Metastable Silicide Research Articles

Formation of metastable aluminum silicide as intermediate stage of Al-Si alloy crystallization

The mechanism of Al-Si alloy crystallization from an amorphous state is still unclear. Despite the absence of equilibrium compounds for this binary system, there are several experimental evidences confirming the formation of metastable silicide at annealing of amorphous Si mixed with Al. Classification of this metastable structure is a missing puzzle piece in the description of Al-Si alloy crystallization kinetics. Here, we considered the properties of aluminum silicide Al2Si structure, which is a probable candidate for the role of the observed metastable compound. Our investigation was based on the atomistic simulations with the interatomic potential developed recently and DFT approach. All used methods revealed that there are several crystal structures of Al2Si with close geometry and relatively low formation energies. The chemical ordering in such structures is similar to the one in Si-Al melt. We also showed that the combination of these structures allows to form a crystal with different degree of ordering.

Identification of nickel silicide phases on a silicon surface from Raman spectra

We have demonstrated the effectiveness of Raman spectroscopy for monitoring nickel silicide formation processes on the surface of silicon wafers, with deposition of a composite metal layer (nickel, platinum, and vanadium) under industrial process conditions in microelectronics. The observed shift of all the NiSi lines toward lower energies is associated with formation of the metastable silicide phase Ni1−x Pt x Si, which leads to the presence of stresses in the lattice as a result of the increase in the distances between atoms.

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 ɛ-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 ɛ-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, Fe 3Si 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-FeSi 2 and α-FeSi 2). 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, Fe 3Si, ε-FeSi and β-FeSi 2.

Calorimetric investigation of the formation of metastable silicides in Au/a-Si thin film multilayers

Metastable phase formations were studied in Au/a-Si thin film multilayers using differential scanning calorimetry and x-ray diffraction. Two nonequilibrium crystalline silicides were found to form below 500 K. Each phase formed by a different mechanism, and the competing growth of the two phases over the temperature range of 375 to 500 K, was found to depend greatly on thickness and grain size in the Au layers. At higher temperatures (500–600 K), these metastable phases decomposed and the a-Si crystallized by metal-induced crystallization to yield a phase mixture of Au and x-Si. The enthalpy of crystallization of a-Si was measured from the calorimetry data to be −12.1±1.2 kJ/mol.

In-situ investigation of the formation of eutectic alloys in the systems silicon–silver and silicon–copper

An in-situ electron microscopic investigation of the formation of eutectic alloys in the systems amorphous Si/particle Ag and amorphous Si/particle Cu showed that the formation of eutectics is preceded by metal diffusion into amorphous silicon with the formation of metastable amorphous metal silicide. Decomposition of the formed metastable amorphous metal silicide results in the evolution of polycrystalline silicon. An oriented formation of liquid eutectic alloy is observed in the systems crystalline (100) Si/particle Ag and (100) Si/particle Cu. In these cases, at the metal–silicon eutectic/silicon crystal interface the formation of silicide phases and a dislocation structure are observed. Investigations allowed us to propose a scheme for the formation of eutectic alloys in the studied systems. The scheme involves the diffusion of metal atoms into the silicon crystal structure, leading to the formation of a metastable silicide or solid solution, the occurrence of strain at the interface and, as a consequence, the formation of defects of different types. The subsequent process is accompanied by synchronous dissolution of metal atoms into the liquid eutectics and by diffusion from the liquid eutectics to the solid crystalline silicon phase, giving rise to metastable silicide, so that an intermediate diffusion zone is always present at the interface between the liquid eutectics and crystalline silicon.

Nanocrystals in Fe-P-Si-Mn alloys

Annealing Fe–P–Si–Mn amorphous alloys is found to produce a few nanocrystalline phases, with a strength gain no higher than that in Fe–P–Si alloys. This crystallization behavior is accounted for by the formation of two metastable silicides whose growth rates during annealing are higher than those of the equilibrium phases.

Surface electronic structure of metastable FeSi(CsCl)(111) epitaxially grown on Si(111)

We report an investigation on the electronic structure of metastable, epitaxial FeSi films grown on Si(111). The electronic structure of the metastable silicides was probed with angle-resolved photoemission, and compared with theoretical calculations. We identify the silicide as FeSi crystallizing in the CsCl structure. Its surface is Fe terminated, and presents a prominent, strongly dispersing surface state at a binding energy of -3.5 eV in \ensuremath{\Gamma}-bar. Its origin lies in the truncation of Fe bonds at the surface, and thus it has a major Fe ${\mathrm{d}}_{{\mathrm{z}}^{2}}$ content.

