bluuebunny/crossref_metadata_2025 huggingface dataset:
Suppression of current through an Esaki–Tsu GaAs/AlAs superlattice by millimeter wave irradiation
Authors: Schomburg E. , Ignatov A. A. , Grenzer J. , Renk K. F. , Pavel’ev D. G. , Koschurinov Yu. , Melzer B. Ja. , Ivanov S. , Schaposchnikov S. , Kop’ev P. S.
Published: July 2002

We report the observation of suppression of the dc current through an Esaki–Tsu GaAs/AlAs superlattice (that shows a negative differential conductance due to Bloch oscillations of miniband electrons) caused by irradiation with millimeter wave radiation (frequency 78 GHz). A theoretical analysis of the dc current indicated that the high-frequency electric current followed the high-frequency field with a response time smaller than the period (∼10−11 s) of the field. Our experiment, with the superlattice at room temperature, demonstrates that the Esaki–Tsu superlattice is suitable for ultrafast millimeter wave detection and other high-frequency applications.
Quasi-phase matched surface emitting second harmonic generation in poled polymer waveguides
Authors: Otomo A. , Stegeman G. I. , Horsthuis W. H. G. , Möhlmann G. R.
Published: July 2002

Quasi-phase matched (QPM) surface emitting second harmonic generation was demonstrated with nonlinear/linear multilayer waveguides in poled polymer based devices. The nonlinear/linear multilayer film was fabricated with a 4-dimethylamino-4′-nitrostilbene (DANS) side chain polymer and a cross-linkable clear polymer. Large efficiency improvement was observed with a combination of QPM and strong field parallel poling.
Synthesis of single domain strontium ferrite powder by pulsed laser ablation
Authors: Nawathey-Dikshit Rashmi , Shinde S. R. , Ogale S. B. , Kulkarni S. D. , Sainkar S. R. , Date S. K.
Published: July 2002

Strontium hexaferrite nanoparticles have been successfully prepared from a sintered SrFe12O19 target, using pulsed excimer laser ablation and cold condensation technique. The as-condensed powders do not exhibit the same magnetic properties as the target material. However, annealing at a high temperature of 850 °C for 3 h in air followed by fast cooling yields very high coercivity of 6665 Oe, exhibiting presence of single domain particles. Vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), and x-ray diffraction (XRD) techniques are used to study the magnetism, microstructure, and phase formation in the powders, respectively.
Criterion for selecting phonons and local vibrations involved in the nonradiative relaxation of photoexcited Cr4+ ions in forsterite
Authors: Demos S. G. , Calistru Dana M. , Alfano R. R.
Published: July 2002

The energy of Cr4+ local modes and the energy and dynamics of Mg2SiO4 phonon modes involved in the nonradiative relaxation of photoexcited impurity Cr4+ ions in forsterite crystal were measured. The experimental results suggest that resonance in energy between local vibrations of the impurity ion and lattice (phonon) modes of the host crystal plays a crucial role in selecting the phonons which participate in the nonradiative relaxation of the photoexcited ions in crystals.
Electron impact vibrational excitation cross sections of SiF4
Authors: Nagpal R. , Garscadden A. , Clark J. D.
Published: July 2002

Electron drift velocities in mixtures of SiF4 and Ar have been measured using a pulsed-Townsend type drift tube. A set of vibrational excitation cross sections of electron scattering in SiF4 has been subsequently determined by the swarm analyses of measured transport data in highly dilute SiF4−Ar mixtures. The derived cross sections are consistent with the electron transport properties over an order of magnitude in SiF4 concentration in gas mixtures, thus providing evidence that the main features of their near threshold behavior, and of their absolute magnitude have been captured.
Lattice matching, phase matching, and constituent consistency of optical waveguide Ba2NaNb5O15 films on KTiOPO4
Authors: Yin Z. , Zhang P. , Zhang M. S.
Published: July 2002

Optical waveguide films of barium sodium niobate (BSN) grown on potassium titanyl phosphate (KTP) substrate by pulsed laser deposition (PLD) with a lattice geometry BSN(110)//KTP(001) were studied. A small lattice mismatch of 2.9% under such geometry was obtained, and excellent parallelism is established between the as-grown BSN(110) film and the KTP(001) substrate. Effective refractive index and effective thickness of the BSN films caused by Goos–Hanchen shifts are examined. A phase matching relation of k(1)–2k(0)=0 is obtained from the dispersion curves of effective refractive index of the BSN films versus incident wavelength for various orders of the guided wave modes (m=0, 1, 2, 3, and 4). The measured constituent ratio 2.0:0.82:5.14 of Ba:Na:Nb in the BSN film deviates slightly from the stoichiometric ratio 2:1:5, which is due to longer flight time and a comparatively higher evaporation pressure for sodium.
Numerical analysis of 1.54 μm double-fused vertical-cavity lasers operating continuous-wave up to 33 °C
Authors: Piprek Joachim , Babic Dubravko I. , Bowers John E.
Published: July 2002

The first vertical-cavity surface-emitting laser diodes operating continuous-wave at room temperature at 1.54 μm emission wavelength are analyzed using a comprehensive numerical simulation procedure. These lasers employ strain-compensated InGaAsP multi-quantum wells sandwiched between GaAs/AlGaAs mirrors that are double-fused on the InP spacer layers at both sides. The model includes finite element electro-thermal simulation, transfer-matrix optical analysis, and k⋅p band structure calculations. Internal laser parameters are obtained by fitting experimental data at different heat sink temperatures. Intervalence band absorption is found to be the dominating loss mechanism that restricts cw operation.
Polarization dependence of the electroabsorption in low-temperature grown GaAs for above band-gap energies
Authors: Ruff M. , Streb D. , Dankowski S. U. , Tautz S. , Kiesel P. , Knüpfer B. , Kneissl M. , Linder N. , Döhler G. H. , Keil U. D.
Published: July 2002

