Thin Film Materials Research Articles

The Memristive Properties and Spike Timing-Dependent Plasticity in Electrodeposited Copper Tungstates and Molybdates.

Memristors possess non-volatile memory, adjusting their electrical resistance to the current that flows through them and allowing switching between high and low conducting states. This technology could find applications in fields such as IT, consumer electronics, computing, sensors, and medicine. In this paper, we report successful electrodeposition of thin-film materials consisting of copper tungstate and copper molybdate (CuWO4 and Cu3Mo2O9), which showed notable memristive properties. Material characterisation was performed with techniques such as XRD, XPS, and SEM. The electrodeposited materials exhibited the ability to switch between low and high resistive states during varied cyclic scans and short-term impulses. The retention time of these switched states was also explored. Using these materials, the effects seen in biological systems, specifically spike timing-dependent plasticity, were simulated, being based on analogue operation of the memristors to achieve multiple conductivity states. Bio-inspired simulations performed directly on the material could possibly offer energy and time savings for classical computations. Memristors could be crucial for the advancement of high-efficiency, low-energy neuromorphic electronic devices and technologies in the future.

Summary of High Throughput Methods for Study of Thin Film Alloys

In the fields of semiconductor and electronics, fine processing at the micro and nanoscale is crucial. Thin film materials are an important part of this fine processing and their small molecular structure, ranging from microns to nanometers, is directly linked to achieving high throughput. Breakthroughs in thin film materials have accelerated research on high throughput systems and ultimately enabled mass production.

Investigation of deformation mechanism of SiC–CuNi composite thin film material nanochannels by molecular dynamics simulation

Semiconductor–metal composites are widely used in aerospace and automotive manufacturing, weapon production, and electronic packaging because of their high hardness and strength as well as low coefficient of thermal expansion. In this study, SiC–CuNi composite films produced by single-target and dual-target co-deposition were simulated using molecular dynamics. The films were subjected to high-temperature quenching to obtain an annealing model. The deformation behaviors of the deposition and annealing models during the nanoindentation process were investigated in detail. Then, a characterization method for inspecting the roundness of nanochannels was proposed. In the deposition model, the Ni–Cu model generated amorphous structures at the interface. However, the annealing treatment atomically rearranged the film, producing dense films with improved crystalline quality. Other deposition models are developed using complex twin structures, which play a role in stress distribution and strain diffusion during deformation. Comparative investigation shows that high-temperature quenching can effectively enhance the stiffness of composite films, tightly bonding the atoms at the semiconductor–metal interface. Hardness calculations were performed for all models and the result was that the Cu + Ni model had the highest hardness. Moreover, the formation of nanochannels is affected: the tighter the bond, the more uniform the nanochannels.

The estimation of crack onset strain for brittle films with residual stress on ductile substrates

In this paper, a method predicting crack onset strain in thin film/substrate system with residual stress is proposed. For thin film materials with size effect, the strength criterion is improved by considering the variation of film cohesive strength. The modified strength criterion is more accurate in fitting the crack density than energy criterion. According to the modified strength criterion, the crack onset strains of three kinds SiNx films prepared in our work are 1.21 %, 1.25 % and 1.10 %, respectively. It is proved that the crack onset strain predicted by the modified strength criterion is accurate by bending test. Our work provides a new method for determining the crack onset strain of various composite films more accurately.

Destruction of a magnesium alloy film in the condensed state by an ultrashort laser-driven shock wave

