ENERGETICS OF DISLOCATION DIPOLES IN CAPPED EPITAXIALLY STRAINED LAYERS

Most device structures based on strained epitaxial layers are capped b y a second, unstrained layer to increase the mechanical stability of t he structure. In order to calculate the energies of these structures i t is necessary to synthesize the total energy from the energies of the line defects they contain (interfacial dislocations and dislocation d ipoles). The self energies and interaction energies of dislocations an d dipoles are calculated and their behavior examined as a function of their spacing and the thicknesses of the strained and capping layers. The results confirm the observations that capped strained layers are m ore stable than uncapped ones (of the same strained layer thickness) a nd that capping layers do not need to be thicker than approximately th ree times the strained layer thickness. An expression is deduced for t he total energy of finite, nonuniform arrays of dipoles in capped laye rs and, by analogy with a similar earlier expression for dislocations in uncapped layers, it is concluded that the effect of a nonuniformity in the dipole spacing will be to increase the energy of the system co mpared with that of a uniform array having the same average spacing. T he results in this paper can be used to assess the stability of device s and their rate of degradation by strain relaxation.