Silicidation Reaction Research Articles

Real-Time NMR Investigations of Structural Changes in Silicon Electrodes for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) containing silicon negative electrodes have been the subject of much recent investigation because of the extremely large gravimetric and volumetric capacity of silicon. The crystalline-to-amorphous phase transition that occurs on electrochemical Li insertion into crystalline Si, during the first discharge, hinders attempts to link structure in these systems with electrochemical performance. We apply a combination of static, in situ and magic angle sample spinning, ex situ (7)Li nuclear magnetic resonance (NMR) studies to investigate the changes in local structure that occur in an actual working LIB. The first discharge occurs via the formation of isolated Si atoms and smaller Si-Si clusters embedded in a Li matrix; the latter are broken apart at the end of the discharge, forming isolated Si atoms. A spontaneous reaction of the lithium silicide with the electrolyte is directly observed in the in situ NMR experiments; this mechanism results in self-discharge and potential capacity loss. The rate of this self-discharge process is much slower when CMC (carboxymethylcellulose) is used as the binder.

Impact of channel doping on Schottky barrier height and investigation on p-SB MOSFETs performance

This paper proposes to study the impact of a moderate variation of the channel doping level on the electrical performance of p-type Schottky-barrier (SB) MOSFETs. First, it has been found that a moderate increase of the acceptors doping level leads to a reduction of the Schottky-barrier height (SBH) but does not affect the silicide reaction. In the case of PtSi, the SBH on p-type silicon at 5 × 10 15 cm −3 is 0.15 eV whereas an increase of the doping level by two decades decreases the barrier by 60 meV. The integration of PtSi MOSFETs on moderately doped channel (5 × 10 17 cm −3) was successfully achieved, demonstrating an overall 60% improvement in current drive at L g = 100 nm. This enhanced performance is attributed to the barrier height reduction related to the beneficial band bending induced by p-type dopants. The considered doping levels are still in a sufficiently low range not to affect the carrier mobility in the channel. A complete study, including comparison of I on, I off, immunity against short channel effects (Swing and DIBL), is presented.

Thermal stability in a-Si/HfSiO(N)/Si gate stack structures studied by photoemission spectroscopy using synchrotron radiation

We have investigated thermal stability in amorphous-Si/HfSiO(N) gate stack structures using synchrotron-radiation photoemission spectroscopy. Core-level photoemission spectra depending on annealing temperature have revealed the mechanism of metallization reaction at the upper interface between a-Si cap layer and HfSiO(N) films under ultrahigh vacuum annealing. Silicidation reaction starts by annealing at 700 °C for both HfSiO and HfSiON films. By annealing at 800 °C, metallization reaction is rapidly promoted for the HfSiO film, while the Hf-silicide component changes into the Hf-nitride component due to its thermal stability and metallization reaction mildly proceeds for the HfSiON films.

Formation of Si- and Ge-based Full-Heusler Alloy Thin Films using SOI and GOI Substrates for the Half-metallic Source and Drain of Spin Transistors

The paper presents a novel preparation technique for Si- and Ge-based half-metallic full-Heusler alloy thin films, utilizing silicon-on-insulator (SOI) and germanium-on-insulator (GOI) substrates, respectively. Full-Heusler Co2FeSi (Co2FeGe) alloy thin films were successfully formed by thermally activated silicidation (germanidation) reaction between an ultra-thin SOI (GOI) layer and Co/Fe layers deposited on it. This technique can easily produce fully ordered L21 structure that is necessary for the half-metallicity of full-Heusler alloys. The proposed technique is compatible with metal source/drain formation process in advanced CMOS technology and would be applicable to the fabrication of the half-metallic source/drain of MOSFET type of spin transistors.

Characterization of half-metallic L21-phase Co2FeSi full-Heusler alloy thin films formed by rapid thermal annealing

The authors developed a preparation technique of Co2FeSi full-Heusler alloy thin films with the L21-ordered structure on silicon-on-insulator (SOI) substrates, employing rapid thermal annealing (RTA). The Co2FeSi full-Heusler alloy films were successfully formed by RTA-induced silicidation reaction between an ultrathin SOI (001) layer and Fe∕Co layers deposited on it. The highly (110)-oriented L21-phase polycrystalline full-Heusler alloy films were obtained at the RTA temperature of 700°C. Crystallographic and magnetic properties of the RTA-formed full-Heusler alloy films were qualitatively the same as those of bulk full-Heusler alloy. The proposed technique is compatible with metal source∕drain formation process in advanced complementary metal-oxide semiconductor technology and would be applicable to the fabrication of the half-metallic source∕drain of metal-oxide-semiconductor field-effect transistor type of spin transistors.

