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In linear filtering, high-frequency (white) noise is reduced by apodization, which is the attenuation or elimination of high-order Fourier coefficients followed by an inverse transformation. Unfortunately, apodization requires compromises to be made among noise leakage, information loss, and Gibbs oscillations. These shortcomings are avoided with the corrected maximum-entropy (CME) procedure, but this procedure applies only to Lorentzian or approximately Lorentzian features. We develop a generalized maximum-entropy method based on partial Hilbert transforms that allows CME to be applied to any spectrum, thereby eliminating white-noise completely with no deleterious side effects. As Hilbert transforms are exact Kramers–Kronig replicas of the original endpoint-discontinuity-corrected segment, new spectral processing opportunities are also realized. [ABSTRACT FROM AUTHOR]

Herein, we report the magnetodielectric effect in a single crystal of (CH3NH3)2MnCl4, an organic–inorganic hybrid layered perovskite having a composition of A2MX4 (A = organic anion, M = metal divalent ion, X = halogen cation). Magnetic property measurements reveal that a magnetic phase transition to antiferromagnetism appears at 47 K and a spin-flop transition occurs at 3.5 T in the magnetization (M) curve. We observed a considerable maximum magnetodielectric (MD) effect of 0.4% at 8 K. Furthermore, it was found that the MD effect is proportional to M2 in low magnetic fields and the MD effect starts to saturate at the spin-flop transition field. We proposed a magnetic structure model to explain the mechanism of the peculiar step-like MD effect induced by the spin reorientation. [ABSTRACT FROM AUTHOR]

In spectroscopy, the objective is to obtain information by analyzing spectra that ideally are undistorted and noise-free. In standard Fourier-space filtering, this goal cannot be achieved because of apodization, which forces a trade-off among errors arising from distortion, noise leakage, and Gibbs oscillations. We show that low-order coefficients can be preserved and apodization, and its associated errors eliminated with the corrected maximum-entropy (M-E) filter obtained here. Although the Burg derivation begins as M-E, by making certain assumptions the Burg approach yields a procedure that deconvolves (sharpens) structure in spectra, thereby violating the basic M-E principle of leaving the low-order coefficients intact. The corrected solution preserves these data and projects the trends established by them into the white-noise region in a model-independent way, thereby eliminating apodization and its associated errors. For a single Lorentzian line, the corrected M-E approach has an exact analytic solution, which reveals not only how M-E performs its extension but also why it works particularly well for line shapes resulting from first-order decay processes. The corrected M-E filter is quantitatively superior to any previous filtering method, including recently proposed high-performance linear filters, yet requires only minimal computational effort. Examples, including multiple differentiation, are provided. [ABSTRACT FROM AUTHOR]

Semiconductor interfaces are the backbone of modern optoelectronic devices. In terahertz (THz) science, the narrow region of an interface is crucial in the emission process. However, reports on the direct correlation of THz emission with local interface properties remain scarce owing to the inherent difficulty of using the same sample for nanoscale and macroscale studies. In this study, we combined scanning tunneling microscopy/spectroscopy (STM/STS) and THz emission spectroscopy to study the interface between a highly n + -doped and undoped gallium arsenide (GaAs). Using STS, we identify a carrier density of 1 × 10 15 cm − 3 in the low-temperature-grown GaAs (LT-GaAs) layer, which we used to visualize the energy band diagram at the interface and the surface of LT-GaAs. THz emission intensity is higher in the LT-GaAs/ n + -GaAs structures relative to semi-insulating GaAs owing to the high electric field at the interface regardless of the LT-GaAs layer thickness. Pump fluence dependence of THz showed that the thinner LT-GaAs layers saturate at lower pump fluence compared to thicker LT-GaAs and SI-GaAs. This behavior is explained by the built-in field screening by the photogenerated carriers and the free carriers from the n + -GaAs to the LT-GaAs. Our results demonstrate the utility of STM/STS to the design of semiconductor-based THz emitters. [ABSTRACT FROM AUTHOR]

Prototypical Al/single-wall carbon nanotube (SWNT)-polymer/indium tin oxide thin-film devices that exhibit promising photoconductive and photovoltaic responses in a broad spectral range, typically from 300 to 1600 nm is fabricated. Three devices are observed that include the near-infrared (NIR) light harvesting properties of semi-SWNTs, the electronic transport properties of the SWNTs in combination with that of the polymer matrices and probably the charge/energy transfer processes between the nanotubes and the polymers.

