Iron Silicide Films Research Articles

Composition and thermoelectric properties of structures based on iron silicide grown by pulse laser deposition

Silicon compounds have a wide range of electrical properties. In particular, the possibility of creating thermoelectric converters based on them looks extremely attractive. The use of most silicides as thermoelectrics today is limited by their low efficiency. The development of approaches consisting in the creation of low-dimensional structures using non-equilibrium formation methods is one of the priority directions for improving the properties of thermoelectric generators. Determination of the effect of technological regimes on the structure, phase-chemical composition and thermoelectric properties of metal-silicide structures is a key task, the solution of which will allow creating highly efficient thermoelectric generators based on them. Thin-film structures with a layer thickness of ~50 nm formed at different growth temperatures by pulsed laser deposition on two types of substrates: sapphire and gallium arsenide coated with an Al2O3 nanolayer were studied in this work. On the formed samples, a chemical analysis and a study of the phase composition were performed. Chemical analysis was carried out by X-ray photoelectron spectroscopy with the chemical composition depth profiling. The phase composition was studied by Raman spectroscopy. In addition, analysis of the elements in the films was carried out by X-ray spectral microanalysis based on a scanning electron microscope. To determine the thermoelectric properties of the formed thin-film structures, the temperature dependences of the Seebeck coefficient and the electrical conductivity coefficient were recorded. The dependence of the thermoelectric characteristics of iron silicide films on the phase composition is analyzed. In particular, measurements of the thermoelectric properties of FeSix thin-film structures registered the manifestation of a strong thermoelectric effect in layers with the maximum number of chemical bonds between iron and silicon. The parameters of the growth process at which the most effective formation of iron-silicon chemical bonds is achieved were determined using the method of X-ray photoelectron spectroscopy. Line shifts from the beta phase of iron disilicide were found in the Raman spectra and the reasons for their appearance were proposed

Structural Properties of Fe3Si thin Films at Different Pulsed-Laser-Deposition Modes

The formation and investigation of Fe3Si thin films by pulsed laser deposition is reported. The structural properties depending on the formation parameters are investigated. Three different approaches to the synthesis of iron-silicide thin films by pulsed laser deposition are explored: deposition from a stoichiometric alloy target, the simultaneous co-deposition of pure Fe and Si targets by two lasers and the single-laser cyclic co-deposition of pure targets.

Magnetic properties of iron silicide films ion-synthesized in the field of mechanical stresses

Abstract The effect of the tensile stress applied during ion beam synthesis of thin silicide films on magnetic properties is studied. The magneto-optical Kerr effect revealed the existence of both magnetically anisotropic and isotropic regions in the synthesized thin film. In the isotropic regions, the increase in the coercive force is observed as the external tensile stresses decrease. It is assumed that in this area stress relaxation and the subsequent growth of the finely dispersed silicide phase occur. In accordance with the Herzer model, large values of the coercive force are associated with relatively large crystallite sizes. In the magnetically anisotropic region, the anisotropy field is proportional to the stress applied during implantation that indicates the magnetoelastic nature of the induced anisotropy.

Effect of chemical ordering on optical properties of Fe3Si epitaxial films

Optical characteristics (electron energy loss function, optical conductivity σ, permittivity ε, refractive index n, extinction coefficient k, and absorption coefficient α) of a 30 nm thick epitaxial Fe3Si iron silicide films grown at different silicon substrate temperature (26, 100, 200, 300 ˚C) were determined within E = 0.74–6.46 eV photon energy range using spectroscopic ellipsometry technique. The experimental data are compared to the optical characteristics calculated in the framework of the density functional theory using the GGA–PBE approximation. Variations of the optical characteristics spectra are discussed from the point of view of chemical ordering of DO3 type crystal structure. It is asserted that the electron energy-loss function, optical conductivity and extinction coefficient of the Fe3Si iron silicide films undergo noticeable changes in different spectral ranges over the whole spectrum between 0.74 and 6.46 eV due to variation in the chemical order. Information on the effect of chemical ordering on the optical properties obtained here allows one to carry out quick qualitative analysis of Fe3Si film crystal quality during the synthesis procedures by ellipsometry method in situ.

