Formation of iron silicide nano-islands on Si substrates by metal organic chemical vapor deposition under electron beams

Electron-beam induced chemical vapor deposition (EBI-CVD) of Fe(CO)₅ was performed on both Si (111) and (110) substrates at 673–873 K inside an ultrahigh vacuum transmission electron microscope. The formation of iron silicide islands was observed on both substrates. Cubic silicide nano-rods were formed on Si(111) substrates by EBI-CVD with focused electron beams. The formation of β-FeSi₂ islands was mainly observed on Si(110) substrates by EBI-CVD when the electron beam was broadly spread. It was shown that the size and the intensity of the electron beam played a significant role in EBI-CVD and affected the CVD process extensively.     

The solid state reaction of Fe with the Si(111) vicinal surface: splitting of bunched steps

The solid state reaction of deposited Fe (four monolayers, ML) with vicinal Si(111) substrate induced by subsequent thermal treatment has been studied using scanning tunnelling microscopy. At the lower range of annealing temperatures up to 400?°C the bunched steps of bare substrate are reproduced by the surface of the covering iron silicide layer. At 400?°C the onset of three-dimensional growth of iron silicide islands is observed. In comparison to the samples covered with smaller amounts of Fe it appears at a lower annealing temperature. Above 500?°C the bunched steps split into lower ones but more densely distributed due to proceeding reactions between Fe-rich iron silicide and Si substrate. As a consequence, at 700?°C the well-developed three-dimensional nanocrystallites of iron silicide are randomly distributed on the Si surface. This observation is in contrast to the formation of a regular array of iron silicide crystallites upon deposition of 2?ML of Fe.     

Formation of iron silicide on Si(001) studied by high resolution Rutherford backscattering spectroscopy

The initial stage of the formation of iron silicides has been investigated by high resolution Rutherford backscattering spectroscopy (HRBS). Just after Fe is deposited on Si(001) at room temperature, the deposited Fe atoms dig themselves into subsurface ejecting Si atoms into outer surface. Upon annealing at 300-400 °C, significant number of Si atoms are displaced from their lattice position near the interface between the crystalline Si and the Fe-Si mixing layer. Such displaced Si diffuses into the Fe-Si mixing layer and forms the layer with the composition of Fe:Si≈1:2. This layer is considered to be a precursive state of FeSi₂.     

Transmission electron microscope analysis of epitaxial growth processes in the sputtered β-FeSi2/Si(001) films

The crystallographic orientation relationships and the formation process of β-FeSi₂/Si(001) films were investigated by transmission electron microscopy. A film produced by sputtering pure iron onto a silicon substrate at 600 °C consists of α- and β-FeSi₂ particles. The crystallographic relationships obtained are: (112)_α‖(111)_Si and (101)_β‖(111)_Si or (110)_β‖(111)_Si. The grains of α- and β-FeSi₂ grown inside the substrate adopt the epitaxy to Si(111), irrespective of the surface orientation of the substrate. At 500 °C, on the contrary, there are few α-FeSi₂ grains and some grains of β-FeSi₂ with (100)_β‖(001)_Si [010]_β‖[110]_Si. These results demonstrate that the lower temperature and the higher Fe concentration suppress the formation of α-FeSi₂ and promote the formation of β-FeSi₂ on/below the substrate surface.     

Iron silicide thin film formation at low temperatures

The rate kinetics of the formation of compound phases from thin layers of 1000–1500 Å α-Fe deposited onto single-crystal Si have been studied by MeV ⁴He⁺ backscattering spectrometry. Si is observed to dissolve into the thin α-Fe layer before compound formation. A compound layer of FeSi is produced at about 450°C and FeSi₂ formation begins at about 550°C. The (100) surface of Si is slightly more reactive than the (111) surface. An inert diffusion marker of implanted Xe was used to investigate the relative movement of the two species.X-ray diffractometry identifies the structure of the compound species as identical to bulk FeSi and FeSi₂. The compounds formed on both (111) n-Si and (100) n-Si are apparently polycrystalline and untextured.     

Effect of Fe and Mn additions on microstructure and mechanical properties of spray-deposited Al–20Si–3Cu–1 Mg alloy

Spray deposition is a novel process which is used to manufacture rapidly solidified bulk and near-net-shape preforms. In this paper, Al–20Si–3Cu–1 Mg alloy was prepared by spray deposition technique. The effect of Fe and Mn additions on microstructure and mechanical properties of spray-deposited Al–20Si–3Cu–1 Mg alloy was investigated. The results show that two kinds of intermetallics, i.e. δ-Al₄FeSi₂ and β-Al₅FeSi, is formed in the microstructure of spray-deposited Al–20Si–5Fe–3Cu–1 Mg alloy. With additions of 5% Fe and 3% Mn to Al–20Si–3Cu–1 Mg alloy, the needle shape of Al–Si–Fe intermetallic phases is substituted by the particle shape of Al₁₅(FeMn)₃Si₂ phases. The presence of the intermetallic phases (δ-Al₄FeSi₂, β-Al₅FeSi and Al₁₅(FeMn)₃Si₂) improves the tensile strengths of the alloy efficiently at both the room and elevated temperatures(300 °C).     

