Evidence for deep lattice damage in krypton-implanted titanium, revealed by positrons

Abstract

Samples of titanium implanted at ambient temperature with doses of 15 × 10¹⁶cm⁻² 100keV krypton ions have been studied using a variable-energy positron beam. Doppler broadening measurements on the 511 keV photon spectra arising from positron annihilation at varying depths beneath the sample surface have revealed information on positron traps, trapping rates and diffusion lengths. Trapping over the depth range corresponding to the implanted krypton is consistent with the krypton bubble formation and associated damage detected by transmission electron microscopy.

A new observation of interest is that positron trapping is found well beyond the implanted-ion range. Calculations using the TRIM code suggest that recoils can penetrate this region to give a significant amount of displacement damage. The resulting characteristic dislocation loop sub-structures, either alone or in combination with knocked-in end-of-range krypton atoms, provide the observed positron traps. This picture is consistent with the removal of these traps after annealing to 400°C. Both positron data and electron microscopy show that the bubble regions are unaffected by annealing, even up to 800°C.