Au Ga Research Articles

High- and Low-Energy Photoemission Study of Strongly Correlated Au–Ga–Ce Quasicrystal Approximants: Localized 4f Nature and Disorder Effects

High- and Low-Energy Photoemission Study of Strongly Correlated Au–Ga–Ce Quasicrystal Approximants: Localized 4<i>f</i> Nature and Disorder Effects

Origin of near-failure in Au contacts to polycrystalline β-Ga2O3 at high temperatures using interfacial studies

Suitable contacts to gallium oxide are a controversial topic with contact behavior depending heavily on the pre- and post-processing conditions. Especially for the extreme environment applications such as those involving high temperatures, contact chemistry is varied and severely lacks understanding. Herein, we report on conventional pure Au contacts to polycrystalline β-Ga2O3, used as Schottky contacts, and explore the origin of their near-failure at high temperature up to 850 °C. For this purpose, β-Ga2O3 with Au interdigitated electrodes is subjected to high temperature annealing and their interface chemistry is studied and correlated with device performance for solar-blind photodetection. Around the optimized temperature of 450 °C, the performance of the PDs is found to be maximum, whereas it reduces drastically at 850 °C. Physical damage to the electrodes along with the formation of intermetallic gold-gallium alloy is observed via XPS depth profile studies and found to be the reason for the near-failure of device at extreme conditions. Although the alloy formation begins to slightly appear at 650 °C and reduces the performance, still it does not lead to device breakdown. This study proves that unlike its counterparts GaN and GaAs, which have reported alloy formation at lower temperatures, β-Ga2O3 shows a higher resilience to the formation of Au–Ga alloy and can withstand higher temperatures before the actual device failure is reached. The proposed study shows the stability of standard metal contacts to Ga2O3 based devices, which have far-reaching implications for the future commercialization of wideband gap semiconductor based (opto)electronics.

Magnetic properties investigation of the Au–Ga–Tb 1/1 approximant crystals with single ferromagnetic and successive ferromagnetic/antiferromagnetic transitions

Magnetic properties investigation of the Au–Ga–Tb 1/1 approximant crystals with single ferromagnetic and successive ferromagnetic/antiferromagnetic transitions

Thermoelectric properties of Tsai-type Au–(Al, Ga, In)–Gd approximants

In this work, the thermoelectric properties of Tsai-type Au–(Al, Ga, In)–Gd approximant crystals (ACs) are investigated over a temperature range of 300–773 K. The changes in lattice constants and effective speed of sound suggest a weaker chemical bonding caused by a replacement of Al by heavier Ga or In atoms. Based on electrical conductivity measurements, Au–Al–Gd shows the deepest pseudogap in spectrum conductivity at the Fermi level, followed by Au–Ga–Gd and Au–In–Gd ACs. The lattice thermal conductivity at 300 K is as low as 0.71–0.81 W m−1 K−1 and asymptotically approaches a minimum lattice thermal conductivity of 0.52–0.59 W m−1 K−1 as calculated using the Cahill model. As a result, the highest power factor and dimensionless figure of merit reach ∼120 µW m−1 K−2 and ∼0.008 at ∼700 K for the Au–In–Gd system. Our systematic approach of weakening the chemical bonding nature by heavy atom substitution promises a better thermoelectric performance of Tsai-type ACs.

Post-nucleation evolution of the liquid–solid interface in nanowire growth

We study using in situ transmission electron microscopy the birth of GaAs nanowires from liquid Au–Ga catalysts on amorphous substrates. Lattice-resolved observations of the starting stages of growth are reported here for the first time. It reveals how the initial nanostructure evolves into a nanowire growing in a zincblende 〈111〉 or the equivalent wurtzite〈0001〉 direction. This growth direction(s) is what is typically observed in most III–V and II–VI nanowires. However, the reason for this preferential nanowire growth along this direction is still a dilemma. Based on the videos recorded shortly after the nucleation of nanowires, we argue that the lower catalyst droplet-nanowire interface energy of the {111} facet when zincblende (or the equivalent {0001} facet in wurtzite) is the reason for this direction selectivity in nanowires.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Consequence of Galvanic Displacement Reaction on Digital Photocorrosion of GaAs/Al0.35Ga0.65As Nanoheterostructures

