bluuebunny/crossref_metadata_2025 huggingface dataset:
Precipitation of Au nanoclusters in SrTiO3 by ion implantation
Authors: Wang C. M. , Shutthanandan V. , Zhang Y. , Thomas L. E. , Baer D. R. , Thevuthasan S. , Duscher G.
Published: April 2004

Gold nanoclusters dispersed in single-crystal SrTiO3 (STO) (001) have been prepared by ion implantation at both 300 and 975 K followed by annealing at 1275 K for 10 h. High-resolution transmission electron microscopy (TEM), high-angle annular dark-field imaging in an aberration-corrected dedicated scanning TEM, and image simulations were used to study the morphology, size, and crystallographic orientation of the Au nanoclusters with respect to the STO matrix, as well as the interface structure between the Au nanoclusters and STO. Gold ion implantation at 300 K leads to amorphization of the STO surface layer, which is corrugated to form bumps and valleys on the surface. Annealing at 1275 K for 10 h leads to epitaxial recrystallization of the amorphized layer within which Au clusters of several nanometers and a narrow size distribution were formed. Implantation at 975 K and subsequent annealing produces much larger Au clusters ∼50 nm in diameter and a very wide size distribution. The precipitated Au clusters possess an epitaxial orientation with the STO, such that Au[001]//STO[001] and Au(100)//STO(100). The critical cluster size for transition from strain-matched interfaces to dislocation relaxed interfaces has been found to be ∼7 nm. Cavities formed by condensation of vacancies were faceted along {001} and {011} planes in the STO matrix. It is generally inferred that implantation below a critical temperature may lead to a finer Au cluster size.
Influence of moisture on device characteristics of polythiophene-based field-effect transistors
Authors: Hoshino Satoshi , Yoshida Manabu , Uemura Sei , Kodzasa Takehito , Takada Noriyuki , Kamata Toshihide , Yase Kiyoshi
Published: April 2004

We investigated a field-effect transistor (FET) based on a poly(3-n-hexylthiophene) (P3HT) to determine the influence of moisture on device characteristics and thus gain a deep understanding of the mechanism underlying the susceptibility to air of the operation of FETs of this kind. The fundamental output characteristics, which include effective field-effect modulation and saturation behavior in the output current, remained almost the same for every current–voltage profile in a vacuum, N2 and O2. By contrast, operation in N2 humidified with water resulted in enlarged off-state conduction and deterioration in the saturation behavior, in the same manner as that experienced with exposure to room air. We concluded that atmospheric water had a greater effect on the susceptibility of the device operation to air than O2, whose p-type doping activity as regards P3HT caused only a small increase in the conductivity of the active layer and a slight decrease in the field-effect mobility with exposure at ambient pressure. We discuss the mechanism of the significant distortion in the operation induced by moisture in terms of the difference between the influence of water and O2 on the device characteristics.
Influence of photon recycling on photovoltage spectra of GaSb diodes
Authors: Toušek J. , Toušková J. , Hulicius E. , Šimeček T. , Pangrác J. , Melichar K. , Výborný Z. , Jurka V.
Published: April 2004

A simple theory of photon recycling (PR) was applied to the calculation of the photovoltage spectra of GaSb diodes. The PR is analyzed here as a two stage process with primary electron-hole pair photogeneration and diffusion followed by an optical transport of secondary carriers induced by emission and reabsorption of photons. The latter process is responsible for different behavior of diodes with and without the PR mechanism. Low losses of the secondary processes in comparison with losses in the primary one are assumed. The secondary processes can change the character of the photovoltage spectra significantly, enhancing the signal in the short wavelength region. The calculated photovoltage spectral response agrees well with the experiment giving the possibility to evaluate parameters of the material and of the measured diodes. The diffusion length obtained in this way characterizes only diffusion of electron-hole pairs and the influence of PR can be separated.
Acoustic-phonon-assisted localization of free excitons due to interface roughness in quantum wells
Authors: Oh I.-K. , Singh Jai
Published: April 2004

A theory of localization of free excitons due to interface roughness is presented. Using the perturbation approach, the rate of localization of free excitons through acoustic phonon emission is calculated in GaAs quantum wells as a function of the width of quantum wells, lateral size of well width fluctuation, and center-of-mass kinetic energy. It is found that the rate of localization is sensitive to the center-of-mass kinetic energy of a free exciton. The results agree with those of recent photoluminescence experiments employed to study the localization of free excitons due to roughness in GaAs quantum wells.
Lateral force measurement using a probe fiber as a microlens
Authors: Fukuzawa K. , Itoh S. , Ando T. , Takahashi K. , Zhang H. , Mitsuya Y.
Published: April 2004

We present a lateral force sensing method for small lateral forces that uses an optical fiber probe as a microlens and measures displacement of the laser spot on a position sensitive detector. We demonstrate that arranging the focus point near the object focal point of the fiber improves the signal intensity and our method combined with synchronous detection can provide a detection limit of the order of 1 pm at a bandwidth of 1 Hz. This indicates that the method enables us to measure lateral force of the order of 1 pN. The method is not restricted by measurement frequency as is the case with tuning fork-based lateral force sensing, and will be a useful method in applications that aim to clarify the intermolecular interaction between the probe and sample by measuring the frequency response, such as nanotribology, nanorheology, interfacial science, and nanobiology.
Topological and chemical arrangement of binary alloys during severe deformation
Authors: Lund Alan C. , Schuh Christopher A.
Published: April 2004

