Session B38: Ferroelectric Oxide Heterostructures and Freestanding Membranes

Manipulating polar structures in freestanding antiferroelectric and ferroelectric membranes

Polar oxides display remarkable properties relevant for a wide number of applications such as non-volatile memories or energy conversion (thermal, mechanical, solar, chemical). Over the last 30 years, via different materials engineering processes (e.g. epitaxial strain or defect engineering), researchers have learned to tune and optimize the properties of these materials (1), however their inherent strong chemical bonds have prevented from exploiting the mechanical flexibility and heterointegration possibilities offered by exfoliable materials such as semiconducting dichalcogenides or layered oxides. Nowadays, with the recent developments of sacrificial layers compatible for the growth of perovskite films and for their substrate release, the synthesis of single-crystal membranes of perovskite oxides has become more accessible, enabling their integration CMOS processing or flexible electronics via near room temperature processes, or expanding the strategies for thermal, mechanical or optical actuation of these materials (2).

In this talk I will show recent progress on the synthesis and manipulation of polar ordering in ferroelectric and antiferroelectric nanomembranes and capacitors, fabricated after releasing epitaxial heterostructures grown on compatible oxide substrates. The consequences of the release from the substrate clamping on the polarization switching dynamics and energetics will be discussed for BaTiO₃ ferroelectric capacitors (3, 4), as well as for antiferroelectric PbZrO₃ capacitors, transferred to silicon. Freed from the mechanical constraints from the substrate, the freestanding membranes are susceptible to internal and external strains that allow manipulating their functional responses. In this regard, I will discuss the effect of interlayer stresses and substrate bending for the macroscopic control of electrical properties, and the impact of local strains at spontaneously occurring folding patterns in the membranes, favoring the emergence of complex domain patterns and functional domain walls.

1.        A. Fernandez, et al., Adv. Mater. 34, 2108841 (2022).

2.        D. Pesquera, et al.. J. Phys. Condens. Matter. 34, 383001 (2022).

3.        D. Pesquera, et al.. Adv. Mater. 32, 2003780 (2020).   

Hybrid improper ferroelectricity in A-cation ordered perovskite BaSrBi2O6: a first-principles study

We investigate structural, electronic, and ferroelectric properties of A-cation ordered perovskite BaSrBi₂O₆ using first-principles density functional theory. We systematically search for the metastable structures by constructing the initial structures with A-site orderings and octahedral rotation patterns commonly observed in perovskite oxides. In all of the metastable structures, we find that the breathing distortions observed in the bulk BaBiO₃ and SrBiO₃ are maintained, which stabilizes the charge-ordered insulating phase. More importantly, we identify a polar ground state with layered A-cation ordering and a⁻a⁻c⁺ octahedral rotation pattern with a net polarization of 5.7 μC/cm². Our distortion mode analysis shows that the polarization is induced from the hybrid improper ferroelectricity in which the polar A-site displacements from Ba/Sr layered ordering emerge from the coupling to the octahedral rotations. We believe that our method to identify the metastable structures can be used in a wide range of perovskite oxides and the proposed ferroelectric BaSrBi₂O₆ can be fabricated by layer-by-layer growth techniques.   

Ultraviolet Raman spectroscopy characterization of (Ba,Sr)TiO3 Ruddlesden-Popper films

Variable-temperature ultraviolet Raman spectroscopy was applied to study barium strontium titanate-based, (Ba_xSr_1-xTiO₃)_nBa_xSr_1-xO Ruddlesden-Popper (R-P) thin films with grown by molecular beam epitaxy on SrTiO₃ and TbScO₃ substrates. Pure SrTiO₃ (x = 0) RP series with n varied from 10 to 50, as well as Ba_xSr_1-xTiO₃ – based series with varied x and n = 6, 10 and 20 were investigated. Raman spectra demonstrated that all samples were ferroelectric at low temperatures, with out-of-plane polarization (normal to the film surfaces). Ferroelectric phase transition temperatures, T_c, were determined from the analysis of temperature dependence of Raman intensities of phonon modes. All films studied had T_c values below room temperature, in the approximate range 220-260 K, making them suitable candidates as tunable dielectrics for microwave device applications. Pure (barium-free) (SrTiO3)_nSrO RP films showed weak T_c dependence on the series number n, (230 and 240 K for n = 10 and 20, respectively). In Ba-containing RP series (Ba_xSr_1-xTiO₃)_nBa_xSr_1-xO, T_c varied between 225 and 255 K, depending on Ba composition x and series number n.   

Berezinskii-Kosterlitz-Thouless phases in ultra-thin PbTiO3/SrTiO3 superlattices

We study the emergence of Berezinskii-Kosterlitz-Thouless (BKT) phases in (PbTiO₃)_3/(SrTiO₃)₃ superlattices by means of second-principles simulations. Between a tensile epitaxial strain of ε = 0.25-1 % the local dipole moments within the superlattices are confined to the film-plane, and thus the polarization can be effectively considered as two-dimensional. The analysis of the decay of the dipole-dipole correlation with the distance, together with the study of the density of defects and its distribution as function of temperature, supports the existence of a BKT phase in a range of temperature mediating the ordered ferroelectric (stable at low T), and the disordered paraelectric phase that appears beyond a critical temperature T_BKT. This BKT phase is characterized by quasi-long-range order (whose signature is a power-law decay of the correlations with the distance), and the emergence of tightly bounded vortex-antivortex pairs whose density is determined by a thermal activation process. The proposed (PbTiO₃)_3/(SrTiO₃)₃ superlattice model and the imposed mechanical boundary conditions are both experimentally feasible, making it susceptible for an experimental observation of these new topological phases in ferroelectric materials.

