Multilayer Thin Film Structures Research Articles

Thermoelectric and Optical Properties of Advanced Thermoelectric Devices from Different Multilayer Thin Films

Thermoelectric devices were prepared using different multilayered thin film structures in the order of Ge/Si+Ge, Si+Ge, Si, Sb+Ge, Ge and SiO2/SiO2+Ge by DC/RF Magnetron Sputtering. The thickness of the deposited thin films were measured using Filmetrics UV thickness measurement system. The prepared thermoelectric devices have been modified with 5 MeV Si ions bombardments at the different fluences (doses) using Pelletron high energy beam accelerator at Alabama A and M University to form quantum structures in the multilayer thin films to improve the efficiency of the thermoelectric devices. The conductivity measurements, the fluence dependence of the power factor measurements, the fluence dependence of thermal conductivity measurements and the figure of merit calculations of multilayered thin films at room temperature have been performed. After the thermoelectric devices were prepared, X-ray Diffraction, Scanning Electron Microscopy (SEM) + Energy-Dispersive X-beam Spectroscopy (EDS) and Scanning Transmission Electron Microscopy (TEM) measurements have been done from the cross section of the prepared multilayer thin films to give us some answer for the thickness of the fabricated thermoelectric devices.

Atomic Structure of Multilayered Low-Contrast Fe/Cr Thin Films: Mathematical Formalism and Numerical Experiments

An experimental depth-resolved method for analyzing the local atomic structure of low-contrast multilayered thin films is presented in this work. A combination of X-ray reflectometry and angle-resolved EXAFS spectroscopy is considered. The following methods for solving ill-posed inverse problems are used to determine structural characteristics: the Tikhonov regularization method (for linear integral equations) and the Levenberg–Marquardt algorithm (for nonlinear equations). The proposed algorithms do not require information on the studied system such as the interface width and shape, the thickness of layers of specific elements and the depth at which they are located, and the atomic structure. This allows one to retrieve data on the local atomic structure of individual interface layers and the surface. Model numerical experiments for a Cr/Fe/Cr/Fe/Cr sample were conducted to assess the potential of this method.

A Multilayer Emitter Close to Ideal Solar Reflectance for Efficient Daytime Radiative Cooling.

A passive radiative cooling method has a significant influence on thermal management applications because it can cool without any energy input. This work both experimentally and theoretically demonstrates a multilayer thin film structure with high solar reflectance, which can be applied to passive daytime radiative cooling. The combination of physical vapor deposition and spin-coating prepared the samples, which were also characterized experimentally by spectrometers. On-site measured results show that the emitter can effectively achieve daytime radiative cooling, and the cooling performance can be further improved with the increase of the ambient air temperature. When the emitter is exposed to direct solar radiation (AM1.5) of about 880 W/m2 on a rooftop under dry air conditions, it can achieve an average temperature reduction of about 12.6 °C from the ambient air temperature with nonradiative heat transfer (11 a.m.–1 p.m.). Theoretical simulations reveal that the emitter can still have a certain cooling performance in the presence of significant nonradiative heat exchange and nonideal atmospheric conditions. The influence of ambient air temperature on the cooling performance of the emitter is also theoretically analyzed.

A novel optical polarization conversion structure designed by multi-layered thin film structures at the wavelength of 10.6 μm

Polarization conversion performance is one of the most important design contents of optical devices. At present, there are many ways to implement polarization conversion. However, they are mostly based on the form of split ring resonator or equivalent plasma, which are not conducive to the miniaturization of electromagnetic devices, particularly in the field of laser applications. In this paper, a novel optical polarization conversion structure has been designed by multi-layered PS/PTFE thin film structures at the wavelength of 10.6 μm based on the equivalent medium theory. Then it was verified that the proposed structure can effectively convert the linear polarized wave to circular polarized wave by the EM full-wave simulation. Compared with the current laser polarization manipulation devices, the proposed novel polarization converter has advantages of ultrathin thickness of structure, simple structure, good light transmission and no electromagnetic loss. Although the structural parameters in this paper are designed for EM wave the wavelength of 10.6 μm, the proposed design idea of the polarization conversion structure is also suitable for the electromagnetic wave at other wavelengths.