We explore the alloy structures that can evolve in nonequilibrium driven systems, using two-dimensional atomistic simulations of mechanical alloying. By tailoring the intrinsic system thermodynamics we demonstrate the evolution of alloys with various states of topological and chemical ordering, including random solid solutions with both crystalline and amorphous topology, short-range ordered glasses and crystals, and segregated dual-phase systems. Our observations are broadly consistent with the experimental literature for mechanical alloying of binary systems. Further, while most of our results can be rationalized on the grounds of existing theories for either (i) glass formation and stability or (ii) nonequilibrium systems under external driving, we also observe strong correlations between topological and chemical ordering which have not yet been addressed in the context of driven systems.
CN
x
/
Si
thin heterostructures for miniaturized temperature sensors
Authors: Simeonov S. , Szekeres A. , György E. , Mihailescu I. N. , Perrone A.
Published: April 2004

CN x / Si thin heterostructures were synthesized on Si 〈111〉 substrates by XeCl* excimer laser (λ=308 nm, τFWHM≅30 ns) ablation of nuclear grade graphite targets in 5 Pa nitrogen at room temperature. We investigated the current–voltage and capacitance–voltage characteristics of heterostructures obtained at 77 and 300 K. We monitored their conductance and capacitance as a function of the bias voltage applied, in the 100 Hz–1 MHz frequency range. Our results revealed the formation of deep localized electron states both inside the thin CNx films and at the CNx/Si substrate interface. The investigations evidenced that conduction through the CNx/Si thin heterostructure is of trap-assisted tunneling type. The experimental studies show quite a large decrease in capacitance and increase in conductance with the increase of applied frequency. The capacitance of the CNx/Si thin heterostructures increases with a decrease of the temperature. All the results support the potential development of new types of high sensitivity temperature sensors.
Photonic material for designing arbitrarily shaped mirrors and microcavities in two dimensions
Authors: Miyazaki Hiroshi , Hase Masashi , Miyazaki Hideki T. , Kurokawa Yoichi , Shinya Norio
Published: April 2004

Based on the numerical finding of a two-dimensional photonic material which has large complete photonic gaps and structural uniformity, we propose a photonic plate which can be used to design arbitrarily shaped photonic mirrors and microcavities on a wavelength scale. This paper describes a wavelength-sized parabolic mirror that can collect light very efficiently without loss. In addition, we present circular microcavities of tunable resonance frequencies with high values of quality factor Q.
Continuous-wave operation of AlGaInP/GaInP quantum-well lasers grown by metalorganic chemical vapor deposition using tertiarybutylphosphine
Authors: Dong Jian-Rong , Teng Jing-Hua , Chua Soo-Jin , Foo Boon-Chin , Wang Yan-Jun , Zhang Lian-Wen , Yuan Hai-Rong , Yuan Shu
Published: April 2004

Strained AlGaInP/GaInP multiple-quantum-well laser structures have been grown by metalorganic chemical vapor deposition using teriarybutylphosphine as the phosphorus precursor and ridge waveguide lasers of 4 μm wide have been fabricated. Room temperature continuous-wave lasing has been obtained with an emission wavelength of about 670 nm. A single-facet output power of more than 18 mW has been achieved for an as-cleaved laser chip. It can be concluded that it is feasible to fabricate AlGaInP red lasers using less toxic metalorganic source tertiarybutylphosphine in parallel with conventionally used highly toxic PH3.
Fast nondestructive technique to determine the content of components in a strain-compensated crystalline ternary alloy
Authors: Nikulin A. Y. , Zaumseil P.
Published: April 2004

The x-ray Bragg diffraction intensity profile for a model strain-compensated structure consisting of a thin SiGe alloy layer grown on a thick Si substrate is derived using a Laplace transform interpretation of the kinematical approximation of x-ray diffraction theory. It is shown that in the case of fully strain-compensated crystals a simplified x-ray phase-retrieval technique can be applied to determine the alloy composition from this x-ray diffraction data. An experimental intensity profile from an almost perfectly unstrained SiGe:C/Si structure is analyzed using this method.
Resetting the magnetization of assemblies of nanoparticles with microwaves
Authors: Walton Derek
Published: April 2004

It is shown that, in materials containing magnetic nanoparticles, it is possible to reset the magnetization with negligible heating of the material, provided that sufficient microwave power is available. When nanoparticles of magnetic materials, such as magnetite are exposed to microwaves, provided certain conditions are fulfilled, a strong absorption results that generates magnons, and raises the temperature of the particles. If the particles are embedded in a nonmagnetic matrix, the energy absorption in the matrix occurs via dielectric processes that are often much weaker than the coupling of the magnetic grains to the magnetic component of the microwave electromagnetic field. Under these conditions, essentially all the energy is absorbed by the magnetic grains, and the thermal barrier between the magnetic grains and the matrix leads to a temperature difference between the 2.
Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3
Authors: Herth Simone , Eggersmann Martin , Eversheim Paul-Dieter , Würschum Roland
Published: April 2004