Enhanced growth of low-resistivity cobalt silicide by using a Co/Au/Co trilayer film on Si 0.8Ge 0.2 virtual substrate

Formation of Co germanosilicides on Si 0.8Ge 0.2 virtual substrates with a Co/Au/Co sandwich thin film after different heat treatments has been investigated. The sequence of phase formation is the same as the reaction of blanket Co thin film with (001)Si. The presence of thin interposing Au layers was found to significantly enhance the formation of low-resistivity CoSi 2 on (001)Si 0.8Ge 0.2 substrates. The formation temperature of CoSi 2 phase in the Co/Au/Co/(001)Si 0.8Ge 0.2 samples was lowered by about 200 °C compared to that of Co/(001)Si 0.8Ge 0.2 samples. From TEM and EDS analysis, some of Au atoms were found to diffuse from the original interface position to disperse within the CoSi 2 layers during silicidation reactions. The mechanisms for the enhanced formation of CoSi 2 in the Co/Au/Co/Si 0.8Ge 0.2 system were explained in the context of classical nucleation theory.

Controlling thin film structure for the dewetting of catalyst nanoparticle arrays forsubsequent carbon nanofiber growth

Vertically aligned carbon nanofiber (CNF) growth is a catalytic chemical vapordeposition process in which structure and functionality is controlled by the plasmaconditions and the properties of the catalyst nanoparticles that template thefiber growth. We have found that the resultant catalyst nanoparticle networkthat forms by the dewetting of a continuous catalyst thin film is dependent onthe initial properties of the thin film. Here we report the ability to tailor thecrystallographic texture and composition of the nickel catalyst film and subsequently thenanoparticle template by varying the rf magnetron sputter deposition conditions.After sputtering the Ni catalyst thin films, the films are heated and exposedto an ammonia dc plasma, to chemically reduce the native oxide on the filmsand induce dewetting of the film to form nanoparticles. Subsequent nanoparticletreatment in an acetylene plasma at high substrate temperature results in CNFgrowth. Evidence is presented that the texture and composition of the nickel thinfilm has a significant impact on the structure and composition of the formednanoparticle, as well as the resultant CNF morphology. Nickel films with a preferred(111) or(100) texture were produced and conditions favoring interfacial silicidation reactions wereidentified and investigated. Both compositional and structural analysis of the films andnanoparticles indicate that the properties of the as-deposited Ni catalyst film influences thesubsequent nanoparticle formation and ultimately the catalytic growth of the carbonnanofibers.

Enhanced growth of CoSi 2 thin films on (0 0 1)Si with Co/Au/Co sandwich structures

Formation of cobalt silicides in the Co/Au/Co trilayer films on (0 0 1)Si substrate after different heat treatments has been investigated. The nucleation temperature of low-resistivity CoSi 2 phase in the Co/Au/Co/(0 0 1)Si samples was found to be lowered by about 190 °C compared to what is usually needed for the growth of CoSi 2. The results can be explained using the classical nucleation theory. From energy dispersive X-ray (EDAX) analysis, the Au atoms were found to diffuse from their original position to disperse in CoSi 2 layer and in the grain boundaries of CoSi 2 during silicidation reactions. In addition, compared with the Co/Au/(0 0 1)Si sample, the surface and interfacial roughness of CoSi 2 film was effectively improved by using the Co/Au/Co sandwich structure on (0 0 1)Si.

Nickel-Based Germanosilicide of Heavily-Doped SiGe Films for Low Resistance and High-Temperature Stability

We have studied thermo-electrical properties for Ni-based germanosilicide to understand the influence of temperature on the evolution of sheet resistance and micro-structures of contacts on heavily-doped SiGe grown by reduced pressure chemical vapor deposition. After the deposition of Ni, Ni/Ti, Ni/Pt films on Si0.83Ge0.17 epi layer and subsequently annealing for silicide reaction, we analyzed sheet resistance, surface roughness and reaction interfaces using four point probe method, scanning probe micrograph and transmission electron microscope. Bi-layer metal structures of Ni/Ti and Ni/Pt were investigated to study feasible use for suppressing inappropriate reaction at interface. It is found that bi-layer structure with thin Pt interlayer presented promising properties for germanosilicide of n+-Si0.83Ge0.17 with low sheet resistance, smooth surface morphology and high temperature stability up to 800 oC.

