Superlattice Components Research Articles

Epitaxial superlattices as a framework for stabilizing different LaNiO3 structures

Targeted material design is at the center of research efforts on epitaxial, complex oxide heterostructures. Due to the strong electron-lattice coupling, small structural modifications are often decisive for the resulting macroscopic, physical properties of artificially layered materials. Here we report on a detailed structural analysis of a set of differently stacked LaNiO3−LaGaO3 superlattices by transmission electron microscopy. We find that the relative thickness ratio of the two superlattice sublayers affects the structural network, resulting in different Ni–O bond lengths and Ni–O–Ni angles under the same epitaxial strain conditions. Whereas the bond length values depend mainly on the LaNiO3 layer thickness, bond angles are mainly influenced by the LaGaO3 layer thickness. The orbital polarization determined from x-ray absorption spectroscopy measurements shows that the superlattice with the smallest deviation of both values compared to bulk shows the highest orbital polarization. Therefore, the thickness ratio of the two superlattice components can be regarded as an additional effective tool to tune the functional properties of nickelates. Published by the American Physical Society 2024

Gradient Strain-Induced Room-Temperature Ferroelectricity in Magnetic Double-Perovskite Superlattices.

Single-phase multiferroics suffer from a fundamental contradiction between polarity and magnetism in d0 electronic configuration, motivating studies of unconventional ferroelectricity in magnetic oxides. However, low critical temperature and polarization still need to be overcome. Here, it is reported that the switchable polarization behavior at room temperature in [(La2 NiMnO6 )/(La2 CoMnO6 )]n double-perovskite magnetic superlattice films is achieved by engineering a microstructure with gradient strains, and the ferromagnetic Curie temperature did not show a rapid decrease. The synergy of gradient strains and superlattice components plays a decisive role in inducing ferroelectricity via the tilting or rotation of various oxygen octahedra. Such distortion responses to gradient strains are accompanied by slight magnetic fluctuations, maximizing the preservation of the initial magnetic exchange interactions, which alleviates the contradiction of multiferroic coexistence to a certain extent. This work confirms the room-temperature ferroelectricity in double-perovskite superlattices and provides a preferred strategy for confronting the difficulty of multiferroic coexistence in single-phase materials.

Improved electrochemical activity of the Li2MnO3-like superstructure in high-nickel Li-rich layered oxide Li1.2Ni0.4Mn0.4O2 and its enhanced performances via tungsten doping

Voltage decay of Li- and Mn-rich layered oxides can be effectively suppressed by increasing their Ni contents, which however normally results in a sizable decrease in capacity probably due to the reduced Li2MnO3-like superlattice component. Here, we reveal that the partial replacement of Mn by W in the high-nickel Li-rich (HNLR) layered oxide Li1.2Ni0.4Mn0.4O2 leads to the increase in both Li2MnO3-like superlattice component and Ni2+ proportion due to the presence of W6+. As a result, the doping of W favors the activation of the Li2MnO3-like component with the reversible redox of more oxygen anions and the creation of a multi-cation chemical environment, accompanied by the size reduction of primary particles and the increase in lattice parameters. The reversible redox of more oxygen anions results in much higher capacity and initial Coulombic efficiency. The reduced sizes of primary particles, along with the lattice expansion significantly enhance the electrochemical kinetics, which accounts for superior rate capability and excellent high-rate cycling performance. This study opens a possibility of improving the performance of HNLR oxides by doping appropriate elements to modify and optimize the properties of the Li2MnO3-like component.

Layer-by-layer design of nanostructured thermoelectrics: First-principles study of ZnO:organic superlattices fabricated by ALD/MLD

Crystalline atomic/molecular layer deposited ZnO:organic superlattices form a fundamentally new exciting family of coherent multilayered thermoelectric materials. They retain the n-type electrical transport properties derived from the parent ZnO lattice, while the organic molecular layers reduce the thermal conductivity. The controlled nanostructuring opens up the possibility of improving the thermoelectric characteristics of the parent oxide. Here we employ quantum chemical methods to rationalize our experimental results on the ZnO:organic superlattices and determine the thermoelectric structure–property relationships arising from the nanoscale layer-by-layer engineering of ZnO. Our results reveal the importance of systematic tailoring of the organic superlattice component and provide us with atomic-level guidelines for the rational design of novel hybrid inorganic–organic thermoelectrics.