Iron tracer diffusion was studied in soft-magnetic nanocrystalline Fe90Zr7B3 without any influence of porosity, relaxation, or grain growth. The interfacial diffusion characteristics differ substantially from grain boundaries in metals due to the presence of an intergranular amorphous phase. The reduced diffusivity in thin amorphous layers compared to in the initial amorphous phase indicates the effect of confinement. The indication of a second, fast interfacial diffusion path is found and quantitatively analyzed within the framework of a two interface-type model.
Optical coherent transient effects in magnetoactive GaAs/AlGaAs quantum well structure
Authors: Kapoor S. , Kumar J. , Sen P. K.
Published: April 2004

The effect of an external magnetic field on optical coherent transient phenomena has been analyzed for semiconductor quantum well structures (QWS). The Lüttinger Hamiltonian is used to incorporate the complex valence band structure which is modified due to the application of the magnetic field. We have considered a GaAs/AlGaAs QWS with magnetic field applied in the direction perpendicular to the plane of the well. We get two quantum levels due to confinement, and doubly degenerate heavy-hole and light-hole bands corresponding to each quantum level, thus leading to eight hole subbands. This results in an 8×8 Hamiltonian in an appropriately chosen basis. Energy eigenvalues and eigenvectors have been calculated by numerically diagonalizing the above Hamiltonian. The absorption characteristics and the optical coherent transient phenomena have been studied by solving the effective semiconductor Bloch equations in the weak excitation regime. The polarization induced in conjunction with the results obtained by solving the modified Lüttinger Hamiltonian has been used to analyze the transient characteristics. We observe a blueshift in transition energy with an increase in magnetic field. The features are changed dramatically when one includes band mixing. The crossing of subbands in the absence of band-mixing effects changes over to anticrossing behavior when band mixing is included. Multiple oscillations are observed in the optical nutation signal that can be attributed to band-mixing effects. Absorption spectra also show a significant blueshift with an increase in magnetic field and additional peaks appear when band mixing is included.
Mechanical resonance of quartz microfibers and boundary condition effects
Authors: Chen Xinqi , Zhang Sulin , Wagner Gregory J. , Ding Weiqiang , Ruoff Rodney S.
Published: April 2004

We have measured the mechanical resonance of microscale quartz fibers to qualify the method of obtaining the Young’s modulus of nanowires from their resonance frequency and geometry. An equation for a circular beam with a linearly varying cross-section is derived and used to calculate the resonance frequency shift. We have established a model to discuss the boundary condition effect on the resonance frequency. The Young’s modulus of the quartz fibers has been determined by measuring the resonance frequency, and the geometry, and by applying the model that treats the influence of the type of clamp. The mean value from measurements of the fundamental resonance on 14 different microfibers is 70±6 GPa. This mean value is close to 72 GPa, the Young’s modulus of bulk fused quartz. Four resonance modes were observed in high vacuum and air. The mechanical resonance in high vacuum is linear at the fundamental vibration mode, and nonlinear for higher modes.
A two-dimensional analysis of spurious compressional wave excitation by thickness-shear-mode resonators
Authors: Beigelbeck Roman , Jakoby Bernhard
Published: April 2004

In this article we investigate the spurious excitation of compressional waves by thickness-shear-mode resonators, which are commonly used for chemical sensing and viscosity sensing. In particular, we consider the excitation mechanism due to the nonuniform shear displacement present at the sensitive surface of the resonator. To illustrate the basic mechanism we analytically develop a general solution for a two-dimensional model describing the displacement and the pressure distribution in the liquid obtained in terms of Fourier integrals. We discuss these solutions and derive a useful approximation for the far field representing said compressional modes. We finally illustrate the results considering a practical example and associated numerical results.
Defect density and atomic bond structure of tetrahedral amorphous carbon (ta-C) films prepared by filtered vacuum arc process
Authors: Lee Churl Seung , Shin Jin-Koog , Eun Kwang Yong , Lee Kwang-Ryeol , Yoon Ki Hyun
Published: April 2004

Defect density of tetrahedral amorphous carbon (ta-C) film prepared by filtered vacuum arc process was investigated in a wide range of fraction of sp3 hybridized bond. We could observe a close relationship between unpaired spin density measured by electron spin resonance spectroscopy and their atomic bond structure: the defect density was proportional to the content of sp3 hybridized bond in the film. Near edge x-ray absorption fine structure analysis further showed that the content of the surface C–H bonds presumably due to the absorption of hydrocarbon to the surface dangling bond also increased with increasing content of sp3 hybridized bond. The observed dependence was discussed in terms of the degree of clustering or pairing of the isolated sp2 sites.
Heat and fluid flow in complex joints during gas metal arc welding—Part I: Numerical model of fillet welding
Authors: Zhang W. , Kim C.-H. , DebRoy T.
Published: April 2004