Study of Multilayer Thin Film Structures by Rutherford Backscattering Spectrometry

We have evaluated possibilities of using the method of Rutherford backscattering spectrometry (RBS) for depth profiling of multilayer thin-film structures containing nanodmensional layers of elements with close atomic masses. It is established that RBS measurements can ensure high precision determination of the composition of these multilayer structures, total film thickness, and thicknesses of separate layers. This ability can be used for the input quality control of multilayer structures used in micro- and nanotechnologies.

Theory of dislocation loops in multilayered anisotropic solids with magneto-electro-elastic couplings

This article presents a general theory on dislocation loops in multilayered anisotropic solids with magneto-electro-elastic couplings. First, a novel extended point-force Green's function is developed that not only guarantees the numerical stability but also admits a direct interpretation of the image method in multilayered solids. Then, based on the new Green's function, the classical theory of dislocation loops in pure-elastic infinite solids is systematically generalized by accounting for both the surface/interface conditions and the magneto-electro-elastic couplings. Two major contributions are made in the generalization: (i) the extended displacement and stress fields induced by an arbitrary dislocation loop are derived and expressed concisely as a simple line integral along the closed dislocation curve; (ii) the interaction energy between two arbitrary dislocation loops is also derived and expressed compactly as double line integrals along the two closed dislocation curves. Based on the new line-integral expressions for multilayered solids, special configurations such as straight dislocation segments and elliptic dislocation loops are analytically studied in detail; and more strikingly, elegant line-integral expressions for dislocation loops in bi-materials or in a semi-infinite solid are obtained by simple reduction. All the expressions derived in this article are compared with available solutions in existing literature and the correctness of them is fully verified. As novel applications, the present theory is utilized to calculate the self-energy of interfacial elliptic dislocation loops efficiently, which clearly shows that both the surface/interface conditions and the multi-field couplings have a considerable influence on the self-energy of interfacial dislocation loops. The line-integral based formulae developed here for dislocation-loop problems will not only enrich the theory of magneto-electro-elasticity, but also have potential applications in three-dimensional discrete dislocation dynamics simulations associated with multilayered thin-film structures.

A Phase-Field Model for Multilayered Heterostructure Morphology

Heteroepitaxially grown multilayered thin film structures have been attracted of great interest due to its potential applications in photovoltaic/light emitting/electronics devices. The thin film morphology plays an important role in enhancing its related physical properties. It is not easy to simulate the multi-layered thin film structures due to the influence of the interface/surface fluctuation. However, the phase field method, based on thermodynamics and Cahn-Hilliard diffusion model, can predict the thin film morphologies without tracking the interfaces. In this paper, a new phase field model was developed for predicting multi-layer structures with multi-order parameters. The morphologies with strain distributions of the quantum wells, quantum dots and buffer layers structures were investigated in the current study. We found that the strain distribution has a strong effect on the suface/interface morphologies in the multilayered structures. Some simulation results are consistent with experimental observations.

Light Nanofocusing on the Apex of the Metal Microtip Located Near a Multilayer Thin-Film Structure: Theory and Possible Applications

The optical energy focused on the nanosize spatial domain in the vicinity of the nanoapex of a metal microtip is studied. This focus occurs under the symmetric convergence of the surface plasmon wave to the nanoapex. The metal microtip surface is approximated by the paraboloid of revolution. The focal field distribution near the nanoapex is studied with scanning of the surface of a multilayer thin-film structure. The numerical method for the obtainment of the focal field distribution is considered, and the application of the presented method to the technologies for the creation of stamps and molds for ultraviolet nanolithography are discussed.

Key parameters for enhancing the thermoelectric power factor of PEDOT:PSS/PANI-CSA multilayer thin films.