We have measured the electroabsorption in low-temperature grown GaAs by performing room-temperature transmission experiments in the spectral range from 1.3 to 1.9 eV for different electric fields induced by a voltage applied to a metal-semiconductor-metal structure. The devices were separated from the substrate by using an epitaxial liftoff technique. Therefore, we have been able to observe the electro-optic effect at the fundamental band gap as well as at the split-off band edge. The absorption is clearly polarization dependent at the fundamental band gap but only weakly at the split-off band gap, in agreement with the theory of the Franz–Keldysh effect.
Structural and photocurrent–voltage characteristics of tungsten oxide thin films on p -GaAs
Authors: Yoon Ki Hyun , Lee Jeong Won , Cho Yong Soo , Kang Dong Heon
Published: July 2002

An n-type WO3 thin film on a p-type GaAs substrate has been investigated in terms of its structural, electrical, and photocurrent–voltage characteristics. Crystallization of the monoclinic phases of WO3 occurred at 400 °C as identified by a glancing angle x-ray diffractometer. Above 400 °C, small traces of gallium oxide (Ga2O3)were observed along with higher intensities of the monoclinic peaks. The formation of Ga2O3 was confirmed by a depth profile analysis using an Auger electron spectrometer. Electrical resistivity measured by the van der Pauw method was influenced by the crystalline nature and the interfacial states induced by the diffused atoms from each side of WO3 and GaAs. A high photocurrent density of 7.5 mA/ cm2 at −1.0 V (vs SCE) appeared at the crystalline WO3 on p-type GaAs at 400 °C resulting from effective carrier movement through the interfaces.
Formation of single tips of oxidation-sharpened Si
Authors: Zhang Yongxia , Zhang Yanwei , Sriram T. S. , Marcus R. B.
Published: July 2002

Atomically sharp silicon tips for scanning probe microscope (SPM) and field emitters are typically fabricated by wet or dry etching followed by oxidation sharpening. However, multiple tips instead of the desired single tips often form after the oxidation sharpening step. We analyzed the mechanisms likely to be responsible for forming multiple tips and found that the likely mechanism involves the presence of sharp corners before oxidation sharpening. This analysis led to the development of a processing scheme that eliminates the sharp corners and produces only single tips of oxidation-sharpened silicon.
Measuring Ge segregation by real-time stress monitoring during Si1−xGex molecular beam epitaxy
Authors: Floro J. A. , Chason E.
Published: July 2002

Real-time stress measurements during Si1−xGex/Si molecular beam epitaxy are used to dynamically monitor Ge surface segregation. Segregation during alloy growth produces a change in the surface stress that offsets the coherency stress in the pseudomorphic film. We outline a method for analyzing the stress evolution kinetics to determine the alloy composition profile resulting from segregation.
Single-sided output Sn/Er/Yb distributed feedback fiber laser
Authors: Loh W. H. , Dong L. , Caplen J. E.
Published: July 2002

A distributed feedback laser based on Sn/Er/Yb fiber is demonstrated. Unlike previous Er/Yb DFB lasers, hydrogenation is not required for grating fabrication. Single-sided output operation is obtained by directly splicing one end of the active grating to a broadband high reflecting grating, with 10 mW output power achieved for 110 mW pump power. Although hydrogenation is able to enhance the photosensitivity/grating strength further, this is actually found to degrade the laser efficiency due to increased losses.
A microelectromechanical-based magnetostrictive magnetometer
Authors: Osiander R. , Ecelberger S. A. , Givens R. B. , Wickenden D. K. , Murphy J. C. , Kistenmacher T. J.
Published: July 2002

The principles of operation of a microelectromechanical (MEMS)-based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol-D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated-microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position-sensitive detector. Currently, the sensitivity of this MEMS-based magnetostrictive magnetometer is ∼1μT.
A precise new method to evaluate Monte Carlo simulations of electron transport in semiconductors
Authors: Urban M. , Siegrist M. R. , Asadauskas L. , Raguotis R. , Brazis R.
Published: July 2002

Monte Carlo simulations of the electron drift response to an ac-electric field are used to calculate the power dependent third harmonic generation efficiency for far-infrared radiation. The results are compared with far-infrared frequency tripling experiments. It is shown that the nonlinear optical properties are much more sensitive to parameter changes in the Monte Carlo simulation than conventional drift velocity results. Hence, this is a sensitive method to test Monte Carlo transport simulations.
Coupling of terahertz radiation to a high-Tc superconducting hot electron bolometer mixer
Authors: Gousev Yu. P. , Semenov A. D. , Pechen E. V. , Varlashkin A. V. , Nebosis R. S. , Renk K. F.
Published: July 2002

We report on efficient coupling of THz radiation to a high-Tc superconducting hot electron bolometer that is suitable for heterodyne detection. Our quasi-optical system consisted of a planar self-complementary spiral antenna on a dielectric substrate clamped to an extended hyperhemispherical lens. The antenna was integrated into a co-planar line for broadband intermediate frequency matching. Measurements in the homodyne regime at a frequency of 2.5 THz showed a radiation pattern with a beam width of 1° and a coupling efficiency of ≊0.1. We measured, at an intermediate frequency of 1.5 GHz, an output noise temperature of ≊160 K and estimated for the device, operated in the heterodyne regime, a system noise temperature of ≊3×105 K. We also discuss possibilities of significant improvement of the sensitivity.
Effects of the external electric field from a substrate on Cl2 gas adsorption on SnO2 thin films
Authors: Kunishima Yoshiko Niki , Miyayama Masaru , Yanagida Hiroaki
Published: July 2002

