Emergent Phenomena in Ferroelectric-Dielectric and Ferroelectric-Graphene Heterostructures
Thesis Defense
1:00 pm –
3:00 pm
Jorgensen Hall Room: 247
Virtual Location:
Zoom
Target Audiences:
Contact:
Physics Department, (402) 472-2770, paoffice2@unl.edu
Tianlin Li will defend his thesis topic, “Emergent Phenomena in Ferroelectric-Dielectric and Ferroelectric-Graphene Heterostructures”.
ABSTRACT: This dissertation explores the emergent phenomena in epitaxial ferroelectric oxide-based heterostructures, including the negative capacitance (NC) effect in ferroelectric-dielectric systems, ferroelectric gate-imposed mobility limit in graphene field-effect transistors, and ferroelectric domain-induced graphene-superlattices.
We carry out a phenomenological study of the NC effect in PbZr0.2Ti0.8O3/SrTiO3 capacitors. The Landau free-energy profile predicts a critical thickness ratio for the ferroelectric-dielectric phase transition, consistent with the observed abrupt change in remnant polarization. A static NC state is observed in the low thickness ratio region, which is compromised by the emergence of multidomain in the moderate ratio region. We simulate the transient NC effect near the critical ratio using the nucleation-limited switching model. As the STO content increases, the switching time evolves from a narrow distribution in the ferroelectric state to a significantly broadened distribution in the multidomain state. This study provides critical parameters for the design of NC modes in low power nanoelectronics.
We then explore remote optical (RSO) phonon scattering in graphene back-gated by ferroelectric Ba0.6Sr0.4TiO3. The devices exhibit high field-effect mobility of up to 23,000 cm2V?1s?1, along with nonvolatile modulation of resistance and quantum Hall effect induced by polarization switching. The temperature dependence of resistivity in graphene can be explained by scattering from intrinsic longitudinal acoustic phonons and RSO phonons, with the latter dominated by the mode at 35.8 meV. This study reveals the dominating effect of RSO phonon scattering in limiting the room temperature mobility of ferroelectric gated graphene transistors.
One-dimensional graphene-superlattice under strong Kronig-Penney potential is promising for achieving the electron lensing effect. We realize one dimensional graphene-superlattice via designing periodic domain structures in a ferroelectric backgate. Magnetotransport studies reveal strong transport anisotropy and emergent satellite Dirac points, whose positions scale inversely with the superlattice period, from band reconstruction. The results are well explained by the high Kronig-Penney potential scenario, with the transverse Fermi velocity quenched to about 1% of that for pristine graphene. Our study presents a promising material platform that can host flat band phenomena and electron supercollimation.
ABSTRACT: This dissertation explores the emergent phenomena in epitaxial ferroelectric oxide-based heterostructures, including the negative capacitance (NC) effect in ferroelectric-dielectric systems, ferroelectric gate-imposed mobility limit in graphene field-effect transistors, and ferroelectric domain-induced graphene-superlattices.
We carry out a phenomenological study of the NC effect in PbZr0.2Ti0.8O3/SrTiO3 capacitors. The Landau free-energy profile predicts a critical thickness ratio for the ferroelectric-dielectric phase transition, consistent with the observed abrupt change in remnant polarization. A static NC state is observed in the low thickness ratio region, which is compromised by the emergence of multidomain in the moderate ratio region. We simulate the transient NC effect near the critical ratio using the nucleation-limited switching model. As the STO content increases, the switching time evolves from a narrow distribution in the ferroelectric state to a significantly broadened distribution in the multidomain state. This study provides critical parameters for the design of NC modes in low power nanoelectronics.
We then explore remote optical (RSO) phonon scattering in graphene back-gated by ferroelectric Ba0.6Sr0.4TiO3. The devices exhibit high field-effect mobility of up to 23,000 cm2V?1s?1, along with nonvolatile modulation of resistance and quantum Hall effect induced by polarization switching. The temperature dependence of resistivity in graphene can be explained by scattering from intrinsic longitudinal acoustic phonons and RSO phonons, with the latter dominated by the mode at 35.8 meV. This study reveals the dominating effect of RSO phonon scattering in limiting the room temperature mobility of ferroelectric gated graphene transistors.
One-dimensional graphene-superlattice under strong Kronig-Penney potential is promising for achieving the electron lensing effect. We realize one dimensional graphene-superlattice via designing periodic domain structures in a ferroelectric backgate. Magnetotransport studies reveal strong transport anisotropy and emergent satellite Dirac points, whose positions scale inversely with the superlattice period, from band reconstruction. The results are well explained by the high Kronig-Penney potential scenario, with the transverse Fermi velocity quenched to about 1% of that for pristine graphene. Our study presents a promising material platform that can host flat band phenomena and electron supercollimation.