Thin Ta Research Articles

Enhancing electrical and thermal property of diamond-based Ta2N films through introducing a Ta transition layer

Diamond, renowned for its exceptional heat dissipation properties, stands out as an optimal substrate for tantalum nitride (Ta2N) film resistors. To fully exploit the high thermal conductivity of diamond, we introduce a thin tantalum (Ta) interlayer between the TaN thin film and the polished diamond substrate, aiming to strengthen their bond and enhance the interfacial thermal conductance. Comprehensive morphological and structural analyses, utilizing advanced techniques such as grazing incident X-ray diffraction (GI-XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS), were employed. Electrical properties were measured using a four-point probe, and thermal boundary conductance (TBC) between Ta2N and diamond was assessed through time-domain thermoreflection (TDTR), comparing scenarios with and without the Ta interlayer. Results demonstrate that the Ta interlayer facilitates tantalum carbide (TaxC) formation, improving adhesion and resulting in a remarkable 70 % increase in overall interface TBC. Moreover, the thin Ta interlayer positively influences the overall temperature coefficient of resistance (TCR) of the Ta2N film. Our strategic approach showcases enhanced adhesion and thermal performance, opening promising avenues for elevated functionality and reliability of Ta2N film resistors in high-temperature applications.

Emergence of considerable thermoelectric effect due to the addition of an underlayer in Pt/Co/Pt stack and its application in detecting field free magnetization switching

Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of an adjacent ferromagnetic layer through spin–orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Generally, thin Ta is used as an underlayer to the HM layer for better adhesion and smoothness of the HM layer. Here, we show that Ta addition to the asymmetric stack Pt/Co/Pt gives rise to several compelling effects, viz., thermoelectric effects [particularly, anomalous Nernst effect (ANE)], and enhanced perpendicular magnetic anisotropy which was negligible in a Pt/Co/Pt stack. For this Ta/Pt/Co/Pt stack, the antidamping-SOT values are evaluated after carefully removing the contribution from the ANE and it is found to match the AD-SOT of the Pt/Co/Pt stack. We have observed current-induced field-free magnetization switching Ta/Pt/Co/Pt stack with Co thickness gradient. Furthermore, we have utilized the thermoelectric effects to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method.

Self-assembled RuO2 nanoneedles on Ta/Cu foil for a robust and high-performance supercapacitor electrode

High-density self-assembled RuO2 nanoneedles (NNs) are successfully fabricated on a tantalum (Ta)-coated copper (Cu) substrate via a mild hydrothermal reaction for an electrochemical supercapacitor electrode with a superior energy storage capacity and excellent electrochemical stability. The electrochemical properties of the RuO2 NNs based electrode in a diluted H2SO4 electrolyte are investigated. The extremely thin Ta insertion layer enhances the adhesion between the RuO2 NNs and the copper foil charge collector, thereby significantly enhancing the mechanical and electrochemical energy storage characteristics of the RuO2 NNs electrode. The proposed electrode exhibits a remarkable capacitive storage performance of 1420 F g−1 at a scan rate of 5 mV s−1 and excellent long-term cycling stability (∼ 98% of capacitance is retained at 100 mV s−1 after several hundred cycles). Moreover, the electrode shows a high energy density of ∼ 13 Wh kg−1 at a power density of 27 kW kg−1. The proposed strategy can facilitate the design of binder-free robust nanostructured oxide/hydroxide electrodes and can be extended to prepare high-performance catalysts and battery anodes.

Coexistence of soft and hard magnetic phases in single layer amorphous Tb–Co thin films

Amorphous, ferrimagnetic Tb–Co thin films prepared with a thin Ta underlayer and either a Ta or a Pt overlayer show evidence of both soft and hard magnetic phases. At room temperature, the films exhibit conventional ferromagnetism, but low temperature magnetometry measurements reveal the decoupling of the two magnetic phases with decreasing temperature due to increased anisotropy energy of the hard layer at lower temperatures. Decreasing the film thickness to 2 nm, slightly above the superparamagnetic limit found at 1 nm, a soft, low-density phase was isolated and found to be present in all the films as confirmed with x-ray reflectivity and Rutherford backscattering spectrometry measurements. For greater thicknesses, the bottom layer retains its soft magnetic nature, while the remainder of the film is denser and has strong perpendicular magnetic anisotropy, leading to the exchange-spring behavior when the anisotropy becomes large, either at low temperatures or via a Pt overlayer that adds a strong interfacial anisotropy to the layer. Micromagnetic simulations of a soft/hard bilayer model with the experimentally determined anisotropy and magnetization parameters into a soft/hard bilayer model reproduced the experimental hysteretic behavior very well. These findings demonstrate how the magnetic state and the response of a-Tb–Co films to external fields can be controlled, providing a high degree of tunability that is promising for high-performance nanoscale devices.