Changes in current by Cl2 gas adsorption were measured at 200 °C for SnO2 films sputtered on SiO2/Si substrates, with a varying electric field from the substrate. Properties of Cl2 adsorption, such as sensitivity and reaction rate constant, under a positive (+5 V) substrate bias were almost the same with those under 0 V bias. However, under a negative (−5 V) substrate bias, the current decreased remarkably by Cl2 adsorption and a high sensitivity was obtained. Moreover, the reaction rate constant was found to be about 10 times or more larger than those under 0 V and +5 V with positive biases. It was indicated that an external electric field affects the adsorption behavior of Cl2 on SnO2 surface.
Simple two-slit interference electro-optic coefficients measurement technique and efficient coplanar electrode poling of polymer thin films
Authors: Kalluri Srinath , Garner Sean , Ziari Mehrdad , Steier William H. , Shi Yongqiang , Dalton Larry R.
Published: July 2002

A novel technique using coplanar electrodes for measuring the electro-optic coefficients of in-plane poled polymer thin films is presented. This method does not require waveguiding and uses a simple optical setup. In this technique the index across one slit of a two-slit aperture is modulated by an electric field and the modulated interference pattern of light after it propagates through these slits is detected in the far field. A coplanar electrode pattern was used for both in-plane poling and for measuring the r coefficients in a two-arm push–pull configuration. The maximum poling strength achieved was 200 V/μm. An r33 coefficients of 12.5 pm/V at 1.3 μm, the wavelength dispersion of r33 and the r33/r13 ratio as a function of the poling field, were measured for the PURDR19 electro-optic polymer.
Size effect upon emission dynamics of 1.5 μm quasi-quantum wire distributed feedback semiconductor lasers
Authors: Wang Jian , Griesinger Uwe A. , Adler Frank , Schweizer Heinz , Härle V. , Scholz F.
Published: July 2002

Emission dynamics of 1.5 μm InGaAs/InGaAsP quasi-quantum wire lasers excited by ultrashort optical pulses were investigated. Wire distributed feedback lasers with different nanometer wire widths and distances between the wires, which were realized by dry-etching technique, were compared in detail. The dependence of dynamic behavior on wire width was observed. Detailed theoretical simulations show that, the improvement of dynamic response with decrease of wire width is attributed to a superlinear increase of differential gain with decrease of wire width, which counteracts the negative effects due to decrease of packing density of the active region and increase of electron relaxation time.
Excimer laser crystallization and doping of silicon films on plastic substrates
Authors: Smith P. M. , Carey P. G. , Sigmon T. W.
Published: July 2002

We report the pulsed laser recrystallization and doping of thin film amorphous silicon deposited on oxide-coated polyester substrates. Although our heat-flow simulation of the laser recrystallization process indicates that the plastic is briefly subjected to temperatures above its softening point, we see no evidence of damage to the plastic or film delamination from the substrate. Film grain size is found to vary up to ∼0.1 μm. Electrical characteristics obtained from simple strip line resistors and thin film transistors indicate that device-quality silicon films have been produced on an inexpensive flexible plastic substrate.
Photoinduced magnetic pole inversion in a ferro–ferrimagnet: (Fe0.40IIMn0.60II)1.5CrIII(CN)6
Authors: Ohkoshi Shin-ichi , Yorozu Shinsuke , Sato Osamu , Iyoda Tomokazu , Fujishima Akira , Hashimoto Kazuhito
Published: July 2002

We tried to design the magnet exhibiting magnetic pole (N and S) inversion by photostimuli. The magnetization of Fe1.5IICrIII(CN)6⋅7.5H2O was changed in a photon mode by visible light. A ferro-ferrimagnet (Fe0.40IIMn0.60II)1.5CrIII(CN)6⋅7.5H2O mixed by ferromagnetic (Fe–Cr system showing the change of magnetization by optical stimuli) site and ferrimagnetic (Mn–Cr system showing no optical response) site showed negative magnetization at the temperature lower than compensation temperature (Tcomp=19 K). In this mixed metal cyanide magnet we have succeeded in demonstrating a novel magnetic behavior “photoinduced magnetic pole inversion.” Moreover, the magnetic pole inversion can be induced repeatedly by alternate optical and thermal stimulations.
Elastic moduli of gallium nitride
Authors: Schwarz R. B. , Khachaturyan K. , Weber E. R.
Published: July 2002

The second-order elastic constants of wurtzite phase GaN have been measured with 0.2% accuracy using a resonance ultrasound method. The measurements were done on a 0.29-mm-thick GaN crystal grown by chloride vapor-phase transport and subsequently detached from the substrate. The elastic moduli in units of GPa are: c11=377, c12=160, c13=114, c33=209, and c44=81.4. The elastic moduli are used to calculate the biaxial misfit stresses expected in heteroepitaxial thin films of GaN.
Correlation between latent interface trap buildup and 1/f noise in metal–oxide–semiconductor transistors
Authors: Johnson Melissa J. , Fleetwood Daniel M.
Published: July 2002

