bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of quantum point contacts by engraving GaAs/AlGaAs heterostructures with a diamond tip
Authors: Regul J. , Keyser U. F. , Paesler M. , Hohls F. , Zeitler U. , Haug R. J. , Malavé A. , Oesterschulze E. , Reuter D. , Wieck A. D.
Published: September 2002

We use the all-diamond tip of an atomic force microscope for the direct engraving of high-quality quantum point contacts in GaAs/AlGaAs heterostructures. The processing time is shortened by two orders of magnitude compared to standard silicon tips. Together with a reduction of the line width to below 90 nm, the depletion length of insulating lines is reduced by a factor of two with the diamond probes. The such fabricated defect-free ballistic constrictions show well-resolved conductance plateaus and the 0.7 anomaly in electronic transport measurements.
Positively charged magnetoexciton transitions in a p-doped GaAs/Al0.45Ga0.55As single heterojunction
Authors: Kim Yongmin , Jiang H. W.
Published: September 2002

We report a photoluminescence-detected positively charged exciton (X+) transition from a p-doped GaAs single heterojunction in magnetic fields to 60 T. As the field increased above the quantum limit (10.8 T), the free carrier transition smoothly changes into a neutral exciton (X0) and positively charged exciton (X+) transitions. In comparison to its initial value ∼12 T, the binding energy of the X+ transition is doubled at 58 T. The variation of the X+ binding energy in magnetic fields has B  dependency due to an increasing Coulomb interaction within the excitonic complex.
Energy level engineering in InAs quantum dot nanostructures
Authors: Rebohle L. , Schrey F. F. , Hofer S. , Strasser G. , Unterrainer K.
Published: September 2002

We present an advanced method to tailor the optical and electrical properties of semiconductor quantum dot structures. By embedding vertically stacked quantum dots in a two-dimensional superlattice, the advantages of self-organized growth and of band structure engineering can be combined. The transition energies between the dot levels and the extended states of the superlattice can be adjusted by the period of the superlattice. We apply this scheme for photodetectors made of InAs quantum dots embedded in an AlAs/GaAs superlattice. The dark current of these devices is reduced by more than one order of magnitude compared to the devices without a superlattice.
Trap evaluations of metal/oxide/silicon field-effect transistors with high-k gate dielectric using charge pumping method
Authors: Lee Ga-Won , Lee Jae-Hee , Lee Hae-Wang , Park Myoung-Kyu , Kang Dae-Gwan , Youn Hee-Koo
Published: September 2002

The interface trap properties of metal/oxide/silicon field-effect transistors with high-k gate dielectrics are evaluated by the charge pumping method and related with electrical characteristics of the transistors. It is found that the gate leakage current is very sensitive to the interface traps by which the barrier height of Fowler–Nordheim tunneling is also influenced. The inversion layer mobility is greatly influenced by the charges in the gate insulator as well as by the interface traps. We propose a simple method to extract the geometrical mean value of capture cross sections of traps ranging from 7.03×10−21 to 2.22×10−22 cm2.
X-ray standing wave microscopy: Chemical microanalysis with atomic resolution
Authors: Drakopoulos Michael , Zegenhagen Jörg , Snigirev Anatoly , Snigireva Irina , Hauser Maik , Eberl Karl , Aristov Vitalii , Shabelnikov Leonid , Yunkin Vecheslav
Published: September 2002

We introduce a microprobe technique based on the x-ray standing wave method (XSW) demonstrating that structural analysis can be achieved with chemical sensitivity on a microscopic scale. We apply this XSW microscopy technique to study an epitaxially grown GaAs/Al0.1Ga0.9As/GaAs(001) heterostructure in cross section. We focus the x-ray beam by a refractive lens onto the cleaved sample and analyze the constituent elements within the 4 μm thick Al0.1Ga0.9As layer resolving the substitutional location of Al. The new micro-XSW technique will permit microscopic examinations of the structure of integrated semiconductor devices or microscopic crystalline grains with chemical sensitivity and structural resolution on the pm scale.
Effect of Hf metal predeposition on the properties of sputtered HfO2/Hf stacked gate dielectrics
Authors: Yamamoto Kazuhiko , Hayashi Shigenori , Kubota Masafumi , Niwa Masaaki
Published: September 2002

The electrical and physical properties of reactively sputtered HfO2 thin films on predeposited Hf metal layer were investigated. Compared with a conventional deposition process, i.e., direct deposition of HfO2 on a Si substrate, the HfO2/Hf stacked capacitor exhibits excellent electrical characteristics such as capacitance equivalent thickness (CET) as thin as 1.3 nm with lower leakage current of 2×10−5 (A/cm2) at −1 V gate bias. Based on the structural analysis as well as the electrical characteristics, the interfacial layer thickness is found to be controllable with minimum CET and higher permittivity by optimizing the thickness ratio between HfO2 and Hf. This improvement in electrical characteristics can be explained by the blocking of oxygen diffusion through HfO2 films into the Si substrate due to oxidation of the Hf metal layer itself.
Effect of MgZnO-layer capping on optical properties of ZnO epitaxial layers
Authors: Makino T. , Tamura K. , Chia C. H. , Segawa Y. , Kawasaki M. , Ohtomo A. , Koinuma H.
Published: September 2002

