Ge Alloys Research Articles

Ground state and magnetic properties of the Cr-doped Ni-Mn-(Ga, Ge, In, Sn) alloys: Insights from ab initio study

By using first-principles calculations in combination with supercell approach a series of Ni8Mn2Cr2Z4 (Z = Ga, Ge, In, Sn) are studied with partial replacing of Mn by Cr to investigate the effect of such substitution on their structural and magnetic properties. Two 16-atom supercells with different distribution of Cr in the Mn sublattice and magnetic ordering are considered. Crystal structure optimization shows that the magnetic ground state of Ni8Mn2Cr2Z4(Z = Ga, In) alloys is the state with pairwise antiparallel magnetic moments of Cr atoms. While for Ni8Mn2Cr2Z4 (Z = Ge, Sn) alloys is it ferromagnetic state with parallel magnetic moments of Cr and Mn atoms. Based on ground state energies it is shown that martensitic phase can be realized only in Ni8Mn2Cr2Z4 (Z = Ga, Ge) alloys. For the stable crystal structures with associated magnetic order magnetic exchange constants are calculated, to estimate Curie temperatures with the help of mean-field approximation. Obtained results are in qualitative agreement with experimental and theoretical data.

Theoretical investigations of group IV alloys in the Lonsdaleite phase

The structural, elastic, elastic anisotropic, thermodynamic and electronic properties of Lonsdaleite C, Si and Ge and Lonsdaleite C–Si and Si–Ge alloys are investigated using density functional theory. The elastic anisotropy calculations show that the Lonsdaleite C0.25Si0.75 alloy has the greatest anisotropy in Poisson’s ratio, shear modulus, bulk modulus and Young’s modulus. Through the mixing of carbon and silicon and silicon and germanium at certain proportions, Lonsdaleite C0.25Si0.75 with metallic properties and Lonsdaleite Si0.25Ge0.75 with a direct band gap are obtained, where Lonsdaleite Si0.25Ge0.75 is a narrow direct band gap semiconductor with a band gap of 0.76 eV at the HSE06 hybrid functional level. The minimum thermal conductivity calculations on Lonsdaleite C–Si and Si–Ge alloys show that the minimum thermal conductivities of Lonsdaleite C0.75Si0.25 and Lonsdaleite C0.5Si0.5 are greater than that of diamond C, and the minimum thermal conductivities of Lonsdaleite C–Si and Si–Ge alloys in different directions are also investigated.

Synthesis and optical properties of (GaAs)yGe5-2y alloys assembled from molecular building blocks

Monocrystalline alloys of GaAs and Ge with compositions (GaAs)yGe5–2y have been synthesized following a chemical vapor deposition approach that promotes the incorporation of Ga and As atoms as isolated donor-acceptor pairs. The structural and optical properties show distinct behavior relative to (GaAs)1-xGe2x counterparts produced by conventional routes. Strong band gap photoluminescence is observed in the 0.5–0.6 eV range for samples whose compositions approach the GaAsGe3 limit for isolated Ga-As pairs. In such samples, the Ge-like Raman modes appear at higher frequencies and are considerably narrower than those observed in samples with higher Ge concentrations. These results suggest that the growth mechanism may favor the formation of ordered phases comprising Ga-As-Ge3 tetrahedra. In contrast with the diamond-to-zincblende ordering transition previously reported for III-V-IV alloys, ordered structures built from Ga-As-Ge3 tetrahedra feature III-III and V-V pairs as third-nearest neighbors, and therefore both the III- and V-components are equally present in each of two fcc sublattices of the average diamond-like structure. These bonding arrangements likely lead to the observed optical response, indicating potential applications of these materials in mid-IR technologies integrated on Si.

