Formation of silicon carbide nanocrystals by high-temperature carbonization of porous silicon

Based on X-ray diffraction analysis, Auger spectroscopy, and Raman scattering, it is shown that carbonization of porous silicon at temperatures of 1200–1300°C results in formation of silicon carbide nanocrystals 5–7 nm in size. The growth of 3C-SiC nanocrystals of fixed size d proceeds as follows. Silicon nanocrystals with d = 3–7 nm pass into the liquid phase, thereby effectively participating in the growth of silicon carbide. After the size of a crystallite has achieved a critical value determined by the equality of its melting point and environmental temperature, the crystallite solidifies and virtually ceases to grow. As a result, a nanocrystalline Si-SiC-amorphous SiC heterostructure is obtained.     

Oxide thickness measurements up to 120 Å on silicon and aluminum using the chemically shifted auger spectra

Thicknesses of oxides on Si or Al can be determined up to about 120 Å using Auger electron spectroscopy, without ion-mill depth profiling, by using the ratio of the chemically shifted and unshifted peaks from the oxide and substrate, respectively. Measurement standard deviations of ± 1 Å at oxide thickness of 30 Å and spatial resolution < 10 μm are readily attainable. The absolute accuracy of the present calibration is about ± 30% at 30 Å for SiO₂. A comparison of the measured thickness d with ellipsometry revealed a disagreement which was largest at $\text{d(}\text{ellips.}\text{) = 50}\text{A}\text{?}$, where d(Auger) was 33 Å. We propose that most of this disagreement is a consequence of the finite extent of the oxide/Si interface, and the measurement of different physical parameters in the two techniques. It is demonstrated that the milling rate of SiO₂ (within about 100 Å from the SiO₂/Si interface) can be determined from ion-mill depth profiles alone and the position of the interface in the depth profile can be located within serveral Å. The electron mean free path in SiO₂ at 1615 eV was determined to be 31 ± 9 Å.     

Superparamagnetism and magnetic granulometry of vitroceramics containing iron-lithium ferrite

Magnetic properties of lithium ferrite crystallites distributed in a glassy matrix vary with their average size d: (1) d ? 100 Å: superparamagnetism d and the limits of the distribution have been calculated by fitting magnetisation curves vs applied field. (2) d ? 150 Å: ferrimagnetism with a hysteresis loop maximum for the monodomain size (260 ± 40 Å). (3) 100 ? d ? 150 Å: superposition of the two preceding phenomena.     

Estimation of the deformation-potential constant for <Emphasis Type="Italic">p</Emphasis>-type isotropic polycrystalline silicon through the deformation potential of a <Emphasis Type="Italic">p</Emphasis>-type silicon single crystal

A method of calculating the effective deformation-potential constant E ₁ for holes and longitudinal acoustic phonons in isotropic polycrystalline silicon is suggested. The deformation-potential constant E ₁ is estimated through the deformation potentials a, b, and d of the silicon single crystal. The procedure of averaging of the squared modulus of the hole and acoustic phonon interaction Hamiltonian written in the plane wave basis over the polycrystal ensemble provides the basis for calculation of the constant E ₁ . It is demonstrated that for T = 200–600 K, hole concentration p = (5.0–10.0)∙10¹⁹ cm^–3, and crystallite size d = 300–3000 ?, the deformationpotential constant E ₁ is independent of the hole concentration p, temperature T, and crystallite size d.     

Rapid synthesis and characterization of silicon substituted nano hydroxyapatite using microwave irradiation

Nano sized hydroxyapatites with silicon substitution of three different silicon concentrations were successfully prepared first time by a rapid microwave assisted synthesis method, with a time saving and energy efficient technique. The effects of the Si substitution on crystallite size, particle size and morphology of the powders were investigated. The crystalline phase, microstructure, chemical composition, and morphology and particle size of hydroxyapatite and silicon substituted hydroxyapatites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and dynamic light scattering. The crystallite size and particle size decreases with increase in silicon content and particle morphology spheroidal for pure hydroxyapatite changes to elongated ellipsoidal crystals while silicon substitution increases. Fourier Transform Infrared Spectroscopy analysis reveals, the silicon incorporation to hydroxyapatite lattice occurs via substitution of silicate groups for phosphate groups. Substitution of phosphate group by silicate in the apatite structure results in a small increase in the lattice parameters in both a-axis and c-axis of the unit cell.     