Gas metal arc (GMA) fillet welding is one of the most important processes for metal joining because of its high productivity and amiability to automation. This welding process is characterized by the complicated V-shaped joint geometry, a deformable weld pool surface, and the additions of hot metal droplets. In the present work, a three-dimensional numerical heat transfer and fluid flow model was developed to examine the temperature profiles, velocity fields, weld pool shape and size, and the nature of the solidified weld bead geometry during GMA fillet welding. The model solved the equations of conservation of mass, momentum, and energy using a boundary fitted curvilinear coordinate system. Apart from the direct transport of heat from the welding arc, additional heat from the metal droplets was modeled considering a volumetric heat source. The deformation of the weld pool surface was calculated by minimizing the total surface energy. Part I of this article is focused on the details of the numerical model such as coordinate transformation and calculation of volumetric heat source and free surface profile. An application of the model to GMA fillet welding of mild steel is described in an accompanying article (W. Zhang, C.-H. Kim and T. DebRoy, J. Appl Phys. 95, 5220 (2004)).
Heat and fluid flow in complex joints during gas metal arc welding—Part II: Application to fillet welding of mild steel
Authors: Zhang W. , Kim C.-H. , DebRoy T.
Published: April 2004

A numerical model described in part I [W. Zhang, C.-H. Kim, and T. DebRoy, J. Appl. Phys. 95, 5210 (2004)] was used to investigate the heat transfer and free surface flow during gas metal arc fillet welding of mild steel. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection and the role of various driving forces on convection in the liquid weld pool. The calculated shape and size, finger penetration characteristic and solidified surface profile of the fillet welds were in fair agreement with the experimental results for various welding conditions. The calculated cooling rates were also in good agreement with independent experimental data. The effect of welding parameters on important weld bead characteristics was quantitatively studied using the numerical model. The results reported here indicate a significant promise for understanding and control of gas metal arc fillet welding processes based on fundamental principles of transport phenomena.
Current density distribution in a spin valve determined through in situ conductance measurements
Authors: McCallum A. T. , Russek S. E.
Published: April 2004

The sheet conductances of top-pinned spin valves and single-material films were measured in situ as the thin-film layers were grown. The data were fit to a Boltzmann transport calculation. The electrical conductivity and electron mean free paths were determined for each material by measuring the in situ conductance of thick single-material films. The electron transmission probabilities were deduced for each interface from the theoretical fits to the multilayer data. From these interfacial transport parameters the ratio of current density to electric field, or effective conductivity, was calculated as a function of position for the completed spin valve. It was found that the distribution of current in the spin valve was not very sensitive to the overall amount of diffuse scattering at the interfaces.
Experimental and theoretical investigations of a low-pressure He–Xe discharge for lighting purpose
Authors: Bussiahn R. , Gortchakov S. , Lange H. , Uhrlandt D.
Published: April 2004

Low-pressure cylindrical dc glow discharges in a mixture of helium and 2% xenon are studied by experiment and self-consistent modeling. They can be used for the design of mercury-free vacuum ultraviolet sources and fluorescent lamps for publicity lighting. Experimental diagnostics of the column plasma includes measurements of the axial electric field strength and of the axis densities of the four lowest excited states of xenon. The electric field is determined from probe measurements. The particle densities are derived from the results of tunable diode laser absorption spectroscopy. Experimental investigations are assisted by a self-consistent analysis of the dc positive column plasma. A comparison between calculated and measured values of the axial electric field strength and the densities of excited xenon atoms is presented and discussed. The validated model is used for optimization of the discharge conditions by variation of the discharge current, gas pressure, and tube radius with respect to the radiation power and efficiency of the 147 nm resonance line of xenon. The discussion includes an analysis of the power budget of the column plasma.
Dense arrays of nanopores as x-ray lithography masks
Authors: Knaack S. A. , Eddington J. , Leonard Q. , Cerrina F. , Onellion M.
Published: April 2004

An anodized aluminum oxide nanopore has been used as an x-ray lithography mask to achieve a feature size of ∼35 nm on the polymethylmethacrylate photoresist. The mask was exposed using synchrotron radiation and demonstrates the feasibility of forming large arrays of regular nanostructures.
High-stability ultrathin spin-on benzocyclobutene gate dielectric for polymer field-effect transistors
Authors: Chua Lay-Lay , Ho Peter K. H. , Sirringhaus Henning , Friend Richard H.
Published: April 2004

Using a thermal-crosslinkable siloxane bisbenzocyclobutene, high quality spin-on (solutionprocessable) gate dielectric layers as thin as 50 nm have been fabricated over the semiconductor layer for polymer field-effect transistors. This was demonstrated on a poly(9,9-dialkylfluorene-alt-triarylamine) as p-channel semiconductor, with a surfactantion-exchanged poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate complex as top-gate electrode. The devices operate at a low voltage with a field-effect mobility of few 10−4 cm2/Vs, and can be continuously operated at 120 °C.
Enhancement of boron solid solubility in Si by point-defect engineering
Authors: Shao Lin , Zhang Jianming , Chen John , Tang D. , Thompson Phillip E. , Patel Sanjay , Wang Xuemei , Chen Hui , Liu Jiarui , Chu Wei-Kan
Published: April 2004

The technique of point-defect engineering (PDE), with excess vacancies produced near the surface region by MeV Si ion implantation, has been applied to form ultrashallow junctions with sub-keV B implants. PDE can reduce boride-enhanced diffusion that dominates the enhanced diffusion of ultralow energy B implants. PDE can further sharpen the dopant profile and enhance boron activation. For 1×1015/cm2, 0.5 keV B implant, B solid solubility has been enhanced over a wide temperature range of 750–1000 °C, with an enhancement factor of 2.5 at 900 °C. These features have enabled a shallower and sharper box-like boron junction achievable by PDE in combination with ultralow energy 0.5 keV B implantation.
Low-threshold polymeric distributed feedback lasers with metallic contacts
Authors: Reufer M. , Riechel S. , Lupton J. M. , Feldmann J. , Lemmer U. , Schneider D. , Benstem T. , Dobbertin T. , Kowalsky W. , Gombert A. , Forberich K. , Wittwer V. , Scherf U.
Published: April 2004