A long-term delayed increase in the 1/f noise of p-channel metal–oxide–semiconductor (MOS) transistors is observed in devices that show significant latent interface-trap buildup after exposure to ionizing radiation. During positive-bias postirradiation anneal, the noise increases by more than an order of magnitude above the level observed after irradiation. The increase in noise precedes the latent buildup of interface traps by at least 4.5 days during room-temperature annealing, and by ∼1 h during 100 °C annealing. The time and temperature dependencies of the increases in noise and interface trap buildup are consistent with the thermally activated motion of protons into the near-interfacial region of the oxide, followed by increases in border trap and interface trap densities. These results suggest hydrogen-related species can significantly affect the 1/f noise of MOS devices.
Evidence by transmission electron microscopy of densification associated to Bragg grating photoimprinting in germanosilicate optical fibers
Authors: Cordier P. , Dupont S. , Douay M. , Martinelli G. , Bernage P. , Niay P. , Bayon J. F. , Dong L.
Published: July 2002

We present the first observation by transmission electron microscopy of glass densification induced by Bragg grating inscription in two germanosilicate single-mode optical fibers.
Electrical characteristics of nearly relaxed InAs/GaP heterojunctions
Authors: Chen E. H. , Chin T. P. , Woodall J. M. , Lundstrom M. S.
Published: July 2002

The electrical properties of lattice mismatched InAs/GaP heterojunctions are examined. In spite of a high dislocation density at the heterointerface, the current versus voltage characteristics show nearly ideal behavior with low reverse leakage currents and high breakdown voltages. The forward current varied exponentially with bias displaying ideal factors of 1.10 or less. Band offsets estimated from current–voltage and capacitance–voltage analysis are consistent with previous estimates based on differences in Schottky barrier heights.
Thermoelectric transport in quantum well superlattices
Authors: Broido D. A. , Reinecke T. L.
Published: July 2002

A full theory of thermoelectric transport in superlattices, including the well width and energy dependence of the optical and acoustic phonon scattering and the effects of confinement in raising valley degeneracy is developed. It is shown that these features result in qualitatively significant modifications in the predicted figure of merit of superlattice systems. Results are given for PbTe superlattices, and comments are made on recent experimental results for such systems.
Scaling behavior of 1/f noise in high-temperature superconductor Josephson junctions
Authors: Marx A. , Gross R.
Published: July 2002

Extensive studies of the low frequency 1/f noise in high-temperature superconducting (HTS) Josephson junctions of various types and materials have been performed for a wide range of operating parameters. The origin of the measured voltage fluctuations can be traced back to the trapping and release of charge carriers in trapping centers in an insulating barrier, giving rise to correlated fluctuations of the junction critical current Ic and normal-state resistance Rn. We observed a linear dependence of the normalized critical current and resistance fluctuations on Rn, which suggests a constant density of trapping centers for the HTS Josephson junctions. The scaling of the normalized fluctuations is in good agreement with the previously found scaling relation IcRn∝1/Rn and supports a junction model assuming a leaky tunnel barrier.
Imaging of domain-inverted gratings in LiNbO3 by electrostatic force microscopy
Authors: Bluhm H. , Wadas A. , Wiesendanger R. , Roshko A. , Aust J. A. , Nam D.
Published: July 2002

Ferroelectric domains in LiNbO3 have been investigated by means of electrostatic force microscopy. Polarization-inverted gratings with 4 μm periodicity were fabricated by titanium diffusion into both +c and −c faces of single-domain LiNbO3 crystals. The distribution of the electric field in the vicinity of the sample surface was measured using scanning probe microscopy. The electrostatic force image was found to correlate with the shape of the domain-inverted profile observed by scanning electron and optical microscopies.
Structural limitation to the material strength of electrorheological fluids
Authors: Pan Xiao-Dong , McKinley Gareth H.
Published: July 2002

The variation of the dynamic modulus of a model electrorheological fluid with strain amplitude is shown to closely resemble that of traditional physical gels. Comparison of the in-phase stress component in each system indicates that the material strength of electrorheological fluids in shear is limited by the small strain amplitude to structural failure. An anisotropic network model is proposed for electrorheological fluids, in which the primary structure consists of chains of particles spanning the electrode gap along the field direction, while many-body interactions form a secondary structure of short chains tilted with respect to the field direction and interconnecting the primary chains. A geometrical argument shows that the tensile strain in the secondary structure can be an order of magnitude larger than that in the primary chains. This nonuniform strain distribution poses an inherent structural limitation on the shear material strength of electrorheological fluids.
Preparation of regular arrays of antidots in YBa2Cu3O7 thin films and observation of vortex lattice matching effects
Authors: Castellanos A. , Wördenweber R. , Ockenfuss G. , Hart A. v.d. , Keck K.
Published: July 2002

Optimized sputtered YBa2Cu3O7 (YBCO) thin films on CeO2-buffered sapphire substrates are patterned into square lattices of submicrometer holes (antidots) with diameters of 250–450 nm and lattice parameters of d=500–1000 nm without deterioration of superconducting properties. In the mixed state, matching effects between the Abrikosov vortex lattice and the artificial antidot lattice are observed. These effects are in the form of peaks or cusps in the critical current density recorded as a function of magnetic induction at integers n and specific rationals k/l of the matching field Bm=(Φ0/d2). The experimental results are discussed in the context of existing theories. The existence of multiquanta vortices confined by the holes in YBCO films are considered.
Free-space electro-optic sampling with a high-repetition-rate regenerative amplified laser
Authors: Lu Z. G. , Campbell P. , Zhang X.-C.
Published: July 2002

