Thin Metal Films Research Articles

High-temperature Mo-based metallic glass thin films with tunable microstructure and mechanical behaviors

Developing high-temperature metallic glass thin films (MGTFs) with excellent combination properties is crucial for extending the practical applications of metallic glasses. A high-temperature multicomponent Mo-based MGTF with tunable microstructure prepared by single-target magnetron sputtering was presented in this study. Corresponding mechanical behaviors and thermal stability of MGTFs related to microstructure are systemically explored. By adjusting deposition parameters (pressure and power), the microstructure of as-deposited MGTFs can be altered from the dense homogeneous type to the loose nanoglass type. Such structure evolution can be explained by the competition between the surface diffusion and geometric shadowing effect. MGTFs with dense microstructure possess smaller surface roughness, higher hardness, higher Young’s modulus, and better wear resistance. Moreover, they also possess higher thermal stability where the fully amorphous structure and smooth surface can be well maintained after vacuum annealing at 1123 K for 30 min. By contrast, the MGTF with nanoglass microstructure shows inferior mechanical properties and thermal stability due to plentiful loose interface regions, providing abundant free volumes during deformation and acting as favorable crystal nucleation sites during annealing. The correlation between the microstructure and properties of as-deposited MGTFs is clarified with the universal scaling law of glasses. The annealing treatment distinctly increases the hardness and Young’s modulus of MGTFs. Meanwhile, after annealing, pop-in behaviors occur in the as-annealed MGTFs with dense microstructure but not in the as-annealed MGTF with nanoglass microstructure during the nanoindentation. These phenomena can be rationalized by the annihilation of free volumes during annealing and the evolution of the dynamical variable, shear transition zone, for the plastic deformation in MGTFs.

Mechanisms and control of bending of monocrystalline and polycrystalline films by gallium ion irradiation

The ion beam-induced bending of thin metal films represents a potent technique for fabricating three-dimensional structures using focused ion beam technology. This technique exhibits significant potential in the realm of micro and nano-scale structure fabrication for applications in various novel devices, such as plasmonic detectors. However, achieving greater control is imperative to ensure the reproducible fabrication of intricate structures. Here, we investigate mechanisms of bending and their dependence on conditions using molecular dynamics (MD) simulations for both monocrystalline and polycrystalline gold films. Our findings reveal the distinct differences in the bending behaviors between monocrystalline and polycrystalline films, as well as the significant dependence of bending on irradiation conditions that can be utilized for control purposes. The results show that mass transports are associated with defect formation and migrations, as well as migration of grain boundaries in the polycrystalline case, which play key roles. We observe a slow initial stage of film bending by following the commencement of irradiation, which is attributed to non-uniform stress fields from non-uniform defect generation; however, mass transport becomes increasingly important at higher doses. There is a strong interplay between this mass transport and grain boundaries, leading to differences in the behavior of monocrystalline and polycrystalline films.

Morphological study of depth-controlled high quality holes and lines fabricated on a metal substrate with a thin metal film by picosecond laser pulses

Laser micromachining is a convenient technique to structure and/or functionalize the surfaces of various materials. The use of ultrashort laser pulses has an obvious advantage to minimize the heat effect, which results in the formation of a hole without rims. Nevertheless, it is still difficult to fabricate a line without rims even with ultrashort laser pulses, because multiple irradiation of ultrashort laser pulses with a sufficient spatial overlap promotes the formation of rims and undesired structures. In this work we employ a metal substrate with a thin metal film, and demonstrate the fabrication of depth-controlled high quality lines, not to mention holes, through the selective removal of the film by picosecond laser pulses without forming rims and undesired structures. Morphological study of the target surface reveals that the fabricated structures have unprecedentedly flat bottoms with nearly vertical sidewalls to the surface.

