Complex Oxide Thin Films Research Articles

Understanding mechanisms of thermal expansion in complex oxide thin-films from first principles: Role of high-order phonon-strain anharmonicity

The thermal properties of materials are critically important to various technologies and are increasingly the target of materials design efforts. However, it is only relatively recent advances in first-principles computational techniques that have enabled researchers to explore the microscopic mechanisms of thermal properties, such as thermal expansion. We use the Grüneisen theory of thermal expansion in combination with density functional calculations and the quasiharmonic approximation to uncover mechanisms of thermal expansion in ferroelectric PbTiO3 thin-films. We show that although there are large changes in the elastic constants and Grüneisen parameters with both misfit strain and temperature, surprisingly the temperature evolution of the structural parameters can be modeled using a simple model of linear elasticity; the only inputs required are the misfit strain and bulk PbTiO3 elastic constants. We show that a near-cancellation between different types of anharmonicity gives rise to this behavior. Our results illustrate how different sources of anharmonicity can affect materials’ properties in different ways, even for the same material. The framework used in our study is general and illustrates how different types of anharmonicity can be systematically identified and explored.

Strain-Induced Magnetic Transitions in SrMO2.5 (M = Mn, Fe) Thin Films with Ordered Oxygen Vacancies.

We examine the epitaxial-strain-induced phase transitions in thin films of perovskite-derived SrMnO2.5 and SrFeO2.5 exhibiting ordered oxygen vacancies (OOVs). We find that SrMnO2.5 hosts multiple magnetic transitions to other ordered states, including antiferromagnetic (AFM) and ferromagnetic (FM) orders of E*-AFM, C-AFM, and FM types depending on the compressive or tensile strain state. In contrast, no magnetic transitions occur in thin-film SrFeO2.5 (G-AFM to FM), whereas its bulk phase exhibits a hydrostatic pressure-induced AFM-to-FM transition. We explain the origin of these dependencies on the transition-metal configuration, that is, d4 Mn versus d5 Fe, and the relative orientation of the OOVs in the Ca2Mn2O5-type structure with respect to the epitaxial interface. We find that the magnetic phase stability can be predicted by using exchange striction arguments, with FM (AFM) spin interactions preferring longer (shorter) Mn-O bonds in the square pyramidal MnO5 unit comprising SrMnO2.5. Because the Mn-O bond lengths directly shrink or elongate to accommodate the applied stress without considerable polyhedral rotations, we show that compressive and tensile strain tune the unit cell structure to favor different combinations of exchange interactions that stabilize the various magnetic spin orders. Our study shows that the strong coupling between the OOV structure and spin orders with epitaxial strain is a promising route to achieve picoscale control of functional electronic and magnetic responses in complex oxide thin films.

Resistive Switching Effect of Multiferroic Complex Oxide Solid Solution Thin Films

Resistive switching in multiferroics has attracted increasing attention due to the potential application for next-generation nonvolatile memory and could lead to forms of computing. However, the resistive switching mechanism in solid solution oxides is unclear. In this article, we have successfully fabricated binary 0.72BiTi0.27Fe0.46Mg0.27O3-0.28LaFeO3 (BTFM-LFO) and ternary 0.625BiTi0.27Fe0.46Mg0.27O3-0.25LaFeO3-0.125La2MgTiO6 (BTFM-LFO-LMT) thin films with precise component control by the spin coating method. The solid solution films exhibit obvious ferroelectricity and magnetism at room temperature. Both binary and ternary solid solution films show switchable polarization, weak magnetism, and reversible and repeatable resistive switching effects. Multistage resistive switching behavior was observed. Four resistive switching states were obtained in the binary film; the highest Ion/off is up to 106. The influence of the film composition on the resistive switching effect was discussed. It is considered that the oxide vacancy/valance exchange-induced defects play a dominant role in the resistive switching effect of complex oxide thin films. This work provides an alternative pathway to explore the resistive switching in multiferroic oxides by fabrication of complex solid solution films.

Heteroepitaxial Hexagonal (00.1) CuFeO2 Thin Film Grown on Cubic (001) SrTiO3 Substrate Through Translational and Rotational Domain Matching

Heteroepitaxy of complex oxide thin films is a significant challenge when a large mismatch in the lattice parameters (>8%) and difference in the crystallographic symmetry coexist between the film and substrate. Herein, the heteroepitaxial growth of a hexagonal delafossite CuFeO2 thin film with (00.1) orientation on a cubic perovskite (001) SrTiO3 substrate through translational and rotational domain matching epitaxy is reported. The rotational in‐plane domain orientation relationships are CuFeO2 [11.0]//SrTiO3 [110] and CuFeO2 [2.0]//SrTiO3 [110] with about 10% in‐plane lattice mismatch. The 14.8 nm‐thick (00.1) CuFeO2 thin film shows high‐crystalline quality with a full width at half maximum of rocking curve of about 0.24° and exhibits a possible indirect optical bandgap of 1.43 eV or direct optical bandgap of 1.94 eV. Herein, not only a model system demonstrating translational and rotational domain matching heteroepitaxy of complex oxides is reported, but also a way to thin‐film heterostructures integrating hexagonal delafossite with cubic perovskite materials for functional oxide devices is opened.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Ferroelectricity promoted by cation/anion divacancies in SrMnO3.