Development of a TiAl Alloy by Spark Plasma Sintering

Spark plasma sintering (SPS) is a consolidated powder metallurgy process for which the powder sintering is achieved through an applied electric current. The present article aims to describe the method we employed to develop a TiAl-based alloy adjusted for this SPS process. Owing to its enhanced mechanical properties, this alloy was found to fully match the industrial specifications for the aeronautic and automotive industries, which require a high strength at high temperature and a reasonably good ductility at room temperature. A step-by-step method was followed for this alloy development. Starting from a basic study on the as-SPSed GE alloy (Ti-48Al-2Cr-2Nb) in which the influence of the microstructure was studied, the microstructure-alloy composition relationships were then investigated to increase the mechanical properties. As a result of this study, we concluded that tungsten had to be the major alloying element to improve the resistance at high temperature and a careful addition of boron would serve the properties at room temperature. Thus, we developed the IRIS alloy (Ti-48Al-2W-0.08B). Its microstructure and mechanical properties are described here.

Magnetocaloric Effect with Very Small Magnetic Hysteresis Losses of CoMn1−x Ti x Ge Alloys

The effects of Ti substitution for Mn and heat treatment on structural, magnetic, and magnetocaloric properties of CoMnGe alloy have been investigated using electron microscopy, X-ray diffraction, calorimetric, and magnetic measurements. According to X-ray diffraction measurements, CoMn1−x Ti x Ge alloys are in a single phase hexagonal structure at room temperature. It is found that the as-cast CoMn0.95Ti0.05Ge alloy shows a magnetostructural phase transition close to room temperature. The transition shows a large magnetic entropy change and a small hysteresis in the isothermal magnetic field-dependent magnetization measurements. After annealing, the phase transition temperature decreases slightly and the magnetization is sharply decreasing at Curie temperature. The sharp change of magnetization at phase transition temperature is accompanied by a significant increase in the magnetic entropy change, i.e., magnetic entropy change value for the magnetic field change of 1 T was increased from −3.3 to −6.3 J kg−1 K−1. Moreover, after annealing, hysteresis losses reduced significantly for ∆H = 7 T. Accordingly, the heat treatment has a significant effect on magnetocaloric properties of the CoMn0.95Ti0.05Ge alloy.

A Magnetoresistance Induced by a Nonzero Berry Phase in GeTe/Sb2Te3 Chalcogenide Superlattices

AbstractThe chalcogenide alloy Ge–Sb–Te (GST) has not only been used in rewritable digital versatile discs, but also in nonvolatile electrical phase change memory as a key recording material. Although GST has been believed for a long time not to show magnetic properties unless doped with magnetic impurities, it has recently been reported that superlattices (SLs) with the structure [(GeTe)L(Sb2Te3)M]N (where L, M, and N are usually integers) have a large magnetoresistance at room temperature for particular combinations of L and M. Here it is reported that when [(GeTe)L(Sb2Te3)M]N chalcogenide SL films are thermally annealed at 470 K and cooled down to room temperature under an external magnetic field accompanied by current pulse injections, a large magnetoresistance change (>2500 Ω) is induced. This study shows that the phenomenon has a strong correlation with the GeTe thickness and the periodic structure of the SL films, and that it is induced by the structural phase transition between electrically nonpolar and polar phases in the GeTe layers in the SLs. This study proposes that the relationship between the polar (ferroelectric) phase and the Berry curvature in the SLs is responsible for the magnetoresistance change.

The surface structures growth’s features caused by Ge adsorption on the Au(111) surface

The initial stage of the adsorption of Ge on an Au(111) surface was investigated. The growth and stability of the structures formed at the surface were studied by ultrahigh-vacuum low-temperature scanning tunneling microscopy and analyzed using density functional theory. It was established that the adsorption of single Ge atoms at the Au(111) surface at room temperature leads to the substitution of Au atoms by Ge atoms in the first surface layer. An increasing of surface coverage up to 0.2–0.4 monolayers results in the growth of an amorphous binary layer composed of intermixed Au and Ge atoms. It was shown that the annealing of the binary layer at a temperature of T s ≃ 500 K, as well as the adsorption of Ge on the Au(111) surface heated to T s ≃ 500 K for coverages up to 1 monolayer lead to a structural transition and the formation of an Au–Ge alloy at least in the first two surface layers. Based on experimental and theoretical data, it was shown that the formation of single-layer germanene on the Au(111) surface for coverages ≤1 monolayer in the temperature range of T s = 297–500 K is impossible.

