Au Layer Thickness Research Articles

Epitaxial growth of bottom-crosslinked ZnGa2O4 nanowire arrays on c-plane GaN/Al2O3 substrate

The epitaxial growth of highly ordered ZnGa2O4 nanowire arrays and the construction of connectivity states between the individually segregated nanowires would significantly improve the performance of ZnGa2O4-based devices and expand their application fields. Herein, a strategy to construct high crystalline quality and well-aligned bottom-crosslinked ZnGa2O4 nanowire arrays along seven equivalent crystallographic directions of [111], [111̅], [11̅1], [1̅11], [11̅1̅], [1̅11̅] and [1̅1̅1] on Au-coated (0001) GaN/Al2O3 substrates using chemical vapor deposition (CVD) is proposed. The vapor-liquid-solid (VLS) mechanism dominates the entire growth process. The horizontal nanowires induced to nucleate and grow by catalyst droplets encountered and then turned towards inclined nanowires. Meanwhile, catalyst droplets that failed to move horizontally were enclosed by horizontal nanowires, resulting in the induction of perpendicular nanowire growth. The film-like horizontal nanowires connected all the inclined and perpendicular nanowires to form the bottom-crosslinked nanowire arrays. Additionally, the morphology, structure and optical properties of the ZnGa2O4 nanowire arrays, as well as the influence of Au layer thickness and growth time on the growth of ZnGa2O4 nanowires, were further characterized and thoroughly investigated. The proposed strategy achieved the growth of epitaxial well-aligned ZnGa2O4 nanowire arrays with bottom cross-linking, which is conducive to the application of ZnGa2O4 nanowires in optoelectronics.

A flexible nanostructured multimodal sensor based on surface plasmon resonance

Biosensors are of great significance for human health and safety monitoring, especially the sensor based on Surface Plasmon Resonance (SPR), widely used in biomolecular detection. Improving its sensitivity has always been the focus of research. In this paper, SPR in grating structures was primarily investigated and then a SPR-based nanoscale metal grating sensor was proposed. With Finite Difference Time Domain (FDTD) simulation, optimization of the sensor's duty ratio, thickness of the Gold (Au) layer, and periodic parameters was conducted. Then sensitivity and quality factor of the sensor are analyzed to obtain the optimal sensor parameters. With a grating period of 0.64 μm, Au layer thickness of 30 nm, and sensor's fill factor of 0.625, the sensor achieves a sensitivity of 0.417 μm/refractive index unit (RIU), demonstrating optimal performance. Additionally, Abaqus tensile simulations were performed on the grating imprinting section, validating the uniformity and consistency of strain in the Polydimethylsiloxane (PDMS) substrate-Au film grating structure during stretching. Finally, a combination mold based on acrylic material is designed, effectively addressing the challenging demolding characteristics of cured PDMS material, offering an efficient manufacturing approach for PDMS substrate fabrication. The SPR-based nanoscale grating multimodal sensor designed in this paper exhibits high sensitivity and potential cost-effectiveness in protein molecule detection and flexible stress-strain sensing applications, revealing promising broad prospects for practical utilization.

Enhancing performance of Au-hyperdoped Si photodetectors for infrared detection

Hyperdoping Si with transition metals to form intermediate bands for infrared absorption has attracted attention recently for producing sub-bandgap photoconductivity. In particular, Si hyperdoped with Au has been demonstrated to exhibit optoelectronic response at 1550 nm. However, the reported external quantum efficiencies (EQEs) are low, and the device fabrication processes had not been optimized. In this paper, we demonstrate a significant improvement in sub-bandgap EQE through modification of the material and device fabrication processes. By increasing the Si:Au layer thickness, modification of device design, and formation of Ohmic contacts, the EQE was measured to be as high as 0.44% at 1550 nm, nearly two orders of magnitude higher than previous reports from similar devices. Additionally, the EQE was measured to be in the 10-3 range for wavelengths as long as 2.4 μm. The EQE spectrum showed features that were attributed to defect levels from a substitutional Au acceptor defect. The above bandgap EQE showed gain in one device. Thermal annealing at 300 °C does not improve the efficiency of Si:Au photodiodes. These results demonstrate the viability of Au-hyperdoped Si for infrared detection below the bandgap of Si.

