Matthew Dawber

Physics of thin-film ferroelectric oxides

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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.

Oxygen-vacancy ordering as a fatigue mechanism in perovskite ferroelectrics

We present a paradigm for fatigue in ferroelectric perovskite oxides: That of a structural phase transition in which oxygen vacancies order into two-dimensional planar arrays capable of pinning domain wall motion.

A model for fatigue in ferroelectric perovskite thin films

An analytic expression in closed form is given for “fatigue,” the dependence of P(N), the switched charge per unit area P versus switching event number N, in ABO3 perovskite structure ferroelectric thin films is given. The analysis is based upon Arlt’s model for fatigue in bulk perovskites, which involves preferential electromigration of oxygen vacancies to sites parallel to the electrode–ferroelectric interface plane, together with some arguments by Brennan on the effectiveness of such defect planes in pinning domain walls. The model is applied to PZT/Pt with no adjustable parameters and yields in complete agreement with experimental data the dependence of P(N) at different frequencies, different voltages, and different temperatures. Notably, unlike other proposals in the literature, the model does not involve any charge injection from the electrodes.

Orientation dependence of ferroelectric properties of pulsed-laser-ablated Bi4−xNdxTi3O12 films

Epitaxial (001)-, (118)-, and (104)-oriented Nd-doped Bi4Ti3O12 films have been grown by pulsed-laser deposition from a Bi4−xNdxTi3O12 (x=0.85) target on SrRuO3 coated single-crystal (100)-, (110)-, and (111)-oriented SrTiO3 substrates, respectively. X-ray diffraction illustrated a unique epitaxial relationship between film and substrate for all orientations. We observed a strong dependence of ferroelectric properties on the film orientation, with no ferroelectric activity in an (001)-oriented film; a remanent polarization 2Pr of 12 μC/cm2 and coercive field Ec of 120 kV/cm in a (118)-oriented film; and 2Pr=40 μC/cm2, Ec=50 kV/cm in a (104)-oriented film. The lack of ferroelectric activity along the c-axis is consistent with the orthorhombic nature of the crystal structure of the bulk material, as determined by powder neutron diffraction.

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.

Combining half-metals and multiferroics into epitaxial heterostructures for spintronics

We report on the growth of epitaxial bilayers of the La2∕3Sr1∕3MnO3 (LSMO) half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by pulsed laser deposition. The growth mode of both layers is two dimensional, which results in unit-cell smooth surfaces. We show that both materials keep their properties inside the heterostructures, i.e., the LSMO layer (11 nm thick) is ferromagnetic with a Curie temperature of ∼330K, while the BFO films shows ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic force microscope mappings of BFO/LSMO bilayers for different BFO thicknesses reveal a high and homogeneous resistive state for the BFO film that can thus be used as a ferroelectric tunnel barrier in tunnel junctions based on a half-metal.

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.

Thickness independence of true phase transition temperatures in barium strontium titanate films

The functional properties of two types of barium strontium titanate (BST) thin film capacitor structures were studied: one set of structures was made using pulsed-laser deposition (PLD) and the other using chemical solution deposition. While initial observations on PLD films looking at the behavior of Tm (the temperature at which the maximum dielectric constant was observed) and Tc* (from Curie-Weiss analysis) suggested that the paraelectric-ferroelectric phase transition was progressively depressed in temperature as BST film thickness was reduced, further work suggested that this was not the case. Rather, it appears that the temperatures at which phase transitions occur in the thin films are independent of film thickness. Further, the fact that in many cases three transitions are observable, suggests that the sequence of symmetry transitions that occur in the thin films are the same as in bulk single crystals. This new observation could have implications for the validity of the theoretically produced thi...

Fatigue in artificially layered Pb(Zr,Ti)O3 ferroelectric films

We have performed fatigue tests on lead zirconate titanate (PZT) multilayers having stacks of Pb(Zr0.8Ti0.2)O3/Pb(Zr0.2Ti0.8)O3 with repeated distances of 12 formula groups. The results are compared with single-layer n-type (0.5 at. % Ta-doped) PZT films. We conclude that fatigue is dominated by space-charge layers in each case, but that in the multilayer such space charge accumulates at the layer interfaces, rather than at the electrode–dielectric interface. The model, which includes both drift and diffusion, is quantitative and yields a rate-limiting mobility of 6.9±0.9×10−12 cm2/V s, in excellent agreement with the oxygen vacancy mobility for perovskite oxides obtained from Zafar et al.