THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, July 27, 2020

1:00 PM

via

BlueJeans Video Conferencing

https://bluejeans.com/674108390

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Sarah Lombardo

“In Situ Characterization of Polarization Switching in Antiferroelectric Fluorite-Structure Binary Oxide Thin Films for Logic and Memory Applications”

Committee Members:

Prof. Joshua Kacher, Advisor, MSE

Prof. Asif Khan, Advisor, ECE

Prof. Matthew McDowell, MSE

Prof. Rampi Ramprasad, MSE

Prof. Andrew Kummel, ChE, UCSD

Abstract:

The need for novel, super-high K dielectric gate oxides has substantially increased in recent years. With the EOT in advanced nodes reaching a limit, materials innovation can enable increased dielectric constants in gate oxide stacks beyond the current limit, providing significant enhancement in logic technologies. Capacitance enhancement and super-high K dielectric gate stacks require the use of ferroelectrics (FEs) to be stabilized in an otherwise unstable state, resulting in an effective static negative capacitance.  In well-known perovskite-based ferroelectrics, a FE-DE stack results in a depolarization field changing the ferroelectric into a high energy unstable negative capacitance state, causing a tetragonal to cubic phase transition. However, the equivalent unstable state in fluorite-based FEs (i.e., HfO2- and ZrO2-based FEs) – offering full scalability and CMOS compatibility – is not well-known, and it is the competition between the polar orthorhombic and non-polar monoclinic and tetragonal phases that dictates the overall properties of these films. Additionally, antiferroelectricity has been reported in ZrO2-based thin films, making them promising candidates for high-density energy storage, neuromorphic oscillators, and nonvolatile memory applications. Antiferroelectrics exhibit interesting non-linearities in their charge-voltage characteristics (i.e. double hysteresis loops). From a microscopic perspective, antiferroelectricity is a phenomenon in which an electric field induces a first-order, structural phase transition between a non-polar, ground state and an energetically similar, polar active state.  Since the discovery of antiferroelectricity in ZrO2, it has been well-recognized that the electrical characteristics associated with the field-induced phase transition in these materials can solve some of the most pressing challenges in modern microelectronics (energy efficiency, sub-Boltzmann logic technologies, memory and neuromorphic applications, etc.).  While this sets the stage for post-scaling electronics, the physical origin of antiferroelectricity in HfO2- and ZrO2-based ferroelectrics has yet to be unanimously determined nor the phase transition experimentally visualized.

There are significant gaps in our fundamental understanding of the polycrystalline nature of ALD HfO2/ZrO2-based FE/AFE thin films.  Since polarization correlates with crystal structure, the application of an electric field alters the microscopic features, e.g. grain orientation, phase, size, and sub-grain characteristics (interphase boundaries and domain walls) of these materials. This complex evolution of microstructure enables electrical characteristics such as multi-level cell capabilities for embedded non-volatile memory, analog synapses, and abrupt transitions for artificial neurons.  On the other hand, such evolution of microstructure poses significant challenges to performance including cycle-to-cycle and device-to-device variation, reliability, and endurance. The first step towards identifying structure-performance relationships in these materials is the direct imaging of the polarization switching at the atomic and mesoscopic scales with applied bias. Due to the end of dimensional scaling of transistors, materials innovation is more crucial now than ever before to the advancement of microelectronics and modern computing.  Understanding the crystallographic pathways for ferroelectricity and antiferroelectricity in HfO2- and ZrO2-based thin films will, therefore, provide significant insight into processes necessary to optimize these material properties and enhance device performance while reducing power consumption in post-scaling electronics.