Clustering Phenomena of Implants in Tungsten Observed with THDS

A W(100) single crystal was irradiated with 5-10 keV of different metallic species (Ag, Cu, Cr, Mn, Al and In). Subsequently the crystal was annealed at temperatures ranging from room temperature to 2400 K. Thermal helium desorption spectrometry (THDS) was applied to monitor the dissociation and clustering reactions of defect complexes either formed at room temperature during implantation or during partial annealing at a higher temperature. Strong evidence is found for clustering of Ag if implantation doses of Ag exceed 6 × 10^12 Ag+ cm-2. For the implants Al, Cr, Cu and Mn, substantial clustering was not detected. For In, a conclusion could not be made concerning clustering since He does not bind to In in W at room temperature. Atomistic calculations are presented which indicate that clustering of the implants other than Ag would certainly be detected if the impurity atoms in the cluster all occupy lattice positions. Therefore we believe that thermally activated radiation enhanced diffusion is much more efficient for Ag than for Cu and Mn in W. For Cr and Al we cannot exclude enhanced diffusion, since the binding energies of Cr and Al to Cr- and Al-clusters in W, respectively, will be rather small. Of the implants Cu, Mn and Ag, the latter is the only one being oversized. A rather strong binding of vacancies to Ag is envisaged, which enables transport of the Ag atom, being part of a moving vacancy cluster