Fabrication and characterization of thin 94Zr target with gold backing for nuclear lifetime measurements using DSAM

For lifetime measurements of excited nuclear states in the sub-picosecond range, a thin target with a thick backing of high-Z material is required. To meet these specifications, a thin isotopic94Zr target of thickness 1.14 ± 0.05 mg/cm2, with backing of 4.7 ± 0.2 mg/cm2 thick Au layer, has been fabricated using the cost-effective cold rolling technique at the target laboratory of Inter-University Accelerator Centre (IUAC), New Delhi. The thickness verification and purity check of the fabricated target have been carried out by non-destructive Energy Dispersive X-ray Fluorescence (EDXRF) and Rutherford Backscattering Spectrometry (RBS) techniques. The results of these measurements are in good agreement with each other. The elemental composition, purity and surface morphology have been explored by Energy Dispersive X-ray Spectrometry (EDS) and Scanning Electron Microscopy (SEM) techniques. The fabricated target has been successfully used with a16O ion beam in a recent γ-ray spectroscopy experiment for lifetime measurements using the Doppler Shift Attenuation Method (DSAM) at IUAC, New Delhi.

Hybrid Bio-Nanocomposites by Integrating Nanoscale Au in Butterfly Scales Colored by Photonic Nanoarchitectures

Plasmonic metallic nanoparticles, like Au, can be used to tune the optical properties of photonic nanoarchitectures occurring in butterfly wing scales possessing structural color. The effect of the nanoscale Au depends on the location and the amount deposited in the chitin-based photonic nanoarchitecture. The following three types of Au introduction methods were compared regarding the structural and optical properties of the resulting hybrid bio-nanocomposites: (i) growth of Au nanoparticles inside the nanopores of butterfly wing scales by a light-induced in situ chemical reduction of HAuCl4 in aqueous solution containing sodium citrate, as a new procedure we have developed, (ii) drop-drying of the aqueous Au sol formed during procedure (i) in the bulk liquid phase, and (iii) physical vapor deposition of Au thin film onto the butterfly wing. We investigated all three methods at two different Au concentrations on the wings of laboratory-bred blue-colored male Polyommatus icarus butterflies and characterized the optical properties of the resulting hybrid bio-nanocomposites. We found that the drop-drying and the in situ growth produced comparable redshift in the spectral position of the reflectance maximum associated with the chitin-based photonic nanoarchitecture in the wing scales, while the 5 nm or 15 nm thick Au layers vacuum deposited onto the butterfly wing behaved like an optical filter, without inducing spectral shift. The in situ growth in the photonic nanoarchitecture under intense illumination produced uniform Au nanoparticles located in the pores of the biological template, which is more advantageous for further applications. An additional benefit of this method is that the Au nanoparticles do not aggregate on drying, like in the case of drop-drying of preformed Au nanoparticles from the citrate-stabilized sol.

SPR Sensor Based on a Concave Photonic Crystal Fiber Structure with MoS2/Au Layers.

We propose a surface plasmon resonance (SPR) sensor based on the concave photonic crystal fiber (PCF) coated with molybdenum disulfide (MoS2) and Au layers, which can detect the refractive index (RI) of the analyte. The finite element method (FEM) was used to verify our design, and the loss spectra of the fundamental mode are calculated. Compared with the SPR sensor with only a Au layer, the wavelength sensitivity can be improved by from 3700 to 4400 nm/RIU. Our proposed sensor works in near-infrared band and has a wide RI range from 1.19 to 1.40. The influences of the geometrical parameters of PCF and the thicknesses of Au and MoS2 layers on the loss spectra are discussed in detail, and the maximum wavelength sensitivity of 5100 nm/RIU can be achieved. Meanwhile, a high resolution of 1.96 × 10-5 RIU and the largest FOM of 29.143 can be obtained. It is believed that our findings show the sensor's excellent potential in medical testing, unknown biological detection, environmental monitoring and organic chemical detection.

