We have investigated the photoluminescence of self-assembled Ge clusters deposited on Si(001) by chemical vapor deposition. The formation of precursor clusters is observed both by microscopy and photoluminescence before the onset of the 2D layer-by-layer to 3D island growth mode transition. These precursor clusters which are observed at different growth temperatures are found to be metastable. They have a square base shape with a height approximate to 1-2 nm and a base width approximate to 30-15 nm. They exhibit strong three-dimensional confinement. Beyond the 2D-3D growth transition, we observe the formation of islands with a dome shape which exhibit photoluminescence at lower energies as compared to the precursor clusters. The photoluminescence energy is compared to the one deduced from a simulation which accounts for the three-dimensional confinement of the holes.