Transition Metal Disilicides Research Articles

Directional atomic bonds in MoSi2 and other transition-metal disilicides with the C11b, C40 and C54 structures

The elastic constants of MoSi2 as well as other transition-metal (TM) disilicides with the C11b, C40 and C54 structures are analysed by adopting an approach similar to the valence-force-field approach in order to elucidate the directional nature of atomic bonding. In the present approach, all interatomic forces are resolved into bond-stretching and bond-bending forces and the elastic properties are described with force constants corresponding to these two forces. The values of force constants are compared with each other. The directionality in atomic bonding in C11b disilicides is found to be stronger than any other disilicides with the C40 and C54 structures. Of the two C11b disilicides, the directionality in atomic bonding is stronger for WSi2 than for MoSi2.

Mo5Si3 single crystals: physical properties and mechanical behavior

The materials processing, physical properties and mechanical behavior of an ultrahigh temperature structural silicide, Mo5Si3, have been studied. High purity single crystals of Mo5Si3 have been synthesized by both optical floating zone and Czochralski methods. The thermal and elastic properties of the Mo5Si3 single crystals were experimentally measured. Results show that Mo5Si3 has significant thermal expansion anisotropy along the a and c directions with αc/αa=2.2. The single crystal elastic moduli of Mo5Si3 indicate that it has less elastic anisotropy and lower shear moduli than transition metal disilicides. Tensile stresses of up to 1.8 GPa can develop at grain boundaries after cooling from the melting point due to the thermal expansion mismatch in Mo5Si3, causing grain boundary cracking during processing of polycrystals. Room temperature Vickers indentation tests on (100) and (001) planes have been performed with different indenter diagonal orientations and the orientation dependence of hardness and fracture toughness of Mo5Si3 single crystals have been obtained. The corresponding deformation and fracture modes have been revealed by microscopy studies. Finally, a comparison of Mo5Si3 with other high temperature structural silicides is discussed.

Synchroshear Versus Conventional Shear Mechanisms in Transitionmetal Disilicides with the C40 Structure

AbstractThe deformation behavior of (0001) <1210> basal slip in single crystals of five different transitionmetal disilicides with the C40 structure has been investigated in the temperature range from room temperature to 1500°C in compression. These disilicides are found to be classified into two groups depending on the onset temperature for plastic flow. The low-temperature group, which consists of VSi2, NbSi2 and TaSi2, exhibits the onset temperature for plastic flow around 0.3 T/Tm (melting temperature) and deforms by a conventional shear mechanism. In contrast, the high temperature group, which consists of CrSi2 and Mo(Si,Al)2, exhibits the onset temperature around 0.6T/Tm and deforms by a synchroshear mechanism. Factors affecting the deformation mechanism in these C40 disilicides are discussed in terms of directionality of atomic bonding and the relative stability of the C40 phase with respect to the C11b phase.

Chemical bonding of3dtransition-metal disilicides

The bonding properties of $3d$ transition-metal (TM) disilicides are explored by the use of the discrete variational $X\ensuremath{\alpha}$ cluster method. The calculated density of states of whole $3d$ TM disilicides reproduce well the characters obtained by photoemission spectroscopy. Mulliken's population analysis reveals that the bonding is dominated by the comparable interactions of TM-Si and Si-Si. The main participant of TM-Si bonds shifts from $3d\ensuremath{-}\mathrm{Si}$ bonding for the middle $3d$ TM disilicides to $4sp\ensuremath{-}\mathrm{Si}$ bonding for the late $3d$ TM disilicides.

Phases of cobalt-iron ternary disilicides

Cobalt–iron transition-metal disilicides were investigated by Mössbauer effect and x-ray diffraction in order to determine the concentration range of their homogeneous and separate phase formation. Except at low Co or Fe concentrations, Co and Fe formed separate CoSi2 and FeSi2 phases. Up to 10 at % Co was found soluble in β-FeSi2; Fe dissolved in CoSi2 below 1.5 at % and was positioned at two different sites of cubic symmetry. The results obtained for the phase formation in thin layers of epitaxial CoSi2 on Si implanted with Fe were in agreement with the results obtained for the bulk samples.