Determination of magnetic anisotropies and miscut angles in epitaxial thin films on vicinal (111) substrate by the ferromagnetic resonance

A method for determining magnetic anisotropy parameters of a thin single-crystal film on vicinal (111) substrate as well as substrate miscut angles from angular dependence of ferromagnetic resonance field has been proposed. The method is based on the following: (i) a new approach for the solution of the system of nonlinear equations for equilibrium and resonance conditions; (ii) a new expression of the objective function for the fitting problem. The study of the iron silicide films grown on vicinal Si(111) substrates with different miscut angles confirmed the efficiency of the method. The proposed method can be easily generalized to determine parameters of single-crystal films grown on substrates with an arbitrary cut.

Thermoelectric Properties of Epitaxial β-FeSi2 Thin Films on Si(111) and Approach for Their Enhancement

We have investigated the intrinsic thermoelectric properties of epitaxial β-FeSi2 thin films and the impact of phosphorus (P) doping. Epitaxial β-FeSi2 thin films with single phase were grown on Si(111) substrates by two different techniques in an ultrahigh-vacuum molecular beam epitaxy (MBE) system: solid-phase epitaxy (SPE), where iron silicide films formed by codeposition of Fe and Si at room temperature were recrystallized by annealing at 530°C to form epitaxial β-FeSi2 thin films on Si(111) substrates, and MBE of β-FeSi2 thin films on epitaxial β-FeSi2 templates formed on Si(111) by reactive deposition epitaxy (RDE) at 530°C (RDE + MBE). Epitaxial SPE thin films based on codeposition had a flatter surface and more abrupt β-FeSi2/Si(111) interface than epitaxial RDE + MBE thin films. We investigated the intrinsic thermoelectric properties of the epitaxial β-FeSi2 thin films on Si(111), revealing lower thermal conductivity and higher electrical conductivity compared with bulk β-FeSi2. We also investigated the impact of doping on the Seebeck coefficient of bulk and thin-film β-FeSi2. A route to enhance the thermoelectric performance of β-FeSi2 is proposed, based on (1) fabrication of thin-film structures for high electrical conductivity and low thermal conductivity, and (2) proper choice of doping for high Seebeck coefficient.

Ferromagnetic resonance investigation of magnetic anisotropy of ion-synthesized iron-silicide thin films

40 keV iron ions have been implanted into single-crystal silicon wafers at room temperature. As a result, thin iron-silicide films were synthesized in the near-surface region of substrates. Scanning magnetooptical Kerr effect studies showed that some films had the uniaxial magnetic anisotropy. The coercive force determined in the direction of the easy magnetization for films with the magnetic anisotropy increased with the increase in the implantation fluence. However, ferromagnetic samples became isotropic when the dose of the order of 2.6×1017 cm–2 was reached. Ferromagnetic resonance studies showed that the resonance linewidth in isotropic samples increased with the temperature decrease. It was found that the ferromagnetic resonance linewidth for samples with the uniaxial anisotropy was less than that for isotropic ones. The observed behavior of resonance spectra can be explained on the basis of the model taking into account the effect of thermal fluctuations of the resonance line shape in disperse ferromagnets.

Surface structure and electronic states of epitaxialβ−FeSi2(100)/Si(001) thin films: Combined quantitative LEED,ab initioDFT, and STM study

The surface structure of epitaxial $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{FeSi}}_{2}$(100) thin film grown on Si(001) was analyzed using the quantitative low-energy electron diffraction intensity-voltage (LEED I-V) method, ab initio density functional theory (DFT) calculations, and scanning tunneling microscopy (STM). LEED patterns measured on the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{FeSi}}_{2}$(100) surface reveal two domains of a $p(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction with $p2gg$ diffraction symmetry. The iron-silicide film truncation and atomic surface structure were determined by LEED I-V method: The smallest Pendry's reliability factor ${R}_{P}=0.22\ifmmode\pm\else\textpm\fi{}0.02$ was achieved for the bare $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{FeSi}}_{2}$ film truncated by an Si layer, whereas Si and Fe ad-atom structures were excluded. Significant atomic relaxations within the topmost surface layers were revealed by the LEED I-V method and confirmed by DFT. The simulated STM patterns from the best-fit model agree well with the measured STM images on the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{FeSi}}_{2}(100)/\mathrm{Si}(001)\text{\ensuremath{-}}p(2\ifmmode\times\else\texttimes\fi{}2)$ surface: Four Si atoms on a surface form one bright protrusion on STM patterns. Electronic band structure analysis of the bulk and epitaxial $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{FeSi}}_{2}$(100) was carried out. A bare truncated epitaxial film was found to be metallic. Surface electronic states were identified by a partial $k$-resolved atomic-orbital based local density-of-state analysis.

