Doctor of Philosophy, University of California Berkeley (2018)
Beyond Substrates: Strain Engineering of Ferroelectric Membranes.
Advanced materials (Deerfield Beach, Fla.)
Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO3 , production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75to 425°C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10kV cm-1 and improving switching times to <5ns for a 20m diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1%strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.
View details for DOI 10.1002/adma.202003780
View details for PubMedID 32964567
Strain-induced room-temperature ferroelectricity in SrTiO3 membranes.
2020; 11 (1): 3141
Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.
View details for DOI 10.1038/s41467-020-16912-3
View details for PubMedID 32561835
Freestanding Oxide Ferroelectric Tunnel Junction Memories Transferred onto Silicon.
Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO3 films grown epitaxially on SrTiO3 growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.
View details for DOI 10.1021/acs.nanolett.9b01327
View details for PubMedID 31136184