Formation and properties of thin films of iron silicides on Si(111) Surface: Ab initio simulation

Density functional theory in the generalized gradient approximation has been used to calculate the total energy and model the atomic and electronic structures of thin FeSi films with CsCl type lattice and γ-FeSi₂ films with CaF₂ fluorite type lattice on a Si(111) surface. It is shown that, upon the adsorption of two monolayers of iron atoms on Si(111), the most energetically favorable process is the growth of a γ-FeSi₂ film with CaF₂ type structure. The electronic structure of a silicide film formed upon the adsorption of one monolayer of iron atoms exhibits features that are characteristic of both FeSi and γ-FeSi₂. The density of states calculated for the γ-FeSi₂ well agrees with the experimental photoemission spectra reported in the literature.     

Semiconducting amorphous FeSi2 layers synthesized by co-sputter deposition

We report here on the synthesis of semiconducting amorphous FeSi₂ layers by co-sputter deposition of Fe and Si on silicon (100) wafers. The layers were grown to a thickness of 300-400 nm, at various substrate temperatures. Structural characterisation has shown that the deposited layers have the FeSi₂ stoichiometry and are fully amorphous up to a deposition temperature of 200 °C. Optical absorption measurements have demonstrated that the amorphous FeSi₂ layers have semiconducting properties, with a direct band gap of 0.89-0.90 eV at room temperature (RT). In order to relax the amorphous structure, some samples were irradiated with 200 keV Ar ions. It was found that both an increased deposition temperature and/or ion irradiation induce a higher photo-absorption, which was attributed to establishing a medium range order in the amorphous phase. The applied fabrication routine can be highly efficient for potential applications of this material in large area electronics and for production of solar cells.     

Effect of target compositions on the crystallinity of β-FeSi2 prepared by ion beam sputter deposition method

Targets with the elemental composition of Fe, Fe₂Si and FeSi₂ were employed in the present study to grow β-FeSi₂ film on Si (100) substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi₂ target was employed, a Si-rich phase, α-FeSi₂ (Fe₂Si₅), was predominant at temperatures above 973 K, while β-FeSi₂ phase was observed only in the limited temperature range at around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be excessive to sustain β structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100 nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe₂Si target was employed, where highly (100)-oriented β-FeSi₂ layer of 120 nm in thickness was obtained at 973 K.     

The growth and transformation of Pd2Si on (111), (110) and (100) Si

Thin Pd films on (111), (110), (100) and amorphous Si substrates form [001] fiber textured Pd₂Si in the temperature range 100°–700°C. The degree of texture is a function of substrate orientation, increasing in the order amorphous Si, (100) Si, (110) Si and (111) Si. Only on the (111) Si substrate is the Pd₂Si film epitaxially oriented. Temperature-dependent growth on this orientation can be characterized by [001] textured growth, epitaxial azimuth orientation at the Si interface and progressive layer by layer formation of the mosaic crystal to the thin film surface.During Pd deposition, rapid non-diffusion-controlled growth of epitaxial Pd₂Si on (111) Si occurs at substrate temperatures of 100° and 200°C. An unidentified palladium silicide of low crystallographic symmetry forms during Pd deposition onto a 50°C substrate. The diffusion-controlled growth of Pd₂Si on (111) Si follows a t^0.5 dependence. The velocity constant is

$\text{k = 7 × 10}{}^{\mathrm{?2}}\text{exp}\text{?}\text{29200±800}\text{RT}\text{cm}{}^2\text{/}\text{sec}$

Palladium deposited on 100°C (111) Ge substrates reacts during deposition to form epitaxially oriented Pd₂Ge. However, growth of this phase at higher temperatures results in a randomly oriented film. The transformation of Pd₂Ge to PdGe is kinetically controlled. After a 15 min anneal at 560°±10°C in N₂ only PdGe is detectable on (111) Ge.The high temperature stability of thin film Pd₂Si is controlled by time- temperature kinetics. For a given annealing cycle, the nucleation and growth rates of the PdSi phase are inversely related to the crystalline perfection of Pd₂Si. Decreasing transformation rates follow the order (100), (110), (111) Si. formation of thin film Pd₂Si occurs by the formation of PdSi and subsequent growth of Si within the PdSi phase. After a 30 min N₂ anneal, initial transformation occurs at 735°C on (100) Si, 760°C on (110) Si and 840°C on (111) Si. Extended high temperature annealing produces a two-phase structure of highly twinned and misoriented Si and small PdSi grains that penetrate as much as 3 μm into the Si.     

