The interface properties of the hafnium gate oxide films prepared by direct sputtering of hafnium in oxygen with rapid thermal annealing have been investigated in detail. X-ray photoelectron spectroscopy reveals that the interface silicate layer is a random mixture of Hf-O, Si-O, Hf-Si, and excess Hf and Si atoms. The contributions of these bonds to the composition of silicate layer are governed by the Si/Hf ratio. At low Si/Hf ratio (<2), the silicate layer is a mixture of SiO4 and HfO4 phases. At higher Si/Hf ratio (2-5), the contribution of the HfO4 phase decreases and Hf-Si (silicide) bonds become important. At very high Si/Hf ratio (>9) and close to the substrate, Hf-Si dominates and the high strain Hf-Si bonds govern the electrical properties of the interface. These results explain the observed high interface trap density at the HfO2/Si interface and the soft breakdown behavior which is different from the silicon oxide film. (C) 2004 American Vacuum Society.