Multilayer Thin Films Research Articles

Air-based sputtering deposition of titanium oxynitride-based single, gradient, and multi-layer thin films for photoelectrochemical applications

Titanium oxynitride (TiNxOy) thin films exhibiting tunable physical and chemical properties can be used in many fields. Air-based sputtering deposition of the films with diverse O/N ratios was employed to produce gradient and multilayer films. By solely altering the air/Ar flow ratio, obtained TiNxOy films could change from a crystalline to a mainly amorphous feature. Moreover, the carrier concentration, Hall mobility, and hence resistivity of the films could be modified to a large range. The optical bandgaps of the films could also be tailored to a wide extent. Based on these results, the gradient TiNxOy layer and TiNxOy/TiN(O) multilayer were also deposited on TiN(O)-coated glass substrates for the assessment of photoelectrochemical performance. Compared with the TiNxOy/TiN(O) bilayer with the best photoelectrochemical performance, the photoelectrochemical currents of gradient TiNxOy films could be improved from 99 ± 2 μA⋅cm−2 to 164 ± 2 μA⋅cm−2 by taking advantage of bandgap engineering. Additionally, the photoelectrochemical currents of TiNxOy/TiN(O) multilayer were further improved to 175 ± 6 μA⋅cm−2. This is mainly due to conductive nano TiN(O) layers providing multiple high-transport paths while allowing light transmission. The enhancement mechanisms of the gradient and multilayer films have also been elaborated.

Multilayer thin films of aluminum oxide and tantalum oxide deposited by pulsed direct current magnetron sputtering for dielectric applications

This research describes the synthesis of multilayer thin films of aluminum oxide and tantalum oxide for dielectric applications. The multilayer thin films are made of two, four, or eight oxide layers produced by physical vapor deposition (PVD), specifically mid-frequency pulsed direct current magnetron sputtering. The oxide layers are stoichiometric Al2O3 and Ta2O5 with two specific morphologies observed from cross-section images obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Al2O3 layers exhibit a columnar structure whereas the Ta2O5 layers are uniform and dense. However, the morphology of the Ta2O5 layers changes progressively in the four-layer and eight-layer systems under the influence of the morphology of the Al2O3 layer below. This behavior is induced by the morphological continuity of the interface between two oxide layers. X-ray diffraction (XRD) shows the low crystallinity of these oxide layers due to the experimental conditions used during the magnetron sputtering process, particularly the low deposition temperature. The dielectric behavior of the multilayer thin films is studied by dielectric strength measurements. The results are compared to the values obtained for single layers of Al2O3 and Ta2O5 produced under the same experimental conditions. The two-layer system shows an intermediate value compared to the single layers, higher than Al2O3 and lower than Ta2O5. The dielectric strengths of the four-layer and the eight-layer systems are higher than those measured for the single layers of Al2O3 and Ta2O5. Finally, the morphology and the crystallinity of the multilayer thin films are changed by thermal annealing of these samples at 850 °C under vacuum. The thermal annealing induces crystallization of the Ta2O5 layers and the loss of morphological continuity at the interface between the oxide layers. These modifications result however in a lower dielectric strength for all the multilayer thin films.

ZnO/Ag multilayer for enhancing the catalytic activity against 4-nitrophenol

A multilayer thin film from zinc oxide and silver (ZnO/Ag multilayer thin film) has been synthesized via the technique of pulsed infrared laser deposition at 600 °C to be used as a portable catalyst for the degradation of 4-nitrophenol. The multilayer thin film was formed by two steps; the first one was making a thin film with a 300 nm thickness, followed by a 100 nm thin film from Ag, which was characterized by different characterization techniques. The XRD data demonstrated the presence of hexagonal Ag on ZnO in the formation of ZnO thin films and ZnO/Ag thin films. Also, it showed that crystallite size is decreasing as the Ag concentration rises because of the difference in atomic radius between Zn and Ag atoms in the crystal structure formation of ZnO coated with Ag. The crystal quality of ZnO and Ag was evaluated using photoluminescence (PL). The optical investigation showed the decrease in transmittance after coating ZnO film with Ag was due to ZnO films including more voids than ZnO/Ag films. From SEM images, the morphology of ZnO films was modified by the appearance of brilliant spots, which was related to the coating of the Ag layer as confirmed by elemental analysis. Then, based on the findings of catalytic experiments against 4-nitrophenol, the ZnO/Ag multilayer thin film shows an exceptional potential enhancement in compared to that of ZnO thin film.

