Alloy Thin Films Research Articles

High-throughput membrane deflection characterization of shape memory alloy thin films

Nickel–Titanium (Ni–Ti) shape memory alloy thin films are currently gaining significant attention for small-scale systems, but device integration is still challenging due to its high compositional-dependent behavior. Therefore, it is crucial to identify the optimal parameters required to fabricate desirable samples. In this study, combinatorial high-throughput membrane deflection experiments were performed to investigate the mechanical properties of submicron-thick Ni–Ti thin films. The mechanical properties of freestanding Ni–Ti thin films, sputter-deposited with compositions and thicknesses ranging from 45.7 at. % to 52.7 at. % nickel and 270–530 nm were evaluated. With an increase in the Ni content, the crystallinity and existing phases of the films varied and exhibited an overall increasing trend of elastic modulus, transformation stress, percentage of recoverable strain and hysteresis area. Based on the obtained results, the compositional ratio of Ni–Ti films can be tailored accordingly to meet the different requirements of small-scale devices, and to then extend the employment of this high-throughput methodology in exploring advanced metallic alloys and ceramic composites.

Design of high-entropy films as ultra-violet light reflector

This study proposes the design and implementation of high-entropy alloy (HEA) thin films as materials for the reflector of deep ultraviolet light-emitting devices and marks the first application of HEA in optical field. In the material design phase, an Al-Co-Zn-Ni alloy is designed by considering the reflectivity, phase stability and binary formation energy. Reverse Monte Carlo calculation is employed to establish stable atomic structure. Frequency-dependent dielectric equations are derived from first principles calculations, with dielectric constants incorporating contributions from ions and electrons and combining classical Drude and Lorentz models. The dielectric equations yield optical constants that are used to calculate the reflectance. In the experimental phase, we systematically vary the aluminum content and prepare Al-Co-Zn-Ni thin films ranging from pure aluminum to equimolar compositions. In the deep ultraviolet region, the reflectance of thermally treated Al65Zn12Co12Ni11 and Al50Zn16Co17Ni17 thin films is found superior to that of gold by three times. Structural and crystalline changes with varying aluminum content are found, as well as intriguing phase separation and superlattice formation that persist until equimolar compositions. This unique nanoscale structural variation causes the Young's modulus and hardness of Al-Co-Zn-Ni thin films to initially increase and then decrease with decreasing aluminum content. There is a significant increase in the corrosion potential difference when reducing Al contents in Al-Co-Zn-Ni, indicating improved passivation effects as the equimolar composition is approached. By modulating the aluminum content, the nanoscale structure of Al-Co-Zn-Ni can be adjusted, thereby improving its optical, mechanical, corrosion resistance, and thermal stability properties. This demonstrates the high versatility of this material for practical optical applications.

Composition-Tunable Properties of Cu(Ag) Alloy for Hybrid Bonding Applications

In the present study, the properties of Cu(Ag) alloy films were studied to evaluate their potential use as an alternate material for interconnection in hybrid bonding. Thin alloy films of Cu(Ag) were deposited by pulsed electrochemical deposition (PED) using a sulfuric acid-based bath, rotating disk electrode, and hot entry. Secondary ion mass spectrometry (SIMS) was used to measure the silver content of the films, with us finding that it decreases with increasing duty cycle. Thereafter, bright field scanning transmission electron microscope (STEM) imaging in combination with energy-dispersive x-ray spectroscopy (EDS) was used to visualize the thin film microstructure and to confirm the uniform distribution of silver throughout the film, with no bands being seen despite the pulsed nature of the deposition. Film resistance was measured by a four-point probe to quantify the impact of Ag content on resistivity, with us finding the expected linear relationship with the Ag content in the film. Furthermore, the coefficient of thermal expansion (CTE) of the films was measured using X-ray diffraction, and modulus and hardness were measured via nanoindentation, revealing linear dependences on the Ag content as well. Notably, the addition of 1.25 atom% Ag resulted in a significant increase in the CTE from 17.9 to 19.3 ppm/K, Young’s modulus from 111 to 161 GPa, and film hardness from 1.70 to 3.99 GPa. These simple relationships offer a range of properties tunable via the duty cycle of the pulsed plating, making Cu(Ag) a promising candidate for engineering wafer-to-wafer metal interconnections.