The use of a high-repetition-rate (250 kHz) regenerative amplified laser with μJ pulse energy in a free-space THz beam electro-optic sampling system has produced a significant improvement in both the signal-to-noise ratio (>105) and absolute probe beam photomodulation depth (>19%). Focal plane images of the electric field distribution (strength and polarity) in dipole and quadrupole planar photoconductive emitters are presented. Preliminary results of real-time 2D THz images of moving objects have also been obtained.
Field emission properties of diamond films of different qualities
Authors: Fox N. A. , Wang W. N. , Davis T. J. , Steeds J. W. , May P. W.
Published: July 2002

Field emission properties of diamond films were studied by macroscopic I–V measurement. A lower turn-on field and a higher emission current were observed for diamond films produced by higher methane concentration, or with higher density of defects, introduced by ion implantation. However, diamond films of poorer quality experience a severe reliability problem. Cold implantation followed by rapid thermal or laser annealing produced diamond emitters with a turn-on field as low as 5 V/μm and the desired reliability.
Potential controlled stripping of an amorphous As layer on GaAs(001) in an electrolyte: An in situ x-ray scattering study
Authors: Scherb G. , Kazimirov A. , Zegenhagen J. , Schultz T. , Feidenhans’l R. , Fimland B. O.
Published: July 2002

We demonstrate that an amorphous As layer deposited as protection on a GaAs(001) surface grown by molecular beam epitaxy can be removed via reductive etching in an electrolytical cell at sufficiently negative electrode potentials. Employing a specially constructed electrochemical cell filled with H2SO4, we monitored the stripping process of the 50 nm As cap over a period of hours in situ with x-ray diffraction. Our results suggest that, using this potential controlled stripping, smooth and well ordered GaAs(001) surfaces can be obtained in an aqueous electrolyte.
High-gain excitonic lasing from a single InAs monolayer in bulk GaAs
Authors: Goñi A. R. , Stroh M. , Thomsen C. , Heinrichsdorff F. , Türck V. , Krost A. , Bimberg D.
Published: July 2002

We report the observation of highly efficient laser emission from a single InAs layer with an effective thickness of 1.5 monolayers (ML) embedded in bulklike GaAs. Lasing action is obtained at the wavelength of the InAs thin-layer luminescence (870 nm) by cw optical pumping with a threshold power density of 0.9(3) kW/cm2 at 10 K. Gain measurements yield a very high material gain of 1.0(5)×104 cm−1 for the InAs layer when pumped with ∼10 kW/cm2 at low temperatures. The 0 dimensional character of the emission as determined from cathodoluminescence and the absence of band-gap renormalization with increasing pump level speak for an excitonic mechanism of population inversion.
Ultranarrow electroluminescence spectrum from the ground state of an ensemble of self-organized quantum dots
Authors: Huffaker D. L. , Graham L. A. , Deppe D. G.
Published: July 2002

Data are presented on the electroluminescence from an ensemble of self-organized quantum dots excited at low current densities. The ensemble contains ∼105 dots, which produce a ground state spectral emission with a 14 K linewidth of ∼1 meV at low current density (∼5×10−2 A/cm2). While the spectra show clearly discrete energy levels, we suggest that obtaining a single ground state emission from the ensemble may be due to interdot electronic coupling. Spectral broadening decreases for decreasing current density due to electronic state filling, even for the lowest current densities studied.
200 MHz optical signal modulation from a porous silicon light emitting device
Authors: Balucani M. , La Monica S. , Ferrari A.
Published: July 2002

The light emission frequency response of an extremely stable Schottky (Al-porous silicon) light emitting device is presented. The construction steps, critical for device stability, are also presented. The device, when it is reverse biased in breakdown conditions, shows a white light emission visible in normal daylight. The emission mechanism is supposed to be the radiative transition of hot electrons generated in the breakdown process. The optical signal modulation has been measured up to 200 MHz and a simple electrical model is presented in order to explain the dynamic behavior of the device. The device speed seems to be limited by the junction capacitance rather than by an intrinsic physical limit of the emission mechanism.
A novel tool for mapping composition distributions in semiconductor microstructures—application to InxGa1−xP quantum wires
Authors: Dua P. , Cooper S. L. , Cheng K. Y.
Published: July 2002

A technique is described which employs resonant Raman scattering for nondestructive, quantitative analysis of alloy composition distributions and their volume fractions in semiconductor microstructures. Use of this technique is demonstrated via application to extract the wire and barrier region compositions and the shape of the composition modulation profile of narrow (∼150 Å) InxGa1−xP multiquantum wire array grown via a strain-induced laterally ordered process.
Growth and characterization of InNxAsyP1−x−y/InP strained quantum well structures
Authors: Bi W. G. , Tu C. W.
Published: July 2002

In this work, we propose the material InNxAsyP1−x−y (InNAsP) on InP for long-wavelength laser applications, where the unique feature of the smaller lattice constant of nitrides together with the large electronegativity of nitrogen atoms has been utilized in reducing the system strain while increasing the conduction-band offset by putting N into a compressively strained material system. InNAsP/In(Ga)(As)P strained quantum well (QW) samples were grown by gas-source molecular beam epitaxy with a rf nitrogen plasma source. Very sharp and distinct satellite peaks as well as Pendellosung fringes are observed in high-resolution x-ray rocking curves for these QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. Photoluminescence (PL) measurements on InNAsP/InP single QWs with different well widths as well as on InNAsP/InGaAsP multiple QWs reveal strong PL emissions in the range of from 1.1 to 1.5 μm, demonstrating their suitability for long-wavelength applications.
GaAs/AlGaAs quantum well photodetectors with a cutoff wavelength at 28 μm
Authors: Perera A. G. U. , Shen W. Z. , Matsik S. G. , Liu H. C. , Buchanan M. , Schaff W. J.
Published: July 2002

