Improved electrical and thermal properties of nickel silicides by using a NiCo interlayer

We have investigated the effects of a NiCo interlayer on the electrical and thermal properties of nickel silicide as a function of the annealing temperature. For the interlayered samples, 3 nm-thick NiCo(10 at.% Co) films are electron-beam evaporated on Si substrates, on which 27 nm-thick Ni films are deposited without breaking the vacuum. It is shown that all the samples exhibit a distinctive increase in the sheet resistance at temperatures above 900 ^°C. However, the NiCo interlayer sample produces the lowest sheet resistance at 900 ^\circC. X-ray diffraction results show that the Ni only and NiCo interlayer samples produce NiSi and NiSi₂ phases, while NiCo full samples give NiSi and Ni_1−xCo_xSi₂ phases. Scanning electron microscopy results exhibit that for all the samples, the surfaces become degraded with numerous arbitrarily-shaped spots, corresponding to areas uncovered by the silicides. The areal fractions of the silicides for the Ni only, NiCo full, and NiCo interlayer samples are about 57%, 72%, and 81%, respectively. The temperature dependence of the electrical properties of the silicide samples is explained in terms of the formation of resistive phases and the agglomeration of the silicide.     

Enhanced formation of periodic arrays of low-resistivity NiSi nanocontacts on (0 0 1)Si0.7Ge0.3 by nanosphere lithography with a thin interposing Si layer

In this study, we demonstrated significant enhancement of the formation of low-resistivity NiSi nanocontacts with controlled size on (0 0 1)Si_0.7Ge_0.3 substrates by combining the nanosphere lithography with the use of a new Ni/a-Si bilayer nanodot structure. Low-resistivity NiSi with an average size of 78 nm was observed to be the only silicide phase formed in samples annealed at 350-800 °C. The presence of the interposing Si layer with appropriate thickness was found to effectively prevent Ge segregation and maintain the interface stability in forming NiSi nanocontacts on (0 0 1)Si_0.7Ge_0.3. As the annealing temperature was increased to 900 °C, amorphous SiO_x nanowires were observed to grow from silicide nanocontact regions. The NSL technique in conjunction with a sacrificial Si interlayer process promises to be applicable in fabricating periodic arrays of other low-resistivity silicide nanocontacts on Si_1−xGe_x substrates without complex lithography.     

Simultaneous formation of contacts and diffusion barriers for VLSI by rapid thermal silicidation of TiW

The formation of (Ti_xW_1–x)Si₂/(Ti_xW_1–x)N, by rapid thermal processing of Ti_xW_1–x on Si in an N₂ ambient is investigated. An activation energy of 1.7 eV is obtained for silicide formation. A distinct snow-ploughing of As atoms is observed during silicide formation whereas the interfacial B concentration decreases with increasing silicide formation temperature. The diffusion barrier properties of the (Ti_xW_1–x)Si₂/(Ti_xWi_1–x)N stack in contact with Al is investigated upon post-metal annealing. No interaction between the layers is found for temperatures as high as 475°C after 60 min. The improved thermal stability of the (Ti_xW_1–x)N layer in contact with Al is attributed to nitrogen blocking of the grain boundaries.     

Formation of interfacial iron silicides on the oxidized silicon surface during solid-phase epitaxy

The early stages of iron silicide formation in the Fe/SiO _x /Si(100) ternary system during solid-phase epitaxy are studied by high-resolution (～100 meV) photoelectron spectroscopy using synchrotron radiation. The spectra of core and valence electrons taken after a number of isochronous heat treatments of the samples at 750°C are analyzed. It is found that the solid-phase reaction between Fe and Si atoms proceeds in the vicinity of the SiO _x /Si interface, which metal atoms reach when deposited on the sample surface at room temperature. Iron silicide starts forming at 60°C. Solid-phase synthesis is shown to proceed in two stages: the formation of the metastable FeSi interfacial phase with a CsCl-like structure and the formation of the stable β-FeSi₂ phase. During annealing, structural modification of the silicon oxide occurs, which shows up in the growth of the Si⁺⁴ peaks and attenuation of the Si⁺² peaks.     

AES and factor analysis study of silicide growth at the Pd/c-Si interface

We have measured the evolution of a palladium/silicon interface under consecutive annealing periods, performed at 200°C in UHV conditions. The interface was analyzed by means of Auger electron spectroscopy combined with factor analysis applied in a sequential way. We found that silicide appears only after annealing and evolves until all the palladium is consumed. A silicon compound different from silicide, identified as Pd_xSi with x<2 is found at the interface Pd/Si and Pd₂Si/Si, before and after annealing respectively.     

