Electron-spin-resonance monitoring of Si dangling-bond-type interface defects is used to study ultrathin (similar to 20 Angstrom) Si-oxide films grown by vacuum ultraviolet (VUV)-enhanced oxidation of Si at 300 K. 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 (111) and (100) Si/SiO2, respectively. As compared to standard thermal Si/SiO2, two major differences emerge: the VUV Si/SiO2 interface is under substantially enhanced stress, while no P-b1 defects are discerned in VUV (100) Si/SiO2. It is inferred that P-b1 generation requires a minimum amount of oxide relaxation. Microscopic understanding is provided for the known inferior electrical interface quality threatening low-thermal-budget oxide fabrication. (C) 2000 American Institute of Physics. [S0003-6951(00)00834-2].