Aluminum Thin Films Research Articles

Superconducting nanostrip single photon detectors fabricated of aluminum thin-films

We systematically investigated the detection performance of Al nanostrips for single photons at various wavelengths. The Al films were deposited using magnetron sputtering, and the sophisticated nanostructures and morphology of the deposited films were revealed through high-resolution transmission electron microscopy. The fabricated Al meander nanostrips, with a thickness of 4.2 nm and a width of 178 nm, exhibited a superconducting transition temperature of 2.4 K and a critical current of approximately 5 μA at 0.85 K. While the Al nanostrips demonstrated a saturated internal quantum efficiency for 405-nm photons, the internal detection efficiency exhibited an exponential dependence on bias current without any saturation tendency for 1550-nm photons. This behavior can be attributed to the relatively large diffusion coefficient and coherence length of the Al films. By further narrowing the nanostrip width, the Al-SNSPDs remain capable of effectively detecting single telecom photons to facilitate practical applications.

КВАНТОВЫЙ РАЗМЕРНЫЙ ЭФФЕКТ В ЧИСТЫХ ПЛЕНКАХ АЛЮМИНИЯ

In the modern world, there's a notable trend towards the active miniaturization of electronic devices. With technological advances enabling the manipulation of nanostructures, there's an increasing focus on exploring quantum effects pivotal to such designs. One distinguishing feature of nanostructures is the quantum nature of the electron's energy spectrum. This spectrum becomes discrete in directions where electrons move. Depending on the direction of this confinement, structures can be categorized as nanoplates, quantum wires, or quantum dots. The properties of such structures can significantly differ from those observed in large-scale systems. When discussing superconductivity, particular emphasis is placed on its macroscopic quantum properties.The influence on electronic wave functions is reflected in the characteristics of the superconducting state on broader scales. The Bardeen-Cooper-Schrieffer (BCS) theory is frequently utilized to analyze these nanostructures. The Gor’kov equations method serves as a potent tool for tasks related to the BCS theory. For instance, it can determine the parameters of the superconducting state, critical temperature, and current. Components of these equations, like Green's functions, are associated with various system properties. Research in the early stages of superconductivity studies revealed that the critical temperature (Tc ) – the temperature at which a material transitions to a superconducting state – can differ significantly between thin films and bulk materials. Intriguingly, reducing the film's thickness can both decrease (e.g., in niobium) and increase (e.g., in aluminum) the Tc value. This study delves into the quantum size effect in thin aluminum films, paving the way for materials with higher transition temperatures. Such advancements can simplify and make the maintenance of superconducting systems more cost-effective. In this study, a theoretical relationship between the critical temperature of a thin aluminum film and its thickness was derived. The Green's function method was chosen, which hadn't been previously employed for this computation. This approach offers greater potential compared to other superconductivity theory methods, presenting extensive avenues for theoretical exploration in this domain. The authors are confident that this work will contribute to further research on quantum dimensional effects in low-dimensional superconducting structures.

Room-temperature, RF-activated, quantum effects via engineered defect sites in thin-film AlN, Al2O3, and SnOx

The behavior of a sine wave propagated through thin films of aluminum nitride (AlN), aluminum oxide (Al2O3), and tin oxide (SnOx) with engineered buried defect sites may suggest quantum excitation and defect-mediated waveform modulations. Two distinct methods to induce these buried defects, etch pattern defects (EPD) and indentation pattern defects (IPD), were employed to detect these interactions. All the experiments were conducted at room temperature (21 °C) over a frequency range between 5 and 1000 kHz. In addition, EPD and IPD devices were composed of AlN, Al2O3, and SnOx. An inverse relationship between excitation frequency and voltage is observed for all devices. All these devices exhibited a relaxation time ranging between 0.2 and 0.75 µs. Devices without these engineered defect sites preserve the waveform integrity, emphasizing the impact of the buried defect sites. This research focuses on the relationship between defect type, excitation frequency, and voltage to understand the deeper mechanisms at play in these quantum defect-driven wave alterations in AlN, Al2O3, and SnOx thin films.

