Session L09: Dielectric and Ferroic Oxides - Nanostructures and Surface

Domain dynamics in low strain BaTiO3 thin films

Phase transitions dynamics and pre-equilibrium phenomena are difficult to detect, raising questions about the relative role of kinetics versus thermodynamics in the final ordered state. Ferroic materials are good examples of symmetry breaking phase transitions. In particular, ferroelastic layers under epitaxial strain minimize the elastic energy by forming periodically ordered domains. We have directly observed that the equilibrium domain structure in these materials is achieved by halving of the domain periodicity, sequentially and multiple times. The process is reversible, displaying periodicity doubling as the temperature is increased. The resulting periodicity halving/doubling cascades follow a 2ⁿ size-scaling (kinetic) law that, combined with the thermodynamic limit (Roytburd law), yields the actual domain size. These behaviour shows the universality of the "edge of chaos" formalism (by which oscillatory systems are known to go through period-doubling cascades before reaching the chaotic dynamical regime) applies to spatially periodic patterns. It does provide an experimental link between maximum adaptability that is believed to take pace at the "edge of chaos" and maximum susceptibilty at order-disorder phase transitions. Implications for adaptable electronics and neuromorphic computing will be discussed.   

Electro-optic and ferroelectric properties of epitaxial 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Ti0.8Zr0.2)O3 thin films grown by pulsed laser deposition.

Ferroelectric oxides have long been used for signal processing in optical communication system for their excellent linear and nonlinear optical properties. Recently, (1-x)(Ba_0.7Ca_0.3)TiO₃-xBa(Ti_0.8Zr_0.2)O₃ ceramics have aroused great attention for its large ferroelectric response and strong electro-optic effect in the vicinity of the morphotropic phase boundary. In this work, we report our efforts on epitaxial growth of 0.5(Ba_0.7Ca_0.3)TiO₃-0.5Ba(Ti_0.8Zr_0.2)O₃ (BCZT-50) thin films on GdScO₃ (GSO) substrates by pulsed laser deposition. Structural characterizations show that the films are epitaxial and remain fully strained up to 200 nm. We have carefully investigated the ferroelectric properties of these films and observe large dielectric constant values of up to 1500. We also report our efforts on the measurement of electro-optic coefficients.   

Seeking Reduced Charge Offset Drift Using Confined, Plasma Oxidized AlOx in SETs

As an essential material for quantum computing applications, aluminum oxide (AlO_x) is extensively employed as the gate dielectric in semiconducting quantum information (QI) devices and as the tunnel barrier in superconducting QI architectures. However, AlO_x is frequently reported to have a broad distribution of electrically active defects and high density of interacting two-level systems (TLSs). Here we study the charge offset stability of single-electron transistors (SETs) fabricated with plasma oxidized, cobalt confined AlO_x tunnel junctions. The performance of metal-based SETs is critically limited by the charge offset drift, which is significantly influenced by the electrical stability of the tunnel barrier. Our tunnel junctions have two physical differences from those of thermal oxides: 1) plasma oxidation has been shown to be more uniform and stoichiometric for AlO_x; and 2) high oxygen content is confined within the insulating regions by using a Co/ AlO_x /Co structure to provide a barrier against oxygen diffusion. These two differences lead to the expectation that better charge offset stability be measured on these devices as compared to typical thermally oxidized devices with unconfined oxygen.   

Ferroelectricity in BaTiO3 and SrTiO3 Nanaoparticles

Nanoparticles of SrTiO3 and BaTiO3 with well define sizes have been found to exhibit ferroelectricity at sizes which are inconsistent with standard models. Detailed measurements of optical and structural properties are being conducted to ascertain the nature of the ferroelectric state and the atomic structure which supports it.   