Synthesis of β-FeSi2/Si heterojunctions for photovoltaic applications by unbalanced magnetron sputtering

We report here the possibility of the growth of semiconducting FeSi₂ layers on Si(100) substrates by depositing iron with unbalanced magnetron sputtering. The originality of the study is the achievement of heterojunction without any further treatment of the deposited films. Pure iron is deposited on Si(100) substrates with unbalanced magnetron sputtering for the production of β-FeSi₂/Si heterojunctions. Prior to coating process the substrates are cleaned with neutral molecular source. Microstructure of β-FeSi₂ films were investigated by X-Ray Diffraction analysis and Raman Spectroscopy. Dark current-voltage characteristic of the deposited coatings showed a rectifying behavior for the β-FeSi₂/Si heterojuctions. Open-circuit voltage (V_oc) and short-circuit current density (J_sc) were measured under 100 mWcm^− 2 illumination and a V_oc of 360 mV and J_sc of 180 μAcm^− 2 were measured. The illumination of the silicon side gave higher photosensitivity than the illumination of the iron silicide side.     

Increase in the density of β-FeSi2 nanoclusters on a Si(111) surface by means of Si(111) √3 × √3R30°-B reconstruction

The formation of iron disilicide (β-FeSi₂) nanoclusters as a result of solid-state epitaxy at T = 500–700°C and an iron coverage of 0.05–0.5 monolayer on a boron-modified Si(111)√3 × √3 R30° surface has been studied by scanning tunneling microscopy. It is established that the number density of β-FeSi₂ nanoclusters on the Si(111) √3 × √3 R30°-B surface significantly exceeds the density of silicide clusters formed on the atomically clean Si(111) surface with a 7 × 7 reconstruction for the analogous iron coverages and annealing temperatures. At the same time, the density of point defects and clusters possessing metallic conductivity on the Si(111) √3 × √3 R30°-B surface is several orders of magnitude lower than on the Si(111)7 × 7 surface treated under identical conditions.     

Effect of filament temperature and deposition time on the formation of tungsten silicide with silane

The effect of filament temperature and deposition time on the formation of tungsten silicide upon exposure to the SiH₄ gas in a hot wire chemical vapor deposition process was studied using the techniques of cross-sectional scanning electron microscopy and Auger electron spectroscopy. At a relatively low temperature of 1500 °C, the decomposition of WSi₂ phase and the diffusion of Si towards the silicide/W interface produce a thick W₅Si₃ layer. The diffusional nature leads to a parabolic rate law for silicide growth. An exponential decrease of silicide thickness with temperature between 1600 and 2000 °C illustrates the dominance of Si evaporation at higher temperatures (T ≥ 1600 °C) over the silicide formation.     

Structural properties of the PdSi interface: An investigation by reflection high energy electron diffraction

The first stages of the formation of silicide were studied at various temperatures during palladium deposition onto an Si(111) 7 × 7 surface using the reflection high energy electron diffraction technique.For temperatures below 200°C, the interaction of palladium with silicon leads to the formation of three-dimensional Pd₂Si crystallites.At more elevated temperatures (300–400°C) two-dimensional phases were distinguished that are denoted as follows: Si(111) √3 R(30°) Pd and Si(111) 2√3 R(30°) Pd.     

Initial stages of silicon-iron interface formation

The formation of a silicon-iron (Si/Fe) interface has been studied in situ by the method of high-resolution photoelectron spectroscopy using synchrotron radiation. The experiments were performed under ultrahigh vacuum conditions (at a residual pressure of 3 × 10^?10 Torr) in a range of Si coating thicknesses within 0.04–0.45 nm. It is established that the process begins with the formation of a FeSi silicide and Fe-Si solid solution on the iron substrate surface. As the Si coating thickness increases, the solid solution converts into ferromagnetic (Fe₃Si) and nonmagnetic (FeSi) silicides. It is shown that thermostimulated solid-state reactions leading to the transformation of FeSi and Fe₃Si silicides into a semiconducting β-FeSi₂ silicide start at a temperature close to 600°C.     

