Overshoot graded layers for mismatched heteroepitaxial devices

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 vapor-phase 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 x 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.