Optical losses in waveguides comprising metallic contacts are thought to be a major hurdle to the realization of organic laser diodes. We demonstrate here that careful tuning of the waveguide mode in flexible distributed feedback lasers can allow lasing action to occur in organic thin films in the presence of contacting electrodes with virtually no difference when compared to metal free devices. A metallic electrode is most suited as the bottom contact between the polymer and the substrate as it reduces mode leakage into the substrate and enhances modal gain. In contrast, a thin transparent electrode such as a metal oxide is preferable for the top electrode, where confinement is not a problem.
Stimulated emission and time-resolved photoluminescence in rf-sputtered ZnO thin films
Authors: Özgür Ü. , Teke A. , Liu C. , Cho S.-J. , Morkoç H. , Everitt H. O.
Published: April 2004

Stimulated emission (SE) was measured from ZnO thin films grown on c-plane sapphire by rf sputtering. Free exciton transitions were clearly observed at 10 K in the photoluminescence (PL), transmission, and reflection spectra of the sample annealed at 950 °C. SE resulting from both exciton-exciton scattering and electron hole plasma formation was observed in the annealed samples at moderate excitation energy densities. The SE threshold energy density decreased with increasing annealing temperature up to ∼950 °C. The observation of low threshold exciton-exciton scattering-induced SE showed that excitonic laser action could be obtained in rf-sputtered ZnO thin films. At excitation densities below the SE threshold, time-resolved PL revealed very fast recombination times of ∼74 ps at room temperature, and no significant change at 85 K. The decay time for the SE-induced PL was below the system resolution of <45 ps.
Experimental investigation of Hall mobility in Ge/Si quantum dot superlattices
Authors: Bao Y. , Balandin A. A. , Liu J. L. , Liu J. , Xie Y. H.
Published: April 2004

We report results of measurements of Hall mobility in a set of doped and undoped GexSi1−x/Si quantum dot superlattices (x=0.50 and 0.73). The dome-shaped Ge quantum dots have the characteristic base size of 40 nm and height of about 4 nm. The molecular beam epitaxy grown structures consist of 5–20 layers of Ge quantum dots separated by 20-nm-thick Si layers. The position of δ doping varies for different samples. The average measured in-plane Hall mobility for p-type structures is 140 cm2 V−1 s−1 at 300 K and 2.4×103 cm2 V−1 s−1 at 77 K. Relatively large values and temperature dependence suggest that in given quantum dot structures the carrier transport is likely of the band conduction type rather than hopping type. These results are important for proposed optoelectronic and thermoelectric application of quantum dot superlattices.
Ion energy distribution change at the transition of power-coupling modes in an immersed-coil-type inductively coupled Ar discharge
Authors: Kang Dae-Hwan , Lee Dong-Kak , Kim Ki-Bum , Lee Jung-Joong , Joo Junghoon
Published: April 2004

The power-coupling mode was changed from a capacitively coupled (E-mode) to an inductively coupled (H-mode) with an increase of rf input power in an immersed-coil-type inductively coupled Ar discharge at 1 mTorr. It was identified that the bimodal-shaped ion energy distribution (IED) of the capacitively coupled discharge was dominant below 100 W (E-mode), while the single peak due to the inductively coupled discharge became dominant above 250 W (H-mode). In addition, it was possible to quantitatively determine the relative amounts of capacitively and inductively coupled components at both modes, making the analysis of IED one of most effective methods to investigate the power-coupling mode transition in an inductively coupled plasma system.
Measurement of Ga implantation profiles in the sidewall and bottom of focused-ion-beam-etched structures
Authors: Park C.-M. , Bain J. A. , Clinton T. W. , van der Heijden P. A. A. , Klemmer T. J.
Published: April 2004

Ga implantation during focused-ion-beam (FIB) milling of metallic magnetic materials for magnetic recording heads was examined using Auger electron spectroscopy (AES). The Ga concentration profiles were characterized in two directions: in the depth direction (parallel to the incident beam) and in the lateral direction (perpendicular to the incident beam). The sample for AES analyses in the depth direction was prepared by FIB etching of a plated Ni78.5Fe21.5 film surface. The AES depth profile showed a maximum implanted Ga concentration of ∼20 at. % at a distance of ∼15 nm, and the tail of the implantation profile reached more than 100 nm. The sample for the lateral direction analyses was prepared by cutting a cross-sectional specimen out of the air-bearing surface of a working recording head. The lateral profile showed a maximum implanted Ga concentration of 47 at. % at the surface, with a lateral implantation depth of ∼10 nm. These results were compared with results predicted by a Monte Carlo implantation simulation package (TRIM). The simulated lateral profile was reasonably consistent with experimental observation, but the simulated depth profile agreed with experiment less well. Specifically, it did not predict the observed concentration levels observed as deep as 100 nm, which are believed to be due to dynamical sputtering effects.
Displacement measurement in real time using the attenuated total reflection technique
Authors: Shi Jian , Cao Zhuangqi , Zhu Jun , Shen Qishun
Published: April 2004