Among the conducting polymers, poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been extensively investigated for organic thermoelectric device applications owing to its high electrical conductivity (σ), flexibility and easy processability. The thermoelectric (TE) power factor – a factor that determines the efficiency of a thermoelectric material, is very critical in developing high-efficiency thermoelectric devices. The TE power factor of PEDOT:PSS requires further enhancement in realizing efficient organic TE devices. Recently, we have reported a layer-by-layer deposition technique to deposit PEDOT:PSS and poly aniline-camphor sulfonic acid (PANI-CSA) forming a PEDOT:PSS/PANI-CSA multilayer (ML) thin film structure with an enhanced thermoelectric power factor up to 49 μW m−1 K−1. However, there exist several ambiguities regarding the parameters that control the TE power factor in (ML) thin films. In order to identify the parameters that control the TE power factor of ML thin films, PEDOT:PSS/PANI-CSA ML thin films have been deposited by varying the deposition conditions such as spin speed, the number of layers, solvent treatment, and thickness of each layer. A thermoelectric power factor up to 325 μW m−1 K−1 is achieved by properly optimizing the spin speed, number of layers, and the thickness of each layer in ML thin films. The enhanced thermoelectric power factor is the result of multiple factors such as stretching of PEDOT chains, structural conformation change from benzoid to quinoid, and excess PSS removal from the top of the PEDOT:PSS layer through solvent treatment and at the PEDOT:PSS/PANI-CSA interface. Our study provides the basis for realizing an enhanced thermoelectric power factor of organic thermoelectric multilayer structures consisting of ultra-thin polymer thin films similar to inorganic superlattices having 2D confinement.

A new matrix formulation of the Maxwell and Dirac equations

Presented in this paper is a new matrix formulation of both the classical electromagnetic Maxwell equations and the relativistic quantum mechanical Dirac equation. These new matrix representations will be referred to as the Maxwell spacetime matrix equation and the Dirac spacetime matrix equation. Both are Lorentz invariant. Key to these new matrix formulations is an 8-by-8 matrix operator referred to here as the spacetime matrix operator. As it turns out, the Dirac spacetime matrix equation is equivalent to four new vector equations, which are similar in form to the four Maxwell vector equations. These new equations will be referred to as the Dirac spacetime vector equations. This allows these new vector equations to be as readily solved as solving a set of Maxwell vector equations. Based on these two new matrix approaches, two computer programs, encoded using Matlab software, have been developed and tested for determining the reflection and transmission characteristics of multilayer optical thin-film structures and multilayer quantum well-and-barrier structures. A listing of these software programs may be found in supplemental material associated with this article. Numerical results obtained based on the use of these computer programs are presented in the results section of this article.

Al2O3 thin film multilayer structure for application in RRAM devices

This study investigated the effect of resistive switching on Pt/Al2O3/Cu/Al2O3/ITO multilayer structures grown by RF/DC magnetron sputtering. A reproducible unipolar switching free forming was found only for the negative voltage. The SET and RESET processes occurred at very low voltage values, which may be advantageous for practical applications. The ON/OFF ratio was approximately five orders of magnitude for more than 103 s, which is desirable for nonvolatile memories such as resistive random-access memories (RRAMs). We explain the unipolar behavior of the characteristic (I–V) curves in terms of the formation and rupture of conductive filaments, connected via a Cu metal layer.

(Invited) Antireflective and Sub-Band Pumped Photoconductive Solution Processed ZnO and Al:ZnO Quasi-Superlattice Films

Zinc oxide (ZnO) and Al-doped ZnO are important optoelectronic materials. ZnO in particular has a wide band gap (Eg ~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping using several metals, including Al and Sn. This gives ZnO a key role in optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies. We demonstrate that crystalline, epitaxial-like and highly ordered ZnO and Al:ZnO (AZO) quasisuperlattice thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating, which crystallize with near epitaxy with a pronounced c-plane texture. We also show the visible and near-infra red (VIS-NIR) light spectroscopy of ZnO and Al:ZnO multi-layered thin film structures grown on oxidized silicon substrates and also define the growth conditions and processing to provide tunable antireflection coatings of ZnO and AZO. Lastly, we show measurements demonstrating sub-band optical pumping of AZO QSLs that enable significant photoconductivity, whereas negligible photoconductance is observed in similarly grown ZnO QSLs. In AZO QSLs, conduction band electrons from valence band pumping at incident irradiation > Eg, are not required to initiate or enhance photoconductivity. The photoconductance scales with optical pump power and points to electron mobility possibly from transitions within the band gap of Al:ZnO or from localized photo-processes in Al-doped ZnO. References D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, J. Vac. Sci. Technol. A, 35, 061517 (2017).D. Buckley, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 99 (2017).D. Buckley, R. McCormack and C. O'Dwyer, J. Phys. D: Appl. Phys., 50, 16TL01 (2017).D. Buckley, R. McCormack, D. McNulty, V. Z. Zubialevich, P. J. Parbrook and C. O'Dwyer, ECS Trans., 77, 75 (2017).