Size-dependent coalescence of nanobubbles in pure water

Previously, we experimentally demonstrated that similar-sized bulk nanobubbles (NBs) preferentially coalesce when the NB size is ∼100 nm. To explain this peculiar NB coalescence phenomenon, we performed a nanoparticle trajectory analysis to determine the size distribution profile and time-dependent concentration of the bulk NBs generated by pressurizing a porous alumina thin film with ordered straight nanoholes. Furthermore, we propose a physical model based on the hypothesis that the repulsive energy increases when two NBs establish contact, and that coalescence occurs when the kinetic energy of the NBs overcomes the repulsive energy. We simulated the observed NB-size distribution by using the proposed model. The simulation results explained the observed NB-size distribution and lifetime of the NBs. They further indicated that NB coalescence was suppressed for small NBs with diameters of approximately 100 nm or less, resulting in their long lifetimes.

Gettering of iron by aluminum oxide thin films on silicon wafers: Kinetics and mechanisms

Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the bulk metal concentrations. Aluminum oxide thin films from plasma-enhanced atomic layer deposition (PE-ALD) have been reported to getter iron from silicon wafers. However, its gettering mechanism and kinetics remain unclear. In this study, by experimentally monitoring the kinetics of iron reduction in the silicon wafer bulk, aluminum oxide gettering of iron is shown to be caused by a segregation mechanism. Fitting the experimental iron reduction kinetics by the simulation of a segregation gettering process based on various diffusion scenarios suggests that the gettering kinetics is limited by both the diffusivities of iron in the silicon wafer bulk and in the aluminum oxide film. The activation energy of the segregation gettering process (negative meaning exothermic reaction) is estimated to be −0.47 ± 0.16 eV for the investigated as-deposited PE-ALD aluminum oxide film at 550–900 °C, and −0.35 ± 0.06 eV at 400–900 °C for the same film after a 400 °C forming gas anneal (FGA), i.e., after activating the passivation effect of the film. Capacitance–voltage measurements of the films indicate a higher surface defect density in the as-deposited films as compared to the FGA-activated films, which suggests a possible correlation between the surface defect density and gettering.

Development of Schottky barrier field-effect transistors (SB-MOSFET) with ultra-low thermal budget

This work presents the development of SB-MOSFET transistors incorporating thin HfO2 as a thin gate dielectric deposited by thermal atomic layer deposition (ALD) at low temperature. Aluminum thin films were used as the source/drain (S/D) and gate (G) electrodes, using e-beam evaporation at high-vacuum conditions. Even though low subthreshold slope (SS) values of 111 mV/dec and threshold voltage (Vth) of 0.58 V were obtained using a low thermal budget of 150 °C for transistor fabrication, scanning electron microscopy (SEM) analysis highlights the importance of proper mask alignment in order to enhance electrical performance and thus, reproducibility of the electrical characteristics of this device. This device can be fully integrated into silicon after standard Complementary Metal-Oxide-Semiconductor (CMOS)-compatible processing, so that it could be easily adopted into Front-End-Of-Line or even in Back-End-Of-Line stages of an integrated circuit, where low thermal budget is required and where its functionality could be increased by developing additional and fast logic.

Study of the Effect of Radiation Exposure on Grain Size and Mechanical Properties of Thin-Film Aluminum

Study of the Effect of Radiation Exposure on Grain Size and Mechanical Properties of Thin-Film Aluminum

Coverage-dependent desorption kinetics of water on a well-ordered alumina thin film surface.

