In situ energy dispersive X-ray diffraction study of iron disilicide thermoelectric materials

The dynamic phase transformation and structure of rapidly solidified Fe_1−xCo_xSi₂ (0.02?x?0.06) thermoelectric materials were in situ investigated under high temperatures and high pressures by energy dispersive X-ray diffraction using synchrotron radiation. The FeSi₂ alloys which solidified as α-Fe₂Si₅ and ε-FeSi eutectic structures, were transformed to the semiconducting β-FeSi₂ phase upon heating by the main reaction α+ε→β and the subsidiary reaction α→β+Si. The low heating rates and Co contents were found to be beneficial for the β phase formation. The decomposition temperature of β→α+ε was weakly dependent on heating rate, but significantly suppressed by the high pressures.     

Formation of ultrathin iron silicide layers on the single-crystal silicon surface

The solid-phase synthesis of iron silicides on the Si(100)2 × 1 surface with a 5-ML-thick iron film deposited at room temperature was studied by high-resolution photoelectron spectroscopy with the use of synchrotron radiation. Computer simulation of the measured Si 2p spectra revealed the formation of silicides in this system already under annealing at a temperature of 60°C. The process of formation consists in successive syntheses of three iron silicide phases, more specifically, monosilicide ε-FeSi, metastable disilicide γ-FeSi₂, and disilicide β-FeSi₂. The temperature ranges of existence of these phases were determined. Silicon was found to segregate on the γ-FeSi₂ surface.     

Quick preparation and thermal transport properties of nanostructured β-FeSi2 bulk material

This paper reports that the nanostructured β-FeSi2 bulk materials are prepared by a new synthesis process by combining melt spinning(MS) and subsequent spark plasma sintering(SPS).It investigates the influence of linear speed of the rolling copper wheel,injection pressure and SPS regime on microstructure and phase composition of the rapidly solidified ribbons after MS and bulk production respectively,and discusses the effects of the microstructure on thermal transport properties.There are two crystalline phases(α-Fe2Si5 and ε-FeSi) in the rapidly solidified ribbons;the crystal grains become smaller when the cooling rate increases(the 20 nm minimum crystal of ε-FeSi is obtained).Having been sintered for 1 min above 1123 K and annealed for 5 min at 923 K,the single-phase nanostructured βFeSi2 bulk materials with 200-500 nm grain size and 98% relative density are obtained.The microstructure of β-FeSi2 has great effect on thermal transport properties.With decreasing sintering temperature,the grain size decreases,the thermal conductivity of β-FeSi2 is reduced remarkably.The thermal conductivity of β-FeSi2 decreases notably(reduced 72% at room temperature) in comparison with the β-FeSi2 prepared by traditional casting method.     

Optical properties of single-phase β-FeSi2 films fabricated by electron beam evaporation

Single-phase semiconducting iron disilicide (β-FeSi₂) films on silicon substrate were fabricated by electron beam evaporation (EBE) technique. For preventing the oxidation of Fe film, silicon/iron/silicon sandwich structure films with different thickness of silicon and iron were deposited and then annealed at different temperatures. X-ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR) measurements were carried out to study the phase distribution and crystal quality of the films. Single-phase β-FeSi₂ with high crystal quality was achieved after annealing at 800 °C for 5 h. An apparent direct bandgap E_g of approximately 0.85-0.88 eV was observed in the β-FeSi₂ films. It is considered that the silicon/iron/silicon sandwich structure is suited for formation of single-phase β-FeSi₂ with high crystal quality.     

Evolution under thermal annealing of Mn-doped iron disilicides obtained by ball milling

Phase formation in the Mn doped $\upbeta $ -FeSi₂ system (Fe_1???x Mn _x Si₂, with 0.00 ≤?x?≤ 0.24) was studied using X-ray diffraction and Mössbauer spectroscopy. Samples were prepared by the simultaneous mill of pure Si, Mn and Fe under Ar atmosphere followed by an annealing at 1,123 K during 4 h at 1 × 10^???7 Torr. After milling, an admixture of $\upbeta $ -FeSi₂, $\upalpha $ -FeSi₂ and $\upvarepsilon $ -FeSi phases was present while $\upalpha $ -FeSi₂ disappeared after annealing, resulting $\upbeta $ -FeSi₂ the main phase. Depending on Mn concentration, small amounts of $\upvarepsilon $ -FeSi and Si segregation were also observed. A preferential substitution of Fe atoms by Mn ones in the Fe_II site of the $\upbeta $ -FeSi₂ regular lattice was inferred from the Mössbauer results.     

