Researchers have developed a fabrication technique for single-crystalline thin-film arrays of an organic ferroelectric small molecules working as a memory device by using a solution process under ambient pressure at room temperature.
For fabrication of organic ferroelectric devices, one of the problems to be solved is to make a homogeneous thin film. The developed printing technique that stimulates thin-film formation from a solution allows formation of highly uniform single-crystalline thin films of organic ferroelectrics. The thin-film device fabricated by the developed technique worked as a memory device with only 3 V which is lower than operation voltage of various memory devices. The developed technique is expected to accelerate the research and development on low power consumption device of printed electronics such as ferroelectric memories and nonvolatile semiconductor FETs.
Details of the results will soon be published online in a German scientific journal, Advanced Materials.
Active R&D of the "printed electronics", which applies printing technologies to the production of electronic devices by forming precise, high-quality, metallic and/or semiconducting patterns, has been conducted globally. So far, several printing methods have been enthusiastically developed to fabricate metallic wires and semiconductor layers for transistors, although the development of printing techniques for other types of materials has not been conducted enough. Ferroelectric materials could reduce the power consumption of electronic devices such as ferroelectric memories in IC cards and nonvolatile semiconductor FETs. Therefore, it is required to develop patterning techniques for ferroelectric thin films though printing technologies.
Ferroelectric materials are generally composed of inorganic materials so that it was considered to be difficult to apply a printing process. Although organic ferroelectric polymer materials are applicable to a printing process, their ferroelectric characteristics are inferior to those of inorganic materials. In recent years, research and development of organic ferroelectrics composed of small molecules have advanced. Some new organic materials showing superior characteristics comparable to inorganic ones have been found. Though thin-film formation of these materials is indispensable for making them into devices, it is difficult to form thin films of the materials. Therefore, it was desired to develop a fabrication technique of uniform thin film without any pinholes through a printing process.
AIST has been promoting the research and development of organic ferroelectric small molecules composed of light elements, which contain no rare metal nor toxic lead and could be suitable for print production technologies. It has developed many organic ferroelectrics, including a binary component molecular compound with excellent ferroelectric properties (AIST press release on January 24, 2005) and a single component material that exhibits the best ferroelectric properties at room temperature (AIST press release on February 12, 2010). To build them into devices, it is necessary to fabricate pinhole-free, uniform thin films with oriented molecules. These demands motivated the researchers to search an appropriate compound and to adopt an advanced printing technique.
This study is supported by the Japan Science and Technology Agency through CREST, as "Creation of Materials Science for Advanced Ferroelectrics of Organic Compounds." (Research Period： FY2011 - FY2015)
The researchers selected 2-methylbenzimidazole (MBI) as a promising candidate for an organic ferroelectric material (Fig. 1a). MBI is one of the hydrogen-bonded organic ferroelectric materials, is soluble in organic solvents, shows polarization reversal at a low coercive electric field (few tens kV/cm), and exhibits excellent ferroelectric properties at room temperature. Within a single crystal, remnant polarization P would appear in two orthogonal directions. In devices to which voltage is applied in a direction normal to the thin film, P should have a component normal to the thin film. MBI is expected to grow in plate-like crystals having a desired polarization direction.
Figure 1b shows the schematics of the developed thin film print fabrication process under ambient pressure at room temperature. First, the surface of a 1 cm square SiO2/Si substrate was treated with hydrophilic/hydrophobic patterning consisting of a 100 µm line and space (L&S) structure. An array of crystalline thin films can be formed on the hydrophilic regions by shearing the solution of MBI with a flat blade and successive drying. Synchronized light extinction by rotating cross-polarizers in the crossed-Nicols optical micrographs indicated a high degree of crystallographic alignment of these thin plate-like crystals (Fig. 1c).
