A. Roelofs

Piezoresponse force microscopy of lead titanate nanograins possibly reaching the limit of ferroelectricity

Single ferroelectric lead titanate (PTO) grains down to 15 nm were fabricated by chemical solution deposition. Varying the dilution of the precursor solution leads to different grain sizes between 15 and 200 nm. The grain-size-dependent domain configuration was studied using three-dimensional piezoresponse force microscopy (PFM). It is found that the PTO grains in a dense film contain laminar 90° domain walls, whereas separated PTO grains show more complicated structures of mainly 180° domain walls. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the superparaelectric phase which has no spontaneous polarization. Recent calculations showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4–14 nm.

Differentiating 180° and 90° switching of ferroelectric domains with three-dimensional piezoresponse force microscopy

Three-dimensional (3D) piezoresponse force microscopy is applied in order to differentiate 90° and 180° domain switching in PbTiO3 (PTO) thin films. The 3D domain configuration is recorded both statically, revealing the surface crystallographic orientation of PTO films on the nanometer scale, and dynamically by simultaneously mapping the in-plane and out-of-plane hysteresis loops. We show that exclusively 180° switching occurs, also switching only half of the grain volume.

Imaging three-dimensional polarization in epitaxial polydomain ferroelectric thin films

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90° domain wall is observed. Nucleation of new reversed 180° domains at the 90° domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time.

Depolarizing-field-mediated 180° switching in ferroelectric thin films with 90° domains

Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr0.2Ti0.8O3 thin films is studied using three-dimensional piezoresponse force microscopy (PFM). It is found that, under an electric field induced between the PFM tip and the bottom electrode, the 180° switching occurs in both c and a domains. After the removal of this field, the spontaneous reversal of the out-of-plane and in-plane polarizations back to the initial orientations takes place, evolving via heterogeneous development of antiparallel 180° domains. The switching of in-plane polarization inside a domains and the preferential formation of reversed 180° domains at 90° domain walls are explained by the effects of the depolarizing fields caused by transient polarization charges appearing on these domain walls.

Direct hysteresis measurements of single nanosized ferroelectric capacitors contacted with an atomic force microscope

Direct hysteresis measurements on single submicron structure sizes were performed on epitaxial ferroelectric Pb(Zr,Ti)O3 thin films grown on SrTiO3 with La0.5Sr0.5CoO3 (LSCO) electrodes. The samples were fabricated by focused-ion-beam milling resulting in pad sizes down to 200 nm×200 nm. The influence of parasitic capacitance of the measurement setup was eliminated by applying an enhanced compensation procedure. No size effects were observed in capacitors milled down to 400 nm×400 nm. Thus, a published increase of Pmax1 for decreasing pad size can be explained by the parasitic influence of the setup. Finally, the inaccuracy of increasing coercive voltage due to the coating of the cantilever of the atomic force microscope is discussed.

Towards the limit of ferroelectric nanosized grains

Ferroelectric random access memories are non-volatile, low voltage, high read/write speed devices which have been introduced into the market in recent years and which show the clear potential of future gigabit scale universal non-volatile memories. The ultimate limit of this concept will depend on the ferroelectric limit (synonymous superparaelectric limit), i.e. the size limit below which the ferroelectricity is quenched. While there are clear indications that 2D ferroelectric oxide films may sustain their ferroelectric polarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone J M 1999 Appl. Phys. Lett. 75 856), the limit will be quite different for isolated 3D nanostructures (nanograins, nanoclusters). To investigate scaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO3 (PTO) and Pb(ZrxTi1−x)O3 grown by a self-assembly chemical solution deposition method. Preparing highly diluted precursor solutions we achieved single separated ferroelectric grains with grain sizes ranging from 200 nm down to less than 20 nm. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the paraelectric phase which has no spontaneous polarization. Recent calculations (Zhong W L, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) and experiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J. Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4.2–20 nm.