Two novel Ge phases and their Si[sbnd]Ge alloys with excellent electronic and optical properties

The physical properties of Si12, Ge12 and their alloys (Si8Ge4 and Si4Ge8) in the oC12 and hP12 phases are reported in this work, including their structural properties, stability, mechanical properties (elastic constant, elastic modulus, B/G ratio), mechanical anisotropy properties (the anisotropy in Poisson's ratio, shear modulus, and Young's modulus; shear anisotropic factor; universal elastic anisotropy index), thermal properties (Debye temperature and minimum thermal conductivity), electronic properties (electronic band structure) and optical properties (absorption). Owing to the band structures of Si12, Ge12, Si8Ge4, and Si4Ge8 in the oC12 and hP12 phases are direct band gap or quasi-direct band gap, and the optical property of these alloys and two novel Ge allotropes in oC12 and hP12 are all better than that of diamond Si, oC12-Si12 and hP12-Si12, so they can be used as candidate materials for photovoltaic devices and thin-film solar cell applications. In addition, we proposed the two metastable germanium allotropes in the hP12 and oC12 phases first time, and our calculations demonstrate that they should be mechanically, thermodynamically and dynamically stable at ambient pressure.

Effect of Ge addition on wettability, copper dissolution, microstructural and mechanical behavior of SnCu–Ge solder alloy

This paper explores the effects of Germanium (Ge) addition on the wetting behavior, copper dissolution, IMC formation, microstructure and mechanical behavior of SnCu alloy. Static copper dissolution was conducted as a function of contact time and in the temperature range of 260–300 °C. The results show that the Ge doped alloy has the lowest copper dissolution rate compared to SnCu, SAC305, and very much comparable with SnPb solder thus making it suitable alloy for the wave soldering process. Two solderability standards measurement (JIS vs. IPC) have been executed on different surface finishes such as OSP, ENIG, ImAg commonly used in industrial soldering process. The results showed that addition of Ge significantly improves the wetting/spread performance for all the surface finishes (ENIG, OSP and ImAg). Contact angles range from 17°–30°, the lowest contact angles being observed on ENIG surface. Microstructural analysis showed Ge addition refine the grain structure of SnCu–0.01Ge alloy. A thin IMC layer was observed, and IMC growth was not significant after multiple reflows as compared to Sn–Cu alloy. Similarly, Ge addition increases the hardness value due to uniform microstructure of SnCu–Ge alloy. The tensile testing results also showed an increase in tensile strength and tolerable ductility.

Bauschinger Effect and Back Stress in Gradient Cu-Ge Alloy

Using surface mechanical attrition treatment (SMAT), a gradient structure composed of two gradient structure (GS) layers and a coarse grain (CG) layer was generated from a Cu-5.7 wt pct Ge alloy, significantly improving the yield strength of the sample. Unloading–reloading tests showed an unusual Bauschinger effect in these GS samples. The back stresses caused by the accumulated geometrically necessary dislocations (GNDs) on the GS/CG border increased with increasing strain. As found by electron backscatter diffraction (EBSD), the GNDs are mainly distributed in the gradient structured layer, and the density of the GNDs increase with increasing SMAT time. The effect of the back stress increased with increasing SMAT processing time due to the increase in the strain gradient. The pronounced Bauschinger effect in a GS sample can improve the resistance to forward plastic flow and finally contributes to the high strength of GS samples.