Optical characteristics of an epitaxial Fe3Si/Si(111) iron silicide film

The dispersion of the relative permittivity ɛ of a 27-nm-thick epitaxial Fe3Si iron silicide film has been measured within the E = 1.16–4.96 eV energy range using the spectroscopic ellipsometry technique. The experimental data are compared to the relative permittivity calculated in the framework of the density functional theory using the GGA-PBE approximation. For Fe3Si, the electronic structure and the electronic density of states (DOS) are calculated. The analysis of the frequencies corresponding to the transitions between the DOS peaks demonstrates qualitative agreement with the measured absorption peaks. The analysis of the single wavelength laser ellipsometry data obtained in the course of the film growth demonstrates that a continuous layer of Fe3Si iron silicide film is formed if the film thickness achieves 5 nm.

The formation of magnetic silicide Fe3Si clusters during ion implantation

A simple two-dimensional model of the formation of magnetic silicide Fe3Si clusters during high-dose Fe ion implantation into silicon has been proposed and the cluster growth process has been computer simulated. The model takes into account the interaction between the cluster magnetization and magnetic moments of Fe atoms random walking in the implanted layer. If the clusters are formed in the presence of the external magnetic field parallel to the implanted layer, the model predicts the elongation of the growing cluster in the field direction. It has been proposed that the cluster elongation results in the uniaxial magnetic anisotropy in the plane of the implanted layer, which is observed in iron silicide films ion-beam synthesized in the external magnetic field.

Thermoelectric properties of sputtered iron-silicide

We have investigated thermoelectric properties of oxygen-included iron-silicide thin films prepared by facing target sputtering for applying them to thermoelectric devices. The highest Seebeck coefficient of 17.84 μV/K are obtained. It is the first attempt to investigate thermoelectric properties of oxygen-included iron-silicide films.

Study of ultrathin iron silicide films grown by solid phase epitaxy on the Si(001) surface

Ultrathin films of iron silicide have been grown by high-temperature annealing of 0.14-to O.5O-nm-thick Fe films deposited on the Si(001) surface at room temperature. It has been found that annealing leads to the formation of nanoislands of iron silicide on the surface, so that their type depends on the thickness of the Fe film. High-energy electron diffraction and atomic force microscopy measurements have revealed that the deposition of Fe films less than 0.32 nm thick on the Si(001) surface stimulates epitaxial growth of both three-dimensional β-FeSi2 and two-dimensional γ-FeSi2 islands. It has been found that, for Fe coverages of more than 0.32 nm thick, a complete transition to solide phase epitaxy is observed only for two-dimensional β-FeSi2 islands. The effect of prolonged annealing at 850°C on the morphology of the surface of the iron silicide film has been investigated.

X‐ray photoelectron spectroscopy of SPE‐grown bcc‐Fe, polycrystal and β‐FeSi2 phases on Si(111) surfaces

AbstractUltrathin films of iron silicides were grown on Si(111) surfaces by solid phase epitaxy (SPE); iron deposition and subsequent annealing, and analyzed using in situ high energy‐resolution X‐ray photoelectron spectroscopy (XPS). We analyzed peaks using Lorentzian and Donaich‐Šunjić curves and estimated the validity of both fittings. Under the sequence from bcc‐Fe to polycrystal phase (the mixture of Fe3Si and β‐FeSi2), and then β‐FeSi2 phase with annealing process, the slight chemical shifts of Fe 2p3/2 toward higher binding energy (BE) and the reduction of peak asymmetry for Fe 2p3/2 and Fe 2p1/2 were observed. We attributed this behavior to the reduction of the metallicity in iron silicide films. Copyright © 2008 John Wiley & Sons, Ltd.

Magnetic linear dichroism in photoemission from an ultrathin iron silicide film

The effect of magnetic linear dichroism in photoemission of Fe 3p electrons was used to investigate the magnetic properties of the Si(100)2 × 1 surface on which iron films up to 10 monolayers thick were deposited at room temperature under ultrahigh vacuum. The experiments were performed with linearly polarized light (at a photon energy of 135 eV) incident at an angle of 30° to the surface. The photoelectron spectra were measured in a narrow solid angle oriented along the normal to the sample surface for two opposite magnetization directions which were parallel to the surface plane and perpendicular to the polarization vector of the light wave. An analysis of the data obtained showed that the effect has a threshold character and appears after deposition of eight Fe monolayers, when the ferromagnetic silicide Fe3Si is formed on the surface.

