Multilayer Thin Films Research Articles

Achieving Good Temperature Stability of Dielectric Constant by Constructing Composition Gradient in (Pb1-x,Lax)(Zr0.65,Ti0.35)O3 Multilayer Thin Films.

Ferroelectrics with a high dielectric constant are ideal materials for the fabrication of miniaturized and integrated electronic devices. However, the dielectric constant of ferroelectrics varies significantly with the change of temperature, which is detrimental to the working stability of electronic devices. This work demonstrates a new strategy to design a ferroelectric dielectric with a high temperature stability, that is, the design of a multilayer relaxor ferroelectric thin film with a composition gradient. As a result, the fabricated up-graded (Pb,La)(Zr0.65,Ti0.35)O3 multilayer thin film showed a superior temperature stability of the dielectric constant, with variation less than 7% in the temperature range from 30 °C to 200 °C, and more importantly, the variation was less than 2.5% in the temperature range from 75 °C to 200 °C. This work not only develops a dielectric material with superior temperature stability, but also demonstrates a promising method to enhance the temperature stability of ferroelectrics.

Epitaxial growth and microstructure of TiN/CrOxNy periodic multilayers

The effect of epitaxial growth on the mechanical properties of periodic multilayer coatings is an important research topic. The focus of this study is the microstructure and mechanical properties of TiN/CrOxNy periodic multilayer thin films consisting of modulated TiN layers. All samples were prepared by magnetron sputtering with varying sputtering powers for the TiN layer, but the CrOxNy layer remained constant. The grazing incidence X-ray reflectometry (GIXRR) and transmission electron microscopy (TEM) results demonstrate the well-defined periodic structure of the thin films, and an accumulating distortion was observed as more layers were added along the growth direction. The results show that the TiN/CrOxNy multilayers possessed a face-centred cubic (fcc) crystal structure and exhibited a (200) preferred orientation of TiN. The thickness of the TiN layers affected the crystalline orientation of the layers. Alternate-orientation epitaxial growth of CrOxNy layers along the TiN layers occurred when N2 was introduced. The introduction of nitrogen and the thickness of the TiN template layers significantly influenced the microstructure and crystallinity of the deposited nanomultilayers. The hardness of the multilayer films was determined by the nanoindentation. The TiN/CrOxNy periodic multilayer prepared from the TiN layer by direct current (DC) sputtering power had the highest nano hardness value, 28.7Gpa.

Optical and Electrical Properties of Mo-doped Zr:ZnO Multilayer Thin Films for Photosensor Applications

Optical and Electrical Properties of Mo-doped Zr:ZnO Multilayer Thin Films for Photosensor Applications

Defects controlled stress engineering in Al-doped ZnO transparent multilayered thin films

Defects controlled stress engineering in Al-doped ZnO transparent multilayered thin films

Dielectric and ferroelectric properties of multilayer BaTiO3/NiFe2O4 thin films prepared by solution deposition technique

In this work different lead-free multilayered structures, composed of perovskite BaTiO3 and spinel NiFe2O4 thin layers, were obtained by solution deposition method. Structural characterization of the sintered thin films confirmed the well-defined layered structure with overall thickness from 160 to 600 nm, crystalline nature of perovskite BaTiO3 and spinel NiFe2O4 phases without secondary phases (after sintering below 900 °C) and grains on nanometer scale. Dielectric properties of the multiferroic multilayer BaTiO3/NiFe2O4 thin films were analyzed in temperature and frequency range from 30 °C to 200 °C and 100 Hz to 1 MHz, respectively. In comparison to the pure BaTiO3 films, the introduction of ferrite layer reduces dielectric response and increases low frequency permittivity dispersion of the multilayer thin films. The multilayer samples have shown relatively low dielectric loss with stronger contribution of conductivity at higher temperatures, and characteristic broad peak representing “relaxation” of the interface charge accumulation.

