Ge Phase Research Articles

Elastic Phases of GexSbxSe100–2x Ternary Glasses Driven by Topology

Topology offers a practical set of computational tools to accurately predict certain physical and chemical properties of materials including transformations under deformation. In network glasses with increased cross-linking three generic elastic phases are observed. We examine ternary Ge(x)Sb(x)Se(100-2x) glasses in Raman scattering, modulated DSC and volumetric measurements, and observe the rigidity transition, x = x(c)(1) = 14.9% that separates the flexible phase from the Intermediate phase, and the stress transition, x = x(c)(2) = 17.5% that separate the intermediate phase from the stressed rigid one. Raman scattering provides evidence of the structural motifs populated in these networks. Using size increasing cluster agglomeration, we have calculated the rigidity and stress transitions to occur near x(c)(1)(t) = 15.2% and x(c)(2)(t) = 17.5%, respectively. Theory predicts and experiments confirm that these two transitions will coalesce if edge-sharing Ge-tetrahedral motifs were absent in the structure, a circumstance that prevails in the Ge-deficient Ge7Sb(x)Se(93-x) ternary, underscoring the central role played by topology in network glasses. We have constructed a global elastic phase diagram of the Ge-Sb-Se ternary that provides a roadmap to network functionality. In this diagram, regions labeled A, B, and C comprise networks that are flexible, rigid but unstressed, and stressed-rigid, respectively.

Room-temperature ferromagnetism of all-epitaxial β-Fe–Ge/diamond–Ge/β-Fe–Ge trilayers

We report on the first all-epitaxial ferromagnet/inorganic semiconductor/ferromagnet hybrid heterostructure that exhibits (i) a Ge barrier of diamond crystal structure, (ii) room-temperature ferromagnetic electrodes and (iii) very smooth interfaces. Both bottom- and top-Fe–Ge electrodes exhibit tiny in-plane magnetic anisotropies dominated by a magnetocrystalline contribution of six-fold symmetry originating from the hexagonal symmetry of the B82 (Ni2In) β-Fe–Ge phase. A key result is the absence of any magnetic coupling between these soft-magnetic electrodes for Ge barrier thickness as low as ∼2.5 nm, which allows us to easily tune the parallel and antiparallel magnetic alignments by applying suitably small magnetic fields. This confirms the beneficial use of H-surfactant in order to drastically reduce the roughness of the Ge barrier, as revealed by our scanning tunneling microscopy and transmission electron microscopy measurements. This new all-epitaxial ferromagnet/semiconductor hybrid system appears, therefore, to be a promising candidate for the realization of magnetic tunnel junctions with a single crystal semiconductor barrier that are fully compatible with Si-based technology.

Experimental determination of the Ta–Ge phase diagram

Abstract In the present work, the Ta–Ge phase diagram has been experimentally studied, considering the inexistence of a Ta–Ge phase diagram in the literature. The samples were prepared via arc melting and characterized by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD). The intermetallics phases βTa3Ge, αTa3Ge, βTa5Ge3 and TaGe2 where confirmed in this system. Three eutectics reactions where determined with the liquid compositions at 20.5; 28.0; 97.0 at.% Ge. The phases βTa3Ge and βTa5Ge3 solidifies congruently while TaGe2 is formed through a peritectic transformation. The temperature of the Ta-rich eutectic (L ↔ Tass + βTa3Ge) was measured by the Pirani-Alterthum method at 2440 °C and the Ge-rich eutectic (L ↔ TaGe2 + Gess) by DTA at 937 °C.

Kinetic Origin of Divergent Decompression Pathways in Silicon and Germanium

Silicon and germanium transform from diamond to β-tin structure under compression, but upon decompression they turn into metastable BC8 Si and ST12 Ge phases, respectively, instead of returning to the lowest-enthalpy diamond structure. Here we explore by first-principles calculations the atomistic mechanism underlying this intriguing phenomenon. We identify a body-centered tetragonal structure in I4(1)/a (C(4h)(6)) symmetry as a precursory state of the BC8 Si phase formed via a double cell bond-rotation mechanism with a low kinetic barrier. Kinetics also play a central role in selecting the decompression pathway in Ge via a trinary cell bond-twisting reconstruction process toward the ST12 Ge phase. In both cases, transformation back to energetically more favorable diamond structure is inhibited by the higher enthalpy barrier. These results explain experimental findings and highlight the kinetic origin of the divergent decompression pathways in Si and Ge.