Laser-driven shock wave phenomena in a sub-micrometer Mg–4Al–2Zn alloy film are studied using spectral interferometry with spatial and temporal (1 ps) resolution. Upon irradiating the film through a glass substrate by 500 fs laser pulses, the ultrashort elastic compression pulses with the peak stress up to 4.6 GPa at a propagation distance of 0.5 μm were generated. Depending on the laser fluence, either spall fracture near the rear surface in the solid state or cavitation near the metal–glass interface in the liquid state was observed. The spall strength of the solid Mg alloy and the upper limit of the cavitation threshold in the melt at the strain rate of ∼109 s−1 were extracted from the free surface velocity history. The depth of fracture initiation was retrieved from the instant of the spall pulse exit, and the thickness of the molten layer was estimated to be 100–160 nm depending on laser fluence. The investigation of the residual morphology by scanning electron and atomic force microscopies revealed the presence of melting and nucleation within the irradiated area. The experimental findings are of interest for predicting the behavior of magnesium alloys in the condensed state at extremely high strain rates, for studying the physics of metastable states and for simulating the interaction of ultrashort laser pulses with thin film materials.

A Real-Time Supervisory Control Strategy for Roll-to-Roll Dry Transfer of 2D Materials and Printed Electronics

With the advancement of manufacturing technology in transfer printing and two-dimensional materials fabrication, roll-to-roll (R2R) dry transfer by mechanical peeling is becoming a promising process for efficient transfer of thin film materials from a donor substrate to a target substrate for device fabrication. As a key process variable, the peeling angle determines the loading condition of the R2R dry transfer process and thus the quality of the transferred material. Controlling the peeling angle is challenging in a R2R process because of the adhesion energy and peeling front velocity in addition to the flexible substrates involved in the peeling process. In this article, a real-time supervisory control strategy is developed for the R2R mechanical peeling process. A strain energy-based peeling process model is employed to estimate the peeling front velocity and the adhesion energy. The real-time estimation is then used to generate reference signals for web tension controllers to track the desired peeling angles. The proposed control strategy is demonstrated with both simulation and experimental results.

Physical-Vapor-Deposited Metal Oxide Thin Films for pH Sensing Applications: Last Decade of Research Progress.

In the last several decades, metal oxide thin films have attracted significant attention for the development of various existing and emerging technological applications, including pH sensors. The mandate for consistent and precise pH sensing techniques has been increasing across various fields, including environmental monitoring, biotechnology, food and agricultural industries, and medical diagnostics. Metal oxide thin films grown using physical vapor deposition (PVD) with precise control over film thickness, composition, and morphology are beneficial for pH sensing applications such as enhancing pH sensitivity and stability, quicker response, repeatability, and compatibility with miniaturization. Various PVD techniques, including sputtering, evaporation, and ion beam deposition, used to fabricate thin films for tailoring materials' properties for the advanced design and development of high-performing pH sensors, have been explored worldwide by many research groups. In addition, various thin film materials have also been investigated, including metal oxides, nitrides, and nanostructured films, to make very robust pH sensing electrodes with higher pH sensing performance. The development of novel materials and structures has enabled higher sensitivity, improved selectivity, and enhanced durability in harsh pH environments. The last decade has witnessed significant advancements in PVD thin films for pH sensing applications. The combination of precise film deposition techniques, novel materials, and surface functionalization strategies has led to improved pH sensing performance, making PVD thin films a promising choice for future pH sensing technologies.

N-type CdTe:In for photovoltaics: in situ doping, type verification and compensation effects

We explored the in-situ doping of cadmium telluride thin films with indium to produce n-type absorbers as an alternative to the near-universal choice of p-type for photovoltaic devices. The films were grown by close space sublimation from melt-synthesised feedstock. Transfer of the indium during film growth was limited to 0.0014%–0.014%—unless reducing conditions were used which yielded 14%–28% efficient transport. While chunks of bulk feedstock were verified as n-type by the hot probe method, carrier type of thin film material was only able to be verified by using hard x-ray photoelectron spectroscopy to determine the Fermi level position within the band gap. The assignment of n-type conductivity was consistent with the rectification behaviour of a p-InP/CdTe:In junction. However, chloride treatment had the effect of compensating n-CdTe:In to near-intrinsic levels. Without chloride, the highest dopant activation was 20% of the chemical concentration of indium, this being for a film having a carrier concentration of n = 2 × 1015 cm−3. However, the activation was often much lower, and compensation due to over-doping with indium and native defects (stoichiometry) are discussed. Results from preliminary bifacial devices comprising Au/P3HT/ZnTe/CdTe:In/CdS/FTO/glass are presented.

