Si films were deposited on Ge substrates at 400 degrees C by two different Physical Vapor Deposition techniques : (A) ion beam sputtering and (B) magnetron sputtering. The intrinsic stresses in the as-deposited films were measured to be compressive and much greater in samples (A), about - 1500 to - 2000 MPa than in samples (B), about - 300 to - 500 MPa. The substrates were subsequently exposed to thermal treatments for varying times at 800 degrees C. In the lower stressed (B) samples, the films had relaxed and reduced the overall curvature of the structure whereas in the high stresses (A) samples, an irreversible large increase in the substrate curvature was found to occur. This indicated that plastic deformation in the Ge substrates itself had occurred.