We have developed an experimental and analytical setup for thermal desorption spectroscopy of solid water films on surfaces. We obtain the coverage-dependent desorption kinetics of water molecules from a well-defined ultra-thin alumina/NiAl(110) surface in the coverage range of 0-2 monolayers. We use a novel deconvolution technique to eliminate the pumping delay of water vapor in the vacuum system, which has previously hindered the accurate estimation of desorption kinetic parameters, such as activation energy and pre-exponential factor. The coverage-dependent Arrhenius analysis reveals that the desorption activation energy decreases with increasing coverage in the sub-monolayer range, indicating that the water-water interaction is not attractive. We also find that the pre-exponential factor for the second layer is higher than that for the sub-monolayer. We explain this difference in terms of transition state theory and propose that entropic effects play a significant role in water desorption kinetics.

Preparation of Alumina Thin Films by Reactive Modulated Pulsed Power Magnetron Sputtering with Millisecond Pulses

This paper aims to investigate the quality of thin alumina films deposited on glass samples using magnetron sputtering in the reactive modulated pulsed power mode (MPPMS) and evaluate the process productivity. The aluminum target was sputtered in Ar/O2 gas mixtures with different fractions of oxygen in the total gas flow, in the fixed pulsed voltage mode. The pulse-on duration was varied between 5 and 10 ms, while the pulse-off time was 100 or 200 ms. The dependences of mass deposition rate and discharge current on the oxygen flow were measured, and the specific deposition rate values were calculated. Prepared coatings had a thicknesses of 100–400 nm. Their quality was assessed by scratch testing and by measuring density, refractory index, and extinction coefficient for different power management strategies. The strong influence of pulse parameters on the coating properties was observed, resulting in a maximum density of 3.6 g/cm3 and a refractive index of 1.68 for deposition modes with higher duty cycle values. Therefore, adjusting the pulse-on and pulse-off periods in MPPMS can be used not only to optimize the deposition rate but also as a tool to tune the optical characteristics of the films. The performance of the studied deposition method was evaluated by comparing the specific growth rates of alumina coatings with the relevant data for other magnetron discharge modes. In MPPMS, a specific deposition rate of 200 nm/min/kW was obtained for highly transparent Al2O3, without using any dedicated feedback loop system for oxygen pressure stabilization, which makes MPPMS superior to short-pulse high-power impulse magnetron sputtering (HiPIMS) modes.

Controlling Nucleation Sites for Metal Oxide Film Growth on Glassy Carbon via Electrochemical Preoxidation.

Coating electrode materials with metal oxide thin films can improve the performance of electrocatalysts and charge storage materials. Atomic layer deposition (ALD) enables the deposition of conformal, uniform films on a wide range of electrodes; however, an even film depends on the availability of nucleation sites directly on the electrode surface. Here, we show that the electrochemical oxidation of glassy carbon electrodes prior to the deposition of alumina thin films by ALD leads to more uniform electrochemically passivating films. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) demonstrate that film uniformity increases with the increasing potential of preoxidation until 2.50 V versus Ag/AgCl, at which point the films are fully passivating and appear continuous by SEM. Further increasing the potential of preoxidation leads to uniform but less consistently passivating alumina films. These findings show that electrochemical preoxidation is a rapid and readily tunable strategy for controlling oxygenic nucleation sites and therefore the growth of thin metal oxide films on glassy carbon electrodes.

Mechanical properties and water vapour corrosion behaviour of Al x CoCrFeNi high-entropy alloys.

