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Notes: We report a detailed transmission electron microscopy and x-ray double-crystal diffractometry study of the lattice-mismatch-induced defect structures in InP grown on (100) GaAs substrate and vice versa by chemical beam epitaxy. A rough estimate of the dislocation densities in GaAs on InP is 2×1010 cm−2 at the interface and 5×107 cm−2 at the surface of the epilayer. The corresponding values in InP on GaAs are slightly lower as expected for the compressive stress state for InP. The majority of the dislocations lie on the {111} slip planes with 1/2 [110]- and 1/2 [101]-type Burgers vectors. A cross-grid-type interfacial misfit dislocation array is not observed. Instead, a complicated dislocation structure near the interface, consisting of overlapping pyramidal dislocation tangles (PDT) similar to those observed previously in InGaAs on InP caused by interfacial misfit particles, is presented. The interfacial dislocations form a cellular structure in GaAs on InP and a random structure in InP on GaAs. A Moiré fringe spacing study of InP on GaAs indicates a localized change in composition at the interface, possibly due to As incorporation or GaAs/InP intermixing. The formation of PDT defects and the variation in composition at the interface suggest a breakdown of layer-by-layer growth in the initial stage of growth which results in island nucleation. A dislocation mechanism for the PDT formation is also proposed. All epilayers prepared by chemical beam epitaxy (CBE) without two-stage growth are specular. X-ray rocking curve linewidth measurement shows a general reduction in the linewidth with increasing growth temperature and is insensitive to the substrate misorientation. Fine surface morphology revealed by Nomarski interference microscopy shows no correlation with x-ray linewidth. Results on the reduction of dislocation by varying growth temperature, substrate misorientation angle, and using AlGaAs/GaAs superlattice barriers for dislocation propagation are presented and their effectiveness are discussed. A realistic scheme to achieve an unwarped wafer with low dislocation density is proposed.