Size effects and magnetization of (Fe/Si)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> multilayer film nanostructures

The temperature dependence of the magnetization of (Fe/Si) _n multilayer films with nanometer layers is investigated. The films are prepared through thermal evaporation under ultrahigh vacuum onto Si(100) and Si(111) single-crystal substrates. It is revealed that the thickness of individual iron layers in (Fe/Si) _n multilayer films affects the magnetization and its temperature dependence. The inference is made that this dependence is associated with the formation of a chemical interface at the Fe-Si boundaries. The characteristics of the chemical interface in the (Fe/Si) _n films are estimated.     

Electrical and optical properties of pulse plated CdS<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>Te<Subscript>1-<Emphasis Type="BoldItalic">x</Emphasis></Subscript> films

Thin films of CdS_xTe_1-x were deposited by the pulse electrodeposition technique using cadmium sulfate, sodium thiosulfate, and tellurium dioxide on titanium and conducting glass substrates. Structural studies indicated the formation of polycrystalline films possessing hexagonal structure. The resistivity varies from 53 Ω cm to 8 Ω cm as the stochiometric coefficient “x” value decreases from 1 to 0. The carrier concentration increases with CdTe concentration. It is observed that as the post-heat treatment temperature increases, the photosensitivity also increases. It is observed that a post-heat treatment temperature of 550 °C results in high photosensitivity as well as low light resistance. The optical constants, refractive index (n) and extinction coefficient (k) were evaluated from the transmission spectra of the films of different composition.     

Growth and luminescence properties of single-crystalline films of <Emphasis Type="Italic">R</Emphasis>AlO<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Lu,Lu-Y,Y, Tb) perovskite

The conditions and the mechanisms of crystallization by liquid-phase epitaxy of undoped and Ce³⁺-doped RAlO₃ (R = Lu, Lu-Y, Y, Tb) single-crystalline films on YAlO₃ single-crystalline substrates at a considerable mismatch between the lattice constants of the single-crystalline films and the substrate are analyzed. The maximum values of this mismatch at which the stable growth of the single-crystalline films is observed are determined. It is demonstrated that transition layers are present between the substrates and the grown single-crystalline films in which the difference between the lattice parameters of the single-crystalline film and the substrate is diminished. The optical and luminescence characteristics of the undoped and Ce³⁺-doped RAlO₃ (R = Y, Y-Lu, Lu, Tb) single-crystalline films, as well as the scintillation characteristics of the (Lu-Y)AlO₃: Ce single-crystalline films under excitation by alpha-particles of the ²³⁹Pu source (5.15 MeV), are investigated.     

An investigation into ultra-thin pseudobinary oxide (TiO<Subscript>2</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1-x</Subscript> films as high-<Emphasis Type="Italic">k</Emphasis> gate dielectrics

As potential gate dielectric materials, pseudobinary oxide (TiO₂)_x(Al₂O₃)_1-x (0.1≤x≤0.6) films (TAO) were deposited on Si (100) substrates by pulsed-laser deposition method and studied systematically via various measurements. By a special deposition process, including two separate steps, the TAO films were deposited in the form of two layers. The first layer was deposited at room temperature and the second layer was completed at the substrate temperature of 400 °C. Detailed data show that the properties of the TAO films are closely related to the ratio between TiO₂ and Al₂O₃. The existence of the first layer deposited at room temperature can effectively restrain the formation of the interfacial layer. And according to the results of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy performed on the films, no other information belonging to the silicon oxide could be observed. For the (TiO₂)_0.4(Al₂O₃)_0.6 film, the best result has been achieved among all samples and its dielectric constant is evaluated to be about 38. It is valuable for the amorphous TAO film as one of the promising dielectric materials for high-k gate dielectric applications. PACS 77.55.+f; 73.40.Qv; 81.15.Fg     

Superconducting films with <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript>=39 K prepared from stoichiometric MgB<Subscript>2</Subscript> targets

Oriented films of MgB₂ high-T_c superconductor are synthesized by pulsed laser sputtering of stoichiometric MgB₂ targets with subsequent annealing of the amorphous Mg-B material deposited onto MgO(111) substrates. The critical temperature of the films depends on the purity of the targets sputtered. The purification of boron powder in a vacuum makes it possible to minimize the content of impurities in the targets and to prepare MgB₂ films exhibiting a critical temperature above 39 K and a sharp inductive transition. A high room-temperature to residual resistivity ratio (more than 3) indicates the good quality of the films.     