A simple displacement measurement based on the attenuated total reflection technique is proposed and experimentally demonstrated. By analyzing the fringe shift of the reflective pattern recorded on a CCD camera, a slight displacement, which corresponds to a change in the waveguide thickness, can be determined in real time with adequate accuracy. The measurement range of this method is about 240 μm, and its accuracy can reach 0.05 μm.
Frequency-dependent bifurcation point between field-cooled and zero-field-cooled dielectric constant of LiTaO3 nanoparticles embedded in amorphous SiO2
Authors: Kohiki Shigemi , Kawakami Shintaro , Nogami Shinichiro , Takada Syozo , Shimooka Hirokazu , Okui Masato , Fukushima Sei , Mitome Masanori , Bando Yoshio
Published: April 2004

Splitting between the field-cooled dielectric constant and the zero-field-cooled dielectric constant was observed for a diluted system of LiTaO3 nanoparticles (diameter ≈30 Å) embedded in amorphous SiO2. At the applied field frequency of 100 kHz, the real part of the field-cooled dielectric constant diverged from that of the zero-field-cooled one at ≈380 °C. The bifurcation point of the history-dependent dielectric constant rose from ≈310 to ≈540 °C upon increasing the field frequency from 10 to 1000 kHz. Bulk LiTaO3 powders showed no splitting in the history-dependent dielectric constant and the maximum at 645 °C in the real part of the dielectric constant, despite the variation of frequency. Both the splitting of the history-dependent dielectric constant and the frequency dependence of the bifurcation point suggest that the LiTaO3 nanoparticles with a single-domain structure were in the superparaelectric state as a consequence of insignificant cooperative interactions among the nanoparticles in the diluted system. The energy barrier of ≈0.9 eV separating two (+p and −p) polarization states corroborated the potential of the LiTaO3 nanoparticle for ultrahigh-density recording media applications.
Photosensitive properties of a tin-doped sodium silicate glass for direct ultraviolet writing
Authors: Milanese Daniel , Ferraris Monica , Menke Yvonne , Olivero Massimo , Perrone Guido , Gawith Corin B. E. , Brambilla Gilberto , Smith Peter G. R. , Taylor Elizabeth R.
Published: April 2004

In this letter, the fabrication of channel waveguides by direct UV writing into a bulk tin-doped multicomponent silicate glass is presented. Different laser powers and scan rates were employed and optimized to avoid surface ablation. The best results were obtained at a laser power of 95 mW and at a scan speed of 10 mm/min, where a refractive index change of 1.5×10−3 was estimated and an attenuation loss of 3.3 dB/cm was measured using the cutback method. A morphological investigation of the glass end facets was performed in order to assess the surface effect of the laser exposure.
Delta doping superlattice structures for amplitude modulators: Observation of the Stark effect and improvement of the chirp
Authors: Tribuzy C. V.-B. , Pires M. P. , Landi S. M. , Borgström M. , Souza P. L.
Published: April 2004

A Stark shift for a GaAs/AlGaAs multiple quantum well structure containing a nipi delta-doping superlattice is observed. Photocurrent measurements revealed a Stark shift that varied with increasing applied electric field, from being equivalent to that of an undoped structure to 30% below. Even though these values are still below those predicted theoretically, they demonstrate the possibility of fabricating amplitude modulators using these novel structures. Furthermore, the obtained chirp parameter is within the desired [−1,0] range for long-distance optical communication, contrary to the high positive values for the undoped structure, showing that such device may have a better overall performance.
Grazing incidence x-ray diffraction and atomic force microscopy investigations of germanium dots grown on silicon (001) by successive depositions of germanium through a thin silicon oxide layer
Authors: Derivaz M. , Noé P. , Dianoux R. , Barski A. , Coati A. , Garreau Y. , Alandi C.
Published: April 2004

Structural properties of high density, nanometric size germanium dots grown on a silicon (001) surface covered by a very thin (1.2 nm thick) silicon oxide layer have been investigated by in-situ grazing incidence x-ray diffraction (GIXD) and ex-situ atomic force microscopy. Growth under molecular nitrogen partial pressure of 10−5 Torr yielded a high density (∼4×1010/cm2) of dots. The dot size can be progressively increased by successive depositions of germanium. GIXD investigations show that dots grow in epitaxial relationship to the silicon (001) substrate and that after few successive depositions, the dots are composed of pure and fully relaxed germanium.
High electrostriction and relaxor ferroelectric behavior in proton-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer
Authors: Guo Shishang , Zhao Xing-Zhong , Zhou Qifa , Chan Helen L. W. , Choy C. L.
Published: April 2004

Effect of proton irradiation on poly(vinylidene fluoride-trifluoroethylene) 56/44 mol % copolymer has been studied using a Mach–Zehnder heterodyne interferometer, polarization hysteresis loop and dielectric constant measurements. The electrical-field-induced strain response of the copolymer after proton bombardment follows a quadratic electrostriction relationship. The electrostrictive coefficient at 5 kHz is similar to that obtained with a bimorph-based strain sensor, but at a much lower electric field. The polarization hysteresis after proton irradiation is reduced considerably and the dielectric constant exhibits typical relaxor behavior, which is suggested as the cause of the observed results.
Correlated structural and optical characterization of ammonothermally grown bulk GaN
Authors: Bai J. , Dudley M. , Raghothamachar B. , Gouma P. , Skromme B. J. , Chen L. , Hartlieb P. J. , Michaels E. , Kolis J. W.
Published: April 2004

