Au Diffusion Research Articles

Transport, microstructure and mechanical properties of Au diffusion-doped Bi-2223 superconductors

We have investigated the effect of the Au-diffusion on the mechanical and transport properties of the (Bi,Pb) 2Sr 2Ca 2Cu 3Oy (Bi-2223) superconducting samples with different annealing times of 10, 20 and 50 h. The samples are prepared by the conventional solid-state reaction method in the polycrystalline bulk form. Doping of Bi-2223 was carried out by means of Au diffusion during sintering from an evaporated Au film on pellets. The experimental works in this study consist of dc electrical resistivity and critical current density measurements for electrical and superconducting properties, microhardness measurements for mechanical properties, powder X-ray diffraction (XRD) for phase analyses (phase ratio) and lattice parameters, and scanning electron microscopy (SEM) for microstructure examination. These measurements showed that Au-doping, in comparison with the undoped samples, increased the critical transition temperature, critical current density and enhanced formation of high- T c phase. Additionally, microhardness and grain size were also improved with increasing amount of diffusion. Moreover, the diffusion-annealing time decreased the number and size of voids and increased the transition temperature. The experimental results of microhardness measurements were analyzed using the Kick's law, modified proportional specimen resistance (MPSR) model and the Hays–Kendall (HK) approach. Kick's law did not give useful knowledge of the origin of the indentation size effect. It was observed that the load independent microhardness values were determined based on the MPSR and HK models, and found to be similar with each other. The possible reasons for the observed enhancement in transport and mechanical properties due to Au diffusion are discussed.

Comparative Studies on Low-Resistance Pd-Based Ohmic Contacts on p-GaAsSb

Ohmic contacts based on , , , , and metallizations have been formed on highly doped thick p-type . All the ohmic contacts with Pd as a bottom layer exhibited better electrical characteristics than conventional metallization at as-deposited conditions. It shows that Pd forms a more favorable metal-semiconductor junction with the layer compared with Ti. When Ir or Pt was incorporated on top of the bottom Pd layer, ohmic characteristics of and were improved by annealing at by the effect of Ir and Pt with high work function. When , , and ohmic metallizations were annealed at temperatures higher than , gradual degradation of ohmic properties was observed. To improve the thermal properties of the Pd-based ohmic contacts, an Mo layer was inserted beneath the top Au layer as a diffusion-barrier layer. The metallization scheme withstood higher annealing temperatures than the ohmic metallizations without the Mo layer. X-ray photoelectron spectroscopy was also carried out to investigate the interactions between the multilayer metallizations and the ternary GaAsSb compound semiconductor. The Mo layer was found to be effective in blocking Au diffusion into the semiconductor layer when it was combined with an Ir layer.

Anomalous diffusion of Au in mega-electron-volt Au implanted SiO2∕Si(100)

Thermal annealing induced redistribution behavior of Au (3 MeV, 6.0×1015 ions cm−2), implanted into SiO2∕Si(100) substrates, has been investigated using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy (XTEM). Sequential annealing in the temperature range of 550−750 °C has been found to result in rejection of Au atoms from Si toward the SiO2∕Si interface. Above 750 °C a significant fraction of the implanted Au atoms has been found to back diffuse into deeper regions, well below the projected range Rp of Au. Direct annealing of a sample at 850 °C also shows a similar anomalous diffusion of Au into deeper regions, well below Rp. However, direct annealing at 1050 °C has been found to result in an enhanced accumulation of Au at a deeper layer. XTEM micrographs for the same sample show the presence of Au-rich nanoparticles and dislocations, decorated with Au-rich nanoparticles in this region. Trails of Au-Si liquid nanodroplets along with dislocations extending into this region have also been observed. The observed enhanced accumulation of Au in the deeper layer has been explained as due to efficient gettering of diffusing Au atoms at dislocations in this region, together with the migration of Au-Si liquid nanodroplets into it.

