Outer Au Layer Research Articles

A self-calibrating flexible SERS substrate incorporating PB@Au assemblies for reliable and reproducible detection.

The precise quantitative analysis using surface-enhanced Raman spectroscopy (SERS) in an uncontrollable environment still faces a significant obstacle due to the poor reproducibility of Raman signals. Herein, we propose a facile method to fabricate a self-calibrating substrate based on a flexible polyvinyl alcohol (PVA) film comprising assemblies of Prussian blue (PB) and Au NPs (PB@Au) for reliable detection. PB cores were coated with an Au shell through simple electrostatic interaction, forming core-shell nanostructure PB@Au assemblies within the PVA film. The outer Au layer provided identical trends in enhancement for both the PB core and neighboring targets while PB cores served as an internal standard (IS) to correct signal fluctuations. The prevention of competitive adsorption on the metal surface between targets and ISs was achieved. The proposed PVA/PB@Au film exhibited enhanced stability of Raman signals after IS correction, resulting in improved spot-to-spot and batch-to-batch reproducibility with significantly reduced standard deviation (RSD) values from 11.42% and 25.02% to 4.43% and 9.39%, respectively. Simultaneously, a higher accuracy in the quantitative analysis of 4-mercaptobenzoic acid (4-MBA) and malachite green (MG) was achieved with fitting coefficient (R2) values improving from 0.9675 and 0.9418 to 0.9974 and 0.9832, respectively. Moreover, the PVA/PB@Au film was successfully applied to detect residual MG in real fish samples. This work opens up an avenue to improve the reproducibility of Raman signals for flexible SERS substrates in the detection of residues under various complex conditions.

Hydrodynamic motion of guiding elements within a magnetic switchyard in fast ignition conditions

Magnetic collimation via resistivity gradients is an innovative approach to electron beam control for the cone-guided fast ignition variant of inertial confinement fusion. This technique uses a resistivity gradient induced magnetic field to collimate the electron beam produced by the high-intensity laser–plasma interaction within a cone-guided fast ignition cone-tip. A variant of the resistive guiding approach, known as the “magnetic switchyard,” has been proposed which uses shaped guiding elements to direct the electrons toward the compressed fuel. Here, the 1D radiation-hydrodynamics code HYADES is used to investigate and quantify the gross hydrodynamic motion of these magnetic switchyard guiding elements in conditions relevant to their use in fast ignition. Movement of the layers was assessed for a range of two-layer material combinations. Based upon the results of the simulations, a scaling law is found that enables the relative extent of hydrodynamic motion to be predicted based upon the material properties of the switchyard, thereby enabling optimization of material-combination choice on the basis of reducing hydrodynamic motion. A multi-layered configuration, more representative of an actual switchyard, was also simulated in which an outer Au layer is employed to tamp the motion of the outermost guiding element of the switchyard.

Enhancing T1 magnetic resonance imaging contrast with internalized gadolinium(III) in a multilayer nanoparticle

Multifunctional nanoparticles for biomedical applications have shown extraordinary potential as contrast agents in various bioimaging modalities, near-IR photothermal therapy, and for light-triggered therapeutic release processes. Over the past several years, numerous studies have been performed to synthesize and enhance MRI contrast with nanoparticles. However, understanding the MRI enhancement mechanism in a multishell nanoparticle geometry, and controlling its properties, remains a challenge. To systematically examine MRI enhancement in a nanoparticle geometry, we have synthesized MRI-active Au nanomatryoshkas. These are Au core-silica layer-Au shell nanoparticles, where Gd(III) ions are encapsulated within the silica layer between the inner core and outer Au layer of the nanoparticle (Gd-NM). This multifunctional nanoparticle retains its strong near-IR Fano-resonant optical absorption properties essential for photothermal or other near-IR light-triggered therapy, while simultaneously providing increased T1 contrast in MR imaging by concentrating Gd(III) within the nanoparticle. Measurements of Gd-NM revealed a strongly enhanced T1 relaxivity (r1 ∼ 24 mM-1⋅s-1) even at 4.7 T, substantially surpassing conventional Gd(III) chelating agents (r1 ∼ 3 mM-1⋅s-1 at 4.7 T) currently in clinical use. By varying the thickness of the outer gold layer of the nanoparticle, we show that the observed relaxivities are consistent with Solomon-Bloembergen-Morgan (SBM) theory, which takes into account the longer-range interactions between the encapsulated Gd(III) and the protons of the H2O molecules outside the nanoparticle. This nanoparticle complex and its MRI T1-enhancing properties open the door for future studies on quantitative tracking of therapeutic nanoparticles in vivo, an essential step for optimizing light-induced, nanoparticle-based therapies.