Density functional simulations of hexagonal Ge2Sb2Te5at high pressure

We investigated the structural transformations of the hexagonal phase of Ge${}_{2}$Sb${}_{2}$Te${}_{5}$ under pressure by means of ab initio molecular dynamics with a variable simulation cell. To overcome the enthalpy barriers between the different phases we used metadynamics techniques. We reproduced the hexagonal-to-$\mathrm{bcc}$ transformation under pressure found experimentally. The $\mathrm{bcc}$ phase retains a partial chemical order, as opposed to a second $\mathrm{bcc}$ phase we generated by pressuring the amorphous phase. This structural difference is suggested to be responsible for the memory effect uncovered experimentally, the $\mathrm{bcc}$ phase reverting to the amorphous or to the hexagonal phase upon decompression, depending on the type of precursor phase it originates from.

Destabilization of LiH by Li Insertion into Ge

Lithium hydride has high hydrogen capacity (12.7 mass %), but could not be considered as practical hydrogen storage media because of being very stable (required 900 °C for 0.1 MPa desorption pressure). Recently, C and Si have been found suitable to reduce the stability of LiH. This motivates us to investigate the properties of other alloys of Li, formed with the other elements. In the present work, Li3.75Ge (Li15Ge4) alloy was synthesized by mechanical milling, which transformed into Li4.2Ge (Li21.1875Ge5) and Li3.5Ge (Li7Ge2) phases during the vacuum heating at 400 °C. Hydrogenation of thus formed alloys at 400 °C under 3 MPa hydrogen pressure during PCI experiment transforms this mixed phase into Li2GeH0.5 (Li4Ge2H) and LiH phase. A remarkable decrease in the desorption temperature (∼300–450 °C) is observed by preparing the above alloy with Ge as observed from TG-DTA-MS experiment. The enthalpy of the reaction has also been calculated using the van’t Hoff plot. The present work concluded with the establishment of a direct relationship between hydrogen storage parameters and electrochemical parameters using the Nernst equation and van’t Hoff equation. A good agreement is found between the values of required potential for lithiation/delithiation as obtained by two methods.

Evidence for theR8Phase of Germanium

The formation of R8 germanium is reported. The β-Sn phase is first induced by the indentation of amorphous germanium (a-Ge) and the resultant phases on pressure release are characterized by Raman scattering. The expected Raman line frequencies for the various phases of Ge are determined from first-principles calculations using density functional perturbation theory of the zone-center phonons in the diamond, ST12, BC8, and R8 Ge phases. In addition to the R8 phase, traces of BC8 may also be present following pressure release.

Self-assembled Ge islands and nanocrystals by RF magnetron sputtering and rapid thermal processing: The role of annealing temperature

Germanium (Ge) nanostructures were fabricated through the rapid thermal processing of radio frequency sputtered Ge on silicon (Si). The substrates were unheated during the growth, resulting in a post-growth Ge deposited layer that was 250nm thick with a surface that had no evidence of nanostructure formation. The samples subsequently underwent rapid annealing from 400°C to 800°C for 15s. Dramatic nanostructuring was observed at the surface following annealing above 600°C. Scanning electron microscopy showed that as annealing temperatures increased, the Ge layer evolved into circular islands. The sizes of the Ge islands increased and the islands became more uniform and dense at 700°C. Viable Ge nanocrystals were obtained at 600°C and 700°C given the increased annealing temperatures that improved crystallinity. The calculated Raman line shape based on the phonon confinement model agreed well with the experimental spectrum which constituted crystalline Ge nanostructures with an estimated size of 2.65–3.5nm. In addition, no Ge–Si intermixing was observed at the interface. High resolution X-ray diffraction revealed the tetragonal Ge phases and the samples were of polycrystalline structures. The annealing temperature also enhanced photo currents of the fabricated metal semiconductor metal photodetector. These results suggest that 700°C for 15s is an optimum annealing temperature for the production of viable crystalline Ge nanostructures.