Ga(sub 1-x)Mn(sub x)N epilayers with well-ordered ferromagnetic structure are grown on sapphire (0001) substrates, using the plasma enhanced molecular-beam epitaxy technique. A Mn acceptor level of Ga(sub 1-x)Mn(sub x)N layer with low Mn content was found to be located around 330 meV above the top of the valence band, suggesting that Mn-bound holes in group-III nitrides exhibit effective-masslike behavior.

Single-crystal metastable Bl-NaCl-structure delta-TaN(sub x) layers with x ranging from 0.94 to 1.37 were grown on MgO(001) at T(sub s)=600degreeC by UHV magnetically unbalanced reactive magnetron sputter deposition. All layers exhibited an epitaxial cube-on-cube relationship with the substrate: (001)(sub TaN)//(001)(sub MgO) and [100](sub TaN)//[100](sub MgO)

Time-of-flight scattering and recoiling spectrometry was used in studying low-keV ion channeling in hydrogen. Ion channeling is the manipulation of a beam of energetic ions into open spaces between close-packed rows or planes of atoms in a crystal. Results indicate that H atoms preferentially populated the threefold fcc sites at a height of 0.9 plus/minus 0.1 angstrom above the first-layer Pt atoms. The corresponding lenght of the Pt-H bond had the same value.

A study on the use of GaN and other III-V nitrides for optoelectronic applications investigated the chemical and structural changes of the sapphire substrate upon a nitridation process by constricted-plasma source molecular beam epitaxy. The transformation of Al2O3 to aluminum nitride and AlNxO1-x was observed using x-ray photoelectron spectroscopy for chemical analysis and transmission electron microscopy for structural analysis.

We critically test the capabilities of the effective-medium approximation (EMA) and alloy models to describe multilayer samples with gradual interfaces by analyzing spectroscopic ellipsometric (SE) data of two AlGaAs samples grown expressly for this purpose. The dielectric functions [variant_greek_epsilon] of the interfaces are calculated in the EMA and alloy models, and the interfaces themselves simulated either as a single layer of Al0.5Ga0.5As or a stack of layers of AlxGa1-xAs with x increasing or decreasing between 0.1 and 0.9 in increments of 0.1. The EMA essentially fails completely for either interface representation. For the alloy model the stepwise-graded representation is significantly better, not only simulating the data more accurately but also yielding thicknesses in essential agreement with those obtained by cross-sectional transmission electron microscopy. The results highlight the types of errors that are encountered with the different models, and show that the analysis of SE data can provide information about these interfaces. [ABSTRACT FROM AUTHOR]

We studied an appropriate annealing condition of Pr0.7Ca0.3MnO3 (PCMO) thin film that can show good resistive switching characteristics for resistance random access memory applications and also tried to elucidate the mechanism of the resistive switching of PCMO thin film at room temperature. We could observe that crystal structure of PCMO film was changed by oxygen annealing. From x-ray photoelectron spectroscopy measurements, we can conclude that the excess of oxygen by oxygen annealing of PCMO film leads to an increase of Mn4+ content at the PCMO surface with a subsequent change in the Mn4+/Mn3+ ratio at the PCMO surface. The resistance ratio of high resistance state and low resistance state was increased by oxygen annealing of PCMO thin film. This can be explained as follows. The change of the Mn4+/Mn3+ ratio at the PCMO surface by oxygen annealing leads to the change of characteristics of Au-PCMO interface domain and therefore results in the change of the resistance ratio. [ABSTRACT FROM AUTHOR]