Preparation and magnetic property of the composite of nitrogen-doped carbon nanotubes decorated with nickel nanoparticles

A magnetic composite of nitrogen-doped carbon nanotubes (CN_x) decorated with nickel nanoparticles was synthesized by a chemical precipitation and deoxidization method. The decorated CN_x were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD pattern showed that CN_x, nickel nanoparticles and little nickel oxides coexisted in the composite, TEM observation indicated that nickel nanoparticles were highly dispersed on the outer walls of CN_x, Magnetic measurements by VSM demonstrated that the saturated magnetization and remanence of CN_x were improved, while the coercivity was lowered after decorating with nickel nanoparticles.     

The interface reaction of high-k La2Hf2O7/Si thin film grown by pulsed laser deposition

The La₂Hf₂O₇ films have been deposited on Si (1 0 0) substrate by using pulsed laser deposition (PLD) method. X-ray diffraction (XRD) demonstrates that the as-grown film is amorphous and crystallizes after 1000 °C annealing. The interface structure is systematically studied by Synchrotron X-ray reflectivity (XRR), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Silicide, silicate and SiO_x formations from interfacial reaction are observed on the surface of the Si substrate in the as-grown film. The impact of silicide formation on the electrical properties is revealed by capacitance-voltage (C-V) measurements. By post-deposition annealing (PDA), silicide can be effectively eliminated and C-V property is obviously improved.     

Effect of nickel on the magnetic state of dysprosium in Dy<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript>x</Subscript>-Ni bilayer films

This paper reports on the results of investigations into the temperature and spectral dependences of the magnetic circular dichroism in Dy_1?xNi_x-Ni bilayer films prepared through thermal sputter deposition of components under ultrahigh vacuum. The distribution of the components over the layer thickness is examined by Auger spectroscopy. The nickel content x in Dy_1?xNi_x layers varies from 0.005 to 0.06. It is shown that, in the temperature range 80–300 K, the contribution made to the magnetic circular dichroism by a Dy_1?xNi_x layer in a bilayer film with a nickel content higher than the threshold value is approximately equal to the magnetic circular dichroism observed in an isolated Dy_1?xNi_x film at temperatures below the temperature of the phase transition to a ferromagnetic state (～100 K). This phenomenon is explained by magnetic ordering in the Dy_1?xNi_x layer of the bilayer film due to the combined effect of two factors, namely, the incorporation of nickel into a dysprosium layer and the presence of a continuous nickel sublayer in the film.     

Positron beam studies of cobalt silicides

Silicide formation in Co/Si thin structures synthesized using thermal evaporation, sputter deposition and ion implantation, has been investigated using depth-resolved positron annihilation spectroscopy (PAS) together with other corroborative experimental techniques. S vs. E_p curves and S-W correlation plots have revealed important processes such as defect annealing, interdiffusion, silicide formation and recrystallization of amorphous Si. These studies have shown that there exist differences in the formation temperature of the silicide phases, the sequence of silicide phase formation and defect generation owing to the nature of the deposition methods employed.     

Lattice diffusion of impurity enhanced by a surface heterogeneous reaction

It is shown that a heterogeneous surface reaction affects impurity distribution in the bulk of a solid body. An analytic expression for the concentration of crystal defects as a function of coordinate and reaction time and a diffusion equation in view of this expression were derived. Numerical simulation shows that the impurity concentration varies as ～exp(-(x/σ)^β. Analytic results were compared with experimental data for phosphorus diffusion upon forming nickel silicide.     

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe_0.5Ni_0.5)_100?xB_x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.     

Chemical order and magnetic properties of the Ni2−xMnSb system

The results of extensive magnetisation, X-ray and neutron powder diffraction measurements on the intermetallic compound series Ni_2-xMnSb, for 0?x?1, are reported. For x ?0.4, a high degree in the C1_b structure is observed, but for x?0.3, some disorder is evident. The series is ferromagnetic, with Curie temperatures rising from 368 to 732 K with decreasing nickel concentration. The magnetic moments all lie in the range (4.0±0.3)μ_B, with a maximum at a composition near x=0.45. The magnetic moments are largely associated with the ordered manganese sites, but it is possible that a small negative moment ?0.25μ_B may be associated with the “nickel sites” for x?0.3. This latter moment may be accounted for by disordered Mn atoms antiferromagnetically aligned.     