The investigation of electronic, mechanical and lattice dynamical properties of PdCoX (X = Si and Ge) half-Heusler metallics in α, β and γ structural phases: an ab initio study

PdCoX (X = Si and Ge) alloys which are XYZ type half-Heusler alloys and also have face centred cubic MgAgAs-type structure which conforms to space group, have been investigated in different atomic arrangements which are called α, β and γ phases, using local spin density approximation in the density functional theory as implemented in VASP (Vienna Ab Initio Simulation Package) software. Both of the alloys are considered in ferromagnetic order. After the investigation of stable structural phase for these alloys, their full structural, electronic, magnetic, mechanical, and dynamical properties have been examined in this structural phase. The calculated electronic band structure and the total electronic density of states of our alloys indicated metallic behaviour. The estimated elastic constants show that these are stable and show anisotropic behaviour mechanically in β and γ phases. Also, the calculated phonon dispersion curves show that PdCoX (X = Si and Ge) alloys are stable dynamically in the same structural phases.

First Principles Modeling of Cathodic Reaction Thermodynamics in Dilute Magnesium Alloys

The effect of dilute alloying additions on the cathodic reaction thermodynamics occurring on the magnesium basal (0001) surface was studied using a first principles approach. The stability of alloying additions on the Mg(0001) surface was considered as a function of surface energy and shown to be a function of implicit solvation. The work function of the alloyed surfaces was also largely affected by the use of an implicit solvent, with alloyed surfaces becoming increasingly noble and the pure magnesium surface becoming less noble after solvation. The cathodic reaction was considered as a three part sequence: (1) water dissociation, (2) hydrogen diffusion, and (3) hydrogen evolution. The water dissociation reaction became endothermic for Ge, In, Sb, and Sn dilute alloying, whereas Ca, Sc, Y, Ti, and Zr enhanced the already favorable water dissociation reaction. Two mechanisms of preventing hydrogen evolution were considered, with early period elements (Ca, Sc, Ti, Y, and Zr) preventing hydrogen evolution f...

Phase separation and subsequent solidification of peritectic Fe–Cu–Ge alloys subjected to substantial undercooling processing

Liquid ternary Fe50-x/2Cu50-x/2Gex (x = 5 and 15) alloys were highly undercooled by glass fluxing technique, and the achieved maximum undercoolings are 300 K (0.18TL) and 268 K (0.16TL), respectively. At small undercoolings, these two alloys solidify in a normal pathway of peritectic solidification. If the alloy undercooling continues to increase beyond a certain value, metastable liquid phase separation takes place. The critical undercooling for Fe47.5Cu47.5Ge5 alloy is 70 K and that for Fe42.5Cu42.5Ge15 alloy is 110 K. At larger undercoolings, the homogeneous liquid alloy separates into an Fe-rich zone and a Cu-rich zone. In such a case, the undercooled alloy displays a monotectic solidification structure. The rapid solidification of the Fe-rich zone is the keystone of alloy solidification, and the growth velocity of the primary phase rises with increasing alloy undercooling according to a power law relation. Within the experimental undercooling ranges, the primary γFe phase of Fe47.5Cu47.5Ge5 alloy attains a maximum growth velocity of 19 m/s, while the growth velocity of the primary Fe0.84Ge0.16 phase reaches 11 m/s in undercooled Fe42.5Cu42.5Ge15 alloy. Moreover, energy dispersive spectroscopy (EDS) analysis shows that the Fe-rich and Cu-rich liquid phases have a similar affinity with solute Ge.

Estimation of the spontaneous magnetization and the universal curve in MnCo1−xNbxGe alloys with long-range interactions

We report on the critical behavior, spontaneous magnetization, and universal curve of MnCo1−xNbxGe alloys. Results demonstrate that the replacement of Co by Nb produces a decrease in the Curie temperature, TC, from 259.3 K to 240.3 K. Critical exponents obtained by the modified Arrott plot technique, Kouvel-Fisher method, and critical isothermal analysis are close to the theoretical prediction of the mean-field model values, revealing the characteristic of long-range ferromagnetic interactions. The values of spontaneous magnetization Msp estimated from |ΔSM| vs. M2 plots are in good agreement with the classical extrapolation from the Arrott curves, proving the validity and usefulness of this approach to estimate the Msp. Moreover, the experimental magnetic entropy changes measured for different fields collapse onto a universal curve, confirming the universal behavior of the magnetocaloric effect in this system.