Combinatorial Growth of Vertically Aligned Nanocomposite Thin Films for Accelerated Exploration in Composition Variation

Combinatorial growth is capable of creating a compositional gradient for thin film materials and thus has been adopted to explore composition variation mostly for metallic alloy thin films and some dopant concentrations for ceramic thin films. This study uses a combinatorial pulsed laser deposition method to successfully fabricate two‐phase oxide–oxide vertically aligned nanocomposite (VAN) thin films of La0.7Sr0.3MnO3 (LSMO)‐NiO with variable composition across the film area. The LSMO‐NiO compositional gradient across the film alters the two‐phase morphology of the VAN through varying nanopillar size and density. Additionally, the magnetic anisotropy and magnetoresistance properties of the nanocomposite thin films increase with increasing NiO composition. This demonstration of a combinatorial method for VAN growth can increase the efficiency of nanocomposite thin film research by allowing all possible compositions of thin film materials to be explored in a single deposition.

Leveraging of both positive and negative magnetocaloric effects in ZnFe2O4 layers

The novel magnetocaloric thin film material with a tunable magnetic transition is the cornerstone of magnetic refrigeration, which plays a crucial role in cooling on-chip devices. Zn-ferrite (ZnFe2O4) occupies a significant place due to its interesting grain size-controlled magnetism. In this study, we investigate the magnetocaloric properties of Zn-ferrite layers consisting of a mixture of superparamagnetic (SPM), ferrimagnetic (FiM), and bulk-type antiferromagnetic (AFM) grains. Due to the co-existence of different magnetic grains, a crossover from the conventional magnetocaloric effect (MCE) to the inverse magnetocaloric effect (IMCE) is observed. The magnetic entropy change (-ΔSM) exhibits high-temperature positive values around the Curie temperature (TC) and low-temperature negative values around either the Néel temperature (TN) or blocking temperature (TB). The maximum value of -ΔSM (0.10 J/kg K) is almost double in AFM-dominant samples compared to SPM-dominant samples, while maintaining a relative cooling power (RCP) of 11.57–14.82 J/Kg under applied fields of 0 – 2T. FiM-dominant samples, with a wide working temperature window around room temperature, are suitable for large RCP applications. Thus, the discovery of new IMCE materials like single layers of Zn-ferrite is equally important in the search for suitable conventional MCE materials for magnetic refrigeration devices, which can exhibit multiple -ΔSM and RCP peaks in low and high temperatures.

Synthesis, Structural, Optical, and Electrical Characterization of Biochitosan/Na0.5Bi0.5TiO3 Composite Thin-Film Materials.

The purpose of this research work was to synthesis bioderived nanocomposite films by incorporating Na0.5Bi0.5TiO3 (NBTO) nanoparticles into a chitosan matrix. The NBTO nanoparticles were synthesized using a traditional solid-state technique. Then, through a solution-casting approach, flexible composite films were fabricated using chitosan polymer. The study presents a range of compelling findings. For structural and morphological insights, scanning electron microscopy (SEM) reveals a fascinating morphology where NBTO nanoparticles are uniformly dispersed and interlocked with other particles, forming interconnected grains with significant interspaces within the chitosan matrix. For the optical properties, the spectral response within the 300-800 nm range is primarily governed by light scattering attributed to NBTO particles with diameter sizes ranging from 100 to 400 nm, as well as the distinctive bandgap exhibited by the NBTO phase. The investigation of dielectric properties demonstrates that composite films exhibit markedly higher dielectric values in comparison to pure chitosan films. It is noteworthy that an increase in the NBTO content results in a corresponding increase in dielectric values, enhancing the versatility of these materials. Local piezoelectric measurements utilizing piezoresponse force microscopy confirm the expected piezoelectric and ferroelectric behavior of NBTO particles when dispersed within the chitosan matrix. This research introduces a novel class of biocompatible nanocomposite materials, combining impressive structural attributes, enhanced dielectric properties, and piezoelectric capabilities. The outcomes of this study hold substantial promise for advanced applications in opto- and piezoelectric technologies, marking a significant advancement in biologically sourced materials with multifunctional properties.