The lattice parameters, crystal orientations, and directions of spontaneous polarization of the MBI film were determined by synchrotron X-ray diffraction measurements at the Photon Factory of KEK. A single diffraction spot (in the dashed red circle) was observed for each diffraction plane (Fig. 2a) suggesting the formation of a single crystal. Figures 2b and 2c show schematics of the molecular packing structure and crystal orientation on the substrate. It was found that one of the hydrogen-bonded chains is directed perpendicular to and another is parallel to the substrate surface, respectively. It means that the principal polarization axes are tilted by 45 degrees relative to the substrate surface. As the spontaneous polarization has a component perpendicular to the substrate, it may be possible to reverse the polarization in electrode/ferroelectric/electrode layered structure by applying voltage between upper and lower electrodes.
A capacitor-type device using the plate-like crystals with about 1 μm thickness exhibited quasi-rectangular loops in the electric polarization (P) versus electric field (E) hysteresis experiments without additional thermal annealing (Fig. 3a). The devices exhibited polarization switching at a very low voltage of about 3–4 V at 10 Hz. The fatigue characteristics of switching were evaluated at frequencies of 10, 100, and 1000 Hz. The ferroelectric properties could be maintained until hundreds of thousands cycles at 1000 Hz (Fig. 3b). The researchers expected that the fatigue characteristics could be improved by optimizing the device structure.
Piezoresponse force microscope (PFM) characterization provides microscopic information about the polarization reversal. Figure 4a shows various sizes of polarization reversal domains obtained by applying a constant DC bias of +20 V to the tip with a pulse duration varying from 10 to 1000 ms for a 1.0 µm thick film. The minimum domain size was ≈500 nm, whereas it increases logarithmically with increasing a pulse duration (Fig. 4b). This domain was found to be stable for at least 40 h under ambient pressure at room temperature. Phases of PFM images reveal that the polarization changes by not 90 degrees but 180 degrees (Fig. 4c).
The researchers aim to develop manufacturing technologies of all-printed electronics devices by combining the developed printing technique for thin film formation and other printing techniques for fabricating metal wires and semiconductor thin films.
Physicists discover large-magnitude elasto-optic effect in ferroelectric materials
An international group of physicists discovered a phenomenon of large magnitude in an unexpected class of materials that can lead to a variety of devices used in optical systems.
That phenomenon – the elasto-optic effect – characterizes the formation of a periodic variance of light refraction when an acoustic wave propagates in optical materials, said Yurong Yang, a research assistant professor at the University of Arkansas who led the research.
"We found a significantly large elasto-optic effect in thin films made of materials called ferroelectrics," Yang said, "which are usually considered for their changes in mechanical energy into electrical energy and vice versa, as well in multiferroelectric thin films, which are commonly investigated because of the possible control of their magnetic response by electric input, as well as of their electric response by magnetic input."
The research group published its findings in a paper in Physical Review Letters, the journal of the American Physical Society. A second paper describing the research was published in Nature Communications, an online journal published by the journal Nature.
"Those discoveries of a large elasto-optic effect in ferroelectrics and multiferroelectrics therefore broaden the potential of these materials since they can now be put in use to also control their optical responses by elastic property," said Laurent Bellaiche, Distinguished Professor of physics at the U of A, "which suggests exciting device opportunities arising from this overlooked coupling in these classes of materials."
Promising ferroelectric materials suffer from unexpected electric polarizations
(Phys.org) —Electronic devices with unprecedented efficiency and data storage may someday run on ferroelectrics—remarkable materials that use built-in electric polarizations to read and write digital information, outperforming the magnets inside most popular data-driven technology. But ferroelectrics must first overcome a few key stumbling blocks, including a curious habit of "forgetting" stored data.
Now, scientists at the U.S. Department of Energy's Brookhaven National Laboratory have discovered nanoscale asymmetries and charge preferences hidden within ferroelectrics that may explain their operational limits.
