Ge K-edge Research Articles

Nanometer Resolution XANES Imaging of Individual PC-RAM Devices

ABSTRACTWe introduce a technique to permit x-ray absorption spectroscopy studies focusing on individual phase-change (Ge2Sb2Te5) memory cells in fully integrated PC-RAM structures. Devices were investigated employing an x-ray nanobeam of only about 300 nm diameter, which could be fully contained within the spatial extent of the active area within a single device cell and enabled us to investigate individual devices without interference from non-switching material surrounding the area of interest. By monitoring the fluorescence signals of tungsten and germanium at a photon energy corresponding to the Ge K-edge absorption edge white line position, we were successful in producing 2D area maps of the active cell region, which clearly show the imbedded tungsten heater element and the switched region of the phase change material. Additionally, position dependent changes in the phase change material could be traced by taking an array of XANES spectra at the Ge K-edge on and in the vicinity of individual devices.

In Situ X-ray Absorption Spectroscopy Study of Si1–xGexO2 Dissolution and Germanium Aqueous Speciation under Hydrothermal Conditions

The dissolution of Si(1-x)Ge(x)O(2) solid solutions under hydrothermal conditions was studied by in situ X-ray absorption spectroscopy. Experiments were performed at the Ge K-edge using a high-pressure cell mounted on the FAME beamline of the European Synchrotron Radiation Facility. Spectra in both transmission and fluorescence mode were collected in isobaric conditions (100 and 150 MPa) up to 475 °C. The local atomic structure around the Ge atom was investigated as a function of the temperature and in pure water and sodium hydroxide solutions. In pure water, the solubility of the cristobalite-type Si(0.8)Ge(0.2)O(2) increases with the temperature and the Ge atom is in 4-fold coordination. In a sodium hydroxide aqueous solution, a complex between Ge and Na atoms forms and gives rise to precipitation of sodium germanates. Under these conditions, the Ge content in the solution decreases with increasing temperature. These results show that a sodium hydroxide aqueous solution, usually used for quartz crystal growth, is not suitable for Ge-containing crystals. The dissolution kinetics and phase transformation of the solid solution were studied as a function of the atomic fraction of Ge. Ge-rich solid solutions dissolve and transform to stable phases faster than Ge-poorer composition, giving rise to important variations of the Ge content in solution.

Molecular dynamics simulations of EXAFS in germanium

Abstract Classical molecular dynamics simulations have been performed for crystalline germanium with the aim to estimate the thermal effects within the first three coordination shells and their influence on the single-scattering and multiple-scattering contributions to the Ge K-edge extended x-ray absorption fine structure (EXAFS).

Influence of hydrogen on thermally induced phase separation inGeO/SiO2 multilayers

The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide inGeOx/SiO2 multilayers(x≈1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorptionspectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminishwith increasing annealing temperature, showing complete phase separation at approximately450 °C using inertN2 ambient. Theuse of reducing H2 in the annealing atmosphere influences the phase separation even atan early stage of the disproportionation. In particular, the temperatureregime where the phase separation occurs is lowered by at least50 °C. At temperaturesabove 400 °C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of thereduction of GeO2 by H2.

Microscopic origin of demixing in Ge 20Se xTe 80− x alloys

The structure of Ge 20Se x Te 80− x ( x = 5, 10, 15, 20) glasses has been investigated by X-ray and neutron diffraction as well as Ge and Se K-edge extended X-ray absorption fine structure (EXAFS) measurements. Experimental datasets have been fitted simultaneously by the reverse Monte Carlo simulation technique. It has been found that all components obey the 8-N rule. Se binds preferentially to Ge. At x = 20, experimental data can be fitted without Se–Te bonding, but Se–Se bonds appear, and the Te–Te coordination number starts to increase. These observations show that though Se and Te are completely miscible in the liquid and in the solid phases the presence of Ge induces nanoscale phase separation.

Permanent Ge Coordination Change Induced by Pressure in La2O3-B2O3-GeO2 Glass

In this study, we report the results of an in situ X-ray absorption spectroscopy investigation of the La2O3–B2O3–GeO2 (LBG) glass compressed in a diamond anvil cell until 17 GPa at ambient temperature. A pressure-induced coordination change of germanium from fourfold to sixfold is shown here. The analysis of Ge K-edge X-ray absorption near-edge structure spectra supports a model of mixing of fourfold and sixfold coordinated Ge sites during the compression/decompression cycle. This transformation proves to be irreversible: about 25% of sixfold coordinated Ge still exists at ambient pressure. This phenomenon underlines a polymorphism property of the LBG glass, whose origin lies in the complexity of composition.