Second harmonic generation and simplified bond hyperpolarizability model analyses on the intermixing of Si/SiGe stacked multilayers for gate-all-around structure

Si/SiGe stacked multilayers are key elements in fabrication of gate-all-around (GAA) structures and improvement of electrical properties, with the evolution of the Si/SiGe interfaces playing a crucial role. In this work, a model is developed based on the simplified bond hyperpolarizability model (SBHM) to analysis the anisotropic reflective second harmonic generation (Ani-RSHG) on a three-period stacked Si/Si1−x Ge x multilayer, which builds on Si(100) diamond structures. The C 4v symmetry of the Si(100) structure enables the second harmonic generation (SHG) contribution from the bonds to be simplified and the effective hyperpolarizabilities of the interfacial and bulk sources to be obtained. The effective interface dipolar and bulk quadrupolar SHG hyperpolarizabilities in the Si1−x Ge x sample with various Ge concentration profiles are modeled by interpreting the concentration of a component element as the probability of the element occupying an atomic site. On the basis of the developed model, the Ani-RSHG spectra of the as-grown samples with various Ge ratios for each layer and the samples annealed at 850 °C and 950 °C are analyzed to inspect the change in Ge distribution and its gradient in depth. The ani-RSHG analysis on as-grown samples showed difference in Ge distribution in samples with the multi Si/SiGe structure, which is not well observed in synchrotron x-ray diffraction (XRD) spectra. For the annealed samples, the response to changes in Ge concentration and its gradient in depth reveal the Si/Si1−x Ge x interface intermixing. Results of high-angle annular dark-field scanning transmission electron microscopy and energy dispersive x-ray spectroscopy agree well with the Ani-RSHG with SBHM findings. Compared with the Raman and synchrotron XRD spectra, the Ani-RSHG with SBHM simulation result demonstrates much better response to changes in compositions of the Si/Si1−x Ge x stacked multilayered structures, verifying the potential for characterizing the concentration distribution in stacked multilayered thin films for GAA structures.

Lamination of inkjet-printed Ag electrodes for highly patternable and customizable polymer light-emitting diodes

Inkjet printing that ensures superior patternability and customizability can be fully exploited as top electrodes in thin-film-based light-emitting devices. However, direct inkjet printing onto multi-layered thin films severely damages the underlying functional layers due to intermixing with the ink solvents, resulting in poor device efficiencies. Here, we report a lamination of inkjet-printed Ag electrodes as a top cathode for polymer light-emitting diodes (PLEDs). The organometallic Ag ink was printed on surface-modified elastomers with stable wettability, and drop spacing was further optimized for low surface roughness. The lamination of Ag cathodes establishes intimate contact between the electrodes and the functional layers, allowing light emission of PLEDs along the printed patterns. Furthermore, additional heat treatment improves the electron injection, resulting in a reduction of turn-on voltage and higher current/power efficiencies. To illustrate the practicability of our laminated PLEDs, we demonstrate passive-matrix driven PLEDs with all-inkjet-printed electrodes by laminating the top cathode arrays crossing the bottom inkjet-printed anodes.

