GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, October 2^nd, 2020
3:00 PM

via

BlueJeans Video Conferencing

https://bluejeans.com/336468853

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Cooper A. Voigt

"An Investigation of the Role of Defects in the Thin-Film Synthesis Mechanism and Functional Properties of α-In₂Se₃"

Committee Members:

Prof. Eric M. Vogel, Advisor, MSE

Prof. Brent K. Wagner, GTRI

Prof. Nazanin Bassiri-Gharb, ME, MSE

Prof. Mark D. Losego, MSE

Prof. Asif I. Khan,  ECE

Abstract:

α-In_2­Se₃ is a semiconducting, two-dimensional van der Waals material that was discovered to be ferroelectric down to the monolayer thickness in 2017. This unique combination of properties makes this material promising for improving the performance of ferroelectric tunneling devices being considered for a beyond CMOS computing paradigm. Promising performance of α-In_2­Se₃-based devices has already been demonstrated in the literature; however, most of these studies have explored exfoliated α-In_2­Se₃ flakes and only a few have demonstrated thin-film synthesis of the ferroelectric phase. Furthermore, despite the fact that defects are ubiquitous in materials and often strongly coupled to a material’s properties, there are no reports on how defects affect the thin-film synthesis mechanism and the functional properties of α-In₂Se₃­. This proposal is organized into four main research aims designed to address these gaps in the literature.

The first aim of this proposal is to improve understanding of the thin-film synthesis mechanism of α-In₂Se₃ to gain control of the defect density of α-In₂Se₃ films. α-In₂Se₃ thin-films will be synthesized via molecular beam epitaxy (MBE). Substrates spanning a variety of defect densities and crystalline surface structures will be used as substrates. A metal-organic Se source to achieve a monatomic Se flux, and an effusion cell sublimation of polyatomic Se species will be used to compare the effect of different precursors. The morphology of the α-In₂Se₃ crystals will be characterized by SEM and AFM and the stoichiometry and crystalline phase will be characterized by XPS and Raman spectroscopy, respectively, to gain insight into the synthesis kinetics and thermodynamics.

The second aim of this proposal is to elucidate the surface electronic structure of α-In₂Se₃ and select heterostructures via combined X-ray/Ultraviolet photoelectron spectroscopy (XPS/UPS) and angle-resolved XPS. Defects can introduce additional electronic states in the bandgap of a material, modifying the space-charge layer and its coupling to the polarization and therefore are important to understand for engineering electronic devices. α-In₂Se₃ thin-films of varying defect density will be synthesized on vdW semiconductor, insulating and metallic substrates.

The third aim of this proposal is to understand how defects, screening and dimensionality affect the polarization domain dynamics in α-In₂Se₃ via piezoresponse force microscopy. Intrinsic dipole locking, free charge-carriers at room temperature and intercorrelated in- and out-of plane polarization make α-In₂Se₃ a unique platform for studying polarization domain dynamics. Defects are also well known to influence domain dynamics in other ferroelectric systems and therefore it is critical to understand their effect in α-In₂Se₃ to enable engineering of ferroelectric devices.

The fourth aim of this proposal is to correlate the synthesis parameters and measured materials properties to electrical characterization of α-In₂Se₃-based ferroelectric Schottky diodes. Each aim will deepen understanding of α-In₂Se₃ and together will enable engineering of ferroelectric devices with superlative performance.