We report an optical spectroscopic method to monitor NiAs-type MnAs (α-MnAs) nanocrystals in (Ga,Mn)As diluted magnetic semiconductors. We utilize Mn ion implantation of low temperature (LT) GaAs epitaxial thin films followed by rapid thermal annealing (RTA) to yield embedded ferromagnetic α-MnAs nanoclusters in a GaAs:Mn matrix. As-implanted samples are paramagnetic and become ferromagnetic with Curie temperature of ∼320 K after RTA at 750 °C. No peaks of potential secondary phases could be observed in x-ray diffraction measurements. However, in optical spectra, the annealed samples show resonant absorption at 0.9 eV photon energy, due to resonant surface plasma oscillation of spherical metallic phases embedded in LT GaAs. Since the absorption peak position in the photon energy has a direct relation to the value of the plasma frequency of metallic inclusions, the metallic clusters in LT GaAs are identified as α-MnAs nanocrystals by comparing them with simulations based on Maxwell–Garnett theory. We suggest that this optical method can be applied to various granular systems and diluted magnetic semiconductors as a nondestructive way to detect and quantify metallic nanoclusters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose a way to enhance the strength of self-generated magnetic field from laser-plasma interactions by incorporating the combined role of pulse asymmetricity and plasma inhomogeneity. The pulse asymmetry combined with the plasma inhomogeneity contributes for strong nonlinear current within the pulse body; consequently, a stronger magnetic field can be produced. The nature of self-generated magnetic field is "Quasistatic" that means the self-generated magnetic field varies on the time scale of the period of laser radiation. Our results show that the magnetic-field generated by a temporally asymmetric laser pulse is many-folds higher than the magnetic-field generated by a symmetric laser pulse in plasmas. The present study predicts the generation of magnetic field of the order of 15 T for the laser intensity of ~1014 cm-2. Our study might be applicable to improve the accelerated bunch quality in laser wakefield acceleration mechanism. [ABSTRACT FROM AUTHOR]

A surface plasmon scanning near-field optical microscope was constructed. The device uses a small scatterer as a probe to avoid problems associated with an aperture and enhance the signal and improve the collection efficiency. A vibrating probe and lock-in detection can be used to improve the signal-to-noise ratio. Moreover, the use of surface plasmon resonance properties is expected to heighten spectroscopic sensitivity.

Van der Pauw Hall effect and Shubnikov-de Haas measurements of Si-delta-doped Al(0.27)Ga(0. 73)As/GaAs quantum wells revealed the presence of a two-dimensional electron gas in the quantum wells. Application of fast Fourier transformations to the measurements indicated the existence of two oscillatory frequencies and two subbands in the quantum well. Temperature-dependent photoluminescence and capacitance-voltage profiling yield values for subband energies and show that the GaAs wells resemble asymmetrical potential wells.

Investigations of microstructure of (GaAs)1-x(Si2)x alloys grown on GaAs (001) and (GaAs)1-x(Si2)x/GaAs strained layer superlattices (SLS) buffer layer on GaAs (001) reveal that the alloys have X-ray diffraction peak widths equal to those of GaAs substrates. Their lattice constants linearly vary between values for GaAs and Si. The (GaAs)1-x(Si)x/GaAsSLS buffer layers permit the growth of dislocation free alloys, with the absence of interfacial spinodal decomposition.

Growth of silicon on Ge(001) 2 times 1, using cyclic gas-source molecular beam epitaxy from silicon hydride, follows a mixed Stranski-Krastanov mode. An analysis of such layers using reflection high-energy electron diffraction, Auger electron spectroscopy, electron-energy-loss spectroscopy, scanning tunneling microscopy and high-resolution cross-sectional transmission electron microscopic studies following silicon deposition reveal surface segregation and growth-mode transitions, occurring in the initial phase of Si growth.

The growth process, microstructure and properties of heteroepitaxial wurtzite and zinc-blende structure GaN fabricated by molecular beam epitaxy were investigated. The results showed that the incident N2 ion/Ga flux ratio and the kinetic energy had a strong influence on film growth kinetics. Increased N2+ acceleration energies during the deposition process resulted in higher residual defect density values.

We present a detailed experimental study on how magnetic and electronic properties of Rb,K-intercalated C60 encapsulated inside carbon nanotubes called peapods can be derived from 13C nuclear magnetic resonance investigations. Ring currents do play a basic role in those systems; in particular, the inner cavities of nanotubes offer an ideal environment to investigate the magnetism at the nanoscale. We report the largest diamagnetic shifts down to -68.3 ppm ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon intercalation. The metallization of intercalated peapods is evidenced from the chemical shift anisotropy and spin-lattice relaxation (T1) measurements. The observed relaxation curves signal a three-component model with two slow and one fast relaxing components. We assigned the fast component to the unpaired electrons charged C60 that show a phase transition near 100 K. The two slow components can be rationalized by the two types of charged C60 at two different positions with a linear regime following Korringa behavior, which is typical for metallic system and allow us to estimate the density of sate at Fermi level n(EF). [ABSTRACT FROM AUTHOR]

