Céline Lichtensteiger

Improper ferroelectricity in perovskite oxide artificial superlattices

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to ‘improper’ ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the ground-state of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of εr ≈ 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications, is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only.

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.

Electric-field tuning of the metal-insulator transition in ultrathin films of LaNiO3

Epitaxial ultrathin films of the metallic perovskite LaNiO3 were grown on (001) SrTiO3 substrates using off-axis rf magnetron sputtering. The film structure was characterized and their electrical properties investigated. Films thinner than 8 unit cells display a metal-insulator transition at a thickness dependent characteristic temperature. Hall measurements revealed p-type conduction, which was confirmed by electric field-effect experiments. Large changes in the transport properties and the metal-insulator transition temperature were observed for the thinnest LaNiO3 films as the carrier density was electrostatically tuned.

Modern Physics of Ferroelectrics: Essential Background

Principles of ferroelectricity and information about ferroelectric materials and their applications are reviewed. The characterization of ferroelectric behavior through measurement of electrical hysteresis is discussed in detail. The main families of ferroelectric oxides, including perovskite compounds and solid solutions, lithium niobate, layered oxides, magnetic ferroelectric oxides, and electronic ferroelectrics are presented and their crystal structures and polarizations given. The effects of pressure and epitaxial strain on perovskites are described. Recent advances in the understanding of ferroelectricity in thin films, superlattices and nanostructures are mentioned. Finally, an overview of applications of feroelectric materials, both established applications and those under development, is included.

Electrostatic Coupling and Local Structural Distortions at Interfaces in Ferroelectric/Paraelectric Superlattices

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.

Direct evidence for ferroelectric polar distortion in ultrathin lead titanate perovskite films

X-ray photoelectron diffraction is used to directly probe the intracell polar atomic distortion and tetragonality associated with ferroelectricity in ultrathin epitaxial PbTiO{sub 3} films. Our measurements, combined with ab initio calculations, unambiguously demonstrate noncentrosymmetry in films a few unit cells thick, imply that films as thin as three unit cells still preserve a ferroelectric polar distortion, and also show that there is no thick paraelectric dead layer at the surface.

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.

Spectroscopic mapping of local structural distortions in ferroelectric PbTiO3/SrTiO3 superlattices at the unit-cell scale

The local structural distortions in polydomain ferroelectric PbTiO

Ferroelectric Size Effects

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