Towards the Development of a Z-Scheme FeOx/g-C3N4 Thin Film and Perspectives for Ciprofloxacin Visible Light-Driven Photocatalytic Degradation

Thermally synthesized graphitic carbon nitride (g-C3N4) over pulsed laser deposition (PLD) produced urchin-like iron oxide (FeOx) thin films were fabricated via in situ and ex situ processes. Materials characterisation revealed the formation of the graphitic allotrope of C3N4 and a bandgap Eg for the combined FeOx/g-C3N4 of 1.87 and 1.95 eV for each of the different fabrication strategies. The in situ method permitted to develop a novel petal-like morphology, whereas for the ex situ method, a morphological mixture between FeOx bulk and g-C3N4 was observed. Given the improved optical and morphological properties of the in situ film, it was employed as a proof of concept for the direct photocatalysis and photo-Fenton removal of ciprofloxacin antibiotic (CIP) under visible light irradiation. Improved photocatalytic activity (rate constant k = 8.28 × 10−4 min−1) was observed, with further enhancement under photo-Fenton conditions (k = 2.6 × 10−3 min−1), in comparison with FeOx + H2O2 (k = 1.6 × 10−3 min−1) and H2O2 only (k = 1.3 × 10−4 min−1). These effects demonstrate the in situ methodology as a viable route to obtain working heterojunctions for solar photocatalysis in thin-film materials, rather than the more common powder materials.

Inducing Circularly Polarized Luminescence by Confined Synthesis of Ultrasmall Chiral Carbon Nanodot Arrays in Pyrene-Based MOF Thin Film.

Combining the features of the host-guest system and chirality is an efficient strategy to achieve circularly polarized luminescence (CPL). Herein, well-defined chiral carbon nanodot (chirCND) arrays were confined-synthesized by low-temperature calcination of a chiral amino acid loaded metal-organic framework (MOF) to induce high CPL. An achiral porous pyrene-based MOF NU-1000 thin film as the host template was prepared by a liquid-phase epitaxial layer-by-layer fashion, and chiral amino acids as the carbon sources could be confined in the porous MOF and carbonized to homogeneous and ultrasmall chirCND arrays, resulting in a chirCNDs@NU-1000 thin film (l-CNDsx@NU-1000; x = l-cysteine (cys), l-serine, l-histidine, l-glutamic acid, and l-pyroglutamic acid). The results show the pristine chirCNDs by directly carbonizing chiral amino acids hardly endow them with a CPL property. By contrast, benefiting from the arrayed confinement and coordination interaction between chirCNDs and NU-1000, the chirality transfer on the excited state of chirCNDs@NU-1000 is enabled, leading to strong CPL performance (a high luminescence dissymmetry factor glum of l-CNDscys@NU-1000 thin film reached 1.74 × 10-2). This study of chirCNDs encapsulated in fluorescent MOF thin films provides a strategy for developing uniform chiral carbon nanoarrays and offers chiral host-guest thin-film materials for optical applications.

Engineering the thin film characteristics for optimal performance of superconducting kinetic inductance amplifiers using a rigorous modelling technique.