Evidences of a ~1 nm thick metallic-like ferroelectric BaTiO3-δ film at room temperature

Requirements of multi-functionalities in thin-film systems have led important findings of unique physical character and degree of freedom which exist only in film forms. As growth technique gets advanced, one can decrease the film thickness even a-few nano-meter (~nm) scale where its unique physical character still appears. Among those intriguing film systems, ferroelectrics have been of interest. As a prototype ferroelectric, electrical properties of ultra-thin BaTiO₃ (BTO) films have extensively studied, which is found that ferroelectricity sustains down to ~ nm thick films as theoretically predicted. However, efforts on determination of the minimum thickness in ferroelectric films has been hindered by large leakage current. In this study, we used ~ nm thick BTO films showing metallic-like behaviour around room temperature (RT). We found that the metallic-like BTO film has ferroelectricity at RT even in ~2 unit-cells thick. Observation of such ultra-thin conducting ferroelectric will enlarge its applicable fields, leading realization of new functional devices and investigations of further physical phenomena.   

Manipulating the depolarizing field in a ferroelectric BaTiO3 – based heterostructure

The demand for ever-smaller devices has been approaching the fundamental limits of ultrathin ferroelectric films. In the low-thickness regime, maintaining a large, stable and switchable ferroelectric polarization relies on the control of the strain state, thickness, interface termination and electrostatic conditions. Achieving a robust polarization or a controlled domain state remains, however, challenging. Imperfect charge screening at interfaces results in non-cancellation of internal fields that can in extreme case annihilate ferroelectricity. Taking (BaTiO₃-SrRuO₃) capacitor-like heterostructures as a model system, we directly access the polarization and the domain state during the film deposition using optical second harmonic generation [1]. We observe a previously elusive impact of the evolving electrostatic environment on the BaTiO₃ domain state simultaneously with the growth. The initial phase of the top-electrode deposition is accompanied by temporary enhancement of built-in fields in the ferroelectric layer resulting in 180° domain formation. We discuss ways to manipulate the depolarizing field and control the polarization during the growth as it presents a possible route towards a novel class of oxide-electronic devices. [1] G. De Luca et al. (in press). Nat. Commun.   

Atomic level study of charge-compensations at ferroelectric interfaces

Understanding the charge-compensation mechanism at oxide hetero-interfaces is important for fundamental science and also for practical applications. Here, using high resolution STEM, we found that the depolarization field induced by polarization discontinuity at (110) BiFeO₃/GdScO₃(010)_O (where the subscript “O” denotes orthorhombic indices) heteor-interfaces are compensated by different mechanisms evolving with film thickness. Thin films with thickness <30nm were screened by 180° and 109° ferroelectric stripe domains, where the vortex arrays at the end of the 109° domain walls were also found. Interestingly, thicker films with ~70nm thicknesses were compensated by an interfacial layer exhibiting exact in-plane polarizations with several nanometers thicknesses, which has not been observed before. Our results are important for understanding the interfacial charge compensations of ferroelectric films and thus designing novel ferroelectric based electric devices.   

First Principles Dielectric Slab Model for Dielectric and Piezoelectric Response in Superlattices

Superlattice systems, experimentally realizable with the development of layer by layer epitaxial growth techniques, continue to be of great interest in the development of new or enhanced material functionalities. Searching this enlarged space of realizable materials can be facilitated by models that allow prediction of superlattice properties from knowledge of the bulk properties of the constituent layers. In the dielectric slab model, introduced in 2003 for $\mathrm{BaTiO_3/SrTiO_3}$ superlattices, the superlattice properties are determined by the bulk responses of the constituents to the electrical and mechanical boundary conditions in the superlattice. The displacement field formulation due to Stengel and collaborators allows for a definitive version of this model that incorporates electric field effects beyond the linear approximation. Here, we present first-principles results for the bulk properties, including strain and polarization, as a function of displacement field for constituent compounds including $\mathrm{PbTiO_3}$, $\mathrm{BaTiO_3}$ and $\mathrm{SrTiO_3}$, and discuss the application of the model to obtain dielectric and piezoelectric responses of superlattices based on these constituents.   