High-resolution x-ray-photoemission study of metastable Fe silicide core-electron states.

The core-electron states of metastable pseudomorphic Fe silicides, epitaxially grown on Si(111), have been measured by monochromatized x-ray photoemission. These silicides have been grown by molecular-beam epitaxy with composition ranging from FeSi up to ${\mathrm{FeSi}}_{2}$. Si 2p and Fe 2${\mathit{p}}_{3/2}$ core lines have been compared to those measured on both polycrystalline and epitaxially grown stable \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ and \ensuremath{\varepsilon}-FeSi layers. Metastable silicide related Si 2p and Fe 2${\mathit{p}}_{3/2}$ core line binding-energy shifts, with respect to \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ or clean Si(111), have been interpreted as arising from electron charge transfer from Si to Fe, in agreement with recent calculations. Si 2p and Fe 2${\mathit{p}}_{3/2}$ binding energies as well as line-shape evolutions, over a wide composition range, give strong support to a model with a nearly cubic structure, evolving from FeSi to ${\mathrm{FeSi}}_{2}$, in which Fe atoms are randomly distributed over the Si cages.

Iron environment in pseudomorphic iron silicides epitaxially grown on Si(111).

Pseudomorphic iron-silicide phases grown on Si(111) have been studied by means of low-energy electron-diffraction, x-ray photoelectron-diffraction, and surface extended x-ray-absorption fine-structure experiments at the Fe K edge (7110 eV). These silicides have been epitaxially grown by codeposition of Fe and Si onto a room-temperature Si(111) substrate with silicide stoichiometry ranging from FeSi to ${\mathrm{FeSi}}_{\mathit{x}}$ (x\ensuremath{\sim}2). In all cases, they were found to be epitaxial as attested to by a (1\ifmmode\times\else\texttimes\fi{}1) low-energy electron-diffraction pattern observed after room-temperature deposition. X-ray-absorption measurements reveal that Fe atoms are coordinated with eight Si atoms for all metastable silicides with bond lengths of 2.34--2.37 \AA{} and with Fe atoms with bond lengths in the 2.68--2.75-\AA{} range. Fe-Si and Fe-Fe bond lengths as well as coordination numbers are found to increase with Fe contents within the silicide. These experiments confirm the formation of an epitaxial cubic and metastable CsCl-type FeSi upon deposition of Fe and Si in the 1:1 ratio onto a room-temperature Si(111) substrate. Furthermore, all data recorded from ${\mathrm{FeSi}}_{2}$ grown at room temperature or annealed at high temperature, are consistent with CsCl-derived or \ensuremath{\alpha}-derived ${\mathrm{FeSi}}_{2}$ structures. Finally, the present data are inconsistent with the formation of a ${\mathrm{CaF}}_{2}$-type ${\mathrm{FeSi}}_{2}$ structure.

High-resolution electron microscopy study of αFeSi 2 heteroepitaxy on Si(111)

During the course of orthorhombic βFeSi 2 heteroepitaxy on Si substrates under molecular beam epitaxy conditions, several ultra-thin pseudomorphic metastable silicide layers have been observed, e.g. a cubic CsCl phase, a cubic CaF 2 phase and tetragonal αFeSi 2, recently and surprisingly stabilized at low temperature. We report here on their phase transition towards the stable βFeSi 2 phase via two main kinetic paths, clearly demonstrated for the first time thanks to high-resolution electron microscopy measurements and image simulation. Our results emphasize the relevant influence of the local morphology at the interface in favoring one or another kinetic path, although the common general behavior of the interface development is well reflected in the sequential formation on Si substrate of: epitaxially strained FeSi 2 layer; relaxed FeSi 2 layer mediated by dislocation formation; stable βFeSi 2.

Fe-Silicides Formed with Molecular Beam Epitaxy and Studied with Conversion Electron Mössbauer Spectroscopy

ABSTRACTFe-sulicides were formed by annealing MBE-deposited thin 57Fe layers with thicknesses between 20 Å and 60 Å on (7x7) reconstructed Si(111) substrates. During the growth the substrate was held at room temperature. The silicide formation upon annealing in the temperature range of 200°C to 900°C was studied in-situ with RH-EED. Samples were studied with in-situ CEMS (Conversion Electron Mossbauer Spectroscopy) as well as with ex-situ CEMS after covering with Ag to prevent oxidation. RBS/Channeling was used to study the epitaxial quality and the structure of these layers.Using the 57Fe nuclear probe, it is shown that silicide formation occurs at the interface already at room temperature. Metastable silicides with cubic structure are observed in the low temperature annealing range, and characterized by their Mossbauer parameters. At higher temperatures stable ε-FeSi and β-FeSi2 are formed.