GaAs/AlGaAs semiconductor nanoheterostructures have found attractive application in the field of biosensing based on the effect of digital photocorrosion (DIP). The sensitivity of semiconductor–nanoheterostructure-based biosensors depends on the precision of controlling the process of DIP, which is highly sensitive to the surface presence of electrically charged biomolecules. To explore further this process, we have investigated the role of a galvanic displacement (GD) reaction on DIP of GaAs/Al0.35Ga0.65As (001) nanoheterostructures. Deposition of ionic gold on the GaAs surface induces spontaneous electron transfer between the semiconductor and ionic gold, which affects the photocorrosion of GaAs/AlGaAs layers observed simultaneously with the photoluminescence effect. The immediate consequence of the electron transfer from GaAs toward Au3+ is a significantly increased rate of DIP. At the same time, the formation of neutralized gold nanoparticles and Au–Ga alloy takes place on the surface of photocorroding nanoheterostructures. In the presence of 2.8% solution of ammonia and 0.1 mM gold chloride, a significantly reduced rate of deposition of gold nanoparticles is observed. This allows achieving layer-by-layer removal of the investigated material, which is sensitive to perturbations induced by surface immobilized electrically charged molecules. We have elaborated various factors stimulating photocorrosion of the GaAs/Al0.35Ga0.65As nanoheterostructures and we demonstrate the biosensing potential of an innovative GD-based DIP sensor.

Effect of Ca element on oxygen content, wetting and spreading properties of Au–Ga filler metal

To meet the low oxygen content requirement on filler metals used in high reliable vacuum brazing of power device, Ca was doped into the Au–Ga filler metal, and the effects of Ca on the oxygen content, the wetting and spreading properties of the filler metal were investigated. The results reveal that Ca can effectively decrease the oxygen content in the Au-Ga-Ca filler metal, and the wettability and spreadability of the Au-Ga-Ca filler metal on the Cu substrate were significantly improved. With 0.004 wt % of Ca addition, the oxygen content in the filler metal decreased to 0.0032 wt % and the spreading area increased to a peak value of74.6 mm2. Proper amount of Ca addition can also effectively restrain the growth of intermetallic compounds (IMC) layer at the vacuum brazed filler metal/Cu interface and improve the shear strength of the joints. For the Cu/Au-Ga-0.004Ca/Cu joints vacuum brazed at 480 °C and a vacuum degree of 5 × 10−3 mbar, the IMC/filler metal interface is flat and the IMC layer is uniform in thickness, and shear strength of the joint was 48.3 MPa, 49.5% higher than the Cu/Au–Ga/Cu joint. It is demonstrated that the Au-Ga-0.004Ca filler metal has the best combination property, and high ratio of brazed area can be obtained with Au-Ga-0.004Ca filler metal at a proper vacuum degree in brazing of real components.

Thermodynamic characterization of solder Au–Ga alloys

Differential thermal analysis, Oelsen calorimetry as well as optical microscopy are used for thermodynamic characterization of Au–Ga solder alloys. Mechanical and electrical characteristic of the investigated alloys is done by measuring of hardness, micro hardness and electro conductivity. Obtained activities and activity coefficients at 873K are smaller than the unit, which means there is a negative deviation of the gallium activity from the Raoult law. Partial mixing and excess Gibbs's energy of gallium is negative for all investigated alloys and do not reach −1800 J/mol for the excess energy and −4400 J/mol for the mixing energy which indicates good miscibility regardless of the existence of an intermetallic compound (AuGa2) in the alloy that microstructural analysis is shown. The obtained good miscibility is excellent for solder characteristics of the investigated alloys.

Kinetics of Au-Ga Droplet Mediated Decomposition of GaAs Nanowires.

