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Integrated ferroelectric oxides for energy conversion devices

Periodic Reporting for period 1 - FERROENERGY (Integrated ferroelectric oxides for energy conversion devices)

Reporting period: 2019-02-08 to 2021-02-07

The rapid development of the semiconductor-based technology has enabled today’s functionalities and convenience that seemed impossible just a generation ago. Today, we rely upon numerous electronic devices that are pervasive around us to augment, accelerate, and alleviate countless tasks. In order to maintain the current technological progression into the future, and to make this progress sustainable for the next generation, the performance of these devices must be improved in a more energy-efficient way.

Oxide perovskites – a family of materials displaying a vast diversity of physical properties – are a promising alternative for creating the superior technologies that could ensure this extended progress. Despite their promise, the two main obstacles currently impeding their implementation are:

- Continued lack of a complete understanding of the microscopic phenomena governing the properties, and

- The difficulty in integrating such materials with existing processes in the semiconductor industry.

A commonly used strategy to explore and manipulate perovskite properties consists on synthesizing nanoscale single crystal films, which allow for complete control over structure-properties relationships. This is specially relevant in systems showing polar order (e.g. ferroelectrics) where the electrical polarization is directly connected to structural distortions. The structural control in nanometric films is typically given via the growth coherence on single-crystal substrates, however - besides the synthesis incompatibility with semiconductor processes- this strategy imposes two restrictions:

- Mechanical clamping: The strong binding between the substrate and the film negatively affects the intrinsic functionality of the latter (the function is “clamped” by the rigid substrate).

- Thermal connection with the substrate: The much larger thermal mass of the substrate causes the ultrafast thermalization of the films, difficulting the direct measurement of temperature changes caused by external stimuli (stress, electric/magnetic fields)

This Project exploits a novel fabrication process, inspired by the manipulation of single atomic layers such as graphene, but applied in a completely new way: to produce freestanding oxide nanomembranes. These membranes can be easily integrated in structurally and chemically incompatible substrates, such as silicon or polymers, and devices can be fabricated on these technologically relevant platforms, thus opening the way to develop a new generation of multifunctional nanoelectronic devices compatible with CMOS processes and integrable in flexible platforms.

The membranes can also be manipulated and studied in novel ways beyond the current possibilities of clamped epitaxial films. For example, much larger strains (and strain gradients) can be applied by stretching (or bending) the freestanding membranes, or one can directly explore the thermal response of the membranes by isolating microscopic regions from the thermal contact with a substrate. These novel strategies make it possible to explore new physical phenomena that so far was out of reach for the oxides research community, and will contribute to find new routes towards the optimization of energy conversion capabilities of future devices based on oxide perovskites.
During the initial period of the action, several oxide perovskites were synthesized as thin films and membranes integrated in silicon and flexible polymers. Three lines of work were devised from here:

1 - Strain engineering of ferroelectric membranes: The main line of research here was the demonstration of the manipulation of structure and ferroelectric properties of the membranes by using the interlayer stress in tri-layer membranes, as well as by tuning the composition of the electrode and functional layers. This lead to ferroelectric capacitor devices integrated on silicon and flexible polymer platforms with strain-engineered properties, namely tunable ferroelectric transition temperatures, polarization values, switching voltages and switching speeds. The latter two hold promising for ultrafast, low-voltage operation in ferroelectric memories. In addition to interlayer stress, induced stress by bending the flexible polymer platforms was proved an efficient tool for mechanical manipulation of the membrane properties, demonstrating large dielectric tunability under low stress application.

2 - New pathways to explore the physics of ferroelectrics with complex microscopic order (relaxors). Relaxor membranes were transferred to grids for examination in Transmission Electron Microscopy. Ongoing experiments at Stanford University (Aaron M. Lindenberg' group) utilizing ultrafast x-ray scattering measurements are expected to reveal new insights into the complex dynamics of these materials.

3 - Direct measurement of electrocaloric effects on ferroelectric membranes. This line of work required the fabrication of freestanding ferroelectric capacitors and measurement of electrocaloric effect via infrared imaging. The transfer of capacitor devices to thermally insulating and holey substrates (microfabricated in the lab) was performed during the secondment period at the Barcelona Institute of Microelectronics (IMB-CNM). Preliminar measurements of ferroelectric polarization/capacitance in the fabricated devices resulted unsatisfactory and further optimization of the fabrication process is required in order to perform infrared experiments. The collaboration with IMB-CNM has been agreed to continue during the returning phase of the project. In parallel, calorimetry experiments on ferroelectric membranes are being performed in collaboration with Autonomous University of Barcelona.
The main beyond-state-of-the-art progress obtained in the reporting period consisted on demonstrating:

1 - A high-yield fabrication method to produce high-quality single-crystal nanometric membranes of several ferroelectric oxide materials, of high interest to the researchers working in the oxide perovskites community, where there is an increasing interest in the membrane fabrication process to further exploit and manipulate the oxides functionalities and integrate them on platforms typically incompatible by conventional synthesis methods.

2 - The excellent performance of CMOS-integrated ferroelectric capacitor devices. In particular, ultrafast operation and low power consumption, making these prototypical devices highly appealing for next generation non-volatile storage devices.

3 - Highly-efficient stress manipulation of dielectric properties in ferroelectric devices integrated on flexible polymer substrates via bending-induced modification of domain microstructure. This demonstration provides a promising path for applications on nanosensors.

The fabrication strategy developed during the initial reporting period will be further exploited during the returning phase, to elaborate novel protocols to explore electrothermal and electromechanical coupling in ferroelectric membranes, providing further insight into the range of applicability of these materials in relevant technological platforms. Ongoing calorimetry studies on thermally isolated membranes, and electrical actuation experiments on freestanding capacitors are expected to expand on the functionalities achievable via the developed methodologies.
Strain-engineering of dielectric properties in integrated single-crystal ferroelectric membranes