Anisotropy of amorphous nanogranular composites CoNbTa-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> and CoFeB-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

A comparative investigation of the magnetic properties of amorphous nanogranular composites (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} and (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} has been performed in the subpercolation region at temperatures in the range 4.2–300 K. The thermomagnetic dependences in the range 4.2–300 K and the processes of magnetization reversal and remanent magnetization relaxation at liquid-helium temperatures have been studied. It has been established that the average anisotropy constants of amorphous nanograins are equal to 3.6–7.0 kJ/m³ for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites and 5–8 kJ/m³ for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites. The fundamental differences in the concentration dependences of the anisotropy constant K _eff and the coercive force H _C have been revealed for the two systems under investigation. It has been demonstrated that, as the concentration of the metal phase increases, the quantities K _eff and H _C increase for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites and decrease for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites.     

High quality,high-<Emphasis Type="Italic">k</Emphasis> gate dielectric: amorphous LaAlO<Subscript>3</Subscript> thin films grown on Si(100) without Si interfacial layer

High-k dielectric amorphous LaAlO₃ thin films have been grown on n-type Si(100) substrates by laser molecular beam epitaxy. The interfacial characteristics of the LaAlO₃ films on Si were measured by high-resolution transmission electron micrography and X-ray photoemission spectroscopy. A sharp interface without a silica inter-layer between the LaAlO₃ film and Si substrate was observed. Atomic force microscopy measurements indicated that the root-mean-square surface roughness within a 2m×2m area of the LAO films was 0.12 nm. The flatband voltage and fixed charge density of the 8 nm thickness LaAlO₃ films were about 0.355 V and 1.8×10¹²/cm², respectively. The leakage currents of the LaAlO₃/Si(100) samples with different film thicknesses of 5, 8 and 12 nm were 0.45, 0.23 and 0.05 mA/cm², respectively, at a +1 V dc bias voltage. LaAlO₃ appears to be one of the most promising high dielectric constant materials for use in future ultra large scale integrated devices. PACS 77.55+f; 77.84.Bw; 73.40.Qv     

Structure and dielectric properties of ultra-thin ZrO<Subscript>2</Subscript> films for high-<Emphasis Type="Italic">k</Emphasis> gate dielectric application prepared by pulsed laser deposition

ZrO₂ thin films have been prepared on Pt-coated silicon substrates and directly on n-Si(100) substrates by the pulsed laser deposition (PLD) technique using a ZrO₂ ceramic target under different deposition conditions. X-ray diffraction showed that the films prepared at 400 °C in 20 Pa oxygen ambient remained amorphous. Differential thermal analysis was carried out to study the crystallization behavior of ZrO₂. The dielectric constant of ZrO₂ was determined to be around 24 by measuring a Pt/ZrO₂/Pt capacitor structure. Sputtering depth profile X-ray photoelectron spectroscopy was used to investigate the interfacial characteristics of ZrO₂/n-Si stacks. A Zr silicate interfacial layer was formed between the ZrO₂ layer and the silicon substrate. The equivalent oxide thickness (EOT) and leakage current densities of the films with 6.6 nm physical thickness post-annealed in O₂ and N₂ ambient were investigated. An EOT of 1.65 nm with a leakage current of 36.2 mA/cm² at 1 V gate voltage for the film post-annealed in N₂ has been obtained. ZrO₂ thin films prepared by PLD have acceptable structure and dielectric properties required by a candidate material of high-k gate dielectrics. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Optical properties of nonstoichiometric ZrO<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> according to spectroellipsometry data

Amorphous nonstoichiometric ZrO_x films of different composition have been synthesized by the method of ion-beam sputtering deposition of metallic zirconium in the presence of oxygen at different partial oxygen pressures in the growth zone, and their optical properties have been studied in the spectral range of 1.12–4.96 eV. It is found that light-absorbing films with metallic conductivity are formed at the partial oxygen pressure below 1.04 × 10^–3 Pa and transparent films with dielectric conductivity are formed at the pressure above 1.50 × 10^–3 Pa. It is shown that the spectral dependences of optical constants of ZrO_x films are described well by the corresponding dispersion models: the Cauchy polynomial model for films with dielectric conductivity and the Lorentz–Drude oscillator model for films with metallic conductivity.     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.     

