Invalidity of graded buffers for InAs grown on GaAs (0 0 1)—A comparison between direct and graded-buffer growth

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.