Room-Temperature Preparation of Ta Ions-Containing Ionic Liquid and its Vapor Deposition toward Ta-Oxide Film Coating

Ta ions-containing solutions, which are brown in color with no precipitation, were successfully prepared through an electroelution process with ionic liquid (IL). An as-delivered Ta metal plate covered with a passivation oxide film could be easily eluted even at room temperature by simply applying an anodic potential of, e.g. +2.2 V vs Ag in [Bmim][PF6] IL. According to the quantity of electric charge required for oxidation of Ta, most Ta ions in the IL were suggested to be in an oxidation state of +5, which was also confirmed by X-ray photoemission spectroscopy (XPS). Ta ions in IL were found to thermally evaporate together with IL molecules by heating in a vacuum, forming a deposit of the Ta ions-containing IL on a substrate. The Ta concentrations in the deposits were reduced uniquely by about one order of magnitude from those in the original bulk source through the evaporation process under the present conditions. Furthermore, a possibility of the formation of thin film-like Ta oxide from such a Ta ions-containing IL deposit and its bulk droplet prepared on substrates by annealing in air at 1000 °C will be discussed.

Hydrogenation Kinetics of Metal Hydride Catalytic Layers.

Catalyzing capping layers on metal hydrides are employed to enhance the hydrogenation kinetics of metal hydride-based systems such as hydrogen sensors. Here, we use a novel experimental method to study the hydrogenation kinetics of catalyzing capping layers composed of several alloys of Pd and Au as well as Pt, Ni, and Ru, all with and without an additional PTFE polymer protection layer and under the same set of experimental conditions. In particular, we employ a thin Ta film as an optical indicator to study the kinetics of the catalytic layers deposited on top of it and which allows one to determine the absolute hydrogenation rates. Our results demonstrate that doping Pd with Au results in significantly faster hydrogenation kinetics, with response times up to five times shorter than Pd through enhanced diffusion and a reduction in the activation energy. On the other hand, the kinetics of non-Pd-based materials turn out to be significantly slower and mainly limited by the diffusion through the capping layer itself. Surprisingly, the additional PTFE layer was only found to improve the kinetics of Pd-based capping materials and has no significant effect on the kinetics of Pt, Ni, and Ru. Taken together, the experimental results aid in rationally choosing a suitable capping material for the application of metal hydrides and other materials in a hydrogen economy. In addition, the used method can be applied to simultaneously study the hydrogenation kinetics in thin-film materials for a wide set of experimental conditions.

Atomic separation in Co/Cu/Co magnetic structures study by hybrid X-ray reflectivity – X-ray standing wave approach

The detailed characterization of nanometer-thin film structures is key to the in-depth understanding and property control of such objects. Here we present the analysis of Co/Cu/Co multilayer grown on and covered by thin Ta layers at individual layer thicknesses below 5 nm. The prime focus of the analysis was to detect the threshold Cu thickness required to separate effectively the magnetic Co layer. We found that a 4 nm Cu layer can effectively separate Co layers, but that 2.8 nm of Cu is not sufficient. To solve such structural characterization tasks, we adapted a hybrid X-ray reflectivity – X-ray standing wave free-form analysis technique. We applied that to the study of the Co layers separation by Cu and now it can be used to perform any task of this kind and also for other material combinations.

Perpendicular magnetic anisotropy and its voltage control in MgO/CoFeB/MgO junctions with atomically thin Ta adhesion layers

We study the perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) effect in MgO/Ta/CoFeB/MgO junctions. A few monolayers (3–5 Å) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼10 Å) CoFeB films with large PMA on top of MgO. In the MgO/Ta/CoFeB/MgO junctions, the diffusion of Ta to the upper CoFeB/MgO interface is effectively eliminated, and superior thermal stability up to 400 ∘C is demonstrated. Detailed nanostructural analyses reveal the appearance of Co–Fe compositional variation in the CoFeB layer associated with the insertion of Ta layer, which explains the observed changes in PMA and VCMA. Our results will facilitate the development of VCMA-based memory devices and also pave the way toward controlling the magnetic and electric properties of CoFeB thin films through subnanometer-scale compositional modulation.

