Oblique Deposition Research Articles

Angular control of multi-mode resonance frequencies in obliquely deposited CoZr thin films with rotatable stripe domains* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51871117 and 51671099), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT-16R35), and the Gansu Provincial Science Foundation for Distinguished Young Scholars, China (Grant No. 20JR10RA649).

We investigate the angular-dependent multi-mode resonance frequencies in CoZr magnetic thin films with a rotatable stripe domain structure. A variable range of multi-mode resonance frequencies from 1.86 GHz to 4.80 GHz is achieved by pre-magnetizing the CoZr films along different azimuth directions, which can be ascribed to the competition between the uniaxial anisotropy caused by the oblique deposition and the rotatable anisotropy induced by the rotatable stripe domain. Furthermore, the regulating range of resonance frequency for the CoZr film can be adjusted by changing the oblique deposition angle. Our results might be beneficial for the applications of magnetic thin films in microwave devices.

Enhancing High-Frequency Properties of Nanocrystalline Sputtered Fe Thin Films by Using MnIr Underlayer and Oblique Deposition

In this work, the single-layer Fe and the bilayer MnIr/Fe thin films were fabricated on Si (100) substrate using the oblique-angle sputtering technique. The structure and magnetic properties of the as-deposited films were investigated. MnIr biased layer deposited at the normal incidence angle develops an exchange bias field of 5.2 Oe which leads to an increase in the uniaxial anisotropy field of Fe layer from 4.36 to 23.66 Oe and the ferromagnetic resonance frequency from 1.02 to 1.46 GHz. It is also found that the resonance frequency of the bilayer MnIr/Fe rises sharply from 1.46 to 5.05 GHz as the incident angle increases from 0 to 36°.

Interference suppression of light backscattering through oblique deposition of high-reflectivity multilayers: a theoretical analysis.

We demonstrate a theoretical approach whereby light backscattering toward the incident beam can be suppressed entirely for a high-reflectivity, rough-surfaced multilayer mirror fabricated using oblique deposition, such that the interface relief is replicated at a certain angle β to the sample normal. The mirror comprises two parts: a main (lower) multilayer consisting of N identical bi-layers growing at the angle βML to the mirror normal, and an additional bi- or tri-layer forming the topmost section of the mirror, which grows at another angle βBL. We show that choosing appropriate growth angles βML and βBL results in a disappearance of backscattering toward the incident beam due to the destructive interference of waves scattered from the main multilayer and uppermost bi- or tri-layer. The conditions for the scattering suppression are formulated, and the suitability of different mirror materials is discussed.

Morphology-dependent optical and wetting behavior of GLAD PTFE thin films

E-beam evaporation equipped with glancing angle deposition arrangement (GLAD) was used to fabricate nanostructured polytetrafluoroethylene (PTFE) films in a single-step coating process. Three sets of PTFE coatings were prepared using various oblique angles, e-beam currents, and deposition times to explore the effects of deposition parameters on the properties of PTFE nanostructured coatings. Water contact angle (WCA) of the coatings was found to be enhanced with the increase in all the above process parameters. Optical transmittance of the coatings was also found to be improved with the above parameters except for in the case of an increase in thickness of the films (in very high thickness region), where the transmittance was degraded due to light scattering from the sample surface. After all the optimizations, the ~ 130-nm-thick GLAD PTFE sample prepared with highest e-beam current was found to be more suitable for antireflection and self-cleaning applications. The single-sided coating demonstrates a very high average transmittance of ~ 95.6% (with a wideband transmittance spectrum among the others) in the visible and NIR wavelength range with excellent self-cleaning nature (WCA ~ 156°, sliding angle ~ 10°). The trend of measured WCA with respect to surface roughness follows the Cassie–Baxter model. Overall, the study demonstrates the possibility of fabricating highly transparent self-cleaning coatings using the GLAD technique, which is potentially useful for fabricating protecting cover glasses in solar panels.