Fabrication of Aluminum Oxide Thin-Film Devices Based on Atomic Layer Deposition and Pulsed Discrete Feed Method

This study demonstrates the low-temperature (<100 °C) process for growing a thin silica buffer layer and aluminum oxide by atomic layer deposition (ALD) in the same reaction chamber. Heterogeneous multilayer thin films are prepared by a dual-mode equipment based on atomic layer deposition and plasma-enhanced chemical vapor deposition (PECVD) techniques. The pulse discrete feeding method (DFM) was used to divide the precursor purging steps into smaller intervals and generate discrete feeds, which improved the saturated distribution of gas precursors, film density and deposition selectivity. The experimental results show that the process method produces a uniform microstructure and that the best film uniformity is ±2.3% and growth rate is 0.69 Å/cycle. The thickness of aluminum oxide film has a linear relationship with the cyclic growth number from 360 to 1800 cycles. Meanwhile, the structural and mechanical stress properties of aluminum oxide thin films were also verified to meet the requirements of advanced thin-film devices.

Optical characteristics of a multilayer solar sail and their effect on its heliocentric motion

The purpose of this work is to determine the influence of the optical characteristics of a thin multilayer solar sail on its orbital motion. First of all, the influence of the coefficients of reflection, scattering and absorption is investigated. These coefficients are computed on the basis of a mathematical model of the optical characteristics of a multilayer epitaxial thin film which is a solar sail itself. The paper considers the effect of temperature changes and sail surface degradation on its optical properties. Modeling of changes in optical characteristics is carried out drawing on an example of heliocentric flight from the Earth to Mercury applying locally optimal control laws. Applying the transfer matrix method and considering the distribution of solar spectral radiation optical parameters for two distinct constructions of a solar sail were obtained (aluminum and silver front coating).

Characterization of Structure, Morphology, Optical and Electrical Properties of AlN–Al–V Multilayer Thin Films Fabricated by Reactive DC Magnetron Sputtering

Composite thin films of the AlN–Al–V type, grown by magnetron sputtering, were analyzed by several complementary diagnostic methods. The power of the magnetron was used as a variable parameter, while gas flows, chamber pressure, and substrate temperature remained unchanged during the film fabrication. According to grazing incidence X-ray diffraction (GIXRD) results, in most cases, it was possible to obtain an (002)-oriented aluminum nitride (AlN) layer in the films, although, with an increase in the magnetron power to 800 W, the formation of X-ray amorphous AlN was observed. Similarly, according to the Raman results, the width of the peak of the vibrational mode E1, which characterizes the correlation length of optical phonons, also significantly increased in the case of the sample obtained at 800 W, which may indicate a deterioration in the crystallinity of the film. A study of the surface morphology by atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that the AlN film grows in the form of vertically oriented hexagons, and crystallites emerge on the surface in the form of dendritic structures. During the analysis of the AFM roughness power spectral density (PSD-x) functions, it was found that the type of substrate material does not significantly affect the surface roughness of the AlN films. According to the energy–dispersive X-ray spectroscopy (SEM-EDS) elemental analysis, an excess of aluminum was observed in all fabricated samples. The study of the current-voltage characteristics of the films showed that the resistance of aluminum nitride layers in such composites correlates with both the aluminum content and the structural imperfection of crystallites.

Nanostructured PEDOT-based multilayer thin films with high thermoelectric performances

Intrinsically conductive polymers are considered as one of the most promising candidates for thermoelectric (TE) materials due to their outstanding properties. Prior studies have been primarily focused on conducting polymers such as polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). In particular, currently available cationic water-soluble conjugated polymers are limited because of difficulties in synthetic routes. Herein, a positively charged PEDOT nanoparticles (PEDOT:NPs) dispersed in water are synthesized to serve as a structure directing agent for the development of hierarchical architectures with high flexibility and TE performances. A completely organic composite is fabricated by alternately depositing layers of aqueous solutions of PEDOT:NPs and double-walled carbon nanotubes (DWNT) stabilized with PEDOT:PSS via a layer-by-layer methodology. A 20 bilayer thin film (≈ 2.1 µm thick), comprised of a PEDOT:NPs/DWNT-PEDOT:PSS repeating sequence, exhibits high electrical conductivities of up to 744 S cm−1 and a high Seebeck coefficient up to 83 µV K−1. As a result, the multilayer thin films achieve a power factor of 512 µW m−1 K−2. These excellent TE properties can be attributed to the formation of a three-dimensional conjugated network with a highly ordered structure, which facilitates carrier transport within the film. This organic composite, based on a newly synthesized conducting polymer, with high flexibility and power factor offers a promising route to utilize efficient thermoelectric devices on flexible surfaces.

