at 20 keV. The samples were treated at 250 degrees C for different annealing times. Thermal desorption (TD) measurements of He are also presented