The influence of defects on the optical properties of a single Ge quantum well deposited on a Si substrate and on a diode structure containing a Si/Ge multilayer structure was investigated. In order to change the density of optically active defects, the as-grown samples were exposed to post-growth treatments: atomic hydrogen passivation and irradiation with 2.0MeV protons to fluences in the range 2×1012 to 1×1014cm −2 . The optical and structural properties were investigated by photoluminescence and X-ray diffraction and reflection measurements. An unexpectedly high radiation hardness was observed for the as-grown Ge quantum wells.