C54 TiSi2 Research Articles

Preparation of TiSi2 Powders with Enhanced Lithium-Ion Storage via Chemical Oven Self-Propagating High-Temperature Synthesis.

Although silicon has highest specific capacity as anode for lithium-ion battery (LIB), its large volume change during the charge/discharge process becomes a great inevitable hindrance before commercialization. Metal silicides may be an alternative choice because they have the ability to accommodate the volume change by dispersing Si in the metal matrix as well as very good electrical conductivity. Herein we report on the suitability of lithium-ion uptake in C54 TiSi2 prepared by the “chemical oven” self-propagating high-temperature synthesis from the element reactants, which was known as an inactive metal silicide in lithium-ion storage previously. After being wrapped by graphene, the agglomeration of TiSi2 particles has been efficiently prevented, resulting in an enhanced lithium-ion storage performance when using as an anode for LIB. The as-received TiSi2/RGO hybrid exhibits considerable activities in the reversible lithiation and delithiation process, showing a high reversible capacity of 358 mAh/g at a current density of 50 mA/g. Specially, both TiSi2 and TiSi2/RGO electrodes show a remarkable enhanced electrochemical performance along with the cycle number, indicating the promising potential in lithium-ion storage of this silicide. Ex-situ XRD during charge/discharge process reveals alloying reaction may contribute to the capacity of TiSi2. This work suggests that TiSi2 and other inactive transition metal silicides are potential promising anode materials for Li-ion battery and capacitor.

Fabrication of Ti-Si-Ti Metal-Semiconductor-Metal Photodetectors Using Low Temperature Rapid Thermal Annealing

The electrical properties of Ti-Si-Ti Metal-Semiconductor-Metal (MSM) photodetector were studied as a function of annealing temperature, using Rapid Thermal Annealing (RTA) process. Low temperatures were used at the RTA (200-350°C) in order to avoid the formation of silicides.We observed a decrease in the dark current on samples annealed between 200 and 300°C. The lowest dark current was obtained in the sample annealed at 250°C (4.8 nA), which is one order of magnitude lower than as-deposited sample (53.5 nA). The sample annealed at 350°C had an increase in dark current (82.9 nA). This behavior of the dark current can be explained by the increase in the barrier height at 200-300°C annealing temperature range, due to increase of the thickness of the amorphous interdiffused Ti-Si interfacial layer, and decrease in the barrier value at sample annealed at 350°C, due to pre formation of C49 TiSi2.

Correction to: Separation and enrichment mechanism of C54–TiSi2 from hypoeutectic Ti–65 wt.% Si alloy during directional solidification via alternating electromagnetic fields

The original version of this article unfortunately contained a mistake. Figure 7 was incorrect. The corrected figure is given below.

Separation and enrichment mechanism of C54–TiSi2 from hypoeutectic Ti–65 wt.% Si alloy during directional solidification via alternating electromagnetic fields

The effects of directional solidification parameters and the coupling of directional solidification parameters and alternating electromagnetic fields on separation and enrichment of the C54–TiSi2 phase were investigated in a directionally solidified hypoeutectic Ti–65 wt.% Si alloy. The results indicated that by increasing the pull-down velocity at a given position within the ingot, the cooling rate, growth rate, and temperature gradient of ingot could be increased. At a pull-down velocity near 5 μm/s, the temperature gradient, cooling rate, and growth rate decreased with increasing the thickness of the C54–TiSi2-rich layer. Electromagnetic fields enhanced mass transfer at pull-down velocities of 5, 10, 15, and 20 μm/s, with resulting enriched layer thicknesses of 15, 10, 10, and 5 mm, respectively. By increasing the percentage of Ti in the Ti–Si alloy from 25 to 35 wt.%, the thickness of the C54–TiSi2-rich layer was increased from 2.5 to 3.3 cm. However, the maximum C54–TiSi2 content obtained experimentally in this layer decreased from 92.06 to 79.49 mass%.

Lithium Insertion in Nanostructured Si1-xTix Alloys

Nanostructured Si1-xTix alloys (0 ≤ x ≤ 0.3) prepared by ball milling were studied as negative electrode materials in Li cells. The alloys comprised a nanocrystalline and amorphous Si phase and a nanocrystalline C49 TiSi2 phase. When x ≥ 0.15 the nanocrystalline Si phase was completely eliminated and such alloys consisted only of amorphous Si and the C49 TiSi2 phase. The alloys with x ≥ 0.15 also completely suppressed the formation of Li15Si4 during cycling, resulting in good capacity retention, and, unlike other Si transition metal alloys, without introducing noticeable cell polarization. The observed capacity of the Si1-xTix alloys suggested that the TiSi2 is an inactive phase toward Li. The Si0.85Ti0.15 composition had a high capacity of 1710 Ah/L with 96% capacity retention after 50 cycles, making it attractive for use in Li-ion batteries.

