Critical Scaling of Domain Dynamics in Ferroelectric Nanoelements

The potential for ferroelectric materials to influence the future of small scale electronics cannot be overstated. At a basic level, this is because ferroelectric surfaces are charged, and so interact strongly with charge-carrying metals and semiconductors - the building blocks for all electronic systems. Since the electrical polarity of the ferroelectric can be reversed, it can both attract and repel charges in nearby materials, exerting complete control over both the charge distribution and movement within the device. It should be no surprise, therefore, that microelectronics industries have already looked very seriously at harnessing ferroelectric materials in a variety of applications, from solid state memory chips (ferroelectric random access memories, or FeRAMs) to field effect transistors (ferroelectric field effect transisitors, or FeFETs). In all such applications, switching of the direction of the polarity of the ferroelectric is the most important aspect of functional behaviour. The mechanism for switching invariably involves the field-induced nucleation and growth of domains. Domain coarsening, through domain wall propagation, eventually causes the entire ferroelectric to switch its polar direction. It is therefore the existence and behaviour of domains under the influence of an external bias field that determine the switching response, and ultimately the performance of the ferroelectric in any given electronic device. Understanding domains and domain dynamics is therefore the key to fully understanding switching behaviour and eventually rationalizing and predicting device performance.However, integrating ferroelectrics into commercial devices has not been altogether straightforward. One of the major issues has been that the properties associated with ferroelectrics, in bulk form, appear to change quite dramatically and unpredictably when at the nanoscale: new modes of behaviour, and different functional characteristics appear. For domains, in particular, the proximity of surfaces and boundaries has a dramatic effect: surface tension and depolarizing fields both serve to increase the equilibrium density of domains, and domain walls, such that minor changes in scale or morphology at the nanoscale can have major ramifications for domain redistribution. Given the importance of domains in dictating the overall switching characteristics of a device, the need to fully understand how size and morphology affect domain behaviour in small scale ferroelectrics is obvious. That the near future plans for microelectronic ferroelectric devices are to move from simple planar 2D to more complex 3D architectures, only increases the imperative for study. This proposal seeks to map and understand the manner in which reduced size and increased morphological complexity affect the switching behaviour of small scale ferroelectrics. Our revolutionary approach will be to make devices in which single crystal ferroelectric material has been machined to thin film dimensions using focused ion beam milling (FIB). 'Stroboscopic Piezo-Force Microscopy (PFM)' will be used to map the dynamics of domain wall motion during in-plane switching, induced by an external electric field dropped between coplanar electrodes. Observations made on nanoscale domain dynamics can then be meaningfully correlated to the measured 'macroscopic' functional behaviour of the devices. Using FIB to machine holes and slits into the thin ferroelectric slabs will allow us to directly investigate the manner in which physical defects alter the nucleation and propagation of domain walls. The study will also be extended to investigate axial switching of discrete FIBed single crystal ferroelectric nanowires with and without topographic complexity (in terms of notches, antinotches and kinks). Prior support on static domain states in passive ferroelectric nanoshapes has enabled this research, but there is no overlap - this new work concerns domain dynamics in active devices.

This collaboration will boost progress in high impact fundamental exploratory work on meso and nanoscale ferroelectrics. The ongoing work at QUB on nanoscale single crystals is already unique in the world at the moment, and has already generated results that have had wide interest, been published in high impact general science journals (and been highlighted in 'Chemistry World' for example) and induced invitations to speak on the topic at prestigious major international conferences, with attendees from all aspects of Materials Science and Physics (MRS and APS Meetings), from academia and industry. The impact of the work is strong and goes well beyond the immediate professional circle carrying out similar research. The same is true for the ongoing developments and insights generated by Prof Gruverman at UNL using the 'stroboscopic' PFM to analyze switching dynamics in conventional thin film meso and nanoscale structures. The combination of these two state-of-the-art strands of research to allow for domain dynamic mapping in shape engineered single crystal nanoscale ferroelectrics, and the expected resulting fundamental insight that should be generated, should have tremendous impact, leading to high value science of fundamental interest and of use for future applications of ferroelectrics in microelectronics. Thus, industrialists involved in the production of devices incorporating meso and nanoscale ferroelectrics should gain a great deal. Small scale ferroelectrics are of great interest for a range of electronic, transducer and energy harvesting devices, such as non-volatile memory, dynamic memory, field-effect transistors, high charge-density capacitors, piezoelectric nanopositioning devices and ultrahigh frequency resonators; crucially these are generally not mature technologies (although FeRAM has been in production for a number of years) and industrialists therefore need to keep an eye on the advances in science produced by the academic community. Indeed, many of the international conferences in which progress on our work is presented are heavily populated by industrialists. The wider society should ultimately benefit from the research we propose. Electronic devices incorporating small scale ferroelectrics are already responsible for the memory systems used in many contactless Smart Cards, such as those used on the public transport networks in Tokyo. Games consoles, such as the PlayStation series have used ferroelectric memory devices. Equally, ultrahigh resolution inkjet printers of the future, or nanoscale vaccine injection systems might rely on controlling the domain dynamics in ferroelectric nanotubes. Improvements in technology based on improved basic understanding of ferroelectric behaviour at the nanoscale should help expand these kinds of applications, and play a part in improving the quality of life for many. To ensure that the impact of the work is maximised, we will continue to publish in high impact general journals, and continue to disseminate the work through conferences as widely as possible, and to as many different audiences as possible. We will also continue to cooperate in dialogue with industry and we will advertise progress and key findings on our website. Chance approaches, such as that recently made by Frost and Sullivan, a company that informs research, technology and business development executives about new developments in high-tech materials which may show commercial promise, will continue to be taken. We are always aware of the potential for exploitation, and before releasing results and insights we review opportunities for patent protection of IP. This is done in conjunction with the dedicated QUB support team involved in academic IP, and we will co-ordinate with similar offices in UNL. We have prior success in inter-university protection of IP (we currently hold a joint QUB-Cambridge patent which resulted from previous EPSRC support).