Au-Ge System Research Articles

Synthesis, structure, and physical properties of a Y–Au-Ge 1/1 Tsai-type quasicrystal approximant and Y14(Au,Ge)51 with the Gd14Ag51 structure type

Exploration of the Au–Ge rich part of the pseudo-binary section Yx(Au0.70Ge0.30)100-x (3 < x < 18) in the Y–Au-Ge system afforded the intermetallic compounds Y3(Au0.7054Ge0.2946)19-x (x ≈ 0.38) and Y14(Au0.8313Ge0.1687)51, which are new representatives of the cubic 1/1 Tsai-type approximant crystal (YbCd6 parent type, space group Im3‾.) and the hexagonal Gd14Ag51 structure type (space group P6/m), respectively. Y3(Au0.7054Ge0.2946)19-x decomposes peritectically into Y14(Au,Ge)51 phase and melt at around 700 °C. In this respect the Y–Au-Ge system deviates from the Y–Au–Si system which firstly contains a second type of 1/1 approximant crystal phase with higher Y content, Y3.25(Au,Si)18-x (x ≈ 0.11) and, secondly, adjacent at higher Y composition there is a stoichiometric and ordered tetragonal phase YAu3Si. Y3(Au,Ge)19-x and Y14(Au,Ge)51 exhibit bulk superconductivity of conventional type-II BCS type albeit with very low transition temperatures (below 1 K), which is very similar to the corresponding compounds in the Y–Au–Si system.

Off-Eutectic Au–Ge Die-Attach—Microstructure, Mechanical Strength, and Electrical Resistivity

In this paper, off-eutectic Au–Ge joints were formed between Si substrates to investigate their high-temperature compatibility. High-quality joints made with small bond pressure, 53 kPa, were fabricated. The joints comprised three different types of morphologies: 1) a layered structure of Au/Au–Ge/Au; 2) a layered structure of Au/Au–Ge/Au where some sections of the central Au–Ge band have been replaced by an Au section that extended across the entire section; and 3) a roughly homogenous Au layer comprising the primary $\alpha $ phase. The average Ge concentration was 10 ± 2 at%. Joints formed with a higher bond line pressure, 7.6 MPa, were of a reduced quality with voids and cracks at the original bond line. Annealing at 400 °C for 1000 h transformed the microstructure into an Au–Ge–Si compound with Au precipitates. The shear strength of the fabricated joints was found to be at least 50 MPa, and the fracture mode was an adhesive fracture between the adhesion layer and the die or substrate. Heated dies detached from the substrates at 460 °C, i.e., more than 100 °C above the eutectic melting point of the binary Au–Ge system. Electrical resistivity measurements confirmed a melting process at the eutectic melting point by an abrupt increase in resistivity.

Theoretical analysis of the axial growth of nanowires starting with a binary eutectic droplet via vapor-liquid-solid mechanism

A series of theoretical analysis is carried out for the axial vapor-liquid-solid (VLS) growth of nanowires starting with a binary eutectic droplet. The growth model considering the entire process of axial VLS growth is a development of the approaches already developed by previous studies. In this model, the steady and unsteady state growth are considered both. The amount of solute species in a variable liquid droplet, the nanowire length, radius, growth rate and all other parameters during the entire axial growth process are treated as functions of growth time. The model provides theoretical predictions for the formation of nanowire shape, the length-radius and growth rate-radius dependences. It is also suggested by the model that the initial growth of single nanowire is significantly affected by Gibbs-Thompson effect due to the shape change. The model was applied on predictions of available experimental data of Si and Ge nanowires grown from Au-Si and Au-Ge systems respectively reported by other works. The calculations with the proposed model are in satisfactory agreement with the experimental results of the previous works.

