Low-temperature (LT) passivation methods (<450oC) for decreasing defect densities in the material combination of silica (SiOx) and silicon (Si) are relevant to develop a diverse technology (e.g. electronics, photonics, medicine), where defects of SiOx/Si cause losses and malfunctions. Many device structures contain the SiOx/Si interface(s), of which defect densities cannot be decreased by the traditional, beneficial high temperature treatment (> 700oC). Therefore, the LT passivation of SiOx/Si has been, since long, a research topic to improve applications performance. Here, we demonstrate that a LT (<450oC) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiOx/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and pre-oxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now not reported crystalline Si oxide phase. This crystalline SiOx phase can explain the observed decrease in the defect density by halve. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by factor of two.