Role of diffusion-annealing time on the mechanical properties of bulk Bi-2223superconductors diffusion-doped with Au

In order to investigate the role of Au doping and diffusion-annealingduration on the mechanical properties of Bi–Pb–Sr–Ca–Cu–O,Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy superconductors were prepared by standard solid-state reaction methods. Doping ofBi-2223 was carried out by means of gold diffusion during sintering from an evaporatedgold film on pellets. The effect of the diffusion-annealing duration on the mechanical andsuperconducting properties of these compounds was investigated using scanning electronmicroscopy, dc resistivity and hardness measurements. These measurements indicated thatthe Au doping and diffusion-annealing time increased the superconducting transitiontemperature, Vickers hardness, Young’s modulus, yield strength, fracture toughness valuesand improved the grain connectivity. The mechanical properties of the samples werefound to be load dependent. In addition, we calculated the load independenthardness, Young’s modulus, yield strength and fracture toughness of the samples.These mechanical properties of the Au-doped samples were dependent on thediffusion-annealing time. Possible reasons for the observed improvements in thesuperconducting and mechanical properties due to Au diffusion are discussed.

High‐temperature diffusion barriers from Si‐rich silicon‐nitride

AbstractBy radiotracer diffusion experiments, it is shown that the 31Si diffusivity in Si‐rich nonstoichiometric amorphous silicon‐nitride layers (Si3+xN4−x) sharply drops to low values at a temperature of 1075 °C and is unmeasurable at lower temperatures. A similar behavior was found for Au and Ge diffusion in Si3+xN4−x, however, at lower temperatures. Utilizing this diffusion‐blocking effect, thin films of Si3+xN4−x were employed as efficient diffusion barriers for preventing the diffusion of Fe and other constituents from a steel substrate into a Cu(In,Ga)Se2 absorber layer of a thin‐film solar cell. The reduced diffusion consequently allows the employment of higher process temperatures, which can lead to larger grains and an improved cell performance. Copyright © 2006 John Wiley & Sons, Ltd.

Diffusion of Gold into InAs Nanocrystals

Pouring in: The picture shows an STEM image of 7-nm Au-InAs metal–semiconductor hybrid nanoparticles where a gold core (yellow) is surrounded by an amorphous InAs shell (pink). These nanoparticles form by diffusion of Au into InAs particles.

Growth of Au-catalysed Si nanowires by low pressure chemical vapour deposition onSi(100) and amorphous Si surfaces

Au-mediated Si nanowires (SiNW) have been grown at low temperatures(500–560 °C) on crystalline Si(100) and amorphous Si surfaces by means of low pressure chemical vapour depositionfrom Si2H6 in the 0.05–1.2 mbar range. The influence of the substrates on the nanowire (NW) growthand morphology has been investigated by means of x-ray photoelectron spectroscopy andscanning electron microscopy. No NW growth has been observed on the Au coveredamorphous Si surfaces. On both substrates, the NW exhibit inhomogeneous sidewalls and anew morphology showing NW entrenchment which has been explained as a consequence ofvapour–liquid–solid growth termination due to Au diffusion on the SiNW sidewalls.

Composite Ohmic Contacts to SiC

Composite ohmic contacts designed for SiC devices operating in air at 350°C have been studied. Ohmic contacts to n- and p-4H-SiC were protected against inter-diffusion and oxidation by Ta-Si-N layers obtained by sputter deposition from a TaSi2 target in a mixture of Ar and N2. Platinum was sputter-deposited at 250°C to promote adhesion between the Ta-Si- N barrier layer and a thick Au cap layer. Platinum also acts as a barrier to the diffusion of Au. The electrical and mechanical characteristics of the composite contacts were stable after hundreds of hours of annealing in air at 350°C. We report the effects of thermal aging on the specific contact resistance and the semiconductor sheet resistance, and the results of wire bond pull and shear tests following aging for Ta-Si-N / Pt / Au stacks deposited on both SiO2 dielectric layers and the ohmic contact layers.