A novel method for improving interfacial joining strength of vacuum brazed TiAl/GH3536 thin-walled structure by Au coating

Lightweight components and materials based on the joining of dissimilar materials have attracted wide attention in aerospace field. To obtain the optimized micromorphology and interfacial microstructure of the brazed joints, a new method of Au layer deposition on TiAl base metal was proposed to assist the brazing of TiAl plate to GH3536 thin-walled structure. Significantly, the design of Au layer affected the brazing fillet, dissolution of GH3536 core and the evolution of interface, and then the maximum force under tensile loading of the hetero-thin-walled structure. The effect of Au layer thickness on the interfacial microstructure and mechanical properties was discussed in detail. Upon the deposition of an Au layer with a concentration of 0.2 wt% onto the TiAl plate, the load-bearing capacity of the brazed thin-walled structure was observed to undergo a significant enhancement. Specifically, the maximum force sustained under tensile loading was measured to be 391 N, thereby exhibiting a remarkable increase of 117 % relative to that of the Au-free system.

Effect of Au thin films thickness on the wetting characteristics of eutectic Pb-Sn solder on Ni/Ag solder bath

Wetting of Pb-Sn solder on Au/Ni surface is experimentally studied as a function of the thickness of Au coating on Ni buffer layer. While Au coating is known to increase the wettability of Pb-Sn solder on Ni surface, the effect of Au layer thickness was not examined systematically. This study shows that i) an increase in Au layer thickness from 0.15 to 0.3 µm decreases the surface coverage (spread) of the solder and ii) there is a compositional change in Pb-Sn solder on Au/Ni surface due to formation of a Sn rich region at the solder - Ni interface during a reflow process of the solder. The thickness of Sn rich region near the interface increases with increasing the Au layer thickness. A driving force for the formation of the Sn rich phase is explained using the negative mixing enthalpy of Au-Sn. Since an increase in the Sn content of Pb-Sn solder increases the surface energy and viscosity of the solder melt, an increase in the thickness of the Sn rich region at the solder-Ni interface suppressed the wetting of the solder.

Two-temperature model for ultrafast melting of Au-based bimetallic films interacting with single-pulse femtosecond laser: Theoretical study of damage threshold

Ultrashort-pulse laser excitation and associated thermal damage of single-layer metallic films have been extensively studied both theoretically as well as experimentally. However, study of ultrafast heat transfer mechanism in bimetallic films, a topic of immense current interest for designing next-generation optical components with better thermal resilience, is limited to a low fluence regime. Moreover, available results for damage threshold (DT) of bimetallic films have several ambiguities. The current work is an attempt towards development of comprehensive theory for the DT of Au-based bimetallic films, analyzing its dependence on associated physical parameters and bridging the gap in existing literature. This has been achieved by developing fully implicit two-temperature model-based Python code. Excellent agreement of the predicted temperature profiles with recently reported [E. L. Gurevich, Y. Levy, S. V. Gurevich, and N. M. Bulgakova, Phys. Rev. B 95, 054305 (2017)] femtosecond laser irradiated Au film validates the code. Subsequently, the code is utilized to investigate the influence of different metallic substrates, $M$ ($M$ = Ni, Cr, Cu) on ultrafast melting damage of $\mathrm{Au}\text{/}M$ films excited by single-pulse femtosecond laser. Simulations carried out for increasing Au layer thickness revealed a nonmonotonic variation of DT for all substrates. This study also brought out an important observation that for every substrate metal, there exists an optimum Au thickness for maximizing the incipient and the complete melting threshold. Maximum achievable enhancement for $\mathrm{Au}\text{/}M$ films over pure Au are found to be 30%, 22%, and 14% with Cu, Ni, and Cr substrates, respectively. The observed distinction in maximum DT among three different $\mathrm{Au}\text{/}M$ targets is explained by developing appropriate thermophysical models. Finally, an empirical function is proposed here for expressing nonmonotonic variation of two-layer DTs. Very good agreement between the substrate-specific peak positions predicted by the proposed function and effective electron diffusion length arising from thermophysical analysis makes the modeling robust and applicable for a wide range of laser wavelengths as well as arbitrary combinations of bimetallic films.