Photoluminescence (PL) and reflectivity spectra of ZnO epilayers capped with MgxZn1−xO layers (up to x=0.36) are reported. These capped films were epitaxially grown on lattice-matched ScAlMgO4 substrates by laser molecular-beam epitaxy. A photoluminescence spectrum from the ZnO layer taken at 5 K shows emission bands at 3.389, 3.376, and 3.362 eV. The two higher bands are due to A- and B-free exciton emissions and the lowest band is due to emission of a neutral-acceptor bound exciton (I6). The linewidth of the I6 emission in our uncapped sample (0.8 meV) is significantly smaller than that in the capped one (6 meV). This is probably due to strain applied across the ZnO layer because of the difference in lattice constant between the two layers. The spectral assignment of the free exciton emissions is strengthened by a comparison with a temperature-dependent PL study and a reflectivity study. With an increase in temperature, the intensity of the bound exciton emission line decreased drastically and became comparable to that of free exciton lines. Free exciton emissions are already detectable at the liquid He temperature for the capped samples, which is never achieved in the uncapped ones grown under identical conditions. This indicates the high degree of purity of the capped films.
Electric current rectification by an all-organic electrochemical device
Authors: Chen Miaoxiang , Nilsson David , Kugler Thomas , Berggren Magnus , Remonen Tommi
Published: September 2002

An all-organic printed electrochemical rectifier is reported. The device is based on a patterned layer of poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) that interfaces a patterned electrolyte top layer. Overlap between the electrolyte layer and the conducting polymer pattern results in the formation of two electrochemically active areas within the conducting polymer pattern. When bias voltage is applied across the conducting polymer pattern, the PEDOT in the negatively biased areas is reduced electrochemically, while the PEDOT in the positively biased area is further oxidized. Reducing PEDOT from its p-doped, pristine state to the neutral state results in a marked loss of electrical conductivity. Due to the unsymmetrical device geometry, the current through the device may be shut off for one polarity of applied bias voltage with an electrical current rectification ratio of 100 compared to the opposite polarity. The output characteristics of a corresponding half wave rectifier as well as those from a full wave bridge rectifier show stable performance at frequencies below 15 Hz.
Effect of competing energies on the transition noise of oriented magnetic media
Authors: Hee C. H. , Wang J. P.
Published: September 2002

There has been some disagreement in observation of the effect of oriented media on the recording performance from earlier and recent studies. In this letter, we have identified the reasons behind these disagreements by studying the effect of the energy ratio of local anisotropy (Ku) and long range magnetostatic interaction energies (Kd) (Q=Ku/Kd). The magnetization reversal mechanism is observed to be different in the recorded bit transition of different magnetic properties (Q) of the media. The results on the effect of Q and the linear density of various oriented media are presented and discussed in detail. It is observed that the transition for high Q media does not follow arctan transition form. Oriented high Q media show better recording performance than isotropic media, even at higher linear density.
Subcritical microwave streamer discharge at the surface of a polymer foil
Authors: Popović S. , Vušković L. , Esakov I. I. , Gratchev L. P. , Khodataev K. V.
Published: September 2002

An innovative type of subcritical streamer discharge, generated by bursts of focused microwave (MW) radiation in transverse electromagnetic mode with wavelength of 10 cm, pulse duration of 40 μs, and intensity of 5.6×104 W/cm2, was initiated at the surface of a polymer foil at 420 Torr in air. The MW field of a quasioptical beam was three times lower than the critical breakdown field. Exposure of a polymer foil to the discharge caused a transition from hydrophobicity to hydrophilicity.
Determination of thermal focal length and pumping radius in gain medium in laser-diode-pumped Nd:YVO4 lasers
Authors: Song Feng , Zhang Chaobo , Ding Xin , Xu Jingjun , Zhang Guangyin , Leigh Matthew , Peyghambarian Nasser
Published: September 2002

A method to determine the focal length of a thermal lens and pumping laser beam waist in the gain medium in laser-diode-pumped solid-state lasers is presented. This method, using resonator transform circle theory, is both simple and reliable. The measured focal length of the thermal lens is used to calculate the beam waist of pumping laser inside the gain medium. The effect of the thermal lens on the output power is also measured and analyzed.
Dielectric and ferroelectric anisotropy of intergrowth Bi4Ti3O12–PbBi4Ti4O15 single crystals
Authors: Goshima Yu , Noguchi Yuji , Miyayama Masaru
Published: September 2002

Intergrowth Bi4Ti3O12–PbBi4Ti4O15 single crystals were grown, and their dielectric and ferroelectric anisotropy were investigated along the a and c axes. Dielectric measurements revealed that the Curie temperature was 550 °C, which was lower than that of Bi4Ti3O12 (675 °C) and PbBi4Ti4O15 (570 °C). Remanent polarization was observed for intergrowth oxides not only along the a axis but also along the c axis, and the c-axis component is suggested to originate from Bi4Ti3O12 layers in the intergrowth structure.
Antiparallel ruthenium coupling in doped La1.2Sr1.8Mn2−xRuxO7
Authors: Weigand F. , Gold S. , Schmid A. , Geissler J. , Goering E. , Dörr K. , Krabbes G. , Ruck K.
Published: September 2002

X-ray magnetic circular dichroism measurements in the hard and soft x-ray region have been performed at Mn K and L2,3 edges of the ruthenium (with x=0.05, 0.1 and 0.5) doped Ruddlesden–Popper-phase La1.2Sr1.8Mn2−xRuxO7. In combination with magnetometry, clear evidence of unexpected antiferromagnetic sublattice coupling between manganese and ruthenium was found. A simple coupling model of Mn+3/Mn+4 and Ru+3/Ru+4 ions in the high-spin state is proposed which explains the antiparallel alignment and the technological interesting increase of the Curie temperature.
Improved brightness of 380 nm GaN light emitting diodes through intentional delay of the nucleation island coalescence
Authors: Koleske D. D. , Fischer A. J. , Allerman A. A. , Mitchell C. C. , Cross K. C. , Kurtz S. R. , Figiel J. J. , Fullmer K. W. , Breiland W. G.
Published: September 2002