On the nature of differences in the Ni charge states in barium and strontium titanates

XAFS studies of nickel-doped solid solution Ba_1–xSr_xTiO₃ show that the Ni charge state changes from 4 in SrTiO₃ to ~2.5 in BaTiO₃ as x is varied. First-principles electronic structure calculations show that nickel creates an impurity band in the forbidden gap of BaTiO₃ and SrTiO₃. Calculations of the formation energy of the oxygen vacancies explain the difference between the Ni charge states in these compounds by the different formation energies of these vacancies.     

Magnetic properties of Ni0.8Cu0.2−xGex solid solutions

The results of magnetic measurements performed on Ni_0.8Cu_0.2-xGe_x solid solutions, with x ? 0.10 are reported. The saturation magnetization, Curie temperature and effective nickel moments decrease nearly linear when substituting Cu by Ge. The ferromagnetic resonance measurements show that the g values are not dependent on composition. Finally, the magnetic behaviour of nickel in these solid solutions is analysed.     

Magnetic properties of xNiO. (1-x)[2B2O3.PbO] glasses

The results of magnetic studies on xNiO.(1-x)[2B₂O₃.PbO] glasses with 0 ? x ? 40 mol.% are reported. These results evidenced that the nickel ions, in this glass system, are in a divalent state and magnetically isolated. The magnetic moments, μ, decrease from a value 3.31 μ_B for the 5 mol.% NiO to a value 2.90 μ_B for 40 mol.% NiO. These variations can be due to the result of a statistical average of the nickel sites distribution.     

Electronic band structure of pseudo-binary AlB2-like hexagonal silicides SrNixSi2−x as novel low-TC superconductors

The very recently discovered pseudo-binary hexagonal silicide SrNi_xSi_2−x, which exhibits low-T_C superconductivity, was examined theoretically to understand the effect of the unusual doping type (partial replacement of Si by Ni) on the electronic band structure of this material. Besides, the possible factors of the stabilization of the hexagonal AlB₂-type structure of SrSi₂ upon substitution of Ni for Si, and the solubility limit of Ni in SrNi_xSi_2−x are discussed in terms of competing Si–Si, Si–Ni, and Ni–Ni bonds.     

Growth of rare earth silicides on silicon

Dysprosium metal layers deposited on (100)Si substrates were annealed in situ at 300 °C for different time intervals in order to produce DySi_2−x layers at several stages of growth. Electron microscopy work showed that silicon reacts with Dy metal through the simultaneous growth of an amorphous interlayer and a polycrystalline phase to form DySi_2−x. Compressive stresses due to volumetric changes during the reaction deform the produced silicide layer and induce roughness at the surface and interface.     

Self supported nickel antimonides based electrodes for Li ion battery

Self-supported nickel antimonides/Ni architectured electrodes were prepared by solid state reaction from Ni thin film, Ni foam and Ni nanorods. This specific design is expected to optimize both NiSb_x/Ni-current collector and NiSb_x/electrolyte interfaces of the electrode in the Li ion battery. This new electrode preparation process is based on solid state reaction of antimony with the nickel architectured substrate. Preliminary electrochemical tests of the as-obtained self supported antimonide electrodes show improvement in the capacity retention of the NiSb_x active material.     

Structural position and charge state of nickel in SrTiO3

The properties of nickel-doped strontium titanate are studied using X-ray diffraction and XAFS spectroscopy. It is shown that, independently of preparation conditions, the most stable phases in the samples are single-phase SrTi_{1 ? x} Ni _x O₃ solid solution and NiTiO₃ which can coexist. According to the EXAFS data, in the single-phase SrTi_0.97Ni_0.03O₃ sample the nickel atoms substitute the titanium atoms and are on-center ones. In this case, no distortions of the oxygen octahedron which would appear in the presence of oxygen vacancies in the nickel environment were detected. An analysis of the XANES spectra shows that the nickel charge state in NiTiO₃ is 2+, whereas in the SrTi_{1 ? x} Ni _x O₃ solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent Ni in the SrTi_{1 ? x} Ni _x O₃ solid solution. This doping seems to be the most promising for solar energy converters based on the bulk photovoltaic effect.     

Structural and electrical properties of nanometric Ni-Cu ferrites synthesized by citrate precursor method

Nanometric nickel copper ferrites Ni_1−xCu_xFe₂O₄, 0≤x≤0.45 were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single phase cubic spinel structure. The lattice parameter (a) is increased with increasing Cu²⁺ ion substitution. The crystallite size was calculated from XRD data and compared with that obtained from TEM micrographs. A significant increase in the density is observed with increasing Cu content. The IR absorption spectra were used for the detection and confirmation of the chemical bonds in spinel ferrites. The dielectric constant ε′ and dielectric loss showed a decrease with increasing frequency for all samples. The decrease in the ac conductivity was ascribed to the increase in hopping length.