Large coercivity of 13 kOe in L10-ordered CoPt on Si/SiO2 substrates by hydrogen annealing

L10-ordered CoPt with a large coercivity (H c) of 13 kOe was demonstrated on Si/SiO2 substrates by hydrogen annealing. Equiatomic 11.2 nm thick (Co/Pt)4 multilayer thin films were fabricated by electron-beam evaporation and were annealed at 500 °C–900 °C for 10–90 min under an Ar/H2 mixed gas atmosphere. The annealing temperature and time dependences of the crystal structures, magnetic properties, and surface morphologies of the films were systematically analyzed based on the experimental results obtained from grazing incidence X-ray diffraction (GI-XRD), vibrating sample magnetometer, and scanning electron microscope, respectively. Hydrogen annealing effectively promoted the out-of-plane c-axis orientation of L10-ordered CoPt compared to the vacuum annealing according to the GI-XRD patterns. A maximum H c of 13.3 kOe was obtained in L10-ordered CoPt with angular-outlined isolated grains by hydrogen annealing at 800 °C for 60 min, where the c-axis of L10-ordered CoPt was randomly distributed.

Xe-ion-irradiation-induced structural transitions and elemental diffusion in high-entropy alloy and nitride thin-film multilayers

The study aims to understand the irradiation behavior of multilayer coatings composed of high-entropy materials. Here, we report the structural stability and elemental segregation of high-entropy TiNbZrTa/CrFeCoNi metallic and nitride multilayer coatings under 3-MeV Xe20+ ion-irradiation at room temperature and 500 °C, respectively. Transmission electron microscopy analysis shows that the microstructure of nanocrystalline CrFeCoNi high-entropy-alloy sublayers are not stable and readily transforms into amorphous state at 500 °C and/or under irradiation conditions. The elemental distribution, acquired by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy mode, shows preferential diffusion of Co and Ni into TiNbZrTa sublayers, while Fe and Cr preferentially remain within the previous CrFeCoNi sublayers. TiNbZrTaN/CrFeCoNiNx nitride multilayers exhibit a higher crystallinity and structural stability as well as resistance to diffusion at high-temperature and/or irradiation conditions than their TiNbZrTa/CrFeCoNi metallic multilayer counterparts. These findings are explained by atomic size differences, the difference in Gibbs free energy of the mixing system, and interstitial-solute-induced chemical heterogeneity. Our findings thus provide a design strategy of high entropy nitride for nuclear fuel cladding.

TMM-Fast, a transfer matrix computation package for multilayer thin-film optimization: tutorial.

Achieving the desired optical response from a multilayer thin-film structure over a broad range of wavelengths and angles of incidence can be challenging. An advanced thin-film structure can consist of multiple materials with different thicknesses and numerous layers. Design and optimization of complex thin-film structures with multiple variables is a computationally heavy problem that is still under active research. To enable fast and easy experimentation with new optimization techniques, we propose the Python package Transfer Matrix Method - Fast (TMM-Fast), which enables parallelized computation of reflection and transmission of light at different angles of incidence and wavelengths through the multilayer thin film. By decreasing computational time, generating datasets for machine learning becomes feasible, and evolutionary optimization can be used effectively. Additionally, the subpackage TMM-Torch allows us to directly compute analytical gradients for local optimization by using PyTorch Autograd functionality. Finally, an OpenAI Gym environment is presented, which allows the user to train new reinforcement learning agents on the problem of finding multilayer thin-film configurations.

Thermal Concentration on Thermoelectric Thin Film for Efficient Solar Energy Harvesting

Thermoelectric generators can directly harvest and convert ambient thermal energy into electricity, which makes it ideal for thermal energy conversion. However, the limited working temperature gradient developed by direct solar radiation severely restricts the performance and the application of solar thermoelectric generators. Here, we report a multilayer thin film integrating a solar selective absorbing coating and a thermoelectric layer, where an in-plane temperature gradient was established. The temperature gradient was relatively large since the absorbed solar energy could only flow through the restricted cross-section of the thin film, representing a high thermal concentration. The fabricated thin-film solar thermoelectric generators (100 mm × 15 mm) achieve an open-circuit voltage of about 300 mV, and an output power of 0.83 μW under AM 1.5G conditions. Our work opens up a promising new strategy to achieve the simple and cost-effective conversion of solar energy into electricity by thermal concentration.

