We present a study of Ge islands formation on Si(100) substrates using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Samples were prepared by magnetron sputtering of a 5 nm thick Ge layer in a very high vacuum on Si(100) substrate held at different temperatures. The vertical cut (perpendicular to the surface) of the experimental 2D GISAXS pattern has been fitted using a Guinier approximation. The optimum temperature for the islands formation was 650 degrees C. At this temperature, islands grow in conical shape with very similar dimensions; however, inter-island distances varied significantly. (c) 2005 Elsevier B.V. All rights reserved.