In order for germanium (Ge) to replace silicon in advanced MOSFET channels, proper passivation of Ge is required. For this purpose, an ultrathin epitaxial Si cap was grown on Ge(001), and we applied second harmonic generation (SHG) in order to probe the Si/Ge interface quality. SHG indicates a better interface quality for a growth temperature of 500°C rather than 450°C. Similarly, a better quality of the interface is observed upon replacing the conventional H2 carrier gas with N2. Additionally, from the SHG signal, we were able to extract both the thickness of the native SiO2 layer (~4 monolayers (MLs)] and the thickness of the strained Si layer (relaxation at ~12 MLs). These results are important for building Ge-based electronic components.