Relaxation of 110 nm thick Si0.72Ge0.28 epitaxial layers grown on Si (100) substrate by molecular beam epitaxy have been studied by a variable energy slow positron beam. He-implantation at 18 keV energy and 2-3 x 10(16) ions cm(-2) fluence and subsequent annealing has been used to create a defect zone with large cavities in the substrate, at approximately 180-240 nm from the surface, in order to enhance strain relaxation in the SiGe layer. Positrons detected a significant difference in the cavity profile between single implantations with 2 x 10(16) and 3 x 10(16) ions cm(-2) fluence. e(+) trapping in the SiGe layer and at the interface was found to be below the limit of the resolution of the slow positron technique. (C) 2002 Elsevier Science B.V. All rights reserved.