Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Yuhang Ren

Committee Members

Steven Greenbaum

Ying-Chih Chen

Karl Sandeman

Jun Ouyang

Subject Categories

Atomic, Molecular and Optical Physics | Condensed Matter Physics | Optics | Physics | Statistical, Nonlinear, and Soft Matter Physics

Keywords

Nonlinear Optics, Perovskites, Dielectrics, Strain Distribution, Second Harmonic Generation, Pump Probe Spectroscopy, Interfacial Ferroelectricity

Abstract

Dielectric and ferroelectric perovskite films have been model energy storage structures for their low-dielectric loss, extremely high charge-discharge speed, and good temperature stability, yet there is still much to understand about the material’s limitations. This dissertation presents a detailed understanding of the strain-induced ferroelectricity at the boundary between a strontium titanate (SrTiO3) ultrathin film epitaxially grown on a germanium (Ge) substrate through optical second harmonic generation (SHG), and the polydomain distribution in the Zr-doped BaTiO3 (BZT) films by time-resolved pump-probe spectroscopy.

First, SHG measurements were performed to reveal interfacial ferroelectricity in the epitaxial SrTiO3/Ge (100) heterostructure. An in-plane tensile strain from the Ge substrate introduced a collective atomic displacement of Ti4+ ions near the STO/Ge interface responsible for the generation of an in-plane ferroelectric polarization. These results are further confirmed by macrostructural analyses including TEM, XRD and electron diffraction along with phase field modeling (PFM). Ultrathin STO can be integrated on semiconductors with enhanced functional properties relevant to emerging nanoelectronic devices.

Second, sub-picosecond acoustic pulses were employed to investigate the polydomain structures in the ferroelectric BZT films. The long-living oscillations in the time-resolved reflectivity spectroscopy are attributed to the coherent acoustic phonons propagating through the BZT film and the STO substrate. The observation of longitudinal acoustic (LA) and transverse acoustic (TA) modes is attributed to the coexistence of mixed tetragonal and rhombohedral polydomain structures. The presence of polydomain structures is further confirmed through TEM and XRD measurements. These adaptive nanodomains allow for effective accommodation and energy performance optimization of the ferroelectric BZT films.

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