Direct Fabrication of Electronic Circuits on Wooden Surfaces

AbstractEquipping otherwise passive surfaces with electronic functionality enables advanced interactive robotics, consumer products, sensor skins, and structural health monitoring. Concurrently, the rapidly growing number of electronic devices fuels the search for sustainable materials and processes that aid in reducing electronic waste. Wood is CO2‐neutral, omnipresent in the construction industry, in furniture, musical instruments, or packaging, yet so far, its potential for direct integration with electronics remains largely unexplored. Complications arise as traditional methods of equipping wood with electronics often compromise structural integrity and thus limit applications requiring load‐bearing capabilities. Here, seamless fabrication methods that allow the direct enhancement of wooden surfaces with electrically conducting structures, sensors, and microelectronic components based on screen printing of conducting inks or physical vapor deposition of thin metal films in conjunction with laser engraving are presented. Such electronic circuits imperceptibly operate on the surface of structural elements or as parts of decorative wooden furniture. These types of electronic wooden surfaces enable touch‐sensing applications, monitoring temperature, or the curing of varnishes without compromising functionality and mechanical stability. This multidisciplinary approach opens up new avenues for the development of smart wooden structures with embedded electronics, revolutionizing the way it is monitored, controlled, and interacted with wood‐based constructions.

Laser welding of fiber array units.

We report the results of fabricating fiber array unit (FAU) connectors using a near IR laser welding process, locking fibers in proper position on planar glass substrates and forming strong glass-to-glass bonds, followed by final assembly using lower coefficient of thermal expansion (CTE) epoxies. A thin metal film deposited on the glass substrate provides the absorption required to attain interfacial temperatures suitable for glass-to-glass bonding. This method allows the elimination of dedicated expensive V-groove plates while still maintaining very good fiber placement accuracy. The use of epoxy is minimized to simply securing macro packaging components and protecting fibers from environmental pressure, temperature, and humidity variation. The thermal expansion properties of the epoxy used were essential for the long-term FAU reliability.

Reconstruction of laser-induced microstructures temperature profiles

Metallic microstructures are currently widely used in micro-optoelectronics, sensorics, microfluidics, and other applications. A promising direction in the creation of such microstructures is pulsed laser processing of thin metal films. To improve laser technologies and simulate ongoing processes, data on the temperature fields that arise during laser exposure are needed. The article shows that the temperature distribution on the surface of a material during pulsed laser heating can be obtained from optical, scanning electron or atomic force micrographs. The proposed approach opens up the possibility of estimating temperature fields from archival photographs of various microstructures.

Ultrathin damage-tolerant flexible metal interconnects reinforced by in-situ graphene synthesis

Conductive patterned metal films bonded to compliant elastomeric substrates form meshes which enable flexible electronic interconnects for various applications. However, while bottom-up deposition of thin films by sputtering or growth is well-developed for rigid electronics, maintaining good electrical conductivity in sub-micron thin metal films upon large deformations or cyclic loading remains a significant challenge. Here, we propose a strategy to improve the electromechanical performance of nanometer-thin palladium films by in-situ synthesis of a conformal graphene coating using chemical vapor deposition (CVD). The uniform graphene coverage improves the thin film’s damage tolerance, electro-mechanical fatigue, and fracture toughness owing to the high stiffness of graphene and the conformal CVD-grown graphene-metal interface. Graphene-coated Pd thin film interconnects exhibit stable increase in electrical resistance even when strained beyond 60% and longer fatigue life up to a strain range of 20%. The effect of graphene is more significant for thinner films of < 300 nm, particularly at high strains. The experimental observations are well described by the thin film electro-fragmentation model and the Coffin-Manson relationship. These findings demonstrate the potential of CVD-grown graphene nanocomposite materials in improving the damage tolerance and electromechanical robustness of flexible electronics. The proposed approach offers opportunities for the development of reliable and high-performance ultra-conformable flexible electronic devices.