Short-time piezoelectric measurements in ferroelectric thin films using a double-beam laser interferometer

An evolution of the double-beam laser interferometer used for piezoelectric measurements in ferroelectric thin films is reported. Measuring the d33 hysteresis of a ferroelectric material using lock-in technique with large time constants requires a varying bias field to be applied to the sample over a long period of time. This long-term application leads to electrical stress during the measurement. We present a measurement technique using a different source for the applied field and a varied method for averaging the interferometric response. The measurement time for a complete d33 hysteresis will be shortened down to several seconds. Also, the cycle frequency becomes comparable to electrical hysteresis measurements. Experimental results on quartz and Pb(Zr(X),Ti(1−X))O3 are given to demonstrate the capabilities of the interferometer and the new measurement method.

Effects of the top-electrode size on the piezoelectric properties (d33 and S) of lead zirconate titanate thin films

The effects of a decreasing top electrode size on the electric and piezoelectric properties of tetragonal Pb(ZrX,Ti1−X)O3 thin films are investigated. The effective piezoelectric small-signal coefficient d33,eff and the piezoelectric large signal-strain S are measured using a double-beam laser interferometer. Both properties are found to decrease rapidly with decreasing size of the used Pt top electrode for the investigated dimensions of 5mmto100μm edge length (square pads). While the loss of d33,eff is as high as 75%, the influence on the relative permittivity is only small. The source of the pad size effect on the measured piezoelectric properties is found to be the mechanics of the layered structure commonly used for piezoelectric measurements (Pt∕PZT∕Pt∕TiO∕SiO2∕Si), [PZT,Pb(Zrx,Ti1−x)O3] which is verified by finite element simulations.The effects of a decreasing top electrode size on the electric and piezoelectric properties of tetragonal Pb(ZrX,Ti1−X)O3 thin films are investigated. The effective piezoelectric small-signal coefficient d33,eff and the piezoelectric large signal-strain S are measured using a double-beam laser interferometer. Both properties are found to decrease rapidly with decreasing size of the used Pt top electrode for the investigated dimensions of 5mmto100μm edge length (square pads). While the loss of d33,eff is as high as 75%, the influence on the relative permittivity is only small. The source of the pad size effect on the measured piezoelectric properties is found to be the mechanics of the layered structure commonly used for piezoelectric measurements (Pt∕PZT∕Pt∕TiO∕SiO2∕Si), [PZT,Pb(Zrx,Ti1−x)O3] which is verified by finite element simulations.

In-situ compensation of the parasitic capacitance for nanoscale hysteresis measurements

Abstract Ferroelectric capacitors of submicron sizes for nonvolatile memory applications are entering the structure size of nanotechnology. Therefore the signal level for hysteresis measurements is getting much smaller than the influence of the parasitic capacitance of the measurement setup, which is caused by the cantilever of a scanning force microscope (SFM) used for contacting. Our novel compensation method significantly increases the signal to noise ratio by active cancellation of the parasitic capacitance of the setup during the measurement. From measurements and simulations the parasitic capacitance of an SFM has been determined to be 170 fF. This is about two orders of magnitude higher than the capacitance of a ferroelectric capacitor of submicron size. The new compensation method will be demonstrated on single ferroelectric PbZr x Ti 1− x O 3 (PZT) submicron capacitors.

Electrical measurements on capacitor sizes in the submicron regime for the characterization of real memory cell capacitors

Abstract Pb(Zr,Ti)O3 (PZT) and SrBi2Ta2O9 (SBT) are promising candidates for the use as cell capacitor materials in ferroelectric non-volatile memory devices (FeRAMs). For years it has been an outstanding challenge to perform electrical measurements on samples with pad sizes which are equal to the pad size of real cell capacitors of integrated memory devices. Up to now either larger sample capacitors or an array of several hundred capacitors in parallel could be investigated with typical measurement techniques [1]. But, both measurements can hardly detect the failure of a single capacitor in the submicron range.