Interface enhanced precessional damping in spintronic multilayers: A perspective

In the past two decades, there have been huge developments in the understanding of damping in multilayered thin films and, more generally, in spin-transport in spintronic systems. In multilayered ferromagnetic (FM)/non-magnetic (NM) thin-film systems, observations of ferromagnetic resonant precession show a strong increase in the fundamental damping when the FM thin films are layered with heavy metals, such as Pt. These observations led to significant theoretical developments, dominated by the “spin-pumping” formalism, which describes the enhancement of damping in terms of the propagation or “pumping” of spin-current across the interface from the precessing magnetization into the heavy metal. This paper presents a perspective that introduces the key early experimental damping results in FM/NM systems and outlines the theoretical models developed to explain the enhanced damping observed in these systems. This is followed by a wider discussion of a range of experimental results in the context of the theoretical models, highlighting agreement between the theory and experiment, and more recent observations that have required further theoretical consideration, in particular, with respect to the role of the interfaces and proximity-induced magnetism in the heavy metal layer. The Perspective concludes with an outline discussion of spin-pumping in the broader context of spin-transport.

NEUTRON: Neural particle swarm optimization for material-aware inverse design of structural color

SummaryDesigning optical structures for generating structural colors is challenging because of the complex relationship between the optical structures and the color perceived by human eyes. Machine learning-based approaches have been developed to expedite this design process. However, existing methods solely focus on structural parameters of the optical design, which could lead to suboptimal color generation because of the inability to optimize the selection of materials. To address this issue, an approach known as Neural Particle Swarm Optimization is proposed in this paper. The proposed method achieves high design accuracy and efficiency on two structural color design tasks; the first task is designing environment-friendly alternatives to chrome coatings, and the second task concerns reconstructing pictures with multilayer optical thin films. Several designs that could replace chrome coatings have been discovered; pictures with more than 200,000 pixels and thousands of unique colors can be accurately reconstructed in a few hours.

Enhanced quadrupole effects for atoms in surface spiral beams with a thin dielectric waveguide

Recently, some physical research has recognized the coupling of optical phase singularity light with quadrupole-active atomic transitions. We show that a Laguerre-Gaussian beam, when completely internally reflected at the dielectric/vacuum interface, can generate a well-defined surface spiral beam with a two-dimensional intensity distribution confined to a sub-wavelength region near the interface outside of the dielectric. The quadrupole interaction has been shown to be significantly enhanced in the case of coating the planar surface of a dielectric prism using multilayer dielectric thin films of different thicknesses. The mechanical action on the atom due to the optical forces characterized by optical quadrupole transitions could greatly facilitate the revolution of optical manipulation of neutral atoms along the dielectric/vacuum interface. The typical optical quadrupole potential, evaluated for the atomic transition, corresponds to λ=675.0nm with ΓQ=7.8×105s-1, in Cesium atoms, which is a dipole-forbidden but quadrupole-allowed transition. The parameters used to compute the optical quadrupole potential are similar to those in recent experiments. The factors controlling the general enhancement are pointed out and discussed.