Particle-assisted III-V semiconductor nanowire growth and applications thereof have been studied extensively. However, the stability of nanowires in contact with the particle and the particle chemical composition as a function of temperature remain largely unknown. In this work, we use in situ transmission electron microscopy to investigate the interface between a Au-Ga particle and the top facet of an ⟨1̅1̅1̅⟩-oriented GaAs nanowire grown via the vapor-liquid-solid process. We observed a thermally activated bilayer-by-bilayer removal of the GaAs facet in contact with the liquid particle during annealing between 300 and 420 °C in vacuum. Interestingly, the GaAs-removal rates initially depend on the thermal history of the sample and are time-invariant at later times. In situ X-ray energy dispersive spectroscopy was also used to determine that the Ga content in the particle at any given temperature remains constant over extended periods of time and increases with increasing temperature from 300 to 400 °C. We attribute the observed phenomena to droplet-assisted decomposition of GaAs at a rate that is controlled by the amount of Ga in the droplet. We suggest that the observed transients in removal rates are a direct consequence of time-dependent changes in the Ga content. Our results provide new insights into the role of droplet composition on the thermal stability of GaAs nanowires and complement the existing knowledge on the factors influencing nanowire growth. Moreover, understanding the nanowire stability and decomposition is important for improving processing protocols for the successful fabrication and sustained operation of nanowire-based devices.

Influence of Silicon on the Nucleation Rate of GaAs Nanowires on Silicon Substrates

Despite the unavoidable presence of silicon atoms in the catalyst alloy droplets during the vapor–liquid–solid growth of III–V nanowires on silicon substrates, it remains unknown how the nucleation of nanowires is affected by these foreign atoms. In this work, we present the first attempt to quantify the nanowire nucleation rate versus the silicon concentration in the droplet. We calculate the chemical potential difference per Ga–As pair in the quaternary Au–Ga–As–Si liquid alloy droplet and in solid state, and compare it to the ternary Au–Ga–As droplet without silicon. This allows us to compute the nucleation rates of GaAs nanowires versus the silicon concentration under different conditions. We find that the presence of silicon in the droplet decreases the nucleation probability of GaAs nanowires for gallium-rich droplets (with the gallium contents cGA greater than 0.6) and increases it for gold-rich droplets (cGA < 0.6). The model is used to explain our experimental data for hydride vapor phase epitaxy...

Controlled Growth of Heterostructured Ga/GaAs Nanowires with Sharp Schottky Barrier

Because of the inevitable Fermi level pinning on surface/interface states of nanowires, achieving high-performance nanowire devices with controllable nanoscale contacts is always challenging but important. Herein, single-crystalline heterostructured Ga/GaAs nanowires with sharp hetero-Schottky interfaces have been successfully synthesized on amorphous substrates by utilizing Au nanoparticles as catalytic seeds via chemical vapor deposition. These nanowires are found to grow with the hemispherical Au7Ga2 catalytic tips following the vapor–liquid–solid mechanism. During the growth, simply by manipulating the source and growth temperatures, the Ga precipitation rate from Au–Ga alloy tips as well as the reaction rate of Ga precipitates with As can be reliably controlled in order to tailor the length (0–170 nm) of Ga nanowire segments obtained in the heterostructure. When configured into field-effect transistors, these Ga/GaAs NWs exhibit the p-type conductivity with a sharp hetero-Schottky barrier of ∼1.0 eV at the atomically connected Ga segment/GaAs NW body interface, in which this barrier height is close to the theoretical difference between the GaAs Fermi level (5.1–5.3 eV) and the Ga work function (∼4.3 eV), suggesting the effective formation of nanoscale contact by minimizing the Fermi level pinning, being advantageous for advanced nanoelectronics.

N-type doping and morphology of GaAs nanowires in Aerotaxy

Controlled doping in semiconductor nanowires modifies their electrical and optical properties, which are important for high efficiency optoelectronic devices. We have grown n-type (Sn) doped GaAs nanowires in Aerotaxy, a new continuous gas phase mass production technique. The morphology of Sn doped nanowires is found to be a strong function of dopant, tetraethyltin to trimethylgallium flow ratio, Au–Ga–Sn alloying, and nanowire growth temperatures. High temperature and high flow ratios result in low morphological quality nanowires and in parasitic growth on the wire base and surface. Alloying and growth temperatures of 400 °C and 530 °C, respectively, resulted in good morphological quality nanowires for a flow ratio of TESn to TMGa up to 2.25 × 10−3. The wires are pure zinc-blende for all investigated growth conditions, whereas nanowires grown by metal-organic vapor phase epitaxy with the same growth conditions are usually mainly Wurtzite. The growth rate of the doped wires is found to be dependent more on the TESn flow fraction than on alloying and nanowire growth temperatures. Our photoluminescence measurements, supported by four-point probe resistivity measurements, reveal that the carrier concentration in the doped wires varies only slightly (1–3) × 1019 cm−3 with TESn flow fraction and both alloying and growth temperatures, indicating that good morphological quality wires with high carrier density can be grown with low TESn flow. Carrier concentrations lower than 1019 cm−3 can be grown by further reducing the flow fraction of TESn, which may give better morphology wires.