As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagnetic electrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed in manganite/(Ba, Sr)TiO3/manganite MFTJs at low temperatures and room temperature four-resistance state devices were also obtained. To enhance the TER for potential logic operation with a magnetic memory, La0.7Sr0.3MnO3/BaTiO3/La0.5Ca0.5MnO3 /La0.7Sr0.3MnO3 MFTJs were designed by utilizing a bilayer tunneling barrier in which BaTiO3 is ferroelectric and La0.5Ca0.5MnO3 is close to ferromagnetic metal to antiferromagnetic insulator phase transition. The phase transition occurs when the ferroelectric polarization is reversed, resulting in an increase of TER by two orders of magnitude. Tunneling magnetoresistance can also be controlled by the ferroelectric polarization reversal, indicating strong magnetoelectric coupling at the interface. [ABSTRACT FROM AUTHOR]

Condensers are flow architectures needed to provide high rates of condensation (or cooling) per unit volume, in enclosures with fixed volume. Their design has not changed from configurations consisting of the banks of horizontal tubes. In this paper, we outline a free path to evolving the design by exploring new features of flow configuration: flattened tubes, multiple tube sizes, arrays of flattened tubes, vertical tubes with turbulent film flow, forced convection condensation instead of gravity driven condensation, and the optimal length of a horizontal tube, i.e., the number of tubes in a column aligned with vapor cross flow. We show that the condensation density can be increased sizably by varying freely and without bias the morphology of the flow system: the shapes and arrangement of the cooled surfaces on which condensation occurs. The evolution of technology is described in terms of the special time direction of the useful (purposeful) changes in the configuration (shapes, arrangements) of surfaces on which flow/condensation occurs. This explains what "evolution" means. It is an important step for physics, not just technology. [ABSTRACT FROM AUTHOR]

Employing graphites with distinctly different mean grain sizes, we study the effects of polycrystallinity on the pattern formation by ion-beam sputtering. The grains influence the growth of the ripples in a highly anisotropic fashion; both the mean uninterrupted ripple length along the ridges and the surface width depend on the mean size of the grains, which is attributed to the large sputter yield at the grain boundary compared with that on the terrace. In contrast, the ripple wavelength does not depend on the mean size of the grains, indicating that the mass transport across the grain boundaries should efficiently proceed by both thermal diffusion and ion-induced processes. [ABSTRACT FROM AUTHOR]

The technique of x-ray photoelectron spectroscopy is used to determine a suitable annealing condition for [Pr.sub.0.7][Ca.sub.0.3]Mn[O.sub.3] (PCMO) thin film, which can provide good resistive switching characteristics for random access memory applications. The results show that oxygen annealing can completely change the crystal structure of the PCMO film and hence focuses towards elucidating the resistive switching mechanism of the PCMO film.

We report temperature-dependent optical properties of epitaxial CdO thin films grown by metal-organic vapor phase epitaxy on sapphire substrates. Dielectric function [variant_greek_epsilon] = [variant_greek_epsilon]1 + i[variant_greek_epsilon]2 spectra for CdO were extracted from the multilayer modeling of ellipsometric data, using a set of Tauc-Lorentz oscillators from 0.74 to 6.43 eV in the temperature range between 24 and 650 K. Temperature dependence of the energy for the major optical structures in the [variant_greek_epsilon] spectra was analyzed by using Varshni's approximation. Raman scattering (RS) spectroscopy was used to characterize the vibrational properties of CdO from 77 to 500 K. Several RS peaks were observed in the wavenumber range from 100 to 1000 cm-1. Peak positions, accurately determined by a series of Gaussian-Lorentzian mixed line profiles, exhibit a weak linear dependence on temperature. [ABSTRACT FROM AUTHOR]

The structural and reflectance properties of large gallium (Ga) droplets and GaAs islands grown by droplet epitaxy (DE) were presented. The reflectance results of self-assembled large GaAs islands by DE suggest the possibility of a novel method for antireflective coating. The diameter, height, density, and aspect ratio of large Ga droplets were investigated up to the scale of optical size. After GaAs island growth, the reflectance of s-polarization at 70° on in-situ measurement was reduced up to approximately 2%-20% in the wavelength range of 350-900 nm. For large GaAs islands, reduction of reflectance for s-, p-polarization at 20°-80° and reduction of reflectance at normal incidence was presented. This result shows that a layer of self-assembled large GaAs islands by DE can be a good candidate for an antireflector for high-quality optoelectronic devices. [ABSTRACT FROM AUTHOR]

