The effect of Ti⁺ ion implantation on the anatase-rutile phase transformation and resistive switching properties of TiO$2$ thin films

We investigate here the structural phase transformation and electrical resistive switching properties of TiO$2$ thin films (80 nm) after their self-ion implantation with 50 keV Ti$+$ ions at several fluences. UV-Raman, grazing incidence x-ray diffraction (GIXRD), transmission electron microscopy, x-ray photoelectron spectroscopy, and atomic force microscopy techniques have been utilized to investigate the modifications in thin films. Both, the as-grown and ion implanted, films display mixed phases of rutile (R) and anatase (A). Surprisingly, however, a phase transition from A to R is observed at a critical fluence, where some anatase content transforms into rutile. This A to R transformation increases with additional fluence. The critical fluence found by GIXRD is slightly smaller (⁠$1×1013$ ions/cm$2$⁠) than from UV-Raman (⁠$1×1014$ ions/cm$2$⁠), indicating the first initiation of phase transformation probably in bulk. All the films contain anatase in nanocrystalline form also and the phase transformation seems to take place via aggregation of anatase nanoparticles. Thin films also show the presence of oxygen vacancies (O$V$⁠) Ti$3+$⁠, whose number grows with fluence. These O$V$ as well as thermal spikes created during Ti$+$ ion implantation are also crucial for the A-R transition. After implantation at the highest fluence, TiO$2$ thin films show bipolar resistive switching behavior. The development of conducting filaments, formed by the migration of many oxygen vacancies generated during ion implantation, can be responsible for this behavior.