Thermal and structural characteristics of a eutectic Au-Ge alloy

Abstract The Au-Ge alloy is useful for bonding applications in microelectronic and optoelectronic packages. Also, recent investigations show that Au-Ge system could present a lead-free solder candidate. The results of characterization of the eutectic alloy from binary Au-Ge system are presented in this paper. Experimental research was done using thermal analysis (differential thermal analysis, DTA) and structural investigation by light optic microscopy (LOM) and scanning electron microscopy with EDS (SEM-EDS). The eutectic temperature of 359 °C was detected using thermal analysis, while eutectic structure was confirmed using microstructural analysis. Obtained results were compared with recent literature references and good mutual agreement was noticed.

Dynamics of thermally induced assembly of Au nanoislands from a thin Au layer on Ge(001)

The dynamics of morphology changes upon post-deposition annealing of a thin film of 6 ML Au deposited on a (2 × 1) reconstructed Ge(001) surface was studied in a systematic way by scanning electron and atomic force microscopy. It has been found that annealing of the initial, continuous Au layer leads to the development of nanoislands which are crystalline and epitaxial to the Ge(001) substrate surface. The nanoislands formed at annealing temperatures lower than the bulk eutectic temperature for the Au–Ge system (634 K) are rectangular, with a flat top, while the nanoislands developed by annealing at temperatures higher than the bulk eutectic temperature have more complex, octagonal shapes. Moreover, the octagonal Au nanoislands are nearly half-buried in the Ge substrate and surrounded by a crystalline Ge rim.

Generation of AuGe nanocomposites by co-sparking technique and their photoluminescence properties

The feasibility of spark discharge technique for preparing metal–semiconductor nanocomposites is demonstrated. In the AuGe system, Au shows only 10−3 atomic percent solid solubility in Ge, whereas 3.1 at.% Ge is soluble in Au. During the co-sparking, Au is used as anode material; the cathode is composed of Ge. The relative atomic percent of Au and Ge in the initially generated mixture can be changed by changing the charging current to the capacitor used to trigger the sparking. Depending upon the atomic ratio of Au and Ge in the initial mixture, AuGe agglomerates form AuGe composite nanoparticles on subsequent sintering, in which AuGe alloy nanoparticles are found dispersed in a Ge matrix. The size of the dispersed AuGe alloy nanoparticles depend on the relative atomic concentration of Au and Ge in the initial mixture as well as on the sintering temperature. AuGe alloy nanoparticles dispersed in the Ge matrix are observed to exhibit an intense photoluminescence between 550 and 600 nm.

Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces

We report on the phase separation in Au–Ge system leading to the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under ultra high vacuum (UHV) conditions (≈3 × 10−10 mbar) on clean Si(100) surfaces. For this study, ≈2.0 nm thick Au samples were grown on the substrate surface by molecular beam epitaxy. Thermal annealing was carried out inside the UHV chamber at temperature ≈500 °C and following this, nearly square shaped AuxSi1-x nano structures of average length ≈48 nm were formed. A ≈2 nm Ge film was further deposited on the above surface while the substrate was kept at a temperature of ≈500 °C. Well ordered Au-Ge nanostructures where Au and Ge residing side by side (lobe-lobe structures) were formed. In our systematic studies, we show that, gold-silicide nanoalloy formation at the substrate (Si) surface is necessary for forming phase separated Au-Ge bilobed nanostructures. These results show that the Au–Ge bonding is unstable in nature. Electron microscopy (TEM, STEM–EDS, SEM) studies were carried out to determine the structure of Au–Ge nano systems. Rutherford backscattering spectrometry measurements show gold inter-diffusion into substrate while it is absent for Ge.

ChemInform Abstract: M3(Au,Ge)19 and M3.25(Au,Ge)18 (M: Ca, Yb): Distinctive Phase Separations Driven by Configurational Disorder in Cubic YCd6‐Type Derivatives.

AbstractTwo cleanly separated nonstoichiometric phases M3Au14.4Ge4.6 (I) and M3.25Au12.7Ge5.3 (II) (M: Ca, Yb) exist in the M—Au—Ge systems and are characterized by single crystal XRD and DFT electronic structure calculations.