A series of ammonothermally grown bulk GaN crystals containing stacking faults has been characterized using structural [transmission electron microscopy (TEM) and synchrotron white-beam x-ray topography (SWBXT)] and optical [low-temperature photoluminescence (PL)] methods. Strong correlations are found between structural and optical properties. In particular, the occurrence of low-temperature PL peaks observed in the 3.30–3.45 eV range correlates with the observation of basal plane stacking faults by TEM (all of which were bounded by Shockley partial dislocations). In addition, the full width at half-maximum of the neutral donor-bound exciton PL peak correlates with the extent of mosaicity revealed on SWBXT Laue patterns recorded from the same crystals.
Double-biprism electron interferometry
Authors: Harada Ken , Tonomura Akira , Togawa Yoshihiko , Akashi Tetsuya , Matsuda Tsuyoshi
Published: April 2004

Electron holography based on two electron biprisms was developed. The upper biprism was installed just on the image plane of the objective lens, and the lower one was set between the crossover point and image plane of the magnifying lens. This system was able to control two important parameters of the hologram—fringe space and width of interference region—independently. The system enabled us to perform electron holography and interferometry more flexibly. We confirmed the good performance of the system and did preliminary applications using a 1-MV field-emission electron microscope.
Temperature dependence and bowing of the bandgap in ZnSe1−xOx
Authors: Polimeni A. , Capizzi M. , Nabetani Y. , Ito Y. , Okuno T. , Kato T. , Matsumoto T. , Hirai T.
Published: April 2004

We investigated the temperature dependence of the bandgap of untreated and hydrogen-irradiated ZnSe1−xOx (x=0.23%–0.90%) alloys by photoluminescence from T=10 K to room temperature. The variation of the bandgap energy with T is similar to that of ZnSe, and does not depend on the oxygen concentration. This indicates that oxygen incorporation in ZnSe does not lead to the carrier localization observed for nitrogen incorporation in GaAs and GaP. Correspondingly, no interaction between hydrogen and oxygen is observed in hydrogenated ZnSe1−xOx.
Effect of thermal processing on mobility in strained Si/strained Si1−yGey on relaxed Si1−xGex (x<y) virtual substrates
Authors: Jung Jongwan , Yu Shaofeng , Olubuyide Oluwamuyiwa Oluwagbemiga , Hoyt Judy L. , Antoniadis Dimitri A. , Lee Minjoo L. , Fitzgerald Eugene A.
Published: April 2004

Annealing effects on hole and electron mobility in dual-channel structures consisting of strained Si and Si1−yGey on relaxed Si1−xGex layers (x=0.3/y=0.6, and x=0.5/y=0.8) were studied. Hole mobility decreases sharply, but electron mobility is quite immune to annealing conditions of 800 °C, 30 min or 900 °C, 15 s. The hole mobility decrease is more severe in dual-channel structures with higher Ge contents. Hole mobility degradation is a direct result of Ge outdiffusion from the Si1−yGey layer, and the resulting decreased Ge content. Ge diffusion preferentially towards the Si1−xGex buffer layer, rather than the Si cap layer, is a reason that electron mobility is highly immune to such annealing.
Continuous-wave emission from midinfrared IV–VI vertical-cavity surface-emitting lasers
Authors: Fürst J. , Pascher H. , Schwarzl T. , Böberl M. , Springholz G. , Bauer G. , Heiss W.
Published: April 2004

Continuous-wave emission of two midinfrared PbSe vertical-cavity surface-emitting laser structures is presented. For both structures, optically pumped cw laser emission is observed at a wavelength of about 7.9 μm. Stimulated emission is found up to temperatures of 80 K. We achieved internal threshold pump intensities of down to 25 W/cm2, which is two orders of magnitude smaller than reported so far. The linewidth of the laser emission is only 18 μeV (0.9 nm) with a strong narrowing as compared to the linewidth of the cavity resonance. Continuous-wave output powers are up to 350 μW.
Semiconductor coupled-resonator optical waveguide laser
Authors: Mookherjea Shayan
Published: April 2004

The coupled-resonator optical waveguide (CROW) has been introduced recently as a single-mode waveguiding structure in which the group velocity of light is controllable, and nonlinear interactions between waves can be enhanced by orders of magnitude. A semiclassical description of lasing in CROWs is presented for typical semiconductor media such as GaAs. The threshold conditions for steady-state lasing are presented and evaluated numerically. The lasing threshold can be lowered by reducing the group velocity, thus the CROW laser is an alternative to photonic crystal lasers for realizing lower threshold lasers. Furthermore, the laser wavelength can be tuned with a fine resolution by changing the coupling between the constituent resonators.
Dislocation- and crystallographic-dependent photoelectrochemical wet etching of gallium nitride
Authors: Gao Y. , Craven M. D. , Speck J. S. , Den Baars S. P. , Hu E. L.
Published: April 2004