"The positive or negative polarizations in these ferroelectric materials should be incredibly easy to switch, but the reality is much stranger," said Brookhaven Lab physicist Myung-Geun Han, lead author on the new study. "To our surprise, opposing electronic configurations only allowed for polarization in one direction—a non-starter for reading and writing data."
The researchers used a suite of state-of-the-art techniques—including real-time electrical biasing, electron holography, and electron-beam-induced current measurements—to reveal never-before-seen electric field distributions in ferroelectric thin films, which were custom-grown at Yale University. The results, published in Nature Communications, open new pathways for ferroelectric technology.
Most electronic devices rely on ferromagnetism to read and write data. Each so-called ferromagnetic domain contains a north or south magnetic polarity, which translates into the flipping 1 or 0 of the binary code underlying all digital information. But ferromagnetic operations not only require large electric current, but the magnets can flip each other like dominoes when packed together too tightly—effectively erasing any data.
Ferroelectrics, however, use positive or negative electric charge to render digital code. Crucially, they can be packed together with domains spanning just a few atoms and require only a tiny voltage kick to flip the charge, storing much more information with much greater efficiency.
"But ferroelectric commercialization is held up by material fatigue, sudden polarization reversal, and intrinsic charge preferences," said Brookhaven Lab physicist and study coauthor Yimei Zhu. "We suspected that the origin of these issues was in the atomic interactions along the material's interface—where the ferroelectric thin film sits on a substrate."
The scientists examined ferroelectric films of lead, zirconium, and titanium oxide grown on conductive substrates of strontium, and titanium oxide with a small amount of niobium—chosen because it exhibits large polarization values with well-defined directions, either up or down. The challenge was mapping the internal electric fields in materials thousands of times thinner than a human hair under actual operating conditions.
Brookhaven scientists hunted down the suspected interface quirks using electron holography. In this technique, a transmission electron microscope (TEM) fired 200,000-volt electron wave packets through the sample with billionth-of-a-meter precision. Negative and positive electric fields inside the ferroelectric film then attracted or repelled the electron wave and slightly changed its direction. Tracking the way the beam bent throughout the ferroelectric film revealed its hidden charges.
"Rather than an evenly distributed electric field, the bending electron waves revealed non-uniform and unidirectional electric fields that induced unstable, head-to-head domain configurations," Han said. "For the first time, we could see these unusual and jagged polarizations mapped out in real space and real time."
These opposing polarizations—like rival football teams squaring off aggressively at the line of scrimmage—surprised scientists and challenged assumptions about the ferroelectric phenomenon.
"These results were totally unexpected based on the present understanding of ferroelectrics," Han said.
The asymmetries were further confirmed by measurements of electron-beam-induced current. When a focused electron beam struck the ferroelectric sample, electric fields within the film-substrate interface revealed themselves by generating additional current. Other techniques, including piezoresponse force microscopy—in which a sub-nanometer tip induces a reaction by pressing against the ferroelectric—also confirmed the strange domains.
"Each technique demonstrated this intrinsic polarization preference, likely the origin of the back-switching and poor coding performance in these ferroelectrics," Han said. "But these domain structures should require a lot of energy and thus be very unstable. The interface effect alone cannot explain their existence."
The scientists used another ultra-precise technique to probe the material's interface: electron energy loss spectroscopy (EELS). By measuring the energy deposited by an electron beam in specific locations—a kind of electronic fingerprint—the scientists determined the material's chemical composition.
"We suspect that more oxygen could be missing near the surface of the thin films, creating electron pockets that may neutralize positive charges at the domain walls," Han said. "This oxygen deficiency naturally forms in the material, and it could explain the stabilization of head-to-head domains."
This electron-swapping oxygen deficiency—and its negative effects on reliably storing data—might be corrected by additional engineering, Han said. For example, incorporating a "sacrificial layer" between the ferroelectric and the substrate could help block the interface interactions. In fact, the study may inspire new ferroelectrics that either exploit or overcome this unexpected charge phenomenon.