Hafnium oxide (HfO₂) has attracted much interest as high-k material of choice for gate oxide replacement in future CMOS technologies and for its use in optical coating technology. The determination of optical properties, like refractive index and band-gap is focus of intense research, since the optical constants of HfO₂ depend on the physical microstructure and the deposition methods and conditions. In particular, HfO₂ ultrathin films use to show significantly different properties than the bulk material.

In the present work the optical characterization of very thin HfO₂ films deposited by plasma ion assisted deposition and annealed at different temperatures is carried out. The characterization is performed using ellipsometric measurements in the spectral range form 1.5 to 8 eV and employing two different approaches: i) fitting of the data using a dispersion model to describe the optical constants of HfO₂, and ii) the direct inversion of the ellipsometric data. In particular, two dispersion models are tested: Tauc-Lorentz and Cody-Lorentz. These models, intended to describe the optical interband transitions of amorphous materials, can be extended to include of sub-gap absorption in terms of an exponential Urbach tail. The validity of the models is analyzed according to the fitting quality and the confidence limits and correlation of the parameters defining the dispersion model. On the other hand, the direct data inversion is shown to be useful to provide additional information like the presence of sub-gap absorption and point out features associated to the crystallinity of the material.

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