Background: Kinetic Inductance Travelling Wave Parametric Amplifiers (KITWPAs) are a variant of superconducting amplifier that can potentially achieve high gain with quantum-limited noise performance over broad bandwidth, which is important for many ultra-sensitive experiments. In this paper, we present a novel modelling technique that can better capture the electromagnetic behaviour of a KITWPA without the translation symmetry assumption, allowing us to flexibly explore the use of more complex transmission line structures and better predict their performance. Methods: In order to design a KITWPA with optimal performance, we investigate the use of different superconducting thin film materials, and compare their pros and cons in forming a high-gain low-loss medium feasible for amplification. We establish that if the film thickness can be controlled precisely, the material used has less impact on the performance of the device, as long as it is topologically defect-free and operating within its superconducting regime. With this insight, we propose the use of Titanium Nitride (TiN) film for our KITWPA as its critical temperature can be easily altered to suit our applications. We further investigate the topological effect of different commonly used superconducting transmission line structures with the TiN film, including the effect of various non-conducting materials required to form the amplifier. Results: Both of these comprehensive studies led us to two configurations of the KITWPA: 1) A low-loss 100 nm thick TiN coplanar waveguide amplifier, and 2) A compact 50 nm TiN inverted microstrip amplifier. We utilise the novel modelling technique described in the first part of the paper to explore and investigate the optimal design and operational setup required to achieve high gain with the broadest bandwidth for both KITWPAs, including the effect of loss. Conclusions: Finally, we conclude the paper with the actual layout and the predicted gain-bandwidth product of our KITWPAs.

In-plane tunnel spectroscopy of cuprate superconductors with nano-scale Josephson junctions

Superconducting tunnel spectroscopy has been used for decades to gather valuable information about the electronic density of states and the energy gap in superconductors. For thin film materials, this has, for the most part, only been possible in directions perpendicular to the substrate. For anisotropic materials like the cuprate superconductors, this has hindered knowledge about the density of states in the a–b plane of the film where superconductivity is strongest. The advent of helium ion beam fabricated planar Josephson junctions has fundamentally changed how we can perform tunnel spectroscopy. In this work, we utilize nanoscale Josephson tunnel junctions orientated along different crystalline directions and report the anisotropy of the superconducting energy gap of cuprates at both the micro- and nanoscale. The smaller nanoelectrodes exhibit more variation that roughly correlates with the granularity of the film. We also observe that the gap energy is inversely proportional to the material's conductivity in the voltage state.

Transfer-free synthesis of large-area 2D boron films on non-wetting liquid gallium substrate

Two-dimensional (2D) boron films have been studied for their attractive physical and chemical properties in diverse technologies. However, the synthesis of large scale and high-quality boron films is still challenging, which limit its applications in high-performance devices. In this work, thin 2D boron films have been synthesized by chemical vapor deposition (CVD) on non-wetting liquid gallium (Ga) spheres by using boron and boron oxide powders as solid sources. The thin films were directly exfoliated onto SiO2/Si substrates to prepare smooth and uniform boron films by means of mechanical exfoliation method. The peak of boron at 618 cm−1 wave number is obtained by Raman characterization and the boron peak at 188.8 eV is obtained by XPS, which indicates that the film is elemental boron film. By controlling the intercalation chemistry, we obtained large-area 2D polycrystalline boron films with a thickness of 0.8 nm by using transfer-free Synthesis, which avoids the introduction of impurities. The research promotes the application of 2D boron thin film materials in electrical and optical devices.

Microstructure, creep properties, and electrical resistivity of magnetron sputtering deposited SAC305 thin films

This paper investigates the surface morphology, mechanical properties, and electrical resistivity of 96.5Sn–3.0Ag–0.5Cu (SAC305) thin films deposited on Si and SiO2 substrates through RF magnetron sputtering. Various deposition parameters were tested using both DC and RF power sources at different pressures and powers to produce robust continuous films. The most optimal surface morphology, with an average grain size of ∼1 μm and a thickness of ∼2.2 μm, was accomplished at a pressure of 2.4 mTorr and 200 W power. After polishing, a uniform thickness of 1800 nm with a mean roughness (Ra) of 14.9 nm was obtained. The samples contained polycrystalline β-Sn grains at (200) diffraction planes with a preferred orientation 2θ of 30.70°. Although the XRD pattern did not indicate any Ag peaks, weak peaks of Ag3Sn were observed at 2θ of 37.60° and 39.59°, corresponding to diffraction planes (020) and (211), respectively. The electrical resistivity of the SAC305 thin film deposited on the SiO2 substrate and of the bulk SAC305 samples were measured as 19.6 and 13.7 μΩ cm, respectively. It was noted that changes in hold time at peak loads or the rate of loading in the creep experiments did not significantly influence the creep properties of the SAC305 bulk or thin film material.