Smart city compatible thin film solar cell based on extraordinary transmission and metallic patch nanoantenna

An effective performance enhancement model for the thin film solar cell conjointly based on extraordinary transmission and nanoantenna is proposed and investigated. The absorber layer of the extraordinary transmission based solar cell contains a metallic thin film with periodic holes. Maximum extraordinary transmission is accomplished as the metallic film has the same refractive index on both sides. Increased light transmission causes the absorber layer to absorb more light, which increases short circuit current density and subsequently the efficiency of the thin film solar cell. The presented analysis demonstrates that adding a square nano patch on the top surface of the absorber layer can further boost the extraordinary transmission. The extraordinary transmission is increased because of the formation of cavity nanoantenna. Cavity nanoantenna increases the coupling of light into the hole due to the excitation of surface plasmons polaritons. The proposed design of solar cell exhibits around 98% absorption and the short circuit current density is increased by a factor of 3.23. The study has been carried out using finite difference time domain (FDTD) method.

Ultrasensitive and ultrastretchable metal crack strain sensor based on helical polydimethylsiloxane.

The majority of crack sensors do not offer simultaneously both a significant stretchability and an ultrahigh sensitivity. In this study, we present a straightforward and cost-effective approach to fabricate metal crack sensors that exhibit exceptional performance in terms of ultrahigh sensitivity and ultrahigh stretchability. This is achieved by incorporating a helical structure into the substrate through a modeling process and, subsequently, depositing a thin film of gold onto the polydimethylsiloxane substrate via sputter deposition. The metal thin film is then pre-stretched to generate microcracks. The sensor demonstrates a remarkable stretchability of 300%, an exceptional sensitivity with a maximum gauge factor reaching 107, a rapid response time of 158 ms, minimal hysteresis, and outstanding durability. These impressive attributes are attributed to the deliberate design of geometric structures and careful selection of connection types for the sensing materials, thereby presenting a novel approach to fabricating stretchable and highly sensitive crack-strain sensors. This work offers a universal platform for constructing strain sensors with both high sensitivity and stretchability, showing a far-reaching significance and influence for developing next-generation practically applicable soft electronics.

Characterization of Thin AlN/Ag/AlN-Reflector Stacks on Glass Substrates for MEMS Applications

Thin metal layers such as silver (Ag) are being utilized for various optical and plasmonic applications as well as for electrical purposes, e.g., as transparent electrodes in display devices or solar cells. This paper focuses on optical MEMS applications such as the Fabry–Pérot interferometer (FPI). Within such filters, reflector materials such as distributed Bragg reflectors (DBRs) or subwavelength gratings (SWGs) have been widely used so far, whereas metallic thin films (MTFs) were limited in application due to their comparatively higher absorption. In this paper, thin sputtered Ag layers with thicknesses of 20, 40 and 60 nm on glass substrates have been investigated, and it is shown that the absorption is very low in the visible spectral range (VIS) and increases only in near-infrared (NIR) with increasing wavelength. Thus, we consider Ag-thin layers to be an interesting reflector material at least for the VIS range, which can be easily fabricated and integrated. However, Ag is not inert and stable when exposed to the atmosphere. Hence, it needs a passivation material. For this purpose, AlN has been chosen in this contribution, which can be deposited by sputtering as well. In this contribution, we have chosen thin AlN layers for this purpose, which can also be deposited by sputtering. Thus, various AlN/Ag/AlN-reflector stacks were created and patterned by lift-off technology preferably. The fabricated reflectors were characterized with respect to adhesion, stress, cohesion, homogeneity, and most importantly, their optical properties. It was found that the thickness of the AlN can be used to adjust the reflectance–transmittance ratio in the VIS range, and influences the adsorption in the NIR range as well. Based on the measured values of the reflectors with 40 nm Ag, an exemplary transmission filter characteristics has been predicted for a wavelength range from 400 to 800 nm. Both the maximum transmittance and the full width at half maximum (FWHM) can be tuned by variation of the AlN thickness from 20 to 60 nm.

