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

Various ions have been implanted into [<11(2)over bar 0>] Be single crystals at temperatures of 77 K and 293 K with fluences between 1 x 10(14) at/cm(2) and 2.5 x 10(15) at/cm(2) and ion energies between 160 and 300 keV. Ions with different size mismatch energies, Delta H-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 Delta H-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 Delta H-size is the quantitative parameter controlling the different observations at the various experimental conditions.