Ge Phase Research Articles

Formation mechanism of Ge nanocrystals embedded in SiO2studied by fluorescence x-ray absorption fine structure

This paper reports that the Ge nanocrystals embedded in SiO2 matrix are grown on Si(100) and quartz–glass substrates, and the formation mechanism is systematically studied by using fluorescence x-ray absorption fine structure (XAFS). It is found that the formation of Ge nanocrystals strongly depends on the properties of substrate materials. In the as-prepared samples with Ge molar content of 60%, Ge atoms exist in amorphous Ge (about 36%) and GeO2 (about 24%) phases. At the annealing temperature of 1073 K, on the quartz–glass substrate Ge nanocrystals are generated from crystallization of amorphous Ge, rather than from the direct decomposition of GeO2 in the as-deposited sample. However, on the Si(100) substrate, the Ge nanocrystals are generated partly from crystallization of amorphous Ge, and partly from GeO2 phases through the permutation reaction with Si substrate. Quantitative analysis reveals that about 10% of GeO2 in the as-prepared sample are permuted with Si wafer to form Ge nanocrystals.

Self-organization of a 2D lattice on a surface of Ge single crystal after irradiation with Nd:YAG laser

Experimentally observed self-organization of a 2D lattice on the surface of Ge single crystal after irradiation by pulsed Nd:YAG laser is reported. The 2D lattice consists of nano-size hills arranged in a pattern of C6i point group symmetry and is characterized by translational symmetry with the period of 1μm. Calculations of time-dependent distribution of temperature in the bulk of the Ge sample are presented to explain the phenomenon. The calculations show that overheating of the crystal lattice occurs at laser radiation intensities exceeding 30MW/cm2. According to synergetic ideas, the presence of the non-equilibrium liquid phase of Ge and a huge gradient of temperature (∼3×108K/m) can lead to self-organization of the 2D lattice similar to Benard cells.

Composition Dependence of Work Function in Metal (Ni,Pt)–Germanide Gate Electrodes

The composition ratio dependence of electrical (work function and resistivity) and structural properties in metal (Ni,Pt)–germanide gate electrodes was investigated for metal–oxide–semiconductor (MOS) devices. X-ray diffraction and cross-sectional transmission electron microscopy clearly revealed that metal–germanide MOS gate electrodes with a single Ni–germanide (Ni–Ge) or Pt–germanide (Pt–Ge) phase can be formed by controlling the thicknesses of the deposited metal and Ge. The resistivities of both Ni–Ge and Pt–Ge were lower than that of conventional polycrystalline silicon (poly-Si) gate electrodes. From the capacitance-voltage characteristics, the work functions of the Ni–Ge gate electrodes were measured to be from 4.6 to 4.9 eV, and those of the Pt–Ge gate electrodes were from 4.9 to 5.3 eV. With increasing metal (Ni,Pt) content, the work function of Ni–Ge decreased, while that of Pt–Ge increased. This composition dependence of the work function of the metal–germanide can be explained by considering the electronegativity of the pure metals that are often used.

GeCu Thin Films for Inorganic Write-Once Media

The Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100-x</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> thin films (x=50 at.%-69 at.%) were deposited on nature oxidized Si (100) wafer by dc co-sputtering of Ge and Cu targets. Microstructures was analyzed by X-ray diffractometer. The optical and thermal properties were measured from static test. It was found that the as-deposited phase was single supersaturated epsiv-Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ge phase and it was transformed to Ge and epsiv-Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ge coexisting phases after annealing at 400degC. The reflectivity of as-deposited film was higher than that of annealed film

Phase constitution and solidification characteristics of undercooled Ag-Cu-Ge ternary eutectic alloy

Liquid Ag38.5Cu33.4Ge28.1 ternary eutectic alloy was undercooled by a great degree up to 175 K(0.22 TE). X-ray diffraction analysis reveals that its rapidly solidified microstructure is composed of (Ag) solid solution phase, (Ge) semiconducting phase and η(Cu3Ge) intermetallic compound phase. With the increase of undercooling, there occurs a transition from the cooperative growth of three eutectic phases to the preferential nucleation and growth of (Ge) phase. The (Ag) and η phases usually grow to form pseudobinary lamellar eutectic structure. When the undercooling exceeds 80 K, the structural morphology of primary (Ge) phase transfers from faceted blocks to branched dendrites. Some (Ge) dendrites look like flowers, which usually have 5—8 lobes. In addition, the number of lobes tends to increase as undercooling increases. The crystal surfaces of the flower-like (Ge) dendrite are {111} planes and the dendrite grows along the 〈100〉 preferred growth direction.

