The photoluminescence (PL) and structural characterization of Ge/Si quantum dots (QD) samples grown by MBE is presented. The structural characterization was performed by means of high-resolution X-ray diffraction (reciprocal space mapping and rocking curves) and model-fitting of X-ray reflectometry. The observation of a diffraction peak associated with the QDs made possible the determination of their average Ge concentration. The PL measurements identified two subsets of QD. The dependence of the QD emission intensity on the measurement temperature can be well explained by the calculated density of states published previously [9].