Scanning X-ray nanodiffraction from ferroelectric domains in strained K0.75Na0.25NbO3 epitaxial films grown on (110) TbScO3

Scanning X-ray nanodiffraction on a highly periodic ferroelectric domain pattern of a strained K0.75Na0.25NbO3 epitaxial layer has been performed by using a focused X-ray beam of about 100 14;nm probe size. A 90°-rotated domain variant which is aligned along [1 2]TSO has been found in addition to the predominant domain variant where the domains are aligned along the [12]TSO direction of the underlying (110) TbScO3 (TSO) orthorhombic substrate. Owing to the larger elastic strain energy density, the 90°-rotated domains appear with significantly reduced probability. Furthermore, the 90°-rotated variant shows a larger vertical lattice spacing than the 0°-rotated domain variant. Calculations based on linear elasticity theory substantiate that this difference is caused by the elastic anisotropy of the K0.75Na0.25NbO3 epitaxial layer

Ferroelectric domains in potassium sodium niobate thin films: impact of epitaxial strain on thermally induced phase transitions

Gegenstand dieser Arbeit ist die experimentelle Untersuchung der Verspannungs-Temperatur-Phasenbeziehungen in epitaktischen KxNa1-xNbO3 Dünnschichten, sowie deren Zusammenhang mit ferro- und piezoelektrischen Eigenschaften. Die präsentierten Ergebnisse ermöglichen es KxNa1-xNbO3 Dünnschichten für neuartige technologische Anwendung zu optimieren. Zunächst wird eine detaillierte strukturelle Untersuchung der ferroelektrischen Domänenstruktur in epitaktischen K0.7Na0.3NbO3 Schichten auf (110) TbScO3 vorgestellt. Eine Analyse der ferroelektrischen Domänenstruktur mittels lateral aufgelöster Piezoresponse-Kraftmikroskopie (PFM) zeigt vier Arten von Superdomänen. Durch die ergänzende Untersuchung der zweidimensionalen und dreidimensionalen Abbildung des reziproken Raumes mittels hochauflösender Röntgenbeugung (HR-XRD) wird nachgewiesen, dass dieses Domänenmuster mittels monokliner Einheitszellen in einem MC Domänenmodell beschrieben werden kann. Im Anschluss an die strukturelle Untersuchung wurden die elektromechanischen Eigenschaften der KxNa1-xNbO3 Schichten auf (110) TbScO3untersucht. Mittels Doppelstrahl-Laserinterferometrie (DBLI) wurde ein makroskopischer effektiver piezoelektrischer Koeffizient von bis zu d33,f = 23 pm/V nachgewiesen. Zudem wurden Oberflächenwellen-Experimente (SAW) durchgeführt. Diese zeigten außergewöhnlich hohe Signalstärken. Um die Temperatur der ferroelektrischen Phasenübergänge gezielt einstellen zu können, wurde der Zusammenhang zwischen epitaktischer Verspannung und der Phasenübergangstemperatur untersucht. Dazu wurden KxNa1-xNbO3 Schichten mit unterschiedlicher Verspannung gewachsen. Die Änderung der Domänenstruktur und der piezoelektrischen Eigenschaften aufgrund von Temperaturänderung wurde in-situ durch temperaturabhängige PFM, HR-XRD und DBLI Messungen untersucht. Die Untersuchung zeigte, dass die Übergangstemperatur des Übergangs von der MC- in die c-Phase mit zunehmender kompressiver Verspannung kontinuierlich um mehr als 400 °C abnahm.The subject of this thesis is the experimental investigation of the strain-temperature-phase relations in epitaxial KxNa1-xNbO3 thin films and their connection to ferro- and piezoelectric properties. This will enable the optimization of KxNa1-xNbO3 layers for novel technological devices. First, a detailed structural investigation of the ferroelectric domain structure in epitaxial K0.7Na0.3NbO3 films on (110) TbScO3 is presented. An analysis of the ferroelectric domain structure with laterally resolved piezoresponse force microscopy (PFM) reveals four types of superdomains. By complementary two-dimensional and three-dimensional high resolution X-ray reciprocal space mapping this domain pattern is proven to be describable by an MC domain structure with monoclinic unit cells. Subsequently to the structural investigation, the electromechanical properties of KxNa1-xNbO3 layers on (110) TbScO3 were investigated. Double beam laser interferometry (DBLI) revealed a macroscopic effective piezoelectric coefficient of up to d33,f = 23 pm/V. Furthermore, surface acoustic wave (SAW) experiments were performed. They exhibited extraordinary signal intensities. In order to be able to selectively tune such phase transition temperatures, the correlation between epitaxial strain and the phase transition temperature was investigated. For this purpose, KxNa1-xNbO3 films with different compressive strain conditions were grown. The change of domain structure and piezoelectric properties upon temperature variation was investigated in-situ by temperature-dependent PFM, HR-XRD and DBLI measurements. The transition temperature between the MC- and c-phase was shown to continuously decrease by more than 400 °C with increasing compressive strain

