Domain control in strained lead-free alkaline niobate thin films to tune piezoelectric coefficients
Final Report Abstract
During the project the combination of bulk oxide crystal growth and epitaxy of thin films in the same institution (IKZ) turned out to be very fruitful to tune layer properties in a targeted way. We have gained many fundamental and promising results for the hitherto unknown material system KxNa1-xNbO3. This was achieved on the basis of the strain-phase diagram calculated within this project. Knowledge of the strain-phase diagram together with the linear elasticity theory in combination with MOVPE as deposition method, that provides almost perfect films, impressively demonstrated the potential of the strain engineering method to tune properties of ferroelectric oxide layers in a controlled way. This was discussed on the basis of two selected examples. On the one hand, we have predicted and verified the coexistence of (100)pc and (001)pc oriented, monoclinic domains with Pm symmetry in strained K0.9Na0.1NbO3 films on NdScO3 substrates leading to a complex herringbone pattern. The monoclinic symmetry exhibits an inherent flexibility in domain arrangement which is not given in tetragonal, orthorhombic or rhombohedral systems. It enables a variable domain ordering and thus an additional degree of freedom in domain engineering. This emphasizes the outstanding role of monoclinic phases. First (local) values of the piezoelectric coefficient are promising for lead-free, only few tens of nanometer thin ferroelectric films. On the other hand, compressively strained KxNa1-xNbO3 thin films epitaxially grown on different rareearth scandate substrates resulted in a periodic stripe domain pattern consisting of pure MC domains or a superposition of MC and MA domains depending on the amount incorporated lattice strain. The symmetry plane of these monoclinic phases determines the permissible propagation directions of surface acoustic waves which are – as a direct result – strongly selective. Comparatively high SAW transmission values could be achieved taking into account the very low film thickness of 30 nm. The concentration of the surface wave energy at a few nm makes these layers tentatively sensitive for surface modifications (e.g. in biosensors). The strong influence of the compressive lattice strain enables a large shift of the phase transition temperature of up to 400 K. So that the transitions can be shifted in such a way that functional properties like electromechanical coefficients or dielectric permittivities can be maximized for the desired operating temperature. Differences in films grown by MOVPE and PLD were investigated by SAW and C-V experiments. Although both films show the same anisotropic SAW behavior, the ferroelectric characteristics are different: While classical ferroelectric behavior and the formation of periodic stripe domains were verified for the MOVPE films, relaxor type behavior and irregularly arranged domains were observed for the PLD films. This was tentatively explained by structural disorder in the PLD films, which is caused inherently by the deposition method and by partial plastic lattice relaxation of the film structure. Our results underline that improved or novel ferro- and piezoelectric properties of lead-free materials can be obtained by an intentional modification of the film structure. A systematic tailoring requires a detailed knowledge of the relation between lattice strain, phase formation and phase transition temperature. In view of ferro-/piezoelectric properties and flexibility, the lead-free material KxNa1-xNbO3 is considered to be very promising, also with regard to applications like sensors or labon-chip devices.
Publications
- Monoclinic MA domains in anisotropically strained ferroelectric K0.75Na0.25NbO3 films on (110) TbScO3 grown by MOCVD, J. Appl. Cryst. 49, 375 (2016)
J. Schwarzkopf, D. Braun, M. Hanke, A. Kwasniewski, J. Sellmann and M. Schmidbauer
(See online at https://doi.org/10.1107/S1600576716000182) - Strain Engineering of Ferroelectric Domains in KxNa1-xNbO3 Epitaxial Layers, Front. Mater. 4, 26 (2017)
J. Schwarzkopf, D. Braun, M. Hanke, R. Uecker and M. Schmidbauer
(See online at https://doi.org/10.3389/fmats.2017.00026) - Strain engineering of monoclinic domains in KxNa1-xNbO3 epitaxial layers: a pathway to enhanced piezoelectric properties, Nanotechnology 28, 24LT02 (2017)
M. Schmidbauer, D. Braun, T. Markurt, M. Hanke and J. Schwarzkopf
(See online at https://doi.org/10.1088/1361-6528/aa715a) - Ferroelectric monoclinic phases in strained K0.7Na0.3NbO3 thin films promoting selective surface acoustic wave propagation, Nanotechnology 29, 415704 (2018)
L. von Helden, M. Schmidbauer, S. Liang, M. Hanke, R. Wördenweber and J. Schwarzkopf
(See online at https://doi.org/10.1088/1361-6528/aad485) - Hierarchy and scaling behavior of multi-rank domain patterns in ferroelectric K0.9Na0.1NbO3 strained films, Nanotechnology 29, 015701 (2018)
D. Braun, M. Schmidbauer, M. Hanke and J. Schwarzkopf
(See online at https://doi.org/10.1088/1361-6528/aa98a4)