Elastic Properties of the Intermetallic Compound ReSi 2

Structural properties are crucial for understanding deformation behavior of a solid, e.g., dislocation slip systems, twinning modes, etc. A knowledge of elastic constants is essential for many practical applications related to the mechanical properties of a solid: load-deflection, thermoelastic stress, internal strain (residual stress), sound velocities, dislocation core structure, and fracture toughness. Further, the elastic properties may provide insight on the atomic bonding and resistance to deformation by shear. In this investigation, the elastic properties of ReSi{sub 2} were evaluated using resonant ultrasound spectroscopy (RUS) and are compared to the elastic properties of other transition metal disilicides.

Vibrational study of transition metal disilicides, MSi2 (M=Nb, Ta, V, Cr)

Polarized Raman spectra of transition metal disilicides, MSi2 (M=Nb, Ta, V, Cr), of hexagonal structure were measured on single crystals. The transmittance IR spectrum of a polycrystalline CrSi2 film is also reported. The vibrational modes are described in terms of symmetry and atomic motions. The effect due to the metal substitution on the spectra is discussed. © 1998 John Wiley & Sons, Ltd.

Plastic deformation of single crystals of transition metal disilicides

Transition metal disilicides, in particular MoSi 2 are candidates for structural materials to be used in oxidizing environments at temperatures higher than the upper limit for Ni-base superalloys (1100–1200°C). This paper presents a survey of studies on the deformation of transition metal disilicides with the C11 b and its derivative structures and the C1 structure using their single crystals. Emphasis is put on revealing the current status of our knowledge of fundamentals of their plastic deformation, e.g. operative deformation modes and their critical resolved shear stresses and their variation with crystal orientation and temperature.

Electronic structure of the Nowotny chimney-ladder silicideRu2Si3s

We report ab initio calculations for the electronic structure of the Nowotny chimney-ladder silicide ${\mathrm{Ru}}_{2}$ ${\mathrm{Si}}_{3}$ in the orthorhombic low-temperature phase. We find ${\mathrm{Ru}}_{2}$ ${\mathrm{Si}}_{3}$ to be a semiconductor with a direct band gap of about 0.45 eV. Since this gap is a p-d gap, the oscillator strength for a direct transition is expected to be of sizable magnitude. Also, the calculated effective masses of hole and electron states suggest that ${\mathrm{Ru}}_{2}$ ${\mathrm{Si}}_{3}$ is a very promising material for various applications in semiconductor technology. The electronic structure is controlled by the hybridization of Si p states with Ru d states and shows similarities to the group-IV transition-metal disilicides (${\mathrm{CrSi}}_{2}$ , ${\mathrm{MoSi}}_{2}$ , ${\mathrm{WSi}}_{2}$ ) and to transition-metal-rich silicides. The calculations are based on the density-functional theory in local-density approximation and are performed by means of the full-potential linearized-augmented-plane-wave method.

Elastic properties of C40 transition metal disilicides

Room-temperature and low temperature elastic properties of hexagonal C40 transition metal disilicides, NbSi2 and TaSi2, have been studied using Resonant Ultrasound Spectroscopy (RUS). All five independent elastic stiffness constantscij for NbSi2 and single crystals have been obtained. The temperature dependence of thecij is normal but not large. The orientation dependence of the Young's and shear moduli was examined in comparison with other transition metal disilicides. The room temperature shear moduli in the {0001} plane, with values of 145.3 and 143.7 GPa for NbSi2 and TaSi2 respectively, are low relative to those in the equivalent pseudo hexagonal {110} close-packed plane for tetragonal C11b MoSi2 and WSi2. The isotropic elastic constants for polycrystalline materials were also calculated. The results show that the various moduli are all much higher than those of the constituent elements. The room temperature Poisson's ratios of NbSi2 and TaSi2 are 0.18 and 0.19, respectively, which are smaller than those of the constituent elements and smaller than most materials. The Debye temperatures,θD, were estimated to be 688 K for NbSi2 and 552 K for TaSi2. The elastic properties of C40 VSi2, NbSi2, TaSi2, and CrSi2 and C11b and WSi2 are compared and the possible influence on mechanical behavior discussed.