Ultraviolet light emitting diodes (LEDs) have been grown using metalorganic vapor phase epitaxy, while monitoring the 550 nm reflected light intensity. During nucleation of GaN on sapphire, the transition from three-dimensional (3D) grain growth to two-dimensional (2D) coalesced growth was intentionally delayed in time by lowering the NH3 flow during the initial high temperature growth. Initially, when the reflectance signal is near zero, the GaN film is rough and composed of partly coalesced 3D grains. Eventually, the reflected light intensity recovers as the 2D morphology evolves. For 380 nm LEDs grown on 3D nucleation layers, we observe increased light output. For LEDs fabricated on GaN films with a longer recovery time an output power of 1.3 mW at 20 mA current was achieved.
Batch-fabricated spin-injection magnetic switches
Authors: Sun J. Z. , Monsma D. J. , Abraham D. W. , Rooks M. J. , Koch R. H.
Published: September 2002

A method is developed for the fabrication of sub-100 nm current-perpendicular spin-valve junctions with low contact resistance. The approach is to use a batch-fabricated trilayer template with the junction features defined by a metal stencil layer and an undercut in the insulator. The spin-valve thin film stack is deposited afterwards into the stencil, with the insulator undercut providing the necessary magnetic isolation. Using this approach, reproducible spin-current-induced magnetic switching is demonstrated for junctions down to 50 nm×100 nm in size.
Direct tunneling leakage current and scalability of alternative gate dielectrics
Authors: Yeo Yee-Chia , King Tsu-Jae , Hu Chenming
Published: September 2002

We explore the scaling limits of alternative gate dielectrics based on their direct tunneling characteristics and gate leakage requirements for future complementary metal–oxide–semiconductor technology generations. Important material parameters such as the tunneling effective mass are extracted for several promising high-κ gate dielectrics. We also introduce a figure of merit for comparing the relative advantages of gate dielectric candidates. Using an accurate direct tunneling gate current model and specifications from the International Technology Roadmap for Semiconductors, we provide guidelines for the selection of gate dielectrics to satisfy the off-state leakage current requirements of future high-performance and low power technologies.
Oxidation of metals at the chromium oxide interface
Authors: Cheng Ruihua , Borca C. N. , Pilet N. , Xu Bo , Yuan L. , Doudin B. , Liou S. H. , Dowben P. A.
Published: September 2002

Metal thin-film deposition, over the Cr2O3 surface of CrO2 thin-film substrates, exhibits a redox reaction at the interface. The transition metal forms an oxide in combination with the reduction of the near-surface chromium oxide to Cr2O3. The insulating barrier layer Cr2O3 increases with the formation of Pb3O4 in Pb/Cr2O3/CrO2 and CoO in Co/Cr2O3/CrO2 junctions, respectively.
Imaging and engineering the nanoscale-domain structure of a Sr0.61Ba0.39Nb2O6 crystal using a scanning force microscope
Authors: Terabe K. , Takekawa S. , Nakamura M. , Kitamura K. , Higuchi S. , Gotoh Y. , Gruverman A.
Published: September 2002

We have investigated the ferroelectric domain structure formed in a Sr0.61Ba0.39Nb2O6 single crystal by cooling the crystal through the Curie point. Imaging the etched surface structure using a scanning force microscope (SFM) in both the topographic mode and the piezoresponse mode revealed that a multidomain structure of nanoscale islandlike domains was formed. The islandlike domains could be inverted by applying an appropriate voltage using a conductive SFM tip. Furthermore, a nanoscale periodically inverted-domain structure was artificially fabricated using the crystal which underwent poling treatment.
Effect of charge exchange ions upon Langmuir probe current
Authors: Sternovsky Zoltan , Robertson Scott
Published: September 2002

The ion current to a Langmuir probe in low-density partially ionized plasma is enhanced by the collection of ions originating from charge–exchange collisions in the probe sheath. This enhancement is modeled by finding the volume integral of the charge–exchange rate in the sheath weighted by the probability that the slower charge–exchange ions are collected. Experiments in a hot-filament argon discharge show that the current enhancement and its dependence upon neutral pressure, plasma density, and probe bias voltage agree well with the model.
Synthesis and characterization of single-wall carbon nanotube–amorphous diamond thin-film composites
Authors: Schittenhelm H. , Geohegan D. B. , Jellison G. E. , Puretzky A. A. , Lance M. J. , Britt P. F.
Published: September 2002

Thin-film single-wall carbon nanotube (SWNT) composites synthesized by pulsed laser deposition (PLD) are reported. Ultrahard, transparent, pure-carbon, electrically insulating, amorphous diamond thin films were deposited by PLD as scratch-resistant, encapsulating matrices for disperse, electrically conductive mats of SWNT bundles. In situ resistance measurements of the mats during PLD, as well as ex situ Raman spectroscopy, current–voltage measurements, spectroscopic ellipsometry, and field-emission scanning electron microscopy, are used to understand the interaction between the SWNT and the highly energetic (∼100 eV) carbon species responsible for the formation of the amorphous diamond thin film. The results indicate that a large fraction of SWNT within the bundles survive the energetic bombardment from the PLD plume, preserving the metallic behavior of the interconnected nanotube mat, although with higher resistance. Amorphous diamond film thicknesses of only 50 nm protect the SWNT against wear, providing scratch hardness up to 25 GPa in an optically transmissive, all-carbon thin-film composite.
Cross-sectional imaging of pendeo-epitaxial GaN using continuous-wave two-photon microphotoluminescence
Authors: Schuck P. J. , Grober R. D. , Roskowski A. M. , Einfeldt S. , Davis R. F.
Published: September 2002

