Effect of free surface and interface on thermal annealing of dislocation loops in silicon

Abstract

The denuded zones free from dislocation loops formed near a free surface or an interface in ion-implanted specimens subjected either to rapid thermal annealing or annealing in a furance were studied. The point defects cluster to form dislocation loops. Smaller dislocation loops coalesce by glide and climb processes to form bigger loops. Free surfaces and interfaces give rise to interaction with the dislocation loops, thus exerting an image force. Transmission electron microscopy observations in the annealed specimens are presented to show that dislocation loops situated within twice the diameter from the free surface are annealed out, thus creating a denuded zone. In order to explain the formation of denuded zones, we have analyzed the forces of interaction between a square dislocation loop and either a free surface or a second phase with a planar interface including the Poisson’s ratio effects. The elastic field of a square loop is a reasonable approximation to represent the actually observed circular loop. Analytical closed-form expressions for the stress and displacement fields associated with a square dislocation loop in a two-phase medium are used to calculate the image force acting on the dislocation loop. Further considerations of the equilibrium of the dislocation loop under the action of the image force exerted by the free surface and the opposing lattice frictional stress at the annealing temperature enable us to determine the dimensions of the denuded zone that are found to be in agreement with the experimental results. These results further show that a soft second phase (over layer) attracts the dislocation loops whereas a hard second phase repels them.