Recrystallization effects in spray-pyrolyzed Nb2O5 thin films induced by 100 MeV O7+ swift heavy ion beam irradiation

Abstract

In this work, the influence of 100 MeV O7+ high energy swift heavy ion beam irradiation at varied fluences on the modification of properties of spray-deposited thin niobium pentoxide (Nb2O5) film is reported. The XRD patterns indicate tetragonal structure for pristine polycrystalline Nb2O5 film and partial amorphous nature for the irradiated samples in corroboration with the Raman spectra of the respective samples. The crystallite size is observed to reduce overall compared to that of the pristine film, however, a peaking is observed for 5x1012 ions/cm2 fluence within the irradiated films which is due to the high energy irradiation-induced defect formation and recrystallization effects. It is important to note that the defect density increases with fluence and recrystallisation occurs at 5x1012 ions/cm2, with further amorphization induced at much higher fluences. The generation of additional defect levels lowers the optical band gap. The direct and indirect band gaps were found to reduce upon irradiation compared to that of the pristine Nb2O5 film. The atomic force microscopic analysis reveals decrease in surface roughness of the irradiated samples from 5.9 nm to 10.1 nm compared to the pristine sample. Oxygen ion beam changes their morphology into network-like structures in a controllable manner which mainly depends on electronic energy loss occurring within the Nb2O5 lattice. Variations in transport parameters measured using Hall effect reveal that lower fluence yields less damage accumulation and does not cause significant change in properties of the sample. As fluence increases, vivid modifications originate from the production of significant density of point defects. At the critical fluence, recrystallisation is observed. These results are discussed in detail using the calculated parameters like crystallite size, Urbach energy, defect density and by correlating them with the observed optical and electrical data obtained from different techniques.