Multilayer structure for highly transmissive angle-tolerant color filter

We propose transmissive color filter which can retain the same perceived color over a wide range of incident angle in the visible spectrum. The enhanced transmission is caused by Tamm plasmons (TP) based on multilayer thin film structure which consists of distributed Bragg reflector (DBR) and intermediate metal layer. High transmission of 60%∼70% efficiently improves the utilization of light energy and contributes to enhancing the color purity. In addition, the color filter is able to display distinctive color over the whole visible spectrum by changing the thickness of dielectric and metal layer. Without etching complex nano-patterns, the multilayer structure can be fabricated by depositing films successively with different materials on the substrate. The presented transmissive color filter exhibits the characteristic of high transmission and angle-tolerance. It has great potential for large-scale application, such as liquid crystal display, imaging sensor, and anti-counterfeiting.

Snapshot multispectral polarization imaging using a photonic crystal filter array.

A new filter array and a demosaicking method for snapshot multispectral polarization imaging are proposed in this paper. The proposed filter array is a thin-film wavy multilayer structure regarded as a photonic crystal that can be fabricated using the autocloning method. The multispectral polarization filter array is developed by altering the wave structure of the photonic crystal at each pixel. In addition, we propose a demosaicking method for multispectral polarization images by considering snapshot imaging as a linear model. In the experiments, we evaluated the recovered spectrum error in some color charts and showed various demosaicked images such as multispectral polarization images, specific-band degree of linear polarization images, polarized RGB images, and non-polarized RGB images.

Semitransparent Decorative Coatings Based on Optical Interference of Metallic and Dielectric Thin Films for High Temperature Applications

This paper introduces a thin film multilayer structure composed of dielectric and metallic layers that allows for a wide range of aesthetic appearances using the phenomenon of optical interference. In addition, this multilayer structure allows the reflection and transmission coefficients to be controlled independently. The application of these decorative coatings to induction stoves is also studied. The aim is to provide an attractive aesthetic appearance for the transparent glass-ceramic, and allow the visualization of blue and white lighting systems. Moreover, degradation of these decorative coatings is studied at high temperatures, so as to ensure that the coating does not change its aesthetic appearance during normal operation of the stove. It has been found to be necessary to use dielectric materials with low diffusion coefficients of oxygen, or not containing oxygen, to prevent oxidation of the metal layers when subjecting the coating to high temperatures.

Layer-by-layer modification of thin-film metal–semiconductor multilayers with ultrashort laser pulses

The surface modifications in a multilayer thin-film structure (50-nm alternating layers of Si and Al) induced by a single Gaussian-shaped femtosecond laser pulse (350 fs, 1028 nm) in the air are investigated by means of atomic-force microscopy (AFM), scanning electron microscopy (SEM), and optical microscopy (OM). Depending on the laser fluence, various modifications of nanometer-scale metal and semiconductor layers, including localized formation of silicon/aluminum nanofoams and layer-by-layer removal, are found. While the nanofoams with cell sizes in the range of tens to hundreds of nanometers are produced only in the two top layers, layer-by-layer removal is observed for the four top layers under single pulse irradiation. The 50-nm films of the multilayer structure are found to be separated at their interfaces, resulting in a selective removal of several top layers (up to 4) in the form of step-like (concentric) craters. The observed phenomenon is associated with a thermo-mechanical ablation mechanism that results in splitting off at film–film interface, where the adhesion force is less than the bulk strength of the used materials, revealing linear dependence of threshold fluences on the film thickness.