Magnetic anisotropies in oblique columnar growth of FeCoB films

We report on the evolution of anisotropies of FeCoB thin films deposited by magnetron sputtering with different oblique angles increasing from α = 0° to α = 75°. The oblique incidence deposition using the Fe43Co43B14 target leads, under chosen conditions, to columnar growth without bundling. This columnar structure results in a correlated increase in the anisotropy field and of the resonance frequency values, observed with an increasing oblique incidence angle from α = 0° to α = 45°. However, these values decrease from α = 45° to α = 75°. To get a better understanding of the reversal magnetization process, in plane angular distribution of different magnetic parameters was carried out, and the role of easy axis dispersion was especially investigated. The optimized conditions of deposition, leading to controlled high resonance frequencies, could be useful in the design of FeCoB-based microwave devices.

Fabrication and characterization of porous WO3 thin film as a high accuracy cyclohexene sensor

In the last decade, an increase of attention to monitoring the hazardous gases and vapors in industry, laboratories, and homes has attracted extensive interest in developing gas sensors. This research investigates the use of porous WO3 thin film as a high accuracy cyclohexene sensor. Oblique deposition and subsequently the post-annealing process were employed to produce the mentioned device. X-ray diffraction (XRD), atomic force microscopy (AFM), and physical adsorption isotherm were used to investigate the crystallographic structure, surface morphology, and porosity of the sample, respectively. Sensing parameters including response amplitude, operating temperature, response and recovery times, detection limit, selectivity, reproducibility, and stability were also studied to find a deep-perception of the device sensing performance. The results disclose that the prepared device in this research shows a decent and fast response to the object and can be introduced as a reliable cyclohexene sensor.

The influence of Cu spacer morphology in Cu/Py/Cu/Co/IrMn spin valves with induced non-collinear spin structures

Non-collinear exchange couplings between Py and Co layers (0°⩽φ⩽90°) were achieved in Py(10)/Cu(5)/Co(10)/IrMn(8) (thickness values in nm) spin valves by modifying their layer morphologies using oblique deposition in a confocal sputtering setup. Two series of spin valves were prepared by keeping the sample holder fix during the depositions, except for the Cu spacers that were deposited using either fixing (Series-I) or spinning mode (Series-II). Series-I (Series-II) has Cu spacer layers composed by oriented (non-oriented) grains and Cu/Co interfacial roughnesses relatively rougher (smoother), favoring lower (higher) interlayer exchange couplings and exchange bias fields. The magnetization easy axis of Py and the unidirectional anisotropy axis followed the expected directions, while the Co uniaxial axis is surprisingly dephased by (15±5)° for 0°⩽φ<90°. The angular offset between the Co uniaxial axis and the Py uniaxial (unidirecional anisotropy) axis was more pronounced in Series-I, indicating that the Cu spacer roughness played an important role in the interlayer exchange couplings and also in the Co spin reorientation effect. The magnetoresistance magnitudes were similar to those found in the literature for ordinary Py/Cu/Co multilayers. However, it reduces as the φ-angle increased, an effect associated with the enhancement of the interface roughness due to the misalignment of the oriented grains of adjacent layers.

Silver thin film deposited 3-dimensional gold nanorod arrays for plasmonic catalysis

Plasmonic metal nanoparticles are of great importance in heterogeneous catalysis applications including partial oxidations, dehydrogenations, and hydrocarbon reforming. Although utilization of such nanoparticulate metals seems to be a simple and cheap approach for catalysis, their low surface area, un-controlled aggregation, and limited recovery and recycling abilities prevent their further applications. Herein, to overcome these limitations, vertically aligned, highly ordered, and unidirectional 3-dimensional (3D) gold nanorod (AuNR) arrays were fabricated through an oblique angle vapor deposition method. The plasmonic features and catalytic efficiencies of 3D AuNR arrays were further improved by combining them with a thin layer of silver having varying thicknesses. The catalytic activity of fabricated samples was evaluated by the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 in the absence and presence of laser illumination with different wavelengths. All the results indicate that the current Ag@AuNR substrates are expected to have a broad impact in a range of fields including plasmonic catalysis, sensing, and many others.