Processes of silicide formation in the Fe/Si(111)7 × 7 system

The initial stages of the formation of iron silicides in the Fe/Si(111)7 × 7 system in the course of solid-phase epitaxy are investigated using high-resolution photoelectron spectroscopy (～100 meV) with synchrotron radiation. The spectra of the Si 2p core and valence-band electrons obtained after deposition of iron coverages of up to 28 monolayers on the surface of the sample and subsequent isochronous annealings at 650°C are measured and analyzed. It is shown that the first to form under Fe deposition is an ultrathin film of the metastable silicide FeSi with a CsCl-type structure, on which a layer of the Fe-Si solid solution with segregated silicon grows. At coverages in excess of 10 monolayers, an iron film grows on the surface of the sample. Annealing of a silicon crystal coated with a Fe layer leads to the sequential formation of two stable silicide phases, namely, the ?-FeSi and β-FeSi₂ phases, in the near-surface region of the sample. It is found that the process of solid-phase synthesis of the ?-FeSi phase passes through the stage of transformation of the iron film into the Fe-Si solid solution.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Iron disilicide formation by Au-Si eutectic reaction on Si substrate

We have investigated the growth of iron disilicide on Au-coated Si(0 0 1) substrates and its photoluminescence behaviour. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy observations revealed that the Si surface above 380 °C was melted as a result of the Au-Si eutectic reaction and that coarse island disilicide grains with sizes of several micrometres were formed on the Si surface. The full width at half maximum of 0.056° on the rocking curve of α-FeSi₂004 was observed on the sample deposited at 800 °C, and indicated the high crystal quality in perfection of orientation. The photoluminescence spectrum of β-FeSi₂ grains, which were deposited at 750 °C, was observed. The melted Si surface contributed to the improved crystallinity of α-FeSi₂ and β-FeSi₂.     

Anisotropic layered high-temperature thermoelectric materials based on the two-phase CrSi2-β-FeSi2 system

The feasibility of synthesizing a wide spectrum of multiphase microstructurally ordered high-temperature thermoelectrics with highly anisotropic thermoelectric parameters is demonstrated with an aluminum-doped CrSi₂-β-FeSi₂ system the composition of which varies from Cr_0.1Fe_0.9Si_2?x Al _x to Cr_0.9Fe_0.1Si_2?x Al _x (x = 0.0–0.4). Doping of either phase (CrSi₂ and β-FeSi₂) is viewed as a promising way for synthesizing n- and p-type domains inside the same sample.     

Microdisperse Iron Silicide Structures Produced by Implantation of Iron Ions in Silicon

Iron implanted and subsequently annealed n-type Si(111) was studied by conversion electron Mössbauer spectroscopy for phase analysis and Auger electron spectroscopy for sputter depth profiling and element mapping. During implantation (200 keV, 3 × 10¹⁷ cm^?2, 350°C) a mixture of β- and α-FeSi₂ is firmed and after the subsequent annealing (900°C for 18 h and 1150°C for 1 h) a complete transition to the β- and the α-phase can be detected. The as-implanted profile has Gaussian shape and is broadening during annealing at 900°C to a plateau-like profile and shows only a slight broadening and depth depending fluctuations of the iron concentration after the 1150°C annealing. With scanning Auger electron spectroscopy the lateral iron and silicon distribution were investigated and show for the sample annealed at 900°C large separated β-FeSi₂ precipitates which grow due to the process of Ostwald ripening. At 1150°C additionally coalescence of the precipitates occur and a wide extended penetration α-FeSi₂ network structure is formed.     

