Deuterium retention in sputter-deposited W-B layers: in-situ implantation and ion beam analysis during annealing

• Deuterium retention in W-B films depends strongly on boron content. • Deuterium retention in W-B mixtures does not scale linearly with the B concentration • W:B = 2:1 films show lowest D retention, below that of pure tungsten. • High D implantation fluences can cause surface enrichment of W for B-rich W-B mixtures. • Boron suppresses bubble formation during D implantation and annealing. Boronization in tokamak devices with tungsten (W) plasma facing components (PFC) may lead to the formation of mixed layers of W and boron (B) that can affect wall retention of plasma fuel species. In this study, deuterium (D) retention was investigated in W-B thin films with different stoichiometries as well as in pure W and B, grown on silicon (Si) substrates by means of magnetron sputter deposition. After pre-characterization, the layers were implanted with 1 keV D 2 + ions to a fluence of 7 × 10 17 D/cm 2 , followed by in-situ ion beam analysis. The samples were annealed to temperatures between 400–600 °C and in-situ ion beam analysis measurements were performed before, during and after the annealing process by simultaneous Elastic Recoil Detection Analysis and Rutherford Backscattering Spectrometry. The different B concentrations in the films led to significant differences in D retention, where higher boron concentrations resulted in higher deuterium retention immediately after implantation. After annealing, the lowest amount of retained D was observed for a W-to-B ratio of 2:1, with an areal density of 8 × 10 13 D/cm 2 , about three times lower than for pure W. The highest retention of around 5 × 10 16 D/cm 2 after annealing to 600 °C was found for the pure B-film. Ex-situ electron microscopy techniques revealed significant morphological modifications due to implantation and/or annealing, including bubble formation (W film), W surface enrichment (B-rich film) and crack formation (W-rich film).