The charge-discharge characteristics and diffusion mechanism of Ti-Si-Al thin film anode using an electrically induced crystallization process

In this study, an Al-Si-Ti multilayer thin film structure is designed as the anode of a lithium ion battery. The novel structure restricts the expansion of Si during charge-discharge, and its battery capacity can reach 1112 mA h g−1 after a 100-cycle charge-charging test under a 0.2 C charge-discharge rate without annealing. Notably, after a 200 °C vacuum annealing process, the cyclic capacity of the anode rises to 1208 mA h g−1 through crystallization of the Al and Ti buffer layer. However, its thermal diffusion behavior in the Al/Si or Ti/Si interfaces seriously reduces the performance and restricts the expansion of Si. The electrically induced crystallization (EIC) process not only performs crystallization but also controls the interfacial stability, after which its capacity can obviously improve to 1602 mA h g−1 after 100 cycles. Using EIC, the electron flow drives the Cu and Al atoms to endow the Si matrix with doping properties and further increases the electron conductivity of the anode. This result demonstrates that the EIC process is a suitable post-treatment process for multilayer anodes and provides a reference for future battery designs.

Near unity ultraviolet absorption in graphene without patterning

Enhancing the light–matter interaction of graphene is an important issue for related photonic devices and applications. In view of its potential ultraviolet applications, we aim to achieve extremely high ultraviolet absorption in graphene without any nanostructure or microstructure patterning. By manipulating the polarization and angle of incident light, the ultraviolet power can be sufficiently coupled to the optical dissipation of graphene based on single-channel coherent perfect absorption in an optimized multilayered thin film structure. The ultraviolet absorbance ratios of single and four atomic graphene layers are enhanced up to 71.4% and 92.2%, respectively. Our research provides a simple and efficient scheme to trap ultraviolet light for developing promising photonic and optoelectronic devices based on graphene and potentially other 2D materials.

Towards a 20 kA high temperature superconductor current lead module using REBCO tapes

Most of the large fusion devices presently under construction or in operation consisting of superconducting magnets like EAST, Wendelstein 7-X (W7-X), JT-60SA, and ITER, use high temperature superconductor (HTS) current leads (CL) to reduce the cryogenic load and operational cost. In all cases, the 1st generation HTS material Bi-2223 is used which is embedded in a low-conductivity matrix of AgAu. In the meantime, industry worldwide concentrates on the production of the 2nd generation HTS REBCO material because of the better field performance in particular at higher temperature. As the new material can only be produced in a multilayer thin-film structure rather than as a multi-filamentary tape, the technology developed for Bi-2223-based current leads cannot be transferred directly to REBCO. Therefore, several laboratories are presently investigating the design of high current HTS current leads made of REBCO. Karlsruhe Institute of Technology is developing a 20 kA HTS current lead using brass-stabilized REBCO tapes—as a further development to the Bi-2223 design used in the JT-60SA current leads. The same copper heat exchanger module as in the 20 kA JT-60SA current lead will be used for simplicity, which will allow a comparison of the newly developed REBCO CL with the earlier produced and investigated CL for JT-60SA. The present paper discusses the design and accompanying test of single tape and stack REBCO mock-ups. Finally, the fabrication of the HTS module using REBCO stacks is described.

Application of Two-Wavelength X-Ray Optical Scheme for Combined Measurements of X-Ray Specular Reflection and Diffuse Scattering to Study Multilayered Thin Film Structures

The complex study results of the parameters of TiN/Ti diffusion-barrier structures using the methods of relative X-ray reflectometry and diffuse scattering of X-ray radiation implemented by a two-wavelength X-ray optical measurement scheme are presented. It is shown that this scheme, by a single measurement, enables studying two different diffuse scattering regions, which increases the correctness and unambiguity of the analysis that was carried out. The considered complex of methods makes it possible to solve the ambiguity of a density/roughness type by the solution of the inverse reflectometry problem and to calculate the parameters of buried layers in the structures which were studied.