Size-effects in the ferroelectricity of BaTiO3 single nanoparticles

It is known that new functionalities can emerge in materials when its physical dimensions are constrained. It has been reported that in thin films, the ferroelectric response of materials can be drastically enhanced via strain effects. Then, a natural approach is to reduce the dimensions even more, in the form of nanoparticles. Here we report on novel properties that emerge when a canonical ferroelectric material, like Barium Titanate, BaTiO₃ (BTO) is synthetized in the form of NPs. We used the hydrothermal method for the fabrication and obtained NPs with a size distribution around 200 nm. Our results indicate that our particles are mostly single phase with a minor contribution of defects or minority phases (less than 1%). Measurements of the piezoelectric response in single Np using PFM microscopy indicate that at these scales, the BTO particles shows an enhancement of the ferroelectric properties compared with bulk. Other novel size-effects will be discussed.   

Emergent Electronic and Dielectric Properties of Interacting Nanoparticles at Finite Temperature

Lead chalcogenide nanoparticle (NP) solids have been successfully integrated into certified solar cells and represent promising platforms for the design of novel photoabsorbers for photo-electrochemical cells. While much attention has been drawn to improving efficiency and device performance through altering the character of the individual NPs, the role of interactions between NPs is not yet well understood. Using first-principles molecular dynamics and electronic structure calculations [1], we investigated the combined effect of temperature and interaction on functionalized lead chalcogenide NPs. We show that at finite temperature, interacting NPs are dynamical dipolar systems, with average values of dipole moments and polarizabilities substantially increased with respect to those of the isolated building blocks. In addition, we show that the interacting NPs exhibit slightly smaller fundamental gaps that decrease as a function of temperature, and that the radiative lifetimes of both isolated NPs and the solids are greatly reduced at finite temperature compared to T=0.

[1] www.qboxcode.org   

Modulation of Heteroepitaxial Metal Organic Framework Crystallization by Local Chemical Environment via Organic-Step Interaction

Heterogeneous nucleation mitigates uncontrolled bulk nucleation by reducing the interfacial energy of the new nucleus, leading to smaller nucleation barriers and reducing the supersaturations required for nucleation. This approach has the benefit of controlling over crystal number, location, and orientation by manipulating the degree of chemical and/or structural match between the substrate and new phase. To understand the formation of hybrid MOF-semiconductor materials, we used in situ AFM to study the nucleation of ZIF-8 on ZnO faces. The results show that on ZnO (001) methylimidazole molecules poison the terrace regions preventing ZIF formation except along the step edge, the ZIF-8 exhibits large percentage of (111) face on ZnO (001) face due to symmetry match. In contrast, on (100) the linker configuration support ZIF-8 nucleation and epitaxial film form due to lattice match. However, the ZIF-8 crystal on step edge shows no orientation control. The control of MOF nucleation on substrate with different nutrient ions symmetry provide a general way to synthesize MOF film with uniform orientation.   

Role of dimensionality on thermodynamic properties in layered calcium titanates

Negative thermal expansion of materials is important for applications such as the fabrication and operation of miniature electronic devices and optical systems. Previously, it was shown that Ca₃Ti₂O₇ with the layered Rudlesden-Popper (RP) structure (n=2 member of A_n+1B_nO_3n+1) exhibits pressure-tunable negative thermal expansion and pressure-independent softening of the bulk modulus due to a quasi-two-dimensional vibration [Huang et al, Phys. Rev. Lett. 117, 115901 (2016)]. Here, we evaluate the structural, lattice dynamical, and thermodynamic properties with layer thickness n=1,2,3, and ∞, in the calcium titanate family using the self-consistent quasi-harmonic approximation within density functional theory. This allows us to formulate layer thickness dependent models of the anharmonic lattice properties that may be used for property design in other chemistries.