Layers Of Different Thicknesses Research Articles

Effect of near-field coupling on far-field inelastic scattering imaging of gold nanoparticles

The optical near-field enhancement induced by coupling between noble nanoparticles andthe substrate has been studied by a far-field imaging method. The longitudinalmode of the incident laser is revealed to contribute to the coupling. The far-fieldimages of individual gold nanoparticles exhibit a peanut-shaped pattern; thesewere constructed by the intensity of inelastically scattered light. The couplingbetween gold nanoparticles and the silicon substrate leads to the patterned image.By tuning the separation between the gold nanoparticles and substrate usingSiO2 layers of different thickness, the coupling efficiency decreases with the thickness of theSiO2 layer.

Power factor of very thin thermoelectric layers of different thickness prepared by laser ablation

Room temperature conductivity and the Seebeck coefficient of thin layers prepared by laser ablation from Bi2Te3 target were explored. The power factor was calculated for samples prepared at substrate temperature of 360°C with the density of the laser beam 5 J cm−2 and at substrate temperature of 410°C with the density of the laser beam 2 J cm−2 during the deposition. Oscillations of the conductivity and the power factor with the layer thickness were observed at room temperature. The oscillations of conductivity were also verified at the temperature of 77 K. The period of oscillations depends on the preparation conditions. This behavior has been theoretically explained by the quantum size effect in the layers containing different phases and in addition, it was demonstrated by the X-ray Diffraction measurement.

Electro‐modulation enhancement in the InGaNAs/GaAs quantum well structures

AbstractElectrooptical properties of InGaNAs/GaAs quantum well structures were investigated by photoreflectance spectroscopy. The MBE grown structures consist of a central InGaNAs well, GaAs barriers, and GaAsN strain compensating buffer layers of different thickness. More quantum states and extended wave functions in the system with strain relief GaAsN barriers due to broader and lower confinement profile. Particular electro‐modulated spectral features corresponding to excitonic interband transitions are enhanced. Compared with the numerically calculated quantum state energies and wave functions, the electromodulation enhanced transitions were those involving quantum states with wave functions distribute in wider regions and extend more from the InGaNAs well into the space charge region near interfaces. It was the internal electric field near the heterointerfaces strongly enhanced the electro‐modulation efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of Distribution of Trap States in Silicon-on-Insulator Layers by Front-Gate Characteristics in n-Channel SOI MOSFETs

We characterized the distribution of trap states in silicon-on-insulator (SOI) layers in epitaxial layer transfer (ELTRAN) wafers and in low-dose separation by implanted oxygen (SIMOX) wafers. We measured the front- and back-gate characteristics of MOSFETs with SOI layers of different thicknesses. We used the current-Terman method to estimate the trap states at the gate oxide (GOX)/SOI interface and at the SOI/buried oxide (BOX) interface separately. As a result, we concluded that the high-density trap states in the SOI layers in SIMOX wafers cause a gate-voltage shift, which is attributed to the charged trap states only in the inversion layer. We also found that the trap states are distributed within about 30 nm from the SOI/BOX interface in the SOI layer in SIMOX wafers, which indicates that the distribution of trap states originates from the oxygen implantation that is peculiar to the SIMOX process.

Reflective performance of Ir film in vacuum ultraviolet wavelength region

This paper theoretically and experimentally investigates the reflective performance of Ir films in the vacuum ultraviolet wavelength region. Ir reflecting layers of different thicknesses on various substrates are calculated and fabricated by the ion-beam-sputtering technique. Their reflectance in the 115 nm to 140 nm wavelength region was measured continuously by a reflectometer located at the National Synchrotron Radiation Laboratory. The testing results show that the reflectance of Si substrates is quite sensitive to the deposited Ir layer thickness, while the reflectance on a quartz or a BK7 glass substrates is higher. The energy of the sputtering ion beam exerts a significant influence on the reflectance of the layer, and the postannealing can cause a substantial decrease in the reflectance. For normal incidence, the reflectance of an Ir film on BK7 glass can reach as high as 30%.