Direct observations of the behavior of single silicon atoms on a metal surface

The behavior of single silicon adatoms on the W {110} plane has been successfully studied for the first time. Single atom diffusion parameters are found to be E_d = 0.70 ± 0.07 eV, and $\text{d}{}_0\text{= 3.08 × 10}{}^{\mathrm{?4}}\text{× 10}{}^{\mathrm{\pm 1.28}}\text{solcm}{}^2\text{s}$. The field desorption behavior of Si atoms is similar to that of metal adatoms. SiSi adatom-adatom interaction shows nonmonotonic distance dependence, but the repulsive region around 3.2 Å is much weaker than those found in metal adatom interactions.     

Spin-wave resonance in Co/Pd magnetic multilayers and NiFe/Cu/NiFe three-layered films

The spectrum of standing spin waves has been detected by the ferromagnetic resonance method in NiFe(740 Å)/Cu/NiFe(740 Å) three-layered film structure in the perpendicular configuration for the copper thickness d _Cu ≤ 30 Å. At thicknesses d _Cu > 30 Å, the resonance absorption curve is a superposition of two spinwave resonance spectra from individual ferromagnetic NiFe layers. For Co/Pd multilayer films, united spinwave responance spectra have also been observed at thicknesses of the paramagnetic palladium layer up to d _Pd < 30 Å. The partial exchange stiffness has been calculated for a spin wave propagating across the Pd layer (A _Pd = 0.1 × 10^?6 erg/cm). This value is always positive (up to the critical thickness of the palladium interlayer d _Pd < d _c) or equal to zero (d _Pd > d _c).     

A theoretical deduction of the shape and size of nanocarbons suitable for hydrogen storage

We evaluated the adsorption energy of a hydrogen molecule in nanocarbons consisting of graphene sheets. The nanocarbon shapes were a pair of disks with separation 2d, a cylinder with radius d, and a truncated sphere with radius d. We obtained the adsorption energy in the form of a 10–4 Lennard–Jones function with respect to 1/d. The values of the potential depth (D) and equilibrium distance (d _e), respectively, were 94 meV and 2.89 Å for the disk pair, 158 meV and 3.14 Å for the cylinder, and 203 meV and 3.37 Å for the sphere. When d=d _e, the adsorption energy of the disk pair (cylinder) became deeper than ?0.9D, and it approached ?D when the radius (length) increased to more than twice its separation (radius). The adsorption energy of the sphere was increased from ?D to ?0.5D when the radius of the opening increased from 0 to d _e. These results suggest that porous carbon materials can increase the adsorption energy by up to ～200 meV if the carbon atoms are arranged on a spherical-like surface with ～7 Å separation. This may lead to practical hydrogen storage for fuel cells.     

Hot́ hole anisotropic effect in silicon and germanium

Anisotropic effect of the hole drift velocity in silicon and germanium has been investigated with the time of flight technique by applying the electric field parallel to the <100 and <111 crystallographic axis. The measurements were performed for electric fields ranging from 10 to 3 × 10⁴V/cm and temperatures from 40 to 200°K. The results indicate that the anisotropic effect v_d<100/v_d<111 increases with decreasing temperature and increasing electric field, and reaches a saturation value at high electric fields (? 10⁴V/cm). The maximum anisotropic effect for Ge is 1.25 at 40°K and for Si is 1.2 at 45°K. A qualitative analysis of the experimental data indicates that the anisotropic effect is due to the warped heavy-valence-band shape.     

Effect of the size factor on the magnetic properties of manganite La0.50Ba0.50MnO3

Nanocrystalline manganite La_0.50Ba_0.50MnO₃ was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ～30 nm to ～7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ～3.787 to ～3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ～30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T _C from ～264 to ～331 K and in the spontaneous magnetic moment σ _S from ～1.52 to ～3.31 μ_B/f.u. The anomalous behavior of the magnetic properties of the manganite La_0.50Ba_0.50MnO₃ obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn³⁺-O-Mn⁴⁺ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.     

