Dynamics of Kinetically Limited Strain and Threading Dislocations in Temperature- and Compositionally Graded ZnSSe/GaAs (001) Metamorphic Heterostructures
We have investigated the evolution of the strain and threading dislocation density in metamorphic compositionally and temperature-graded ZnSySe1−y buffer layers. Linear variation in composition in conjunction with temperature grading may allow control over the relaxation process. Previously, we reported the development of a general kinetic model based on dislocation flow, which accounted for the time evolution of the strain relaxation in semiconductor structures under kinetically limited conditions, including interactions of threading and misfit defects. In this work, we studied ZnSySe1−y/GaAs (001) heterostructures with linear compositional grading and a convex-upward (type A), linear (type B) or convex-downward (type C) temperature grading profile. The thermal budget available for relaxation in these types of structures is controlled by the temperature grading profile, made up of combinations of linear ramps and constant-temperature sections. In all cases, the temperature was varied from T0 (400°C to 600°C) at the substrate interface to TF = 300°C at the surface. We also investigated the effect of varying the compositional gradient in the range from 0.18%/μm to 1.6%/μm. Structures with higher average temperature (greater thermal budget) and/or higher grading coefficient exhibited greater extent of relaxation and therefore reduced residual strain. Furthermore, controlling the extent of strain relaxation enabled optimization of the dislocation densities in these heterostructures.