Conformation-dependent thermoelectric power factor of multilayer nanocomposites

Polyelectrolyte multilayer thin films, assembled by sequentially depositing bilayer (BL) of poly(allyamine hydrochloride) (PAH) and double-walled carbon nanotubes (DWNT) stabilized in polyacrylic acid (PAA) using the layer-by-layer assembly technique, are studied in an effort to establish the relationships between the conformational changes and thermoelectric performances. Controlling the charge density of weak polyelectrolytes with assembly pH conditions gives rise to a different physical, structural, and mechanical properties. The 7.5PAH/3.5(DWNT-PAA) films exhibit an exponentially-grown thickness behavior due to in-and-out diffusion of the partially charged polyelectrolytes with coiled polymeric conformation, which results in randomly distributed nanotubes within the multilayers. Films made with the same sequences, but with extended polymeric conformation due to high charge density along the polymer chain (PAH at pH 6 and PAA at pH 6.5), show the linear growth behavior with a uniformly oriented nanotube structure and relatively higher elastic modulus. The precise control of the polymeric conformation upon charge density at the nanoscale level significantly affects the thermoelectric performances. A 14 BL 6PAH/6.5(DWNT-PAA) film exhibits an electrical conductivity of 417 S cm−1 and Seebeck coefficient of 85 μV K−1. This translates to a power factor of 301 μW m−1 K−2, which is 75 times higher than the same films made at pH 7.5/3.5.

Magnetron sputtered NiAl/TiBx multilayer thin films

Transition metal diboride-based thin films are currently receiving strong interest in fundamental and applied research. Multilayer thin films based on transition metal diborides are, however, not yet explored in detail. This study presents results on the constitution and microstructure of multilayer thin films composed of TiBx and the intermetallic compound NiAl. Single layer NiAl and TiBx and NiAl/TiBx multilayer thin films with a variation of the individual layer thickness and bilayer period were deposited by D.C. and R.F. magnetron sputtering on silicon substrates. The impact of the operation mode of the sputtering targets on the microstructure of the thin films was investigated by detailed compositional and structural characterization. The NiAl single layer thin films showed an operation mode-dependent growth in a polycrystalline B2 CsCl structure with a cubic lattice with and without preferred orientation. The TiBx single layer thin films exhibited an operation mode independent crystalline structure with a hexagonal lattice and a pronounced (001) texture. These TiBx layers were significantly Ti-deficient and showed B-excess, resulting in stoichiometry in the range TiB2.64–TiB2.72. Both thin film materials were deposited in a regime corresponding with zone 1 or zone T in the structure zone model of Thornton. Transmission electron microscopy studies revealed, however, very homogeneous, dense thin-film microstructures, as well as the existence of dislocation lines in both materials. In the multilayer stacks with various microscale and nanoscale designs, the TiBx layers grew in a similar microstructure with (001) texture, while the NiAl layers were polycrystalline without preferred orientation in microscale design and tended to grow polycrystalline with (211) preferred orientation in nanoscale designs. The dislocation densities at the NiAl/TiBx phase boundaries changed with the multilayer design, suggesting more smooth interfaces for multilayers with microscale design and more disturbed, strained interfaces in multilayers with nanoscale design. In conclusion, the volume fraction of the two-layer materials, their grain size and crystalline structure, and the nature of the interfaces have an impact on the dislocation density and ability to form dislocations in these NiAl/TiBx-based multilayer structures.

Investigation of Interfacial Microstructures of TiN/AlN Multilayers by Neutron Reflectometry

Three alternate TiN/AlN nanostructured multilayers with the TiN layer thicknesses of 60, 70, and 120 Å and the AlN layer thickness of 10 Å were fabricated using dc reactive magnetron sputtering. Microstructural characterizations of the three nano-scale films were performed using nonpolarized specular neutron reflectometry. The results showed that the three TiN/AlN multilayer thin films were typical superlattice films and the thicknesses of the TiN layer and AlN layer in the multilayers were consistent with the design thickness nearly. The interface roughness was asymmetric in all the samples. The interface of AlN growing on TiN was much sharper than that of TiN growing on AlN and the latter was the diffusion interface in the TiN/AlN multilayer films.