We demonstrate the longest (λc=28.6 μm) far-infrared quantum well photodetectors (QWIPs) based on a bound-to-bound intersubband transition in GaAs/AlGaAs. The responsivity is comparable with that of mid-infrared GaAs/AlGaAs and InGaAs/GaAs QWIPs. A peak responsivity of 0.265 A/W and detectivity of 2.5×109 cmHz/W at a wavelength of 26.9 μm and 4.2 K have been achieved. Based on the temperature dependent dark current and responsivity results, it is expected that similar performance can be obtained at least up to 20 K.
Superconducting YBa2Cu3O7−x thin films on polycrystalline ferrite for magnetically tunable microwave components
Authors: Jia Q. X. , Findikoglu A. T. , Arendt P. , Foltyn S. R. , Roper J. M. , Groves J. R. , Coulter J. Y. , Li Y. Q. , Dionne G. F.
Published: July 2002

Superconducting YBa2Cu3O7−x (YBCO) thin films with a surface resistance of 0.86 mΩ at 10 GHz and 76 K have been grown on polycrystalline ferrite yttrium iron garnet (YIG) substrates. The chemical and structural mismatches between YBCO and YIG are solved by using a double buffer layer of biaxially oriented yttria-stabilized zirconia (YSZ) and CeO2, where YSZ is deposited by an ion-beam-assisted-deposition technique. The YBCO films are c axis oriented with an in-plane mosaic spread [full width at half maximum of an x-ray φ-scan on (103) reflection] of less than 8°. The films have a superconductive transition temperature above 88 K with a transition width less than 0.3 K, giving a critical current density above 106 A/cm2 in self field at 75 K. At 75 K in an external magnetic field of 1 T perpendicular to the film surface, the films maintain a critical current density over 2×105 A/cm2.
Metalorganic molecular beam epitaxy of GaAsN with dimethylhydrazine
Authors: Qiu Y. , Jin C. , Francoeur S. , Nikishin S. A. , Temkin H.
Published: July 2002

Epitaxial layers and superlattices of GaAsN/GaAs were grown by metalorganic molecular beam epitaxy using dimethylhydrazine, triethylgallium, and conventional arsenic sources. The incorporation of nitrogen into the solid was investigated as a function of the substrate temperature and the flux of dimethylhydrazine and modeled assuming formation of an adduct. Growth of GaAsN is characterized by an activation energy of 0.97 eV arising from a difference between activation energies of the adduct sticking coefficient, EB∼1.27 eV, and the adduct formation, EA∼0.3 eV. Nitrogen incorporation of 3% is obtained at a growth temperature of 400 °C. High-resolution x-ray diffraction and photoluminescence data demonstrate excellent quality of epitaxial layers and superlattices grown with dimethylhydrazine.
Single molecule detection and underwater fluorescence imaging with cantilevered near-field fiber optic probes
Authors: Talley Chad E. , Lee M. Annie , Dunn Robert C.
Published: July 2002

Tapping-mode near-field scanning optical microscopy (NSOM) employing a cantilevered fiber optic probe is utilized to image the fluorescence from single molecules and samples in aqueous environments. The single molecule fluorescence images demonstrate both the subdiffraction limit spatial resolution and low detection limit capabilities of the cantilevered probe design. Images taken as a function of tip oscillation drive amplitude reveal a degradation in the resolution as the amplitude is increased. With all cantilevered probes studied, however, a minimum plateau region in the resolution is reached as the drive amplitude is decreased, indicating that the tapping mode of operation does not reduce the optical resolution. Images of fluorescently doped lipid films illustrate the ability of the probe to track small height changes (<1.5 nm) in ambient and aqueous environments, while maintaining high resolution in the fluorescence image. When the tip is immersed in water (1.3 mm), the cantilevered NSOM tip resonance, 25–50 kHz, shifts approximately 100–150 Hz, the amplitude dampens less than 40% and the Q factor is reduced from 300–500 to 100–200.
Electromechanical behavior of BaTiO3 from first principles
Authors: García Alberto , Vanderbilt David
Published: July 2002

Using an effective Hamiltonian parametrized from first principles, Monte Carlo simulations are performed in order to study the piezoelectric response of BaTiO3 in the ferroelectric tetragonal phase as a function of temperature. The effect of an electric field on the phase behavior is also illustrated by a simulation of the transformation of a rhombohedral domain into a tetragonal one under a strong field.
Electronic states tuning of InAs self-assembled quantum dots
Authors: Garcı́a J. M. , Mankad T. , Holtz P. O. , Wellman P. J. , Petroff P. M.
Published: July 2002

We demonstrate the dimensional tuning of InAs self-assembled quantum dots (QDs) by changing the growth kinetics during the capping of InAs islands with GaAs. Modifying the growth sequence during the capping of InAs islands, allows us to tune the thickness and lateral dimensions of the QDs while keeping the wetting layer thickness constant. Using the same method but embedding the tuned InAs islands into AlAs layers allows to further blueshift the photoluminescence emission to higher energies while keeping the wetting layer thickness constant. The main process responsible for the QDs size modification is consistent with a kinetically controlled materials redistribution of the InAs islands that minimizes the energy of the epitaxial layers at the start up of the GaAs capping deposition.
Electromigration-induced failure of metallic thin films due to transgranular void propagation
Authors: Gungor M. Rauf , Maroudas Dimitrios
Published: July 2002