We investigate the effect of polar Sr–O vacancy pairs on the electric polarization of SrMnO3 (SMO) thin films using density functional theory (DFT) calculations. This is motivated by indications that ferroelectricity in complex oxides can be engineered by epitaxial strain but also via the defect chemistry. Our results suggest that intrinsic doping by cation and anion divacancies can induce a local polarization in unstrained non-polar SMO thin films and that a ferroelectric state can be stabilized below the critical strain of the stoichiometric material. This polarity is promoted by the electric dipole associated with the defect pair and its coupling to the atomic relaxations upon defect formation that polarize a region around the defect. This suggests that polar defect pairs affect the strain-dependent ferroelectricity in semiconducting antiferromagnetic SMO. For metallic ferromagnetic SMO we find a much weaker coupling between the defect dipole and the polarization due to much stronger electronic screening. Coupling of defect-pair dipoles at high enough concentrations along with their switchable orientation thus makes them a promising route to affect the ferroelectric transition in complex transition metal oxide thin films.

Fabrication of GdxFeyOz films using an atomic layer deposition-type approach

The growth of complex oxide thin films with atomic precision offers bright prospects to study improved properties and novel functionalities.

Effect of Subcycle Arrangement on Direct Epitaxy in ALD of LaNiO3

Monolithic device integration of crystalline complex oxide thin films can open for smarter and more sustainable devices in electronics and energy technology. However, the facile integration of such...

Numerical modeling of the plasma plume propagation and oxidation during pulsed laser deposition of complex oxide thin films

Pulsed laser deposition is widely used to synthesize complex oxide thin films with advanced functional properties because of the versatility of the process, although the actual processes and mechanisms that take place are highly dynamic and nontrivial. Detailed understanding of the plume dynamics is required to achieve enhanced control of the growth of complex oxide thin films. However, analytical models of the plasma plume proposed in previous studies, the so-called shockwave model, drag model, and adiabatic thermalization model, only apply to a specific pressure regime and only focus on the propagation of the plasma front. Numerical modeling of the plasma dynamics has previously been pursued for the one-dimensional propagation of a Si plasma plume in a noble background gas. Here we have explored a more generalized numerical model for the dynamics of a ${\mathrm{TiO}}_{2}$ plasma plume by extending to three dimensions, multiple elements in the plasma plume, and including chemical reactions in an oxygen environment without the use of adaptive parameters, other than used for the description of the initial plume shape. Comparison between the simulations and the self-emission measurements of the ${\mathrm{TiO}}_{2}$ plasma plume shows good agreement. Our model enables detailed simulation of the oxidation state of the arriving particles (Ti, TiO, and ${\mathrm{TiO}}_{2}$) on the substrate surface depending on oxygen background pressure. Interestingly, low pressures of about 0.02 mbar result in fast oxygen particles arriving prior to the slower titanium particles, which could significantly influence the oxidation state of the initial substrate surface. The enhanced number of collisions for higher pressures of about 0.1 mbar leads to a large amount of the low-mass oxygen particles coming to a standstill 1--2 cm away from the ablated target. Furthermore, the three-dimensional nature of our model has enabled simulation of the lateral variations in composition of the deposited particles on the substrate surface. Although negligible variations in the Ti:TiO and Ti:${\mathrm{TiO}}_{2}$ ratios are present for high pressures over deposition areas with diameters up to 5 cm, significant variations can be observed for low pressures. These insights could play an important role in upscaling pulsed laser deposition from a scientific laboratory-based scale to an industrial large-area scale.