Design, Fabrication and Characterization of n-Si Columnar Structures for Solar Cell Applications

Background: Glancing Angle Deposition (GLAD) provides oblique deposition and substrate motion to engineer thin film microstructures in three dimensions on nano scale. Using this technique zigzag, chevrons, staircase, post, helical and various type of nanostructures including 3-D multilayers can be obtained from various metals with controllable morphologies. The aim of the study is to increase surface porosity and junction using GLAD method area for thin film solar cells and therefore to increase p-n junction area. This provides efficient charge separation and strong light absorption. Methods: Glancing angle deposition using e-beam evaporation technique has been employed to create 3- D silicon nano-structures on the surface. Al and Ag contact layers were deposited by thermal evaporation technique. Hole-conductor polymer PEDOT: PSS was spin coated onto n type silicon thin film. Reflectance spectra were measured using UV-VIS spectroscopy. Scanning electron microscopy was used to image surface and cross-section with and without PEDOT: PSS. Also, transmission spectra of PEDOT: PSS was measured using UV-VIS spectroscopy. Surface wettability properties and contact angles of silicon samples were measured by contact angle measurement with water. Results: Columnar structures possess less reflection compared to the flat surface depending on surface porosity. This phenomenon shows that these structures can be used as anti-reflection coatings for solar cells and optical devices to decrease reflectivity and increase light harvesting with higher efficiency. Contact angle decreases when surface roughness increases therefore we can see that columnar structures are more hydrophilic compared to dense films. Flat silicon has 98° contact angle while columnar structures have 71° and 61°. PEDOT: PSS exhibits high transparency in the range from 200 to 1100 nm of wavelength of light, which resembles to solar radiation inside the atmosphere. Also, SEM images of the samples show that silicon columnar structures form better contact with PEDOT: PSS than flat surface. Conclusion: GLAD technique has been used to achieve homogenous rough surface by e-beam evaporation. Both cross-sectional and top-view SEM images show that columnar structures have higher porosity than flat surfaces. The response of UV-VIS spectroscopy shows that columnar structures have less reflection due to highly porous surface. With increasing incident flux angle, antireflection property of the surfaces was enhanced by surpassing the surface reflection. Due to the reduced hydrophobicity of porous structures, organic polymer can be distributed homogenously in between the columnar structures with increased p-n junction interface area. PEDOT: PSS is highly conductive, and it is highly transparent material in the range of the wavelength typically seen in the solar radiation. This makes it easier for light to reach to Si interface to generate electrons and holes. These results provide better understanding of Si- based heterojunction solar cells efficiency improvement with surface modification. This study also shows dependency of optical and electrical activity to surface geometry and surface porosity.

Biaxially Textured Titanium Thin Films by Oblique Angle Deposition: Conditions and Growth Mechanisms

Growing highly crystalline nanowires over large substrate areas remains an ambiguous task nowadays. Herein, a time‐efficient and easy‐to‐handle bottom‐up approach is demonstrated that enables the self‐assembled growth of biaxially textured Ti thin films composed of single‐crystalline nanowires in a single‐deposition step. Ti thin films are deposited under highly oblique incidence angles by electron beam evaporation on amorphous substrates. Substrate temperature, angle of the incoming particle flux, and working pressure are varied to optimize the crystallinity in those films. Height‐resolved structure information of individual nanowires is provided by a transmission electron microscopy (TEM) nanobeam, high‐resolution TEM, and electron diffraction. Ti nanowires are polycrystalline at 77 K, whereas for ≥300 K, single‐crystalline nanowires are tendentially found. The Ti crystals grow along the thermodynamically favored c‐direction, but the nanowires’ tilt angle is determined by shadowing. Biaxially textured Ti thin films require a certain temperature range combined with highly oblique deposition angles, which is proved by X‐ray in‐plane pole figures. A general correlation between average activation energy for surface self‐diffusion and melting point of metals is given to estimate the significant influence of surface self‐diffusion on the evolution of obliquely deposited metal thin films.