A technique utilizing continuous-wave two-photon absorption has been developed for optically sectioning and imaging deep into GaN structures. Imaging at depths greater than 20 μm below the surface of a coalesced pendeo-epitaxial GaN sample is demonstrated. Free and donor-bound excitonic emission in this sample appears to originate at the surface, acceptor-bound exciton transitions are strongest in the top bulk portion of the sample, and subgap luminescence is most intense deep in the sample. The depth resolution of the imaging system is measured to be 1.75 μm near the GaN surface.
Extraction of height probability density of random rough surfaces from the central δ-peak of angle-resolved light scattering using the optical inversion algorithm
Authors: Cheng Chuanfu , Qi Dongping , Teng Shuyun , Chen Jianping , Jia Tianqing , Xu Zhizhan
Published: September 2002

We developed a method for extracting the height probability density of random surfaces by light scattering. Theoretical results show that height probability function is the Fourier transform of the central δ-peak in the light waves of angle-resolved light scattering. Experimentally, we measure the δ-peak intensities at a different angle of incidence in a simple setup. We then reconstruct the phase distributions of the δ-peak versus the perpendicular component of the wave vector from the intensities using the Gerchberg–Saxton inversion algorithm, and then extract the height probability density of the samples. Two different types of samples are measured to verify the validity of the method, and the results are compared with those obtained by atomic force microscopy.
Epitaxial thin films of the giant-dielectric-constant material CaCu3Ti4O12 grown by pulsed-laser deposition
Authors: Si W. , Cruz E. M. , Johnson P. D. , Barnes P. W. , Woodward P. , Ramirez A. P.
Published: September 2002

Pulsed-laser deposition has been used to grow epitaxial thin films of the giant-dielectric-constant material CaCu3Ti4O12 on LaAlO3 and SrTiO3 substrates with or without various conducting buffer layers. The latter include YBa2Cu3O7, La1.85Sr0.15CuO4+δ, and LaNiO3. Above 100–150 K, the thin films have a temperature independent dielectric constant as do crystals. The value of the dielectric constant is of the order of 1500 over a wide temperature region, potentially making it a good candidate for many applications. The frequency dependence of its dielectric properties below 100–150 K indicates an activated relaxation process.
Single-molecule near-field optical energy transfer microscopy
Authors: Trabesinger W. , Kramer A. , Kreiter M. , Hecht B. , Wild U. P.
Published: September 2002

The nano-optical interaction between a sharp tip and a single dipolar emitter is investigated. Changes of the excited state lifetime and the fluorescence rate of single molecules are recorded simultaneously as a function of the tip position relative to the molecule. A subdiffraction-limited area of decreased fluorescence and shortened lifetime is observed for gold-coated Si3N4 tips. The results are discussed in terms of molecular fluorescence in a system of stratified media. The outlined methodology holds promise for applications in ultrahigh-resolution near-field optical imaging at the level of single fluorophores.
Growth and properties of (001) BiScO3–PbTiO3 epitaxial films
Authors: Yoshimura Takeshi , Trolier-McKinstry Susan
Published: September 2002

The ferroelectric and piezoelectric properties of (001) BiScO3–PbTiO3 epitaxial films were investigated. Epitaxial films 1 μm thick were grown by pulsed laser deposition using ceramic targets of (1−x)BiScO3–xPbTiO3 (x=0.5) at substrate temperatures of 700 °C on (100) SrRuO3/(100)LaAlO3 substrates. Well-saturated hysteresis loops with a remanent polarization of 36 μC/cm2 were observed. The films had a room temperature dielectric constant of 800, tan δ=0.09 and a maximum permittivity of 3600 at 460 °C. The room temperature e31f piezoelectric coefficient was −9 C/m2. The piezoelectric properties obtained were comparable to those of Pb(Zr,Ti)O3 or relaxor ferroelectric-PbTiO3 films.
Mechanism for grain size softening in nanocrystalline Zn
Authors: Conrad Hans , Narayan J.
Published: September 2002

A technique (laser deposition of Zn/W multilayers) for producing stable nanocrystalline (nc) Zn films free of undesirable imperfections down to a grain size of d=6 nm is presented. These films exhibited grain size softening (inverse Hall–Petch effect) for d⩽11 nm, providing support that this can be a real phenomenon in nc solids. The mechanism responsible for the softening was concluded to be thermally activated grain boundary shear.
Wide-field, real-time depth-resolved imaging using structured illumination with photorefractive holography
Authors: Ansari Z. , Gu Y. , Siegel J. , Jones R. , French P. M. W. , Nolte D. D. , Melloch M. R.
Published: September 2002

We have demonstrated real-time, wide-field depth-resolved imaging by combining the techniques of structured illumination and photorefractive holography. These proof of principle experiments illustrate the potential for providing real-time three-dimensional photorefractive holographic imaging with both reflected light and fluorescence.
Microstructural properties of Eu-doped GaN luminescent powders
Authors: Contreras O. , Srinivasan S. , Ponce F. A. , Hirata G. A. , Ramos F. , McKittrick J.
Published: September 2002