Preparation of ZnO Piezoelectric Thin-Film Material for Ultrasonic Transducers Applied in Bolt Stress Measurement

The measurement of bolt preload by using ultrasound can be accurate, convenient, and can realize the real-time monitoring of the change in the residual axial stress of a bolt during use. In order to realize the ultrasonic measurement of bolt preload, the use of zinc oxide (ZnO) piezoelectric thin-film material as an ultrasonic transducer material to stimulate an ultrasonic signal on the bolt is a feasible solution. In this paper, we choose to use RF magnetron sputtering technology to prepare ZnO piezoelectric thin-film materials and study the effects of sputtering power and target substrate distance on the structure and ultrasonic properties of ZnO piezoelectric thin films during the preparation process, in order to lay the foundation for realizing the application of ZnO piezoelectric thin films in the field of bolt preload measurement. The experimental results show that too-large sputtering power or too-small target substrate distance will result in the particles having too much kinetic energy during sputtering and exhibiting a structure of multiorientation growth, which excites ultrasonic longitudinal–transverse waves. A sputtering power of 600 W, sputtering time of 4 h, and target substrate distance of 100 mm are ideal experimental parameters for a ZnO piezoelectric thin-film material to be excited by an ultrasonic longitudinal wave signal, and its ideal operating frequency is 41 MHZ. These research results of bolt stress detection demonstrate good application prospects.

Strain-induced self-rolled-up microtubes for multifunctional on-chip microfluidic applications.

On-chip microfluidics are characterized as miniaturized devices that can be either integrated with other components on-chip or can individually serve as a standalone lab-on-a-chip system for a variety of applications ranging from biochemical sensing to macromolecular manipulation. Heterogenous integration with various materials and form factors is, therefore, key to enhancing the performance of such microfluidic systems. The fabrication of complex three-dimensional (3D) microfluidic components that can be easily integrated with other material systems and existing state-of-the-art microfluidics is of rising importance. Research on producing self-assembled 3D architectures by the emerging self-rolled-up membrane (S-RuM) technology may hold the key to such integration. S-RuM technology relies on a strain-induced deformation mechanism to spontaneously transform stacked thin-film materials into 3D cylindrical hollow structures virtually on any kind of substrate. Besides serving as a compact microfluidic chamber, the S-RuM-based on-chip microtubular architecture exhibits several other advantages for microfluidic applications including customizable geometry, biocompatibility, chemical stability, ease of integration, uniform field distributions, and increased surface area to volume ratio. In this Review, we will highlight some of the applications related to molecule/particle sensing, particle delivery, and manipulation that utilized S-RuM technology to their advantage.

Study of doped Cu on adjusting the TiZrV phase structure and improving the low temperature activated gettering performance

In this paper, we focus on studying the low-temperature activated bulk TiZrV getter materials instead of thin film materials to meet the high reliability requirements of vacuum devices. The research was carried out by introduce Cu element to improve the activated characteristic and gettering performance. The XRD and in-situ XPS results showed the addition of Cu could effectively reduce the diffusion temperature and increase the diffusion rate of O atoms by modifying the phase structure of TiZrV getter. An appropriate amount of Cu was beneficial for stabilizing high-temperature β-Zr phase at room temperature, which had a larger interstitial void compared to that of the low-temperature α-Zr phase. The TiZrVCu getter with high proportion of β-Zr phase displayed 55.0% higher gettering performance than that of TiZrV with activated at 350 °C for 10 min. The proposed strategy of improving the low-temperature activated gettering performance offers promising opportunities for exploiting potential new low-temperature activated bulk getter materials.