Effect of a Fe 3Si buffer layer for the growth of semiconducting β-FeSi 2 thin film on stainless steel substrate

This paper reports a Fe 3Si buffer layer for the growth of semiconducting β-FeSi 2 film on stainless steel (SS) substrate. It was formed through the silicidation reaction of Fe and Si at a Fe-rich interface at an elevated temperature of 600 °C. β-FeSi 2 film was deposited on it at low temperature around 400 °C. Auger electron spectroscopy depth profile showed that the inter-diffusions of Fe, Si and impurities contained in the SS substrate were effectively blocked by the buffer layer. The adhesive ability to SS substrate and crystallinity of β-FeSi 2 film were clearly improved by the introduction of the buffer layer, thus also induced the decrease of residual carrier concentration, which was revealed by capacity–voltage measurement.

Roughness improvement of the CoSi 2/Si-interface for an application as buried silicide

The material CoSi 2 is preferred for the fabrication of buried silicide films between silicon device layer and buried oxide of SOI substrates for BICMOS integrations. Such an application needs excellent quality of the interface between the silicide and the silicon device layer. Using the conventional cobalt salicide process the roughness and waviness of the interface is too large for a device application. In this presentation three technologies to improve the CoSi 2/Si-interface quality were characterized. Using the first technology a very thin single crystalline CoSi 2 film was fabricated on a silicon substrate. This film acts as initial layer to produce thicker single crystalline silicide films. By the second technology an interlayer between cobalt and the silicon substrate was used to mediate an epitaxial CoSi 2 growth. Different types and materials were tested. Using the third technique a sacrificial layer of polycrystalline silicon between cobalt and the silicon substrate was consumed during the silicidation reaction. This method gives the best results with interface roughness values of less than 1 nm. The interface roughness was measured after CoSi 2 removal using AFM. A possible epitaxial growth of the silicide films was investigated with XRD analysis. Cross sectional SEM images were prepared to analyze the interface waviness and the CoSi 2 structure.

Formation Kinetics and Work Function Tuning of Pd2Si Fully Silicided Metal Gate

The formation kinetics of Pd2Si for fully silicided (FUSI) gate formation and the work function tuning of a Pd2Si FUSI gate by impurity predoping were investigated. It has been found that the morphology and phase of a formed FUSI layer depend not only on the silicidation annealing temperature but also on the heating ramp-up rate and the presence of impurities. Fast ramp-up annealing was necessary to avoid defect formation, such as voids in the silicide film at the oxide interface, and to obtain a homogeneous silicide film containing only Pd2Si phase. The most severe effect on the silicidation reaction, that is the increase in defect formation, was brought about by As predoping. The work function of the Pd2Si FUSI gate was modulated by impurity pileup at the Pd2Si/SiO2 interface, as in the case of the NiSi FUSI gate. However, the work function shifted in the opposite direction to that of the NiSi FUSI gate for As, P, Sb, and BF2. A wide range of work function shift, comparable to that of the NiSi FUSI gate, has been obtained for a Pd2Si FUSI gate.

Measurements of metal gate effective work function by x-ray photoelectron spectroscopy

We developed a method to measure effective work function of metal gate in contact with high-k dielectric. The method was based on the deposition of ultrathin (1–5nm) continuous and very uniform layers of metal on high-k gate stack (HfO2) by pulsed laser deposition and in situ measurements by x-ray photoelectron spectroscopy. This approach was applied to investigate the evolution of work function during silicidation reaction between Ni overlayer and thin Si∕HfO2∕Si(100). The effect of dopants on fully silicided gate formation and work function modulation was also explored. The effective work function of NiSi, Ni2Si, and Sb-doped Si gate electrodes on HfO2 dielectric was found to be 4.4, 4.5–4.7, and 4.2eV, respectively. Ge interlayer at the NiSi∕HfO2 interface showed no significant effect. These values were in a good agreement with C-V measurements performed ex situ on metal-oxide-semiconductor capacitors.

A new solution route to hydrogen-terminated silicon nanoparticles: synthesis,functionalization and water stability

Hydrogen-capped silicon nanoparticles with strong blue photoluminescencewere synthesized by the metathesis reaction of sodium silicide, NaSi, withNH4Br. The hydrogen-capped Si nanoparticles were further terminated with octylgroups and then coated with a polymer to render them water-soluble. Thenanoparticles were characterized by TEM, FT-IR, UV–vis absorption andphotoluminescence. The Si nanoparticles were shown to have an average diameter of3.9 ± 1.3 nm and exhibited room temperature photoluminescence with a peak maximum at 438 nmwith a quantum efficiency of 32% in hexane and 18% in water; the emission wasstable in ambient air for up to 2 months. These nanoparticles could hold greatpotential as a non-heavy-element-containing quantum dot for applications in biology.