Electronic states of alloying elements in MoSi 2 compound

The alloying effects on the electronic structure of MoSi 2 were investigated using the DV-X α molecular orbital method. It was found that there were strong covalent SiSi and MoSi bonds in MoSi 2. The nature of the chemical bonds between atoms was modified by alloying. Two alloying parameters, the bond order Bo and the d-orbital energy level Md, were determined for various alloying elements in order to estimate the alloying behavior of MoSi 2. The solid solubilities of alloying elements in MoSi 2 could be described in terms of these two parameters. A new crystal structure map of transition metal disilicides was also made by using the Bo and the Md parameters. This map was proved to be useful in examining any possibilities of the structural modification of disilicides by alloying.

Chemical bonding of transition metal disilicides

First-principles molecular orbital calculations of 3d and 4d transition-metal (TM or M) disilicides are conducted using model clusters composed of nearly 20 atoms, with special attention to their chemical bonding. M is positively charged and Si is negatively charged for early TM disilicides; however the direction of charge transfer changes between Fe and Co in the 3d TM disilicides. The net charge of M decreases almost linearly with increasing atomic number except for Sc3Si5 (defective ScSi2). The bond overlap population between M-Si decreases and that between Si-Si increases with rising atomic number of M. The energy distribution of Si 3s and 3p components is found to play an important role in the magnitude of both Si-Si and M-Si bonds. The Si-Si bond strength in Sc3Si5 is exceptionally strong among these disilicides. With regard to net charge and bond overlap population, the results for 4d TM disilicides are qualitatively the same as those for 3d TM disilicides.

Silicide Metallization of Aluminum Nitride Substrates for High-Temperature Microelectronics

AbstractA novel metallization for aluminum nitride substrates to package silicon carbide integrated circuits for use at temperatures up to 600°C was investigated. Chemical equilibrium calculations were used to determine the chemical compatibility of several refractory and transition metal disilicides with AIN. Tungsten disilicide, niobium disilicide, and titanium disilicide were selected for thin film deposition studies. WSi2, NbSi2, and TiSi2 thin films were deposited by RF sputtering on AIN substrates and heat treated at 900°C, 1000°C, and 1200°C in an argon atmosphere. Sheet resistivities were measured and interface stabilities and structures were characterized by scanning and transmission electron microscopy imaging, electron diffraction, and energy dispersive x-ray microanalysis spectroscopy. The results show that metal silicides appear to be promising as metallization for aluminum nitride for use at temperatures above 600°C.

Structure of the layer compound ZrSiTe

Zirconium silicon telluride, ZrSiTe, crystallizes is a layer-type structure which is related to the PbFCl type. The Zr atom is coordinated by four Si and four Te atoms. The Si and Te atoms are arranged in square planes. These planes are situated at opposite sides of the Zr atom; the coordination polyhedron may be described as a distorted quadratic antiprism. The distances between the Si atoms [2.614 (1) A] are longer than that of a Si-Si single bond (2.35 A), but comparable with the Si-Si separations in transition metal disilicides. The Te-Te separations within the Te 4 plane give no indication of significant interactions

The influence of germanium substitution on the phase stability of 3d transition metal disilicides

Few studies have been up to now devoted to the metallization of Si-Ge alloys and most of them concerns the sequence of phases formation for thin films interactions. In order to obtain a better understanding of these metallization processes, we have determined the thermodynamic equilibrium state by a systematic analysis of the ternary phase diagrams in the region of interest, which is the quasi-binary between the non-metal richest phases in both M-Si and M-Ge systems. The results show that as a function of the number of d electrons of the 3d metal, three different situations can be found: total miscibility (Ti), partial miscibility (V, Fe) or immiscibility (Cr, Mn, Fe, Co). These results can then be used to analyse and in some cases to predict the phase formation during the interaction of a 3d metal with a Si-Ge thin film.

Calculated structural properties of CrSi2, MoSi2, and WSi2.