GaN powders doped with europium have been prepared using Eu and Ga nitrates and N2H4 as reactants. The resulting particles have dimensions ranging from 0.5 to 1.0 μm. The crystalline structure was studied by transmission electron microscopy, and it consisted of single crystals with a hexagonal (wurtzite) structure containing small cubic domains (zinc blende) and a high density of stacking faults, all aligned along the [0001] and 〈111〉 directions, respectively. Cathodoluminescence measurements show strong light emission in the red region. This luminescence corresponds to transitions of Eu with the strongest emission in the 611 nm line, which is associated to the Eu3+ 4f transition from5D0 to7F2. These results demonstrate the feasibility of GaN:RE powders for luminescent applications.
Vibrational mode analysis of β-phase poly(vinylidene fluoride)
Authors: Li J. C. , Wang C. L. , Zhong W. L. , Zhang P. L. , Wang Q. H. , Webb J. F.
Published: September 2002

The infrared spectrum of β-phase poly(vinylidene fluoride) (PVDF) film (30 μm) in the region from 4000 to 400 cm−1 has been measured. The vibrational frequencies of all-trans chains of PVDF have been calculated from first principles based on a restricted Hartree–Fock self-consistent field method. The calculated result is in good agreement with the experiment. All the vibrational modes were divided into three types, only one of them which is in the region from 1330 to 1020 cm−1, corresponds to a changing dipole moment, and may be responsible for the spontaneous polarization.
Near-field photoluminescence imaging of single semiconductor quantum constituents with a spatial resolution of 30 nm
Authors: Matsuda K. , Saiki T. , Nomura S. , Mihara M. , Aoyagi Y.
Published: September 2002

High-resolution photoluminescence (PL) imaging of semiconductor quantum dots (QDs) was demonstrated using a low-temperature near-field scanning optical microscope. We systematically evaluated the spatial resolution for various fiber probes with different aperture diameters ranging from 30 to 135 nm. We achieved a spatial resolution of 30 nm (∼λ/30:λ=930 nm) in the PL imaging of self-assembled InAs QDs due to both improvement in probe preparation and optimization of the sample structure. The spatial resolution obtained in this study is on the scale of semiconductor quantum constituents and will make it possible to map out and manipulate the wave function in quantum-confined systems.
Growth of SiCAlN on Si(111) via a crystalline oxide interface
Authors: Tolle John , Roucka R. , Crozier P. A. , Chizmeshya A. V. G , Tsong I. S. T. , Kouvetakis J.
Published: September 2002

Growth of single-phase SiCAlN epitaxial films with the 2H–wurtzite structure is conducted directly on Si(111) despite the structural differences and large lattice mismatch (19%) between the two materials. Commensurate heteroepitaxy is facilitated by the conversion of native and thermally grown SiO2 layers on Si(111) into crystalline oxides by in situ reactions of the layers with Al atoms and the H3SiCN precursor, forming coherent interfaces with the Si substrate and the film. High-resolution transmission electron microscopy and electron energy-loss spectroscopy show that the amorphous SiO2 films are entirely transformed into a crystalline Si–Al–O–N framework in registry with the Si(111) surface. This crystalline interface acts as a template for nucleation and growth of epitaxial SiCAlN. Integration of wide-band-gap semiconductors with Si is readily achieved by this process.
Laser-induced lateral epitaxy in fully depleted silicon-on-insulator junctions
Authors: Dezfulian Kevin K. , Krusius J. Peter , Thompson Michael O. , Talwar Somit
Published: September 2002

Junction formation by laser-induced lateral epitaxy was studied on a fully depleted silicon-on-insulator substrate (25–30 nm Si on 375 nm silicon dioxide). Selective laser melting of amorphous films with a 35 ns 308 nm XeCl laser pulse was characterized in situ using transient conduction and optical reflectance techniques. Lateral epitaxy from a channel edge was observed for 146 nm after a 300 mJ/cm2 irradiation. The initial 28 nm of epitaxy was nearly defect free, followed by an increasing density of twins and ultimately terminating in an amorphous quench. The microstructure is discussed as a lateral equivalent of laser-induced amorphization of bulk Si.
Truly random number generator based on turbulent electroconvection
Authors: Gleeson J. T.
Published: September 2002

We report an approach for generating truly random digital numbers. This approach is based on probing the chaotic and unpredictable fluctuations that take place in a nematic liquid crystal when it is undergoing turbulent, electrically induced, convective flow. This technique generates a stream of numbers that passes standard benchmark tests for randomness. A device utilizing this technique is not only inexpensive to construct, but also tamper proof. We discuss various strategies for improving the operating characteristics, primarily the bit generation rate, of a device constructed on this principle.
Separation of reflected shock waves due to the secondary interaction with vortices: Another mechanism of sound generation
Authors: Inoue Osamu , Takahashi Tomoyuki , Hatakeyama Nozomu
Published: September 2002

The development of a flow field and the generation of sound due to the interaction between a shock wave and two vortices are simulated numerically. The two vortices are initially set either in tandem or parallel to the incident shock wave. The two-dimensional, unsteady, compressible Navier–Stokes equations are solved by a finite difference method. The results show that reflected shock waves, which are generated by the interaction between the incident shock wave and a vortex, interact with the other vortex and produce new acoustic waves, in addition to the waves observed in a single vortex case. For sufficiently strong vortices, the waves thus produced interact with the vortices again and produce further acoustic waves. The results suggest that separation of reflected shock waves due to the secondary interaction with vortices may be one of the key mechanisms in the generation of acoustic waves in supersonic turbulent flows.
Large threshold hysteresis in a narrow AlGaAs/GaAs channel with embedded quantum dots
Authors: Schliemann A. , Worschech L. , Reitzenstein S. , Kaiser S. , Forchel A.
Published: September 2002