Perpendicular Magnetic Anisotropy and its Voltage Control in MgO/CoFeB/MgO Junctions with Atomically Thin Ta Adhesion Layers

We study the perpendicular magnetic anisotropy (PMA) and the voltage-controlled magnetic anisotropy (VCMA) effect in MgO/Ta/CoFeB/MgO junctions. A few monolayers (3 − 5 A) of Ta acts as an adhesive layer, allowing for the formation of flat and ultrathin (∼ 10 A) CoFeB films with large PMA on top of MgO. In the MgO/Ta/CoFeB/MgO junctions, the diffusion of Ta to the upper CoFeB/MgO interface is effectively eliminated, and superior thermal stability up to 400◦C is demonstrated. Detailed nanostructural analyses reveal the appearance of Co-Fe compositional variation in the CoFeB layer associated with the insertion of Ta layer, which explains the observed changes in PMA and VCMA. Our results will facilitate the development of VCMA-based memory devices and also pave the way toward controlling the magnetic and electric properties of CoFeB thin films through subnanometer-scale compositional modulation.

Effect of Ta insertion between Pt and CoFeB on interfacial magnetic anisotropy in Pt/CoFeB/MgO multilayer thin-film stack

The effect of a thin Ta layer inserted between Pt and CoFeB layers on perpendicular magnetic anisotropy (PMA) with MgO barrier layer has been studied. The crystallinity was studied by performing high-resolution x-ray diffraction (HR-XRD) technique. While the crystal peak of Pt is observed in all sample stack that is oriented as (111) face-centered cubic crystal structure, a very weak and broad α-Ta (110) peak is observed when Ta layer thickness is above 0.8 nm. Magneto-optical Kerr effect (MOKE) measurements show that the PMA could be enhanced by inserting Ta layer between Pt and CoFeB layer. The magnetically dead layer thickness (tdead) and the interfacial anisotropy energy density (Ki) of the various Ta-inserted layer thickness were found as 0.135 nm; 0.75 erg/cm2, 0.141 nm; 1.02 erg/cm2, 0.187 nm; 1.15 erg/cm2, for tTa = 0.0, 0.5, and 1.0 nm, respectively. Both tdead and Ki increase with Ta-inserted layer thickness in Pt/Ta(t)/CoFeB/MgO multilayer film stack. The sources of interfacial magnetic anisotropy can be hybridization, crystallinity properties, and/or B/Ta diffusion effect at the interfaces in Pt/Ta(t)/CoFeB/MgO multilayer film stack.

Direct Measurement of the Be7 L/K Capture Ratio in Ta-Based Superconducting Tunnel Junctions

We report a high-statistics measurement of the L/K orbital electron capture ratio in ^{7}Be embedded in cryogenic Ta. The thin Ta film formed part of a high-resolution superconducting tunnel junction radiation detector that was used to identify the signals from different decay channels. The measured L/K capture ratio of 0.070(7) is significantly larger than the only previous measurement of this quantity and the theoretical predictions that include in-medium effects. This value is a uniquely sensitive probe of the 1s and 2s orbital overlaps with the nucleus and is of relevance to nuclear and atomic physics, as well as Li production in novae and other astrophysical scenarios. This is the first experiment that uses superconducting tunnel junctions for nuclear-recoil detection, opening a new experimental avenue for low-energy precision measurements with rare isotopes.

Relationship between 〈111〉-oriented Cu film and thin Ta–W–N barrier

We examined a thin Ta–W–N film as an underlying material candidate for the Cu(111) orientation. It became clear that a 5-nm-thick Ta–W–N film leads to the high orientation of Cu(111). It was difficult to understand this mechanism, but it was clarified by introducing a measurement system with high resolution. The structure of the Ta–W–N film was based on fcc-TaN with a slightly expanded lattice and showed a (111) orientation, resulting in a lattice match with Cu(111). It is newly confirmed that this result is due to the thin Ta–W–N film having a (111) orientation and favorable lattice matching with Cu(111). Moreover, the Ta–W–N film showed good properties as a diffusion barrier for protecting against Cu diffusion. These results will be useful for future metallization technology.