The linear augmented-plane-wave method has been applied to calculate the valence-electron contribution to the total energy of the hexagonal (C40) and tetragonal (C${11}_{\mathit{b}}$) phases of the group-VI transition-metal disilicides ${\mathrm{CrSi}}_{2}$, ${\mathrm{MoSi}}_{2}$, and ${\mathrm{WSi}}_{2}$ in the local-density approximation (LDA). In agreement with experiment, the results show that the tetragonal C${11}_{\mathit{b}}$ phase is the lower-energy structure for both ${\mathrm{MoSi}}_{2}$ and ${\mathrm{WSi}}_{2}$ (by 0.11 and 0.22 eV/formula unit, respectively). However, the LDA fails to replicate the observed switch to the stable hexagonal C40 phase in ${\mathrm{CrSi}}_{2}$, leaving a reduced but positive C40-C${11}_{\mathit{b}}$ structural-energy difference of 0.05 eV/formula unit. The calculated lattice parameters for the observed stable phases are in excellent agreement (\ensuremath{\sim}0.01--0.03 \AA{}) with measured values. Somewhat large discrepancies (\ensuremath{\sim}0.1--0.3 \AA{}) are found for the c lattice parameters of the metastable hexagonal ${\mathrm{MoSi}}_{2}$ and ${\mathrm{WSi}}_{2}$ compounds.

Some transport properties of single crystals of group Va transition metal disilicides

Although transition metal disilicides are used in the form of thin films, single crystals are needed to study their intrinsic properties. Single crystals of Group Va transition metal disilicides, namely VSi 2, NbSi 2 and TaSi 2, have been successfully grown by Czochralski pulling from a levitated melt. The good quality of our crystals is shown by the high residual resistance ratios (VSi 2: RRR 20; NbSi 2: RRR 220; TaSi 2: RRR 400). The three compounds crystallize in the same hexagonal structure (C40, space group P6 222). Crystallographic and temperature dependences of the resistivity and of the magnetoresistance ( H ⊥ ⩽ 7.4 T) have been measured from 4.2 K up to room temperature. All the three materials show a metallic behaviour and some anisotropy, which is of the order of 2 for the three compounds. The magnetoresistance measurements show that VSi 2 and TaSi 2 are compensated metals with relatively small charge carrier concentrations (VSi 2: n ≈ 0.15 electrons/formula unit; TaSi 2: n ≈ 0.20–0.25 electrons/formula unit).

Optical and electronic properties of 5th-column transition metal disilicides

We performed optical reflectivity measurements on the whole spectral range from 0.01 up to 6 eV and ellipsometric spectral measurements in the range 1.4–5 eV on the three isoelectronic and isocrystalline metal disilicides VSi 2, NbSi 2 and TaSi 2. We analyzed the optical response in order to obtain the dielectric functions and we correlated the results with the available information on the electronic states. Both intraband and interband properties have been discussed and the numerical values of the plasma frequency, the scattering time and the energy of interband transitions were reported. Evidence is given to some general trends for the electronic properties of disilicides in passing from 3d to 5d metals.

X-ray-emission studies of chemical bonding in transition-metal silicides.

We present Si ${\mathit{L}}_{2,3}$ emission-band spectra of a series of 3d and 4d transition-metal (TM) silicides, together wtih Si K emission-band spectra of four 3d TM disilicides. The data are compared with augmented-spherical-wave density-of-states (DOS) calculations, and good agreement is found. The trends we find are explained with a general scheme for chemical bonding in TM silicides. The differences between the experimental data and the calculated DOS curves are tentatively attributed to self-energy effects.

Energetics of C11b, C40, C54, and C49 structures in transition-metal disilicides

The relative energies of the related C11b, C40, and C54 crystal structures of group IV–VII transition-metal disilicides are obtained by ab initio self-consistent band-structure calculations using the augmented-spherical-wave (ASW) method. The structural energy differences among these three structures correlate strongly with d-band filling, with C40 being stabilized relative to C54 and C11b relative to C40 as the transition-metal d-electron count increases. The C40/C11b energy difference is <0.05 eV/atom only for CrSi2 and MoSi2. Relative C11b/C40/C54 energies are similar in magnitude to those obtained in previous studies of L12/D022/D023 competition in transition-metal aluminides.1,2 Calculations of the C49/C54 energetic competition are inaccurate; the differences in atomic coordination in these two structures are probably too large for the computational method to handle accurately. The total-energy results are interpreted by a detailed analysis of the electronic density-of-states (DOS) distributions. The stable structures do not correlate as strongly with DOS effects in the vicinity of the Fermi level as in the aluminides.