Ultrafast electron dynamics in Au/Fe/MgO(001) analyzed by Au- and Fe-selective pumping in time-resolved two-photon photoemission spectroscopy: Separation of excitations in adjacent metallic layers

The transport of optically excited, hot electrons in heterostructures is analyzed by femtosecond, time-resolved two-photon photoelectron emission spectroscopy (2PPE) for epitaxial Au/Fe/MgO(001). We compare the temporal evolution of the 2PPE intensity upon optically pumping Fe or Au, while the probing occurs on the Au surface. In the case of Fe-side pumping, assuming independent relaxation in the Fe and Au layers, we determine the hot electron relaxation times in these individual layers by an analysis of the Au layer thickness dependence of the observed, effective electron lifetimes in the heterostructure. We show in addition that such a systematic analysis fails for the case of Au-side pumping due to the spatially distributed optical excitation density, which varies with the Au layer thickness. This work extends a previous study [Beyazit et al., Phys. Rev. Lett. 125, 076803 (2020)] by new data leading to reduced error bars in the determined lifetimes and by a non-linear term in the Au-thickness dependent data analysis which contributes for similar Fe and Au film thicknesses.

The Design of a Glycerol Concentration Sensor Based on an LRSPP Hybrid Photonic Biosensor.

A refractive index sensor based on an on-chip silicon nitride (Si3N4) ridge waveguide long-range surface plasmon polariton (LRSPP) is theoretically designed. The waveguide sensor consists of a gold film to enable the plasmonic resonance on top of a Cytop polymer layer. A proper finite element method was used to design and optimize the geometric parameters at the optical wavelength of 633 nm. In addition, the spectral performance was evaluated using the transfer matrix method from 580 to 680 nm. The redshifted interference spectrum results from an increasing analyte refractive index. The sensitivities of 6313 dB/cm/RIU and 251.82 nm/RIU can be obtained with a 400 nm wide and 25 nm thick Au layer. The proposed sensor has the potential for point-of-care applications considering its compactness and simplicity of construction.

Ultrafast transport and energy relaxation of hot electrons in Au/Fe/MgO(001) heterostructures analyzed by linear time-resolved photoelectron spectroscopy

In condensed matter, scattering processes determine the transport of charge carriers. In case of heterostructures, interfaces determine many dynamic properties like charge transfer and transport and spin current dynamics. Here we discuss optically excited electron dynamics and their propagation across a lattice-matched, metal-metal interface of single crystal quality. Using femtosecond time-resolved linear photoelectron spectroscopy upon optically pumping different constituents of the heterostructure we establish a technique which probes the electron propagation and its energy relaxation simultaneously. In our approach a near-infrared pump pulse excites electrons directly either in the Au layer or in the Fe layer of epitaxial Au/Fe/MgO(001) heterostructures while the transient photoemission spectrum is measured by an ultraviolet probe pulse on the Au surface. Upon femtosecond laser excitation, we analyze the relative changes in the electron distribution close to the Fermi energy and assign characteristic features of the time-dependent electron distribution to transport of hot and non-thermalized electrons from the Fe layer to the Au surface and vice versa. From the measured transient electron distribution we determine the excess energy which we compare with a calculation based on the two-temperature model and takes diffusive electron transport into account. On this basis we identify a transition from a super-diffusive to a diffusive transport regime to occur for a Au layer thickness of 20-30~nm.

Au-coated Ag alloy bonding wires with enhanced oxidation resistance for electronic packaging applications

PurposeThe purpose of this paper is to develop high-performance Au-coated Ag alloy wires (ACAA wires) and demonstrate the effect of Au coating layers on the bonding performance and oxidation resistance for stable and reliable electronic packaging applications.Design/methodology/approachACAA wire with a diameter of approximately 25 µm and Au layer thickness of approximately 100 nm were prepared by the continuous casting, plating and wire drawing method. The bonding performance of the ACAA wires were studied through bonding on 3,535 chips. The oxidation resistance of ACAA wires and Ag alloy wires (AA wires) were comparatively studied by means of chemical oxidation tests, accelerated life tests and electrochemical tests systematically.FindingsACAA wires could form axi-symmetrical spherical free air balls with controllable diameter of 1.5∼2.5 times of the wire diameter after electric flame-off process. The ball shear strength of ACAA wire was higher than that of AA wires. Most importantly, because of the surface Au coating layer, the oxidation resistance of ACAA wires was much enhanced.Research limitations/implicationsACAA wires with different lengths of heat affected zone were not developed in this study, which limited their application with different loop height requirements.Practical implicationsWith higher bonding strength and oxidation resistance, ACAA wires would be a better choice than previous reported AA wire in chip packaging which require high stability and reliability.Originality/valueThis paper provides a kind of novel ACAA wire, which possess the merits of high bonding strength and reliability, and show great potential in electronic packaging applications.

