Using transmission electron microscopy images, the threading dislocation density was studied near the surface of a buffer layer consisting of an Al0.10In0.90Sb matrix layer and two AlxIn1-xSb interlayers grown on a GaAs (001) substrate. The aluminum composition x in the interlayer was varied from 0.10 to 0.30 with an increment of 0.05. The buffer layer with two Al0.25In0.75Sb interlayers showed the lowest threading dislocation density above the top interface of the second interlayer. An InSb quantum well grown on a 1.5-mu m-thick buffer structure with two Al0.25In0.75Sb interlayers has a 21% higher electron mobility at 65 K than an InSb quantum well grown on a 1.5-mu m-thick Al0.10In0.90Sb buffer without an interlayer. (c) 2007 American Vacuum Society.