Anisotropy in elastic properties of TiSi2 (C49, C40 and C54), TiSi and Ti5Si3: an ab-initio density functional study

We present a comparative study of the anisotropy in the elastic properties of the C49, C54 and C40 phases of TiSi2, as well as orthorhombic TiSi and hexagonal Ti5Si3. The elastic constants, elastic moduli, Debye temperature and sound velocities are computed within the framework of density functional theory. The computed values of the elastic constants and moduli are found to be in excellent agreement with available experimental values. The average elastic moduli, such as Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio, of polycrystalline aggregates are computed using the computed elastic constants of single crystals. The anisotropy in elastic properties is analyzed using estimates of shear anisotropic factors, bulk modulus anisotropic factors and variations in Young’s and bulk moduli in different crystallographic directions. Among the Ti–Si phases, the computed directional Young’s modulus profiles of C49 TiSi2 and C40 TiSi2 are found to be quite similar to those of bulk Si and Ti, respectively. In addition to the elastic properties, the electronic structure of five Ti–Si phases is studied. The density of states and planar charge density profiles reveal mixed covalent–metallic bonding in all Ti–Si phases.

Structural characterization of dual ion implantation in silicon

〈100〉 Si wafers were dual implanted at room temperature with Fe+C and Ti+C with fluences of 2×1017cm−2. The samples were annealed in vacuum at 800°C and 1000°C respectively, and characterized in scanning electron microscope (FEG-SEM), grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS). The scanning electron microscopy characterization showed that both annealings generated precipitates, with sizes within the range of 10–100nm at 800°C and 1–10μm for the 1000°C annealing. The GIXRD measurements revealed the presence of different silicides phases. For the Fe+C implantation β-FeSi2 was observed at 800°C while at 1000°C α-FeSi2 and SiC were found. The Ti+C sample at 800°C showed simultaneously the presence of four different phases, both metastable C49 and stable C54 silicide TiSi2, poly-Si and SiC. At higher temperatures the metastable C49-TiSi2 silicide phase was no longer observed, all the others remaining. The XPS analysis confirmed the existence of the SiC at 1000°C temperature, and showed that the initial carbon clusters get richer in Si with the increase of temperature to form SiC. It was also possible to see that among all present species, C was the one that oxidized the most with increasing temperature.

C49 TiSi2 on silicon: Preparation by rapid thermal processing and optical properties

We have synthesized a novel TiSi2(C49)/Si silicon-technology-based semiconductor structure which possesses both photoelectric and luminescent properties and is potentially attractive for use in optoelectronic couples. According to IR spectroscopy results, increasing the synthesis temperature of C49 titanium disilicide increases its reflectance by 15% at wavenumbers from 500 to 1000 cm−1 and by 30% in the range 3000–4000 cm−1. Under excitation in the range 360–390 nm, the C49 TiSi2 films have been observed for the first time to exhibit luminescence between 400 and 750 nm, which correlates with the spectral distribution of their photosensitivity.

Anisotropic growth of titanium disilicide nanocrystals on Si(113) surface

Growth of titanium disilicide (TiSi2) particles on a Ti-deposited Si(113) surface was observed with a scanning tunneling microscope. The direction of the particle growth was specifically related to the anisotropy of the high-index surface. The particles were identified as C54 nano-crystals, and their orientation was determined from the morphology of the silicide facets. The Si(113)-specific growth orientation is explained in terms of the lattice mismatch between C54 TiSi2 and the reconstructed surface structure.

Influence of Substrates on the Formation of the TiSi Nanowire by Atmosphere Pressure Chemical Vapor Deposition

TiSi nanowires were deposited on both Si(111) and glass substrates by using SiH4, TiCl4 and N2 as the Si, Ti precursors and diluted gas respectively through atmosphere pressure chemical vapor deposition (APCVD) method. Effects of the substrates on formation of the nanowires were investigated. The results show that the nanowires can be formed on both Si(111) and glass substrates at ratio of SiH4/TiCl4 of 4. However, the quantities of the TiSi nanowires that formed with glass substrate are less than that with Si(111) substrate. The nanowires formed with glass substrate has length of 2-3 microm and diameters of 15-25 nm while that is 4-5 microm and 25-35 nm respectively with Si(111) substrate. Great quantities of the titanium silicide nanowires with relative higher contents of the C54 TiSi2 crystalline phase underneath can be obtained through improving the deposition conditions.

