[en] We have grown modulation-doped In_0.53Ga_0.47As/In_0.52Al_0.48As heterostructures on GaAs substrates using compositionally step-graded In_xGa_1-xAs buffers. Triple-axis x-ray diffraction measurements indicate nearly complete and isotropic strain relaxation in the buffer, lattice matching of the active layers with the top of the buffer, and no significant epilayer tilt. The temperature dependence and the photoresponse of the electron mobility suggest that transport in the heterostructures is limited principally by remote ionized-impurity scattering, with mobility values comparable to those of heterostructures grown lattice-matched to InP. copyright 1996 American Vacuum Society