Dynamic solidification mechanism of ternary Ag-Cu-Ge eutectic alloy under ultrasonic condition

The dynamic solidification of ternary Ag38.5Cu33.4Ge28.1 eutectic alloy within a 35 kHz ultrasonic field is investigated and compared with both its equilibrium solidification by DSC method and its rapid solidification in drop tube. The volume fractions of the primary (Ge) phase and pseudobinary (Ag+ɛ2) eutectic solidified within ultrasonic field are larger than those formed under equilibrium state, whereas that of ternary (Ag+ɛ2+Ge) eutectic exhibits the reverse trend. During rapid solidification, the liquid alloy droplet directly solidifies into ternary (Ag+ɛ2+Ge) eutectic if its diameter is smaller than 350 μm. The ultrasound stimulates the nucleation of alloy melt and prevents the bulk undercooling. With the increase of sound intensity, the primary (Ge) phase transfers from faceted dendrites to nonfaceted blocks with blunt edges, and its grain size is remarkably reduced. Both pseudobinary (Ag+ ɛ2) and ternary (Ag+ɛ2+Ge) eutectics experience a morphological transition from regular to anomalous structures. This indicates that their cooperative growth mode is replaced by independent growth of eutectic phases under the combined effects of cavitation and acoustic streaming. The ultrasound also shows a prominent coarsening effect to the pseudobinary (Ag+ɛ2) and ternary (Ag+ɛ2 +Ge) eutectics.

Superconductivity in β-Tin Germanium

The superconducting properties of the β-tin germanium under pressure have been investigated by the first-principles calculations based on density functional perturbation theory. The electron–phonon coupling strength as a function of pressure was calculated, and it deceases with the application of pressure. Using the calculated electron–phonon coupling strength, the superconducting transition temperature T c has been estimated. Assuming a nominal Coulomb repulsion parameter of 0.1, the calculated T c is in excellent agreement with experimental data. The T c correlates with electron–phonon coupling strength, indicating phonon-mediated superconductivity in β-tin Ge phase. Our findings have great implications to other Group IVa elements.

Aluminum–germanium wafer bonding of (AlGaIn)N thin-film light-emitting diodes

Eutectic aluminum–germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2″ wafer level at a bond temperature of 470 °C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1–2 kN/cm2. The formation of a liquid Al–Ge phase is evident from cross-section analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.

Ca10Pt7Tt3 (Tt = Si, Ge): New Platinide Phases Featuring Electron-Rich 4c–6e Bonded [Pt7Tt3]20– Intermetalloid Clusters

Two new phases Ca(10)Pt(7)Tt(3) (with Tt = Si, Ge) were obtained by reacting stoichiometric mixtures of the elements at high temperature. Their structures were refined from single crystal X-ray diffraction data. They are isostructural and crystallize in the Ba(10)Al(3)Ge(7) type structure, space group P6(3)/mcm (No. 193) with a = b = 8.7735(3) Å, c = 13.8260(5) Å, V = 921.66(6) Å(3), Z = 2 for Tt = Si, and a = b = 8.7995(6) Å, c = 13.9217(14) Å, V = 933.56(16) Å(3) for Tt = Ge phase. The most interesting structural features in these phases are the propeller shape {Pt(7)Tt(3)} (Tt = Si, Ge) intermetalloid clusters in a D(3h) local symmetry. LMTO electronic structure calculations and COHP analyses reveal that both Ca(10)Pt(7)Tt(3) (Tt = Si, Ge) phases are charge optimized, which is not predicted by the classical Zintl concept and the octet or Wade-Mingo's rules, but rather by a more complex bonding model based on the unprecedented electron-rich 4c-6e multicenter bonding. The clusters are best described as three-condensed trigonal planar [TtPt(3)](8-) units, resulting in a central Pt atom also with a trigonal planar coordination of three symmetrical equivalent Si/Ge atoms that are further connected to two terminal Pt atoms each. The "trefoil" electron-rich multicenter bonding is proposed here for the first time, and may be viewed as a unique bonding feature with potential relevance for the catalytic properties of the noble metal platinum.

Elastic softening of alloy negative electrodes for Na-ion batteries

Accurate description of the mechanical behavior of crystalline Na alloys is essential in establishing their electrochemical performance as well as their viability as anodes in Na-ion batteries. Using first principles simulations, we have investigated the intrinsic elastic properties of crystalline Na–M (M = Sn, Pb, Si and Ge) phases observed during Na intercalation. We have obtained the complete set of concentration-dependent anisotropic elastic constants as well as the average macroscopic elastic moduli of polycrystalline structures. We find that sodiation of pure M phases leads to a remarkable elastic softening that results in up to 75% deterioration of the elastic moduli. Our analysis of the electronic charge distribution demonstrates that the elastic softening during sodiation originates from a transition to weaker ionic interatomic bonding. Our results highlight the significance of the concentration dependence of the elastic moduli for the analysis of deformation behavior of Na alloy anodes of Na-ion batteries during sodiation and desodiation.

