In situ real time single wavelength ellipsometry and spectroscopic ellipsometry were used to monitor electron cyclotron resonance plasma enhanced chemical vapor deposition of SiO2 on Si and Ge at substrate temperatures from floating temperature (<50 °C) to 400 °C. Dielectric layers were evaluated by capacitance–voltage measurements. Single wavelength ellipsometry and x-ray photoelectron spectroscopy results show that oxidation occurred along with SiO2 deposition at the initial stage of plasma enhanced chemical vapor deposition. The real time oxidation analysis shows that the subcutaneous oxidation follows parabolic growth kinetics during SiO2 deposition. The parabolic rate coefficient is proportional to the voltage drop across the subcutaneous oxide layer, and yields temperature activated transport. The subcutaneous oxidation of Ge yields a Ge rich oxide that degrades the electrical properties of metal–oxide capacitors. A thin Si layer deposited at room temperature before SiO2 deposition protects the Ge surface from undesirable oxidation during SiO2 deposition. The required thickness of Si layer is predicted. Electron cyclotron resonance plasma oxidation at 400 °C after SiO2 deposition reduces the slow trap states. An Al/SiO2/Si/Ge structure shows improved electrical properties of metal–oxide Ge capacitors. For c-Si substrates, a rapid thermal anneal of the deposited SiO2 in high vacuum can effectively eliminate the OH group incorporation, and thus reduces both interface trapped charge and slow trap states.