A spherical shell target scheme for laser-driven neutron sources

A scheme for neutron production is investigated in which an ultra-intense laser is irradiated into a two-layer (deuterium and aurum) spherical shell target through the cone shaped entrance hole. It is found that the energy conversion efficiency from laser to target can reach as high as 71%, and deuterium ions are heated to a maximum energy of several MeV from the inner layer surface. These ions are accelerated towards the center of the cavity and accumulated finally with a high density up to tens of critical density in several picoseconds. Two different mechanisms account for the efficient yield of the neutrons in the cavity: (1) At the early stage, the neutrons are generated by the high energy deuterium ions based on the “beam-target” approach. (2) At the later stage, the neutrons are generated by the thermonuclear fusion when the most of the deuterium ions reach equilibrium in the cavity. It is also found that a large number of deuterium ions accelerated inward can pass through the target center and the outer Au layer and finally stopped in the CD2 layer. This also causes efficient yield of neutrons inside the CD2 layer due to “beam-target” approach. A postprocessor has been designed to evaluate the neutron yield and the neutron spectrum is obtained.

Fabrication of co-axial field emitter tips for scanning probe energy loss spectroscopy

We report on the fabrication of a co-axial tip for application to scanningprobe energy loss spectroscopy (SPELS). The device consists of a 23.3 µm tall tip on a 76 µm tall mesa with amultilayer Si/Au/HfO2/Au structure;the outer Au and HfO2 layers are stripped from the apex of the tip. The inner Au layer is used as a field emittinglayer and the outer Au layer is grounded to screen the electric field between the tip and thesubstrate. The co-axial tip shows comparable field emission characteristics to electrochemicallyetched tungsten tips. The SPELS spectra of graphite obtained with the new tips showπ andσ plasmon peaks and intense secondary electron emission peaks. It is anticipated that suchco-axial tips will present a significant advantage for future angular resolved SPELSmeasurements.

Au/ITO dual-layer-coated optical fiber probe for multifunctional scanning tunnelingmicroscopy

We have fabricated an Au/ITO (indium-tin-oxide) dual-layer optical fiber probe with functionsfor high-quality scanning tunneling microscope (STM) imaging and near-field excitationand detection of STM-induced electroluminescence (STML). The inner ITO layer is kept∼150 nm in thickness to ensure the conductivity of the fiber probe. The outer Au layer is kept∼13 nm in thickness to obtain a better combination of high electron injection efficiency and opticaltransparency. High-quality STM images, and STML corresponding to maximum quantum efficienciesof 7 × 10−4 and 1 × 10−4 photons/electron are obtained on Au(111)/mica and p-GaAs(110) surfaces at room temperature (RT),respectively. The outer Au layer on the Au/ITO-coated fiber probe shows aneffective role in improving the quality of STM imaging, the tip apex geometryand durability of the fiber probe, the stability and reproducibility of STML andenhancing the intensity of plasmon-mediated STML from the Au(111) surface but itshows no effect in enhancing the intensity of STML from the GaAs(110) surface.

