Strain-phase relations in lead-free ferroelectric KxNa1-xNbO3 epitaxial films for domain engineering

The aim of this thesis is to demonstrate the potential of thin films for technical applications by tuning the ferroelectric film properties on the basis of the strain-phase diagram. For this purpose, a fundamental understanding of the relation between incorporated lattice strain by epitaxial growth of thin films and the evolution of ferroelectric phases has been pointed out. As an exemplary lead-free material, potassium sodium niobate (KxNa1-xNbO3) is considered. This material is of high technological interest due to its large coupling constant and Curie temperature. Furthermore, calculations predict the appearance of monoclinic phases under anisotropic epitaxial lattice strain. They are attractive due to their inherent flexibility for the arrangement of the electrical polarization vector yielding e.g. huge piezoelectric responses and flexible domain wall formation. However so far, thin films have rarely been investigated and the influence of lattice strain on ferroelectric properties has not been studied systematically. In this work, thin films were grown by metal-organic chemical vapor deposition technique (MOCVD) on different oxide single-crystalline substrates. For a targeted choice of appropriate film-substrate combinations, theoretical considerations with regard to film orientation and phase symmetry are essential. To predict the energetically most favorable film unit cell orientation on a substrates, linear elasticity theory was used in this thesis. For engineering the domain structure, a misfit strain-misfit strain phase diagram was calculated for potassium niobate by means of the Landau-Ginzburg-Devonshire (LDG) theory with major accuracy compared to existing predictions. The result manifests a diversity of different, mainly monoclinic phases for KNbO3. By means of the calculated misfit strain-phase diagram, particular KxNa1-xNbO3 compositions were epitaxially grown by MOCVD with nearly perfect structural ordering and stoichiometry. Use of different 0.1° off-oriented (001) SrTiO3, (110) DyScO3, (110) TbScO3, (110) GdScO3 and (110) NdScO3 single-crystalline substrates provides an experimental verification of the calculated strain-phase diagram. This was realized for the first time for the promising KxNa1-xNbO3 material system. As a proof of concept, two different film-substrate combinations have been investigated in detail mainly with the piezoresponse force microscope (PFM). Together with elaborated x-ray diffraction measurements a detailed analysis of the ferroelectric domain structure inherently coupled to the crystal symmetry was possible. First, K0.75Na0.25NbO3 on TbScO3 with nearly uniaxial, medium compressive lattice strain leads to periodically ordered, monoclinic MA stripe domains. A second 90° rotated variant occurs only to a minor fraction which is attributed to the small strain energy density difference of both (001)pc orientations. Second, K0.9Na0.1NbO3 deposited on NdScO3 displays a combination with degenerated strain energy densities for the (100)pc and (001)pc orientation. The result is a nested ferroelectric herringbone pattern with alternating, monoclinic a1a2/Mc domains. Moreover, the domain wall inclination angle differs significantly from those of other symmetries and was discussed as a function of potassium concentration x in the framework of a model established by Bokov and Ye yielding a first experimental proof. Furthermore, the composition x=0.90 on NdScO3 enabled the first investigation of the hierarchy and scaling behavior of complex, monoclinic multi-rank pattern.

Author dc:creator

• Braun, Dorothee

Advisor dc:contributor.advisor

• Schwarzkopf, Jutta

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Identifier URI

http://dx.doi.org/10.14279/depositonce-6434

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/7159