Threading Dislocations in S-Graded ZnS_xSe_1-x/GaAs (001) Metamorphic Buffer Layers

Abstract

Metamorphic semiconductor devices are commonly fabricated with linearly-graded buffer layers, but equilibrium modeling studies suggest that S-graded buffers, following a normal cumulative distribution function, may enable lower threading defect densities. The present work involves a study of threading dislocation density behavior in S-graded ZnSxSe1-x buffer layers for metamorphic devices on mismatched GaAs (001) substrates using a kinetic model for lattice relaxation and misfitthreading dislocation interactions. The results indicate that optimization of an S-graded buffer layer to minimize the surface threading dislocation density requires adjustment of the standard deviation parameter and cannot be achieved by varying the buffer thickness alone. Furthermore, it is possible to tailor the design of the S-graded buffer layer in such a way that the density of mobile threading dislocations at the surface vanishes. Nonetheless, the threading dislocation behavior in these heterostructures is quite complex, and a full understanding of their behavior will require further experimental and modeling studies.