We investigated the effects of InAlAs graded-buffer growth for InAs layers in association with a fundamental mechanism of reduction in threading dislocation (TD) density by the graded buffers. Four InAs layers were grown on semi-insulating GaAs (0 0 1) substrates by direct and graded-buffer growth using molecular beam epitaxy. The materials were characterized by using atomic force microscopy (AFM), plan-view transmission electron microscopy (PTEM), and X-ray diffraction (XRD). We found that the graded-buffer growth is ineffective for the InAs on GaAs; it is invalid to reduce the InAs near-surface TD density in comparison with the direct growth, despite its effectiveness for InGa(Al)As below the intermediate indium contents. This can be attributed to a driving force of upward moving of TDs above the intermediate indium contents due to alloy hardening, while the direction of the driving force is downward below the intermediate indium contents. (c) 2006 Elsevier B.V. All rights reserved.