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.

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Physics of thin-film ferroelectric oxides

Ferroelectric oxide materials have offered a tantalizing potential for applications since the discovery of ferroelectric perovskites more than 50 years ago. Their switchable electric polarization is ideal for use in devices for memory storage and integrated microelectronics, but progress has long been hampered by difficulties in materials processing. Recent breakthroughs in the synthesis of complex oxides have brought the field to an entirely new level, in which complex artificial oxide structures can be realized with an atomic-level precision comparable to that well known for semiconductor heterostructures. Not only can the necessary high-quality ferroelectric films now be grown for new device capabilities, but ferroelectrics can be combined with other functional oxides, such as high-temperature superconductors and magnetic oxides, to create multifunctional materials and devices. Moreover, the shrinking of the relevant lengths to the nanoscale produces new physical phenomena. Real-space characterization and manipulation of the structure and properties at atomic scales involves new kinds of local probes and a key role for first-principles theory.

http://science.sciencemag.org/content/sci/303/5657/488.full.pdf

Ferroelectricity at the Nanoscale: Local Polarization in Oxide Thin Films and Heterostructures

We report the observation of ferroelectricity in capacitors based on hafnium-zirconium-oxide. Hf0.5Zr0.5O2 thin films of 7.5 to 9.5 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remnant polarization of 16 μC/cm2 and a high coercive field of 1 MV/cm were observed. Further proof for the ferroelectric nature was collected by quasi-static polarization-voltage hysteresis, small signal capacitance-voltage, and piezoelectric measurements. Data retention characteristics were evaluated by a Positive Up Negative Down pulse technique. No significant decay of the initial polarization state was observed within a measurement range of up to two days.

Ferroelectric Zr0.5Hf0.5O2 thin films for nonvolatile memory applications

Theoretical models for small ferroelectric particles predict a progressive decrease of the Curie temperature, spontaneous lattice strain, and polarization until the critical size corresponding to t ...

High dielectric constant and frozen macroscopic polarization in dense nanocrystalline BaTiO3 ceramics

Ferroelectric materials have remained one of the major focal points of condensed matter physics and materials science for over 50 years. In the last 20 years, the development of voltage-modulated scanning probe microscopy techniques, exemplified by Piezoresponse force microscopy (PFM) and associated time- and voltage spectroscopies, opened a pathway to explore these materials on a single-digit nanometer level. Consequently, domain structures and walls and polarization dynamics can now be imaged in real space. More generally, PFM has allowed studying electromechanical coupling in a broad variety of materials ranging from ionics to biological systems. It can also be anticipated that the recent Nobel prize [“The Nobel Prize in Chemistry 2016,” http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2016/ (Nobel Media, 2016)] in molecular electromechanical machines will result in rapid growth in interest in PFM as a method to probe their behavior on single device and device assembly levels. However, the br...

Ferroelectric or non-ferroelectric: Why so many materials exhibit "ferroelectricity" on the nanoscale

Free ferroelectric nanoparticles in the order of 10 nm undergo a size driven phase transition into a paraelectric phase. However, in all applications, especially in ferroelectric random access memories, ferroelectric nanograins are integrated into a circuit. They are therefore exposed to new electromechanical boundary conditions e.g. substrate strain and screening of the depolarization field in the electrodes. Carefully adapted to the respective material, some of the extrinsic effects can be used to stabilize ferroelectricity and to shrink the ultimate size. The system performance is very sensitive to the fabrication and processing procedures.

Nanosize ferroelectric oxides – tracking down the superparaelectric limit

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.

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

New measurement techniques enable the electrical characterization of single ferroelectric capacitors with electrode areas below 1 µm2. This is in the range of the cell size of a typical capacitor in ferroelectric non volatile memory devices. Usually, a scanning force microscope (SFM) is used to contact these capacitors. However, a compensation procedure is necessary to extract the polarization of the capacitor material from the parasitic influence of the measurement setup. The subtraction of an open measurement with a lifted cantilever is not sufficient, because lifting the cantilever changes its parasitic capacitance to the bottom electrode of the wafer. Therefore, we developed a finite element model of a SFM cantilever to calculate its parasitic capacitance. This model enabled us to calculate the parasitic capacitance which has to be subtracted from the measurement data dependent on geometrical parameters of the cantilever and parameters defined by the measurement setup, e.g. the distance from the wafer in the lifted position. Our simulations show that the angle of the cantilever to the wafer surface has to be taken into consideration, whereas the size and shape of the cantilever tip can usually be neglected. The results lead to a simulation-enhanced compensation procedure which is applied for example on a measurement of a ferroelectric sample capacitor with an electrode area of 0.09 µm2. Furthermore, a triax shielding concept is proposed to reduce the main influence of the parasitic capacitance of the cantilever.

Compensation of the Parasitic Capacitance of a Scanning Force Microscope Cantilever Used for Measurements on Ferroelectric Capacitors of Submicron Size by Means of Finite Element Simulations

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.

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

We report on the integration of fully functional ferroelectric PbTiO3 nanostructures of typically less than 100nm lateral extension into a low-k dielectric hydrogen silsesquioxane film. Chemical mechanical polishing of the dielectric layer down to an overall thickness below the nanoparticles height exposes the structures. After confirmation of the piezoelectricity of individual embedded grains, gold electrode pads are deposited to characterize several of these grains in parallel. Evidence of ferroelectric switching is observed and discussed within an equivalent circuit model. This paves the way to a better integration and statistical analysis of ferroelectric nanostructures.

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T. Schmitz

Papers

Nanosize ferroelectric oxides – tracking down the superparaelectric limit

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

Compensation of the Parasitic Capacitance of a Scanning Force Microscope Cantilever Used for Measurements on Ferroelectric Capacitors of Submicron Size by Means of Finite Element Simulations

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

Integration of ferroelectric lead titanate nanoislands for direct hysteresis measurements