Al x CoCrFeNi (x = 0.1, 0.5 and 1) high-entropy alloys (HEAs) were prepared using a spark plasma sintering (SPS) technique combined with aerosol powder. Their microstructure and phase constituents were characterized using an X-ray diffractometer and SEM, and their tensile properties, hardness and compactness were tested. The results show that the crystal structure of the Al x CoCrFeNi HEAs changed significantly with the Al content, from the original single face-centered cubic FCC phase (Al0.1CoCrFeNi) to an FCC + BCC structure (Al0.5CoCrFeNi), and then to FCC + BCC + sigma (σ) phase structures (AlCoCrFeNi). Chemical composition analysis showed that the crystal structure transformation was related to the segregation caused by the increased Al content. The hardness of the Al x CoCrFeNi HEAs increases with increasing Al content, and the hardness of AlCoCrFeNi reaches a maximum of 507.3 HV. The tensile properties of the alloy show a trend of first increasing and then decreasing with increasing Al content, and the yield strength, ultimate tensile strength and elongation of the Al0.5CoCrFeNi alloy reach maximum values of 527.4 MP, 943.3 MPa and 28.2%, respectively. The fracture mechanism of the Al0.1CoCrFeNi and Al0.5CoCrFeNi alloys is typical ductile fracture, while that of the AlCoCrFeNi alloy is cleavage fracture. The compactness of the alloy increases with the Al content. The samples were also subjected to high-temperature water vapour corrosion, and corrosion products such as Al3Fe5O12, CoCr2O4 and NiCr2O4 were found in the Al0.1 and Al0.5 alloys, whereas no oxide peaks were detected using XRD for the Al1 alloy. It was also presumed that a very thin alumina film was generated on the surface of the Al1 alloy, preventing the oxidation of the sample, in combination with the analysis of SEM, EDS and XPS behaviour.

Mechanism of Infiltration between Alumina and Polyurea for Stretchable Patterned Thin Film

Conventional display panels could be replaced by OLED displays, which are a leading and popular technology in display industry. While OLEDs are quite stretchable, they are vulnerable to oxygen and moisture, and currently, there is no proper barrier film that can protect them from these elements while also maintaining high stretchability. In this study, a horizontally patterned organic/inorganic layer that can increase the stretchability of the film while also providing the necessary protection is proposed. The final structure of the thin film will consist of two layers: a stripe-patterned organic/inorganic layer created using area-selective ALD/MLD processes, followed by an organic layer.To achieve the research objectives, the degree of infiltration of the inorganic layer into the organic layer is crucial. In this study, the mechanism behind the generation of the interface layer between alumina and polyurea is investigated. Trimethylaluminum (TMA) and H2O were used as precursors for fabricating alumina by ALD, while 1,4-phenylene diisocyanate (PPDI) and ethylene diamine (ED) were used for fabricating polyurea by MLD. The porous structure of the polymer allows ALD precursors to spread into it. During TMA pulsing on the polymer layer, the degree of infiltration is affected by the type of chemical bond in the polymer. Previous research has shown that TMA precursors are more likely to infiltrate polymers with C=O bonds into the polymer layer. Therefore, polyurea, which contains a carbonyl functional group (-C(=O)-), is suitable for this research. While the conditions for polyurea MLD are fixed, five chamber temperature conditions for depositing alumina were selected: 30℃, 50℃, 70℃, 90℃, and 110℃. Alumina was deposited by ALD on 30nm of polyurea which was deposited on a Si wafer.From X-ray reflectivity (XRR) data, it was observed that three main types of thin films were formed. As the temperature increased, the density and thickness of the alumina layer forming the thin film changed. To verify the change in the alumina thin film, FTIR and XPS were used to compare the peaks representing the bonds of Aluminum and Oxygen.

Fabrication of ZnO-encapsulated porous alumina structure utilizing a two-step anodizing technique for memristor applications

Highly ordered porous alumina thin films with various aspect ratios were fabricated by a two-step anodizing technique. The dependence of the high resistance state to low resistance state ratio on the aspect ratio of the pores was observed in the memristors utilizing porous alumina. The reduction of threshold voltage and improvement in stability were achieved by properly encapsulating i-ZnO inside the porous alumina as the switching medium. A significant improvement in the ratio of high resistance state to low resistance state was achieved by i-ZnO encapsulation in porous alumina compared to the conventional planer switching layer of ZnO alone.

