Microstructure and strain effects on ferroelectric and transport properties of hafnium oxide thin films

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Nontechnical: Ferroelectrics such as hafnium oxide have promising applications in data storage and memory processing. These materials have a spontaneous electrical polarization that can be switched by applying an electric field, akin to ferromagnetism. The PIs will study the fundamental properties of hafnium oxide and the performance of hafnium oxide-based devices. The ultimate aim is to develop ferroelectric memory devices with superior performance. Graduate and undergraduate students will benefit from education and training in the science and technology of ferroelectric materials and devices. Students from underrepresented groups will be provided valuable educational experiences that enrich their professional development. Outreach programs aim at K-12 students and Nebraska residents through open public events, such as the State Science Olympiad and Nanocamp. These activities will integrate research with community engagement.Technical:The discovery of ferroelectricity in HfO2 films has recently attracted enormous interest due to its best compatibility with CMOS technology among known ferroelectrics. Thin films of HfO2-based ferroelectric materials exhibit robust switchable polarization and low leakage, and thus have a huge potential for being used in nonvolatile memories and ferroelectric field-effect transistors with enhanced performance. To realize this potential in practice, systematic studies of the structure-property-device performance relationship in these materials are required. In contrast to conventional perovskite ferroelectrics, the mechanism of ferroelectricity in HfO2-based films is far from being understood. The microstructure and strain conditions likely control the formation of the ferroelectric phase in polycrystalline HfO2 films, but there is no clear method for stabilizing it in a monocrystalline film. This proposal aims at achieving a comprehensive understanding of how the interplay between the film microstructure and interfacial stress affects the stability of the ferroelectric phase, polarization reversal dynamics, electronic transport behavior, and the related device performance. This project studies (1) theory-driven fabrication of the HfO2-capacitor structures with controlled microstructure and electromechanical boundary conditions stabilizing the ferroelectric state; (2) the effect of film microstructure on the polarization reversal mechanism in the epitaxial, textured, and polycrystalline HfO2-based films; and (3) the polarization-controlled resistive switching in the HfO2-based ferroelectric tunnel junctions as a function of the ferroelectric barrier microstructure. The effect of epitaxial strain on stabilizing the ferroelectric phase in single-crystalline orthorhombic and rhombohedral HfO2 films will be explored using theoretical modeling based on density functional theory and verified by electrical and structural characterization of the HfO2 films grown using pulsed laser deposition on the substrates and buffer layers. The time-bias-dependent switching behavior will be investigated as a function of the HfO2 films microstructure via direct observation of the domain structure evolution by piezoresponse force microscopy (PFM) and measurements of the device-level integrated transient currents by pulse testing methods. Local probe microscopy in conjunction with structural characterization methods will be used to relate the tunneling electroresistance effect to the polarization state stability and microstructure of the HfO2-based tunnel junctions. Thus, the intellectual merit of this project will be elucidation of the relationship between the polar phase stability, film microstructure and interface strain, clarification of the mechanism of polarization reversal, and demonstration and quantification of the resistive switching in the HfO2-based ferroelectric films and device structures.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.