Au sputter-deposited ZnO nanofibers with enhanced NO2 gas response

A selective NO2 gas sensor was fabricated using Au-decorated ZnO nanofibers. A layer of Au was sputter-deposited on the synthesized ZnO nanofibers. The thicknesses of the Au layers were varied as 3, 6, 9, 12, and 15 nm for optimization. Annealing of the samples at a high temperature converted the Au layer to isolated Au nanoparticle (NPs) dispersed on the ZnO nanofibers. Different characterizations including structural, chemical, and morphological confirmed that the composition and morphology of the Au-decorated ZnO nanofibers were appropriate for gas sensing. The sensor with an Au layer thickness of 9 nm demonstrated an optimal response at 350 °C. The enhanced sensing response stemmed from the optimal amount of Au, generation of Au–ZnO Schottky junctions, and catalytic effect of Au NPs. This study highlights the need for the optimization of the amount of Au NPs to achieve improved response to a target gas.

Ohmic contact morphology improvement with reduced resistance using Si/Au/Ti/Al/Ni/Au (AlGaN) and Si/Au/Ti/Al/Ni/Au (InAlN) stack layers in III-Nitride HEMTs

This article reports a Ti/Al-based ohmic contact utilizing a thin interfacial Au layer for improved morphology, edge acuity and low contact resistance for applications to III-Nitride high electron mobility transistors (HEMTs). Conventional Ti/Al contacts are based on a metal stack of the form Ti/Al/X/Au, where X is any suitable barrier layer such as Ni. The formation of ohmic contact in GaN and its alloy systems is governed by the formation of TiN during annealing; Au inter-diffusion during annealing also assists the formation of TiN islands and aids in reducing the contact resistance. Furthermore, inter-metallic phase formation between Ti and Al lowers the contact resistance. It is observed that the contact surface morphology strongly depends on the top Au layer thickness. Insertion of a very thin Au layer (2–3 nm) has drastically improved the surface quality with the achievement of contact resistance as low as 0.36 Ω mm and a specific contact resistance of 2.4 × 10−6 Ω cm2 on AlGaN/GaN HEMT. The reduction in contact resistance is due to the formation of more conducting intermetallic phases during annealing, which is assisted due to the thin Au layer insertion. Further reduction in the contact resistance to 0.13 Ω mm was also achieved with the introduction of Si as a dopant layer in the reported metal scheme. The measured rms surface roughness was reduced to ∼8 nm from 0.12 µm in comparison to the conventional ohmic contact.

Thermo-reliability of PVD Cr/Au on sapphire substrates for high-temperature sensors

Thermo-reliability of physical vapor deposition (PVD) Cr/Au on sapphire substrates was systematically investigated to improve the routing interconnection for high-temperature sensors. Cr and Au films were sequentially deposited on sapphire through magnetron sputtering. Samples with various Au layer thicknesses were subjected to annealing experiments at different temperatures and times. The temperature and processing time affected the dimension of Au particles and changed the surface roughness. With the increase in the temperature and treatment time, the agglomeration of Au particles increased, and the diameter enlarged substantially. Furthermore, the agglomeration effect is enhanced with the increase in the Au film thickness. However, the annealing time under the same temperature had a limited effect on the resistance. All the samples exhibited a low resistance at 700 °C. Moreover, compared with 100/500 nm Cr/Au films, the resistance of 100/1000 nm Cr/Au films slightly changed before and after annealing at 500 °C. Thus, thermo-reliability of PVD Cr/Au was beneficial for high-temperature sensors. The results of this study provided theoretical support for the practical applications of Cr/Au films.

