Synthesis And Properties Of Ferroelectric Perovskite Oxide Thin Films

The growth of oxide thin films has long been of great interested for materials scientists for the application in advanced technologies such as nanoelectronics, photovoltaics and sensors. Atomic layer deposition (ALD) is known for being a non-expensive, low-vacuum and low-temperature deposition method able to conformally coat even high-aspect-ratio materials and structures. ALD has been mainly applied to prepare binary oxides such as ZnO and TiO2, and the synthesis of functional complex oxides such as BiFeO3 and CoFe2O4 has little been studied. In the first part of this work, the application of ALD to the preparation of complex oxide thin films based on the Bi-Fe-O system is explored. The important aspects of the ALD approach such as (a) the growth of amorphous Bi-Fe-O by ALD, (b) the epitaxial crystallization of the multiferroic perovskite BiFeO3 films on different substrates, (c) appearance of impurity phases in the films are addressed. Ultimately, the dissertation aims at dispelling the widely held notion that atomic layer deposition is not appropriate for attaining high-quality chemically complex oxide films in the epitaxial form, demonstrating the applicability as an inexpensive, facile, and highly scalable route. The second part of this work is devoted to the preparation and characterization of new ferroelectric thin films as photovoltaic materials. Large optical band gaps of ferroelectric oxides make the optical absorption impractical for solar cell applications. Recently a new strategy for band gap lowering by doping the perovskite KNbO3 with Ba on the A-site and Ni on the B-site resulting in the generation of Ni2+-O vacancy pairs has been shown to significantly increase the optical absorption without loss of ferroelectricity. Using pulsed laser deposition, the synthesis approach to thin films of these new oxides was developed despite significant challenges for the stoichiometry control. The chemical doping approach to the reduction of the band gap was extended to produce new visible-light-absorbing ferroelectric thin films in another perovskite oxide. These materials showed a switchable photovoltaic effect under the visible light illumination along with the retention of the ferroelectric order. This dissertation demonstrates the feasibility of thin film deposition of aforementioned doped ferroelectric oxide perovskites and their ferroelectric photovoltaic properties, which can be extended to other ferroelectric oxides, and provide important information on the synthesis of such materials as thin films.