We have studied the formation mechanisms and the structural features of Ge islands grown by Reduced Pressure-Chemical Vapor Deposition onto Si(001) substrates. The size, the shape, and the density of Ge islands change drastically when altering parameters such as the growth temperature or the Ge coverage (in-between 3 and 15 monolayers (ML)). For temperatures either equal to 600 degrees C or 650 degrees C, pyramids with 1105 facets are nucleated first. They then gradually change into larger size domes as the amount of deposited Ge increases. Most probably because of longer Ge atoms diffusion lengths, the islands are less numerous (by a factor of 5) and larger (25% increase in diameter and height) at 650 degrees C than at 600 degrees C. At 550 degrees C, Ge hut clusters are nucleated first; then, small domes appear as the number of Ge monolayers increases. Those islands, although much denser, are rather smaller than the counterparts at higher temperatures. We have also studied Si-capped Ge islands (grown at 600 degrees C) in photoluminescence. The Ge No Phonon (NP) and Transverse Optical (TO) lines shift to lower energies as the amount of Ge increases, which is most probably due to bigger islands which are richer in Ge when capped. For 15.3 ML, the NP line and its TO replica are centered at 9.5 K on 835 meV and 795 meV, respectively. (c) 2004 Elsevier B.V. All rights reserved.