We have studied the use of overshoot graded layers for the control of the dislocation density in mismatched heteroepitaxial layers. Graded ZnS y Se 1-y structures were grown on GaAs (001) by photoassisted metalorganic vaporphase epitaxy (MOVPE) and characterized by high-resolution x-ray diffraction (HRXRD). All samples had a uniform top layer of ZnS 0.014 Se 0.986 , and various graded layers were incorporated between the substrate and the uniform top layer; these included forward-graded (FG) and reverse-graded (RG) buffers. Some structures incorporated overshoot at the interface with the uniform top layer (FGO and RGO buffers). Among the FG samples, those with overshoot exhibited better crystal quality and lower dislocation densities than those without. This is expected because the mismatched interface between the graded layer and the top ZnS 0.014 Se 0.986 can affect the bending over of threading dislocations for the production of misfit dislocations, indirectly promoting annihilation and coalescence reactions. An overshoot interface with 0.1% mismatch was found to remove 2 · 10 8 cm -2 dislocations from the top device layer. Overshoot did not reduce the dislocation density in RG structures, but this may be because the sign of the overshoot caused the generation of new dislocations rather than interactions between existing ones. For growing a high-quality device layer with minimal defect density, it appears that steep forward-graded layers with overshoot may be best in this material system.

We demonstrate an x-ray rocking curve method which allows detection of an asymmetry in the dislocation densities in an heteroepitaxial (001) zinc blende semiconductor layer. These dislocations exist on two types of slip systems with their misfit dislocation line segments oriented along either a [1−10] direction (type A) or a [110] direction (type B). An imbalance in the densities of dislocations on these slip systems produces an observable azimuthal variation in the rocking curve width for symmetric x-ray reflections. An approximate quantitative model allows the estimation of the dislocation densities on the two types of slip systems.

We report an experimental and modeling study of ZnS y Se 1Ày /GaAs (001) structures, all of which comprised a uniform top layer of ZnS 0.014 Se 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 ½1 " 10 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 9 10 8 cm À2 ), but a forward-graded (FG) structure (grading coefficient of 27 cm À1 ) exhibits 20% more dislocations on the type B slip systems (D A % 1.6 9 10 8 cm À2 and D B % 1.9 9 10 8 cm À2 ) and a steep forwardgraded structure (grading coefficient of 54 cm À1 ) exhibits 50% more type B dislocations (D A % 2 9 10 8 cm À2 and D B % 3 9 10 8 cm À2 ). 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.

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