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We present an infrared spectroscopy and X-ray diffraction study of hafnium oxide gate dielectric films deposited from hafnium tetra–tert-butoxide, Hf(OC(CH3)3)4. We characterize the crystal phase as a function of thickness and detect the chemical state of impurities in this high-permittivity (high-κ) material. The HfO2 films are composed of monoclinic crystallites in an amorphous matrix. The crystalline fraction increases with increasing film thickness. Infrared spectroscopy is used to obtain information about impurities, which may help to uncover the nature of electrical defects. We detect and quantify for instance the presence of incorporated hydroxyl groups or water that may be responsible for the deviation from ideal HfO2 stoichiometry. The concentration of residual C&z.sbnd;H bonds is low. However, carbon is incorporated in an oxidized form, possibly as carbonate. [Copyright &y& Elsevier]