The effect of various types of buffer layers on the generation and propagation of dislocations in epitaxial layers of high composition (x=0.85) AlxGa1−xAs grown by metalorganic chemical vapor deposition (MOCVD) on horizontal Bridgman (HB) and liquid-encapsulated Czochralski (LEC) substrates is examined. Bulk epilayers of high composition (x=0.85) AlxGa1−xAs and graded-barrier quantum-well laser structures with confining layers of the same composition were grown simultaneously on high-quality/low etch-pit density (EPD) HB substrates and comparatively lower quality/high EPD LEC substrates with one of four types of compositionally graded and/or superlattice buffer-layer structures. The bulk material was characterized by delineation and measurement of surface EPD and the observation of overall surface morphology. Data are also presented on the device characteristics of graded-barrier quantum-well laser diodes grown with these same buffer layers in order to determine the correlation between dislocation density and laser threshold current. The various buffer-layer structures were seen to be effective in reducing the defect density and improving the surface morphology of high composition epilayers grown on both HB and LEC substrates. The threshold-current density of the laser diodes, however, was independent of both the type of prelayer and/or substrate utilized.