A theoretical analysis is presented of the failure of metallic thin films due to electromigration-induced morphological evolution of transgranular voids. Self-consistent dynamical simulations emphasize the important roles of the anisotropy of void surface diffusivity, the strength of the applied electric field, and the void size. Our simulations predict formation of stable faceted voids, formation of wedge-shaped voids through a facet selection mechanism, as well as failure due to propagation of slitlike features from void surfaces, in excellent agreement with recent experimental observations.
Temperature dependence of field emission characteristics of phosphorus-doped polycrystalline diamond films
Authors: Sugino Takashi , Kuriyama Kenji , Kimura Chiharu , Kawasaki Seiji
Published: July 2002

Temperature dependence of the field emission characteristics is investigated for the phosphorus(P)-doped polycrystalline diamond film in comparison with that of the boron(B)-doped one. The threshold voltage decreases with increasing temperature for the P-doped diamond film, while no variation in the threshold voltage occurs for the B-doped diamond film. It is considered that an increase of the ionized donor concentration with increasing temperature leads to a reduction in the tunnel barrier width at the interface between the diamond and the cathode, resulting in an enhancement of the emission current. Field emission characteristics in the higher voltage region are featured by the space charge limited current. The activation energy estimated from the Arrhenius plot of the emission current suggests the upward band bending at the diamond surface.
Spontaneous splitting of ferromagnetic (Ga, Mn)As valence band observed by resonant tunneling spectroscopy
Authors: Ohno H. , Akiba N. , Matsukura F. , Shen A. , Ohtani K. , Ohno Y.
Published: July 2002

Current–voltage characteristics of AlAs/GaAs/AlAs double barrier resonant tunneling diodes with ferromagnetic p-type (Ga, Mn)As on one side and p-type GaAs on the other have been studied. A series of resonant peaks have been observed in both polarities, i.e., injecting holes from p-type GaAs and from (Ga, Mn)As. When holes are injected from the (Ga, Mn)As side, spontaneous resonant peak splitting has been observed below the ferromagnetic transition temperature of (Ga, Mn)As without magnetic field. The temperature dependence of the splitting is explained by the the spontaneous spin splitting in the valence band of ferromagnetic (Ga, Mn)As.
Growth and second harmonic generation characterization of Er3+ doped bulk periodically poled LiNbO3
Authors: Bermúdez V. , Capmany J. , Garcı́a Solé J. , Diéguez E.
Published: July 2002

Samples of Er3+ doped bulk periodically poled lithium niobate have been grown by the Czochralski method. The efficiency, tuning and thermal tolerances of infrared to green second harmonic generation have been measured, showing good accordance with a 4%–5% dispersion in the domain period length.
Lattice deformation and magnetic properties in epitaxial thin films of Sr1−xBaxRuO3
Authors: Fukushima Noburu , Sano Kenya , Schimizu Tatsuo , Abe Kazuhide , Komatsu Shuichi
Published: July 2002

Crystal structure and magnetic properties in epitaxially grown Sr1−xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1−xBaxRuO3 exhibits a simple perovskite structure in the whole region of the Ba/Sr ratio, in contrast to the complex hexagonal layered perovskite of Ba-rich bulk Sr1−xBaxRuO3, which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from pseudocubic in SrRuO3 to a highly distorted tetragonal lattice in BaRuO3. Electronic properties such as conductivity and magnetization were examined. A metal–insulator transition was not observed in this system, and metallic conductivity was maintained in the whole region of Ba concentration. Ferromagnetic ordering at 160 K seen in bulk SrRuO3 was observed to be suppressed in the Sr1−xBaxRuO3 films with increasing tetragonal deformation and Curie temperatures decreased to 50 K in BaRuO3.
Band gap and band alignment of strain reduced Si/Si1−x−yGexCy multiple quantum well structures obtained by photoluminescence measurements
Authors: Hartmann R. , Gennser U. , Sigg H. , Grützmacher D. , Ensslin K.
Published: July 2002

The effect of strain compensation in Si/SiGeC multiple quantum wells (MQWs) is investigated. Photoluminescence measurements on strain-reduced Si/SiGeC layers indicate a type-I band alignment. Values for the conduction-band and valence-band offsets of nearly strain-free Si/SiGeC MQWs are presented. Variation of the strain in the pseudomorphic layers reveals a lowering of the gap energy for exactly strain-compensated SiGeC compared to compressive SiGeC. A model is developed which explains this energy behavior in terms of band offsets and confinement shifts. The band alignments of strain-reduced Si/SiGeC and Si/SiC are compared.
Comparison between optical techniques for the measurement of the surface electric field in (100) oriented GaAs
Authors: Yang Z. , Chen Y. H. , Wong Yuqi
Published: July 2002

Three optical spectroscopic techniques commonly used for the measurement of the surface electric field (SEF) of semiconductors, namely photoreflectance (PR) near the E0 energy and near the E1 energy, and the linear electro-optic (LEO) effect of reflectance difference spectroscopy near the E1 energy, are compared on a series of (100) oriented GaAs layered structures. The LEO strength is found to be linearly proportional to the SEF obtained from the PR oscillation period near the E0 energy for samples with a single space charge region in which the field is uniform over the light penetration depth near E1(∼20 nm). The LEO strength remains proportional to the SEF for samples with multiple space charge regions while the PR period near E0 no longer reflects the true SEF. The SEF value estimated from the line width of PR near the E1 energy is quite unreliable because both the electric field and the impurity scattering broaden the line.
Tunnel-junction-connected distributed-feedback vertical-cavity surface-emitting laser
Authors: Korshak A. N. , Gribnikov Z. S. , Mitin V. V.
Published: July 2002

