X-ray standing wave analysis of the effect of isotopic composition on the lattice constants of Si and Ge

The x-ray standing wave (XSW) technique is used to measure the isotopic mas s dependence of the lattice constants of Si and Ge. Backreflection allows s ubstrates of moderate crystallinity to be used while high order reflection yields high accuracy. The XSW, generated by the substrate, serves as a refe rence for the lattice planes of an epilayer; of different isotopic composit ion. Employing XSW and photoemission, the position of the surface planes is determined from which the lattice constant difference Deltaa is calculated . Scaled to DeltaM = 1 amu we find (Deltaa/a) of -0.36 x 10(-5) and -0.88 x 10(-5) for Ge and -1.8 x 10(-5) and -3.0 X 10(-5) for Si at 300 and 30 K, respectively.