Electrodeposited Fe2CoSn Thin Film with Enhanced Structural and Magnetic Properties

Single-phase near-stoichiometric Fe2CoSn Heusler alloy thin film of thickness 320 nm and crystallite size of 22 ± 1 nm was electrodeposited on low-cost Cu substrate. Heat-treated Fe2CoSn film exhibited an X-type inverse Heusler alloy structure along with high saturation magnetization (∼5 μ B/f.u.), high effective anisotropy constant (∼106 erg/cc), and high Curie temperature (983 K). Apart from demonstrating a procedure to obtain considerably thin electrodeposited Fe2CoSn films with high crystalline order, the electronic density of states close to the Fermi level have also been evaluated for its stable structure for the first time.

Atomistic mechanism of AlCu thin film alloy growth on trenched Si substrate

Molecular dynamics simulation was employed to study the growth mechanisms of an AlCu thin film on a trenched Si(001) substrate. The atomic interactions between Al, Cu, and Si atoms were described by using the Modified Embedded Atom Method (MEAM). This investigation focused on examining the effect of both incident energy and trench heights on the structure, and the morphology of deposited AlCu thin film. The results reveal that the growth mode follows an island mode, and the AlCu film surface exhibits both non-flat and non-uniform texture. The increase in the trench height leads to a more pronounced change in surface morphology and roughness. However, a higher incident energy can significantly reduce the surface roughness due to the mobility of deposited atoms. In addition, a mixing region was observed at the interface between the film and substrate. It is revealed that increasing the incident energy leads to deeper penetration of Al and Cu atoms into the substrate. More importantly, Al atoms penetrate deeper than the Cu atoms. On the other hand, deposition on a trench with 15 monoatomic layers leads to the formation of voids in the film. However, these voids disappear with higher incident energy. The microstructural evolution during deposition film was also analyzed based on the radial distribution function. The results of this function indicate that changes in trench height and the energy of incident adatoms do not significantly influence the structure of the film.

Guided acoustic waves in thin epitaxial films: Experiment and inverse problem solution for NiTi

Despite the fundamental and technological importance of the elastic constants, a suitable method for their full characterization in epitaxial films is missing. Here we show that transient grating spectroscopy (TGS) with highly k-vector-selective generation and detection of acoustic waves is capable of determination of all independent elastic coefficients of an epitaxial thin film grown on a single-crystalline substrate. This experimental setup enables detection of various types of guided acoustic waves and evaluation of the directional dependence of their speeds of propagation. For the studied model system, which is a 3μm thin epitaxial film of the NiTi shape memory alloy on an MgO substrate, the TGS angular maps include Rayleigh-type surface acoustic waves as well as Sezawa-type and Love-type modes, delivering rich information on the elastic response of the film under different straining modes. The resulting inverse problem, which means the calculation of the elastic constants from the TGS maps, is subsequently solved using the Ritz–Rayleigh numerical method. Using this approach, tetragonal elastic constants of the NiTi film and their changes with the austenite→martensite phase transition are analyzed.

