The electron mobility in AlxIn1-xSb/InSb quantum-well structures grown on GaAs substrates is reduced due to the presence of crystalline defects. In structures grown by molecular beam epitaxy, we observe three kinds of defects: hillocks, oriented abrupt steps, and square mounds. The hillocks and oriented abrupt steps are caused by lattice mismatch. The square mounds arise only when AlxIn1-xSb is grown and probably originate near the AlxIn1-xSb/buffer layer interface. The optimum V/III growth rate ratio fur reducing the square-mound density is approximate to 1.1 at a substrate temperature of similar to 440 degrees C.