Phase diagrams for understanding gold-seeded growth of GaAs and InAs nanowires

Phase diagrams are useful tools to study the phase equilibria of nanowire materials systems because the growth of nanowires is accompanied by phase formation and phase transition. We have modeled the phase equilibria of the As–Au–Ga ternary system by means of the CALPHAD method. This method is a well-established semi-empirical technique for thermodynamic modeling in which Gibbs energy functions with free parameters are defined for all phases in a system followed by adjusting these parameters to the experimental data. Using the resulting As–Au–Ga thermodynamic database, four vertical cuts of this ternary system are calculated and all show good agreement with experiments. This ternary system is particularly useful for predicting the state of the Au seed alloys when growing GaAs nanowires and we discuss such predictions. Similar calculations are performed for Au-seeded InAs nanowires. We show that the vapor–liquid–solid (VLS) growth fails for InAs nanowires, while GaAs nanowires can grow from a liquid particle. Our calculations are in agreement with experimental data on the growth of Au-seeded GaAs and InAs nanowires.

Layered Structures and Disordered Polyanionic Nets in the Cation-Poor Polar Intermetallics CsAu1.4Ga2.8 and CsAu2Ga2.6

Gold intermetallics are known for their unusual structures and bonding patterns. Two new compounds have been discovered in the cation-poor part of the Cs–Au–Ga system. Both compounds were obtained directly by heating the elements at elevated temperatures. Structure determinations based on single-crystal X-ray diffraction analyses revealed two structurally and compositionally related formations: CsAu1.4Ga2.8 (I) and CsAu2Ga2.6 (II) crystallize in their own structure types (I: R3, a = 11.160(2) A, c = 21.706(4) A, Z = 18; II: R3, a = 11.106(1) A, A, c = 77.243(9) A, Z = 54) and contain hexagonal cationic layers of cesium. This is a unique structural motif, which has never been observed for the other (lighter) alkali metals in combination with Au and post transition elements. The polyanionic part is characterized in contrast by Au/Ga tetrahedral stars, a structural feature that is characteristic for light alkali metal representatives, and disordered sites with mixed Au/Ga occupancies that occur in both str...

Enthalpies of mixing of liquid Ag–Ga, Au–Ga and Ag–Au–Ga alloys

A literature overview of Ag-Au-Ga system shows that the information about the liquid phase of this system does not exist. Therefore, using MHTC 96 Setaram high temperature drop calorimeter, partial and integral enthalpies of mixing of liquid alloys were determined at first in the binary systems Ag-Ga, Au-Ga and finally in ternary Ag-Au-Ga system. Next, the ternary alloys were investigated along two sections at two temperatures: 1223K and 1323K. Experimental results were used to find ternary interaction parameters by applying the Redlich–Kister–Muggianu model for substitutional solutions, and consequently a full set of parameters describing the concentration dependence of the enthalpy of mixing was derived. The experimental results indicate that the heat of mixing in this system is temperature independent, at least in the measured temperature range.

Density-functional investigation of gold cluster anions doped with gallium: Au n Ga– (1 ⩽ n ⩽ 8)

The equilibrium geometric, relative stabilities, electronic and magnetic properties of small AunGa– clusters (1 ⩽ n ⩽ 8), in comparison with pure gold cluster anions are investigated systematically within the framework of the first-principles density functional calculations at the PW91PW91 level. The calculations reveal that the impure gallium atom changes the structure of pure gold clusters. The most stable structures of AunGa– (4 ⩽ n ⩽ 8) prefer three-dimensional structures. The VDE, fragmentation energies and the secondorder difference energies and the HOMO–LUMO gaps for the lowest-energy structures of AunGa– and Au-n+1 (1 ⩽ n ⩽ 8) clusters show an even-odd oscillation along with the cluster size. Meanwhile the above results indicate the doping gallium atom enhance the stability of the clusters. It is must be pointed out that Au2Ga– cluster is the most stable. Finally, we research the natural population analysis (NPA) of Au-n+1 (1 ⩽ n ⩽ 8) clusters. The results indicate that Ga atom attracts electron from the Au atom except for the Au Ga–cluster.

