This paper reports our detailed investigation of the microstructural evolution of Ge/Si(1 0 0) nanoscale islands grown by deposition of pure Ge onto Si(1 0 0) substrates using molecular beam epitaxy. Substrate temperatures during growth were varied in the range 400 less than or equal to T less than or equal to 700degreesC, although we have mostly concentrated our attention here on studying samples grown at T = 550degreesC, 650degreesC and 700degreesC. Atomic-force microscopy was first used ex situ to document the shape and size evolution of the Ge/Si(1 0 0) islands as a function of the growth conditions (Ge coverage, substrate temperature). A range of transmission electron microscopy techniques including energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy was then used to further investigate the local microstructure and composition of the islands. Substantial Si diffusion into the Ge islands was measured, and trenches extending well into the Si substrate were also observed at the bases of larger clusters grown with T greater than or equal to 550degreesC. Plan-view imaging of selected samples using both bright-field and dark-field imaging modes identified the critical size for dislocation formation. Cross-sectional high-resolution imaging enabled the strain-relieving dislocations to be identified, and also confirmed the multi-facetted and often asymmetrical shapes of larger islands. (C) 2003 Elsevier B.V. All rights reserved.