Radiation-tolerant ferroelectric materials for multifunctional devices

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Chin, Evelyn
Bassiri-Gharb, Nazanin
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Ferroelectric materials have switchable, spontaneous polarization in addition to strong dielectric, pyroelectric and piezoelectric response. In thin films form, these materials are leveraged for numerous microelectronic devices, including mechanical logic elements, optical sensors and transducers, precision positioners, energy harvesting units, nonvolatile memory storage, and microelectromechanical systems (MEMS) sensors and actuators. Ferroelectric materials have also become attractive for use in devices for radiation-hostile environments (e.g. aerospace, medical physics, x-ray/high energy source measurement tools, nuclear monitoring systems) due to their relatively high radiation tolerance. An increased understanding of material properties responsible for radiation tolerance will allow for development of materials for the next generation of radiation-tolerant, multifunctional devices. Lead zirconate titanate (PZT), one of the most commonly used ferroelectric materials for microscale applications, is widely known for its high polarization and piezoelectric response. However, increasing demand for smaller device footprint has pushed research efforts on PZT thin films towards their limitations, creating a need for new material systems to exceed the current standards. In this thesis, two material systems are explored as radiation tolerant ferroelectric alternatives to PZT: 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) and Hf0.5Zr0.5O2 (HZO). PMN-PT films exhibit strong piezoelectric response, exceeding that of PZT, making them a strong candidate for next generation piezoelectric MEMS devices. Additionally, the large amount of chemical, polar, and structural heterogeneities in this material imply a large degree of entropy, which could result in accommodation of radiation-induced defects and enhanced radiation tolerance. HZO thin films exhibit strong polarization properties at only a few nanometers in thickness. Combined with its CMOS compatibility and the potential to fabricate complex 3D structures using atomic layer deposition, HZO has become an attractive material for (ferroelectric) non-volatile memory applications. Total ionization dose (TID) studies, using gamma-radiation doses up to 10 Mrad(Si), were performed to understand the radiation tolerance of PMN-PT and HZO thin films. Processing-structure-property relations were explored to identify the material characteristics responsible for both high functional response and high radiation tolerance. PMN-PT thin films were confirmed to exhibit equivalent or superior radiation tolerance in dielectric, polarization, and piezoelectric response than PZT thin films, largely unaffected by microstructural differences. Although the HZO thin films suffered significantly from aging, the films fabricated via plasma-enhanced atomic layer deposition exhibited superior radiation tolerance in polarization response than PZT thin films. The studies illustrate different pathways for concomitant enhanced functionality and higher radiation tolerance in ferroelectric thin films.
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