Multiferroic BiFeO3 films and nanostructures for hybrid energy harvesters

As part of the MSCA-ITN ENHANCE project, which aims to develop and integrate lead-free hybrid energy harvesters for the car industry, the following work has been focused on the deposition by metal-organic vapor phase deposition of multiferroic BiFeO3 films and nanostructures. BiFeO3 is a multiferroic material and can maintain its properties in extreme environments thanks to its Curie and Neel temperatures well above room temperature. Photoelectric, pyroelectric, and piezoelectric properties are the most appealing characteristics for creating a BiFeO3 based hybrid energy harvester. Different deposition techniques have been used to obtain thin films of BFO and its derived systems. The main deposition techniques applied to the production of BFO films are chemical solution deposition (CSD), pulsed laser deposition (PLD), sputtering and sol-gel. So far, compared to other methods, Metal-Organic Chemical Vapor Deposition (MOCVD) has been relatively unexplored and never used to develop BiFeO3 deposition on silicon to fit the standard of the MEMs industry. In this context, the present thesis has been focused on the deposition of thin-films of pure and doped BiFeO3 systems using a Metal-Organic Chemical Vapor Deposition (MOCVD) approach. MOCVD deposition technique offers a large number of advantages compared to other thin-films production methods with a wide choice of industrial precursors, high-quality substrates, and upscale opportunities. The carried work aims to bring a methodology for the synthesis of BiFeO3 systems thin films that are compatible with conventional industrial processes. Along with studying the deposition protocols within the University of Catania, national and international collaborations have been crucial for the analysis of thin-film structures and functional properties. The versatility conceded by the deposition of thin film by MOCVD allowed the production of a large quantity of simple and complex BiFeO3 systems. As material chemical composition has a critical influence on its properties, precursors’ thermal behavior has been checked by thermogravimetric analysis (TGA) for complex doped systems. The understanding of the films’ chemical compositions and homogeneity has been obtained by energy-dispersive X-ray analyses (EDX) and X-ray ray photoelectron spectroscopy (XPS). The polycrystalline or epitaxial nature of the films has been controlled by X-ray diffraction (XRD) and transmission electron microscopy (TEM) in the case of intricate nanostructures. Morphological study of the materials has been done by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Coupled to the AFM, piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS) have permitted the imaging of ferroelectric domains and the confirmation of the films’ piezoelectric properties. The first part of the thesis is dedicated to the deposition of BiFeO3 on Si-based substrates and how to optimize its integration as a functional device. The second chapter is focused on the depositions of BiFeO3 epitaxial films on SrTiO3 single crystal. As doping is the most common and simple way to tune its properties, BiFeO3 A-site doping with rare earth elements has been intensively studied. This work focused on dopant from the lanthanide series with Neodymium (Nd), Samarium (Sm), Dysprosium (Dy), and Ytterbium (Yb). Along with depositions tuning, film morphologies, compositions, and functional properties have been investigated. The A-site of the BiFeO3 perovskite structure can be substituted, but B-site doping is also possible. Ba and Ti co-doping of the A- and B- site of the BiFeO3 has been explored. Impact of precursors ratio used during the deposition and in-depth structural characterization of the self-growth solid-solution or nanocomposite films are presented. The last part of the investigation is about the use of cobalt (Co) as a dopant. In the case of cobalt doping leading to a solid-solution, Co is substituting the B site of the perovskite and has an important influence on the material magnetic properties. Careful control of the process conditions allows the deposition of BiFeO3-CoFe2O4 nanocomposite thin-films in a single step MOCVD process.