We studied on the thermal stability improvement of Si(1-y)C(y) against high temperature annealing by using SiO(2) cap layers. The Si(1-y)C(y) films with a carbon concentration of 1.0% were grown at 620 degrees C by a gas source MBE. From XRD measurements, the layer with a carbon concentration of 0.79% formed in the Si(1-y)C(y) films without SiO(2) cap layers after annealing at 900 degrees C and it indicated the segregation of carbon atoms. The 100-nm SiO(2) cap layers effectively prevented the segregation of carbon atoms when the Si(1-y)C(y) film thickness was less than its critical thickness. X-ray reciprocal lattice space maps around (115) reciprocal lattice points supported these phenomena, showing that the SiO(2) cap layer improved the thermal stability of Si(1-y)C(y) films against annealing at 900 degrees C. (c) 2008 Elsevier B.V. All rights reserved.