Metal organic chemical vapor deposition of multifuctional perovskitic oxides: from giant K to colossal magnetoresistance materials, from ionic conductors to multiferroics.

Abstract

In the last years, mixed oxides with perovskite structure ABO3 (A=alkaline metal or alkaline earth or rare-earth metal, B= transition metal) have attracted increasing interest due to their several magnetic and electrical properties, which make them particularly interesting candidates in a variety of technological applications. Many of the physical properties of perovskites depend crucially on crystal structure and on the possibility to vary their composition by the partial substitution of cations occupying the A and/or B sites. In fact such substitutions are responsible for the distortion of perovskite octahedra and /or oxygen vacancy creation, which affect the functional property. In this context, the following work focuses on some different perovskite materials such as calcium copper titanate (CaCu3Ti4O12)(1-3), bismuth ferrite (BiFeO3)(4,5), lanthanum cobaltite (LaCoO3),(6) barium ceriate (BaCeO3) and Ca-doped praseodymium manganites (Pr1-xCaxMnO3) (6), which possess challenging physical properties of scientific and technological interest. Suitable Metal Organic Chemical Vapor Deposition (MOCVD) approaches have been optimized to fabricate the high quality films of the above mentioned complex oxides. The structural and morphological characterization of films have been carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Additional information regarding the epitaxial growth of thin film have been obtained using transmission electron microscopy (TEM) and pole figure analysis. Chemical composition has been assessed through energy dispersive X-ray analysis (EDX), while the X-ray photoelectron spectroscopy (XPS) has been used to evaluate the element oxidation states and to confirm the sample purity. The experimental results obtained by structural, morphological and compositional characterizations have shown that good quality perovskite thin films have been satisfactorily and reproducibly deposited through the present MOCVD approach. The epitaxial pure CaCu3Ti4O12 thin films have been obtained on different substrates, single crystal SrTiO3 (100) and on La0.9Sr1.1NiO4/LaAlO3 conductor stack. The study has focused on the interplay between kinetic and thermodynamic factors in stabilizing the CaCu3Ti4O12 phase. The functional properties of the Pr1-xCaxMnO3 thin films have been investigated through magnetic measurements. This data evidence the main characteristics of the thins films, i.e. the presence of a FM transition temperature that has been correlated to the transport properties of the film. The approach used to obtain LaCoO3 thin film has the advantage of being a very simple method for the easy production of lanthanum cobaltites and makes it possible to control simultaneously the evaporation rates of the La and Co precursors, thus avoiding any problems related to the crystallite size, resulting in a constant evaporation rate even for very long deposition time. In regards to BiFeO3 thin films, the piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS) investigations suggest that within the present MOCVD approach, obtained films possess good piezoelectric and ferroelectric properties. In particular the obtained data indicate that deposition temperature plays a crucial role since films obtained at high temperature possess better ferroelectric properties.