Current gain and external quantum efficiency modeling of GeSn based direct bandgap multiple quantum well heterojunction phototransistor

This paper aims to provide the performance characteristics of proposed, strain balanced direct band gap multiple quantum wells (MQWs) hetero phototransistor (HPT) made of SiGeSn/GeSn alloys grown on Si substrate which is compatible with recent CMOS fabrication technology. This also presents a comprehensive comparison of this proposed structure with the existing HPT structure made of indirect gap Ge/SiGe MQWs. Alloys of Ge and Sn grown on Si platform shows about tenfold increase in absorption over Ge at C and L-bands due to direct nature of band gap in GeSn. Initial work begins the solution of continuity equation to solve the different terminal current densities and optical gain of the multiple quantum well structure. Main analysis was concentrated on finding the external quantum efficiency depending on the doping variations of emitter and base, base width etc. Finally the photocurrent density variations are estimated for the structure and compared with existing indirect band gap HPT. The calculated values for direct band gap GeSn HPT device are found to be comparable with those for indirect band gap SiGe device to flourish as a potential candidate of photo detectors for the present day telecommunication network.

Rapid Alloying and Nanostructuring of Silicon–Germanium Powder Mixture by Spark Plasma Sintering

This work reports on rapid alloying and nano-structuring of Si80Ge20 alloy by spark plasma sintering (SPS). Elemental silicon (Si) and germanium (Ge) powders were ball milled for 10 h under optimized process parameters. The alloying and densification of Si80Ge20 powders were performed using SPS at temperature range of 900–1200 °C. X-ray diffraction and High resolution transmission electron microscope studies were done to characterize the powders and sintered SiGe alloy. Scanning electron microscope and energy dispersive spectroscopy were carried out to understand the surface morphology and elemental quantification of nanostructured SiGe alloy. The reaction kinetics at different SPS temperatures in Si80Ge20 alloying were reported. It was observed that the increase in SPS temperature up to 1200 °C simultaneously increased the Si(Ge) alloying kinetics and bulk density. Raman spectroscopy study confirmed the Si(Ge) alloying.

A comparative first-principles study of the structural and electronic properties of the liquid Li-Si and Li-Ge alloys.

We have performed a comparative first-principles study on the structural and electronic properties of the liquid Li1-xSix and Li1-xGex alloys over a range of composition from x = 0.09 to 0.50. Our calculations showed that Si and Ge atoms can exhibit very distinct local bonding characteristics as they were alloyed with the Li atoms in the liquid state, where Si atoms tended to form a variety of covalent bonding configurations while Ge atoms predominantly appeared as the isolated anions in the liquid alloys. These differences in bonding characteristics were reflected in their electronic density of states, in which the liquid Li1-xGex alloys have a lower degree of s-p hybridization with narrower distributions of the 3s and 3p states than the liquid Li1-xSix alloys. Our calculations also showed that the optical conductivities of these two liquid alloys can undergo a transition from the Drude-like metallic nature to the semiconductor-like character as the Si/Ge content increases from 0.09 to 0.22. However, as the Si/Ge content further increases to 0.50, the liquid Li1-xGex alloys may transit to exhibit the Drude-like metallic nature, while the liquid Li1-xSix alloys can still hold the semiconductor-like character. Moreover, our calculations revealed that the dc conductivities of these liquid alloys are predominantly determined by the number of total electronic states at the Fermi level. As the liquid Li1-xSix alloys are within the composition range between 0.20 and 0.50, the increment of the states at the Fermi level with increasing the Si content is nearly identical to the amount of the Li states decreased, leading to an almost unchanged number of total electronic states at the Fermi level. However, since Ge atoms do not favor forming covalent bonding in the liquid alloys to keep the Fermi level at a minimum of the density of states, the liquid Li1-xGex alloys would have more electronic states at the Fermi level and thereby higher dc conductivities than the liquid Li1-xSix alloys within the same composition range.

