Beyond Substrates: Strain Engineering of Ferroelectric Membranes

Abstract

Integrating the diverse functionalities of complex oxides into semiconductor [1-7] and flexible [8-12] electronics is a major technological challenge that has motivated extensive work. But despite considerable effort, the structural, chemical, and thermal mismatches between such substrates and complex-oxide materials often yield films with considerably worse crystal quality than that attained on single-crystal perovskite substrates. Alternative strategies for hetero-integration via substrate release and transfer, mimicking the fabrication processes of van-der-Waals heterostructures, [13-15] are just now beginning to yield single-crystal oxide films on arbitrary substrates, [16-21] thus circumventing the constraints of epitaxial growth. Moreover, these detached films no longer experience mechanical constraint from the substrate, giving more flexibility in structural manipulation and heterostructure assembly. Strain control over the lattice structure is particularly impactful in ferroelectrics where the polarization is directly connected to structural distortions. [22,23] In thin films, strain can define the optimal operating temperature regime [24-26] or domain configuration [27-29] that will boost electro-mechanical or thermal functionalities, 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 BaTiO 3 , production of precisely strain-engineered, substratereleased 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 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm −1 and improving switching times to <5 ns for a 20 µm 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. Perovskite ABO 3 oxides can display an immense number of phases and functions by merely changing the A-and B-site cations. Even within a single chemistry, multiple phases can be in competition, and their stability can be tuned statically by fixing The ORCID identification number(s) for the ...

Beyond Expectation: Advanced Materials Design, Synthesis, and Processing to Enable Novel Ferroelectric Properties and Applications

“…All these synthesis processes avoid strong bonding of the films with the substrate, thus expanding the combinatorial approaches to fabricate multifunctional devices. Beyond this, the film-transfer methods also provide opportunities to explore new phases/properties when approaching the two-dimensional limit [75,76], target enhancement of properties typically clamped by the substrate in epitaxial systems [72], and provide mechanical control of properties and domain structures beyond the current limitations of heteroepitaxy [77,78]. These processing techniques are a "hot" field of work and new exciting studies are expected to appear in the next few years.…”

The role of lattice dynamics in ferroelectric switching

“…Further, BFO follows a two-step switching pathway consisting of out-of-plane (109º) and in-plane (71º) steps, thereby, offering an ideal system to study how substrate constraints impact switching. These measurements are also complemented by similar measurements in the literature [26] on thin films of ferroelectric BTO, a prototypical tetragonal ferroelectric, which is used to highlight how substrate clamping impacts a variety of ferroelectric materials. The data reveal a clear impact on the switching voltage (a measure of the barrier energy) as well as the switching dynamics (as manifested by changes in the switching time).…”

“…While there have been a large number of studies of quasi-static switching behavior and equilibrium properties of thin films [16]- [20], there have been fewer studies of the limits and timescales of ultrafast switching [21]- [25], and even fewer on the role of lattice dynamics in influencing ferroelectric switching [26]. A key question is how to quantify the role of the substrate in dictating the switching process and whether switching can be studied experimentally without the influence of the substrate.…”

“…In order to account for any such effects, we normalized all switching times to the measured RC-time constant for each device (Fig. 3d) [21], [26]. Clear decreases in switching time persist even after such normalization verifying that the observed changes are from the mechanical clamping and not changes to extrinsic-circuit parameters.…”

Strain Engineering: A Pathway for Tunable Functionalities of Perovskite Metal Oxide Films

“…Recently, more approaches for strain engineering have come to the fore. Examples include mechanically straining free-standing single-crystalline membranes [ 8 , 14 ] grown on the buffered substrate where the buffer layer can be etched off thereafter, and the ion implantation of species such as helium [ 15 ] and nitrogen [ 16 ] to strain the lattice. Perovskite materials have also garnered interest in a wide range of applications such as solar cells, photo/electrocatalysis, photopolymerization, thermoelectrics, resistive switching devices, etc.…”