Fast, semianalytical approach to obtain the stray magnetic field above a magnetic skyrmion

We present a fast, matrix-based semianalytical method to calculate the stray magnetic field and its derivative above spin textures with cylindrical symmetry. A magnetostatic Green's function approach is used to obtain accurate fields and magnetic forces, which are confirmed using micromagnetic simulations. The developed method can be used to quickly analyze and fit experimental measurements of the stray magnetic field (measured, for example, via nitrogen vacancies in diamond) or magnetic forces (measured via magnetic force microscopy) above a thin film or patterned element. Calculations for magnetic skyrmions show that these techniques have the potential to distinguish between Bloch and N\'eel walls for skyrmions in single-layer as well as in multilayer thin films where the wall structure evolves with depth.

Investigation of Exchange Bias in Irmn/Nife Bilayer Thin Films Using Magnetoresistance Technique

A bottom pinned Si/SiO2/Ta(20)/IrMn(15)/NiFe(10)/Ta(5) film has been fabricated using an ion beam sputtering system at room temperature. Post-deposition magnetic annealing was performed on sputtered multilayer thin film in a vacuum of 2´10-6 mbar at 250˚C for one hour in an in-plane magnetic field of 1.8 kG. The X-ray diffraction and X-ray reflectivity measurements have been performed for the structural and thickness analysis, respectively. The roughness of individual layers was determined using XRR measurements. The longitudinal MOKE measurements have been performed to measure the exchange bias, coercivity, and magnetic anisotropy at room temperature. The magnetoresistance measurement technique is employed to demonstrate the thermal evaluation of the exchange bias effect, training effect, and magnetization reversal asymmetry at room temperature and low temperature. The magnetoresistance measurement revealed a large exchange bias, pronounced magnetization reversal asymmetry, and enhancement in the training effects at 20 K. The experimental result exhibits the occurrence of temperature-dependent change from uniaxial to biaxial exchange anisotropy.

Enhanced breakdown strength of ferroelectric–dielectric multilayered thin films by blocking oxygen vacancies through linear dielectric layer

Enhanced breakdown strength of ferroelectric–dielectric multilayered thin films by blocking oxygen vacancies through linear dielectric layer

Decoupling opposed thermoelectric properties

We create GeTe/Sb2Te3 multilayer thin films by alternating GeTe and Sb2Te3 to enhance thermoelectric properties. Crystal structure of the sample was characterized from amorphous to crystalline via annealing. The GeTe/Sb2Te3 multilayer thin film can significantly improve thermopower factor from ∼0.1 μW/mK2 to 815.1 μW/mK2 because the interfacial effect breaks down the trade-off between Seebeck coefficient and conductivity. GeTe/Sb2Te3 exhibits high Seebeck coefficient while maintaining large electrical conductivity under proper annealing at 575 K. Thus, a high power factor of 815.1 μW/mK2 obtained by the multilayer structure, which blocks carriers and rationally tunes the carrier density, leads to an enhanced Seebeck coefficient and power factor for thermoelectric devices. Improvement of electrical conduction originates from interfacial bonding and intermixing at a particular thickness ratio during element diffusion process. Results of microstructure characteristics revealed that the existence of hexagonal Ge2Sb2Te5 phases may drive the improvement of thermoelectric performance and provide us with a new way of enhancing thermoelectric devices.

An investigation of electromagnetic interference shielding effectiveness with multilayer thin films material

This study prepared a thin films composite material sputtered with multi-layer good conductors on the surface of lightweight condensation polymer, which can be applied to electromagnetic interference shielding materials in the range of 1–6 GHz. Sub-micrometer grade silver (Ag), nickel (Ni), titanium (Ti), and graphite films were sputtered on a nylon (PA12) substrate by physical vapor deposition. The shielding effectiveness of the multilayer thin films composite was measured in accordance with a standard procedure, and the shielding index of the multilayer thin films composite was calculated. Three different electromagnetic interference shielding structures were tested in this study and gave statistically similar results with electromagnetic interference shielding effectiveness varying from 58 dB to 35 dB. The prediction results of a composite mathematical simulator were developed, including the theoretical formula model and the comprehensive prediction results of the multiple regression model, which are conducive to the development and evaluation of related composite materials in the future.