A polynomial model of transmission and reflection of electromagnetic monochromatic plane waves in lossy, non-magnetic multilayer thin films subjected to an external transverse voltage

Simulation of the electrical and optical response of a multilayer thin film composed of lossless material coupled with adjacent lossy material in an alternating arrangement when applying a transverse voltage across the multilayer thin film is conducted using a polynomial approach. The modelling of the lossy–lossless multilayer thin film is a generalization of our previous work on multilayer thin film made up of alternating lossy–lossy materials. It is the propagation matrix of the electromagnetic wave in the thin film that governs its propagation, while the interface matrix represents the coupling between layers at an interface. The present solution models the multilayer thin film as an effective capacitor constructed from a series of coupled capacitors, with every layer being considered as a capacitor coupled to the next. A transverse voltage can affect the amounts of electric charges that accumulate at the interface between adjacent ‘capacitors’. The present model is constructed to describe nonmagnetic, lossy and lossless materials. With the aid of a home-developed code implementing the model, the reflection and transmission of multilayer Ge/MgO thin films are simulated. By tuning the transverse electric potential, geometrical and electrical parameters of an arbitrary lossy–lossless multilayer thin film, the code is capable of predicting nontrivial optical responses in terms of $$T\left(\lambda \right), R\left(\lambda \right), {\phi }_{T}\left(\lambda \right), {\phi }_{R}(\lambda )$$ . The code can serve as a useful tool for designing and optimizing lossless-lossy or lossless-lossless multilayer thin films to deliver desired optical functions.

Design of organic/inorganic multilayer water vapor barrier thin films deposited via plasma polymerization for encapsulation

Plasma-enhanced chemical vapor deposition was used to deposit organic/inorganic encapsulation thin films on polyethylene terephthalate (PET) substrates by controlling the flow rate of oxygen (O2) and hexamethyldisiloxane. The control of gas flow can deposit different film structures such as a network structure of inorganic thin films and linear and cage-like structures of organic thin films, which are stacked on one another to form a multilayer water vapor barrier film. A calcium (Ca) test was established to measure the WVTR of thin films with a measurement limit of 3.05 × 10−3 g/m2/day. Surface profilometry, ultraviolet–visible spectrometry, and radius of curvature are applied to analyze the physical and optical properties. By depositing different monolayer film thicknesses on the PET, the encapsulation capability also varies. The WVTR of 3-pair (sub/buffer (50 nm)/barrier (100 nm)) multilayer thin films is 7.05 × 10–3 g/m2/day, but that of the 6-pair (sub/buffer (25 nm)/barrier (50 nm)) multilayer thin films decreased to less than the limit of the Ca test after the thickness of the monolayer was reduced. In the case of stacking 4-pair (sub/buffer (50 nm)/barrier (100 nm)) multilayer thin films, the single-layer film is too thick to withstand bending with a curvature radius of 24.3 mm, which may cause film cracks and increase its WVTR. After thinning the thickness of the single-layer, the 8-pair (sub/buffer (25 nm)/barrier (50 nm)) stack with the same total thickness withstands a curvature radius of 24.3 mm, and its WVTR also exceeds the limit of the established Ca test.

