Structure evolution, bandgap, and dielectric function in La-doped hafnium oxide thin layer subjected to swift Xe ion irradiation

High-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy provide information on the structural evolution, dielectric function, and bandgap values of nanocrystalline 10nm thick lanthanum doped hafnia (La:HfO2) layers in TiN/La:HfO2/TiN/SiO2/Si irradiated with 24, 46, 72, and 160MeV (0.2-1.2MeV/u) Xe ions. Swift heavy Xe ions were expected to create significant atomic rearrangements when passed through a solid losing energy mainly through electronic excitation and ionization of the target atoms. Local heating and subsequent rapid cooling in the region around the ion track can lead to re-solidification with the formation of a new stable crystalline phase, and direct crystal-to-crystal transformations are possible. The structure evolution of hafnia nanocrystals from the orthorhombic Pbcm in the pristine layer to the tetragonal P4(2)/nmc phase in the 160MeV Xe ion irradiated layer was observed. The mixture of tetragonal and orthorhombic phases was found in samples irradiated with ions of intermediate energies. Textured hafnia layers were formed as a result of ion irradiation. The changes in plasmon line shape and the blueshift of the plasmon energy-loss peak from 14.9eV in the pristine layer to 15.4eV in 46MeV and 15.9eV in 160MeV Xe ion irradiated hafnia reflect structural transformations, the increase in the Hf coordination number, and crystal orientations. Valence-electron energy-loss spectroscopy measurements showed a slight increase in the bandgap value from 6.1eV in the pristine sample to 6.2eV and 6.3eV in irradiated samples with 46 and 160MeV Xe ions, respectively, and dielectric functions changed insignificantly in irradiated hafnia layers.