Thermal Diffusivity of Metallic Thin Films: Au, Sn, Mo, and Al/Ti Alloy

The thermal diffusivity of Au, Sn, Mo, and Al0.97Ti0.03 alloy thin films, which are commonly used in microelectromechanical (MEMs) system applications, is measured by two independent methods — the ac calorimetric and photothermal mirage methods. Both methods yield similar results of the thin-film thermal conductivity, but the uncertainty of the mirage technique is found to be relatively large because of the large temperature increase during the measurement. The measured thermal diffusivities of the thin films are generally lower than those of the same bulk material. Especially, the Al0.97Ti0.03 thin film shows a pronounced thermal conductivity drop compared with bulk Al, which is believed to be mainly due to impurity scattering. Comparison of the thermal conductivity with the electrical conductivity measured by the standard four-probe technique indicates that the relation of thermal and electrical conductivities follows the Wiedemann–Franz law for the case of Au and Sn thin films. However, the Lorentz number is significantly larger than the theoretical prediction for the case of Al0.97Ti0.03 and Mo thin films.

Influence of ion induced amorphicity on the diffusion of gold into silicon

It is experimentally demonstrated that, after ion irradiating 60nm thick Au films on Si substrates with 230keV Ar+ ions, annealing conditions can be found leading to strong diffusional contrasts between bombarded and unbombarded areas. While Au readily diffuses into the bombarded part of the sample at 310°C, its diffusion is still completely blocked under identical conditions in the unbombarded parts. Clear evidence is provided that this diffusional contrast is due to bombardment induced amorphization of the underlying Si substrate. The amorphous Silicon (a-Si), however, has to extend right to the Au∕Si interface, since any intermediate crystalline layer will suppress the diffusional contrast. An example for this latter situation is realized by performing the ion bombardment prior to the evaporation of the top Au layer leading to a still crystalline Si surface layer, which is found to act as a barrier against Au diffusion at 310°C. In accordance with the idea that a-Si, independent of its specific preparation, causes the observed Au diffusion enhancement, the effect is also found for a-Si prepared by evaporation at ambient temperature. In that case an even higher Au diffusion coefficient is obtained than for Si amorphized by ion bombardment pointing to subtle structural differences between both types of amorphous Si.

Reliability of BGA Solder Joints on the Au/Ni/Cu Bond Pad-Effect of Thicknesses of Au and Ni Layer

A systematic experimental work has been carried out to investigate the effect of thicknesses of the Au and Ni layers in the bond pad metallization on the reliability of ball grid array (BGA) solder joints as well as to understand the mechanism of Au diffusion to the solder interface. BGA solder balls of Sn-37 wt.% Pb have been bonded on BGA solder bond pads of Au/electrolytic Ni/Cu by reflowing at 225 degC for 0.5 min. The thickness of the Ni layer was varied from 0.35 to 2.8 mum, while the thickness of Au layer was varied from 0.1 to 1.3 mum. Solid state aging at 150 degC up to 1000 h has been carried out to simulate the ultimate interfacial reactions during the operational life of electronic devices. Cross-sectional studies of the solder-to-bond pad interfaces have been conducted by scanning electron microscopy equipped with an energy-dispersive X-ray analyzer to investigate the interfacial reaction phenomena. Ball shear tests have been carried out to obtain the mechanical strength of the solder joints and to correlate shear behavior with the interfacial reaction products. After the shear tests, fracture surfaces have also been investigated to understand the fracture modes. It has been found that Au embrittlement in the BGA Sn-Pb solder joints on Au/Ni/Cu BGA bond pads can be reduced by using either a thin Au layer or a thin Ni layer in the trilayer metallization of Au/Ni/Cu. It is easy to understand why Au embrittlement is reduced when a thin Au layer in the BGA bond pad is used; however, contrary to the usual expectation, it has been realized in this paper that a very thin Ni metallization is an effective means to maintain the mechanical integrity of the BGA SnPb solder joint. It is revealed that the out-diffusion of the Cu atoms during the aging period changes the chemistry and morphology of the intermetallic compounds (IMCs) at the solder interface, contributing to a significant reduction in brittleness. During aging at 150 degC, the thin Ni layer facilitates Cu diffusion from the bond pad toward IMCs. As a result, the brittle continuous layers of (Au,Ni)Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> are transformed into a nodular-shaped discontinuous layer of AuSn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and a layer of (Au,Cu,Ni) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6 </sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , respectively. New interfacial reaction products with the modified morphology change the typical scenario of Au embrittlement at the BGA solder interface and thus improve the interfacial joint strength significantly