Existence of a stable compound in the Au-Ge alloy system

First-principles electronic structure calculations predict the existence of a crystalline compound in the Au-Ge system. The structure is found by matching the theoretically determined local atomic structure in the liquid state with that for experimentally known crystal structures in other alloys. Subsequently, the best matching crystalline structures were structurally optimized using first-principles methods. Surprisingly, although Au-Ge is known as a non-compound-forming system, a crystal structure was found to be more stable than the terminal phases by about 6 meV/atom at $T=0\text{ }\text{K}$. Possibly, this structure can be prepared by a suitably chosen substrate such as ${\text{Pt}}_{5}{\text{P}}_{2}$.

Nanoscale Structural Engineering via Phase Segregation: Au−Ge System

A tunable structural engineering of nanowires based on template-assisted alloying and phase segregation processes is demonstrated. The Au-Ge system, which has a low eutectic temperature and negligible solid solubility (<10(-3) atom %) of Au in Ge at low temperatures, is utilized. Depending on the Au concentration of the initial nanowires, final structures ranging from nearly periodic nanodisk patterns to core/shell and fully alloyed nanowires are produced. The formation mechanisms are discussed in detail and characterized by in situ transmission electron microscopy and energy-dispersive spectrometry analyses. Electrical measurements illustrate the metallic and semiconducting characteristics of the fully alloyed and alternating Au/Ge nanodisk structures, respectively.

Chemical composition anomalies of nanotubes stretched from Au-Si(Ge) liquid alloy by an intense electric field

Nanotubes are stretched by intense electric fields (few volts per angstrom) supplied by an eutectic Au0.73Ge0.27 or Au0.82Si0.18 alloy liquid metal ion source. Chemical composition anomalies are observed in the stretched nanotubes. In the Au-Ge system, nanotubes are the smaller two-shell pure gold nanowires. In the Au-Si system, nanotubes have same geometric structures as previously but now silicon atoms can substitute gold atoms. Nanotubes are mainly formed by SiAu3 alloy zones separated by smaller pure gold parts. Our results are consistent with local modifications of the chemical composition of the alloy liquid in the nanotube vicinity submitted to intense electric field gradients.

Phase Equilibrium and Nucleation in VLS-Grown Nanowires

Phase diagrams accounting for capillarity and surface stress in VLS-grown nanowires have been calculated, and linearized forms for the compositions of the solid and liquid are given. The solid-vapor interfacial energy causes a significant depression of the liquidus, and the impurity concentration in the wire decreases with decreasing wire diameter. Nucleation calculations give upper bounds on the nucleation temperature and liquid supersaturation during growth that are consistent with measurements in the Au-Ge system.

Conditions for subeutectic growth of Ge nanowires by the vapor-liquid-solid mechanism

The requirement of the presence of a liquid for the widely accepted vapor-liquid-solid (VLS) mechanism suggests that nanowire growth should be carried out at temperatures above the catalyst-nanowire eutectic melting point. In this paper, we examine the nanowire growth at the deep subeutectic temperatures often employed for germanium nanowire growth with Au catalyst nanoparticles. The effects of germane partial pressure on the low temperature limit for Au-catalyzed Ge nanowire growth by chemical vapor deposition from GeH4(g) were studied thoroughly, both with and without initial heating above the bulk eutectic temperature. A model to explain the temperature and pressure limits of subeutectic VLS nanowire growth was developed. The interdependence of the pressure and temperature required for good quality germanium nanowire growth may also apply in other cases of nanowire growth, beyond the Au–Ge system.

