Titanium assisted dual site doping for enhancement of ferroelectric properties in Ca doped Bismuth Ferrite thin films

The present work explores the novel use of Ca and Ti as substituents for Bi and Fe, respectively, in Bismuth ferrite (BFO) to capture the advantage of the dopants to enhance its ferroelectric properties. The motivation for incorporating Ti is to facilitate Ca doping and compensate for the unbalanced charges generated by aliovalent Ca2+ ions. The structural properties were examined by X-ray diffraction, where the doublet peaks of BFO (104) and (110) merge, indicating a perturbation of the rhombohedral structure and this is supported by Raman studies. This substitution also affects the grain size, restricting growth to below 70 nm. The mutual charge compensation could have stabilized defect concentration, leading to a largely unaffected Fermi level position and thus the bandgap (around 2.6 eV). XPS data showed increased levels of Fe2+ concentration due to Ti replacing Fe3+ as well as incomplete charge compensation by Ti. Essentially, it was only the 2.5% Ca, Ti doping (with 86% Rhombohedral phase and 14% orthorhombic phase) which showed distinct resistive switching and well-saturated loops. However, it must be noted that the applied field levels were restricted due to high leakage current. The 2.5% Ca, Ti doping film showed saturation and remnant polarisation of around 40 μC/cm2. It was observed that with further increase in doping concentration, the ferroelectric property was depleted due to the replacement of the Bi (source of ferroelectricity) and the perturbation to the structure.