Tunable rapid electron transport in titanium oxide thin films

Rapid electron transport triggers many novel physical phenomena and becomes a critical point for high-speed electronics. Two-dimensional electron gas (2DEG) has drawn great attention due to its high electron mobility, and this has been observed in different materials, such as semiconductor, oxide interfaces, and 2D materials. In this paper, we report, for the first time, the existence and electrical manipulation of 2DEG in the Schottky quantum well, which was formed in the titanium oxide thin films. We take the asymmetry interface electron scattering effect into consideration when studying the electrical transport properties of our multilayer thin films. We found electrons would be transferred from the low-mobility semiconducting and metallic conductive channels to the high-mobility 2DEG conductive channel with an in-plane applied electric field. Therefore, electron concentration and mobility of the 2DEG formed in the Schottky quantum well could be tuned, and the nano-devices exhibited non-linear voltage–current curves. The differential resistivity of the nano-devices could decrease by two orders with increasing electric field at room temperature. Weak electron localization of electrons was experimentally observed in our nano-devices at low temperature, which further demonstrated the existence of 2DEG in the Schottky quantum well. Our work will provide us new physics about the rapid electron transport in the multilayer thin films and bring novel functional devices for the modern microelectronic industry.

Effect of Ag Layer Thickness and Interference of Cu-SnO2/Ag/Cu-SnO2 (CTO/Ag/CTO) Multilayer Thin Film on the Electrical and Optical Properties

Effect of Ag Layer Thickness and Interference of Cu-SnO2/Ag/Cu-SnO2 (CTO/Ag/CTO) Multilayer Thin Film on the Electrical and Optical Properties

Transparent-conductive and infrared-shielding WO3/Ag/WO3 multilayer heterostructures

Multilayer thin films are used as functional coatings in a variety of applications. In optoelectronics, they are used as transparent conducting electrodes that possess high electrical conductivity and visible transmittance. Another application is infrared-blocking energy-saving coatings. These two applications are usually explored separately. In this work, multilayers based on thermally evaporated tungsten oxide/silver/tungsten oxide (WO3/Ag/WO3) coatings were investigated for both applications. The properties of these multilayers were optimized to fulfill the requirements of both applications simultaneously. First, the properties of single-layer Ag and WO3 were investigated. Then, the properties of the multilayers were examined as a function of the thicknesses of the metallic and dielectric layers. The optimum structure consisted of a WO3/Ag/WO3 multilayer with thicknesses of 25/16/25 nm characterized by an electrical resistivity of 67 μΩ cm, a sheet resistance of 10.2 Ω/□, and a mean visible transmittance of 75 %. Such properties were suitable for transparent conductive applications. The infrared-blocking performance was manifested by a temperature reduction of 14 °C for solar radiation passing through a glass window.

DC pulsed plasma magnetron sputtering of CdO/Cu/CdO multilayers thin films for self-cleaning and optoelectronic applications

In this study, CdO/Cu/CdO multilayers thin films were organized on glass substrates with different Cu intermetallic layer thickness engaging DC plasma magnetron sputtering. The optoelectronic properties and structural characteristics of the multilayers at various Cu intermetallic layer thicknesses which were varied from 4 to 16 nm were explored. The calculated band gap was reduced from 2.66 eV to 2.48 eV as the Cu intermetallic layer thickness increased from 4 to 16 nm. The refractive index and coefficient of extinction of CdO/Cu/CdO multilayers increased with increasing the Cu intermetallic layer thickness. The resistivity is reduced from 1.8 × 10−2 Ω cm for CdO single layer to reach a value of 2.7 × 10−4 Ω cm for CdO/Cu (16 nm)/CdO multilayer. Further, the sheet resistance is decreased from 1000 to 13.8 Ω/sq. with the variation in Cu intermetallic layer thickness from 0 to 16 nm. CdO/Cu (4 nm)/CdO multilayer film recorded the best figure of merit (2.3 × 10−4 Ω−1). After sunlight illumination for the multilayers, the surface wettability was improved and the contact angle recorded lowest value of nearly 24° for CdO/Cu (8 nm)/CdO and CdO/Cu (12 nm)/CdO.

Elastic broadband antireflection coatings for flexible optics using multi-layered polymer thin films

A precise and facile fabrication of robust and flexible polymeric thin film multilayers with optical quality is achieved by iCVD.

Influence of plasma kinetic energy during the pulsed laser deposition of borophene films on silicon (100).