We report on the synthesis of Fe-doped ZnO with nominal composition of Zn0.99Fe0.01O by using a coprecipitation method. X-ray diffraction and selective area electron diffraction studies reveal a single phase wurtzite crystal structure without any secondary phase. Field emission transmission electron microscopy measurements infer that Zn0.99Fe0.01O have nanorod-type microstructures. Magnetic hysteresis measurement performed at different temperatures show that Zn0.99Fe0.01O exhibits a weak ferromagnetic behavior at room temperature. A detailed investigation of the electronic and local structure using O K-, Fe L3,2 near edge x-ray absorption fine structure suggests that Fe is substituting Zn in ZnO matrix and is in Fe3+ state. [ABSTRACT FROM AUTHOR]

We report the results of a study of the optical gain properties of binary blend films of regioregular poly(3-hexylthiophene) (RR-P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Efficient optical gain is observed in the 10-20 wt. % RR-P3HT composition range and can be tuned to the ∼650 nm window utilised for polymer optical fibre data-communications, with maximum gain ∼50 cm-1. At higher P3HT fractions, gain thresholds increase substantially, consistent with the observed concomitant improvement in photocharge generation. Distributed feed back lasers with one-dimensional gratings exhibit pump pulse thresholds as low as 8 nJ (26 μJ cm-2, 2.17 kW cm-2), lower than many other polymer-based gain media in this wavelength range. They also provide relatively high slope efficiencies >2%. The confluence of efficient optical gain with electronic properties that are conducive to charge carrier injection and transport is relatively novel and is expected to be a necessary requirement for the achievement of electrically pumped lasing. [ABSTRACT FROM AUTHOR]

Micropatterned ordered arrays of cobalt (Co) anti-dots on a uniform thin nickel (Ni) underlayer, known as bilayered magnetic anti-dot lattices (BMALs), were fabricated using photolithography and a controlled wet-etching process. The magnetization reversals in a rhomboid BMAL under the application of a field along 0° (i.e., easy axis) and 90° (i.e., hard axis) were investigated using field-dependent magnetic-force microscopy, and the angular dependence of squareness (Mr/Ms) and coercivity (Hc) were studied using magneto-optical Kerr effects. Although the magnetic reversals were dominated by domain-wall motions or domain rotations, reflecting the easy and hard axis characteristics, various other complex reversal processes were also found that depends on the history of the field application. The angular dependence of Mr/Ms and Hc in a BMAL system was completely different from that in a single-layer, Co magnetic anti-dot lattice. Interestingly, a unidirectional and uniaxial component representing the overall magnetic anisotropy was revealed in the BMAL system. The details of these complicated magnetization behaviors were investigated and elaborated. [ABSTRACT FROM AUTHOR]

The Fe/MgO/Fe magnetic tunnel junction with C modified interfaces has been studied based on the first principle density function theory method under a finite bias voltage for thin (five layers) and thick (ten layers) MgO barriers. Positive and negative tunneling magnetic resistance (TMR) ratios are obtained as a function of the interface structure. We found that the tunneling conductance is highly nonlinear for asymmetric systems with C at one side of the barrier, and even a sign reversal of the TMR as a function of the bias was found to be in agreement with experiments. [ABSTRACT FROM AUTHOR]

The transport and magnetic properties of 10 wt % malic acid and 5 wt % nanocarbon doped MgB2 have been studied by measuring the resistivity (ρ), critical current density (jc), connectivity factor (AF), irreversibility field (Hirr), and upper critical field (Hc2). The pinning mechanisms are studied in terms of the collective pinning model. It was found that both mean free path (δl) and critical temperature (δTc) pinning mechanisms coexist in both doped MgB2. For both the malic acid and nanocarbon doped samples, the temperature dependence of the crossover field, which separates the single vortex and the small bundle pinning regime, Bsb(T), shows that the δl pinning mechanism is dominant for temperatures up to t(T/Tc)=0.7 but the δTc pinning mechanism is dominant for t>0.7. This tendency of coexistence of the δl and the δTc pinning mechanism is in strong contrast with the pure MgB2, in which the δTc pinning mechanism is dominant over a wide temperature range below Tc. It was also observed that the connectivity factor, active cross-sectional area fraction (AF), are 0.11 and 0.14 for the nanocarbon and the malic acid doped MgB2, respectively, indicating that there are still rooms for further improving jc performance. [ABSTRACT FROM AUTHOR]