Nanowire array fabricated by Al–Ge phase separation

Phase-separated Al–Ge films, composed of Al nanowire array with diameters ranging from less than 10 to 18 nm embedded in an amorphous Ge matrix, have been prepared by a sputtering method. Similar to lateral phase separation of Al–Si system, sputtered Al and Ge form an Al nanowire array surrounded by a Ge matrix during film growth when the preparation conditions are optimized. Removal of the Al nanowires from the phase-separated Al–Ge films by etching in acid solution provides nanoporous films with a pore density on the order of 10 15 pores/m 2. Template-assisted growth of Ni nanowires into the nanopores was carried out to increase the potential applications of the phase-separated Al–Ge films.

Phase transition in multicomponent semiconductor Cd1−xMnxGeP2 under hydrostatic pressure up to 7 GPA

Electrical resistivity and Hall coefficient in the samples of Cd1−xMnxGeP2, where x=0÷ 0.19, have been measured under hydrostatic pressures up to 5 GPa. All measurements were carried out at T=300 K. A dissociation of Cd1−xMnxGeP2 solid solution was observed and the formation of separated phases of CdP2 and Ge were found at P=3.2 GPa for the Mn fraction x<0.19. The further increase of Mn fraction stabilized a crystal structure and for the compound of Cd0.81Mn0.19GeP2 a reversible phase transition was observed at P=3.5 GPa.

Magnetic phases in Ge–Fe thin films containing high Fe content

Magnetic phases in evaporated thin films ofGe100−xFex (x in at.%) alloyswith x = 36.6 and 51.8are examined. At x = 36.6, a single amorphous phase is formed. The magnetic phase at room temperatureappears to be ferromagnetic, and it transforms into a spin-glass phase at 20 K.The magnetic phases are more complicated at the higher Fe composition ofx = 51.8, which is mainly due to the presence of Fe precipitates in the amorphousmatrix. An analysis based on the superparamagnetism indicates thatthe effective composition of the amorphous phase is estimated to bex = 39.4, and this amorphous phase shows a similar magnetic behaviour to that observed in thex = 36.6 sample with the exception of a slightly higher spin freezing temperature of 30 K. Due to theFe precipitates, an additional superparamagnetic behaviour is observed with a blockingtemperature of 160 K. Magnetization saturation in the spin-glass state occurs at a relativelysmall applied field, which indicates that the direct antiferromagnetic exchange couplingbetween Fe ions is weak.

Structure and magnetic properties of mechanically alloyed Ni–Ge and Co–Ge alloys

Mechanically synthesised Ni and Co-base alloys exhibit interesting magnetic properties, suggesting their potential applications. The equilibrium Ni–Ge and Co–Ge phase diagrams reveal the existance of Ni 5Ge 3 and Co 5Ge 3 phases, both exhibiting high and low-temperature modifications. The purpose of this work was to synthesise by mechanical alloying the Ni 5Ge 3 and Co 5Ge 3 alloys and to study their structure and magnetic properties. Mechanical alloying processes performed for Ni 62Ge 38 and Co 62Ge 38 powder mixtures resulted in the formation of single phase Ni 5Ge 3 and Co 5Ge 3 nanocrystalline alloys after 20–50 h of processing. Ni 62Ge 38 alloy was ferromagnetically ordered for all milling times. In the case of Co–Ge alloy, magnetization measurements showed an evolution from ferromagnetic to paramagnetic ordering upon milling.

High-Curie-temperature ferromagnetism in self-organized Ge1−xMnx nanocolumns

The emerging field of spintronics would be dramatically boosted if room-temperature ferromagnetism could be added to semiconductor nanostructures that are compatible with silicon technology. Here, we report a high-TC (>400K) ferromagnetic phase of (Ge,Mn) epitaxial layer. The manganese content is 6%, and careful structural and chemical analyses show that the Mn distribution is strongly inhomogeneous: we observe eutectoid growth of well-defined Mn-rich nanocolumns surrounded by a Mn-poor matrix. The average diameter of these nanocolumns is 3nm and their spacing is 10nm. Their composition is close to Ge(2)Mn, which corresponds to an unknown germanium-rich phase, and they have a uniaxially elongated diamond structure. Their Curie temperature is higher than 400K. Magnetotransport reveals a pronounced anomalous Hall effect up to room temperature. A giant positive magnetoresistance is measured from 7,000% at 30K to 200% at 300K and 9T, with no evidence of saturation.