Phase separation and nanocrystal formation in GeO

The temperature-induced phase separation (disproportionation) and Ge nanocrystal formation in bulk amorphous germanium monoxide (a-GeOx,x≈1) are studied both in situ and ex situ by measurements of the x-ray absorption near edge structure at the Ge K-edge and x-ray diffraction. The considerable amount of suboxides contained in the native a-GeO samples decreases with increasing annealing temperature. The phase separation sets in at a temperature of 260±20 °C and is almost completed at a temperature of 450±18 °C before nanocrystal formation occurs. Ge nanocrystals of a few nanometers in diameter are observed for an annealing temperature of 509±15 °C. The time dependence of the phase separation and the effect of different annealing procedures are discussed. The presented results provide important information for the production of Ge nanocrystals embedded in amorphous oxide matrices which are relevant for optoelectronic applications.

Temperature-induced obliteration of sub-oxide interfaces in amorphous GeO

Abstract The temperature-induced phase separation of native amorphous germanium monoxide, a-GeO, into Ge and GeO2 was studied by measurements of the Ge K-edge employing partial fluorescence yield detection. In the native a-GeO samples a significant amount of sub-oxides was found. This sub-oxide content decreases with increasing annealing temperature. At elevated temperatures the formation of nanocrystals was observed. The results indicate a structure of a-GeO which consists of nanoscaled Ge and GeO2 clusters separated by sub-oxide interfaces.

Tetragonal Phase in Ge Doped HfO2 Films on Si Investigated by X-ray Absorption Spectroscopy

AbstractThe stabilization of the tetragonal phase of 5 nm thick HfO2 films by Ge doping is investigated using x-ray absorption spectroscopy around O and Ge Kedges and by Rutherford backscattering spectrometry. We show that Ge concentrations higher than ˜5at.% are not stable during rapid thermal anneal at temperatures as low as 750°C and that the tetragonal phase of HfO2 is achieved at this Ge concentration.

Temperature dependence of EXAFS Debye-Waller factor in the high pressure perovskite SrGeO3

The temperature dependence of EXAFS Debye-Waller factors in SrGeO3 perovskite, which synthesize in a cubic anvil type apparatus under 9GPa and 1000K, was investigated with the cumulant expansion method. The Sr and Ge K-edge EXAFS spectra were measured in transmission mode at various temperatures from 30 to 300K. Effective pair potentials, V(u) =αu2/2, were evaluated by harmonic model, the potential coefficient α for Sr-O, Ge-O, Ge-Sr, Sr-Sr and Ge-Ge distances in SrGeO3 perovskite were 2.6 e/VÅ2, 10.3 e/VÅ2, 6.5 e/VÅ2, 4.3 e/VÅ2 and 8.6 e/VÅ2, respectively. Ge-O bond has the largest value. The coefficients for Sr-O bond and Sr-Sr distances are small values in composition with other cubic perovskite-type compounds. It is suggested that the amorphouszation occurs at relatively lower temperature of 500K in SrGeO3 perovskite because these values are very small. In AII+BIV+O3-type perovskite, the potential coefficient for the bond distance which associated B site atom is larger than that for the others distances and this tendency is also seen in high pressure perovskite such as CaGeO3.

Atomistic observation of photo-expansion and photo-contraction in chalcogenide films by in situ EXAFS

In situ EXAFS measurements have been performed to study the changes in the local structure of a-As2S3 (around As and S K-edges) and a-GeSe2 (around Ge and Se K-edges) thin films. To study the effect of illumination on the local structure and environment, the structural parameters (nearest neighbor distances, Debye–Waller factor) are estimated for as-deposited films, during illumination and after illumination for both these compositions. For As2S3 films, there is an expansion in the sulfur nearest neighbor distances and a small contraction in the Arsenic nearest neighbor distance during illumination. For GeSe2 films, both Ge and Se show a contraction in the nearest neighbor distances by illumination. Ordering of the structure (a decrease in the Debye Waller factor) is seen with illumination in both the compositions. a-As2S3 films show larger photo-induced changes and also a larger transient part in the changes as compared to a-GeSe2 films, which is suggested to be due to a more 3d rigid structure in GeSe2. Larger changes are seen around the chalcogen atom as compared to As and Ge atoms. From the results, we are able to confirm photo-expansion in As2S3 films and photo-contraction in GeSe2 at an atomistic level.

The structure of SiO2–GeO2 glasses: A spectroscopic study

Four glasses of the SiO2–GeO2 binary system have been synthesized via a sol–gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO4 and SiO4 tetrahedra, with GeO4 tetrahedra similar to the GeO4 units in GeO2 glass and continuous compositional variations from GeO2-rich regions to SiO2-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge–O–Si angles are close to the Ge–O–Ge mean angle in GeO2 glass, 134° and 130°, respectively. This misfit with the Si–O–Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO2 glasses.

Ge nitride formation in N-doped amorphous Ge2Sb2Te5

The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3Nat.% doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.

Determination of partial pair distribution functions in amorphous Ge 15Te 85 by simultaneous RMC simulation of diffraction and EXAFS data

The atomic structure of amorphous Ge 15Te 85 has been studied by X-ray absorption spectroscopy (EXAFS) at Ge K-edge. EXAFS data were modeled simultaneously with existing X-ray and neutron diffraction measurements by the reverse Monte Carlo (RMC) simulation technique. Combination of the three independent measurements allowed separation of the partial pair correlation functions g GeTe( r) and g TeTe( r) and estimation of the corresponding coordination numbers.