Thermally induced behavior of double-lasing edge-emitting quantum dot (QD) laser is investigated by coupling the electron/hole rate equation model with thermal analysis. The increase in substrate temperature due to laser self-heating causes the gradual and continual degradation of ground-state slope efficiency, roll-over, which eventually leads to a complete loss of ground-state light emission. Early excited-state spectral hole burning is observed, which is attributed to carrier leakage from the excited-state to the ground-state induced by the vigorous ground-state stimulated emission. At elevated temperatures, the enhanced carrier transport/communication yields the electron/hole occupation probabilities approaching quasithermal equilibrium, i.e., thermal equilibration. Spectral analysis also shows that self-heating results in recovery of the ground-state spectral hole burning of electron, which can be explained by the thermal equilibration. Homogeneous broadening optically synchronizes all the inhomogeneously broadened QDs by involving all the carriers at the same mode in different QDs, so that QD laser’s performance becomes more thermally sensitive. The strong coupling between thermally-induced emission and the spectral hole burning is demonstrated. [ABSTRACT FROM AUTHOR]

The microstructural properties of a GaN layer grown on a patterned sapphire substrate (PSS) were studied in detail using transmission electron microscope techniques to determine dislocation and growth behaviors. Regular and uniform recrystallized GaN islands were observed on the protruding pattern. On a flat sapphire surface, the crystallographic orientation relationship of [formula] and [formula] existed between the GaN and the substrate. On the other hand, the orientation relationship of [formula] and [formula] was confirmed among the GaN layer, the recrystallized GaN islands on an inclined sapphire surface and the PSS. The flat surface among the protruding patterns began to fill rapidly with GaN. Then, the GaN gradually overgrew the protruding pattern and coalesced near the summit as the growth time increased. The generation of threading dislocations was observed in the vicinity of the coalescence points near the top of the protruding patterns. [ABSTRACT FROM AUTHOR]

Deposition and annealing behaviors of Al atoms on rough Cu (111) surface were investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. The rough Cu surface was modeled by depositing 5 ML of Cu on Ta (011) substrate. Most Al atoms deposited on the rough Cu surface placed on the atomic steps, preserving the major features of the surface during Al deposition. This behavior was discussed in terms of the smaller barrier of the surface diffusion than Ehrlich–Schwoebel barrier of Al on Cu (111) surface. By annealing at 700 K, significant intermixing between Al and Cu rapidly occurs with decrease in the surface roughness. This behavior reveals that the exchange process of Al with substrate Cu dominates during the initial stage of high temperature annealing. [ABSTRACT FROM AUTHOR]

Many optical models have been used to construct analytic composition-dependent dielectric functions of AlxGa1-xAs alloys. However, these models incorporate various unphysical assumptions to improve their fits to data. Here, we provide the parameters needed to calculate dielectric functions of AlxGa1-xAs for 1.5≤E≤6.0 eV and 0≤x≤1 by means of the parametric model of Johs et al. [Thin Solid Films 313–314, 137 (1998)], which eliminates these problems. A representative example concerning interface analysis is discussed, where it is necessary to construct a dielectric function of an alloy of essentially arbitrary composition. [ABSTRACT FROM AUTHOR]

We investigate the effect of incomplete removal of semiconductor overlayers on critical-point (CP) parameters determined from the analysis of ellipsometric spectra. An approximate analytic expression shows that CP energies and broadening parameters should be relatively unaffected for isolated CPs if the dielectric response of the overlayer varies slowly with energy. The results are confirmed by model calculations for InAs, which show that the energies of the E1 and E1+Δ1 CP structures that are commonly used for compositional analysis of semiconductor alloys are relatively unaffected. We also analyze overlayer-removal data for a series of InxGa1-xAs alloy samples. Consistent with the above, the amplitudes and phases are affected significantly for all CPs, while the energies of the well-separated E1 and E1+Δ1 transitions are relatively invariant. The results show that accurate values of composition can be obtained from the analysis of the E1 and E1+Δ1 CP structures, even if complete removal of overlayers is not achieved. [ABSTRACT FROM AUTHOR]