Preparation of TiSi2 via flash lamp processing of TiN/Ti/Si heterostructures

The structural and phase transformations induced in the TiN/Ti/Si system by flash processing with incoherent light have been studied by transmission electron microscopy and Auger electron spectroscopy. The conditions have been identified under which polycrystalline C49 or C54 TiSi2 films grow.

Spontaneous Growth of Highly Conductive Two‐Dimensional Single‐Crystalline TiSi2 Nanonets

Net result: Two-dimensional nanonets form spontaneously in a chemical vapor deposition reaction. All beams in the nanonet are single-crystalline nanobelts that are connected by 90° joints. The nanonets are roughly 15 nm thick and a few micrometers long and wide (see pictures for three images at different tilting angles), and they assume a C49 TiSi2 structure.

Fabrication of Metal-Semiconductor-Metal Photodetectors Using Low Temperature Rapid Thermal Annealing

The electrical properties of Ti-Si-Ti Metal-Semiconductor-Metal (MSM) photodetector were studied as a function of annealing temperature using the Rapid Thermal Annealing (RTA) process. Low temperatures were used at the RTA (200-350{degree sign}C) in order to avoid the formation of silicides. We observed a decrease in the dark current on samples annealed between 200 and 300{degree sign}C. The lowest dark current obtained was in the sample annealed at 250{degree sign}C (4.8 nA), which is one order of magnitude lower than as-deposited sample (53.5 nA). The sample annealed at 350 {degree sign}C had an increase in dark current (82.9 nA). This behavior of the dark current can be explained by the increase in the barrier height at 200-300 {degree sign}C annealing temperature range (due to increase of the thickness of the amorphous interdiffused Ti-Si interfacial layer) and decrease in the barrier value at sample annealed at 350{degree sign }C (due to pre formation of C49 TiSi2).

Influence of Annealing Environment and Film Thickness on the Phase Formation in the Ti/Si(100) and (Ti +Si)/Si(100) Thin Film Systems

Influence of an annealing environment and film thickness on the phase formation in the Ti(30 nm)/Si(100), [(Ti+Si) 200 nm]/Si(100) thin film systems produced by magnetron sputtering and the Ti(200 nm)/Si(100) thin film system produced by electron-beam sputtering were investigated by X-ray and electron diffraction, Auger electron spectroscopy (AES), secondary ion mass-spectrometry (SIMS) and resistivity measurements. Solid-state reactions in the thin film systems under investigation were caused by diffusion processes during annealing in the different gas environments: under vacuum of 10-4 - 10-7 Pa, flow of nitrogen and hydrogen. It is shown that the decrease of Ti layer thickness from 200 to 30 nm in the Ti/Si(100) film system causes the increase of the transition temperature of the metastable C49 TiSi2 phase to the stable C54 TiSi2 phase up to 1070 K at vacuum annealing. During annealing in the nitrogen flow of the Ti(30 nm)/Si(100) thin film system the C49 TiSi2 is the first crystal phase which is formed at 870 K. For annealings of the [(Ti+Si) 200 nm]/Si(100) thin film system by impulse heating method or for furnace annealings in inert gas atmosphere of N2, Ar, H or higher vacuum (10-5 Pa) the crystallization process has two stages: the first metastable C49 TiSi2 phase is formed at 870 K and then at higher temperatures it is transformed to the stable C54 TiSi2 phase.

Characteristics of the nanoscale titanium film deposited by plasma enhanced chemical vapor deposition and comparison of the film properties with the film by physical vapor deposition

TiSi2 film was grown by plasma enhanced chemical vapor deposition (PECVD) method for lower contact resistance. We obtained a uniform TiSi2 film on Si substrate. The growth rate of TiSi2 on Si substrate is four times higher than that of Ti on silicon dioxide. The thickness ratio of TiSi2∕Si is very high at the early stage of growth. The high ratio, 8.8, results from a long incubation time before the formation of Ti film on silicon dioxide and ignoring the incubation time, thickness ratio is almost constant in the given thickness range. The average of thickness ratio is determined as 4.25. C49 (060) phase was formed on Si substrate by PECVD method and C49 TiSi2 phase was not changed into C54 one by rapid thermal annealing (RTA) at 900°C. We could not observe boron segregation at the TiSi2∕Si interface. On the contrary, we observed inward diffusion of boron into Si substrate during the silicide formation. We obtained 1.21×103Ω of contact resistance by PECVD method. When Ti thickness is less than 1nm, the contact resistance is comparable with that by physical vapor deposition method or lower.