Effect of different oxygen precursors on alumina deposited using a spatial atomic layer deposition system for thin-film encapsulation of perovskite solar cells

An atmospheric-pressure spatial atomic layer deposition system operated in atmospheric-pressure spatial chemical vapor deposition conditions is employed to deposit alumina (AlO x ) thin films using trimethylaluminum and different oxidants, including water (H2O), hydrogen peroxide (H2O2), and ozone (O3). The impact of the oxygen precursor on the structural properties of the films and their moisture-barrier performance is investigated. The O3-AlO x films, followed by H2O2-AlO x , exhibit higher refractive indexes, lower concentrations of OH− groups, and lower water-vapor-transmission rates compared to the films deposited using water (H2O-AlO x ). The AlO x films are then rapidly deposited as thin-film-encapsulation layers on perovskite solar cells at 130 °C without damaging the temperature-sensitive perovskite and organic materials. The stability of the p–i–n formamidinium methylammonium lead iodide solar cells under standard ISOS-D-3 testing conditions (65 °C and 85% relative humidity) is significantly enhanced by the encapsulation layers. Specifically, the O3-AlO x and H2O2-AlO x layers result in a six-fold increase in the time required for the cells to degrade to 80% of their original efficiency compared to un-encapsulated cells.

Thermally stimulated exoelectron emission from the surface of black aluminum layers prepared by PVD methods

There is an increasing trend in the utilization of highly nanostructured materials, especially as light absorbers or chemical gas sensors. However, for most applications a more detailed study of surface processes and phenomena is necessary or at least highly beneficial.The present paper describes the surface processes of black aluminum prepared by different PVD methods (magnetron sputtering at room and liquid nitrogen temperature, and thermal evaporation) studied by thermally stimulated exoelectron emission. The actual investigation of the interaction between surface and adsorbed atoms and molecules was carried out after UV irradiation. For the samples prepared in this way, thermostimulated exoelectron emission was measured in the temperature range: liquid nitrogen temperature to 400 °C. SEM analysis revealed significant differences in the morphology of the layers, which were associated with a change in the position of the maxima in the thermostimulated exoelectron emission spectra.The activation energies for the electron emission were calculated. The binding process on the surface of the individual samples and their dependence on the type of black aluminum thin film preparation were described.The results could determine which type of thin film preparation is suitable for use as chemical gas sensors.

Vivid Colored Cooling Structure Managing Full Solar Spectrum via Near-Infrared Reflection and Photoluminescence.

Colored radiative cooling (CRC) offers an attractive alternative for surface and space cooling, while preserving the aesthetics of an object. However, there has been no study on the CRC using phosphors in regard to vivid coloration, sophisticated performance investigation, retention of properties, functionality, and structural flexibility all at once. Thus, to manage the entire solar spectrum, a colored cooling structure comprising a near-infrared (NIR)-reflective bottom layer and a top colored layer with a phosphor-embedded polymer matrix is proposed. The structure is paintable, vividly colored, hydrophobic, and ultraviolet (UV) and water resistant. In the daytime outdoor measurement, the structure with red, orange, and yellow colors exhibited lower temperature than a control group using commercial white paint by 4.7 °C, 7.2 °C, and 7.4 °C, respectively. After precise theoretical and experimental time-tracing temperature validation, the CRC performance enhancement from NIR reflection and photoluminescence effects was thoroughly analyzed, and a temperature reduction of up to 16.1 °C was achieved for the orange-colored structure. Furthermore, experiments of hydrophobicity infusion and exposure to UV and deionized water verified the durability of the colored cooling structure. In addition, flexible-film-type colored cooling structures were demonstrated using different bottom reflective layers, such as a silver thin film and porous aluminum oxide particle-embedded poly(vinylidene fluoride-co-hexafluoropropylene), suggesting the potential applicability of these colored cooling structures for vivid-colored, functional, and durable CRC.