Growth paths in polycrystalline thin films

The polycrystalline grain microstructure of metallic thin films coarsens during grain growth in a unique way when the initial grain structure contains multiple grains in the film thickness. A regime with fast coarsening is followed by a regime of slow coarsening. At the same time, the grain structure itself undergoes clear structural changes from a bulk-like to a bamboo-like structure. The overall coarsening process evolves continuously, whereas the growth paths of individual grains do not follow the ones observed and predicted in either two- or three-dimensional grain growth.

Research on correlation between metal oxide semiconductor target and magnetron sputtered film based on TFT application

The research on the intrinsic relationship between target quality and TFT (thin film transistor) performance is not perfect for magnetron sputtering processes. The systematic analysis of target-film-device still needs to be improved. This paper studied the internal relationship between metal oxide semiconductor targets, thin films, and TFT devices for the deterioration problem of device performance caused by the failure of the aging target. The experimental results showed that the target surface with a long cumulative sputtering time had apparent macroscopic concave, related to the electromagnetic field distribution. The concave region was the primary source of thin film composition, so the research object of this work was mainly the concave region. A multi-oxide target was used in this work, and its selective sputtering phenomenon was serious, mainly reflected in the sputtering rate of In2O3 grains with higher electrical conductivity than other components. There were many failure sites on the surface of the target, including nodules, holes, coating layers, and cracks. The failure sites would hinder the normal sputtering process and eventually form a vicious circle, failing the target. Due to the decline of target quality, sputtering particles would be in a state of insufficient kinetic energy, resulting in particle agglomeration, element segregation, weak M-O (metal-oxide) bonds, loose accumulation, and surface or block defects. Ultimately, the TFT performance with the aging target showed lower mobility, larger subthreshold swing, increased off-state current, and poorer stability. Clarifying the relationship between target quality and device performance is beneficial for the development of electronic devices.

On Numerical Modelling and an Experimental Approach to Heterojunction Tandem Solar Cells Based on Si and Cu2O/ZnO—Results and Perspectives

A comparative analysis of three advanced architectures for tandem solar cells (SCs) is discussed, respectively: metal oxide, thin film, and perovskite. Plasmonic solar cells could further increase solar cell efficiency. Using this development, an innovative PV technology (an SHTSC based on metal oxides) represented by a four-terminal Cu2O/c-Si tandem heterojunction solar cell is investigated. The experimental and numerical modelling study defines the main aim of this paper. The experimental approach to SHTSCs is analysed: (1) a Cu2O layer is deposited using a magnetron sputtering system; (2) the morphological and optical characterization of Cu2O thin films is studied. The electrical modelling of silicon heterojunction tandem solar cells (SHTSCs) is discussed based on five simulation tools for the optimized performance evaluation of solar devices. The main novelty of this paper is represented by the following results: (1) the analysis suggests that the incorporation of a buffer layer can improve the performance of a tandem heterojunction solar cell; (2) the effect of interface defects on the electrical characteristics of the AZO/Cu2O heterojunction is discussed; (3) the stability of SHTSCs based on metal oxides is studied to highlight the degradation rate in order to define a reliable solar device. Perspectives on SHTSCs based on metal oxides, as well as Si perovskite tandem solar cells with metal oxides as carrier-selective contacts, are commented on.

Bridging the gap between surface physics and photonics.

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Synthesis of ZnS/Al2O3/TaSe2 Core/Shell Nanowires Using Thin Ta Metal Film Precursor

This study introduces a novel approach for fabricating ZnS/Al2O3/TaSe2 heterostructured core/shell nanowires (NWs) through the selenization of a metallic Ta thin film precursor. The synthesis process involves a meticulously designed four-step protocol: (1) generating ZnS NWs on an oxidized silicon substrate, (2) encapsulating these NWs with a precisely controlled thin Al2O3 layer via atomic layer deposition (ALD), (3) applying a Ta precursor layer by magnetron sputtering, and (4) annealing in a Se-rich environment in a vacuum-sealed quartz ampoule to transform the Ta layer into TaSe2, resulting in the final core/shell structure. The characterization of the newly produced NWs using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) was validated using the integrity and composition of the heterostructures. Our method not only establishes a new pathway for the synthesis of TaSe2-based core/shell NWs but also extends the potential for creating a variety of core/shell NW systems with chalcogenide shells by adapting the thin film metal precursor approach. This versatility opens the way for future advancements in nanoscale material applications, particularly in electronics and optoelectronics where core/shell geometries are increasingly important.