Ag–Copper Structure for Bonding Large Semiconductor Chips

Ag-copper dual-layer substrate design is presented. The Ag cladding on the copper substrate is a buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/deg C). Ag is chosen because of its low yield strength, only one-tenth of that of Cu and one-third of the popular Sn3.5Ag solder. Other advantages are high electrical conductivity and high thermal conductivity. To bond Si chips to the Ag layer on copper substrates, Sn-rich solder is used. A fluxless bonding process is designed and developed. The bonding media are Ni/Sn/Au multilayer solder structure plated over Ag. In this design, Ni is a diffusion barrier between Sn and Ag. The thin (100 nm) outer Au layer prevents inner Sn from oxidation. The Si chip is deposited with Cr/Au under bump metallurgy (UBM). The bonding process is performed in 50-mtorr vacuum atmosphere without any flux. Comparing to bonding in air, the oxygen content is reduced by a factor of 15 200. The resulting joints consist of three distinct layers, i.e., Sn-rich layer, Ni3 Sn4 intermetallic compound, and Ni. Scanning acoustic microscopy (SAM) is used to verify the quality of the joint. Microstructure and composition of the joints are studied using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). This technique presents an initial success in overcoming the very large mismatch in thermal expansion between silicon and copper. It can be applied to mounting numerous high-power silicon devices to Cu substrate for various applications such as hybrid automotive and high-voltage power networks.

Fluxless Hermetic Lid Sealing Using Electroplated Sn-Rich Solder

A new fluxless hermetic sealing technique using electroplated Sn-rich soft solder is reported. Specific glass (SCG72) is chosen as lid material to seal alumina packages. It has nominal coefficient of thermal expansion of 7 ppm/deg C, close to that of alumina. Alternatively, sapphire can also be used as the lid material. A thick Sn layer is plated over the Cr/Au patterned glass wafer, followed immediately by thin Au layer. This outer Au layer prevents the inner Sn from oxidation. In bonding, the glass lid is placed over the package rim which has a metallization structure of W/Ni/Au. The fluxless sealing process is performed in vacuum (50 millitorrs) to suppress tin oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Fluxless bonding is valuable in many hermetic sealing applications such as microelectromechanical systems (MEMS), sensors, photonic devices, and imaging devices. Nearly void-free sealed joints are achieved with Sn-rich composition. Helium leakage tests are performed to evaluate hermetic quality. Scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) is used to evaluate the composition and microstructure of the sealing joints. The results show how the Sn-rich solder reacts with the W/Ni/Au on the package. This new sealing process can be applied to nearly all devices and packages that require hermetic sealing.

Fluxless process of fabricating In-Au joints on copper substrates

Based on the oxidation-free fluxless bonding technology, we have developed a bonding process to manufacture In-Au joints on copper substrates. 4 mm/spl times/4 mm Si blank dice and 6 mm/spl times/6 mm copper substrates are used. The dice are deposited with indium-rich Au/In/Cr multilayer structure in a single high vacuum cycle to prevent oxidation. Right after deposition, the outer Au layer interacts with the In layer to form AuIn/sub 2/ intermetallic compound. This compound is quite stable and thus can protect the In layer against oxygen penetration when it is exposed to ambient. On the other hand, it can easily be dissolved by the molten In during the bonding process. The substrate is deposited with Cr and Au. The dice are bonded to the substrates at 180/spl deg/C in inert environment. Nearly void-free joints have been obtained as examined by a 75 MHz Scanning Acoustic Microscope (SAM). Cross sections of several samples are studied using SEM and EDX to identify the microstructure and composition of the joints. Shear test has been performed according to MIL-STD-883C. All the well-bonded devices meet the shear test force requirement. Despite the large mismatch on the thermal expansion coefficient between silicon and copper, no die cracking is observed on the 30 samples produced. To assess further endurance, two samples underwent thermal cycling test between -50 and 120/spl deg/C for 20 cycles, SAM examination indicates that the joints incur little degradation after the test. This bonding method requires neither flux nor scrubbing action. It is thus particularly attractive for bonding devices that cannot be exposed to flux.