Experimental and ab initio studies of the structural and optoelectronic properties of hexagonal wurtzite ZnO1-xSx alloys

The structural and optoelectronic properties of ZnO1-xSx alloys were investigated experimentally as well as theoretically. The ZnO1-xSx alloy thin films at different S concentrations (0.0 < x < 1.0) were deposited on quartz substrates by radiofrequency magnetron sputtering. The alloy composition was monitored with the help of optical emission spectroscopy. The ab initio density functional theory (DFT) based calculations were performed for the hexagonal wurtzite ZnO1-xSx system with the S compositions of x = 0.0, 0.0625, 0.125, 0.25, 0.5, 0.75, 0.875, 0.9375 and 1.0. The X-ray diffraction (XRD) and Raman spectroscopy measurements suggest the hexagonal wurtzite phase for the entire composition range. The films exhibit good crystalline quality with preferential growth along the c-axis. The linear change in the lattice parameter (c) was observed both from XRD analysis and DFT based calculations. The X-ray photoelectron spectroscopy confirms the anionic substitution of oxygen (O2−) and sulphur (S2−) in the hexagonal wurtzite lattice. Field emission scanning electron microscopy and atomic force microscopy were used to study the surface morphology. The optical properties were evaluated through transmittance spectroscopy and spectroscopic ellipsometry. The density of state calculations shows the forbidden region which confirms the semiconducting nature for all the alloy compositions. The energy band gap obtained from the respective Tauc's plots was found to vary from 2.9 to 3.6 eV with a bowing parameter of 3.2 eV. The same trend was observed in the calculated electronic band structures through DFT. The optical constants were evaluated from spectroscopic ellipsometry and the corresponding band gap values were compared with the values obtained from optical transmittance.

Computational approach to identify structural and elastic relationship in metastable crystalline and amorphous alloy thin films: Mo1-xNix and Mo1-xSix case studies

Previously well-established experimental trends of ground-state properties of crystalline and amorphous Mo1-xSix and Mo1-xNix alloys with 0 ≤ x ≤ 1 are predicted by first-principles calculations and inter-relationship with sound velocities and elastic properties are identified. The free energy of mixing, calculated at 300 K provides accurate values of the critical Si and Ni concentrations leading to the crystalline-to-amorphous transition. Specifically, a transition from BCC to amorphous state is predicted for xSi ~ 0.14 and xNi ~ 0.32, while FCC to amorphous transition is observed for xNi ~ 0.77. These structural transitions are accompanied by modifications of the out-of-plane longitudinal and shear elastic constants. In the crystalline region, a pronounced softening of elastic moduli is predicted for both solid solution alloys. While for amorphous Mo-Ni, the elastic constants do not undergo significant changes, for amorphous Mo-Si, they exhibit two distinct behaviors depending on the electronic properties and bonding character. For 0.2 ≤ xSi ≤ 0.7, the metallic character of the amorphous alloys is maintained and the elastic properties are remarkably stable. For xSi > 0.7, a progressive increase in the atomic volume is observed and the amorphous alloys acquire a covalent character and a reduced coordination number. Surprisingly, during this transition, both the longitudinal and shear acoustic velocities continuously increase, despite a progressive softening of the elastic stiffness constants. These calculations provide deep insight at the atomic-scale and reproduce satisfactorily the earlier experimental works of magnetron co-sputtered Mo-Si films, while some disagreement on elastic properties remain for more energetic ion beam sputtered Mo-Ni films, likely partially attributed to point-defects.

Magnetic and anomalous Hall effect investigations of co-sputtered Co2MnGa Heusler alloy thin films

