Tuning of the depolarization field and nanodomain structure in ferroelectric thin films

The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.Comment: 19 pages, 7 figure

Nanoscale domain engineering in SrRuO3_3 thin films

We investigate nanoscale domain engineering via epitaxial coupling in a set of SrRuO$_3$/PbTiO$_3$/SrRuO$_3$ heterostructures epitaxially grown on (110)$_o$-oriented DyScO$_3$ substrates. The SrRuO$_3$ layer thickness is kept at 55 unit cells, whereas the PbTiO$_3$ layer is grown to thicknesses of 23, 45 and 90 unit cells. Through a combination of atomic force microscopy, x-ray diffraction and high resolution scanning transmission electron microscopy studies, we find that above a certain critical thickness of the ferroelectric layer, the large structural distortions associated with the ferroelastic domains propagate through the top SrRuO$_3$ layer, locally modifying the orientation of the orthorhombic SrRuO$_3$ and creating a modulated structure that extends beyond the ferroelectric layer boundaries.Comment: 19 pages, 6 figures, supplementary materials. arXiv admin note: text overlap with arXiv:2304.0694

Full control of polarization in ferroelectric thin films using growth temperature to modulate defects

P.P. and C.W. acknowledge partial support by Swiss National Science Foundation Division II grant 200021_178782. L.R.D. acknowledges support from the US National Science Foundation under grant DMR‐1708615. L.W.M. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE‐AC02‐05‐CH11231 (Materials Project program KC23MP) for the growth and study of defect structures in ferroic materials. A.B.N. gratefully acknowledges support from the Engineering and Physics Sciences Research Council (EPSRC) through grants EP/R023751/1 and EP/L017008/1.Deterministic control of the intrinsic polarization state of ferroelectric thin films is essential for device applications. Independently of the well-established role of electrostatic boundary conditions and epitaxial strain, the importance of growth temperature as a tool to stabilize a target polarization state during thin film growth is shown here. Full control of the intrinsic polarization orientation of PbTiO3 thin films is demonstrated-from monodomain up, through polydomain, to monodomain down as imaged by piezoresponse force microscopy-using changes in the film growth temperature. X-ray diffraction and scanning transmission electron microscopy reveal a variation of c-axis related to out-of-plane strain gradients. These measurements, supported by Ginzburg-Landau-Devonshire free energy calculations and Rutherford backscattering spectroscopy, point to a defect mediated polarization gradient initiated by a temperature dependent effective built-in field during growth, allowing polarization control not only under specific growth conditions, but ex-situ, for subsequent processing and device applications.Publisher PDFPeer reviewe

Switchable tribology of ferroelectrics

Artificially induced asymmetric tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe where down domains having lower friction coefficient than up domains can be used as smart masks as they show slower wear rate than up domains. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is universal across ferroelectrics with different chemical composition and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm-cm) size. These findings establish that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.Peer ReviewedPostprint (author's final draft

InteractiveXRDFit: a new tool to simulate and fit X-ray diffractograms of oxide thin films and heterostructures

InteractiveXRDFit is a custom-made MATLAB program that calculates the X-ray diffracted intensity for oxide thin films and heterostructures based on a library of inorganic materials

Ferroelectricity at the nanoscale : study of size effects in lead titanate thin films

Ce travail porte sur l'étude de l'évolution des propriétés physiques de matériaux ferroélectriques lorsque la taille de ces matériaux est réduite à l'échelle nanométrique. Cette étude représente un défi sur le plan expérimental puisque des films ultraminces de haute qualité cristalline sont nécessaires ainsi que des techniques adaptées à l'étude des propriétés physiques à cette échelle. C'est également un défi sur le plan théorique puisque des phénomènes nouveaux sont attendus à cette échelle et doivent être correctement interprétés. La tétragonalité de films minces de PbTiO3 a été mesurée par diffraction aux rayons x et aux photoélectrons. Sa diminution observée lorsque l'épaisseur des films est réduite, liée à une diminution de la polarisation, a permis de démontrer le rôle déterminant joué par l'écrantage imparfait de la polarisation ferroélectrique, responsable de l'apparition d'un champ dépolarisant et de la réduction de la polarisation dans les films très minces

Investigation of Ferroelectricity in Ultrathin PbTiO3 Films

Using off-axis magnetron sputtering onto metallic Nb-SrTiO3 substrates, we have grown a series of epitaxial c-axis oriented PbTiO3 perovskite films with thicknesses ranging from 460 A ̊ down to 12 A ̊ (about 3 unit cells). Topographic measurements using atomic force microscopy showed that these films are essentially atomically smooth. X-ray measurements allowed us to precisely determine the thickness of the films and the c-axis lattice parameter value, and to confirm epitaxial growth. It is found that the c-axis lattice parameter systematically decreases with decreasing film thickness

Phase transitions in ultra-thin ferroelectric films and fine period multilayers

Phase transition behaviour in ferroelectric materials is strongly dependent on boundary conditions, both electrical and mechanical. In this article, we cover several topics. Beginning with thin films, we look at the suppression of ferroelectricity as film thickness is reduced, covering two situations, the first in which the polarisation decreases and eventually is suppressed, and the second where the system forms domains in order to sustain ferroelectricity. The focus then moves to superlattices, highlighting the way that electrostatic coupling dominates the behaviour of superlattices, but revealing that for very thin layers interfacial effects can communicate through this electrostatic coupling to produce new kinds of macroscopic ferroelectric behaviour