Ohmic Pd/Zn/Au/LaB6/Au contacts on p-type In0.53Ga0.47As: Electrical and metallurgical properties

The development of a shallow and low-resistive contact on moderately doped (p≈5×1018 cm−3) In0.53Ga0.47As is demonstrated. By reducing the layer thicknesses of a conventional Pd/Zn/Pd/Au scheme to a minimum and coupling this system to an outer Au layer via an amorphous LaB6 diffusion barrier, contact resistivities ⩽1×10−6 Ω cm2 were achieved only slightly exceeding that of the conventional scheme (2–4×10−7 Ω cm2). The contact reaction depth, however, could be reduced from several hundred to well below 100 nm, since the LaB6 barrier effectively prevents the outer Au layer from reaction with the semiconductor during contact formation. The influence of Zn content on electrical and metallurgical properties has been studied by varying it over orders of magnitude using both implantation and evaporation as a means of introducing Zn into the metallization. Implanted contacts with low Zn content annealed at 375 °C exhibit a reaction depth as low as 55 nm with the Zn diffusion depth practically coinciding with the reaction depth. If Zn is evaporated, the reaction depth is enhanced and the Zn diffusion depth exceeds the reaction depth noticeably. Secondary ion mass spectrometry demonstrated the onset of ohmic behavior to be correlated with the buildup of high interfacial Zn concentrations suggesting that these contacts conform to the standard model of interfacial doping. Simultaneously a solution phase emerges, reported here first, namely (Pd,Au)12(Ga,In)5As2 based on hexagonal Pd12Ga5As2 as revealed by cross-sectional transmission electron microscopy and x-ray diffraction analysis. Contacts with evaporated Zn exhibit stable resistivity during thermal stressing at 400 °C for 24 h. The stability loss for longer times is correlated with the appearance of (Au,Pd)9In4, a solution phase based on cubic Au9In4, evolving from indiffused Au and replacing increasingly the low-resistive (Pd,Au)12(Ga,In)5As2/p+-In0.53Ga0.47As junction areas.

Microstructural analysis of Au/Pt/Ti contacts to p-type InGaAs

A detailed study on the microstructural changes that occur on annealing of Au/Pt/Ti ohmic contacts to n-type InGaAs has been carried out. The metal layers were deposited sequentially by electron beam evaporation onto InGaAs, doped with Zn to a level of 7 × 1018 cm−3, that was epitaxially grown on InP substrates. The deposition sequence and metal layer thicknesses were: Ti (25 or 30 nm), Pt (25 or 30 nm) and Au (250 or 300 nm). Samples were annealed at temperatures ranging from 250–425 ‡C in a nitrogen atmosphere. As-deposited contacts were Schottky barriers, while a minimum contact resistance of 2 × 10−5 Ω cm2 was obtained by annealing in the 375–425 ‡C range. Annealing resulted in the inward diffusion of Ti and outward diffusion of In and As, leading to the formation of TiAs, metallic In and Ga-rich InGaAs at the Ti/InGaAs interface. The Pt diffusion barrier was effective in preventing In diffusion into the outer Au layer and minimizing Au diffusion to the semiconductor.

Technological aspects of the preparation of Au-Zn ohmic contacts to p-type InP

Au-Zn/Au and X/Au-Zn/Y/Au metallizations (where X=Au, Cr or Ni is a nucleation layer and Y=Cr or Ni is a diffusion barrier), formed by vacuum evaporation of Au-Zn (10 wt%) alloy, were studied for the preparation of ohmic contacts to p-type InP. The metallurgical and electrical properties of such contacts were investigated as a function of deposition parameters (especially the deposition rate of Au-Zn alloy) and alloying conditions. The role of the nucleation layer X on the InP surface and of the barrier layer Y between the Au-Zn and the outer Au layer in contact formation is also discussed.

Correction of Energy Shifts to Form Real Three-Dimensional Images by Microprobe RBS

Data processing for three-dimensional analysis by microprobe Rutherford backscattering (RBS) was studied to form real images of multilayered structures. Bilayered Au stripes on Si were analyzed by a 400 keV He+ microprobe. Unprocessed mapping and tomographic images of the inner Au layer showed wavy shapes due to the energy loss of probe beams in the outer Au layer. A set of 8192 RBS-spectra corresponding to a measured frame of 128×64 points was processed by evaluating amounts of Au and the energy loss in the outer Au layer. The inner Au images were successfully corrected after the processing.