Lithium-Stuffed Diamond Polytype Zn–Tt Structures (Tt = Sn, Ge): The Two Lithium–Zinc–Tetrelides Li3Zn2Sn4 and Li2ZnGe3

In view of the search for alternative structures for Li ion battery materials and electron-poor framework semiconductors for thermoelectric applications, the systems Li-Zn-Tt with Tt = Ge or Sn were investigated. Li3Zn2Sn4 and Li2ZnGe3 were obtained by high-temperature syntheses from the elements. The crystal structures of both phases were determined with single-crystal X-ray diffraction methods and the electronic structure of Li3Zn2Sn4 was analyzed by means of DFT calculations (TB-LMTO-ASA). Both phases show diamond polytype analogous Zn-Tt networks with tetrahedrally four-coordinated Zn and Tt atoms. The new phase Li3Zn2Sn4 crystallizes in space group P6(3)/mmc (No. 194) with lattice parameters a = 4.528(1) Å and c = 22.119(2) Å. Zn and Sn atoms are fully ordered on three sites that constitute a 6H diamond polytype like network. Li2ZnGe3 is also described in space group P6(3)/mmc (No. 194) with lattice parameters a = 4.167(1) Å and c = 6.754(1) Å. The Zn-Ge substructure is a hexagonal diamond (2H polytype) like network. The existence of such a Ge-rich Li-Zn-Ge phase has already been reported, but a full structure determination has not yet been published. No indication for an ordering of Zn and Ge atoms on different sites could be deduced from the X-ray diffraction data. Band structure calculations for Li3Zn2Sn4 indicate that the phase is metallic, with the Fermi level at the flank of a pseudogap in the density of states curve. The topological analysis of the electron localization function (ELF) shows covalent Sn-Sn bonding and lone pair like valence basins for the Sn atoms. Concerning the appearance of the lone pair like ELF basins, a strong influence of the basis set for Li that is employed in the calculations is found.

Tetragonal Allotrope of Group 14 Elements

Group 14 elements (C, Si, and Ge) exist as various stable and metastable allotropes, some of which have been widely applied in industry. The discovery of new allotropes of these elements has long attracted considerable attention; however, the search is far from complete. Here we computationally discovered a tetragonal allotrope (12 atoms/cell, named T12) commonly found in C, Si, and Ge through a particle swarm structural search. The T12 structure employs sp(3) bonding and contains extended helical six-membered rings interconnected by pairs of five- and seven-membered rings. This arrangement results in favorable thermodynamic conditions compared with most other experimentally or theoretically known sp(3) species of group 14 elements. The T12 polymorph naturally accounts for the experimental d spacings and Raman spectra of synthesized metastable Ge and Si-XIII phases with long-puzzling unknown structures, respectively. We rationalized an alternative experimental route for the synthesis of the T12 phase via decompression from the high-pressure Si- or Ge-II phase.

Investigation of the crystallization process and crystal structure of Si-incorporated GeSb phase-change films

The effect of Si incorporation on the crystallization process and crystal structure of Te-free Sb-rich GeSb was investigated in this study. Si concentrations were controlled to 0, 5.1, 9.3, and 12.8 at.% by controlling the sputtering power of the GeSb alloy target (20:80 at.% for Ge:Sb) and Si target. After film deposition, the crystallization process and crystal structure were investigated. Crystallization temperature increased from 320 to 400 °C and the overall crystallinity was decreased with increasing Si concentration. These were analyzed by sheet resistance measurements after thermal annealing and optical contrast measurements by optical static testing. Glass transition temperatures were calculated and increased from 240 to 285 °C with increasing Si concentration. Considering the proportional relation between the glass transition temperature and crystallization temperature, it is thought that more energy is required for crystallization with increased Si concentration. A study of the crystallization process kinetics was conducted by applying the Johnson–Mehl–Avrami model to the optical static test results, which were carried out under a pseudo-isothermal process. The Avrami coefficient was 4.10 and decreased to 3.18 when the crystallization was generated with increased Si concentration from 0 to 12.8 at.%. Therefore, crystallization speed was thought to decrease with increased Si concentration. Based on the results of crystal structure analysis by XRD and HRTEM, the crystal structure of our Sb-rich GeSb PCM was revealed to be a typical Sb structure, i.e., an A7 hexagonal structure with lattice parameters of a = 4.26 A and c = 11.45 A. No crystal phase of Ge or Si was observed and no evidence of the structure change in Sb crystals due to Ge or Si incorporation was observed.