Reduced thermal conductivity of TiNiSn/HfNiSn superlattices

Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of DC sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of $\kappa$ is measured by the 3$\omega$ method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $\kappa$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $\kappa$. We also present an enhanced ZT value for all investigated superlattices compared to the single TiNiSn and HfNiSn films.

Influence of interdiffusion of heteromaterials on X-ray diffraction by superlattice with stacking fault

X-ray dynamic diffraction on a superlattice with a stacking fault between the layers is considered theoretically in the case of interdiffusion of superlattice components. Expressions for the X-ray reflectance within satellite limits, depending on the wave’s phase jump on the stacking fault, its depth and the extent of interdiffusion of superlattice components are obtained.

Elastic superlattices with simultaneously negative effective mass density and shear modulus

We investigate the vibrational properties of superlattices with layers of rubber and polyurethane foam, which can be either conventional or auxetic. Phononic dispersion calculations show a second pass band for transverse modes inside the lowest band gap of the longitudinal modes. In such a band, the superlattices behave as a double-negative elastic metamaterial since the effective dynamic mass density and shear modulus are both negative. The pass band is associated to a Fabry-Perot resonance band which turns out to be very narrow as a consequence of the high contrast between the acoustic impedances of the superlattice components.

Thermal transport in SiGe superlattice thin films and nanowires: Effects of specimen and periodic lengths

We compute the thermal conductivity of superlattice (SL) thin films and nanowires for various SL periods and total specimen lengths using nonequilibrium molecular dynamics. Both types of materials exhibit similar behaviors with respect to SL period but the thermal conductivity of the thin films exhibits a significantly higher sensitivity to the specimen length. Notably, the thermal conductivity of SL thin films is smaller than those of the corresponding nanowires for specimen lengths below approximately 35 nm. These results arise from the complex dependence of the conductivities of the interfaces and the SL components on the specimen size and period. These trends and observations are explained using a simple phonon model that builds on the relationship between the cumulative thermal conductivity and the phonon wavelength.

Characterization of InAs/GaSb type‐II superlattice photodiodes for mid‐wave IR with different mesa sidewall passivation schemes

AbstractWe report on the electrical and material characterization of InAs/GaSb type‐II superlattice (T2SL) mid‐wave infrared (MWIR) photodiodes with different passivation schemes on the mesa‐sidewalls with significant differences in the resulting low temperature dark currents. Devices fabricated by dry etching and passivated with polymerized photoresist, show orders of magnitude lower dark currents as compared to unpassivated devices. The mesa side walls were examined using high resolution transmission electron microscopy (HRTEM) with a special focus on the interface between the superlattice material and the dielectric passivation. Material analysis on nanometer scale at the mesa sidewall interface was performed using energy dispersive X‐ray (EDX) spectrometry. EDX line scans were obtained from interfaces for different passivated and unpassivated devices, using the highly focused electron beam in TEM, to investigate the chemical compositions. The unpassivated and photoresist‐passivated mesas, with different electrical properties, revealed different sidewall morphologies and compositions. An oxygen containing layer was observed in photoresist‐passivated devices covering the whole mesa sidewall. We think this plays a role in reducing surface leakage dark current. In HR‐TEM the mesa sidewall topography reveals preferential etching of one superlattice component as previously observed. Nevertheless, the dielectric material covers the sidewall uniformly (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic Polaritons, Magnetostatic Waves and Effective-Medium Approximation for Antiferromagnetic Superlattice with Impurity in Parallel Magnetic Field

We derive the general effective-medium expression for the surface-guided magnetic polaritons and magnetostatic waves, which propagate in the antiferromagnetic superlattice with antiferromagnetic impurity film, and investigate the influence of the external magnetic field on the energy of localized magnetic polaritons. Similarly as in the free-standing antiferromagnetic film, the spectrum of magnetic polaritons in the presence of an external magnetic field is reciprocal in the sense that the frequency is independent of the direction of propagation. In the system under consideration one finds both the surface polaritons which are strongly localized in the antiferromagnetic film which acts as a waveguide, and waves which are weakly localized within film. The first waves are the pure surface modes or guided modes where excitations have a standing-wave-like character. This important feature of the localized magnetic polaritons enables us to use these antiferromagnetic systems in the technologies for devices (for example, in resonators) that work at wavelengths in the infrared region. Second waves have the very small value of the decay parameter and appear in the regions where the surface mode penetrates into the bulk band, i.e. the magnetic polaritons are weakly localized in the impurity film region. Now we obtain the mixed type mode having both bulk and surface characteristics. Also, the general way in which the dispersion curves vary with the volume fraction of the superlattice components and with impurity film is illustrated in this study.