Oxidation of Fine Tantalum Particles: Metastable Intermediates and Multistep Kinetics

The metastable tantalum suboxides have been found to be a cause of complicated oxidation behaviour of fine tantalum particles when heated in air at moderate temperatures. Fine Ta 0.02–10 μm particles of bimodal size distribution with average volume maxima of 0.12 and 4 μm were prepared via electric explosion of thin Ta wire in Ar atmosphere. The oxidation reaction proceeded via several intermediate suboxides (Ta6O, Ta4O, Ta2O and TaO2) in which chemical and structure transformations caused multistep kinetics of the oxidation process in a narrow temperature interval of 250–570 °C. Within the interval of 250–400 °C, the oxidation process occurred under diffusion control with an apparent activation energy Ea,1 = 31.6 ± 1.2 kJ/mol and the main oxidation products were Ta(O) solid solutions and suboxides Ta6O and Ta4O. In the interval of 400–520 °C, oxidation was accompanied by a low-barrier martensite-like transformation of the lower suboxides resulting in reducing of diffusion limitations of the process (Ea,2 = 4.8 ± 1.2 kJ/mol). In the interval of 520–570 °C, oxidation of the larger Ta particles was completed to form Ta2O (Ea,3 = 64.7 ± 1.9 kJ/mol). An abrupt decay of Ta2O at temperatures greater than ~ 570 °C induced the high-rate oxidation of micron-sized Ta particles, resulting in self-heating of the sample.

Modelling of transition zone formation between thin Si or Ta film deposited on TiNi under low-energy electron beam irradiation

Nickel titanium or nitinol is very often coated by biocompatible and corrosion-resistant silicon and tantalum coatings. One of the methods for the formation of a continuous transition zone between the coating and nitinol is pulsed or continuous electron beam treatment. Surface heating accelerates diffusion processes that facilitate the formation of new phases. However, the interpretation of experimental results is hindered due to the impossibility of directly observing the processes. Here we model the phenomena accompanying the transition zone formation. The mathematical model used takes into account thermal and diffusion phenomena and the stages of the main chemical reactions in the heated area. Numerical studies revealed that the transition zone for the two types of films is formed differently: in the case of silicon the transition zone grows inside nickel titanium, while in the case of tantalum it is formed mainly in the area of the deposited film. The interaction of competing stages determines the absence of regularities in the oxide phase formation under varying treatment conditions.

Tuning of Exchange-Biased Ni-Mn/Fe-Ni Films With High Blocking Temperatures

Effect of a reduction in an antiferromagnetic layers’ thickness and insertion of a Ta spacer on thermal behavior of exchange bias field, coercivity, and on blocking temperature of Ni-Mn-based exchange-biased films is studied. Reduction of a Ni-Mn layer's thickness from 20 to 17.5 nm to 14 and 12 nm gradually decreases the exchange bias field. The dependence of the blocking temperature on the thickness of a Ni-Mn layer shows a finite-size scaling effect with a maximum blocking temperature of 650 K for a 20 nm thick antiferromagnetic layer. Despite that, an unusually stable or even increasing exchange bias is present for some thicknesses in the temperature range up to 400 K. Addition of a Fe-Ni buffer with an increase in Ni concentration in Ni-Mn layer yields a greater exchange-bias-field-to-coercivity ratio and an even better thermal stability of exchange bias, which remains steady in the range from 300 to 475 K. Addition of a very thin Ta spacer (0.1 nm, 0.2 nm, or 0.3 nm thick) significantly decreases the exchange bias field, while only slightly decreasing the blocking temperature. A peak in the coercivity at around 700 K is found for all of the studied films, and is likely due to structural transformations.

Size-tunable skyrmion bubbles in Ta/CoFeB/MgO multilayers

The stabilization of skyrmion bubbles in [Ta(tTa)/CoFeB(1.1 nm)/MgO(1.0 nm)]15/Ta(2.0 nm) magnetic multilayers with perpendicular magnetic anisotropy is investigated. The size of the skyrmion bubbles can be controlled by tuning the Ta interlayer thickness. Because the perpendicular anisotropy constant is large in samples with a thin Ta interlayer, we can observe nanoscale skyrmion bubbles in our system. This finding provides a means to control the skyrmion size without using an external magnetic field. Moreover, we found that skyrmion bubbles can be created by a magnetic tip and subsequently be deleted. Our novel findings provide insight into skyrmion-based spintronic devices.