Self-assembled InGaAs quantum dots have been grown by molecular-beam epitaxy in the spacer of AlGaAs/GaAs modulation-doped heterostructures. The wafer has been further processed by electron-beam lithography and etching techniques in order to realize 70 nm wide channels controlled by lateral side gates. It is found that the drain current threshold differs by up to 2 V with respect to down and up sweeps of the gate voltage. The large hysteresis is attributed to charging and discharging of the quantum dots in the spacer and persists up to 260 K.
Tunnel barrier formation using argon-ion irradiation and single quantum dots in multiwall carbon nanotubes
Authors: Suzuki M. , Ishibashi K. , Toratani K. , Tsuya D. , Aoyagi Y.
Published: September 2002

Argon-ion irradiation has been used to form a tunnel barrier in multiwall carbon nanotubes and single quantum dots have been fabricated with this technique. The periodic Coulomb diamonds have been shown at 10 K and the periodic Coulomb oscillations have been observed up to around 80 K.
Tunable remanent state resonance frequency in arrays of magnetic nanowires
Authors: Encinas Armando , Demand Marc , Vila Laurent , Piraux Luc , Huynen Isabelle
Published: September 2002

The zero-field microwave absorption, or natural ferromagnetic resonance, spectra in arrays of electrodeposited magnetic nanowires is studied as a function of the saturation magnetization of NiCu, NiFe, CoNiFe, and CoFe alloys of several compositions. Measurements show that due to the shape anisotropy, these systems present strong absorption peaks in the absence of an applied magnetic field in the GHz range due to the ferromagnetic resonance. Furthermore, the zero-field resonance frequency is observed to be independent of the wire diameter and density as well as the magnetic history and its value depends only on the material, through the saturation magnetization and the gyromagnetic factor. It is shown that, using different electrolytic solutions and depositing at different electrostatic potentials, the alloy composition can be varied and the remanent state resonance frequency can be tailored quasicontinuously between 4 and 31 GHz.
Optical loss mechanisms in GeSiON planar waveguides
Authors: Osinsky A. V. , Bellman R. A. , Akwani I. A. , Sachenik P. A. , Logunov S. L. , McCamy J. W.
Published: September 2002

A double frequency overtone of the fundamental N–H bond related vibration was found to dominate the optical loss in the 1.5–1.6 μm range in high index GeSiON-based planar waveguides deposited by plasma-enhanced chemical vapor deposition using hydrogen-based precursors. The fundamental N–H bond-related absorption and its double frequency overtone were measured to be ∼560 cm−1 and 1.8 cm−1, respectively, resulting in their ratio of 310±10. The optical loss, extracted from analysis of the N–H-related absorption is consistent with the directly measured optical loss in the waveguide structures. It was shown that using the deuterated precursors results in incorporation of N–D bonds in the GeSiON films. The N–H bond absorption band was eliminated in the deuterated films, leading to low optical losses in the communication band.
Lattice contraction in free-standing CdSe nanocrystals
Authors: Zhang Jia-Yu , Wang Xiao-Yong , Xiao Min , Qu L. , Peng X.
Published: September 2002

Colloidal CdSe nanocrystals (NCs) with high photoluminescent quantum yield are fabricated by a method adopting a large initial Se:Cd ratio of the precursors in the solution. X-ray diffraction and Raman measurements confirm that there exists lattice contraction in the free-standing wurtzite CdSe NCs. The lattice contraction indicates the occurrence of surface optimization/reconstruction during the growth, which results in the high quantum yield of the obtained CdSe NCs.
Determination of the microwave permittivities of nematic liquid crystals using a single-metallic slit technique
Authors: Yang Fuzi , Sambles J. R.
Published: September 2002

A technique is presented for quantifying the microwave permittivities of small quantities of material. Here a 75 μm gap between two 3 mm thick aluminum plates is filled with a nematic liquid crystal. This filled slit is inserted in an absorber aperture. A set of transmission peaks are recorded as a function of incident wavelength for microwaves polarized with their electric field perpendicular to the slit direction. When a voltage is applied between the two plates, the liquid crystal realigns and the shift of the resonant peaks gives the anisotropic permittivities of the nematic material in the microwave region.
Piezoelectric displacement sensing with a single-electron transistor
Authors: Knobel R. , Cleland A. N.
Published: September 2002

We propose a displacement sensing scheme for rf mechanical resonators made from GaAs, based on detecting the piezoelectrically induced charge. By using a single-electron transistor to detect the charge, we calculate that a significantly higher displacement sensitivity can be achieved than by using capacitive displacement sensing, primarily due to the strong piezoelectric coupling strength. We estimate a displacement sensitivity of order 10−17 m/Hz1/2 for a 1 GHz GaAs resonator. Our model solves the coupled electromechanical response self-consistently, including the effects of both dissipative and reactive electronic circuit elements on the resonator behavior.
Growth and characterization of single-crystal GaN nanorods by hydride vapor phase epitaxy
Authors: Kim Hwa-Mok , Kim D. S. , Kim D. Y. , Kang T. W. , Cho Yong-Hoon , Chung K. S.
Published: September 2002