The influence of the Cr–Al foil texture on morphology of adhesive Al 2O 3 layers in monolithic environmental catalysts

The FeCrAl-alloy foils of 0.1 and 0.05 mm thickness, being the supports of metallic monolithic environmental catalysts, were investigated by XRD and TEM to determine the reasons of the differences in morphology of the alumina adhesive layers grown during oxidation. Alumina adhesive layers of stable thickness and outgrowing scale-like crystallites were observed on 0.1 mm foil and alumina layers of different thickness with lower population of outgrowing scale-like crystallites on 0.05 mm foil. αFe was found to be the only phase seen by XRD in both kinds of the foil. The density of dislocations on the surface of 0.1 mm foil was found to be of one order of magnitude higher than this on the surface of the 0.05 mm one. Both kinds of the foil reveal different texturing investigated with X-ray. Different texturing connected with various face development results in various possibilities of growing Al 2O 3 layers. However, the high dislocation density is responsible for the high population of scale-like crystallites on 0.1 mm foil.

Coating of multi-walled carbon nanotube with SnO 2 films of controlled thickness and its application for Li-ion battery

Multi-walled carbon nanotubes (MWCNTs) coated with a smooth and uniform tin oxide (SnO 2) layers of different thickness were prepared by a novel thioglycolic acid assisted one-step wet chemical method. The coatings were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thickness of the SnO 2 coatings can be easily controlled by changing the synthesis conditions, such as pH value of the solution and hydrolysis time. The electrochemical properties of the SnO 2/MWCNTs composites as anode materials for lithium batteries were studied by galvanostatic method. The composites showed high charge capacities and good durability against decay. This could be ascribed to the good dispersion, thin layer and small particle size of SnO 2 on MWCNTs.

Effect of selective layer thickness on pervaporation of composite poly(vinyl alcohol)–poly(sulfone) membranes

Effects of the selective layer thickness of flat composite membranes were studied on pervaporation. Selective layers of different thicknesses were prepared from poly(vinyl alcohol), and the porous support layer was prepared from poly(sulfone). The microstructure, surface roughness and hydrophobicity of the selective PVA layers were determined from SEM, AFM and contact angle measurements, respectively. For each composite PVA–PSf membrane, dehydration of ethanol–water mixtures was conducted over a range of ethanol concentrations (10–85 wt%) in the feed solution. Results of the total flux and selectivity were obtained and statistically analyzed as a function of selective layer thickness. In general, the outcome agrees with the solution-diffusion (S-D) theory: the total flux is significantly affected by the PVA layer thickness, while the selectivity remains nearly unaffected. Using the S-D theory, the mass transfer resistance of the selective layers was calculated and found to increase with thickness. The relatively small change observed in the selectivity has been related to changes in surface properties of the crosslinked membrane, especially when the selective layer thickness is small.

Influence of the Si cap layer on the SiGe islands morphology

Transmission electron microscopy (TEM), atomic force microscopy (AFM), and EDX methods were used to study morphology and chemical composition of SiGe/Si(0 0 1) islands grown at 700 °C and covered at 550 °C and 700 °C by Si layers of different thickness. The samples were grown in ultra high vacuum chemical vapor deposition process (UHVCVD) controlled with in situ reflection of high-energy electron diffraction (RHEED). The islands transformed from initial pyramid and dome shapes to lens shape for 1.4 nm and 3.7 nm cap layer thickness at 550 °C and 700 °C, respectively. An increase of lateral to vertical ratio was observed during the transformation. For the higher depositing temperature the ratio was bigger and was increasing continuously with cap layer thickness. Also, with increasing Si cap layer thickness, the Ge concentration was decreasing, which was more observable for higher capping temperature.

Spin current studies in Fe∕Ag,Au∕Fe by ferromagnetic resonance and time-resolved magneto-optics

A precessing magnetization within a magnetic double layer acts as a peristaltic spin pump which transports spin momentum but no net electric charge. Crystalline Fe single layers Au∕12Fe∕GaAs(001) and double layers Fe12∕(Au,Ag)∕Fe16∕GaAs(001) were prepared by molecular beam epitaxy, where the integers represent the number of Fe atomic layers, and (Au,Ag) represents a set of gold and silver layers of different thicknesses. Ferromagnetic resonance (FMR) was used to investigate spin diffusion in thick Au layers in Au∕12Fe∕GaAs(001) samples. Time-resolved magneto-optical Kerr effect (TRMOKE) measurements are an ideal tool for investigating the propagation of spin currents in these structures. Spin currents generated by the bottom 16Fe layer propagated across the normal metal spacer and resulted in rf excitations in the top 12Fe film. Landau-Lifshitz-Gilbert equations of motion modified by spin pump and spin sink effects were used to interpret the FMR and TRMOKE measurements. The spin diffusion lengths in Au were Ag were found 34 and 170nm, respectively.