The effect of titanium diboride addition on the thermoelectric properties of β-FeSi2 semiconductors

β-FeSi₂-TiB₂ composites with various amounts of TiB₂, from 0 up to 30 vol%, were prepared by hot pressing. The electrical and thermal conductivities, and the Seebeck coefficient were measured as a function of temperature. The results show that the thermal and electrical transport behavior of the composites is different as the volume fraction of TiB₂ is below and above about 0.255. A 5 vol% TiB₂ added sample has higher figure of merit than one without TiB₂ for temperatures above 650 K. The influence of an additional phase, ε-FeSi, formed during the hot pressing, on the thermoelectric properties of the β-FeSi₂-TiB₂ composites was also discussed.     

Phase control of iron silicides using femtosecond laser irradiation

Phase control of Fe–Si amorphous thin film in micro area is demonstrated using femtosecond laser irradiation. A femtosecond laser beam with a high repetition rate over 200 kHz and tightly focused through an objective lens promotes both crystallization and phase transformation from an amorphous phase into crystalline β-FeSi₂, α-FeSi₂, or ε-FeSi phases. Formation of each crystalline phase is possible by changing the pulse energy or the scanning speed of the incident laser beam.     

Mössbauer study of the stability of Fe5Si95 produced by inert gas condensation

Fe₅Si₉₅ specimens with an enhanced solubility of Fe in Si by about twenty-one orders of magnitude relative to crystalline Si at room temperature were prepared by the inert gas condensation technique. The thermal stability of the Fe₅Si₉₅ sample was investigated by Mössbauer spectroscopy. The spectrum of the as-prepared Fe₅Si₉₅ exhibited a broadened many different iron sites in the sample. This material was thermal stable up to temperatures of 673k for one hour. After annealing at 773K for one hour, the intermetallic compound α-FeSi₂ was formed in the annealed sample, probably via the precipitation process. The amount of the α-FeSi₂ phase increased with the annealing temperature. No β-FeSi₂ phase was observes in any of the annealed samples up to an annealing temperature of 1273K For one hour.     

X-ray and luminescent analysis of finely dispersed β-FeSi2 films formed in Si by pulsed ion-beam treatment

Finely dispersed β-FeSi₂ films were formed by implanting Fe⁺ ions with an energy of 40 keV and a dose of 1×10¹⁶ cm⁻² in Si single crystals, followed by nanosecond pulsed ion-beam treatment. The results of glancing incidence x-ray diffraction indicate the formation of a highly grain-oriented film consisting of inclusions of the iron disilicide phase (β-FeSi₂) with a grain size of approximately 40 nm surrounded by a polycrystalline Si matrix. The photoluminescence spectroscopy data reveal that the photoluminescence signal with a peak around 1.56 μm, which is observed up to 210 K, is associated with direct interband transitions in β-FeSi₂ and not with the contribution from the dislocation-induced line D1. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 9, 2001, pp. 1569–1572. Original Russian Text Copyright ? 2001 by Bayazitov, Batalov, Terukov, Kudoyarova.     

Determination of the Hyperfine Parameters of Iron Silicides by Angle Dependent Conversion Electron Mössbauer Spectroscopy on Single Crystals

Hyperfine Interactions - The Mössbauer spectra of single crystals of the iron silicides ε-FeSi, β-FeSi2 and α-FeSi2 are collected by conversion electron Mössbauer...     

NGR study of iron molybdates and iron-cobalt molybdates catalysts

Iron molybdates FM and cobalt-iron molybdates CF _x M (which means Co_1?y Fe _y MoO₄ withy=x/100 6≤x≤67), used as matrix of catalysts for propene mild oxidation, were studied by Mössbauer spectroscopy at 295 K and 4 K. All the spectra exhibit three Fe²⁺ doublets, two of them correspond to β-FeMoO₄ and the third one to α-FeMoO₄. Enhancement of quadrupole splittings of Fe²⁺ in the spectra of the CF _x M catalysts is ascribed to the occurrence of a solid solution of either iron in CoMoO₄ or cobalt in FeMoO₄. In each spectrum at 295 K one or several Fe³⁺ doublets were present, which low temperature spectra allow to be assigned to Fe₂(MoO₄)₃, small particles of Fe₂O₃, solid solution of Fe³⁺ in ferrous molybdates and even Fe₂MoO₄ in some cases. The behaviour of all the iron species of these solids during reduction by hydrogen is described.     

