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Hafnium oxide (HfO 2 ) gate dielectric film was prepared by Hf sputtering in oxygen, and the thermal instability of HfO 2 was investigated by rapid thermal annealing ͑RTA͒ in nitrogen. X-ray photoelectron spectroscopy study reveals that the HfO 2 film is thermally unstable at postmetallization annealing temperatures (Ͼ500°C). The HfO 2 film decomposes and some oxygen atoms are released upon the RTA in nitrogen. In addition, the current-voltage characteristics of the Al/HfO 2 /Si capacitor are also highly unstable at temperatures higher than 300 K. These observations suggest that although HfO 2 has a much higher dielectric constant, it may not be suitable for the gate dielectric application because the postdeposition thermal treatment deteriorates both the physical and the electrical properties of the HfO 2 film.

The atomic structure of amorphous and crystalline hafnium oxide ͑HfO 2 ͒ films was examined using x-ray diffractometry and Hf edge x-ray absorption spectroscopy. According to the x-ray photoelectron spectroscopy and band data calculated by the density functional method, we found that the valence band of HfO 2 consists of three subbands separated by ionic gaps. The upper subband is formed by O 2p, Hf 4f, and Hf 5d states; the intermediate subband is formed by O 2s and Hf 4f states, whereas the lower narrow subband is mainly formed by Hf 5p states. The energy gap of amorphous HfO 2 is 5.7 eV as determined by electron energy loss spectroscopy. The band calculation results indicate the existence of light ͑0.3m 0 ͒ and heavy ͑8.3m 0 ͒ holes in the HfO 2 film and the effective mass of electron lies in the interval of 0.7m 0-2.0m 0 .

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