Dislocations and stress relaxation in heteroepitaxial films

Abstract

The chapter discusses elastic properties of dislocations for a study of stress relaxation in thin layers. It also discusses elastic interactions of a dislocation with a free surface and an interface. It explains elementary properties stemming from the elastic theory of dislocations with the basic concepts of elastic energy; thereby, the line tension and curvature properties of a dislocation line and the Peach–Koehler force on a dislocation can be deduced. In the dislocation theory, distinction between the dislocation properties that stem from the atomic structure of the core and those that stem from the volume external to the core can be made. The core properties govern the nature of the slip systems and the intrinsic mobility. These properties depend on the nature of the crystal considered and can be investigated with the help of atomistic simulations. Outside the core, the atomic distortions are small enough to allow performing estimates within a continuum frame. Within the elastic theory of dislocations, the stress and strain fields surrounding a dislocation line are long-ranged. They decay as 1/r, where r is the distance to the geometrical center of the core. A calculation of the elastic energy shows that most of the energy of a dislocation is stored outside the core. Thus, the elastic theory of dislocations describes the energetic aspects of dislocation behavior, including the forces applied to dislocations at distances larger than one or a few lattice spacing. The chapter further describes the concept of critical thickness for the relaxation of misfit strains in heteroepitaxial films. In heteroepitaxial films, the elastic stresses can be relaxed in many different manners. Several mechanisms can occur at the interface like amorphisation, phase transformation, interdiffusion, reaction, and the formation of point or planar defects, including stacking faults and microtwins.