<Emphasis Type="Italic">B</Emphasis> and <Emphasis Type="Italic">B</Emphasis><Subscript><Emphasis Type="Italic">s</Emphasis></Subscript> decay constants from moments of finite energy sum rules in QCD

We use an appropriate combination of moments of finite energy sum rules in QCD in order to compute the B _q -meson decay constants f _B and . We perform the calculation using a two-loop computation of the imaginary part of the pseudoscalar two point function in terms of the running bottom quark mass. The results are stable against the so-called QCD duality threshold, and they are in agreement with the estimates obtained from Borel transform QCD sum rules and lattice computations.Received: 28 July 2004, Revised: 10 September 2004, Published online: 23 November 2004PACS: 12.38.Bx, 12.38.LgSupported by MCYT-FEDER under contract FPA2002-00612.     

Electrophysical parameters of <Emphasis Type="Italic">c</Emphasis>-oriented Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> films with a low concentration of antistructural defects

Epitaxial c-oriented Bi₂Te₃ films 1.2 μm in thickness are grown by the hot wall method for a low supersaturation of the vapor phase over the surface of mica substrates. The hexagonal unit cell parameters a = 4.386 Å and c = 30.452 Å of the grown films almost coincide with the corresponding parameters of stoichiometric bulk Bi₂Te₃ crystals. At T = 100 K, the Hall concentration of electrons in the films is on the order of 8 × 10¹⁸ cm^?3, while the highest values of the thermoelectric coefficient (α ≈ 280 μV K^?1) are observed at temperatures on the order of 260 K. Under impurity conduction conditions, conductivity σ of the films increases upon cooling in inverse proportion to the squared temperature. In the temperature range 100–200 K, thermoelectric power parameter α²σ of Bi₂Te₃ films has values of 80–90 μW cm^?1 K^?2.     

Photoacoustic spectroscopy of thin films of As<Subscript>2</Subscript>S<Subscript>3</Subscript>, As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and GeSe<Subscript>2</Subscript>

Photoacoustic spectroscopy (PAS) is one of the important branches of spectroscopy, which enables one to detect light-induced heat production following the absorption of pulsed radiation by the sample. As₂S₃, As₂Se₃ and GeSe₂ exhibit a wide variety of photo-induced phenomena that enable them to be used as optical imaging or storage medium and various electronic devices, including electro-optic information storage devices and optical mass memories. Therefore, accurate measurement of thermal properties of semiconducting films is necessary to study the memory density. The thermal conductivity of thin films of As₂S₃ (thickness 100 μm and 80 μm), As₂Se₃ (thickness 100 μm and 80 μm) and GeSe₂ (thickness 120 μm and 100 μm) has been measured using PAS technique. Our result shows that the thermal conductivity of thicker films is larger than the thinner films. This can be explained by the thermal resistance effect between the film and the surface of the substrate.      

Multiband tight-binding theory of disordered A<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C semiconductor quantum dots

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type A _x B_{1- x} C can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp ³ empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of Cd _x Zn_{1- x} Se nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes.     

Colossal resistive switching behavior and its physical mechanism of Pt/<Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Mg<Subscript>0.6</Subscript>Zn<Subscript>0.4</Subscript>O/Pt thin films

A heterojunction structure of p-NiO/n-Mg_0.6Zn_0.4O with an aim to tuning or improving the resistive switching properties was fabricated on Pt/TiO₂/SiO₂/Si substrates by the sol-gel spin-coating technique. The Pt/NiO/Mg_0.6Zn_0.4O/Pt heterojunction thin-film device shows excellent resistive switching properties, such as a reduced threshold current of 1 μA for device initiation, a small dispersion of reset voltage ranging from 0.54 to 0.62 V, long retention time and a high resistance ratio of high-resistance state to low-resistance state about six orders of magnitude. These results indicate that the resistive switching properties can be greatly improved by constructing the p-NiO/n-Mg_0.6Zn_0.4O heterojunction for nonvolatile memory applications. The physical mechanism responsible for colossal resistive switching properties of the heterojunction was analyzed based on interfacial defect effect and formation and rapture of conductive filaments.     

