Structural disorders in an amorphous HfO₂ film probed by x-ray absorption fine structure analysis

The local structural order of amorphous HfO₂ films (a-HfO ₂) was examined using Hf L₃-edge x-ray absorption spectroscopy and fine structure analyses. The fine structure simulation successfully reproduced the spectral evolution of the crystalline-to-amorphous phase transition by reducing the characteristic radius for atomic ordering to ∼3.5 Å. Detailed path-by-path analyses further showed that the vibrational displacement of oxygen atoms in a-HfO₂ films is highly anisotropic showing mainly lateral dispersion perpendicular to a Hf-O bond. This anisotropic structural disorder is responsible for enhancing the dielectric constant accompanying phonon mode softening in the a-HfO₂ film.