Influence mechanism of Au layer thickness on wettability of Sn–Ag–Cu solder on heated ENIG pads

Solder wettability on substrate is an issue for the electronic devices. In this present, the heated Cu pads with ENIG finish was selected as the study object. Effect of Au layer thickness on ENIG surface morphology, Ni distribution and wettability of Sn-3.0Ag-0.5Cu solder was investigated. An acceptable morphology with nanoscale fluctuation for ENIG surface was achieved, with the absence of interface cracking, pits, holes. Ni diffused from Ni layer to Au surface during heating process, then reacted with the ambience to form Ni oxides, the fraction was increased with the increasing Au layer thickness. The Ni oxides inhibited the interfacial reaction between Sn-3.0Ag-0.5Cu and Ni during reflow, and remained to the interface. It resulted in the formation of “barren zones”, where without any intermetallic compounds. With Au layer thickness increased, “barren zones” was shrunk until to disappear. The “barren zones” was responsible to the wettability of Sn solder. The more “barren zones”, the poor wettability.

Crystallization of Ge thin films by Au-induced layer exchange: effect of Au layer thickness on Ge crystal orientation

Au layer thickness dependence (9–34 nm) of Ge crystallization in the metal-induced layer exchange process has been investigated. It has been found that Ge crystals are (111) oriented when the Au layer is as thin as 9 nm, whereas crystal grains are randomly oriented when the Au layer is as thick as 34 nm. The difference is discussed in terms of the difference in the position of nucleation sites of Ge crystals.

Photon spin angular momentum driven magnetization dynamics in ferromagnet/heavy metal bilayers

Thin-film magnetization controlled by optical helicity has been recently reported. Although circularly polarized light has spin angular momentum, helicity-dependent all-optical magnetization switching is mediated by the stochastic thermal process, such as magnetic circular dichroism, and the effect of photon spin angular momentum is considered to be a secondary role. Conversely, the inverse Faraday effect in ferromagnetic thin films and photon spin angular momentum injection into heavy metal thin films have been observed, which can induce torque on metallic thin-film magnets. In this study, we show photon spin angular momentum driven magnetization dynamics in bilayers of Co/(Pt, Au) thin films with various thicknesses. The heavy metal Pt, Au, and ferromagnetic Co layer thickness dependencies of photon spin angular momentum driven torques are discussed in terms of field-like torque owing to the inverse Faraday effect and spin-transfer torque caused by photon spin angular momentum injection into the heavy metal layer with details of optical and magnetic properties. This study provides a better understanding of photon spin angular momentum induced magnetization dynamics in metallic thin-film heterostructures for efficient photon-driven magnetization manipulation.

High-performance indium-free flexible transparent ATO/Au/ATO tri-layer films by magnetron sputtering

Due to the scarcity of indium (In) in the earth and its potential harm to individuals, the development of In-free transparent conductive film is considered crucial. In this work, In-free SnO 2 :Sb/Au/SnO 2 :Sb (ATO/Au/ATO, SAS) tri-layer films with high transparency and conductivity were successfully prepared on polycarbonate (PC) substrates by RF and DC magnetron sputtering at room temperature. The influence of the Au layer thickness on microstructure, electrical and optical performances was systematically studied after fixing the ATO thickness to 50 nm. It was indicated by X-ray diffraction patterns that ATO is amorphous and Au is oriented along (111). The trend of increasing and then decreasing light transmission with Au layer thickness was observed in both experimental and simulation results. The improved figure of merit (FoM, 1.89 × 10 −2 Ω −1 ) was achieved in SAS tri-layer film, the resistivity and average transmittance of which was lowered to 7.50 × 10 −5 Ω cm and 81.4%, respectively, when Au layer thickness is 11 nm. Moreover, the mechanism of the variation of optical and electrical properties at different Au layer thickness was proposed. Particularly, the SAS tri-layer films also exhibit superior flexibility, durability and adhesion. These results demonstrate SAS tri-layer films are promising alternative to ITO in flexible electronics applications.