<Emphasis Type="Italic">J</Emphasis><Subscript><Emphasis Type="Italic">AW,WA</Emphasis></Subscript> functions in Passarino-Veltman reduction

We continue to study a special class of Passarino-Veltman functions J arising at the reduction of infrared divergent box diagrams. We describe a procedure of separation of two types of singularities, infrared and mass singularities, which are absorbed in simple C ₀ functions. The infrared divergences of C ₀’s can be regularized then by any method: photon mass, dimensionally or by the width of an unstable particle. Functions D ₀ are represented as certain linear combinations of the standard C ₀ Passarino-Veltman functions and infrared finite functions J. Then mass singularities are extracted from J to other combinations of C ₀. The rests are free of both types of singularities and are expressed as explicit and compact linear combination of logarithms and dilogarithm functions. The extensive comparison of numerical results with those obtained with the aid of the Loop Tools package is presented.     

Electron paramagnetic resonance of spin-variable metallomesogens [Fe<Emphasis Type="Italic">L</Emphasis><Subscript>2</Subscript>]<Emphasis Type="Italic">X</Emphasis> (<Emphasis Type="Italic">X</Emphasis> = PF<Subscript>6</Subscript>, SCN)

Significant differences in the manifestation of spin-crossover properties of the mesogen compounds [FeL ₂]X with oxysalicylidene-N′-ethyl-N-ethylenediamine ligands L and anions X = PF₆⁻ and SCH⁻ have been found by means of electron paramagnetic resonance. The electron paramagnetic resonance data and the quantum-chemical calculation within the density functional theory enables us to establish that the observed specific features are associated with the incorporation of the SCH⁻ ion into the first coordination sphere of the Fe(III) ion. The role of the transition of the material to the liquid-state phase in the formation of a low-dimensional (two-dimensional) structure with stronger intermolecular interactions has been revealed.     

a-Si/SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> multilayered light absorber for solar cell

40 alternate a-Si/SiN _x multilayer are incorporated as an absorber layer in a p–i–n solar cell. The device is fabricated using hot-wire chemical vapor deposition (HWCVD) technique. The structure of the multilayer film is examined by high resolution transmission electron microscopy (HR-TEM) which shows distinct formation of alternate a-Si and SiN _x layers. The a-Si and SiN _x layers have thickness of ~3.5 and 4 nm, respectively. The photoluminescence (PL) of multilayer film shows bandgap energy of ~2.52 eV, is larger than that of the c-Si and a-Si. Dark and illuminated current–voltage (I–V) characterization of the ML films shows that these ML are photosensitive. In the present work, it is seen that the p–i–n structure with i-layer as ML quantum well (QW) structures show photovoltaic effect with relatively high open-circuit voltage (V _OC). The increment of bandgap energy in PL and high V _OC of the device is attributed to the quantum confinement effect (QCE).     

<Emphasis Type="Italic">SU</Emphasis>(3)-Instantons and <Emphasis Type="Italic">G</Emphasis><Subscript>2</Subscript>,<Emphasis Type="Italic">Spin</Emphasis>(7)-Heterotic String Solitons

Necessary and sufficient conditions to the existence of a hermitian connection with totally skew-symmetric torsion and holonomy contained in SU(3) are given. A formula for the Riemannian scalar curvature is obtained. Non-compact solution to the supergravity-type I equations of motion with non-zero flux and non-constant dilaton is found in dimension 6. Non-conformally flat non-compact solutions to the supergravity-type I equations of motion with non-zero flux and non-constant dilaton are found in dimensions 7 and 8. A Riemannian metric with holonomy contained in G₂ arises from our considerations and Hitchin’s flow equations, which seems to be new. Compact examples of SU(3),G₂ and Spin(7) instanton satisfying the anomaly cancellation conditions are presented.