Thermal Stress Impact on Electrical Conductivity and Microstructure of Thin Aluminum Films Deposited on Polyethylene Terephthalate Substrates

Thermal Stress Impact on Electrical Conductivity and Microstructure of Thin Aluminum Films Deposited on Polyethylene Terephthalate Substrates

Performance evaluation of a single microchannel plate with large length-to-diameter ratio

The conventional microchannel plate (MCP) detector has found widespread application in various fields. This article focuses on investigating the performance of a single MCP with a large length-to-diameter ratio (L/D) of 80:1, particularly for single electron counting. To enhance MCP performance, alumina with a high secondary electron yield (SEY) is coated onto the MCP using atomic layer deposition (ALD). The SEYs of alumina thin films with different thicknesses are measured using a pulsing electron beam method. The 80:1 L/D MCPs operate in electron counting mode, and the optimal alumina thickness is determined through a comparative study of MCP performance before and after coating. The relationships between maximum SEY, primary electron energy, gain, peak-to-valley ratio (P/V), and pulse height resolution (PHR) are analyzed. After alumina coating, the single 80:1 MCP exhibits improved gain, P/V and PHR. The optimal P/V and PHR of a single MCP as functions of the primary electron energy align with the relationship between the SEY of the alumina coating and the primary electron energy. Additionally, the variation of DC gain with extracted charge is investigated. This article provides valuable insights for parameter selection in achieving the optimal working state of MCP and explores the potential application of single electron counting using a single MCP.

Correlation of sputtering power with microstructure and ionic conductivity in lithium aluminum oxide thin films prepared by reactive co-sputtering

Li–Al–O thin films synthesized via the reactive magnetron co-sputtering have the potential to act as solid-state electrolytes in micro-batteries. In this paper, the constant RF power (155 W) and variable DC power (20–50 W) were applied to lithium (Li) and aluminum (Al) metal targets, respectively, in Ar/O2 atmosphere to produce films with adequate ionic conductivity. The influence of DC sputtering power on the composition, microstructure, band gap energy, ionic conductivity, and dielectric loss of films were studied in detail. X-ray photoelectron spectroscopy (XPS) showed a shift of the Li 1s peak towards higher binding energy with increasing DC power. X-ray diffraction (XRD) spectra confirmed the amorphous state for all as-deposited films, while the post-annealed films at 950 °C contain the γ-LiAlO2 phase. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) images revealed that the increasing DC power had a significant impact on the film morphology, enlarging grain size and increasing surface roughness. Energy dispersive X-ray (EDX) spectroscopy showed that increasing the DC power increased the Al content. The optical properties were examined by ultraviolet–visible (UV–vis) spectroscopy, which indicated that increasing the Al content led to a blue shift of the absorption edge and an increase in the band gap energy (5.40–5.68 eV). Increasing the DC power resulted in an improvement of room-temperature conductivity by two orders of magnitude (∼2.85 × 10−9 S cm−1) and a decrease in the activation energy from 0.54 eV to 0.32 eV. These results suggest that composition and microstructure significantly affect the ionic transport in co-sputtered Li–Al–O films.

Evaluation of thermal stresses on thin Al and AZO films deposited on polyethylene terephthalate substrates for flexible electronics applications

Thin aluminum (Al) films are widely used in electronic devices and flexible photovoltaics. In a similar capacity, Aluminum Doped Zinc Oxide (AZO) serves the same purpose. This study examines the sensitivity of both films to thermal stresses when sputtered onto Polyethylene Terephthalate substrates. These stresses include a constant temperature of 100 °C and temperature fluctuations ranging from 0 °C to 100 °C. The resistance of the films was affected by isothermal aging, but not by thermal cycling. Interestingly, thermal cycling led to microcracks in the film, while isothermal aging resulted in a slight increase in bonding and oxygen content in the Al films. For AZO films, imaging did not reveal any noticeable changes in morphology. However, both thermal cycling and isothermal aging led to a minor increase in the resistance of AZO films. These findings highlight the pronounced impact of thermal stress on the performance and structural integrity of both thin films.