Tuning non-Gilbert-type damping in FeGa films on MgO(001) via oblique deposition

The ability to tailor the damping factor is essential for spintronic and spin-torque applications. Here, we report an approach to manipulate the damping factor of FeGa/MgO(001) films by oblique deposition. Owing to the defects at the surface or interface in thin films, two-magnon scattering (TMS) acts as a non-Gilbert damping mechanism in magnetization relaxation. In this work, the contribution of TMS was characterized by in-plane angular dependent ferromagnetic resonance. It is demonstrated that the intrinsic Gilbert damping is isotropic and invariant, while the extrinsic mechanism related to TMS is anisotropic and can be tuned by oblique deposition. Furthermore, the two and fourfold TMS related to the uniaxial magnetic anisotropy and magnetocrystalline anisotropy were discussed. Our results open an avenue to manipulate magnetization relaxation in spintronic devices.

Efficient excitation of exchange dominated spin waves in oblique deposited CoFeB thin films

Spin waves play a vital role as potential data carriers in low power wave-like computing and high-frequency spintronic devices, which stimulate intensive studies on the efficient excitation of spin waves in magnetic thin films. Here, we suggest a controllable approach for the efficient excitation of perpendicular standing spin waves (PSSWs) in a series of CoFeB/Si thin films by the oblique angle deposition technique. It is experimentally demonstrated that the intensity of the PSSW mode can be enhanced significantly with increased oblique angle, and a comparable intensity of spin wave to that of uniform FMR mode is achieved at an oblique angle of 39°. Moreover, the dependence of in-plane anisotropy field Hk, exchange constant A, as well as intrinsic Gilbert damping α on oblique deposition angle are also investigated.

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering.

Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy.Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS.Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.

Nano-structural Characteristics and Optical and Electrical Properties of Obliquely Deposited Cuprous Azide Thin Films

Cuprous azide films were prepared using oblique angle deposition of Cu at 40° and post-annealed with the flow of nitrogen at three different temperatures. This deposition angle provides the highest void fraction according to the published literature on the basis of rearrangement of atoms resulting from the diffusion or thermal vibration and the available crystallographic sites and surface energy on the substrate/growing film surface for relaxation of an adatom. Structural characteristics of the films were obtained using atomic force microscopy and field emission scanning electron microscopy while their crystallography was investigated by x-ray diffraction analysis. Optical spectra of the samples were measured from which optical constants were deduced. The direct band gap energies were obtained and compared with the reported theoretical calculations and experimental results. The band gap energies obtained from the process of plotting (αhυ)2 versus hυ are in agreement with the reported values in the literature. Band gap energies obtained from dT/dλ versus hυ eV method are in agreement with experimental values and with the theoretical band gap energy of 4.1 eV as well as those obtained from (αhυ)2 versus hυ method, while those obtained from dR/dλ versus hυ eV are in good agreement with theoretical band gap energy of 3.86 eV.

Marked Enhancement of Roll-Off Frequency in FeCoN Synthetic Antiferromagnetic Films Deposited by Oblique Incidence.

A series of FeCoN films were successfully deposited on glass substrates in a magnetron sputtering system. Using oblique incidence method and FeCoN/Ru/FeCoN synthetic antiferromagnetic (SAF) structure, two additional anisotropies energy were introduced: oblique incidence anisotropy and exchange anisotropy energy, which marked enhancement of the effective magnetic anisotropy (Hk). The increment of Hk results in a significant improvement in the roll-off frequency of these films. The roll-off frequency of FeCoN/Ru/FeCoN films with SAF structure can reach up to 8.6 GHz. A feasible approach to conveniently controlling Hk of soft magnetic thin films by using oblique deposition and SAF structure can further improve their properties for the potential applications in the high frequency region.

Self-assembled nano-dots structures on Si(111) surfaces by oblique angle sputter-deposition

Controlled surface modification and nano-dots structures over Si(111) surfaces have been produced by oblique angle sputter deposition of 80 keV Ar+ beam. Temporal parameters such as self-assemble, tunability of size and density of fabricated nano-dots exhibit distinct fluence dependence. Crystalline to amorphous (c/a) phase transition for sputter deposited Si(111) surfaces has been observed. RBS/C reveals the non-linear response of damage distribution with Ar ion fluence. Compositional alterations like degree of amorphization, damage distribution and depth profiling of Ar in these nano-structured surfaces has been correlated with the morphological and structural findings. The underlying self-organization mechanism relies in ion beam sputtering induced erosion and re-deposition of Si atoms thereby leading to mass transport inside the amorphous layers. Such nano-structured Si(111) surfaces could be applied as key engineering substrates for surface reconstruction, optoelectronic devices, data storage devices, recording media and photovoltaic applications.