Polarity and dislocation dependence study of photoelectrochemical wet etching on GaN was carried out on lateral epitaxial overgrown nonpolar (112̄0)a-GaN/(11̄02)r-plane sapphire substrate. This LEO nonpolar GaN sample has low dislocation density Ga- and N-faces exposed horizontally in opposite directions, which can be exposed to identical etching conditions for both polarity and dislocation dependence study. It is observed that N-face GaN is essentially much chemically active than Ga-face GaN, which shows the hexagonal pyramids with {101̄1̄} facets on the etched N face. No obvious etching was observed on Ga face in the same etch condition. As for dislocation dependence, the “wing” (low dislocation density) region was etched faster than the “window” (high dislocation density) region. Smooth etched surfaces were formed with the (1̄1̄22̄) facet as an etch stop plane both on Ga and N-wing region.
Tandem dye-sensitized solar cell for improved power conversion efficiencies
Authors: Dürr M. , Bamedi A. , Yasuda A. , Nelles G.
Published: April 2004

The power conversion efficiency of dye-sensitized solar cells crucially depends on the ability of the sensitizer to absorb light in a broad range of the solar spectrum. In order to increase this range of absorption, a tandem structure with two different sensitizer dyes in two different compartments of the cell, respectively, was realized. Overall power conversion efficiencies as high as η=10.5% and short circuit current densities of JSC=21.1 mA/cm2 were achieved under air mass 1.5 illumination with red dye and black dye in the upper and lower compartment of the cell, respectively.
Superstructures of self-assembled cobalt nanocrystals
Authors: Gao Youhui , Bao Yuping , Beerman Michael , Yasuhara Akira , Shindo Daisuke , Krishnan Kannan M.
Published: April 2004

Uniform three-dimensional superstructures of spherical cobalt nanocrystals are produced by the interplay between dipolar interaction and applied magnetic field. An anomalous low-temperature magnetic behavior is observed, indicating that uncompensated surface spins become ordered below 10 K, as evidenced by the presence of two magnetic phases that superimpose in hysteresis loops as compared to measurements at 20 K. The approach discussed here provides a framework for applications such as high-performance mesomagnets, microelectronic and magnetic devices fabrication, and can be extended to other nanocomposite materials fabrication if cobalt particles can act as carriers for other nanoparticles.
Plasmon-enhancement of optical near-field of metal nanoaperture surface-emitting laser
Authors: Hashizume Jiro , Koyama Fumio
Published: April 2004

We propose a double-aperture structure to enhance the optical near-field in metal nanoaperture vertical-cavity surface-emitting lasers (VCSELs). We fabricated 850 nm GaAs VCSELs with subwavelength-sized double metal apertures closed to each other. The optical near-field localized in the metal apertures is strongly enhanced by the excitation of localized plasmon around the metal apertures. The far-field power radiated from the apertures is enhanced to be 16 times larger than that from a single aperture VCSEL. The peak power density is as large as 2.5 mW/μm2 with a spot size of 260 nm. The obtained power density is even larger than that of a conventional single-mode VCSEL without nanoapertures, which is very encouraging for use in high-density data storages.
Optical gain in porous silicon grains embedded in sol-gel derived SiO2 matrix under femtosecond excitation
Authors: Luterová K. , Dohnalová K. , Švrček V. , Pelant I. , Likforman J.-P. , Crégut O. , Gilliot P. , Hönerlage B.
Published: April 2004

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm−1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission.
Structure of 6H silicon carbide/silicon dioxide interface trapping defects
Authors: Meyer David J. , Bohna Nathaniel A. , Lenahan Patrick M. , Lelis Aivars J.
Published: April 2004

We utilize spin-dependent recombination (SDR) to observe deep level trap defects at or very near the interface of 6H silicon carbide and the SiO2 gate dielectric in SiC metal-oxide-semiconductor field effect transistors. The SDR response is strongly correlated to SiC/SiO2 interface recombination currents. The magnitude of the SDR response is correlated with processing-induced changes in interface trap density, an extremely strong indication that we are observing the dominating interface/near interface trapping defects. The SDR response is extremely large, as large as one part in 350.
Optically-induced magnetization of CdMnTe self-assembled quantum dots
Authors: Mackowski S. , Gurung T. , Nguyen T. A. , Jackson H. E. , Smith L. M. , Karczewski G. , Kossut J.
Published: April 2004

We demonstrate that resonant excitation of CdMnTe self-assembled quantum dots creates an ensemble of spin-polarized magnetic polarons at B=0 T. The strong spatial confinement characteristic of quantum dots significantly increases the stability of magnetic polarons so that the optically-induced spin alignment is observed for temperatures >120 K.
High-mobility field-effect transistors based on transition metal dichalcogenides
Authors: Podzorov V. , Gershenson M. E. , Kloc Ch. , Zeis R. , Bucher E.
Published: April 2004

We report on fabrication of field-effect transistors (FETs) based on transition metal dichalcogenides. The unique structure of single crystals of these layered inorganic semiconductors enables fabrication of FETs with intrinsically low field-effect threshold and high charge carrier mobility, comparable to that in the best single-crystal Si FETs (up to 500 cm2/V s for the p-type conductivity in the WSe2-based FETs at room temperature). These FETs demonstrate ambipolar operation. Owing to mechanical flexibility, they hold potential for applications in “flexible” electronics.