Substrate oxygen sponge effect: A parameter for epitaxial manganite thin film growth

The emergent phenomena in complex oxide thin films are strongly tied to the oxygen content, which is often engineered by the oxygen partial pressure during growth. However, such oxygen control by the growth pressure is challenging to synthesize for some oxide films, which requires a subtle control of the oxygen content. A parameter of controlling the oxygen content independent of the growth pressure is desired. Here, we propose a method of controlling the oxygen content of films by engineering the substrate before the growth. The oxide substrate serves as an oxygen sponge, which provides a tunable oxygen environment ranging from oxygen-rich to oxygen-poor for the film growth, depending on the pre-substrate annealing (PSA) conditions. Using manganite as a model system, we demonstrate that this simple PSA method leads to remarkable changes in the structure and physical properties of the as-grown films. This substrate oxygen sponge effect, driven by the large oxygen concentration gradient at high temperatures, can be applied to explore not only emergent interfacial phenomena but also the growth of a variety of functional oxide thin films and nanocomposites.

In situ x-ray and electron scattering studies of oxide molecular beam epitaxial growth

We perform in situ synchrotron x-ray ray diffraction (SXRD)/reflection high energy electron diffraction (RHEED) studies on the growth of complex oxide thin films by molecular beam epitaxy. The unique deposition chamber, located at the Advanced Photon Source, allows the preparation of complex oxide samples with monolayer precision and facilitates the formation of direct correlations between in situ x-ray studies and the more prevalent RHEED investigations. Importantly, because SXRD and RHEED probe different atomic-scale processes during thin film synthesis, their concomitant use enables the extraction of details concerning growth behavior that one cannot determine from either probe alone. We describe the results of such in situ studies on the epitaxial growth of perovskite LaNiO3 on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001). We find that during the earliest stages of growth, the RHEED and x-ray signals do not agree with each other, demonstrating that while regular RHEED oscillations may imply high quality growth, the film–substrate interface can undergo significant changes during deposition due to the occurrence of interdiffusion at the growth temperature.

Ultrathin complex oxide nanomechanical resonators

Complex oxide thin films and heterostructures exhibit a variety of electronic phases, often controlled by the mechanical coupling between film and substrate. Recently it has become possible to isolate epitaxially grown single-crystalline layers of these materials, enabling the study of their properties in the absence of interface effects. In this work, we use this technique to create nanomechanical resonators made out of SrTiO3 and SrRuO3. Using laser interferometry, we successfully actuate and measure the motion of the nanodrum resonators. By measuring the temperature-dependent mechanical response of the SrTiO3 resonators, we observe signatures of a structural phase transition, which affects both the strain and mechanical dissipation in the resonators. Here, we demonstrate the feasibility of integrating ultrathin complex oxide membranes for realizing nanoelectromechanical systems on arbitrary substrates and present a novel method of detecting structural phase transitions in these exotic materials.

Phonon‐Enhanced Near‐Field Spectroscopy to Extract the Local Electronic Properties of Buried 2D Electron Systems in Oxide Heterostructures

AbstractIn the family of functional oxide materials, the interface between LaAlO3 and SrTiO3 (LAO/STO) is an interesting example, as both materials are large‐bandgap insulators in their bulk state but give rise to a confined 2D electron gas (2DEG) when combined through thin‐film deposition. While this 2DEG exhibits remarkable properties, its experimental investigation is mostly limited to destructive or non‐local (i.e. averaging over larger areas) methods until recently. Scanning near‐field optical microscopy is shown to overcome this limitation, detecting buried 2DEGs by using highly confined optical near‐fields. Here, a full spectroscopic approach with phonon‐enhancement and simulations based on the finite dipole model is combined to extract quantitative electronic properties of the interfacial LAO/STO 2DEG. This threefold improvement compared to previous work will enable the quantitative nanoscale, non‐destructive, sub‐surface analysis of complex oxide thin films and interfaces, as well as similar heterostructures.

Anisotropic domains and antiferrodistortive-transition controlled magnetization in epitaxial manganite films on vicinal SrTiO3 substrates

We studied the microstructural evolution and magnetism of ferroelastic La0.9Sr0.1MnO3 (LSMO) epitaxial thin films grown on SrTiO3 (001) substrates with different miscut angles. The substrate miscut angle plays a critical role in controlling the in-plane magnetic anisotropy. The microscopic origin of such magnetic anisotropy is attributed to the formation of anisotropic stripe domains along the surface step terraces. The magnetization in the LSMO films was found to be selectively modulated by the antiferrodistortive phase transition of the SrTiO3 substrate. This phenomenon has been qualitatively explained by a strain modified Stoner–Wohlfarth model. We conclude that the magnetization modulation by the SrTiO3 phase transition depends on h, the ratio of applied magnetic field to the saturation field. Such modulation is only visible with h < 1. The established domain microstructure–anisotropy–magnetism correlation in manganite films can be applied to a variety of complex oxide thin films on vicinal substrates.