Self-assembly of TaC@Ta core–shell-like nanocomposite film via solid-state dewetting: Toward superior wear and corrosion resistance

The improvement of comprehensive properties including hardness, toughness, wear, and corrosion resistance in the transition-metal carbides/nitrides (TMC(N)) films, especially avoiding the trade-off between the hardness and toughness, is strongly required for various applications. Herein, we provide a new strategy, by activating solid-state dewetting during layered deposition, to accomplish the self-assembly of ordered TaC@Ta core–shell-like nanocomposite film consisting of TaC nanocrystallines encapsulated with thin pseudocrystal Ta tissues (∼1.5 nm). The novel core−shell-like structure can simultaneously achieve superhardness (∼45.1 GPa) mainly dominated by the Orowan strengthening mechanism and high toughness attributed to the indenter-induced phase transformation from the pseudocrystal to body-centered cubic α-Ta, together with drastically enhanced wear and corrosion resistance. Furthermore, thin pseudocrystal Ta encapsulated layer (∼1.5 nm) in the TaC@Ta core–shell-like structure helps for promoting the formation of lubricious TaOx Magnéli phase during sliding, thereby further dropping the coefficient of friction relative to TaC monolayer. Apparently, solid-state dewetting may provide a new route to construct ordered TMC(N)@TM core–shell-like nanocomposite structure capable of combining superhardness, high toughness, low friction, and superior wear and corrosion resistance.

Size dependence of the Anderson singularity index and Seebeck coefficient of thin monodisperse nanocluster metal films

The paper presents the results of the study of thin monodisperse Co and Ta nanocluster films deposited by magnetron sputtering method onto highly oriented pyrolytic graphite (HOPG) and silicon dioxide surfaces by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) methods. The evolution of X-ray photoelectron spectra asymmetry of the core levels Co2p, Ta4f as the dependence on typical sizes of nanoclusters are studied. It is found experimentally that the Anderson singularity index α (i.e. asymmetry parameter) increases with decreasing of nanocluster’s size. The correlation between α and Seebeck coefficient is discussed for pure bulk metals. It is shown that Seebeck coefficient of Co nanocluster films consisted of 1.2nm sized nanoclusters can reach 120μV/K. The applicability of the nanocluster films obtained for the development of highly efficient thermoelectric materials is discussed.

Batteries: Nanoscale Heterogeneity of Multilayered Si Anodes with Embedded Nanoparticle Scaffolds for Li‐Ion Batteries (Adv. Sci. 10/2017)

A versatile synthesis approach is proposed for Li‐ion battery anodes composed by intercalated layers of amorphous‐Si thin films and Ta nanoparticles by Mukhles Sowwan and co‐workers in article number 1700180. The porosity, nanomechanical, and electrochemical properties of the Si layers are dictated by the Ta nanoparticle scaffolds, leading to anodes with enhanced charge‐discharge rate and self‐limited capacity. The proposed prototype meets the advantages of nano‐ and thin film technologies.

Control of perpendicular magnetic anisotropy and spin pumping damping in MgO/CoFeB/ Ta/Pt structures

The perpendicular magnetic anisotropy (PMA) and spin pumping induced magnetic damping of CoFeB films can be significantly tuned by the adjacent nonmagnetic layer (Ta or Pt). For the CoFeB/Pt(tPt) structure, both PMA strength and magnetic damping increase with tPt, due to the enhanced interfacial orbital hybridization and spin pumping effects, respectively. By inserting a thin Ta interlayer between CoFeB and Pt, the magnetic easy axis immediately turns into in-plane direction. Meanwhile, time-resolved magneto-optical Kerr effect measurement shows an obvious reduction in the magnetic damping, which can be explained as the increase of spin current backflow and the decrease of interfacial spin-mixing conductivity. As the Ta interlayer thickness (tTa) increases, the effective damping parameter αs measured at saturation field initially decreases and eventually reaches a constant value of 0.027. The αs value at tTa > 5 nm does not change regardless of the top Pt layer, implying the influence of Pt is completely isolated and the spin diffusion length of Ta is around 5 nm. Our findings provide more insights into the control of PMA strength and spin pumping contribution to magnetic damping.