Deposition of highly transparent and conductive Ga-doped zinc oxide films on tilted substrates by atmospheric pressure plasma jet

Oblique angle deposition (OAD) has become a cost-effective method for depositing nanostructured films. Films with various degrees of porosity, anisotropy, and crystallographic texture have been successfully demonstrated. However, most studies were based on the more established processes, such as evaporation and sputtering; oblique angle deposition using atmospheric pressure plasma jet (APPJ) has not been reported. Here we deposit Ga-doped zinc oxides (GZO) on tilted substrate using APPJ, and study how the film properties are affected by the oblique angle and nozzle scanning trajectory. We found that the trajectory plays a key role in determining the optoelectronic properties of the obliquely deposited films. If the nozzle scans from the upstream side of the substrate to the downstream side, the resistivity, carrier concentration and mobility deteriorate considerably near the downstream region. The film becomes more porous with the haze factor rising significantly from <5% to ∼80%, while the total transmittance remains high (>80% at 550 nm). This degradation can be attributed to the “pre-deposition” of the GZO adsorbed particles on the downstream side of the bare glass where the nozzle has not scanned. Such undesirable pre-deposition occurs due to the combined effect of asymmetric plasma jet and gas flow, both pointing toward downstream, compared to axial symmetric pattern observed in the normal case. More importantly, this undesirable phenomenon can be removed by merely reversing the scanning trajectory, i.e. from downstream to upstream. In this case, a dense film has been formed in the downstream region so the subsequent exposure of the biased plasma jet and gas flow do not result in ad-particles deposition. Therefore, highly transparent and conductive OAD films as good as the normally deposited films can be achieved, a unique phenomenon not reported before. Our findings may shed light on the development of curved display technology, and may also be applied to other deposition methods with moving nozzle or moving stage such as spray pyrolysis.

Disordered and Densely Packed ITO Nanorods as an Excellent Lithography-Free Optical Solar Reflector Metasurface

Precise control and stabilization of the operating temperature environment of spacecraft and satellites during their life cycle is of paramount importance to increase device reliabilities and reduce the thermomechanical constraints. Optical solar reflectors are the physical interface between the spacecraft and space, and they are broadband mirrors for the solar spectrum, while having strong thermal emission in the mid-infrared part of the electromagnetic spectrum. Strong light–matter interactions in metamaterials and metasurfaces offer significant advantages compared to the conventional methods in performance, weight, launch, and assembly costs. However, the fabrication complexity of these metastructures due to necessitating lithography hinders their upscaling, reproducibility, large-area compatibility, and mass production. In this regard, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design a metasurface based on disordered and densely packed Indium Tin Oxide (...

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Recovery of Palladium(ii) and Platinum(ii) by Biocrystallization and Biodeposition Using a Pure Culture and Mixed Culture

Despite limited availability of platinum group metals such as palladium (Pd), there is an increasing demand to use them especially as catalysts. However, conventional recovery of these metals has proven detrimental to the environment, therefore, sustainable recycling and production is required. Biological synthesis of Pd (0) has proven to be an innovative method for recovery of palladium. This study aims to investigate the deposition of Pd at different concentrations by a pure isolate of Desulfovibrio desulfuricans and a consortium of Sulphate-reducing bacteria. The reduction was accelerated at the expanse of formate as an electron donor at a pH of 4. The results showed that after 12 hours of incubation a black precipitate of reduced Pd formed, the consortium produced a higher percentage reduction of 90% at a concentration of 2mM while the pure isolate had a percentage reduction of 77%. SEM showed electron oblique deposits on the surface of the bacteria, the deposits were between 40nm and 50nm in size. The use of the consortium proved to be highly efficient for the potential bioremediation of palladium contaminated environments.