Single-crystalline GaN nanorods are formed on a sapphire substrate by hydride vapor phase epitaxy (HVPE). Their structural and optical properties are investigated by x-ray diffraction, scanning and transmission electron microscopy, and cathodoluminescence (CL) techniques. The high density of straight and well-aligned nanorods with a diameter of 80–120 nm formed uniformly over the entire 2 in. sapphire substrate. The x-ray diffraction patterns and transmission electron microscopic images indicate that the formed GaN nanorods are a pure single crystal and preferentially oriented in the c-axis direction. We observed a higher CL peak position of individual GaN nanorods than that of bulk GaN as well as a blueshift of CL peak position with decreasing the diameter of GaN nanorods, which are attributed to quantum confinement effect in one-dimensional GaN nanorods. We demonstrate that the well-aligned, single-crystalline GaN nanorods with high density, high crystal quality, and good spatial uniformity are formed by the HVPE method.
Switching times in electric-field-tunable GaAs/AlAs heterostructures
Authors: Ribeiro F. J. , Capaz R. B. , Koiller Belita
Published: September 2002

Optical experiments on electric-field-tunable AlAs/GaAs heterostructures demonstrate that the optical nature of these structures can be directly controlled by an applied electric field. We present calculations performed within the tight-binding supercell formalism, taking into account the effects of the heterostructure geometry and of the external uniform electric field. We describe the temporal and spatial evolution of the relevant electronic states. The electric-field intensity needed to cause the indirect–direct transition is obtained in good agreement with the experimental results for comparable heterostructure geometry. We present simple and reliable expressions to estimate the switching time scales involved in such processes.
Bright electroluminescence from a conjugated dendrimer
Authors: Ma Dongge , Lupton J. M. , Samuel I. D. W. , Lo Shi-Chun , Burn P. L.
Published: September 2002

Photoluminescence and electroluminescence (EL) from a conjugated dendrimer consisting of three distyrylbenzene units linked by a central nitrogen atom as core and meta-linked biphenyl units as dendrons were investigated. The conjugated dendrimer emits green light and shows photoluminescence quantum efficiency of 9%. Bright electroluminescence was realized by using bilayer devices with blurred interface, which were fabricated by sequentially spin coating a neat dendrimer and a dendrimer doped with 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD). The devices have the following structure: indium tin oxide/3,4-polyethylenedioxythiothene-polystyrenesulfonate/dendrimer/ dendrimer:PBD/Al. By optimizing the concentration of PBD, the maximum brightness and EL quantum efficiency reach 4100 cd/m2 and 0.17%, respectively. This is the best result reported so far on organic light-emitting diodes using dendrimer as an active material with an Al cathode.
Dynamic higher-order correction of acoustic aberration due to material microstructure
Authors: Sharples S. D , Clark M. , Somekh M. G.
Published: September 2002

Material microstructure, such as grains in metals, can perturb ultrasound as it propagates through—or on the surface of—the material. This acoustic aberration affects the accuracy and reliability of ultrasound measurements and is a fundamental limit to resolution for many materials. Using an all-optical approach to generation and detection of surface acoustic waves, we detect the acoustic wave front aberrations, and correct for them by calculating a different generation profile, which is imaged onto the material using a spatial light modulator.
Superconducting properties of MgB2 thin films prepared on flexible plastic substrates
Authors: Kúš Peter , Plecenik Andrej , Satrapinsky Leonid , Xu Ying , Sobolewski Roman
Published: September 2002

Superconducting MgB2 thin films were prepared on 50-μm-thick, flexible polyamide Kapton-E foils by vacuum co-deposition of Mg and B precursors with nominal thickness of about 100 nm and a subsequent ex situ rapid annealing process in an Ar or vacuum atmosphere. In the optimal annealing process, the Mg–B films were heated to approximately 600 °C, but, at the same time, the backside of the structures was attached to a water-cooled radiator to avoid overheating of the plastic substrate. The resulting MgB2 films were amorphous with the onset of the superconducting transition at Tc,on≈29 K and a transition width of approximately 3 K. The critical current density was >7×105 A/cm2 at 4.2 K, and its temperature dependence indicated a granular film composition with a network of intergranular weak links. The films could be deposited on large-area foils (up to 400 cm2) and, after processing, cut into any shape (e.g., strips) with scissors, or bent multiple times, without any observed degradation of their superconducting properties.
High-quality SiO2 film formation by highly concentrated ozone gas at below 600 °C
Authors: Nishiguchi Tetsuya , Nonaka Hidehiko , Ichimura Shingo , Morikawa Yoshiki , Kekura Mitsuru , Miyamoto Masaharu
Published: September 2002

Highly concentrated (>93 vol %) ozone (O3) gas was used to oxidize silicon for obtaining high-quality SiO2 film at low temperature. Compared to O2 oxidation, more than 500 °C lower temperature oxidation (i.e., from 830 to 330 °C) has been enabled for achieving the same SiO2 growth rate. A 6 nm SiO2 film, for example, could be grown at 600 °C within 3 min at 900 Pa O3 atmosphere. The temperature dependence of the oxidation rate is relatively low, giving an activation energy for the parabolic rate constant of 0.32 eV. Furthermore, a 400 °C grown SiO2 film was found to have satisfactory electrical properties with a small interface trap density (5×1010 cm−2/eV) and large breakdown field (14 MV/cm).
Utilization of sputter depth profiling for the determination of band alignment at polycrystalline CdTe/CdS heterointerfaces
Authors: Fritsche J. , Schulmeyer T. , Kraft D. , Thißen A. , Klein A. , Jaegermann W.
Published: September 2002

The band alignment at polycrystalline CdS/CdTe heterointerfaces for thin-film solar cells is determined by photoelectron spectroscopy from stepwise CdTe deposition on polycrystalline CdS substrates and from subsequent sputter depth profiling. Identical values of 0.94±0.05 eV for the valence band offset are obtained.
Detection of a micron-sized magnetic sphere using a ring-shaped anisotropic magnetoresistance-based sensor: A model for a magnetoresistance-based biosensor
Authors: Miller M. M. , Prinz G. A. , Cheng S.-F. , Bounnak S.
Published: September 2002