The Use of Fluorine to Protect β-Solidifying γ-TiAl-Based Alloys against High-Temperature Oxidation

ABSTRACTLight-weight alloys based on intermetallic titanium aluminides (TiAl) are structural materials considered for high-temperature applications, e.g. in aero engines or automotive engines. TiAl alloys of engineering interest consist of two phases, the γ-TiAl and the α2-Ti3Al-phase. Recent developments have led to the so-called TNM alloys (T = TiAl; N = Nb; M = Mo) with an Al-content of 43.5 at.%. These alloys also possess the disordered body centered cubic β-Ti(Al)-phase at elevated temperatures, which ensures a better hot-workability compared to conventional two-phase alloys. However, the relatively low Al content (< 45 at.%) limits the high-temperature capability due to reduced oxidation resistance. This impedes their application in a temperature range above 800°C. The present work shows how the fluorine effect counteracts this disadvantage due to the formation of a protective alumina layer. The performance of the TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.%) is compared with another TNM alloy variant containing additional elements, such as Si and C, and the so-called GE alloy (Ti-48Al-2Cr-2Nb; at.%), which is already in use for turbine blades. The results of isothermal and thermocyclic high-temperature exposure tests of untreated and fluorine treated specimens will be compared. The effect of composition and microstructure of the alloys on the oxidation behavior with and without fluorine treatment are discussed.

TiSi(Ge) Contacts Formed at Low Temperature Achieving Around $2 \,\, \times \,\, 10^{-{9}}~\Omega $ cm2 Contact Resistivities to p-SiGe

This paper reports ultralow contact resistivities ( $\rho _{c})$ achieved on highly doped p-SiGe with two low-temperature contact formation methods. One method combines precontact amorphization implantation with ~500 °C rapid thermal processing (RTP)-based Ti germano-silicidation; $\rho _{c}$ achieved was $\sim 2.9\times 10^{-9}~\Omega \cdot $ cm2. The other method combines codeposited TiSi—Ti:Si =1:1—with ~450 °C RTP-based Ti silicidation; $\rho _{c}$ achieved was $\sim 1.7\times 10^{-9}~\Omega \cdot $ cm2. When $\rho _{c}$ reaches minimum, the TiSi(Ge) alloy is generally amorphous with embedded small crystallites, similar to the previous observations on pure Si substrates.

The properties and structure of Ge[sbnd]Se[sbnd]Te glasses and thin films

Ge30Se70−xTex bulk glasses and thin films for x=0, 10 and 20 were prepared. The properties and structure were characterized by differential scanning calorimetry (DSC), Raman spectroscopy and also by 77Se and 125Te MAS NMR spectroscopy. The optical properties were also measured. The absence of crystalline phases was verified by XRD, while DSC indicated that the glasses were not phase separated. Although the glass transition temperature of both Ge30Se70 glass and thin film is the same, different ΔCp reveal a higher disorder of the thin film. Both the glass transition temperature and ΔCp of Te glasses as well as crystallization temperatures decrease when Te increases due to increasing GeTe2/2 instead of GeSe4/2 structural units thereby reducing 3D interconnection of the glassy network. Based on the 77Se MAS NMR results, the short-range order model was determined as a suitable structural model for Ge30Se70 glass. 77Se and 125Te MAS NMR confirmed that Ge in alloys with Te only reaches oxidation state +II in agreement with GeTe phase diagram and therefore no GeTe4/2 tetrahedra can be present in these glasses and/or films. Raman spectroscopy confirmed that the short-range order of glasses and films is basically the same. The refractive index of thin films increases with tellurium content. The optical band gap of the thin films strongly decreases from 2.02eV to 1.36eV with increasing Te content and simultaneously the increasing structural disorder and width of the band tails increase. Reduction of the oxidation state of germanium to GeII caused by a reaction with tellurium ultimately leads to a reduction in 3D connectivity of the non-crystalline matrix and the loss of the glass-forming ability of the GeSeTe system.