Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 x 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small 'pyramids', small 'd omes' and facetted 'domes' of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of similar to 1 (diameter):1 (height). Observation of truncated- sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported. (C) 2009 Elsevier B. V. All rights reserved.