Control of Si Nanowire Growth by Oxygen

Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets. This allows the droplet volumes to remain constant for longer times and therefore permits the growth of untapered wires. This effect is observed for both gas-phase and surface-bound oxygen, so the source of oxygen is unimportant. The control of oxygen exposure during growth provides a new tool for the fabrication of long, uniform-diameter structures, as required for many applications of nanowires.

RBS and AFM study of Ag and Au diffusion into PET foils influenced by plasma treatment

AbstractMetallization of polymers is essential for their application in microelectronic elements and photonics devices. We performed a study of the diffusion of Ag and Au atoms in polyethyleneterephtalate (PET). Thin metal layers were deposited using the diode‐sputtering technique on polymer foils at room temperature. Simultaneous post‐deposition annealing and plasma treatment was used to induce metal/polymer intermixing. Concentration profiles of diffused metals were determined by Rutherford backscattering spectrometry (RBS) and diffusion coefficients were extracted. The influence of the plasma treatment on the surface morphology was studied using atomic force microscopy (AFM). Ag atoms exhibit deeper penetration into polymer structures after Ar plasma treatment than after Ar + O2 plasma treatment. AFM measurements show more significant changes of surface roughness (increased roughness and porosity) of Ag on PET surfaces caused by the Ar + O2 plasma treatment compared to Au surfaces. In the case of very rough surfaces, a decrease of metal diffusivity is observed probably owing to a reduction of surface concentration. The metal mobility is strongly influenced by temperature, which is increased either by direct heating of the sample holder or indirectly by plasma discharge. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Erratum: Adsorption and diffusion of Pt and Au on the stoichiometric and reducedTiO2rutile (110) surfaces [Phys. Rev. B72, 081407(R) (2005)

Erratum: Adsorption and diffusion of Pt and Au on the stoichiometric and reduced<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>TiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>rutile (110) surfaces [Phys. Rev. B<b>72</b>, 081407(R) (2005)

Study of diffusion barriers for Au metal on liquid phase oxidized GaAs

TiW, TiN, Pd, and Mo as the diffusion barriers (DBs) in Au/DB/GaAs native oxide multilayer structures are investigated. The GaAs native oxides are prepared by liquid phase oxidation, and the results indicate that TiW and Mo films can effectively block Au diffusion at temperatures of up to 550°C for 30min. However, TiN and Pd films can effectively block Au diffusion only at 450°C for 30min. The failure of TiN and Pd appears related to the embedded oxygen in the barrier layers which cause the interdiffusion between Au and the barrier films. In comparison, TiW and Mo show better blocking properties that prevent Au from diffusing into oxide films. They also act as a diffusion barrier even at temperatures above 550°C.

Role of the Oxide Layer on the Performances of a-Si:H Schottky Structures Applied to PDS Fabrication

AbstractIn this work we present results of studies performed on Schottky and MIS (metal-insulator – semiconductor) PSD (position sensitive detectors) structures: substrate (glass)/ Cr (300 nm) / a-Si:H [n] (37 nm) / a-Si:H [i] (600 nm) / SiO2 (1.5 nm – for the MIS) / Au (7 nm). The effect of the interfacial oxide layer between Au and a-Si:H, for the MIS structures, was studied and compared with the Schottky, in order to determine how beneficial it could be for device performances and time degradation. For doing so, the Au thickness of 70Å was deposited by thermal evaporation on an oxide free (Schottky) and oxidized (20Å) (MIS) a-Si:H surfaces. These structures were characterized by SIMS, RBS, SEM and AFM in order to correlate the obtained diffusion profile of Au at the interface and the topography with the presence of the oxide at the interface. The results show that the Au inter-diffuses very easily in the oxide free a-Si:H surface, even at room temperature, degrading the devices performance. On the other hand, the MIS structures, with their interfacial oxide present no structural changes after annealing and the PSD produced are stable. We believe that this effect is associated with the barrier effect of the interfacial oxide that prevents the Au diffusion. The optimized 1D MIS sensors are stable and exhibit a linearity error as low as 0.8 % and sensitivities of 33 mV/cm for a 5 mW spot beam intensity at a wavelength of 532 nm, while the Schottky sensors showed a time degradation of their characteristics.

