Observation of both longitudinal optical and transverse optical phonons of ∼1.3 nm ultrathin silicon dioxide (SiO2) layers formed by immersion in nitric acid shows that the SiO2 density increases by 16% after postoxidation annealing (POA) at 900 °C. For the SiO2 layers without POA, postmetalization annealing (PMA) greatly decreases the SiO2 thickness from 1.3 to 0.2 nm, the effect of which is attributable to the reaction of aluminum with SiO2 to form a metallic mixture of aluminum oxide and Si. For SiO2 layers with POA, PMA decreases the SiO2 thickness to a lesser extent (from 1.4 to 0.9 nm), because of the suppression of aluminum diffusion into SiO2 due to its dense structure. PMA is found to decrease the interface state density but increase the leakage current density. © 2002 American Institute of Physics.