Coating of multi-walled carbon nanotubes with SnO 2 films of controlled thickness

Multiwalled carbon nanotubes coated with SnO 2 layers of different thickness were prepared by a simple wet-chemical method. The samples have been characterized by X-ray diffraction and transmission electron microscopy. The thickness of the coatings can be easily controlled by changing the synthesis conditions, such as pH value of the solution and hydrolysis time. The smoothness and uniformity of the SnO 2 layers were greatly influenced by the addition of thioglycolic acid, which was used as connecter. The samples prepared by using thioglycolic acid showed very smooth and uniform coating layers, without cracks and broken segments. The growth behavior of the coating layer is also discussed.

Thermal activation of bulk and interfacial order in exchange biased systems

In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately ∼50% of the exchange bias, while the remaining 50% is due to the order in the bulk. For both samples, the median blocking temperature was found to be greater for the interfacial spins by ∼25K. The energy barriers to reversal for both bulk and interfacial spins have been calculated from the magnetic data and we find that the distributions are of similar form but with that for the interfacial spins occurring at higher values.

Modeling and design methodology for metal-insulator-metal plasmonic Bragg reflectors

In this paper, we present a modeling and design methodology based on characteristic impedance for plasmonic waveguides with Metal-Insulator-Metal (MIM) configuration. Finite-Difference Time-Domain (FDTD) simulations indicate that the impedance matching results in negligible reflection at discontinuities in MIM heterostructures. Leveraging the MIM impedance model, we present a general Transfer Matrix Method model for MIM Bragg reflectors and validate our model against FDTD simulations. We show that both periodically stacked dielectric layers of different thickness or different material can achieve the same performance in terms of propagation loss and minimum transmission at the central bandgap frequency in the case of a finite number of periods.

Magnetotransport properties of manganite based magnetic tunnel junctions

We have studied spin-dependent tunneling in La2/3Ba1/3MnO3−δ based trilayer magnetic tunnel junctions fabricated by pulsed laser deposition. In the heteroepitaxial trilayer structures, a strained SrTiO3 layer of different thickness was used as a tunneling barrier. For the junctions with a SrTiO3 barrier, the measured tunneling characteristics deviated considerably from the ideal characteristics expected for elastic tunneling through an ideal barrier. The data indicate that inelastic multistep tunneling as described by the Glazman-Matveev model is present. Our analysis indicates that the SrTiO3 barrier layer consists of two different parts: The interface region with the manganite electrodes containing a high density of structural defects, and the central barrier layer with a small density of defects. The observed temperature dependence of the junction resistance can be explained by a series connection of the resistance contributions from the interfacial and central barrier part. The magnetic field dependence of the junction resistance is found to depend strongly on the barrier thickness and can show complicated behavior due to both a complex magnetic state in the junction electrode and magnetic interactions between the ferromagnetic electrodes. A very large tunneling magnetoresistance up to ΔR/Rp≃1200% is observed that, however, depends sensitively on the magnetic history.

Investigation of hafnium-aluminate alloys in view of integration as interpoly dielectrics of future Flash memories

In this paper, we evaluate the potentialities of hafnium-aluminates (HfAlO) materials as possible candidates for the interpoly dielectrics of future Flash memory devices. HfAlO layers of different thicknesses and compositions are integrated in single-layers and in Oxide/HfAlO/Oxide (OHO) triple-layer stacks, and analyzed in terms of coupling and insulating capabilities. We demonstrate that increasing the Hf content allows reducing the leakage current at high voltages but it results in a stronger leakage current at low voltages. We also show that once normalized in electric fields, the leakage current characteristics are independent of the high-k thickness. The electron conduction modes in these materials, at different temperatures, are also investigated. The activation energy increases with the Hf concentration in the HfAlO alloy, resulting in a higher leakage current at elevated temperatures. Finally, it is demonstrated that the conduction in triple-layer stacks is limited by a Poole–Frenkel conduction in the high-k layers, while the trap contribution in the case of single-layers becomes dominant when the HfAlO layer is thicker than 8 nm.

