Asymmetric Dislocation Densities in Forward-Graded ZnS ySe1− y/GaAs (001) Heterostructures.

We report an experimental and modeling study of ZnS _ySe_{1− y}/GaAs (001) structures, all of which comprised a uniform top layer of ZnS_0.014Se_0.986 grown on a compositionally graded buffer layer or directly on the GaAs substrate. High-resolution x-ray diffraction was used to estimate dislocation densities on type A slip systems, with misfit dislocation (MD) line segments oriented along the $$ [1\bar{1}0] $$ direction, and type B slip systems, with MD line segments oriented along a [110] direction. A control sample having no graded buffer exhibits equal dislocation densities on the two types of slip systems ( D_A ≈ D_B ≈ 1.5 × 10⁸ cm⁻²), but a forward-graded (FG) structure (grading coefficient of 27 cm⁻¹) exhibits 20% more dislocations on the type B slip systems ( D_A ≈ 1.6 × 10⁸ cm⁻² and D_B ≈ 1.9 × 10⁸ cm⁻²) and a steep forward-graded structure (grading coefficient of 54 cm⁻¹) exhibits 50% more type B dislocations ( D_A ≈ 2 × 10⁸ cm⁻² and D_B ≈ 3 × 10⁸ cm⁻²). The insertion of an overshoot interface reduced the dislocation densities in the uniform top layer by promoting annihilation and coalescence reactions, but type B dislocations were removed more effectively. Based on equilibrium calculations the overshoot graded layer in the steep graded overshoot structure is expected to exhibit large compressive and tensile strains, with a reversal in the sign of the strain near its middle, which may promote annihilation and coalescence reactions between threading dislocations. [ABSTRACT FROM AUTHOR]