Texture development mechanisms in ion beam assisted deposition

Three-dimensional molecular dynamics simulations of ion beam assisted deposition (IBAD) are performed to determine the mechanisms of crystallographic texture selection during the IBAD of polycrystalline films. A face centered cubic bicrystal consisting of [111] and [110] oriented grains is grown while an ion beam bombards the growing film at normal incidence. As the film grows, the grain boundaries delimiting the [111] and [110] grains move towards each other, eventually pinching off the [111] grain such that the film texture changes from equal densities of [111] and [110] to purely [110]. Examination of single crystals grown in the presence of ion beams shows two important effects: ion beam induced atomic sputtering from the surface and ion beam induced damage are significantly reduced when the ion beam is oriented along channeling directions of the crystals. The first observation suggests that grains with channeling directions aligned parallel to the ion beam grow more quickly than those where they are not aligned. This leads to grain-to-grain variations in the film thickness that increase in magnitude during growth. Variations in thickness result in a shadowing effect that further slows the growth of the less thick (nonaligned) grains-eventually leading to pinch-off of these grains. The second observation suggests that the stored energies within the grains with channeling directions aligned parallel to the ion beam will be lower than that of the nonaligned grains. This difference in stored energy (in the form of crystal defects) is shown to lead to grain boundary migration-a process equivalent to primary recrystallization. Both of these effects can lead to changes in crystallographic texture during film growth and both are observed in the bicrystal simulations. Which mechanism will dominate under a prescribed set of conditions remains to be delineated. (C) 1998 American Institute of Physics. [S0021-8979(98)01020-2].