Influence of vacancy mobility and volume size mismatch energy on the lattice site occupation of implanted species in beryllium single crystals

Abstract

Various ions have been implanted into 〈11 2 0〉 Be single crystals at temperatures of 77 K and 293 K with fluences between 1 × 10 14 at/cm 2 and 2.5 × 10 15 at/cm 2 and ion energies between 160 and 300 keV. Ions with different size mismatch energies, ΔH size, were chosen. In situ Rutherford backscattering and channeling with 1.5 MeV He ions was used to determine the lattice location of the implanted species. At 77 K most of the implanted species show a much higher substitutional fraction, f s, than at 293 K and f s was found to decrease with increasing ΔH size. These effects are attributed to vacancy trapping. At 77 K vacancies are not mobile in beryllium and can be trapped by implanted species only within the collision cascade. At 293 K vacancies are mobile and trapping is effective also during implantation or post-irradiation. Warming 77 K implants to 293 K results in a similar lattice site occupation as observed for 293 K implants. It is discussed that ΔH size is the quantitative parameter controlling the different observations at the various experimental conditions.