The electrical and chemical properties of ultrathin HfON and HfSiON gate dielectrics are investigated as a function of physical thickness. Grazing incidence x-ray diffraction was used to detect phase separation and crystallization of 1.5, 2.0, 2.5, and 4.0 nm films of HfON and HfSiON after a 1000°C-10 s activation annealing. X-ray photoelectron spectroscopy was used to determine the chemical composition of the dielectrics. No evidence of crystallization was detected in 1.5 nm HfON or HfSiON films after the activation annealing. The HfON film showed crystallization at a 2.0 nm thickness whereas the 2.0 nm HfSiON film remained amorphous. © 2006 American Institute of Physics.