Thermal stability and hyperfine interactions of mechanically alloyed Fe–Ge phases

Mechanical alloying method was applied to prepare the Fe 75Ge 25, Fe 62Ge 38 and Fe 33.3Ge 66.7 alloys. X-ray diffraction, differential scanning calorimetry and Mössbauer spectroscopy were used as complementary methods to obtain structural data and hyperfine interaction parameters for the as-milled as well as the heated samples. After mechanical alloying process the disordered α-Fe(Ge) solid solution was formed in the case of Fe 75Ge 25 composition and it was characterized by the broad hyperfine magnetic field distribution with the average field of about 25 T. Heat treatment up to 873 K resulted in the formation of the mixture of three ordered phases and α-Fe(Ge) solid solution. The ordered phases were: metastable D0 3 phase with two hyperfine magnetic fields of 32.8 and 20.2 T and two equilibrium phases—hexagonal ɛ and cubic ɛ′ with fields of about 24.6 and 23.6 T, respectively. After heating up to 973 K this mixture transformed into the mixture of ɛ and ɛ′ phases. In the case of Fe 62Ge 38 composition the hexagonal β-Fe 3Ge 2 phase was obtained during mechanical alloying process. This phase was stable after heat treatment up to 973 K. The as-milled and heated samples were characterized by similar Mössbauer spectra, which were a superposition of the sextets with various hyperfine magnetic fields what reflected different configurations of germanium atoms around 57Fe atoms. In the case of Fe 33.3Ge 66.7 composition the ordered tetragonal FeGe 2 phase was obtained and it was stable during heat treatment up to 973 K. Mössbauer measurements revealed the single paramagnetic line with the isomer shift of 0.30(1) mm/s characteristic for the FeGe 2 intermetallic phase.

The structure and magnetocaloric effect of rapidly quenched Gd 5Si 2Ge 2 alloy with low-purity gadolinium

A magnetic refrigerating material Gd 5Si 2Ge 2 was prepared by arc-melting method under argon atmosphere with low-purity commercial gadolinium. The structure and magnetocaloric effect (MCE) of the as-cast and rapidly quenched Gd 5Si 2Ge 2 alloys were investigated by means of X-ray diffraction and magnetic measurements. The as-cast Gd 5Si 2Ge 2 consists of Gd 5Si 2Ge 2-type, Gd 5Si 4-type, Gd 5(Si,Ge) 3 and Gd(Si,Ge) phases from powder XRD results. The alloys quenched at a velocity of 25 m/s or 40 m/s both adopt in a single phase with orthorhombic Gd 5Si 4-type structure. The lattice parameters under different quenched velocities calculated by least-squares method have no significant difference. After being rapidly quenched at 40 m/s, the Curie temperature of Gd 5Si 2Ge 2 is about 303 K and its maximum magnetic entropy change is 7.25 J/kg K (0–5 T).

A computational thermodynamic model of the Mg–Al–Ge system

The Mg–Al–Ge system has been thermodynamically modeled by combining thermodynamic description of the three constituent binary systems and using a ternary interaction parameter for the liquid phase. Among the three binary systems, Mg–Ge and Al–Ge systems are optimized in this study, whereas the optimized thermodynamic parameters for the Al–Mg system are taken from COST 507 database. The binary excess energy terms are described using Redlich–Kister polynomial model. The ternary invariant points are predicted from the thermodynamic model. The Mg–Al–Ge phase diagram has been modeled for the first time in this work and found to be consistent with the experimental results available in the literature.

Valence Band X-Ray Emission Spectra of Compressed Germanium

We report measurements of the valence band width in compressed Ge determined from x-ray emission spectra below the Ge K edge. The width of the valence band does not show any pressure dependence in the semiconducting diamond-type structure of Ge below 10 GPa. On the other hand, in the metallic beta-Sn phase above 10 GPa the valence band width increases under compression. Density-functional calculations show an increasing valence band width under compression both in the semiconducting phase (contrary to experiment) and in the metallic beta-Sn phase of Ge (in agreement with observed pressure-induced broadening). The pressure-independent valence band width in the semiconducting phase of Ge appears to require theoretical advances beyond the density-functional theory or the GW approximation.