TEM Studies on Phase Stability in Nanometer-Sized Alloy Particles

TEM is one of the most powerful experimental tools for the study of nanometer-sized particles. In the present work, the finite size effect on both the stability of two-phase microstructure and the solid solubility has been examined by in situ TEM using particles in the Au-Ge system. The size effect on the two-phase microstructure is rather small when the size of particles is larger than approximately 10 nm in diameter. However, the effect becomes strong in particles smaller than about 10 nm in diameter and an amorphous structure appears instead of the crystalline two-phase microstructure. The solid solubility in each solid solution in nanometer-sized alloy particles with a two-phase microstructure gradually increased as the particle size decreased. The enhancement of solid solubility was large in gold solid solution as compared with the in germanium solid solution.

Phases around the 1:1:1 composition in the Yb–Au–Ge and Ca–Au–Ge systems

The systems YbAu 2− x Ge x and CaAu 2− x Ge x with 0.7< x<1.3 were structurally examined by X-rays on powders and single crystals. In this range of composition nine phases occur, belonging to six closely related structure types: CeCu 2, AlB 2, EuAuGe (occupation derivative), SrAgGe, α-YbAuGe and β-YbAuGe (the last two being centrosymmetric variants of the CaCuGe type). The equiatomic compound CaAuGe shows a phase transition at 1238 K. YbAuGe crystallizes in three different structural modifications as a function of temperature: α-YbAuGe up to 1003 K, β-YbAuGe in the range 1003 to 1308 K and γ-YbAuGe at T>1308 K. Resistance measurements carried out in the range 14–400 K for the high-temperature forms of YbAuGe and CaAuGe show normal metallic behaviour. Magnetic susceptibility data of YbAuGe are in agreement with the occurrence of divalent ytterbium.

Specific contact resistance of the Ni/AuGe/nGaP system

This work studies the specific contact resistance for Ni/Au-Ge/nGaP system at the rectifying regime, i.e. for which the heat-treatment temperature is below 400°C and the I–V characteristics exhibit a rectifying behavior. The specific contact resistance is first computed by using the generalized majority carrier transport theory derived by Chang and Sze [1]. The computed theoretical results are then used to interpret the experimental data which are obtained by measuring the specific contact resistance, at zero bias, as a function of the temperature for as-deposited and heat-treated Ni/Au-Ge/nGaP Schottky diodes. Au/nGaP Schottky diodes are also fabricated to verify the theoretical results. It is found that the barrier height for the Ni/Au-Ge/nGaP system rises from the as-deposited value, 1.10±0.04 eV, to the value of the Ni/nGaP system, 1.27∓0.02 eV, as the contact is heat-treated at various temperatures up to 360°C, the eutectic point of the Au-Ge system, and drops rapidly as the contact is heat-treated above 360°C. The barrier height rise is believed to be caused by the Ni in-diffusion toward the Au-Ge/nGaP interface during the heat-treatment. The smaller temperature dependence of the specific contact resistance for Schottky diode samples heat-treated above 360°C indicates that after heat-treatment above this temperature, an n + layer is formed on the GaP surface. The theoretically computed results are used to fit the experimentally measured data to obtain the effective n + doping concentrations and barrier heights.

Structure of liquid alloys of the Au-Si and Au-Ge systems

A study of the liquid alloys of the simple eutectic systems Au-Si and Au-Ge at selected compositions has demonstrated a pattern of behaviour very similar to that seen in the related gold-based system Au-Sn. The sharp X-ray diffraction profile of Au-Ge liquid alloys at the gold-rich end demonstrates that the structure, like that of pure Au, is simple, close-packed but with a mean closet distance for neighbouring atoms smaller than that for liquid Au or Ge. This result is also obtained for liquid Au-Si alloys. The packing in the liquid phase can only be interpreted in terms of A-B bonding, i.e. unlike atom interaction, a feature attributed to a high electrochemical factor. With higher Ge content, the X-ray diffraction profile broadens in a manner consistent with a more open liquid structure. The author accounts for the latter in terms of the increasing influence of Ge-Ge nearest neighbour packing.