Ferroelectric ultrathin films and superlattices

Main researchers: Céline Lichtensteiger, Stéphanie Fernandez, Christian Weymann

In ferroelectric ultrathin films, the depolarization field arising from bound charges on the surface of the film and at the interface with the substrate can be partially screened by free charges. In the absence of sufficient free charges, a ferroelectric has several ways of minimizing its energy whilst preserving its polar state, e.g., by forming domains of opposite polarization, or rotating the polarization into the plane of the thin ferroelectric slab. In heterostructures such as ferroelectric (FE)-paraelectric (PE) superlattices, the non-ferroelectric layers may polarize in order to preserve the uniform polarization state and hence eliminate the depolarization fields. If all else fails, the ferroelectric polarization will be suppressed [1].

ferroelectric ultrathin films
Figure 1: The depolarization field arising from unscreened bound charges on the surface of the ferroelectric is generally strong enough to suppress the polarization completely and hence must be reduced in one of a number of ways.

We study the role of the depolarizing field in determining the domain structure and domain dynamics in oxide heterostructures and superlattices composed of ultrathin ferroelectric and paraelectric layers.

Using piezo-force microscopy (PFM), we investigate the intrinsic nanodomain structure of PbTiO3 ultrathin films at room temperature, focusing on the effect of film thickness and the role of the effective screening length.

Superlattices are ideal model systems for investigating the structure and functional properties of such nanodomains. The domain structure can be tailored by exploiting Kittel's law and the electrostatic interactions between the ferroelectric layers. X-ray diffraction and transmission electron microscopy (TEM) combined with electron energy loss spectroscopy were used to study the electrostatic interactions in PbTiO3/SrTiO3 superlattices in detail, revealing highly inhomogeneous polarization and structural profiles which were attributed to ferroelectric nanodomains [2,3,4].

ferroelectric ultrathin films
Figure 2: Top - X-ray intensity along the specular rod of a (6 unit cells|6 unit cells)21 superlattice with SrRuO3 electrodes, showing superlattice peaks in red and substrate peaks in blue. Bottom - Left: High-resolution TEM image of a (6|6) superlattice. Center: sketch of the superlattice with domains, showing the different periodicities in the system. Right: Reciprocal space map for a (10|10) superlattice close to the (113) reflection of the SrTiO3 substrate showing diffuse satellites around the superlattice Bragg peaks due to the periodic domain structure [4].

In oxide heterostructures, we insert ultrathin paraelectric layers between the ferroelectric film and metallic electrodes to tune the depolarizing field and study its effect on the domain structure. Using piezo-force microscopy (PFM), we investigate the intrinsic nanodomain structure of PbTiO3 ultrathin films at room temperature, focusing on the effect of film thickness and the role of the effective screening length [5].

ferroelectric ultrathin films
Figure 3: 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 [5].

A few related publications:

[1] Ferroelectricity in ultrathin-film capacitors
C. Lichtensteiger, P. Zubko, M. Stengel, P. Aguado-Puente, J.-M. Triscone, P. Ghosez and J. Junquera
Ch. 12 in Oxide Ultrathin Films, Science and Technology, Wiley (2011).

[2] X-Ray Diffraction Studies of 180° Ferroelectric Domains in PbTiO3/SrTiO3 Superlattices under an Applied Electric Field
P. Zubko, N. Stucki, C. Lichtensteiger, and J.-M. Triscone
Physical Review Letters 104, 187601 (2010).

[3] Spectroscopic mapping of local structural distortions in ferroelectric PbTiO3/SrTiO3 superlattices at the unit-cell scale
Almudena Torres-Pardo, Alexandre Gloter, Pavlo Zubko, Noémie Jecklin, Céline Lichtensteiger, Christian Colliex, Jean-Marc Triscone, and Odile Stéphan
Physical Review B 84, 220102(R) (2011).

[4] Electrostatic Coupling and Local Structural Distortions at Interfaces in Ferroelectric/Paraelectric Superlattices
P. Zubko, N. Jecklin, A. Torres-Pardo, P. Aguado-Puente, A. Gloter, C. Lichtensteiger, J. Junquera, O. Stéphan, and J.-M. Triscone
Nano Letters 12, 2846 (2012).

[5] Tuning of the Depolarization Field and Nanodomain Structure in Ferroelectric Thin Films
Céline Lichtensteiger, Stéphanie Fernandez-Pena, Christian Weymann, Pavlo Zubko, and Jean-Marc Triscone
Nano Letters 14, 4205−4211 (2014).