Strain relaxed Si1-yCy films were grown on silicon on insulator (SOI) substrates. The S1-yCy films were deposited by a gas-source molecular beam epitaxy (GS-MBE) system using a gas mixture of Si2H6 and C2H2. The crystal structures and surface morphologies were investigated by X-ray reciprocal lattice space mapping and atomic force microscopy (AFM), respectively. The X-ray reciprocal maps showed that the strain relaxation started at a lower Si1-yCy film thickness by using SOI substrates with thinner Si layer as compared to the case of strain relaxed Si1-yCy films grown on bulk-Si substrates. As a result, we have obtained higher strain relaxation ratio. Surface roughness of strain relaxed Si1-yCy films grown on SOI substrates was also evaluated and it was drastically suppressed by using SOI substrates. These results indicate the use of SOI substrates is effective for the growth of high-quality strain relaxed Si1-yCy films. (c) 2006 Elsevier B.V. All rights reserved.