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A high-quality GaAs epilayer was successfully grown on a Si substrate using a-GaAs/a-Si double buffers, which were crystallized by thermal cyclic annealing. Double crystal x-ray diffraction measurements showed that the full width at half maximum of the rocking curve for the GaAs epilayer was only 102 arcs. The effect of a-GaAs/a-Si double buffers was examined in detail by transmission electron microscopy. Transmission electron microscopy characterizations revealed obvious bending behaviour of dislocations in the a-GaAs/a-Si buffered epilayer. Most threading dislocations and planar defects were confined within the crystallized double buffers. The dislocation density was reduced to 2.6 × 10 6 cm −2 at a thickness of 1.0 µm in the first epitaxial layer and was further improved to under 1.3 × 10 6 cm −2 in the second epitaxial layer. Transmission electron microscopy observation and analysis revealed a defect reduction mechanism.