Wafer scale growth of MoS2 and WS2 by pulsed laser deposition

Since the discovery of Graphene, 2D materials have attracted wide research curiosity due to their unique properties and potential applications. The new class of 2D materials such as transition metal dichalcogenides (TMDs) is gaining tremendous popularity due to its intrinsic bandgap, while single-layer graphene lacks a finite bandgap and is considered a bottleneck for various electronic applications. In recent years, MoS2 and WS2 have widely explored TMDs for fundamental properties and applications due to the intrinsic bandgap and high carrier mobility. To demonstrate the realisation of 2D materials, various growth mechanisms were developed, including mechanical exfoliation, liquid-phase exfoliation, chemical vapour deposition (CVD), and so on. However, a bottom-up physical vapour deposition technique, especially pulsed laser deposition (PLD) was routinely utilised for complex oxide thin film growth and can be considered a potential alternative to commonly reported CVD method for realising 2D layers. Large-scale device fabrication of 2D materials and thus the commercialisation of these materials requires a large area growth mechanism. In the present work, the wafer-scale growth of 2D TMDs such as MoS2 and WS2 is discussed. Raman spectroscopy is used to identify the number of layers of the grown thin films.

Modification of magnetocrystalline anisotropy via ion-implantation

The ability to systematically modify the magnetic properties of epitaxial La0.7Sr0.3MnO3 thin films is demonstrated through the use of Ar+ ion implantation. With increasing implant dose, a uniaxial expansion of the c-axis of the unit cell leads to a transition from in-plane toward perpendicular magnetic anisotropy. Above a critical dose of 3 × 1013 Ar+/cm2, significant crystalline disorder exists leading to a decrease in the average Mn valence state and near complete suppression of magnetization. Combined with lithographic techniques, ion implantation enables the fabrication of magnetic spin textures consisting of adjacent regions with tunable magnetic anisotropy in complex oxide thin films.

Lateral Gating of 2D Electron Gas in Cross‐Sectional LaAlO3/SrTiO3

Abstract2D electron gas at LaAlO3/SrTiO3 (LAO/STO) interfaces has emerged as an attractive platform for novel nanoelectronic devices. Control over the active functional interface by electrical or mechanical means in this regard is of special interest. A new means of electric‐field control by lateral gating of the interface and modification of its electronic properties in cross‐sectional samples that allows direct access to depth‐resolved physical‐property measurements is investigated. Local scanning probe measurements in conjunction with electrical characterization allow it to be established that field‐driven reversible migration of oxygen vacancies is the origin of enhancement and suppression of electronic conductivity in LAO/STO. These results point to new possibilities of device property control in complex oxide heterostructures and thin films that contain highly conductive engineered interfaces.

Strain-driven disproportionation at a correlated oxide metal-insulator transition

Metal-to-insulator phase transitions in complex oxide thin films are exciting phenomena which may be useful for device applications, but in many cases the physical mechanism responsible for the transition is not fully understood. Here we demonstrate that epitaxial strain generates local disproportionation of the NiO6 octahedra, driven through changes in the oxygen stoichiometry, and that this directly modifies the metal-to-insulator phase transition in epitaxial (001) NdNiO3 thin films. Theoretically, we predict that the Ni-O-Ni bond angle decreases, while octahedral tilt and local disproportionation of the NiO6 octahedra increases resulting in a small band gap in otherwise metallic system. This is driven by an increase in oxygen vacancy concentration in the rare-earth nickelates with increasing in-plane biaxial tensile strain. Experimentally, we find an increase in pseudocubic unit-cell volume and resistivity with increasing biaxial tensile strain, corroborating our theoretical predictions. With electron energy loss spectroscopy and x-ray absorption, we find a reduction of the Ni valence with increasing tensile strain. These results indicate that epitaxial strain modifies the oxygen stoichiometry of rare-earth perovskite thin films and through this mechanism affect the metal-to-insulator phase transition in these compounds.

High‐Resolution Crystal Truncation Rod Scattering: Application to Ultrathin Layers and Buried Interfaces

AbstractIn crystalline materials, the presence of surfaces or interfaces gives rise to crystal truncation rods (CTRs) in their X‐ray diffraction patterns. While structural properties related to the bulk of a crystal are contained in the intensity and position of Bragg peaks in X‐ray diffraction, CTRs carry detailed information about the atomic structure at the interface. Developments in synchrotron X‐ray sources, instrumentation, and analysis procedures have made CTR measurements into extremely powerful tools to study atomic reconstructions and relaxations occurring in a wide variety of interfacial systems, with relevance to chemical and electronic functionalities. In this review, an overview of the use of CTRs in the study of atomic structure at interfaces is provided. The basic theory, measurement, and analysis of CTRs are covered and applications from the literature are highlighted. Illustrative examples include studies of complex oxide thin films and multilayers.

Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.