Electronic structure and simulation of the dielectric function of β-FeSi2 epitaxial films on Si(111)

The optical functions of iron disilicide (β-FeSi₂) thin epitaxial films are calculated from the reflectance spectra in the energy range 0.1–6.2 eV with the use of the Kramers-Kronig (KK) integral relations. A comparison of the results of calculations from the transmittance and reflectance spectra and the data obtained from the reflectance spectra in terms of the Kramers-Kronig relations indicates that the fundamental transition at an energy of 0.87±0.01 eV is a direct transition. An empirical model is proposed for the dielectric function of β-FeSi₂ epitaxial films. Within this model, the specific features in the electronic energy-band structure of the epitaxial films are described in an analytical form. It is shown that the maximum contributions to the dielectric function and the reflectance spectrum in the energy range 0.9–1.2 eV are made by the 2D M ₀-type second harmonic oscillator with an energy of 0.977 eV. This oscillator correlates with the second direct interband transition observed in the energy-band structure of β-FeSi₂.     

Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

Epitaxial growth and optical characterization of compound semiconductor β-FeSi2 prepared by pulsed laser deposition

β-FeSi₂ thin films were prepared on FZ n-Si (1 1 1) substrates by pulsed laser deposition (PLD). The structural properties and crystallographic orientation of the films were investigated by X-ray diffraction (XRD) analysis. This indicates that β-FeSi₂/Si (2 0 2/2 2 0) and the single-crystalline β-FeSi₂ can be prepared using PLD. In photoluminescence (PL) measurements at 8 K detected by Ge detector, the PL spectra of the samples annealed at 900 °C for 1, 5, 8 and 20 h showed that the PL intensity of the A-band peak increased depending on annealing time in comparison with those of as-deposited samples. The intrinsic PL intensity of the A-band peak at 0.808 eV of the β-FeSi₂ from the 20-h-annealed sample was investigated for the first time by the PLD method detected by an InGaAs detector. This result has been confirmed by temperature dependence and excitation power density of the 20-h-annealed sample with the comparison of other defect-related band peaks of the sample. Cross-sectional scanning electron microscopy (SEM) observation was also performed and the thickness of the thin films was found to be at 75 nm for 20-h-annealed. The thermal diffusion for the epitaxial growth of β−FeSi₂/Si was observed when the compositional ratio of Fe to Si was around Fe:Si=1:2 for 20-h-annealed carried out by energy dispersive X-ray spectroscopy (EDX). We discussed high crystal quality of the epitaxial growth and optical characterization of β-FeSi₂ achieved after annealing at 900 °C for 20 h.     

Hydrothermal synthesis and structural characterization of (1−x)α-Fe2O3-xSnO2 nanoparticles

Structural and morphological characteristics of (1−x)α-Fe₂O₃-xSnO₂ (x=0.0-1.0) nanoparticles obtained under hydrothermal conditions have been investigated by X-ray diffraction (XRD), transmission Mössbauer spectroscopy, scanning and transmission electron microscopy as well as energy dispersive X-ray analysis. On the basis of the Rietveld structure refinements of the XRD spectra at low tin concentrations, it was found that Sn⁴⁺ ions partially substitute for Fe³⁺ at the octahedral sites and also occupy the interstitial octahedral sites which are vacant in α-Fe₂O₃ corundum structure. A phase separation of α-Fe₂O₃ and SnO₂ was observed for x≥0.4: the α-Fe₂O₃ structure containing tin decreases simultaneously with the increase of the SnO₂ phase containing substitutional iron ions. The mean particle dimension decreases from 70 to 6 nm, as the molar fraction x increases up to x=1.0. The estimated solubility limits in the nanoparticle system (1−x)α-Fe₂O₃-xSnO₂ synthesized under hydrothermal conditions are: x≤0.2 for Sn⁴⁺ in α-Fe₂O₃ and x≥0.7 for Fe³⁺ in SnO₂.