Ultrathin magnetic multilayer films for low-field microwave notch filters

Microwave filters that use thin films of ferromagnetic metals are now being established as a valuable option compared to yttrium iron garnet based filters due to their higher frequency response. In these filters the signal propagation is inhibited over a wide frequency band, depending on the applied dc magnetic field. However, the continuous application of an applied field to achieve an operating frequency in the higher gigahertz range increases the power consumption of the device. The main contribution of this article is to provide techniques which significantly boost the operating frequency of notch filters in zero or very low applied magnetic fields. To do this, the authors fabricated high quality epitaxial Fe films which are interlayer exchange coupled through nonmagnetic Si layer of different thicknesses. The films were used in flip-chip geometry on top of a Cu-coplanar waveguide to create band-stop filters. In contrast to filters based on Fe alone, the multilayer filters can operate above 25GHz with a very small applied magnetic field. The observed upshift in frequency is attributed to the induced interlayer exchange coupling energy mediated through the nonmagnetic Si layer between the two Fe layers. These frequency shifts are in good agreement with theoretical calculations of the ferromagnetic resonance modes taking into account anisotropy, exchange, and Zeeman energies.

DLTS measurements of radiation induced defects in epitaxial and MCz silicon detectors

n-Type epitaxial silicon layers of different thickness and resistivity, grown on highly Sb doped CZ-substrate by ITME (Warsaw), and n-type MCz silicon supplied by Okmetic (Finland) were used for the processing of planar diodes at CiS (Erfurt). For the epi-diodes a standard as well as a diffusion oxygenation process was employed. Irradiations had been performed with 26 MeV protons at the cyclotron of the Karlsruhe University and with neutrons at the TRIGA reactor of the Ljubljana University. Microscopic investigations using the DLTS method were done. The correlation of the IO 2 i -defect and the oxygen concentration was studied by a depth-profile measurement. The annealing behavior of the IO 2 i -defect at different temperatures was investigated and the activation energy extracted.

Phonon pressure coefficient as a probe of the strain status of self-assembled quantum dots

The built-in strain in self-assembled quantum dots has large impact on their physical properties, but both its average value and degree of anisotropy are often unknown. The authors demonstrate that the pressure coefficient of optical phonons might be used as probe for the strain status of the dots. This method was applied to the case of Ge dots capped with Si layers of different thicknesses. The authors observe a transition from a strictly biaxial stress situation for uncapped dots to a status of quasihydrostatic strain for cap-layer thicknesses larger than a critical value of the order of the dot height.

Influence of the buffer layer on the growth of superconducting films based onmercury

Thin films of mercury based superconductors were prepared onR-planesapphire with a CeO2 buffer layer by a two-step process involving the deposition of the Hg-freeprecursor and ex situ mercuration in a sealed quartz tube. We usedCeO2 buffer layers of different thicknesses to study their influence on the phase composition andproperties of the final films. We succeeded in preparing a Hg-1223 superconducting phase on thinCeO2 buffered(10 nm) R-plane sapphire with a critical temperatureTC0 up to 122.5 K andcritical current density JC up to 4 × 105 A cm−2.

Structure and surface morphology of MnF2 epitaxial layers grown on grooved and ridged CaF2 (1 1 0) surface

Structural and surface morphology studies of MnF2 epitaxial layers have been performed by x-ray diffraction and atomic force microscopy (AFM). MnF2 layers of different thicknesses (0.12–1.25 µm) were grown by MBE on Si(0 0 1) substrates with the deposited CaF2-buffer layer having a ⟨1 1 0⟩ orientation and a grooved and ridged surface. X-ray diffraction patterns in the 10°–60° angular range have been measured. Profiles of diffraction peaks in two directions—parallel and normal to the diffraction vector—as well as the intensity distribution around 2 2 0 reciprocal lattice points were obtained. It was found that the metastable orthorhombic phase of α-PbO2 type with [1 1 0] growth direction dominates in the films; a much smaller portion of the stable rutile type phase was also detected, though it increases with the film thickness. As follows from the x-ray diffraction data, the films have a mosaic block structure. Parameters of the crystallites were determined using the Williamson–Hall analysis of angular width of the diffraction peaks. It was obtained that the crystallites of the orthorhombic phase have an elongated shape oriented along the direction of the CaF2 ridges. The AFM study confirms a regular distribution of the orthorhombic crystallites and shows the appearance of some irregular shaped crystallites on the top of the thick MnF2 films.