Reaction of Iron with Amorphous Silicon and Crystal Silicon for the Fabrication of Iron Silicides

Iron silicide films with two different structures were fabricated by electron beam evaporation (EBE) technique. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscope (SEM) were carried out to describe the characteristics and structures of the films. It was found that after annealing at 800oC for 5 h, the β-FeSi2 film formed in the sample with the structure of Si/Fe film on silicon substrate, while only FeSi film generated in the sample with the structure of Si/Fe/Si films on silicon substrate. It is considered that the different iron silicides may be due to the different reaction of iron with crystal silicon or amorphous silicon, which is related to diffusion of iron or silicon atoms.

Magnetization reversal of ultrathin Fe film grown on Si(111) using iron silicide template

Ultrathin Fe films were epitaxially grown on Si(111) by using an ultrathin iron silicide film with p(2 × 2) surface reconstruction as a template. The surface structure and magnetic properties were investigated in situ by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), and surface magneto-optical effect (SMOKE). Polar SMOKE hysteresis loops demonstrate that the Fe ultrathin films with thickness t < 6ML (monolayers) exhibit perpendicular magnetic anisotropy. The characters of M-H loops with the external magnetic field at difference angles and the angular dependence of coercivity suggest that the domain-wall pinning plays a dominant role in the magnetization reversal process.

Epitaxially growth of β-FeSi2 thin films on Si(100) substrates from ϵ-FeSi targets with ArF excimer laser deposition

We prepare high quality thin films of β-FeSi2 on silicon substrates by an ArF excimer laser deposition method (ArF-PLD) using ϵ-FeSi alloy targets. Preferentially [100]- oriented β-FeSi2 films were grown on Si(100) surfaces, and the interface between the films and substrates are very smooth. Based on a fact that when iron silicide films are obtained on sapphire substrates in stead of the silicon ones, the films have the same compositions as the target materials, silicon atoms in the β-FeSi2 films must be supplied from the silicon substrates.

β-FeSi2thin film grown on a Si(111) surface with ferromagnetic interface

Thin iron silicide films were grown by solid phase epitaxy method on Si(111) surface. Silicide structure and magnetic properties at 40 K were observed with reflection high-energy electron diffraction and surface magneto-optic Kerr effect in-situ. The bcc-Fe film was grown at 40 K and transformed to polycrystals at 670 K annealing. The SMOKE revealed ferromagnetic property of the polycrystals at 40 K. After 820 K annealing, a β-FeSi2 film was formed. This sample was ferromagnetic at 40 K. We suggest a multilayer structure including a ferromagnetic interface between the β-FeSi2 film and the Si(111) substrate.

Cross-sectional Transmission Electron Microscopy of Interface Structure of β-FeSi2/Si(100) Prepared by Ion Beam Sputter Deposition

The effect of a silicon substrate surface pretreatment on epitaxial iron silicide film formation on a Si(100) substrate with ion beam sputter deposition (IBSD) was investigated. In this study, two surface pretreatment methods namely, thermal etching (TE) and sputter etching (SE) with subsequent thermal annealing, were employed. The interface structure between a β-FeSi2 film and a Si substrate was analyzed by cross-sectional observation using a transmission electron microscope (XTEM). High-resolution XTEM images showed that the lattice of the Si substrate is an almost perfect crystal immediately after the TE treatment. However, the TE treatment results in an undulated interface, and the deposited silicide contained coalesced β-FeSi2 islands. On the other hand, the dislocations and stacking faults produced by radiation damage were observed near the Si substrate surface for the SE treatment. Even though this treatment produced defects, the interface of the SE-treated epitaxial β-FeSi2(100) film had a smooth interface after the deposition at 973 K. It can be concluded that a moderate disorder of the silicon substrate surface treated by SE may well enhance the mixing of Fe and Si atoms for the epitaxial growth of β-FeSi2.

Structure of Iron Silicide film on Si(111) Grown by Solid-Phase Epitaxy and Reactive Deposition Epitaxy

The method dependence of iron silicide grown on Si(111) was studied by scanning tunneling microscopy. The iron silicide films grown by two different methods (solid-phase epitaxy and reactive deposition epitaxy) were compared at a small-film-thickness region below 1 nm. The surface morphology strongly depends on the growth method, which suggests that the strain in the interface region is affected by the growth method.