Effect of Al addition on the microstructure and electronic structure of Hf O2 film

We have investigated the microstructures and electronic structures of a series of hafnium aluminate (HfAlO) films with Al concentration ranging from 0% to 100%. When the films evolve from pure Hf O2 to pure Al2 O3 by increasing the aluminum content, we find changes in their radial distribution functions, which disclose the short-range order of the materials, despite the amorphous nature of all films. The HfAlO films (with AlHf ratio ranging from 0.25 to 5.8) appear to be a single glassy phase of Hf, Al, and O, instead of simple mixtures of Hf O2 and Al2 O3. The Hf (Al)-O, Hf (Al)-Al, and Hf-Hf bonds are observed to be insensitive to the amount of Al in the film, except when the Al concentration is large (AlHf∼5.8), in which case the bonding is similar to that in pure Al2 O3. Although the local symmetry of Hf in amorphous Hf O2 is suggested by the electron energy-loss spectrum taken at an oxygen K edge, it is largely disrupted when Al is introduced. The valence electron energy-loss spectroscopy reveals three distinct evolving features as the Al content increases, which we discuss in terms of the electronic structure of Hf O2.