In order to investigate the mechanism behind bistable resistance switching in NiO thin films, we have done detailed x-ray photon spectroscopy (XPS) and x-ray diffraction Analysis (XRD) on NiO and Ti doped NiO samples fabricated under various conditions. We discovered that a high initial resistivity was required for samples to undergo bistable resistance switching, and the presence of metallic Ni content in these samples was determined by XPS. XRD data also showed that NiO grown with a relative (200) orientation was preferred over those grown with relative (111) orientation. [ABSTRACT FROM AUTHOR]

The dielectric functions of Co2Si, CoSi, and CoSi2 compounds have been measured and compared with the calculated ones. All three compounds have very common features in their density of states, especially near the Fermi level: Co 3d-dominated characteristics with very small amounts of Co and Si sp characteristics. Consequently, the features observed in the optical conductivity (OC) spectra of 0.5–5.0 eV for all the compounds also have very similar characteristics. Since in this energy range both the occupied and empty states involved in the interband-absorption peak formation have a 3d character in an energy range, the major factor contributing to the features observed in the OC spectra is the joint-density-of-states (JDOS) effects, and the optical-transition-matrix elements do not play an important role. Co2Si, whose crystal structure is orthorhombic, has a strong optical anisotropy, as expected. The structural dependence of the electronic band structure for Co silicides has also been evaluated by examining the optical properties of the disordered and the crystalline Co-Si alloy films of the CoSi and CoSi2 stoichiometry. The change in the OC spectra can be explained by “simple” JDOS effects in the DOS curves. [ABSTRACT FROM AUTHOR]

Hydrothermal growth of ZnO on the annealed titanate nanotube films results in the oriented hexagonal-needlelike structures. The size, shape, density, and alignment of ZnO film are significantly affected by annealing temperature and orientation of the beneath titanate layer. It is believed that oxygen and hydrogen vacancies, generated due to dehydration of interlayered OH groups while annealing of the titanate, are responsible for the changes in the morphology of the ZnO. Microscopic observations clearly resolved nanoneedles with the base diameter of ∼150 nm and length of ∼5 μm with lattice spacing of 0.52 nm, indicating single crystalline ZnO and grown along the (0001) direction. A growth model is presented based on the layer-by-layer growth (three-step growth) as a function of growth time (2–6 h). Thicknesses of these three steps were found increasing with growth time. The second step (II) of growth exhibits the same feature as that of the first step (I), i.e., bounded with six crystallographic, lower surface energy facets of {0110} surfaces and capped with {0111} planes, but with higher step thickness and sharper tip. Finally, the obtained ZnO nanoneedles exhibit {0111} facets, the third step (III), with sharp tip in the [0001] direction. [ABSTRACT FROM AUTHOR]

Self-assembled Cu2O nanoparticles embedded in a polyimide (PI) matrix were formed by curing Cu thin films with PI precursor layers. Transmission electron microscopy images and selected area electron diffraction patterns showed that Cu2O nanocrystals were created inside the PI layer. The excitonic peaks corresponding to the transitions from the electronic sublevel to the heavy-hole sublevel in the Cu2O nanocrystals embedded in the PI layer shifted to the lower energy side with increasing curing time, and this behavior is attributed to a decrease in the confinement effects because the size of the nanocrystals increases with the curing time. The activation energy of the electrons confined in the ground state of the Cu2O nanocrystals embedded in the PI layer, as obtained from the temperature-dependent photoluminescence spectra, was larger than that of those confined in the excited state. The present results can help us to improve understanding of the optical properties of Cu2O nanocrystals embedded in a PI layer. [ABSTRACT FROM AUTHOR]

A homogeneous crystalline Sm2Ti2O7 (ST) phase was formed in films grown at temperatures ranging between 100 and 200 °C and subsequently annealed at 900 °C. The ST film had a large dielectric constant of 58, which is similar to that of ST ceramics. The leakage current density of the ST film was low and the Poole-Frenkel emission was suggested as being the leakage current mechanism. The ST film had a negative quadratic voltage coefficient of capacitance (VCC), possibly due to the dipolar relaxation. The 100-nm-thick ST film had a high capacitance density of 5.2 fF/μm2 with a low leakage current density of 1.34 nA/cm2 at 2 V. Its quadratic and linear VCCs were -99.5 ppm/V2 and 11 ppm/V, respectively, with a low temperature coefficient of capacitance of 135 ppm/°C at 100 kHz. These results confirmed the potential for the ST film to be used as a high performance metal-insulator-metal capacitor. [ABSTRACT FROM AUTHOR]