A calorimetric study of thermodynamic properties for binary Cu–Ge alloys

The phase transformation temperatures and enthalpy of fusion for Cu–Ge alloys in the whole composition range are systematically measured by differential scanning calorimetry (DSC). It is found that the undercoolability of liquid Cu–Ge alloys depends mainly on the nucleation ability of primary solid phases. Those liquid alloys solidifying with the preferential nucleation of intermetallic compounds exhibit smaller undercoolings than the others with the primary α(Cu) and (Ge) solid solution phases. Microstructural observation indicates that the ξ, ε and ε1 intermetallic compounds display non-faceted growth mode. In Cu–Ge eutectic alloys, the ξ and ε phases grow into lamellar structures, whereas the ε2 and (Ge) phases form irregular eutectics. The peritectic reactions can rarely be completed, and the peritectic microstructures are usually composed of both the primary and peritectic phases.

Laser generated Ge ions accelerated by additional electrostatic field for implantation technology

The paper presents research on the optimization of the laser ion implantation method with electrostatic acceleration/deflection including numerical simulations by the means of the Opera 3D code and experimental tests at the IPPLM, Warsaw. To introduce the ablation process an Nd:YAG laser system with repetition rate of 10Hz, pulse duration of 3.5ns and pulse energy of 0.5J has been applied. Ion time of flight diagnostics has been used in situ to characterize concentration and energy distribution in the obtained ion streams while the postmortem analysis of the implanted samples was conducted by the means of XRD, FTIR and Raman Spectroscopy. In the paper the predictions of the Opera 3D code are compared with the results of the ion diagnostics in the real experiment. To give the whole picture of the method, the postmortem results of the XRD, FTIR and Raman characterization techniques are discussed. Experimental results show that it is possible to achieve the development of a micrometer-sized crystalline Ge phase and/or an amorphous one only after a thermal annealing treatment.

Laser produced streams of Ge ions accelerated and optimized in the electric fields for implantation into SiO2 substrates

Ge crystals were prepared by means of laser-induced ion implantation technique. A Nd:YAG pulsed laser (repetition rate: 10 Hz; pulse duration: 3.5 ns; pulse energy: ∼0.5 J) was used both as an ion source and to carry out the ablation processes. The optimization of the laser-generated ion beam parameters in a broad energy and current density range has been obtained controlling the electrostatic field parameters. Numerical simulations of the focusing system, performed adopting an OPERA 3D code, and an investigation of the ion characteristics, using the ion time-of-flight method, have allowed to optimize the preparation parameters. The structural properties of the samples were investigated by means of x-ray photoelectron, micro-Raman spectroscopies, and scanning electron microscopy techniques. Experimental results show that, by appropriately varying the ion implantation parameters and by a post-preparation annealing treatment, it is possible to achieve the development of a micrometer-sized crystalline Ge phase and∕or an amorphous one.

New Ternary Germanides La4Mg5Ge6 and La4Mg7Ge6: Crystal Structure and Chemical Bonding

The synthesis, structural characterization, and chemical-bonding peculiarities of the two new polar lanthanum-magnesium germanides La(4)Mg(5)Ge(6) and La(4)Mg(7)Ge(6) are reported. The crystal structures of these intermetallics were determined by single-crystal X-ray diffraction analysis. The La(4)Mg(5)Ge(6) phase crystallizes in the orthorhombic Gd(4)Zn(5)Ge(6) structure type [Cmc2(1), oS60, Z = 4, a = 4.5030(7) Å, b = 20.085(3) Å, c = 16.207(3) Å, wR2 = 0.0451, 1470 F(2) values, 93 variables]. The La(4)Mg(7)Ge(6) phase represents a new structure type with a monoclinic unit cell [C2/m, mS34, Z = 2, a = 16.878(3) Å, b = 4.4702(9) Å, c = 12.660(3) Å, β = 122.25(3)°, wR2 = 0.0375, 1466 F(2) values, 54 variables]. Crystallographic analysis together with linear muffin-tin orbital band structure calculations reveals the presence of strongly bonded 3D polyanionic [Mg-Ge] networks balanced by positively charged La atoms in both stoichiometric compounds. The La(4)Mg(5)Ge(6) compound is related to Zintl phases, showing a prominent density of states pseudogap at the Fermi level. A distinctive feature of the La(4)Mg(5)Ge(6) structure is the presence of Ge-Ge covalent dumbbells; in La(4)Mg(7)Ge(6), the higher Mg content generates a polyanionic network consisting exclusively of Mg-Ge heterocontacts. Nevertheless, the frameworks of the two phases are structurally similar, as is highlighted in this work.