The cobalt-based full Heusler alloy Co2MnGa (CMG) is well known for exhibiting an exotic phenomenon such as magnetic Weyl semimetallic nature with a high Curie temperature of ∼700 K and a giant anomalous Hall effect. Here, we report a detailed study of structural, electrical, and magnetic properties of Co2MnGa thin films (thickness in the 40–10 nm range) grown on Si(100) by the direct-current magnetron co-sputtering technique using Co and MnGa targets. Structural analysis of the samples revealed the polycrystalline nature of these films with B2 type structural ordering. The damping parameter decreases with the increase in film thickness and reaches the minimum value of 6.1 × 10−3 for a 40 nm thin CMG film. These CMG films are magnetically isotropic and soft ferromagnetic in nature. A remarkably high value of anomalous Hall conductivity (AHC) of 1920 S/cm (2 K) is found for the 40 nm thin film, which is comparable to earlier reported values on highly ordered CMG films. Nearly 73% of this AHC value originates from the intrinsic contribution. The AHC and longitudinal conductivity both increase with the film thickness. Different scaling mechanisms are used to compute the intrinsic and extrinsic contributions playing a role in AHC. The analysis of advanced scaling [by Tian et al., Phys. Rev. Lett. 103, 1–4 (2009)] performed on these CMG films suggests the consistency in the enhanced intrinsic AHC value irrespective of the thickness and a decrease in skew scattering contribution with thickness. These results will enhance the understanding about the magnetic and transport properties of Co2MnGa thin films of different thicknesses and suggest it to be a promising material for topospintronic applications.

Post-selenization induced structural phase transition and enhanced thermoelectric properties in AgBiSe2 alloy thin films

Thermoelectric materials have the potential to harness waste heat for energy purposes, and there is considerable scientific and technological interest in managing their thermal properties. BiAgSe2, a narrow bandgap semiconductor, is a strong candidate for use as a thermoelectric material due to its remarkably low thermal conductivity. The compound of AgBiSe2 crystallizes in the hexagonal symmetry of the grown sample at room temperature and transitions to a cubic phase, increasing the post selenization duration at 773K. The granular surface morphology has been observed and slightly decreases with increases the more post selenization treatment. The enhancement of the Seebeck coefficient value from 126 to 177 μV/K due to the phase transition, grain refinement and post-selenization treatment. The improvement of charge carrier concentration and electrical conductivity is due to improving the crystallinity and diffusion of Se atoms in the AgBiSe2 alloy thin film with post-selenization time. The maximum thermoelectric power factor (5.16 μWcm−1K−2) of stable cubic AgBiSe2 alloy thin film has been achieved at room temperature. However, the stable cubic phase of the AgBiSe2 alloy thin film exhibits excellent thermoelectric behaviour, making it a promising material for applications in thermoelectric devices.

Extracting mechanical and microstructural properties of Cu–Zr thin film alloys by MEMS, AFM and ellipsometer

The quantification of the atomic concentration ratios of thin-film metallic alloys having low atomic ordering is challenging, particularly if they are grown on similar metals and possess different surface chemistries. Micromechanical and optical methods have been used to correlate the elemental ratios with the mechanical and optical properties of the films. The room-temperature growth of Cu–Zn thin-film alloys with varying elemental ratios on cosputtered Si substrates was performed to obtain an amorphous film structure. X-ray diffraction patterns confirmed that the grown films exhibited a very short range ordering, suggesting an amorphous structure. The mechanical properties of the films evaluated using microelectromechanical system (MEMS) indicated that the alloy films with moderate Zr concentrations had lower surface stress compared to those with low and high Zr concentrations. Furthermore, spectroscopic ellipsometry was employed to qualitatively assess the relaxation times of free carriers. The results demonstrated a strong correlation between the relaxation times and surface roughness measurements, showing that the microstructure and resistivity characteristics of the alloys align with the Nordheim semiempirical model. The extinction coefficient of the binary alloy film linearly depends on the metallic bulk concentration ratio in a specific metallic ratio range, paving the way for realizing qualitative elemental percentage assessment in the field of metrology.

Effect of Hf alloying on magnetic, structural, and magnetostrictive properties in FeCo films for magnetoelectric heterostructure devices

Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron–cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1−xHfx (x = 0 – 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x ∼ 0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying.