Impact of nucleation conditions on diameter modulation of GaAs nanowires

Diameter-modulated nanowires can be used to impart unique properties to nanowire-based devices. Here, diameter modulation along Au-seeded GaAs nanowires was achieved by varying the flux of the III and V precursors during growth. Furthermore, three different types of [111]B-oriented nanowires were observed to display distinct differences in diameter modulation, growth rate, and cross-sectional shape. These differences are attributed to the presence of multiple distinct Au–Ga seed particle phases at the growth temperature of 420 °C. We show that the diameter modulation behavior can be modified by the growth conditions during nanowire nucleation, including temperature, V/III ratio, substrate orientation, and seed particle size. These results demonstrate the general viability of flow-controlled diameter modulation for compound semiconductors and highlight both opportunities and challenges that can arise from using compound-forming alloys to seed nanowire growth.

ChemInform Abstract: Crystal Structure and Bonding in BaAu5Ga2 and AeAu4+xGa3‐x (Ae: Ba and Eu): Hexagonal Diamond‐Type Au Frameworks and Remarkable Cation/Anion Partitioning in the Ae—Au—Ga Systems.

AbstractThe new polar intermetallic compounds BaAu5Ga2 (I), BaAu4.3Ga2.7 (II), Ba1.0Au4.5Ga2.4 (III), EuAu4.8Ga2.2 (IV), and Eu1.1Au4.4Ga2.2 (V) are prepared from the elements (Ta ampules, 500—1000 °C, 3 h—7 d) and characterized by single crystal XRD, magnetic measurements, and TB‐LMTO‐ASA electronic structure calculations.

Crystal structure and bonding in BaAu5Ga2 and AeAu4+xGa3-x (Ae = Ba and Eu): hexagonal diamond-type Au frameworks and remarkable cation/anion partitioning in the Ae-Au-Ga systems.

Five new polar intermetallic compounds in the Ae-Ga-Au system (Ae = Ba, Eu), BaAu(5)Ga(2) (I), BaAu(4.3)Ga(2.7) (II), Ba(1.0)Au(4.5)Ga(2.4 )(III), EuAu(4.8)Ga(2.2) (IV), and Eu(1.1)Au(4.4)Ga(2.2) (V), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. I crystallizes in the orthorhombic crystal system with a large unit cell [Pearson symbol oP64; Pnma, Z = 8, a = 8.8350(5) Å, b = 7.1888(3)Å, c = 20.3880(7) Å], whereas all other compounds are hexagonal [hP24; P6̅2m, Z = 3, a = 8.54-8.77(1) Å, c = 7.19-7.24(1) Å]. Both structures contain mutually orthogonal layers of Au(6) hexagons in chair and boat conformations, resulting in a hexagonal diamond-like network. Ae atoms and additional (Au/Ga)(3) groups are formally encapsulated by (Au(6))(2) distorted hexagonal prisms formed of three edge-sharing hexagons in the boat conformation or, alternatively, lie between two Au(6) hexagons in the chair conformation. The (Au/Ga)(3) groups can be substituted by Ae atoms in some of the hexagonal structures with no change to the structural symmetry. Tight-binding electronic structure calculations using linear-muffin-tin-orbital methods on idealized models "BaAu(5)Ga(2)" and "BaAu(4)Ga(3)" show both compounds to be metallic with evident pseudogaps near the corresponding Fermi levels. The integrated crystal orbital Hamilton populations are dominated by Au-Au and Au-Ga orbital interactions, although Ba-Au and Ba-Ga contributions are significant. Furthermore, Au-Au interactions vary considerably along different directions in the unit cells, with the largest values for the hexagons in the boat conformation and the lowest values for those in the chair conformation. II revealed that partial substitution of Au atoms in the hexagonal diamond net by a post-transition element (Ga) may occur in this family, whereas the sizes of the (Au/Ga)(3) groups and strong Ba-Au covalent interactions allow for their mutual replacement in the voids.