Formation and Properties of Metastable Silicide Precipitates in Silicon

Formation and Properties of Metastable Silicide Precipitates in Silicon

Nanosecond laser-induced phase transitions and metastable phases quenching

Abstract Pulsed-laser treatment of solid surfaces has two major features: fast heating, up to the melting point, on the nanosecond time scale, and subsequent rapid solidification and cooling. Metastable phases are often quenched in. The way in which one can control experimentally the solidification velocity, which is the main parameter in selecting between the different solid phases that can solidify, is discussed. As a reverse process, a metastable phase retained at room temperature can be heated to its melting point and melted, by-passing any solid-phase transformation or decomposition due to the fast heating rate. Examples are shown, throughout this paper, which relate mainly to the semiconductor field. This is because historically the technique has been developed as a tool to remove damage in ion-implanted semiconductors. Amorphous Si, amorphous Si–As solutions, supersaturated Si-dopant solutions and metastable silicides are sequentially presented both as examples of rapid melting and as examples of l...

The Ti/c-Si solid state reaction: II. Additional measurements by means of RBS, XPS and AES

In a previous paper [Appl. Surface Sci. 40 (1990) 333], we have reported the results of a spectroscopic ellipsometric study of the c-Si/Ti solid state reaction. For this purpose we have grown and heated thin (≈10nm) Ti films on a clean c-Si substrate. The investigation revealed that already at moderate temperatures a metastable silicide (≈350°C), probably a monosilicide, and disilicide (≈450°C) are formed. These two metastable transition states, denoted by state I and II respectively, and the final disilicide (state III, ≈700°C) are additionally studied by means of a number of quantitative techniques, such as RBS, XPS and AES. The results reveal a Si-enriched monosilicide state I, Si: Ti = 1.2 and a stoichiometric but Si segregated disilicide state II; surface composition approximately TiSi 3. The finally obtained disilicide (III) has recrystallized into probably large, flat islands embedded in a c-Si matrix.

Low-Temperature Self Aligned TiSi2'TiN Bilayer by Rapid Thermal Nitridation of Metastable Titanium Silicide in NH3 Ambient

ABSTRACTRapid thermal nitridation (RTN) of metastable Ti silicide in pure ammonia ambient has been shown to result in the formation of a bilayer TiSi 2'TiN structure. This bilayer structure provides an effective self-aligned diffusion barrier against aluminum spiking. Further, the simultaneous formation of TiN on top of the TiSi2 preserves the low resistivity of the silicide layer upon subsequent high temperature process steps. Rutherford backscatttering spectroscopy, Auger electron spectroscopy and four point probe techniques were used to analyze the stoichiometry of the nitrided layer, and to study the kinetics of the nitridation reaction. The nitridation of the metastable silicide film is a substitutional eaction which begins at the surface of the silicide and progresses by substituting nitrogen atoms for silicon. The nitrogen atoms result from dissociation of ammonia The released silicon atoms then diffuse to the silicide'silicon interface and deposit in an epitaxial manner. The benefits of the proposed metallization scheme are substantiated by electrical characterization of the bilayer structure in comparison with a conventional process.

Ion-beam induced metastable Pt2Si3 phase. I. Formation, structure, and properties

A metastable silicide phase with a composition of Pt2Si3 has been obtained by using ion-beam mixing techniques. To form the phase, a thin PtSi film on a Si substrate was first converted into a Si-rich amorphous Pt-Si alloy by implantation with energetic ions through the PtSi-Si interface. The amorphous alloy then transformed into the metastable crystalline phase upon post annealing at 350–500 °C. X-ray diffraction analysis showed that the Pt2Si3 phase has a hexagonal crystal structure with lattice parameters a=3.841 Å and c=11.924 Å and there are ten atoms per unit cell. The phase was unstable at temperatures above 550 °C and transformed back to PtSi and Si. The metastable Pt2Si3 phase was found to exhibit a superconducting transition onset at about 4.2 °K and becomes completely superconductive below 3.6 °K.