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Electrical and material characteristics of hafnium oxynitride (HfON) gate dielectrics have been studied in comparison with HfO2. HfON was prepared by a deposition of HfN followed by post-deposition-anneal (PDA). By secondary ion mass spectroscopy (SIMS), incorporated nitrogen in the HfON was found to pile up at the dielectric/Si interface layer. Based on the SIMS profile, the interfacial layer (IL) composition of the HfON films appeared to be like hafnium-silicon-oxynitride (HfSiON) while the IL of the HfO2 films seemed to be hafnium-silicate (HfSiO). HfON showed an increase of 300°C in crystallization temperature compared to HfO2. Dielectric constants of bulk and interface layer of HfON were 21 and 14, respectively. The dielectric constant of interfacial layer in HfON (∼14) is larger than that of HfO2 (∼7.8). HfON dielectrics exhibit ∼10× lower leakage current (J) than HfO2 for the same EOTs before post-metal anneal (PMA), while ∼40× lower J after PMA. The improved electrical properties of HfON over HfO2 can be explained by the thicker physical thickness of HfON for the same equivalent oxide thickness (EOT) due to its higher dielectric constant as well as a more stable interface layer. Capacitance hysteresis (ΔV) of HfON capacitor was found to be slightly larger than that of HfO2. Without high temperature forming gas anneal, nMOSFET with HfON gate dielectric showed a peak mobility of 71 cm2/Vsec. By high temperature forming gas anneal at 600°C, mobility improved up to 256 cm2/Vsec.