Ultra-thin (similar to2 nm) Si-oxide films, grown by vacuum ultraviolet (VUV) enhanced oxidation of Si at 300 K, were studied using electron spin resonance monitoring of Si dangling bond-type interface defects. As a major impact of VUV photons, large densities (up to similar to 9 x 10(12) cm(-2)) Of P-b and P-b0 centers (interfacial (Si-3=Si .)) are observed in VUV-grown (I 1 1) and (1 0 0) Si/SiO2, respectively. Their features indicate that, as compared to standard thermal Si/SiO2, the VUV Si/SiO2 interface is under much enhanced stress. No P-b1 defects are observed in VUV (1 0 0) Si/SiO2, ascribed to lack of high temperature oxide relaxation. This may appear pertinent as to the understanding of the defect's specific role in the interface structure. Microscopic understanding is provided for the known inferior electrical interface quality threatening low thermal budget oxide fabrication.