Thickness-Dependent Evolution of Piezoresponses and Stripe 90° Domains in (101)-Oriented Ferroelectric PbTiO₃ Thin Films

Abstract

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO)(SrTaAlO) (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/ c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse amplitude indicates that the 50 nm thick PTO films demonstrate a superior piezoresponse. Electron diffraction and contrast analysis clarify that all these (101)-oriented PTO films contain periodic stripe ferroelectric 90° domains. The domain periods increase with the film thickness following Kittel's law. Aberration-corrected STEM imaging reveals that the stripe ferroelectric 90° domains have an alternate arrangement of wide and narrow c domains with polarization directions along [100] for c domains and [001̅] for c domains, forming a "head-to-tail" polarization configuration. Further strain analysis reveals that stripe domains have uniform strain distributions and distinct lattice rotations around domain walls. It is proposed that the periodic arrangement of high-density stripe 90° domains in 50 nm thick PTO films is the main contributor to the superior piezoresponse behavior. These results are expected to provide useful information to understand the domain structures in (101)-oriented PTO thin films and thus facilitate further modulation of the properties for potential applications.

Converse flexoelectricity around ferroelectric domain walls

“…Thus, a PTO film of 50 nm was grown on a (101)-oriented (La,Sr)(Al,Ta)O3 (LSAT(101)) substrate and 90° DWs perpendicular to the film/substrate interface were found ( Supplementary Fig. 3), similar to our recent finding 15 . In this orientation, the two domains beside a 90° DW are in symmetric strain states, so that most 90° DWs are very straight.…”

Crystallographic Orientation and Surface Charge-Tailored Continuous Polarization Rotation State in Epitaxially Ferroelectric Nanostructures

“…At room temperature, PTO is a typical tetragonal ferroelectrics with lattice parameters of a = b = 3.899 Å and c = 4.153 Å, whereas STO exhibits a cubic structure with lattice parameters of a = b = c = 3.905 Å. In contrast to the (001) orientation, STO(101) substrates have two different in-plane crystal axes, [010] and [1̅01], which bring asymmetric biaxial strain for PTO thin films. , The surface topographies of [001]­PTO and [101]­PTO thin films are shown in Figure a,b, respectively. From Figure a, it is seen that the [001]­PTO thin film exhibits a very smooth surface with the surface root-mean-square (rms) roughness of 152 pm.…”

“…So far, most studies on polarization rotation have focused on (001)-oriented films. However, recent studies have revealed that the crystallographic orientation of ferroelectric films has a great effect on the domain configuration, , dielectric property, and ferroelectric switching behavior . Furthermore, theoretically, the temperature-strain phase diagrams of single domain (101)-oriented PTO thin films indicated that the PTO films grown on SrTiO 3 (STO) substrates could be a monoclinic a 2 c phase or an orthorhombic c phase, which means that the polarization rotation may exist in PTO/STO(101) films due to the monoclinic–orthorhombic ferroelectric morphotropic phase boundary…”

Self-Recovery of Defective PbTiO3 Film with Enhanced Piezoelectricity by Homogenizing Annealing

“…9 Moreover, these growth defects generally have a negative impact on the properties of ferroelectric films. 10,11 For instance, it is noticeable that the presence of impurity defects will affect the performance of oxide films prepared by PLD. It has been demonstrated that local phase separation is a general reason for fatigue in ferroelectric films.…”

“…Thus, defects may be involved in ferroelectric thin films if the growth conditions, such as oxygen partial pressure, substrate temperature, and annealing rate, are not manipulated accurately . Moreover, these growth defects generally have a negative impact on the properties of ferroelectric films. , For instance, it is noticeable that the presence of impurity defects will affect the performance of oxide films prepared by PLD. It has been demonstrated that local phase separation is a general reason for fatigue in ferroelectric films.…”