Oxide superlattices for multiferroics: opportunities, issues, and challenges

Multiferroics form a rare class of materials, which simultaneously display various extraordinary properties that are usually absent in the classical case. Consequently, they are potential candidates for various novel devices which are unachievable by the conventional materials utilized for device applications. In this letter, we detail the alternative methodology to design an artificial multiferroic by the superlattice approach, which will circumvent the looming problem of their rarity. Details of the way of selecting the various superlattice components, controlling their properties, the role of interfaces, and the methodology of inducing the coupling between distinct orders are presented. Furthermore, we also discuss the various limitations, issues, and challenges in designing such heterostructures.

Effective-medium approach for surface-localized magnetic polaritons in magnetic (ferromagnetic and antiferromagnetic) superlattices

The general effective-medium dispersion relations are derived for surface-localized magnetic polaritons which propagate parallel to the surface between a superlattice and semi-infinite bulk material, as applied to ferromagnetic and antiferromagnetic superlattices, in the situation when a static magnetic field is applied in the plane of the layers and parallel to the magnetization. The dependence of the energy of the surface waves on the volume fraction of the ferromagnetic superlattice component and the influence of the external magnetic field on the spectrum of the surface magnetic polaritons for the antiferromagnetic superlattice are investigated. The spectrum of the surface-localized magnetic polaritons which appear at the junction of the magnetic (ferromagnetic and antiferromagnetic) superlattice with the magnetic material are more complex, in contrast to the cases of semi-infinite magnetic material or semi-infinite magnetic SL. It is essential that in all cases in the presence of the external magnetic field the spectrum of the magnetic polaritons are non-reciprocal. The properties of surface polaritons are discussed in detail for the system ferromagnetic superlattice (YIG/non magnet)/YAG and the antiferromagnetic superlattice (MnF 2 /ZnF 2 )/FeF 2 .

Interlayer exchange coupling in (Ga,Mn)As-based superlattices

The interlayer coupling between (Ga,Mn)As ferromagnetic layers in all-semiconductor superlattices is studied theoretically within a tight-binding model, which takes into account the crystal, band and magnetic structure of the constituent superlattice components. It is shown that the mechanism originally introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe superlattices, explains the experimental results observed in ferromagnetic semiconductor structures, i.e., both the antiferromagnetic coupling between ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer structures. The model allows also to predict (Ga,Mn)As-based structures, in which an antiferromagnetic interlayer coupling could be expected.

Interlayer exchange coupling mediated by valence-band electrons

The interlayer exchange coupling mediated by valence band electrons in all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied theoretically. A 3D tight-binding model, accounting for the band and magnetic structure of the constituent superlattice components is used to calculate the spin-dependent part of the total electronic energy. The antiferromagnetic coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in agreement with the experimental evidences. The results obtained for the coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also presented.

Surface-enhanced Raman spectroscopy of cadmium sulfide/cadmium selenide superlattices formed on gold by electrochemical atomic-layer epitaxy

The phonon properties of ultrathin CdS/CdSe superlattice films formed on gold by electrochemical atomic-layer epitaxy are characterized by means of surface-enhanced Raman spectroscopy (SERS). Substantial (15–25 cm −1) red-shifts in the CdS phonon frequencies are observed, whereas the CdSe frequencies are essentially unaltered, indicating that substantial crystallographic strain occurs in the former, but not the latter, superlattice component. The findings demonstrate the virtues of SERS for exploring the structure of such solid–solid interfaces with unique monolayer-level sensitivity.