Ferroelectric monoclinic phases in strained K 0.70 Na 0.30 NbO 3 thin films promoting selective surface acoustic wave propagation

We present a detailed analysis of the ferroelectric domain structure of K0.70Na0.30NbO3 thin films on (110) TbScO3 grown by metal–organic chemical vapor deposition. Upon piezoresponse force microscopy and nanofocus x-ray diffraction measurements we derive a domain model revealing monoclinic MC domains. The complex domain pattern is formed out of four co-existing in-plane orientations of the shearing direction of the monoclinic unit cell resulting in four types of superdomains each being composed of well-ordered stripe domains. Finally, we present surface acoustic wave (SAW) experiments that exhibit extraordinary signal intensities given the low thickness of the tested film. Moreover, the SAW propagation is found to occur selectively along the identified shearing directions

Ferroelectric phase transitions in multi-domain K0.9Na0.1NbO3K_{0.9}Na_{0.1}NbO_{3} epitaxial thin films

A high-temperature phase transition in strained ferroelectric K$_{0.9}$Na$_{0.1}$NbO$_3$ thin films epitaxially grown on orthorhombic (110) NdScO$_3$ substrates is identified and investigated by in situ x-ray diffraction and piezoresponse force microscopy. At room temperature, the thin films exhibit a highly anisotropic misfit strain, inducing the occurrence of monoclinic a$_1$a$_2$/M$_C$ phases and manifesting itself in the formation of a highly regular, herringbone-like domain arrangement. With increasing temperature, a ferroelectric-to-ferroelectric phase transition to an orthorhombic a$_1$/a$_2$ phase with exclusive lateral electrical polarization takes place. Within a wide temperature range from 180 °C to about 260 °C, a coexistence of the monoclinic a$_1$a$_2$/M$_C$ room temperature phases and the orthorhombic a$_1$/a$_2$ high temperature phase is observed. Finally, at higher temperatures only the orthorhombic a$_1$/a$_2$ phase, which is arranged in a regular stripe domain pattern, is present. Corresponding simulations of the scattered x-ray intensity patterns show that the orthorhombic unit cells undergo a small in-plane rotation. This leads to four different in-plane orientations of the orthorhombic unit cells and four corresponding variants of superdomains

Temperature Dependence of Three-Dimensional Domain Wall Arrangement in Ferroelectric K0.9_{0.9}Na0.1_{0.1}NbO3_{3} Epitaxial Thin Films

The three-dimensional arrangement and orientation of domain walls in ferroelectric K$_{0.9}$Na$_{0.1}$NbO$_{3}$/(110)NdScO$_{3}$ epitaxial thin films wereinvestigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffractionand theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain patternappears in which there is a periodic arrangement of a$_1$a$_2$/M$_C$ monoclinic phases. Four different types of domain walls are observed, whichcan be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, aperiodic $_1$a$_2$ stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed,both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimentalresults are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreementbetween the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In thistemperature range, a complex interplay of coexisting monoclinic $_1$a$_2$/M$_C$ and orthorhombic $_1$a$_2$ phases takes place