Features of local atomic structure of nanocrystalline disordered Fe–M (M=Ge, Sn, Al, Si) alloys: EXAFS-study

The EXAFS-study (Fe, Sn and Ge K-edges) of disordered by mechanical activation binary supersaturated nanocrystalline solid solutions Fe–Al, Fe–Ge, Fe–Sn and Fe–Si is presented. The EXAFS-spectra are processed by solving the inverse binary problem, using the EXAFS-spectrum of the Fe K-edge only or combining the EXAFS-spectra on two K-edges, Fe and Ge or Fe and Sn. The parameters of partial correlation functions indicate chemical short-range ordering, high local static distortions in the lattice, increasing with metalloid content. The macrostructure of other type is forming through an initial, “local” stage within the bcc lattice.

Size dependent thermal properties of embedded crystalline germanium nanowires

Here we report the size-dependent melting points of crystalline germanium nanowires confined within the pores of hexagonal mesoporous silica templates. A supercritical fluid deposition technique was used to form the nanowire–template composite materials and differential thermal analysis, coupled to thermal gravimetric analysis, was used to determine the melting points of the embedded Ge nanowires with mean diameters ranging from 22 to 85 A. The melting points of the Ge nanowires within the templates were found to be higher than the melting point of bulk germanium (937 °C), typically by 60 °C, and with a broad melting range (∼80 °C). Extended X-ray absorption fine structure (EXAFS) analysis of the Ge K-edge from the nanocomposite materials revealed a linear increase in the Ge–Ge nearest neighbour distance with decreasing nanowire diameter over the size range investigated. In all cases the Ge–Ge first shell distance in the nanowires was greater than in the bulk. This observed lattice expansion can be attributed to crystallographic deformation resulting from the strain imposed on the nanowires by the template. EXAFS studies also revealed an increase in the average number of oxygen atoms at the Ge/silica interface with decreasing diameter due to the increasing surface area. Interfacial effects and interactions at the nanowires/matrix interface are believed to delay the melting point of these systems.

Local structural and optical properties of GeSb phase-change materials

Local structural variation of Ge x Sb 1− x and (GeSb)Si alloy films was studied by extended X-ray absorption fine structure (EXAFS). From X-ray diffraction result, the as-deposited Ge x Sb 1− x alloy films consisted of amorphous phase but the analysis of Fourier transformed Ge K-edge EXAFS spectra indicated that the nearest neighbor of Ge atoms was changed with Ge content. The crystalline Ge x Sb 1− x films obtained by annealing showed the presence of Ge crystalline phase in GeSb solid solution. The calculated interatomic distance between Ge atoms for crystalline GeSb films was similar to that of pure Ge film. The optical reflectivity increased with decrease of Ge content and the Ge rich composition caused slow crystallization time. The effect of Si addition in GeSb alloy was also analysed.

Lead-Cadmium Fluorogermanate Glasses and Transparent Glass-Ceramics. Spectroscopy and Structure

In Lead-cadmium fluorogermanate glasses (PbF2-CdF2-PbGeO3) the addition of metal fluorides to the base PbGeO3 glass leads to a decrease of the glass transition temperature (Tg) and to an enhancement of the ionic conductivity properties. Based on different spectroscopic techniques (19F NMR, Ge K-edge X-ryas absorption and Raman scattering) an heterogeneous glass structure is proposed at the molecular scale, which can be described by fluoride rich regions permeating the metagermanate chains. The temperature dependence of the 19F NMR lineshapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluoride mobility in these systems. Eu3+ emission and vibronic spectra are used to follow the crystallization process leading to transparent glass ceramics.

Ge and Ga K-edge EXAFS analyses on the structure of Ge–Ga–S–CsBr glasses

The local structure of Ge and Ga ions in (1−x)(Ge0.25Ga0.10S0.65)–xCsBr glasses (x=0.00,0.05,0.10 and 0.12) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. CsBr formed [GaS3/2Br]− structural units in glass while Ge ions remained in GeS4/2 tetrahedra, unaffected by CsBr addition. Rare-earth ions can be surrounded by Br ions only when CsBr/Ga ratio in glass became larger than unity.

Relaxed state of GexSi1−x islands embedded in Si

The process of capping Ge islands with Si overlayers is known to have a strong influence on their composition and shape. In this work we have investigated Ge islands on Si produced by chemical vapor deposition covered with Si layers of different thickness. The structural characterization was carried out by X-ray absorption spectroscopy at the Ge-K edge. A noticeable Si uptake by the islands is evident upon capping. Bond length for the first three shells have been analyzed by comparison with models based on the valence force field method. The results evidence that the islands have, on the average, a relaxed state with presumably strained parts in contact with the Si matrix.