High quality relaxed Si0.6Ge0.4 films as thick as 1.0 mu m have been achieved as grown on silicon-on-insulator (SOI) substrates with 200 Angstrom Si(100)-oriented compliant substrates. This represents the thickest relaxed layers grown by this technique. A greater than five order of magnitude reduction in threading dislocation density was achieved by compliant growth compared to growth on unmodified Si(100) substrates. Additionally, for the first time (to the best of our knowledge) we show that it is not necessary to separate the growth and relaxation processes, and relaxation during growth is governed by the nature of the compliant substrate structure that causes dislocations to terminate at the unique crystalline-amorphous SiO2 interface. Results indicate that utilizing SOI as a compliant substrate which is effective in producing high quality Si1-xGex films can be extended to other films, where film relaxation (via dislocation nucleation and growth) cannot be conveniently separated,from the synthesis method (e.g., SiC).