Tuning microstructure and properties of MoNbTaWZr high entropy alloy films by adjusting the parameters in high power impulse magnetron sputtering

Within this study a series of MoNbTaWZr thin films were synthesized using high power impulse magnetron sputtering (HiPIMS) with varying the deposition parameters: duty cycle from 0.5 to 100 % (i.e. DC magnetron sputtering (DCMS)), Ar pressure from 0.5 to 3 Pa and pulse frequency from 25 to 200 Hz. The structure of the deposited films was analyzed by X-ray diffraction and scanning and transmission electron microscopy. In addition, the electrical resistivity and the residual stress of the films was determined. The influence of the deposition parameter variation on structure and properties of the MoNbTaWZr films is discussed on the established framework of film growth conditions achievable with HiPIMS and DCMS and how they can be influenced by adapting the deposition parameters. The performed work is intended to contribute to a comprehensive understanding about synthesis-structure-property relations for refractory high entropy alloy thin films while using MoNbTaWZr films as a model system.

The Optical Properties of Thin Film Alloys of ZnO, TiO2 and ZrO2 with Al2O3 Synthesised Using Atomic Layer Deposition

In this work, the results of ellipsometric studies of thin films of broadband oxides (ZnO, TiO2, ZrO2) and broadband oxides doped with Al2O3 (Al2O3–ZnO, Al2O3–TiO2, Al2O3–ZrO2) are presented. All layers have been produced using the atomic layer deposition method. Ellipsometric studies were performed in the wavelength range of 193–1690 nm. Sellmeier and Cauchy models were used to describe the optical properties of the tested layers. Dispersion dependencies of refractive indices were determined for thin layers of broadband oxides on silicon substrates, and then for layers of Al2O3 admixture. The EDX investigations enabled estimation of the composition of the alloys. The Bruggeman effective medium approximation (EMA) model was used to determine the theoretical dependencies of the dispersion refractive indices of the studied alloys. The refractive index values determined using the Bruggeman EMA model are in good agreement with the values determined from the ellipsometric measurements. The doping of thin layers of ZnO, ZrO2 and TiO2 with Al2O3 enables the creation of anti-reflective layers and filters with a specific refractive index.

Phase decomposition of AlCrFeCoNiCu0.5 HEA thin films by vacuum annealing

The most attractive advantages of high entropy alloy (HEA) thin films are their excellent properties, such as high strength and high corrosion and oxidation resistance, with a lower material cost. However, the thermal stability of the HEA thin films is always controversial. This is critical since the thermal stability of metals and alloys is closely related to the material's performance at high temperatures and also determines the application field of the material. The question of how will the nanocrystalline high entropy alloy thin film perform in terms of thermal stability needs to be clarified urgently. In this research, the thermal stability of AlCrFeCoNiCu0.5 HEA thin film fabricated by cathodic arc deposition was investigated at 500 °C as a function of time during vacuum annealing. X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM) and Transmission Kikuchi Diffraction (TKD) were applied to analyze the chemistry and microstructure of HEA thin films in detail. The results showed that there was no obvious change in the elemental compositions of the films after annealing, but the elemental distribution of the annealed films was different from the as-deposited film, especially at the film surface and film-substrate interface for the 24 h-annealed sample. The existence of the FeCo phase with B2 structure was a significant sign to demonstrate that the spinodal decomposition occurred in the single face-centered-cubic (FCC) phase matrix of the film during annealing. Simultaneously, the segregation of Cu and Cr near the film-substrate interface and the penetration of Ni and Cu towards the Si substrate were clearly observed. These unusual phenomena indicated that the thermal stability of HEA was not always excellent. Also, it was noticed that a few Kirkendall voids forming at the film-substrate interface after 24 h of annealing would affect the adhesion of the film. Additionally, heat recovery and recrystallization were achieved by 500 °C annealing, which were confirmed by the decrease of lattice parameter and the increase of grain size in the 24 h-annealed thin film. Furthermore, the hardness and elastic modulus of the thin films before and after annealing were measured by nanoindentation. The results showed all the thin films exhibited excellent mechanical properties, and the optimal hardness and elastic modulus of 7.7 ± 0.3 GPa and 183.9 ± 4.4 GPa were obtained after 3 h-annealing.