Growth,structure and electrical characteristics of epitaxial nickel silicide from chemically electroless Ni deposition on Si

The epitaxial NiSi₂ has been successfully grown on (100) and (111) silicon single crystals by chemical electroless deposition and isothermal annealing for the first time. Transmission electron microscopy (TEM) was applied to study the structure and orientation relationship of the films and substrates. The nickel silicide formed on both (100) and (111) Si substrates was identified to be NiSi₂ and was found to have epitaxial relationship with the substrates by bright field imaging and selected area diffraction pattern analysiS. Square, hexagonal and irregular dislocation networks were observed. The orientation relationships of silicide phase with respect to (100)Si substrate were identified to be (220)Si (220)NiSi₂, (400)Si (400)NiSi₂ and (001) Si (001)NiSi₂. The orientation relationships of silicide phase with respect to (111)Si substrate were identified to be (220)Si (02¯2)NiSi₂, (3¯11) Si (31¯1) NiSi₂ and [1¯1¯4]Si [1¯1¯2]NiSi₂. The average spacing of interfacial dislocations was about 90 nm for epitaxial silicide formed on (100)Si at 800° C, which is narrower than that on (111)Si. These spacings decreased with increasing annealing temperature.Sheet resistances were measured to be lower than that of poly-NiSi₂ produced from electron gun evaporation with the same annealing condition. A linear relationship between resistivity and thickness was found and discussed.     

Formation and characterization of semiconductor Ca2Si layers prepared on p-type silicon covered by an amorphous silicon cap

Formation of calcium silicide on three types of templates: Si(111)7 × 7, 2D Mg₂Si, and 3D Mg₂Si, was studied during Ca deposition at 120 °C in situ by Auger and electron energy loss spectroscopy, and by differential optical reflectance spectroscopy. A continuous Ca₂Si layer is formed on 2D and 3D Mg₂Si templates; but, on an atomically clean silicon surface (Si(111)7 × 7), a mixture of Ca₂Si with another Ca silicide was found. The growth of a Si cap layer over the Ca silicide layers at about 100 °C studied by in situ methods demonstrated the full embedding of Ca silicide in amorphous silicon, independent of the used template. Transmission electron microscopy, Rutherford backscattering spectrometry, atomic force microscopy, and electrical characterization of Schottky junctions revealed the Ca₂Si and Mg₂Si nanoparticles and the redistribution of Mg and Ca during the silicon cap growth and its effect on the electronic properties of the structures. Reproduction of the experiments on higher doped and better purity substrates is needed to understand better the role of Mg- and Ca-related defects, and defects of silicon generated by the growth process.     

Approaches to growth and study of properties of multilayer silicon-silicide heterostructures with buried semiconductor silicide nanocrystallites

Studies of nanosize (5-50 nm) island formation of Fe, Cr and Mg silicides on atomically clean silicon surfaces with (111) and (100) orientations, silicon growth atop nanosize silicide islands and multilayer repetition of developed growth procedure for all silicides have been carried out. Optimization of growth parameters has permitted to create multilayer monolithic heteronanostructures with buried nanocrystallites of iron and chromium disilicides. Only polycrystalline multilayer heteronanostructures with buried Mg₂Si nanocrystallites have been created after optimization of growth procedures. A new approach to study optical properties of multilayer heteronanostructures has been developed and tested.     

Phase Composition, Orientation, and Substructure of Iridium Silicide Films on Silicon

The phase composition, orientation, and substructure of iridium silicide films produced by electron-beam evaporation of Ir on (001) and (111) Si at substrate temperatures from 300 to 1000°C were studied by electron microscopy and diffraction. The results demonstrate that the sequence of silicide formation upon Ir deposition onto heated substrates is the same as upon heat treatment of Ir films deposited at room temperature. It is shown that oriented growth of Ir₃Si₅ and IrSi_0.7 is possible. The IrSi₃/Si interface is shown to be, to some extent, coherent, with the lattice mismatch being accommodated by dislocations in some directions and by elastic strain in other directions. The likely mechanism for the formation of a dislocation structure at the IrSi₃/Si interface is discussed.     

Magnetocrystalline anisotropy energy and spin polarization of Fe3Si in bulk and on Si(001) and Si(111) substrates

In order to achieve highly efficient spin polarized transport, first of all magnetocrystalline anisotropy energy, which determines the magnetic easy axis, must be understood. The highly precise full-potential linearized augmented plane-wave method is employed to investigate the magnetism and magnetocrystalline anisotropy energy of a ferromagnetic Heusler alloy Fe₃Si on Si(001) and Si(111) substrates. The calculated magnetocrystalline anisotropy energy of bulk D0₃ Fe₃Si was found to depend sensitively on a tetragonal distortion: The magnetization is along the z-axis at c/a < 1 and on the xy plane at c/a > 1. The out-of-plane magnetic easy axis of both Fe₃Si/Si(001) and (111) was calculated to be quite stable with enhanced magnetocrystalline anisotropy energy compared with bulk value. The magnetic easy axis of Fe₃Si/Si(001) and (111) is discussed in detail with single particle energy spectra. The degree of spin polarization is also presented at the interfaces between Fe₃Si and Si. The calculated spin polarizations of Fe₃Si/Si(111) tend to retain the spin polarization of the bulk, whereas they are reduced for the (001) interfaces.