In Situ Metallic Coating of Atom Probe Specimen for Enhanced Yield, Performance, and Increased Field-of-View.

Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatings have been applied ex situ and mostly are not uniform. Here, we report on the controlled focused-ion beam in situ deposition of a thin metal film on specimens immediately after specimen preparation. Different metallic targets e.g. Cr were attached to a micromanipulator via a conventional lift-out method and sputtered using Ga or Xe ions. We showcase the many advantages of coating specimens from metallic to nonmetallic materials. We have identified an increase in data quality and yield, an improvement of the mass resolution, as well as an increase in the effective field-of-view. This wider field-of-view enables visualization of the entire original specimen, allowing to detect the complete surface oxide layer around the specimen. The ease of implementation of the approach makes it very attractive for generalizing its use across a very wide range of atom probe analyses.

Density-controlled electrochemical synthesis of ZnO nanowire arrays using nanotextured cathode

Zinc oxide (ZnO) nanowires fabricated via wet chemical synthesis on flexible polymer substrates are inherently unstable against mechanical bending stress because of their high density and weak adhesion to the substrate. We introduce a novel method for controlling the density of such ZnO nanowire arrays using a three-dimensional corrugated metal substrate. These metal substrates, featuring extruded and recessed patterns fabricated via nanoimprint lithography, were employed as cathodes during the electrochemical deposition of ZnO nanowire arrays. The ZnO nanowire arrays synthesized on the patterned metal thin film exhibited smaller diameters and lower densities compared to those on non-patterned metal films. This reduction in density can be attributed to aligned nucleation and limited growth on the patterned metal surface. Crucially, ZnO nanowires synthesized on patterned metal substrates displayed remarkable mechanical robustness against external forces, a direct consequence of their reduced density. In contrast, nanowires synthesized on non-patterned metal substrates were broken under mechanical bending. Detailed morphological analyses performed after mechanical bending tests confirm that ZnO nanowires synthesized on nanoimprinted metal electrodes exhibited enhanced mechanical characteristics compared to those on non-patterned metal electrodes. These findings clearly demonstrate the promise of utilizing density-controlled ZnO nanowires in piezoelectric devices.

Adhesion strength of ductile thin film determined by cross-sectional nanoindentation

Cross-sectional nanoindentation (CSN) is a powerful tool for measuring the adhesion strength of thin films. However, a quantitative assessment of interfacial properties for metal thin films on brittle substrates remains a challenge due to thin film ductility. In this work, we present a hierarchical analysis method to account for plastic dissipation during delamination. According to our observations, the CSN-induced wedge geometry is more complex than what is currently presented in literature. Therefore, we develop a three-dimensional finite element (FE) modelling strategy to simulate CSN-induced delamination. Furthermore, we present the mode mixity distribution during CSN measurement based on our simulation results. We find that a purely experimental assessment of adhesion strength by CSN for ductile thin films is imprecise due to the adhesion strength-dependent stress-state in this characterization technique. By using our combined experimental and numerical approach, the adhesion strength of EBPVD-deposited Al thin film on SiO2/Si substrate is determined as 0.577 J/m2 and an additional 20 nm thick Ti adhesion layer enhances its adhesion strength to 16.4 J/m2.

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO2 to Formic Acid

AbstractEfficiently transforming CO2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non‐toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO2 (CO2RR) on conventional carbon black‐supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp , , and orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p‐block elements.

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO2 to Formic Acid.

Efficiently transforming CO2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non-toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO2 (CO2 RR) on conventional carbon black-supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp , , and orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p-block elements.