LOW-ENERGY HEAVY-ION IRRADIATIONS OF COPPER AND MOLYBDENUM AT LOW-TEMPERATURES

A specially designed and characterized oil-free cryogenic UHV experimental system has been used to irradiate pre-thinned orientated copper and molybdenum single crystals at 4.2 and 78 K with very high doses (5 x 10(12) to 5 x 10(15) ions cm-2) of low-energy (0.2-20 keV) self-ions and xenon ions. Populations of dislocation loops were produced, after subsequent warm-up to room temperature, within a few hundred angstroms of the irradiated surface. The loop depth distributions were measured using stereo transmission electron microscope (TEM) techniques and correlated with possible displacement damage production and retention mechanisms, including replacement collision sequences (RCSS), incident ion channelling and near-surface loop losses. In copper, both near-surface vacancy-loop populations at depths up to about 100 angstrom and deeper interstitial loops up to about 400 angstrom in depth could be produced. In molybdenum only interstitial loops were visible, at depths up to about 250 angstrom. The depth distributions depended critically on the incident ion mass, ion energy and the specimen surface orientation used. In all the copper and molybdenum irradiations at doses greater than 10(13) ions cm-2 only a very small fraction (not more than 2%) of the point defects theoretically created during the irradiations at 4.2 K were retained in visible loops after warm-up to room temperature. The results are consistent with displacement cascades, initiated by incident ions, injecting some interstitials along RCSS directed away from the irradiated surface, which may subsequently aggregate into interstitial loops. It is concluded that the loop depth distributions are determined more by the fraction of incident ions channelled at the irradiated surface than on RCS orientation effects relative to the surface. The low point defect retention efficiency in the present study is consistent with RCS ranges of only a few tens of angstroms at most. However, the lattice is highly damaged during the irradiation, possibly leading to enhanced RCS defocusing. Irradiation at 78 K provides results consistent with interstitial mobility at this temperature.