Bismuth X-ray absorber studies for TES microcalorimeters

Bismuth's large atomic number and low carrier density makes it an attractive X-ray absorber material for microcalorimeters. Bismuth's long fermi wavelength and long mean free paths have motivated much interest in the fabrication of high quality bismuth films to study quantum size effects. Despite such incentives, fabrication of high quality bismuth films has proven difficult, and measured properties of such films are highly variable in the literature. Implementing a bismuth deposition process for TES (superconducting Transition Edge Sensor) device fabrication presents additional challenges particularly at interfaces due to the inherent granularity and surface roughness of its films, its low melting point, and its tendency to diffuse and form undesired intermetallic phases. We report observations of Bi–Cu and Bi–Au diffusion in our devices correlating with large shifts in T c (superconducting transition temperature). Using SEM and in situ R vs T annealing experiments we have been able to study these diffusion processes and identify their activation temperatures.

Study of Au Diffusion in Nitrogen-Doped FZ Si

Depth profiles of Au in nitrogen-doped FZ p-Si have been studied by the DLTS after diffusion at 680 - 730°C. It was shown that the Au depth profile could be described by trap-limited diffusion. To explain the experimental profiles the existence of two traps with different capture radius should be assumed. A formation of new electrically active center is detected at a depth close to that of sharp decay in Au concentration. This center is assumed to be a result of Au capture by nitrogen-vacancy complexes and could be associated with the substitutional gold neighboring nitrogen and/or vacancy.

The interaction of Al, Ag, Au, and Ti to Pr 2O 3 thin film dielectrics

We studied the thermal stability of various electrode materials on 2 nm Pr 2O 3 dielectric films. Thin Al, Au, Ag, and Ti layers are deposited, by thermal evaporation or by sublimation (case of Ti). The Pr 2O 3 layers were deposited on Si substrates by electron beam evaporation from a Pr 6O 11 powder and by wet chemical deposition. XPS and SR-PES were used to study the effect of annealing on the metal to Pr 2O 3 interaction. XPS results show a strong diffusivity of Al, Ag, and Au in Pr 2O 3 upon annealing to 300 °C with formation of alloying with Si substrate. In contrast, Ti remained stable even upon annealing to 900 °C. SR-PES results show a stable Titanium oxides formation at room temperature. All results demonstrate that Ti is a good diffusion barrier between metal contacts and Pr 2O 3 or can be used as a metal contact.

Growth and structure of small gold particles on rutileTiO2(110)

The growth and structure of small ${\mathrm{Au}}_{n}$ particles $(n=1\text{--}4)$ on a rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface have been examined using gradient corrected density functional theory slab calculations. We present potential energy maps for single Au atoms on the stoichiometric and reduced surfaces. This comparison shows that the presence of oxygen vacancies on $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ drastically alters the adsorption and surface diffusion of single Au atoms, and in turn the growth and structure of Au particles. On the reduced surface, the delocalization of electrons from oxygen vacancies provides a low-energy diffusion channel for Au adatoms along a $\mathrm{Ti}(5c)$ row, while there is no preferential direction in Au diffusion on the stoichiometric surface. The small Au particles bind preferably to the vacancy site, with a sizable adsorption energy that oscillates with the number of constituent atoms by virtue of spin pairing. Based on the comparison of supported and gas-phase Au particles, we also discuss the effect of the particle-substrate interaction on the structure of small Au particles grown on $\mathrm{Ti}{\mathrm{O}}_{2}(110)$.