Nanoscale Ferroelectric Oxides

Abstract

Abstract People Publications Alumni

W. Lee, H. Han, A. Lotnyk, M.A. Schubert, S. Senz, M. Alexe, D. Hesse

A method has been developed to manufacture extremely densely packed ferroelectric data storage devices. Using a finely perforated alumina template, capacitors made of platinum and lead-zirconate-titanate Pb(Zr,Ti)O3 have been manufactured with a density of about 175 billion capacitors per square inch. The template was placed on a platinum-coated magnesium oxide substrate. Thin films of ferroelectric lead zirconate and platinum were then deposited through the holes of the stencil using pulsed laser deposition. When the stencil was removed, there remained an array of nanoscale Pt/Pb(Zr,Ti)O3/Pt capacitor islands that were around 80 nm or less across. They could be individually addressed and switched by piezoresponse force microscopy.

Fig. 1:Scheme of the manufacturing process

Fig. 2: Top image: SEM image of the template (top part) and the nanocondensators (bottom part). Bottom image: TEM cross section image of four Pt/Pb(Zr,Ti)O3/Pt nanocapacitors.

Fig. 3: (a) Piezoresponse hysteresis curve of one single nanocapacitor. (b) Topo-graphy image of the corresponding sample. (c) PFM image before switching. (d) PFM image after positive switching of two capacitors by applying +3Vd.c.. (e) PFM image after negative switching of one of the two previously switched capacitors by applying -3Vdc. The inset in (a) shows the measurement configuration. The scale bar referring to (b)-(e) is 100 nm.

For details, see, e. g. Nature Nanotechnology 3  (2008) 402-407. 

G. Le Rhun, I. Vrejoiu, L. Pintilie, D. Hesse, and M. Alexe

We showed through piezoresponse force microscopy (PFM) investigations that 90° a-domains can significantly move in a continuous epitaxial Pb(Zr0.2Ti0.8)O3 (PZT) thin film on application of a non-uniform electric field (Fig. 1). Following this observation, stripe-shaped top electrodes have been designed in order to induce a non-uniform electric field in the capacitor and make use of the extrinsic contribution of the moving ferroelastic domains (Fig. 2). As a result, a reversible and tunable increase of the piezoelectric coefficient, as well as a significant enhancement of the specific capacitance were obtained (Fig. 3).

Fig. 1: Piezoresponse images of the same 1 µm² area showing the evolution of the 90° domain structure of a 150 nm thick PbZr0.2Ti0.8O3 film as a function of applied d.c. bias voltages. (a) as-grown state, (b) after -6 V d.c., (c) after +3 Vd.c., and (d) after +6 Vd.c..

Fig. 2: SEM image of the stripe-electrode structure fabricated by electron-beam lithography, with a detail

Fig. 3: (a) Piezoelectric loop measured on a single stripe electrode and on a square electrode on the same sample. (b) Specific capacitance as a function of d.c. voltage for the uniform capacitor (square electrode) and the non-uniform capacitor (stripe electode).

For details, see, e. g. Nanotechnology 17  (2006) 3154. 

S. K. Lee, M. Alexe, Woo Lee, K. Nielsch, and D. Hesse

(Left) Scheme of the preparation of epitaxial ferroelectric (Bi,La)4Ti3O12 (BLT) nanostructures by pulsed laser deposition (PLD) using a gold nanotube membrane as a deposition mask. (Center) AFM topography image (10 µm x 10 µm) of an array consisting of epitaxial ferroelectric BLT nanostructures on a SrTiO3 single-crystal substrate. Hexagonal arrangement of the nanostructures with a pitch of 500 nm, and a lateral size of the individual nanostructures of 170 nm. (Right) Piezoelectric hysteresis curve of an individual epitaxial nanostructure obtained by piezoresponse force microscopy (PFM).

For details, see, e. g. Journal of Applied Physics  98 (2005) 124302.  

M. Alexe, C. Harnagea, W. Ma, S.K. Lee, and D. Hesse

The ever-higher integration of microelectronic devices also addresses both questions of technological interest and of fundamental importance. Structuring devices at a submicron scale is an ambitious task intensively researched all over the world. On the other hand the questions to know whether ferroelectric structures that have submicron size (lateral and in height) are still ferroelectric, are still switching, and whether they still possess an equilibrium domain pattern are fundamental questions to be addressed before scaling down ferroelectric devices. Nanostructures as small as 50 nm in size have been fabricated by the Direct Electron Beam Writing method. First results show that these nanostructures are still ferroelectric and are still fully switching.

SEM picture of several arrays of PZT nanostructures with various sizes obtained by direct electron beam writing

Recently, arrays of fully switchable ferroelectric mesoscopic structures of 300 nm in lateral size have been prepared by a modified nanoimprint lithography method.

Topography image of an array of ferroelectric PZT cells prepared by imprint lithography (left) and piezoresponse image of the same array (at the same magnification) after switching serveral cells using d.c. puls applied to the tip (right)

For details, see, e. g. J. Electroceramics 12 (2004) 69-88.

C. Harnagea, G. Le Rhun, and M. Alexe

Voltage-modulated scanning force microscopy (SFM), also called Piezoreponse SFM, is a powerful method for imaging ferroelectric domains. It allows the characterization of ferroelectric thin films at a nanoscopic scale, as well as the investigation of their ferroelectric behavior in dependence on their micro- or nano-structure. The materials under investigation are various bismuth layered perovskite thin films with different crystallographic orientations, as well as ferroelectric nanostructures made of PZT and other materials.

Fig. 1: The well-known lamellar domain structure in simple perovskites. Within micron-sized grains, domains as small as 50 nm in lateral size are present. 90° as well as 180° domain walls are also visible.

Fig. 2: Ferroelectric domains in twinned-epitaxial bismuth titanate films. A rectangular (110)-oriented crystallite embedded in a flat c-oriented matrix is shown in the 3-D surface plot. The color scale corresponds to the piezoresponse signal reflecting the out-of-plane polarization component. Half of the polarization in the structure was oriented upward, by applying a +30V pulse to the bottom electrode. A weaker response of the matrix shows that there is a non-zero component of polarization along the [001] direction.

Fig. 3: Piezoresponse microscopy is a powerful technique that allows to image the in-plane component of the polarization and therefore to obtain information about the three-dimensional distribution of polarization. Shown is the ferroelectric domain structure of a BaTiO3 single crystal: Topography image (upper left), vertical piezoresponse image (lower left), and lateral piezoresponse image (lower right)

For details, see, e. g.
M. Alexe, A. Gruverman
"Nanoscale Characterisation of Ferroelectric Materials"
Springer (2004)