Well width dependence of electron-phonon interaction in ZnSe/ZnSxSe1-x superlattices determined by micro-raman spectroscopy

We report a study of multiphonon resonant Raman scattering in a series of symmetric (ZnSe)_d(ZnS_1-xSe_x)_d superlattices (SL), 20Å ≤ d ≤ 150Å. In addition to confined optical phonons, the energies of interface (IF) modes with in-plane wavevectors have been assessed by means of micro-probe Raman measurements in backscattering from the SL edge. The comparison between one- and two-phonon spectra shows that the electron-phonon interaction is dominated by ZnSe-like IF phonons for d < 50Å, whereas LO₂ phonons prevail for larger well.     

The effect of high iron-ion implantation doses on the X-ray emission spectra of silicon

X-ray emission spectroscopy (Si L _{2, 3} spectra, 3d3s → 2p electronic transition) was employed to study p-and n-type silicon samples implanted with Fe⁺ ions in a pulse mode (the implantation energy was 30 keV, the pulse current was varied up to 0.5 A, the pulse duration was 400 μs, and the ion irradiation doses ranged from 10¹⁴ to 10¹⁷ cm⁻²). The x-ray emission spectra were found to be dependent on the ion irradiation dose and the electron-accelerating voltage that was used in the x-ray studies. By comparing the Si L spectra with the spectra of reference materials and by modeling the former spectra, it was revealed that, as the ion-irradiation dose increases, there occur disordering of the structure, partial amorphization of the sample in a surface layer approximately 7200-? thick, and its subsequent recrystallization (under high irradiation doses). It was shown that this effect is most pronounced in a layer at a depth of ∼1000 ? and is not associated with the formation of iron silicide FeSi in the bulk of the sample but rather is due to the breakage of Si-Si bonds caused by ion implantation under the irradiation doses used. Original Russian Text ? D.A. Zatsepin, E.S. Yanenkova, é.Z. Kurmaev, V.M. Cherkashenko, S.N. Shamin, S.O. Cholakh, 2006, published in Fizika Tverdogo Tela, 2006, Vol. 48, No. 2, pp. 204–209.     

The role of pulse time Toff on porous silicon as template for Au nanoparticles by using the integrated electrochemical technique

Gold nanoparticles (AuNPs) having variety of sizes and shape were prepared using the template synthesis approach. Porous silicon (PSi) was fabricated using the pulsed electrochemical anodization method at different pause times, T_off as template for gold deposition. Choosing suitable pulse parameter produces PSi with higher porosity and smaller crystallite size. SEM showed that the variation of T_off affects the pores formation and the growth of gold nanoparticles while EDX suggested the presence of Au inside the pores structure. Photoluminescence spectra showed emission enhancement and a blue shifted relative to porous silicon before deposited with AuNPs. XRD shows a high degree crystallinity of the samples and the presence of cubic gold with crystalline sizes was around 42 nm.     

Controlled synthesis of photoluminescent Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> nanoparticles from metal-organic polymeric precursor

Bi₄Ti₃O₁₂ (BIT) nanoparticles with a narrow average particle size distribution in the range of 11–46 nm was synthesized via a metal-organic polymeric precursor process. The crystallite size and lattice parameter of BIT were determined by XRD analysis. At annealing temperatures >550 °C, the orthorhombic BIT compound with lattice parameters a = 5.4489 Å, b = 5.4147 Å, and c = 32.8362 Å was formed while at lower annealing temperatures orthorhombicity was absent. Reaction proceeded via the formation of an intermediate phase at 500 °C with a stoichiometry close to Bi₂Ti₂O₇. The particle size and the agglomerates of the primary particles have been confirmed by FESEM and TEM. The decomposition of the polymeric gel was ascertained in order to evaluate the crystallization process from TG-DSC analysis. Raman spectroscopy was used to investigate the lattice dynamics in BIT nanoparticles. In addition, investigation of the dependence of the visible emission band around the blue–green color emission on annealing temperatures and grain sizes showed that the effect of grain size plays important roles, and that oxygen vacancies may act as the radiative centers responsible for the observed visible emission band.     

