Twinning and dislocation pileups in heavily implanted LiNbO3

The nature of a striking pattern of extended defects formed by very high dose implantation of helium in a complex oxide, for example, ferroelectric z-cut LiNbO3, is studied. After irradiation, a high concentration of defects is found to collect and create a network of thick prismatic planar defects, which have typical dimensions of similar to 1.5 mu m and 200 nm parallel and perpendicular to the z axis, respectively. This defect network and its effect on the lattice were studied using a set of x-ray and electron microscopy probes of the lattice structure and spatial variation. Optical microscopy shows that there is strong temperature dependence for forming the network; the density of these extended defects reaches a maximum value for an annealing temperature of 250 degrees C but is fully eliminated by a temperature of 380 degrees C. High-resolution transmission electron microscopy studies indicate that these extended defects are probably localized twinning and dislocation pileups due to plastic deformation of the lattice to relieve He-implantation-induced stress. During this deformation, He accumulates at the twin boundaries. The study also shows that the He interstitials evolve into bubbles causing high stress and resulting in a formation of thick prismatic planar defects. Finally, a mechanism is proposed for defect creation and elimination.