Towards the Further Miniaturization of FeRAM
Ferroelectric Random Access Memory, or FeRAM for short, offers both working memory and data storage in one. This saves on the time and energy needed by conventional computers to transport data between the two units. In addition, the saved data is retained even without a power supply. The write performance and service life of these components are already outstanding and the first FeRAMs are already in use, for example in chip cards or RFID tags. However, comparatively small amounts of data can as yet be stored on a FeRAM, as the space required for storing individual bits is too large. The latest studies using electron microscopes by scientists at Forschungszentrum Jülich and RWTH Aachen University now indicate a way to reduce the size of FeRAM bits by more than a factor of 100. The JARA-FIT members Prof. Rafal Dunin-Borkowski, Prof. Joachim Mayer and Prof. Ulrich Simon with their respective institutes are significantly involved in the investigations.
At the heart of any FeRAM are ferroelectric materials. They consist of tiny crystals that can be polarized by an electric field. During this process, differently charged ions in the crystal lattice are pushed against each other slightly, a state that remains even after the electric field is deactivated. Each polarized area can thus encode one bit of information. The electric field is also used to read out data: if the polarisation was changed during the write-in, this is revealed by a change in current strength during the read-out process. Since the cell content is erased during this read process, it is followed by another write process.
Previous FeRAMs used classical ferroelectrics, such as lead zirconate titanate (PZT), which is also commonly found in piezomechanical actuators. A miniaturisation of such FeRAM structures to below 130 nm has not been possible in FeRAM so far. Because ferroelectric properties were recently discovered in nanocrystalline, 10 nanometre-thick films of hafnium oxide, the researchers from Jülich and Aachen took a very close look at this material. A contrast enhancement method developed in Jülich for ultra-high-resolution electron microscopy enabled them to make visible not only the comparatively easy-to-detect hafnium heavy metal atoms, but also the oxygen atoms. The extremely high-resolution microscopes at the Jülich Ernst Ruska-Centre for high resolution microscopy and spectroscopy with electrons (ER-C) were used, which make it possible to measure the tiniest displacements of the atoms in the crystals.
It emerged that of particular interest are those structures in hafnium oxide where two mirror-image nanocrystals are adjacent to each other. “In these areas, we found stable ferroelectric regions starting at a size of only four cubic nanometres. This is an order of magnitude smaller than the nanocrystals we have produced chemically,” explains Dr. Hongchu Du from RWTH Aachen University, currently a visiting scientist at the Jülich ER-C. As a result, numerous polarized regions can be contained in a nanocrystal and FeRAM structures in the sub-nanometre range become possible.
“Interestingly, the polarisations here occur without the usual associated phase transitions between ferroelectric and paraelectric order,” Hu adds. Such phase transitions do not change the composition of the materials, but rather their electrical properties and patterns. These are accompanied by changes in symmetry properties. This opens up previously unanticipated possibilities for finding suitable structures even in materials that have not been in the spotlight so far. The researchers already have some ideas in this direction.
“In terms of practical applications, the next step could be to integrate individual nanocrystals into circuits based on nanoelectrodes,” explains Prof. Ulrich Simon, Managing Director of the Institute of Inorganic Chemistry at RWTH Aachen University. “We are working on this within the Collaborative Research Centre “Nanoswitches”, funded by the German Research Foundation, and have already been able to develop a comparable approach for metal nanoparticles with colleagues from the Peter Grünberg Institute”.
Multiple polarization orders in individual twinned colloidal nanocrystals of centrosymmetric HfO2;
Hongchu Du et al.;
Matter, published January 6, 2021, DOI: https://doi.org/10.1016/j.matt.2020.12.008