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

Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric-to-ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non-centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.

Incipient Ferroelectricity in Al-Doped HfO2 Thin Films

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

Ferroelectric switching is unambiguously demonstrated for the first time in a III-V semiconductor based material: Al1-xScxN—A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator, and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in Al1-xScxN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 μC/cm2—which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from −0.3% to + 0.4% and a paraelectric transition temperature in excess of 600 °C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors—that of their spontaneous polarization.

AlScN: A III-V semiconductor based ferroelectric

Piezoelectric materials play a crucial role in a large number of devices and applications modern society would not like to miss. Mobile phones and ultrasonic imaging are just the most prominent ones. Since two decades, miniaturization of mechanical devices in silicon technology is a major research direction in engineering known under name of MEMS, which stands for micro-electro-mechanical systems. Piezoelectricity fits very well into this concept and was expected right from the beginning to play its role in MEMS. The breakthrough was made with RF filters in mobile phones working on the principle of standing thickness waves in AlN films. What counts here is acoustic quality and stability. The force champion among piezoelectric thin film materials, Pb(Zr,Ti)O3 gave more problems in processing, and requires more patience to meet requirements and needs for a mass applications. It seems, however, that the breakthrough is imminent. This article attempts to give an overview of the field, highlighting recent achievements, introduce operation principles, and describe some applications.

Recent Progress in Materials Issues for Piezoelectric MEMS

We report on a measurement procedure to separate ferroelectric switching current and dielectric displacement current from the leakage current in leaky ferroelectric thin-film capacitor structures. The ac current response is determined for two adjacent frequencies. Taking advantage of the different frequency dependencies of the ferroelectric switching current, dielectric displacement current and ohmic current, the hysteresis loop is calculated without performing a static leakage current measurement, which causes a high dc field stress to the sample. The applicability of the proposed measurement procedure is demonstrated on a Pt∕Pb(Zr,Ti)O3∕IrO2 ferroelectric capacitor revealing a high leakage current.

/pdf/dynamic-leakage-current-compensation-in-ferroelectric-thin-2mnfnq0lkg.pdf

Dynamic leakage current compensation in ferroelectric thin-film capacitor structures

We present a new measurement method to characterize piezoelectric thin films utilizing a four-point bending setup. In combination with a single- or a double-beam laser interferometer, this setup allows the determination of the effective transverse and longitudinal piezoelectric coefficients e31,f and d33,f respectively. Additionally, the dielectric coefficient and the large signal electrical polarization are measured to add further important characteristics of the film. These data are essential for piezoelectric thin film process specification and the design and qualification of microelectromechanical systems devices.

Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices

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

A two-dimensional numerical model that predicts the reliability of multilayer capacitors (MLCs) during soldering and bending is presented. The Weibull parameters used in the model are based on measurements of soldered MLC devices. The preheating and soldering temperatures have a dominant impact on the failure probability, in comparison to the thickness of the nickel layer, the soft solder geometry, and the number of inner electrodes. Comparison of calculated and measured reliability of three MLC sizes leads to the assumption that residual stresses due to the manufacturing process or size-related microstructure are important.

Finite‐Element Analysis of Ceramic Multilayer Capacitors: Failure Probability Caused by Wave Soldering and Bending Loads

Using BaTiO3 as a piezoelectric model system we compare a finite element model with experimental data to demonstrate the impact of grain topography on the in-plane piezoelectric response at the perimeter. Our findings emphasize the need for a careful consideration of both electric field and piezoelectric tensor orientation. An analysis is given showing that the in-plane piezoresponse is a function of two directions of the electric field, whereas the out-of-plane response is a function of all three directions of the applied field. The effect of an adsorbate layer on the piezoelectric response is quantified with typical material parameters.

/pdf/analysis-of-shape-effects-on-the-piezoresponse-in-52qwyz2jlp.pdf

Klaus Prume

Papers

Dynamic leakage current compensation in ferroelectric thin-film capacitor structures

Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices

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

Finite‐Element Analysis of Ceramic Multilayer Capacitors: Failure Probability Caused by Wave Soldering and Bending Loads

Analysis of shape effects on the piezoresponse in ferroelectric nanograins with and without adsorbates