Temperature dependence of damage ranges in ion implanted metals

Abstract

Damage ranges in ion-implanted crystalline metals have been found to lie significantly deeper than the ranges predicted by the stopping theory for ions in amorphous materials. This phenomenon is especially significant in fee crystals and occurs to a lesser extent in bee metals. Three mechanisms have been proposed to explain these unexpectedly deep damage ranges: the channeling of implanted ions, thermal migration of radiation-induced interstitials, as well as a stress field propagation of defects which initially lie within the range predicted by the LSS theory. In order to obtain some insight into the relevance of these three possible mechanisms it is necessary to study as many possible variables which might influence both the range distribution as well as the level of radiation- induced damage. This study investigates the temperature dependence of the damage. Single crystals of fee copper, nickel and platinum, as well as the bee metal alpha-iron, were implanted off-axially at various temperatures (ranging from 77K to 573K) with 150 keV argon ions using similar dose rates and fluences. Rutherford backscattering of 1-2 MeV alpha particles was used to obtain dechanneling spectra which provided information as to the extent and level of the radiation damage. In previous studies the deep damage ranges have generally been found to increase gradually with implantation temperature. While this investigation discerned a similar trend for iron, no definite temperature dependence of the radiation damage ranges in nickel and copper could be established due to large standard deviations in experimental data. The damage range in platinum was unexpectedly found to decrease with implantation temperature. Especially in the case of platinum, but also possibly in the other three metals, the level of radiation damage in all the samples was found to decrease with implantation temperature. Due to the very large differences observed in the relative damage ranges of the various metals, thermal migration of interstitials is not expected to be the main cause of the unexpectedly deep damage ranges because the migration energies in these solids are quite similar. The large differences in relative damage range may however be incorporated within a stress field defect propagation model. Such a model would also explain the fact that the experimentally obtained damage range in iron does not significantly differ from the LSS range of Ar+ ions in this metal: Because the slip planes in bee metals are less densely packed than in fee metals, the Peierls force opposing glide along these slip planes is greater in bee metals. Within a thermal spike-induced stress field model a gradual increase in radiation damage range with temperature is considered reasonable due to the decrease in Peierls force with increase in ambient crystal temperature. An increase in damage range with temperature can also be expected due to decreased crowdion trajectories resulting in a greater amount of displacement collisions occurring within the collision cascade region. This would lead to an intensification of the thermal spike and subsequently greater thermal expansion, causing defects to be propagated deeper into the crystal the greater the implantation temperature. The decrease in damage levels with temperature that was observed in all the samples investigated can also be explained within the stress field defect propagation model. Due to the fact that insufficient Ar+ ions are expected to penetrate via channeling to depths corresponding to the observed radiation damage ranges (especially for off-axial implantation) channeling is not expected to play a major role in the production of deep damage. This was confirmed by the fact that iron, which has a relative damage range of about 1, was calculated to possess Ar+ channeling half angles of comparable size and orientation to those calculated for nickel and copper, which displayed significantly larger relative damage ranges. However, the low temperature channeling half angles for platinum were calculated to be considerably greater than those of any of the other metals. It is suggested that channeling of Ar+ ions could well be the major cause of the deep damage found in low temperature implanted platinum. Reduced channeling half angles with increase in implantation temperature might then also provide an explanation for the observed decrease in platinum damage range.