Superior electrical and optical quality of GaAs grown on Si with an inserted low-temperature (LT)-GaAs buffer-layer was demonstrated. Photoluminescence intensity was increasing and leakage current of Schottky diodes build on such a structure was decreasing by few orders of magnitude. These observations were correlated with structural studies employing classical and high-resolution transmission electron microscopy (TEM). Bending of the threading dislocations and their interaction was observed at the interface between a cap GaAs layer and the LT-GaAs layer. This dislocation interaction results in the reduction of dislocation density by at least one order of magnitude or more compared for the GaAs layers with the same thickness grown on Si. The surface morphology of the cap GaAs layer is improving as well.