On the role of oxygen in the catalysis of C54 titanium disilicide by Ti5Si3 phase

The influence of the native SiO2 in the phase formation sequence of Ti∕Si system has been studied. Furnace annealing of Ti∕Si couples has been investigated by secondary-ion-mass spectrometry, Rutherford backscattering, and glancing incidence x-ray diffraction. The influence of dilute concentration of oxygen at the interface in altering reaction kinetics was studied. These studies reveal that the highly metallic Ti5Si3 phase forms during the intermediate stages of heat treatment. This phase is found to catalyze the direct formation of the low-resistive C54 TiSi2 phase, completely bypassing its polymorphic C49 counterpart. X-ray-diffraction line profile analysis of Bragg peaks indicates that the formation of the C54 phase is accompanied by a reduction in the microstrain. Subsequent annealing results in the complete transformation to TiSi2 with an increase in the average grain size.

Ab initio comparative study of C54 and C49 TiSi2 surfaces

A theoretical comparison of C54 and C49 TiSi2 surfaces is presented, using ab initio plane-wave ultrasoft pseudopotential method based on generalized gradient approximation (GGA). The different surface energies of TiSi2 have not only been calculated out, but the preferential formation of C49 phase in solid-state reaction could be explained by smaller surface energies and Poisson's ratio of C49 TiSi2 as well. As for polar C54 TiSi2(1 0 0) and C49 TiSi2(0 1 0) surfaces, the Si termination surfaces are more stable.

The Effect of Film Thickness on the C40 TiSi2 to C54 TiSi2 Transition Temperature

In this paper, we investigate the transformation of from the high-resistivity C40 phase to the low-resistivity C54 phase. In particular, the effect of C40 film thickness on the transition temperature, which has not been studied in the literature, is scrutinized. Two types of laser, i.e., (yttrium aluminum garnet) and excimer laser, were used to induce C40 on samples of 350 Å of Ti deposited on (100) Si substrates. It was also found that the thicker C40 (40 nm) induced by the laser required a very high temperature of more than 1000°C and an activation energy of 6.5 eV for the formation of the stable C54 , whereas a thinner C40 (15 nm) produced by the excimer laser required a low temperature of 700°C and an activation energy of 2.5 eV to transform into C54 . The much lower transition temperature for the thin samples is attributed to the compressive strain in these samples.

Interfacial reaction in the growth of epitaxial SrTiO3 thin films on (001) Si substrates

The SrTiO3∕Si interface was investigated by transmission electron microscopy for SrTiO3 films grown on (001) Si by molecular-beam epitaxy with different native oxide (SiO2) removal treatments, and Sr∕Ti flux ratios. The interface and film microstructure were independent of the process used to remove the native oxide, but the interface reactivity was dependent on the Sr∕Ti flux ratio. A low Sr∕Ti flux ratio (∼0.8) resulted not only in a layer of amorphous material at the film∕substrate interface but also in the formation of crystalline C49 TiSi2 precipitates at that interface. These results are consistent with thermodynamic expectations in which it is paramount to maintain separation between TiO2 and the underlying silicon.

Approach to interface roughness of silicide thin films by micro-Raman imaging

The rough interface between a silicide film and a Si substrate induces large junction leakage currents in Si-based devices. In this work, we demonstrate that micro-Raman imaging can be used to characterize the interface roughness, without any special sample preparation. Pure metal (Ni and Ti) thin films and the subsequently formed silicide thin films after annealing (NiSi and TiSi2) were investigated using the intensity of the Si Raman band at 520cm−1. Uniformity results of the pure metal films are in good agreement with those obtained using global four-point-probe measurement. In comparison, the signals obtained from the interfaces between Si and NiSi are nonuniform with large variations. The intensity of the major peak of NiSi at 214cm−1 shows a point-to-point correlation with the intensity of the Si peak at 520cm−1, which reveals that the local grain orientation affects the diffusion velocity of Ni atoms and the growth of NiSi. Images of Ti-silicides show that the interface of the C49 TiSi2 phase is much smoother than that of the C54 TiSi2 phase which is due to different formation mechanisms. Simulation was carried out based on two interfacial models, a two-step interface and a sinusoidal interface, to understand the correlation between the average attenuation of the Si Raman signal and the film thickness.