We have fabricated micron-sized NiFe ring-shaped sensors that show localized detection of the radial component of the dipolar fringing field from a single, partially magnetized, micron-sized NiFe sphere. Specifically, the anisotropic magnetoresistance response to this fringing field is strongly peaked when the sphere is directly above the center of the ring and rapidly decreases to zero when the sphere is outside the ring. Such a device is a model system for a proposed biosensor array architecture that could operate similarly to high-density random access computer memory.
Balanced electronic detection of displacement in nanoelectromechanical systems
Authors: Ekinci K. L. , Yang Y. T. , Huang X. M. H. , Roukes M. L.
Published: September 2002

We describe a broadband radio frequency balanced bridge technique for electronic detection of displacement in nanoelectromechanical systems (NEMS). With its two-port actuation-detection configuration, this approach generates a background-nulled electromotive force in a dc magnetic field that is proportional to the displacement of the NEMS resonator. We demonstrate the effectiveness of the technique by detecting small impedance changes originating from NEMS electromechanical resonances that are accompanied by large static background impedances at very high frequencies. This technique allows the study of important experimental systems such as doped semiconductor NEMS and may provide benefits to other high frequency displacement transduction circuits.
Synthesis, Raman scattering and defects of β-Ga2O3 nanorods
Authors: Gao Y. H. , Bando Y. , Sato T. , Zhang Y. F. , Gao X. Q.
Published: September 2002

Large yield of β-Ga2O3 nanorods with metal Ga tip were efficiently synthesized. They were deposited on surface of amorphous C fibers by decomposition of Ga2O vapor at around 1000 °C, where Ga2O vapor was produced at 1360 °C by a reaction between pure Ga2O3 and active carbon powders. The nanorods had diameters ranging from 10 to 100 nm and lengths of up to several tens micrometers. Twins and edge dislocations having a Burgers vector of 0.0859 Å [2.66, 3.66, 1̄] existed in the nanorods. A redshift of 4–23 cm−1 was found in the Raman scattering spectrum of nanorods compared with that of a pure Ga2O3 powder. This phenomenon was explained qualitatively in terms of the defects in the nanorods.
Structural studies of L10 FePt nanoparticles
Authors: Klemmer T. J. , Shukla N. , Liu C. , Wu X. W. , Svedberg E. B. , Mryasov O. , Chantrell R. W. , Weller D. , Tanase M. , Laughlin D. E.
Published: September 2002

We have studied the lattice parameter changes of L10 FePt nanoparticles annealed to near equilibrium as a function of composition by x-ray diffraction. We have found that the (111) diffraction peak shifts linearly with composition, however, the c parameter mostly changes in the Pt rich compositions and the a parameter mostly changes in the Fe rich compositions with respect to the equiatomic composition. This causes the tetragonality of the L10 structure to be maximized near the Fe 50%/Pt 50% composition. The magnetic properties were measured at room temperature and at 5 K and are correlated to the structural changes occurring as a function of composition.
Single spin states in a defect center resolved by optical spectroscopy
Authors: Jelezko F. , Popa I. , Gruber A. , Tietz C. , Wrachtrup J. , Nizovtsev A. , Kilin S.
Published: September 2002

Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.
Approach for enhanced polarization of polycrystalline bismuth titanate films by Nd3+/V5+ cosubstitution
Authors: Uchida Hiroshi , Yoshikawa Hiroki , Okada Isao , Matsuda Hirofumi , Iijima Takashi , Watanabe Takayuki , Kojima Takashi , Funakubo Hiroshi
Published: September 2002

Thin films of Nd3+-substituted bismuth titanate, (Bi4.00−y,Ndy)Ti3.00O12 (BNT), Nd3+/V5+-cosubstituted bismuth titanate, (Bi4.00−y,Ndy)(Ti3.00−xVx)O12 (BNTV), and La3+-substituted bismuth titanate, (Bi3.25,La0.75)Ti3.00O12 (BLT) were fabricated on the (111)Pt/Ti/SiO2/(100)Si substrates by a chemical solution deposition technique. These films possessed random-oriented polycrystalline structure. The BNT film had larger remnant polarization (Pr) than the BLT film; Pr and coercive field (Ec) of the BNT film with y=0.50 were 32 μC/cm2 and 126 kV/cm, respectively. Furthermore, V5+ substitution improved the Pr value of the BNT film up to 37 μC/cm2 (BNTV film; y=0.50, x=0.02), while the BNTV film had an Ec value of approximately 119 kV/cm which was similar to that of the BNT film. Ferroelectric properties of the Pb-free polycrystalline BNT and BNTV films are comparable with those of conventional Pb-based ferroelectric films like a lead zirconate titanate.
Growth of suspended carbon nanotube networks on 100-nm-scale silicon pillars
Authors: Homma Yoshikazu , Kobayashi Yoshihiro , Ogino Toshio , Yamashita Takayuki
Published: September 2002

We investigated carbon nanotube growth by means of methane chemical vapor deposition on ultrafine silicon patterns prepared by synchrotron-radiation lithography. Grown nanotubes formed suspended bridges between pillars when pillar spacing was comparable to pillar height. Network-like interconnections were obtained on pillar arrays. Nearest-neighbor bridging accounted for more than 80% of all the bridging nanotubes. The self-directed growth between neighboring pillars may be explained by the swing of the nanotube cantilever which contacts a catalyst particle in liquid phase as the nanotube grows. These results confirm the possibility of self-assembled wiring of nanostructures.