    

Polarization induced by oxygen octahedral rotations in strained sodium niobate thin films directly imaged by electron ptychography

Due to its extremely complicated phase diagram, bulk sodium niobate (NaNbO₃) has often been dubbed as the most complex perovskite system. Unlike prototypical ferroelectric perovskites where the B-site displacement is the primary order parameter, the structure and electrical properties of NaNbO₃ are primarily governed by the oxygen octahedral rotations. For strained epitaxial thin films of sodium niobate, this makes it all the more important that the oxygen positions are determined precisely for an accurate determination of the structure and polarization. Reliable atomic scale characterization of light elements has always been a challenging problem in transmission electron microscopy, with traditional methods having well documented artefacts arising from mistilts, thickness variations and residual aberrations. By using the recently developed multislice electron ptychography method, we are able to overcome these artefacts, enabling a precise measurement of oxygen displacements that show ferroelectric textures in strained thin films of NaNbO₃.

    

Tunable Magnetic Scattering and Ferroelectric Switching at the LaAlO3/EuTiO3/Sr0.99Ca0.01TiO3 Interface

Ferroelectric and ferromagnetic orders rarely coexist, and magnetoelectric coupling is even more scarce. A possible avenue for combining these orders is by interface design, where orders formed at the constituent materials can overlap and interact. Using a combination of magneto-transport and scanning SQUID measurements, we explore the interactions between ferroelectricity, magnetism, and the 2D electron system (2DES) formed at the novel LaAlO₃/EuTiO₃/Sr_0.99Ca_0.01TiO₃ (001) heterostructure. We find that the electrons at the interface experience magnetic scattering appearing along with a diverging Curie-Weiss-type behaviour in the EuTiO₃ layer. The 2DES is also affected by the switchable ferroelectric polarization at the Sr_0.99Ca_0.01TiO₃ bulk. While the 2DES interacts with both magnetism and ferroelectricity, we show that the presence of the conducting electrons has no effect on magnetization in the EuTiO₃ layer. Our results provide a first step towards realizing a new multiferroic system where magnetism and ferroelectricity can interact via an intermediate conducting layer.   

Tunneling electroresistance in PbZr0.2Ti0.8O3-based ferroelectric tunnel junctions with SrIrO3, LaNiO3, and La0.67Sr0.33MnO3 electrodes

We report a study of tunneling electroresistance (TER) in epitaxial ferroelectric tunnel junctions (FTJs) composed of ferroelectric PbZr_0.2Ti_0.8O₃ (PZT) barriers and correlated oxide electrodes. Epitaxial heterostructures LaNiO₃ (LNO)/PZT/SrIrO₃, LNO/PZT/La_0.67Sr_0.33MnO₃, and LNO/PZT/LNO) are deposited on SrTiO₃ (001) substrates via off-axis RF magnetron sputtering, with atomically smooth surface and high crystallinity achieved. The heterostructures are fabricated into tunnel junction devices with cross-strip geometry via photolithography followed by Ar ion milling. Switching the polarization of PZT leads to nonvolatile, reversal modulation of the tunneling current. The maximum room temperature TER ratio is observed in the LNO (8 nm)/PZT (4 nm)/LNO (12 nm) device, reaching about 1,000%. We exploit the Brinkman model to fit the tunneling current and investigate the temperature and magnetic field dependences of TER. Our study provides information about the critical material parameters for designing all-oxide epitaxial FTJs.   

Free-standing complex oxide membranes using hybrid MBE

Free-standing oxide films can be used to create symmetry-mismatched and non-equilibrium heterostructures. In this talk, we will present a detailed study for the synthesis of complex oxide nanomembranes with thickness ranging from 1 unit cell to several 100 nm. Combining X-ray diffraction, electrical transport, and dielectric property measurements, we show high quality nanomembranes of SrTiO₃ that can be exfoliated and transferred onto dissimilar substrates. We will discuss the implication of these structures on their physical properties. 

    

Growth mechanism for epitaxial SrTiO3 using remote epitaxy

Lattice mismatch between a crystalline film and substrate can lead to the formation of defects such as dislocations. Remote epitaxy is a promising approach to avoid this. The question of the growth mechanism is, however, still open. For instance, is the remote epitaxy assisted through the interatomic potential of the substrate via the graphene layer, or is it pin-hole-assisted growth? In this talk, we will present a detailed growth study using hybrid molecular beam epitaxy that employs a metal-organic precursor, titanium tetraisopropoxide (TTIP), to supply both Ti and oxygen (without the need for additional oxygen). This approach prevented the oxidation of the graphene layer while providing epitaxial SrTiO₃ films. Films were successfully exfoliated and transferred onto other substrates. Using Raman spectroscopy and high-resolution X-ray diffraction, we show that the transferred SrTiO₃ membrane is single-crystalline and that the graphene layer remained intact to the substrate after exfoliation. We will discuss how pinholes or wrinkles in graphene can influence the growth and surface morphology of the SrTiO₃ film, in addition to discussing the growth mechanism during remote epitaxy.   