Efficient 300 K light-emitting diodes at λ∼5 and ∼8 μm from InAs/In(As1−xSbx) single quantum wells

300 K light-emitting diodes which emit at 5 and 8 μm with quasi-cw output powers of up to 50 and 24 μW, respectively, are reported. The devices have a single molecular beam epitaxy grown InAs/In(As, Sb) quantum well in the active region with a strong type-IIa band alignment giving mid-IR emission at energies up to 64% lower than the alloy band gap. The emission energies are shown to be in good agreement with a k⋅p bandstructure model where Qc, the ratio of the strained conduction-band offset to the band-gap difference between the two strained superlattice components, is found to be ∼2.0.

Investigation of strain relaxation in GaInAs/GaAs superlattices by X-ray diffuse scattering

Two different superlattices (SLs) of Ga 0.8In 0.2As/GaAs grown by molecular-beam epitaxy on GaAs [0 0 1] have been investigated using the measurements on the X-ray diffuse scattering (DS) and X-ray specular reflectivity. The samples differ in the thickness of the GaAs barrier layers being t b=9 nm for sample 1 and 2 nm for sample 2 separating the active Ga 0.8In 0.2As layers of t a=16 and 20 nm, respectively. The X-ray wavelength was chosen just above the Ga absorption edge to get a sufficient scattering contrast between both superlattice components. The large difference in lattice parameter of the (Ga 0.8In 0.2As) and (GaAs) components initiates the relaxation process within samples, which results in two distinguishable states of relaxation, partial for sample S1 and nearly complete for sample S2. The formation of an ordered dislocation network during the relaxation process leads to appearance of microcrystalline domains within multilayers. In contrast to sample S1, the interaction of strain fields across thin barrier layers of sample S2 results in a weak plastic deformation close to the interfaces, displayed as small-height islands. The theoretical simulations of experimental curves were performed in the framework of distorted-wave Born approximation.

Crystal Structure of ann-Paraffin Binary Eutectic Solid. An Electron Diffraction Determination

The crystal structure of a paraffin binary eutectic formed from a nearly 1:1 combination of n-C30H62/n-C40H82 was determined with electron diffraction intensities from epitaxially oriented microareas. A pure n-C40H82 component could be isolated, and its diffraction pattern could be shown to match the structure expected for the pure even-chain paraffin in the Pca21 orthorhombic crystal structure. Although the microarea itself could not be isolated in electron diffraction experiments, the superlattice component of the lamellar row was separated from that of the pure longer chain species. After assigning a crystallographic phase envelope to these reflections to calculate the reverse Fourier transform, the sequence of lamellae suggested a packing model of pure components similar to the one found earlier for the n-C30H62/n-C36H74 binaries. The packing model also agreed well with the observed intensity data, given the suggested separation of the observed diffraction component from the overlapped patterns.

Small angle X-ray diffraction study of a-Si:H/a-Ge:H multilayers: reflectivity modeling and thermal stability

The structural properties of hydrogenated amorphous silicon/germanium superlattices (a-Si:H/a-Ge:H), deposited by the plasma enhanced chemical vapor deposition method, were analyzed by means of small angle X-ray diffraction. The multilayer period and the individual layer thickness, as well as the width of the interface, were determined. The periodicities and interface widths were obtained for samples deposited on different substrates. The experimental reflectivity was compared to theoretical simulations, considering interface roughness, material mixing and non-homogeneous thickness. The thermal stability of the bilayer components was studied by means of heat treatments. A crystallization process occurs after the diffusion of the materials, being dependent on the thickness of superlattice components. The crystallization happens at temperatures around 600°C and is retarded for structures with smaller layer thickness.

197Au Mössbauer study of Au/Ni and Au/Fe metallic superlattices

197Au Mossbauer measurements have been performed for Au/Ni and Au/Fe metallic superlattices at below 75 K. For Au/Ni superlattices, the area ratio in a spectrum between a superlattice component and that of the pure Au buffer layer has been determined at 25, 50 and 75 K. From the area ratios, it is found that the recoil-free fraction of Au in Au(10 A)/Ni(10 A) is larger than that of the bulk Au, suggesting the existence of the supermodulus effect in this superlattice. The197Au Mossbauer spectrum obtained from Au(5 A)/Fe(8 A) is entirely magnetic at 16 K, suggesting the existence of a magnetic hyperfine interaction at197Au nuclei through the transferred electron spin polarization.