Preparation and characterization of ferromagnetic DO3-phase Fe3Si thin films on silicon-on-insulator substrates for Si-based spin-electronic device applications

The authors developed a preparation technique of stoichiometric and off-stoichiometric ferromagnetic Fe3+δSi1−δ thin films with a DO3 phase structure using silicon-on-insulator (SOI) substrates. Ferromagnetic Fe3+δSi1−δ films were formed by thermally activated silicidation reaction between an ultrathin SOI layer and a Fe layer deposited on it, induced by rapid thermal annealing. Their chemical composition was precisely controlled by the relative thickness ratio of the SOI and Fe layers, and highly oriented DO3-phase polycrystalline Fe3+δSi1−δ films were obtained with annealing temperature ranging from 620 to 780°C. The Fe3+δSi1−δ films exhibited ferromagnetic nature with the Curie temperature much higher than room temperature.

Interlayer-Assisted Stability of Germanosilicide for Heavily-Doped n+-Si0.83Ge0.17 Grown by Reduced Pressure Chemical Vapor Deposition

Ni-based germanosilicide process of heavily doped n+- Si0.83Ge0.17 has been investigated to understand the influence of temperature and doping concentration on the evolution of sheet resistance and micro-structures. After annealing Ni film on various Si0.83Ge0.17 epi layers for silicide reaction, the evolution of sheet resistance and surface roughness was analyzed using x-ray diffraction, four pint probe, atomic force microscope, and transmission electron microscope. Especially, bi-layer metal structures were employed to resolve inappropriate degradation associated with heavy doping of phosphorous in Si0.83Ge0.17, from which crucial improvements in structural and electrical properties could be confirmed. Pt interlayer played an important role for uniform germanosilicide at the interface of metal and semiconductor, and resulted in low sheet resistance and relevant thermal stability.

Influence of AlN layers on the interface stability of HfO2 gate dielectric stacks

The influence of thick (∼10nm) AlN overlayers on the interface structure and reactions in Si gate stacks with HfO2 dielectrics was investigated. Annealing caused a reduction of the interfacial SiO2 at the Si interface. At high temperatures (∼1000°C) a silicide reaction was observed at the HfO2∕Si interface. No reactions were observed for stacks processed similarly but with WN or TiN overlayers instead of AlN. The reaction mechanisms, in particular, the role of oxygen deficiency of the HfO2, and the consequences for the electrical properties are discussed.

Effect of Ti alloying in nickel silicide formation

In the presence of interfacial oxide, an addition of 20 at.% Ti to Ni film leads to the formation of a thick TiO x layer at the Ni(Ti)/Si interface upon annealing, preventing the inter-diffusion of Ni and Si hence hindering the Ni silicide formation up to 700 °C. At 800 °C, a mixture of predominant NiSi phase with some NiSi 2 grains facetted in (111) plane was observed. A different mechanism occurred for the sample with minimal oxygen contamination where Ni 3Si 2 was found to be stable up to 900 °C. Nevertheless, Ti addition has delayed the silicidation reaction to 600 °C.

Silicon-Atom Induced Fermi-Level Pinning of Fully Silicided Platinum Gates on HfO2 Dielectrics

Fully silicided platinum gates have been proposed as metal gate electrodes for scaled complementary metal oxide semiconductor field-effect transistors (CMOSFETs). The Pt monosilicide (PtSi) phase was formed on gate dielectrics by a full silicidation reaction at 400°C. PtSi gate electrodes on SiO2 possess a high effective work function of 4.9 eV, which could be used as a possible metal gate electrode for p-MOSFETs, while at the PtSi/HfO2 interface, Fermi-level pinning is observed independent of the PtSi fabrication process. The Fermi-level pinning at the PtSi/HfO2 interface is comparable to that at the poly-Si/HfO2 interface, and Fermi-level pinning does not occur at the Pt/HfO2 interface. These results show that Fermi-level pinning is induced by the presence of silicon atoms at the HfO2 upper interface.

Thermodynamic stability of SiO 2 in contact with thin metal films

Solid-state compound phase formation has been investigated between thin metal films (Cu, Ir, Mo, Pt, Re, Y, Yb, and Zr) and SiO 2 substrates using Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) techniques. The thin-film couples were annealed for time periods ranging from 30 min to 3 h between 320 and 900 °C. It was found that Y, Yb, and Zr react with SiO 2 whereas Cu, Ir, Pt, Re, and Mo do not. Heats of reaction were calculated for all possible combinations of silicide and metal-oxide reaction products. Comparisons with experimental results obtained from this study and the literature show in all cases that metal–SiO 2 reactions only take place when the calculated heat of reaction is negative. This study shows that the results obtained as well as those which could be found from the literature correlate well with the electronegativity of the metal, which offers a convenient empirical method of predicting whether a metal will react with SiO 2 or not. Only metals with a Miedema electronegativity parameter less than 4.45 V reacted with SiO 2.