Influence of thermal spikes on preferred grain orientation in ion-assisted deposition

Theoretical models are presented to explain how thermal spike processes can induce the development of preferred orientation of polycrystalline grains during ion-assisted atomic deposition and thin film growth. Two ion energy regimes are investigated. In the first, higher-energy regime, ions penetrate growing grains and generate defects at rates dependent upon the probability of ion channeling in individual grains. The volume free-energy density of different grains is, therefore, different and the thermal spike arising from each ion impact results in preferential growth of grains with the lowest volume free-energy density. In the second, lower-energy regime, ions do not penetrate nor create defects in grains but the ion-induced thermal spikes centered on the surface can enhance surface atomic migration and lead to preferential growth of grains with the lowest surface free-energy density. The results in these two regimes are compared with the predictions of a model for preferred grain orientation evolution in the absence of ion assistance and where minimization of surface free-energy density is the driving process. Ion energy, ion flux density, depositing atom flux density, and system temperature conditions are established where ion-assisted and non-ion-assisted rates of preferred grain orientation become comparable. It is shown that, in general, the tendency toward preferred orientation is not a simple function of the energy per deposited atom.