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Influence of nickel incorporation and thermal treatment under atmospheric conditions and controlled oxygen flow on the ferroelectric and electrical properties of BiFeO3 thin films obtained by sol-gel

Abstract:

In this work, we study the effect of nickel incorporation (0, 2, 4, and 6) % on the structure of sol-gel BiFeO<inf>3</inf> films. Besides, we used thermal treatments in two ambient, namely atmospheric conditions (CA) and controlled oxygen flow (FO2). The films were deposited by spin-coating from a solution based on the chemical formula [Formula presented] that maintains the charge neutrality of the unit cell. Structural analysis by X-ray diffraction and Raman shows the correct incorporation of nickel into the BiFeO<inf>3</inf> structure. Diffractions related to the rhombohedral R3c phase without the presence of secondary phases were observed for the two thermal treatments. The incorporation of nickel was quantified with energy dispersive spectroscopy. The XPS analysis shows that thermal treatment affects the oxidation states of Fe and Ni, influencing lattice parameters, cell volume, and defect formation. We employ advanced atomic force microscopy (AFM) techniques such as piezoresponse, writing and reading of ferroelectric domains, higher-order harmonics, maps and spectroscopy of conductive AFM, and magnetic force microscopy to evaluate the piezoelectric, ferroelectric, electrical, and magnetic properties of the films. It was found that for all nickel incorporations and both thermal treatments, the samples are ferroelectrics with electromechanical deformations between 10 pm/V and 40 pm/V, which was always dominated by the piezoelectric effect and almost negligible contribution by electrostriction. The FO2 treatment provides more insulating films with fewer morphological defects than the CA treatment. Increasing nickel concentration reduces the morphological grain size and the volume of the crystal cell. For the CA treatment, an accumulation of nickel-rich material was observed at the grain boundaries, resulting in a magnetic response that follows the morphological changes. The behavior of the films with nickel incorporation was related to the emergence of bismuth vacancies (V<inf>Bi</inf><sup>´´´</sup>), and oxygen vacancies (V<inf>O</inf><sup>∙∙</sup>) mainly for the CA treatment. Additionally, macroscopic I-V measurements in the dark, UV light, and UV–visible light revealed photosensitivity in samples with the highest nickel incorporation, which may be helpful in photosensitive ferroelectric devices.