(Bi,Sb)2 Se3 Alloy Thin Film for Short-Wavelength Infrared Photodetector and TFT Monolithic-Integrated Matrix Imaging.

Short-wavelength infrared photodetectors play a significant role in various fields such as autonomous driving, military security, and biological medicine. However, state-of-the-art short-wavelength infrared photodetectors, such as InGaAs, require high-temperature fabrication and heterogenous integration with complementary metal-oxide-semiconductor (CMOS) readout circuits (ROIC), resulting in a high cost and low imaging resolution. Herein, for the first time, a low-cost, high-performance, high-stable, and thin-film transistor (TFT) ROIC monolithic-integrated (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetector is reported. The (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetectors demonstrate a high external quantum efficiency (EQE) of 21.1% (light intensity of 0.76µWcm-2 ) and a fast response time (3.24µs). The highest EQE is about two magnitudes than that of the extrinsic photoconduction of Sb2 Se3 (0.051%). In addition, the unpackaged devices demonstrate high electric and thermal stability (almost no attenuation at 120°C for 312h), showing potential for in-vehicle applications that may experient such a high temperature. Finally, both the (Bi,Sb)2 Se3 alloy thin film and n-type CdSe buffer layer are directly deposited on the TFT ROIC (with a 64×64-pixel array) with a low-temperature process and the material identification and imaging applications are presented. This work is a significant breakthrough in ROIC monolithic-integrated short-wavelength infrared imaging chips.

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.

Growth rate and substrate temperature dependent magnetic and magneto-microstructural properties of sputtered Fe-Co-Al thin films

This work reports the effect of growth rate and substrate temperature on the structural, magnetic and magneto-microstructural properties of Fe-Co-Al alloy thin films. 75 nm thick Fe-Co-Al films were deposited on Si 〈100〉 substrates at room temperature by high power impulse magnetron sputtering at different growth rates viz., 0.67–1.6 Å/s. All the films exhibited a uniform composition of about Fe60Co25Al15 irrespective of growth rates. In addition to this, all the films crystallized in single phase with bcc type structure. Growth rate was found to have a considerable impact on the magnetic properties of Fe-Co-Al thin films. Film grown at 1.3 Å/s exhibited an in-plane magnetic anisotropy with large saturation magnetization combined with low coercivity when compared with other films. To improve the magnetic properties further, Fe-Co-Al films were also processed at different substrate temperatures (Ts) viz., 300, 400 and 500 °C. Substrate temperature of about 300 °C was found to have beneficial effect on the soft magnetic characteristics of Fe-Co-Al films. While the crystal structure did not show any perceptible change with processing conditions, magnetic microstructures traced from magneto optic Kerr microscopy displayed substantial change. Upon increase in substrate temperature, during magnetization reversal process, magnetic domain images traced along the easy magnetization direction magnetization showed transformation from branched band domains (as-deposited) → wide band domains (Ts = 300 °C) → less periodic branched band domains (Ts = 400 °C) → ripple domains (Ts = 500 °C). Attempts were made to correlate the achieved magnetic properties in Fe-Co-Al with magnetic microstructures.

Estimating refractive index of thin-film alloys of Cu, Ag, and Au by extension from those of the main metal

Due to dependence of nanostructural material properties on the deposition conditions, the refractive index (RI) of a thin-metal alloy can vary by over 100×. For a more accurate RI estimation of thin-film alloys under a specific deposition condition, a model of RI for untested alloys is derived by extension from the alloy’s main element properties. One of the key derivations is to simplify RI polynomial equations into monomial expressions through four assumptions. Thus, a ratio method can accurately predict the RI of thin-film alloys from the properties of their main metals under the same deposition condition. The model is valid in the red to infrared spectrum for thin-film alloys MX where M is one of copper, silver, or gold and X is one or more elements with total concentration &lt;10%.

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.