The authors present a spectroscopic study of defects in HfO2, Hf0.8Si0.2O2, Al2O3, and SiO2 dielectric gate stacks. The results indicate that all optically observable dielectric-related defects are associated with the interfacial SiO2 layer rather than the bulk high-k film. The identified defects, located at 2.9, 3.5, 3.9, and 4.75 eV within the dielectric film's band gap, are found to be strongly affected by subsequent postdeposition anneal treatments and trend consistently with recent electron spin. resonance results and theoretical calculations of optical transitions associated with negatively charged vacancies in SiO2 media. The close connection between our results and both atomistic calculations and experimental findings motivates the use of spectroscopic ellipsometry as a potential in-line characterization method for identifying process-induced defects during complementary metal oxide semiconductor device fabrication. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3021045]