Grain fragmentation and phase transformations in hafnium oxide induced by swift heavy ion irradiation

We report on the 100 MeV Ag ion irradiation-induced modifications in hafnium oxide films deposited by radio frequency (RF) magnetron sputtering method. The phase transformations and grain fragmentation within the films induced by ion irradiation have been investigated by employing grazing incident X-ray diffraction (GIXRD) and field emission scanning electron microscope (FESEM). The FESEM analysis confirms the grain fragmentation as function of ion fluence. The grain size varied from 70 to 15 nm with various irradiation fluences. The photoluminescence (PL) at 377 nm and 420 nm indicates the shallow oxygen defects and F-centers present in hafnium oxide films. After irradiation, the sharp peak at 377 nm disappears completely due to the ion-induced reconfiguration of defects. Sharp thermal gradients due to ion-induced electronic excitation can lead to the observed phase transformations and grain fragmentation in films. Particularly, the anisotropic thermal expansion in the nano-grains can cause grain fragmentation. Ion-induced defect annihilation, strain relaxation, phase transformation and grain fragmentation are interdependent processes and their mutual dependency is responsible for the observed changes in the films.