Developing borophene films with good structural stability on non-metallic substrates to maximize their potential in photosensitivity, gas detection, photothermia, energy storage, and deformation detection, among others has been challenging in recent years. Herein, we succeeded in the pulsed laser deposition of multilayered borophene films on Si (100) with β12 or χ3 bonding by tuning the mean kinetic energy in the plasma during the deposition process. Raman and X-ray photoelectron spectroscopies confirm β12 and χ3 bonding in the films. Borophene films with β12 bonding were obtained by tuning a high mean kinetic energy in the plasma, while borophene with χ3 bonding required a relatively low mean kinetic energy. Atomic force microscopy (AFM) micrographs revealed a granular and directional growth of the multilayered borophene films following the linear atomic terraces from the (100) silicon substrate. AFM nanofriction was used to access the borophene surfaces and to reveal the pull-off force and friction coefficient of the films where the surface oxide showed a significant contribution. To summarize, we show that it is possible to deposit multilayered borophene thin films with different bondings by tuning the mean kinetic energy during pulsed laser deposition. The characterization of the plasma during borophene deposition accompanies our findings, providing support for the changes in kinetic energy.

Enhanced dielectric energy storage in multilayer filmsviavalley-type structural design

Realizing ultrahigh breakdown strength and energy density by regulating electron transport and utilizing the ferroelectric property differences of parent materials to slim theP–Eloop of multilayer thin films.

Computational and Experimental Analysis of a Triode Microfuse with a WO3 Heater

In this study, high‐current‐protecting multilayered thin film microfuses are designed and simulated using the MEMS‐based tool of COMSOL multiphysics software and then fabricated and tested in the laboratory. Portable electronic devices are comprised of a secondary battery or DC charge source, and due to short circuit overcurrent, fire, and explosions can ensue. A protecting device should steadily cater to phenomena like overcurrent situations to avoid hazardous circumstances. The primary purpose of this investigation is to design a heater resistor with a negative temperature coefficient (NTC) to function as a low melting point‐based alloy for the fuse element. A lead‐tin (90Pb:10Sn wt.%) alloy has been employed as the low melting point‐based fuse element, and tungsten oxide (WO3) is integrated with the layer as a heater resistor due to its negative temperature coefficient of resistance characteristics. The electro‐thermo‐mechanical behavior is assessed, and a three‐dimensional structural modeling and simulation technique has been performed in both steady‐state and transient conditions with varying physical and electrical parameters. The heat required to melt the fuse depends on heater geometry, and when we applied 2 A current to the 1 : 30 length and width ratio‐based device, the heater achieved 600 K. Experimentally, nearly at 1 A current and above 4 V, the microfuse reached melting temperature and thus has been blown which provides a scope of controlling nearly 4 W of power electronic devices.

Characterization of Electrodeposited Cobalt/Copper (Co/Cu) Granular Multilayered Thin Films onto the Anodic Aluminum Oxide (AAO) Substrates

Herein, we report on the characterization of a series of cobalt/copper granular thin films alloys electrodeposited onto the previously pre-treated aluminum substrates, at different deposition time. And a systematic study of their structural and magneto-optical properties were investigated. The proper deposition potentials of Co, and Cu were obtained from the cyclic voltammetry method. It was observed that the easy axis direction of magnetization was parallel to the film plane for all films. The X-ray diffraction (XRD) results revealed that all films had a mixture of hexagonal close-packed (hcp) and face centered cubic (fcc) phases. The Scanning electron microscope images (SEM) and atomic force microscopy (AFM) showed some parallelepiped and ellipsoid shaped rods with an averaged dimension about of 600 nm length, 600 nm width and 200 nm thickness for a sample electrodeposited at room temperature. These rods are irregularly arranged. Their irregular arrangement could explain the large d-spacing in the sample at this temperature. The magnetic analysis of the granular thin films alloys showed the saturation magnetization increased and the coercivity decreased as the deposition time increased. These results showed that the changes in magnetic and structural properties of granular cobalt/copper alloys deposits were substantially depending on the variation of deposition time. Therefore, the change of the deposition time was seen to control the properties of the alloys deposits and hence their properties could be modified for desired purpose.