Unraveling Tunable Piezoelectricity in Hafnia-based Ferroelectrics

Nontechnical Description Ferroelectric materials have a spontaneous electric polarization that can be reversed by the application of an external electric field. They have many applications in electronic devices, such as non-volatile memories, optical filters and pressure sensors. All ferroelectrics are also piezoelectric. They change shape under an applied electric field and can also convert mechanical stress to electrical energy. Hafnia oxide ferroelectrics are unique in that their piezoelectric behavior depends on how they are made and the electrical cycling history. While these are interesting materials, the body of knowledge about hafnia piezoelectricity is largely empirical. The community lacks a fundamental understanding of precisely how such factors determine the piezoelectric properties. This project directly addresses that gap in knowledge. It brings together an interdisciplinary team to perform systematic experimental studies and theoretical modeling of the piezoelectric behavior of hafnia films with controlled microstructure. The ultimate aim is to achieve tunability of the piezoelectric response of hafnia. This in turn will enable design of the devices with enhanced electromechanical performance. The international nature of this project enhances the research, education, and outreach missions of the University of Nebraska and the Luxembourg Institute of Science and Technology. The project prioritizes an increase in the number, quality, and diversity of students pursuing careers in science and technology. Outreach activities to promote science literacy will help to build a culturally diverse community of scientists and educators and enrich their professional preparation and education experience. Technical Description Hafnia-based ferroelectrics are among the most actively studied groups of materials due to the vast range of fundamentally and technologically captivating properties. One of the most intriguing and unique characteristics of these materials is extremely high sensitivity of their piezoelectric properties on a variety of extrinsic factors, which causes a significant discrepancy between the theoretically predicted piezoelectric behavior and broad variations of the experimentally measured parameters. The current international collaborative project seeks to address this outstanding controversy by achieving a fundamental understanding and deterministic control of the piezoelectric properties of the hafnia-based ferroelectrics by adopting an approach based on synergy between theoretical modeling and systematic testing of the role of the intrinsic and extrinsic factors in the piezoelectric behavior. Experimental studies carried out both at the nanoscale and global levels using a combination of the local probe microscopy and time-resolved synchrotron measurements focus on investigation of the ferroelectric and electromechanical properties of the epitaxial, polycrystalline, and free-standing hafnia thin film capacitors as a function of thickness, composition, substrate and film microstructure as well as on evaluation of the effect of electrical cycling and mechanical strain modulation on evolution and tunability of the electromechanical properties. Theoretical studies involve first-principles modeling of the electromechanical response of thin films and free-standing membranes with the goal to assess a role of the intrinsic and extrinsic factors in piezoelectric tunability and provide guidance for the experimental studies. The project is performed in collaboration with the Luxembourg Institute of Science & Technology supported by the Luxembourg National Research Fund. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.