Record-high minority carrier lifetimes exceeding 10 ns are reported for GaAs grown on Si wafers using compositionally graded GeSi buffers coupled with monolayer-scale control of the GaAs/Ge interface nucleation during molecular beam epitaxy. The GaAs layers are free of anti-phase domain disorder, with threading dislocation densities at or below 2 x 10(6) cm(-2) Secondary ion mass spectroscopy (SIMS) reveals that crossdiffusion of Ga, As and Ge at the GaAs/Ge interface formed on the graded buffers are below detection limits in the interface region. Test diodes yielded excellent I-V characteristics, matching those of GaAs diodes on Ge and GaAs wafers, indicating that to a first order, threading dislocations do not limit device performance. (C) 2001 Elsevier Science B.V. All rights reserved.