Exploring Free-standing PbZr0.2Ti0.8O3 Membranes and its Nonvolatile Gating Effect in Two-dimensional MoS2

We report a comprehensive study of single crystalline free-standing PbZr_0.2Ti_0.8O₃ (PZT) and PZT/(La,Sr)MnO₃ (LSMO) membranes on different base-layers, including Au, LSMO/SrTiO₃ (STO), and 2D semiconductor MoS₂. We deposit 10 to 50 nm epitaxial PZT thin films on Sr₃Al₂O₆ (SAO) or LSMO/SAO buffered STO substrates using off-axis RF magnetron sputtering. After water etching of the SAO buffer layer, we transfer the suspended oxide membranes on designated base layers. We create stripe and dot shaped domains in the PZT and PZT/LSMO membranes and analyze the domain wall roughness and creep behaviors via piezo-response force microscopy, which reveals long range random bond disorder dominated behavior and substrate-dependent dimensionality. We then fabricate 2D MoS₂ transistors sandwiched between 50 nm PZT membrane top-gates and SiO₂ back-gates. Switching the polarization induces nonvolatile current switching in the MoS₂ channel, with a current on/off ratio of ~2.04×10⁵ achieved at room temperature. The PZT membrane top-gated MoS₂ transistors exhibit long retention for over 3 days in both on and off states and robust cycling behavior, making them promising for developing flexible nonvolatile memory applications.   

Twisted complex oxide lateral homostructures

Epitaxial growth, thin film deposition by which the crystalline layers with well-defined orientations grown atop single crystal substrates, is indispensable in condensed matter science and semiconductor industry, given that it delivers high quality films comparable to single crystals. Over the past decades, epitaxial growth has enabled efficient interface and strain engineering of functional materials, which has played a key role in renovating modern science and a wide spectrum of technically important applications. In terms of epitaxial growth, the selection of single crystal substrates determines the foundation template for the deposited materials. Namely, the lattice constrains and crystalline orientations of deposited materials are subject to selected template beneath, thus the allowed epitaxial degrees of freedom is determined once a specific substrate is chosen.

In this talk, using multiferroic BiFeO₃ as a model system, we propose an efficient approach to fabricate twisted oxide lateral homostructures with various conjunction tunability, including crystalline orientation, epitaxial constrain and phase stability. With a patterned prototype device, we further demonstrate that the proposed approach is not only compatible with conventional lithography and etching processes, but also versatile for fabricating various twisted complex materials. Additionally, we show that the lateral homostructures can also be assembled by combining structurally-different polymorphs, forming interfaces with unconventional physical properties. Our results evidence the excellent controllability and unbounded conjunction tunability of the lateral homostructures using the proposed method, allowing epitaxial films to be assembled at particular position in the plane, as if they were artificially “weaved”. Such an approach not only provides a new way to design twisted lateral homostructures, but also depicts an different scene of epitaxial growth.

    

Control of ordering in a polar skyrmion lattice through the ferroelectric proximity effect

Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. Topological solitons such as magnetic skyrmions have long drawn attention as stable quasi-particle-like objects, but the recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened the door for new length scales and electric-field manipulation. Functional phenomena can be distinct from those of normal ferroelectrics, with properties such as collective dynamics, chirality, and negative capacitance. Topologically nontrivial ferroelectric textures are uniquely possible and manipulable by careful control of thin film boundary conditions, allowing for the exploration of phase space and order-disorder transitions with atomic precision. Here, we show controlled ordering of polar skyrmions enabled through electrostatic coupling, controlled by the ferroelectric proximity effect in incipient ferroelectric SrTiO₃.

    

Designing ferromagnetic polar half-metallic oxide superlattices

By implementing hybrid-improper ferroelectric mechanism, our first-principles-based investigation of La³⁺Ni³⁺(3d⁷)O₃/Ca²⁺Ni⁴⁺(3d⁶)O₃ superlattice showed a ferromagnetic polar ground state with half-metallic phase. The system showed a ferromagnetic Ni-Ni interaction, but the Ni-3d’s are interacting antiferromagnetically with O-2p. Due to the antiferromagnetic Ni-O interaction, we noted a reduced total magnetization in the system. The system emerged to be half-metallic due to unpaired eg electrons in Ni-3d. Further, we examined a series of compounds utilizing the trilinear mode coupling and found the same ferromagnetic polar half-metallic phase as the ground state. In addition, we discussed how the meta-stable phases can offer entirely different properties than that of ground state structure. The LnNiO₃/CaNiO₃ superlattices where Ln denotes La, Nd, Gd, Dy, Tm, and Lu with polar ferromagnetic spin degree of freedom through design could be crucial for memory storage and memory transport.