An injection distributed-feedback vertical-cavity surface-emitting laser (VCSEL) with tunnel junctions served as quasi-Ohmic intercontacts (tunnel-junction-connected distributed-feedback VCSEL) is proposed. A periodic structure of vertically stacked double-heterostructure laser diodes connected by low-resistance tunnel junctions forms a vertical distributed-feedback (DFB) laser medium. To minimize the threshold, the DFB structure is placed in a Fabry–Perot cavity designed to match gain layers with the maximums of the optical mode, and the tunnel junctions—with its minimums. The passive regions with tunnel junctions provide effective vertical injection into each active region of this multiple-active-region laser. This DFB VCSEL is expected to have an improved performance, specifically, reduced threshold current and heightened output power.
A photonic switch based on a gigantic, reversible optical nonlinearity of liquefying gallium
Authors: Bennett P. J. , Dhanjal S. , Petropoulos P. , Richardson D. J. , Zheludev N. I. , Emelyanov V. I.
Published: July 2002

Liquefying gallium shows a huge reversible optical nonlinearity which is compatible with waveguide technology and promises to be a breakthrough in broadband, light-by-light modulation at milliwatt operating power levels and frequency band spanning up to several hundred kilohertz.
Si/Pt Ohmic contacts to p-type 4H–SiC
Authors: Papanicolaou N. A. , Edwards A. , Rao M. V. , Anderson W. T.
Published: July 2002

In this letter, we report on the investigation of Si/Pt Ohmic contacts to p-type 4H–SiC. The contacts were formed by a vacuum annealing method at 1100 °C for 3 min, which resulted in specific contact resistivities in the low 10−4 Ω cm2 range. Auger analysis has shown that, at this anneal temperature, there was a uniform intermixing of the Si and Pt, migration of Pt into the SiC, and out-diffusion of C into the metallization layers. Overlayers of Au or Ni/Au on Si/Pt had the effect of decreasing the specific contact resistance and improving the surface morphology of the annealed contacts.
High Tc superconducting second-order gradiometer
Authors: Kittel A. , Kouznetsov K. A. , McDermott R. , Oh B. , Clarke John
Published: July 2002

A planar, second-order gradiometer was fabricated from single-layer YBa2Cu3O7−x films. The gradiometer consists of a symmetric flux transformer with an overall length of 80 mm inductively coupled to a directly coupled magnetometer, and has a baseline of 31 mm. The mutual inductance between the flux transformer and the magnetometer is adjusted mechanically to reduce the response to a uniform magnetic field applied perpendicularly to the plane of the gradiometer to typically 50 ppm. From an independent measurement, the residual first-order gradient response was determined to be at most 1.4% relative to the second-order gradient response.
Improved quality GaN grown by molecular beam epitaxy using In as a surfactant
Authors: Widmann F. , Daudin B. , Feuillet G. , Pelekanos N. , Rouvière J. L.
Published: July 2002

The surfactant effect of In during the growth of GaN by molecular beam epitaxy has been investigated. It has been found that the presence of In modifies the diffusion kinetics in the growing GaN surface, leading to the observation of persistent reflection high energy electron diffraction intensity oscillations, characteristic of layer-by-layer growth. Electron microscopy studies revealed drastic modifications of the GaN structural properties associated with the presence of In during the growth. When grown in the presence of In, GaN exhibits an intense band edge luminescence, free of the component at 3.41 eV which is characteristic of defects associated with growth in N-rich conditions.
Nano-field effect transistor with an organic self-assembled monolayer as gate insulator
Authors: Collet J. , Vuillaume D.
Published: July 2002

We demonstrate the realization and functioning of a hybrid (organic/silicon) nanometer-size field effect transistor (nano-FET) having a gate length of 25 nm. The gate insulator is an organic self-assembled monolayer (SAM) of alkyltrichlorosilanes (∼2 nm thick). We have used densely packed SAMs with functionalized end groups (–CH3, –CH=CH2, –COOH) that all exhibit reduced leakage current density (10−8–10−5 A/cm2). This nano-FET is free of punchthrough down to 50 nm, and shows a good field effect behavior at 25 nm. This demonstrates the compatibility of these SAMs with semiconductor device processes and their wide capability for applications in nanometer-scale electronics.
Hydrogenation disproportionation desorption recombination in Sm–Co alloys by means of reactive milling
Authors: Gutfleisch O. , Kubis M. , Handstein A. , Müller K.-H. , Schultz L.
Published: July 2002

Sm–Co-type alloys were disproportionated by milling in hydrogen at enhanced temperatures. X-ray diffraction confirmed the disproportionation of the SmCo5 and Sm2Co17 phases into Sm hydride and α-Co. This “reactive milling” procedure facilitates the disproportionation of these alloys which are characterized by a very high thermodynamic stability, and therefore are not available for a standard hydrogenation disproportionation desorption recombination treatment. Recombination of the reactively milled powders leads to the formation of the original phases, now with dramatically refined grain sizes of around 25 nm and significant coercivities such as μ0JHC=3.7 T in the case of the SmCo5 alloy. Exchange coupling between the nanoscaled grains resulted in magnetically single phase behavior despite a multiphase microstructure. In particular, for the Sm2Co17 alloy, a remanence enhancement was observed for recombination temperatures ⩽700 °C.