Ferromagnetic and spin-wave resonance in Ni0.8Fe0.2/Dy1−x Cox bilayer films

The standing spin wave spectra of Ni_0.8Fe_0.2(1000–3000 Å)/(Dy_1?x Co_x(700 Å) bilayer exchange-biased films with two different (precompensation Dy_0.2Co_0.8 and postcompensation Dy_0.3Co_0.7) compositions of the hard magnetic layer are analyzed. Measurements are performed at room temperature. It is found that the effective magnetic layer thickness (d _eff=d ₀±Δd), which determines the wave vectors of the first modes in the spectrum, differs from the d ₀ value specified in film technology. The sign of |Δd| ～ 500 Å is governed by the composition of the DyCo hard magnetic layer.     

Fabry-Perot etalons for x-rays: Construction and characterization

A solid Fabry-Perot etalon (FPE) for X-rays has been constructed using sputtering technique. This FPE is made up of two layered synthetic microstructures (LSM's) separated by a carbon spacer. Each LSM consists of fifteen pairs of tungsten/ carbon having a mean period d = 27 Å and deposited onto (111) orientation silicon single crystal substrate. The carbon spacer thickness is 470 Å. The FPE has been characterized by new processes, namely electron microscopy and transmission electron diffraction pattern. The results obtained are in good agreement with those provided by reflectivity measurements using Cu K_α (1.5418 Å) radiation. Another FPE made of two different LSM's has been constructed. The effect of such an asymmetry is investigated.     

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E?E _v=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 Å in comparison with the crystal. The presence of a third maximum (E?E _v=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.     

The proximity effect in an Fe-Cr-V-Cr-Fe system

The proximity effect was studied in a thin-film Fe-Cr-V-Cr-Fe layered system. As the chromium layer thickness (d_Cr) increases at a fixed thickness of iron layers (d_Fe), the dependence of the superconducting transition temperature (T_c) on d_Cr exhibits a maximum at d_Cr ? 40 Å followed by a sharp decrease. Investigation of the dependence of T_c on d_Fe at a fixed d_Cr showed that the depth of penetration of the Cooper pairs into a chromium layer does not exceed 40 Å. Analysis of the results obtained suggests that, at d_Cr ? 40 Å, chromium layers exhibit the transition from a nonmagnetic state to an incommensurate spin density wave state.     

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×10 ¹⁹ cm ^?3 ) and boron (4.1×10 ¹⁸ , 1.34×10 ¹⁹ , and 4.9×10 ¹⁹ cm ^?3 ) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ～10 ¹⁷ cm ^?3 , the relaxation rate ν of the magnetic moment of a _μ Al acceptor in silicon with a high impurity concentration of germanium (9×10 ¹⁹ cm ^?3 ) depends on temperature as ν～T ^q , q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T?30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σ _h ) and electrons (σ _e ) from an Al acceptor center in Si: σ _h ～10^?13 cm² and σ _e ～8×10^?15 cm² at the acceptor (donor) impurity concentration n _a (n _d )～4×10¹⁸ cm^?3.     

HD structural isotope effect in hydrogen-bonded 2-aminoethanol

The microwave spectra of the d₃ species of intramolecularly hydrogen-bonded 2-aminoethanol (ODCH₂CH₂ND₂) as well as the d₃-O¹⁸, d₃-N¹⁵ and three d₂ isotopic forms have been studied. The r_s O-(D)?N distance of 2.8027(5) Å in the d₃ species is 0.0058(7) Å shorter than the O-(H)?N distance found earlier for the d₀ species (R. E. Penn and R. F. Curl, Jr., J. Chem. Phys.55, 651–658 (1971)). The anomalous r_s structural parameters $\text{r}\text{OH = 1.14}\text{A}\text{?}$ and ∠COH = 103.7°, when corrected for the systematic error caused by the O?N “shrinkage” upon deuteration, become 1.00 Å and 108°, respectively.