Structural and magnetic properties of Mn5Ge3 clusters in a dilute magnetic germanium matrix

We have characterized the structural and magnetic properties of low-temperature molecular-beam epitaxy grown Ge:Mn by means of high-resolution transmission electron microscopy (HR-TEM), energy dispersive x-ray spectroscopy, and superconducting quantum interference device (SQUID) magnetometry. We find a coherent incorporation of Mn5Ge3 clusters in an epitaxially grown Ge:Mn matrix, which shows the characteristics of a diluted magnetic semiconductor phase of Mn-doped Ge. The clusters are preferentially oriented with the hexagonal [0001] direction parallel to the [001] growth direction of the Ge:Mn matrix, as determined from both HR-TEM and SQUID measurements.

Local chemical distribution and electronic structure of Ge 1−xT x DMS single crystals (T=Cr, Mn, Fe)

The spatial concentration distribution and local electronic structure of ferromagnetic Ge 1− x T x (T=Cr, Mn, Fe) DMS single crystals have been investigated by using scanning photoelectron microscopy (SPEM), X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). It is found that doped T ions in Ge 1− x T x crystals are chemically phase-separated, suggesting that the observed ferromagnetism arises from the phase-separated T-rich phases in Ge 1− x T x .

Titania-German Nanocomposites for Quantum Dot Solar Cells

ABSTRACTSeveral new photovoltaic semiconductor materials and technologies have been developed due to the increasing need for renewable energy sources. Quantum dot (QD) based solar cell is potentially one of the best contenders. We have developed a thermodynamically stable nanocomposite (stable up to 900°C) titania-germanium (TiO2-Ge) which shows promise as the active layer for QD solar cells. In TiO2-Ge nanocomposites Ge nanodots are distributed in a TiO2 matrix. Due to the 3-D quantum confinement effect, tailoring of the optoelectronic properties is relatively easily done by simply varying the Ge nanodots size. Ge is particularly advantageous since its Bohr radius is relatively large, 25 nm. In this paper results of the variation of the optoelectronic properties of TiO2-Ge nanocomposites as a function of the nanostructure parameters are presented. TiO2-Ge nanocomposite thin films were synthesized using RF magnetron sputtering. The structural studies (by XRD, HRTEM) established the fact that the Ge concentration in the composite target governs the size of the Ge nanodots whereas RF sputtering power controls the density of Ge nanodots in the nanocomposite films. The optical spectroscopic studies showed that the variation of the size of Ge nanodots in the TiO2-Ge films shifts the absorption edge from ∼ 0.66 eV (infrared) to ∼ 2.3 eV (blue-green). Similarly, dark conductivity also varies in a wide range of 10−7 - 10−2 Scm−1 by altering the concentration as well as the structural phase (amorphous or crystalline) of Ge.

Optical, thermal and phase transition studies in Sn1-x Ge x Te

The optical and thermal properties of the mixed semiconducting alloy, Sn1-xGexTe, is studied by photo acoustics, for various Ge concentrations and phase transition for a particular concentration is also studied by the same method. The results are compared with the available literature values and discussed.

Growth of high quality Er–Ge films on Ge(001) substrates by suppressing oxygen contamination during germanidation annealing

Solid-state reactions between Er and Ge (001) under different processing conditions were investigated. Under normal rapid thermal processing (RTP) in high-purity N 2 ambience, the Er–Ge film formation was ‘contaminated’ with Er 2O 3 even at low temperature annealing. Ti capping of Er films before RTP delayed Er 2O 3 formation with the Ti cap acting as a sacrificial layer for the Er underneath. Vacuum annealing of Er films significantly reduced Er 2O 3 formation even after higher temperature annealing. High quality Er–Ge films can thus be formed through solid-state reaction of Er and Ge if oxygen contamination from annealing ambient during RTP is controlled. The Er–Ge phase had low sheet resistance values averaging 3 to 4 Ω/sq. ErGe 1.8 was formed from the solid-state reaction between Er and Ge(001) in vacuum.

Identification of soft phonon modes in Ge–Sb–Te using electron diffraction

The atomistic structure of crystalline Ge–Sb–Te thin film for phase-change optical recording was investigated using transmission electron microscopy and nanobeam electron diffraction. Nonradial diffuse streaks were observed in electron diffraction patterns obtained from laser-induced crystalline phases of Ge–Sb–Te thin films. The intensities of the diffuse streaks were pronounced in particular directions in this alloy. The diffuse streaks were due to low-frequency transverse lattice waves that occur along directions perpendicular to the near neighbor zigzag atomic chains.