Pd/Zn/Pd/Au ohmic contacts to ρ-Type InP

Low resistance ohmic contacts (ρc = 7 x 10-5 Ω-cm2) have been fabricated to Zn-doped p-type InP using an annealed Pd/Zn/Pd/Au metallization. Palladium reacts with InP at low temperatures to form a Pd2InP ternary phase, which is initially amorphous but crystallizes and grows epitaxially on InP. Zinc reacts with some of the overlying Pd to form PdZn (≅250° C), which decomposes at 400-425° C to form PdP2, freeing up Zn to diffuse into Au as well as InP. The contact resistance reaches a minimum as the decomposition reaction takes place. The resultant ohmic contact is laterally uniform and consists of epitaxial Pd2InP adjacent to InP, followed by a thin layer of PdP2 and then the outer Au layer. Further annealing leads to a breakdown of the contact structure,i.e. decomposition of Pd2InP, and an increase in contact resistance.

Evolution of a surface state into an interface state: A probe of the buried epitaxial Cr/Au(100) interface.

Angle-resolved synchrotron-radiation photoemission results are presented that examine the evolution of the Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) surface state at the epitaxial Cr/Au(100) interface as a function of Cr thickness. At submonolayer coverages, two-dimensional Cr islands form and stabilize the (1\ifmmode\times\else\texttimes\fi{}1) termination of the substrate, as judged by the simultaneous detection of both the Au-(1\ifmmode\times\else\texttimes\fi{}1) and -(5\ifmmode\times\else\texttimes\fi{}20) surface states. The Au(100)-(1\ifmmode\times\else\texttimes\fi{}1) surface state persists in the outer Au layer for 1 monolayer (ML) of Cr or Au(100). As the Cr overlayer thickness is increased from 1 to 2 ML, the interface state is modified and its binding energy changes from 1.9 to 2.6 eV. For Cr coverages in excess of 3 ML, the Au state couples with bulk states and vanishes. The Au surface-interface state is reestablished by the growth of an ultrathin Au layer over the 2--3-ML-thick epitaxial Cr layers, and its binding energy shifts back toward ${E}_{F}$. The results are discussed in terms of changes in the surface-interface potentials during the evolution of the overlayer.

VIA-7 sputter Ni-P as an ohmic contact to n-InP, p-InGaAs and as a diffusion barrier

Ni 2 P is a metallic conductor with a bulk resistivity of 32 µΩ . cm. Films of Ni 2 P can be sputter-deposited in both the amorphous and crystalline forms by varying the sputtering parameters. The amorphous to crystalline transition has been found to take place at about 250 °C and sufficient grain growth to exhibit X-ray reflections takes place by 400 °C. In both forms, films of Ni 2 P have been found to form ohmic contacts to n-InP (Sn doped 2 × 1018cm-3) p-InGaAs (Zn-doped 8 × 1018cm-3) with sufficiently low sheet resistivity and specific contact resistance to be useful as a metallization layer in most devices. It was also found that up to 400 °C, amorphous (as sputtered) films of Ni 2 P function as efficient diffusion barriers between an outer Au layer and the III-V substrate. It is possible, therefore, for amorphous (as sputtered) films of Ni 2 P to function simultaneously both as a metallization layer and diffusion barrier.

Metallization scheme for n−GaAs Schottky diodes incorporating sintered contacts and a W diffusion barrier

Using Schottky barrier measurements on n−GaAs diodes metallized with Au/W and Pt/W films, we have shown that thin films of W (∼3000 Å) offer an excellent barrier to diffusion of both Au and Pt atoms at temperatures up to 500 °C. These properties of W are utilized in a Schottky barrier metallization scheme consisting of Au/W/(PtGa/PtAs2)/n−GaAs, where (a) the thin sintered Pt layer provides a large φB (due to PtAs2 at the GaAs interface) and a metallurgically stable conductor−semiconductor interface, (b) the W layer acts as a diffusion barrier, and (c) the outer Au layer provides thermocompression bondability.