Microstructure evolution and crystallisation kinetics of pure and isovalently substituted lead titanate thin films derived by chemical solution deposition

Electroceramic thin films are an essential component in modern technologies of many kinds. In this regard, perovskite materials based on lead titanate (PbTiO3) are especially relevant as functional layers in piezoelectric, ferroelectric, pyroelectric, and other devices. The industrially most important lead titanate derivative nowadays is Pb(Zr1-xTix)O3 (PZT). In order to enhance the efficiency of the films and, hence, of the entire module, detailed understanding of the material and its preparation process as well as knowledge of the fundamental influence factors are crucial. In the case of complex materials such as ternary or quaternary compounds, it is useful to extensively characterize a base material first and successively identify the impact of further constituents. Among the diverse physical and chemical deposition techniques for thin films, chemical solution deposition (CSD) is distinguished by various benefits including easy composition control, high availability of the starting materials, relatively low technical demands, and therefore high cost-effectiveness. As the name implies, the CSD method is based on a solution which contains the constituents of the desired film material as compounds referred to as precursors. In general, the CSD procedure consists of three steps: precursor solution synthesis, film deposition, and thermal treatment. Multiple repetitions of the latter two allows for a stepwise increase of the layer thickness. The scope of this study was the development of CSD preparation routes for lead titanate thin films on platinised silicon substrates and their methodical characterisation. Initially, process parameters were to be found that facilitate the fabrication of homogeneous films with excellent electrical, in particular ferroelectric properties. This task involved neutralising the large mechanical stress that arises upon formation of the tetragonal room temperature phase with a distortion of 6% related to the cubic lattice constants. Different approaches using molecular precursor solutions as well as nanoparticle dispersions were realised. The microstructural and electrical characteristics of the films were determined and correlated to the respective processing parameters. Furthermore, the transformations induced at elevated temperatures were studied including the evolution of intermediate phases and the kinetics of decomposition and crystallisation events. As suggested above, the obtained results serve as a fundament for equivalent examinations of partially substituted lead titanate layers. As an example of such a mixed system, (Pb1 xBax)TiO3 was chosen in this study which represents a solid solution of lead titanate and the well-known and widely used ferroelectric barium titanate. Systematic investigation of the introduced routes substantiated the feasibility of preparing morpho-logically and electrically dense lead titanate thin films by means of the molecular precursor solutions (MP) as well as the nanoparticle dispersions (ND). As for the established MP type routes, the precondition to achieve this goal was the crystallisation of every individually deposited coating and the restriction of their thickness to a maximum of 30 nm each. In contrast, the ND precursors enabled the deposition of crack-free films of more than 100 nm in thickness that were obtained by a single cycle of the CSD routine. In addition, the heating parameters (above all the rate) were attuned to the combustion sequence of the precursors. In this way, continuous layers with a columnar morphology were realised whose grain size amounted to a few hundreds of nanometres. Under optimised processing conditions, the as-prepared layers of both precursor types showed remanent polarisation values of up to 55 µC cm 2. Compared to literature results on CSD derived layers, this value approaches the theoretical magnitude of about 80 µC cm 2 to considerably larger extent. The analysis of the gaseous reaction products evolving during the synthesis of the molecular precursor solutions and the annealing of the films allowed for conclusions on the subsiding chemical conversions. In so doing, the mechanism that gave rise to the formation of a clear sol-gel type solution from the partially solid reactants was identified. Accordingly, the powder lead(II) acetate and a liquid titanium alkoxide reacted to an ester and, more importantly, a compound that comprised a Pb O Ti linkage and thus constituted the basic element of the final PbTiO3 film. Besides the formation of heteroleptic alkoxides by alkoxy group exchange with the alcohol molecules of the applied solvent, this cross-linking to heterometallic oxygen-bridged bonds represented the main chemical transformation under heat-treatment before the organic residues decomposed to carbon dioxide at more elevated temperatures. In analogy to earlier reports on PZT films deposited by CSD, the choice of platinum-coated silicon wafers as substrates led to the evolution of an intermediate alloy layer of lead and platinum during the thermal treatment. In this work on pure PbTiO3 films, it was achieved to specify its composition to alter from the initial stoichiometric phase PtPb to a more Pt rich composition because of diffusion of Pb atoms into the substrate. The formation of the intermetallic phase was preceded by the reduction of the divalent lead ions to the atomic state at the film substrate interface in the course of the organics burnout. When this combustion was complete, the PtxPb phase was decomposed due to the re-oxidation of the lead atoms in consequence of the increased oxygen partial pressure. Creating a persistent reducing atmosphere provided for an extended existence of the intermediate phase. In this way, a preferred [111] orientation of the PbTiO3 films was obtained owing to the minimal lattice mismatch with the (111) lattice plane distances of the PtxPb phase. Kinetic studies of the perovskite crystallisation in the films and the precursors for the first time re-vealed the consistence of the respective activation energies associated with the structural rearrange-ments in the layers and the bulk-like samples. The corresponding value calculated for the ND type precursors was found to be the lowest in the series including the MP routes. Most likely, the development of a nanocrystalline phase of similar composition as PbTiO3 during the nanoparticle synthesis considerably reduced the energy barrier for the perovskite crystallisation. The nucleation of the PbTiO3 grains in the films was determined to take place instantaneously. Subsequently, laminar nuclei formed at the film substrate interface which, after impingement, grew in a diffusion-controlled process one-dimensionally towards the film surface. This proposed model based on the computed kinetic parameters is in good agreement with the columnar morphology of the crystalline films and represents a novel perception of this interrelation. In comparison to earlier studies, the systematic results on the lead titanate films allowed for a more specific evaluation of the impact of isovalent barium substitution. A growing barium content led to the decrease of grain size as well as the tetragonal distortion. Moreover, it also went along with a reduction of the remanent polarization in a way that actually resembled a drop in phase transition temperature to the cubic, non-ferroelectric phase even below room temperature. Depending on the particular composition of the barium titanate precursor solution, the intermediate lead platinum phase was detectable up to an equimolar ratio of lead and barium. Therefore, the nucleation mechanism evidently changed from predominantly heterogeneous at the film substrate interface to increasingly homogeneous in the volume of the amorphous material with increasing barium content.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Dippel, Ann-Christin

Contributors dc:contributor

• Heger, Gernot

• info:eu-repo/classification/ddc/620
• Perowskit
• Titanate
• Ferroelektrikum
• Bleititanat
• Bariumtitanat
• PZT
• Dünne Schicht
• Schichtwachstum
• Keimbildung
• Kristallwachstum
• Kinetik
• Sol-Gel-Verfahren
• Alkoholate
• Pyrolyseprodukt
• Ingenieurwissenschaften
• chemische Lösungsdeposition
• in situ Analyse
• chemical solution deposition
• in situ analysis

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-114024%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A51768