We have investigated by imaging ellipsometry (IE) the crystallization of amorphous silicon (a-Si) at low temperature by Ni-silicide-mediated crystallization on a glass substrate. We observed a significant difference in the ellipsometric image between polycrystalline silicon (poly-Si) and a-Si and confirmed that annealing for longer time leads to increasing area of poly-Si domain. The progress of crystallization is modeled as the growth of a disk in a two-dimensional surface since the crystallized region expanded as a disk shape. The rate of increasing radius of the disk and the average distance between initial nucleus points were obtained by fitting the change of the area of the disk measured by the IE with annealing time. [ABSTRACT FROM AUTHOR]

Prominent deflections are shown to occur in freestanding silicon nitride thin membranes when exposed to a 50 keV gallium focused ion beam for ion doses between 1014 and 1017 ions/cm2. Atomic force microscope topographs were used to quantify elevations on the irradiated side and corresponding depressions of comparable magnitude on the back side, thus indicating that what at first appeared to be protrusions are actually the result of membrane deflections. The shape in high-stress silicon nitride is remarkably flat-topped and differs from that in low-stress silicon nitride. Ion beam induced biaxial compressive stress generation, which is a known deformation mechanism for other amorphous materials at higher ion energies, is hypothesized to be the origin of the deflection. A continuum mechanical model based on this assumption convincingly reproduces the profiles for both low-stress and high-stress membranes and provides a family of unusual shapes that can be created by deflection of freestanding thin films under beam irradiation. [ABSTRACT FROM AUTHOR]

Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial SrRuO3/PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality (c/a∼1.058) is maintained down to 50 Å thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization (ΔP) using a pulsed probe setup and the out-of-plane piezoelectric response (d33) revealed a systematic drop from ∼140 μC/cm2 and 60 pm/V for a 150 Å thick film to 11 μC/cm2 and 7 pm/V for a 50 Å thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180° polydomain state for the thinnest films. Existence of a polydomain state is demonstrated by piezoresponse force microscopy images of the ultrathin films. These results suggest that the limit for a ferroelectric memory device may be much larger than the fundamental limit for ferroelectricity. [ABSTRACT FROM AUTHOR]

The effects of InGaAs/GaNAs barrier combinations were investigated in the metal organic vapor phase epitaxy grown GaInNAs quantum wells (QWs). Significant improvement was made in the optical performance of the 1.3 μm range with the proposed structures. Structural investigation linked with the optical properties reveals that a thin InGaAs layer, placed between GaInNAs QWs and GaNAs layers, plays a role in redistributing the nitrogen within the QWs while maintaining a sharp interface. The amount of diffused nitrogen is proportional to the strain residing in the InGaAs layer, which explains the tendency of blueshift with the increase in GaNAs thickness. [ABSTRACT FROM AUTHOR]

The coexistence of a phase separation and an ordered structure in CdxZn1-xTe epilayers grown on GaAs(001) substrates by using molecular beam epitaxy was investigated. The results of selected-area electron diffraction pattern and transmission electron microscopy measurements showed that ordered structures, such as CuPt-type and CuAu-I-type ordered structures, together with a spinodal-like phase separation were formed in the CdxZn1-xTe/GaAs heteroepitaxial layers. The coexistence of the phase separation and the ordered structures is discussed. [ABSTRACT FROM AUTHOR]

A metallic coating method is used to modify the optical properties of a dielectric photonic lattice and to achieve a near visible photonic band edge. It is experimentally shown that the linear scaling rule of a metallic band edge versus lattice constant holds only for perfect conducting metals. When a metal deviates from a perfect conducting behavior near the plasma wavelength, the metallic photonic band edge is pinned and is nearly independent of lattice constant. For our tungsten photonic lattice, the pinning occurs at λ≈1.5–2 μm. By using a thin copper coating (∼70 nm) to a dielectric photonic lattice, a photonic band edge at λ∼750 nm is observed. This achievement is made possible by the fact that copper is a good conductor at visible wavelengths and the linear scaling rule holds. Finally, this coating method allows for tailoring photonic properties through material engineering at the nanometer scale. [ABSTRACT FROM AUTHOR]