bluuebunny/crossref_metadata_2025 huggingface dataset:
Langmuir probe array for edge plasma study on the compact helical system heliotron/torsatron
Authors: Ohkuni K. , Toi K. , Ohdachi S. , Takagi S. ,   Torus experiment group
Published: July 2002

In order to study plasma behaviors and transport in edge plasma region, a triple probe array is installed on the compact helical system (CHS) heliotron/torsatron. The probe array is designed to measure the radial profiles of electron temperature, electron density, plasma potential and these fluctuations, simultaneously. The standard triple probe technique does not supply reliable results unless all electrodes are on the same magnetic surface. Then we also use the modified triple probe technique based on five electrodes, which provides precise fluctuations of electron temperature. In edge plasma region of neutral beam injection (NBI) heated plasmas on CHS, plasma profiles and their fluctuations are measured by the newly installed triple probe array. In the region of ρ≃0.95∼1.1, Boltzman relationship ñe/〈ne〉≃Ṽs/〈Te〉 is roughly satisfied and temperature fluctuation T̃e/〈Te〉 is by a factor of 3 smaller than density fluctuation ñe/〈ne〉. In sawtoothing plasmas, we also measure these plasma parameters, their gradients, and plasma fluctuations. Electron temperature and the gradient transiently increase associated with each sawtooth crash, but electron density and the gradient show the opposite behavior. Radial electrical field and the gradient are also modulated by sawtooth crashes. Correlation between the increased radial electrical field gradient and fluctuations, however, is unclear in these plasmas.
On the harmonic technique to measure electron temperature with high time resolution
Authors: Boedo J. A. , Gray D. , Conn R. W. , Luong P. , Schaffer M. , Ivanov R. S. , Chernilevsky A. V. , Van Oost G. ,   The TEXTOR Team
Published: July 2002

A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the technical issues relevant to extending the technique to high bandwidth (200 kHz) are discussed. The technique has been implemented in a fast reciprocating probe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is derived for the harmonic currents. Measurement of the probe current by inductive pickup is introduced to improve electrical isolation and bandwidth. The temperature profiles in the boundary plasma of TEXTOR have been measured with high spatial (∼2 mm) and temporal (200 kHz) resolution and compared to those obtained with a double probe. The exact expansion of the probe characteristic in terms of Bessel functions is compared to a computationally efficient power series. Various aspects of the interpretation of the measurement are discussed such as the influence of plasma potential and density fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in plasmas.
Ultrafast frequency sweep heterodyne reflectometer on the Tore Supra tokamak
Authors: Moreau Ph. , Clairet F. , Chareau J. M. , Paume M. , Laviron C.
Published: July 2002

A new O-mode dual frequency heterodyne reflectometer has been installed on Tore Supra [Tore Supra Team, Fusion Technol. 29, 417 (1996)] and it has greatly improved the phase determination due to its high dynamic heterodyne detection and ultrafast sweep capabilities (10 μs) provided by its solid state source HTO. This system operates with O-mode electric field polarization in the range of 26–36 GHz and has been designed for density profile measurements. The reflectometer launches two frequencies separated by 320 MHz simultaneously into the plasma. Heterodyne detection improves the dynamics up to 60 dB, and is associated with a sin/cos detection to allow separate analysis of the amplitude and phase of each reflected signal of both probing waves. Therefore, to calculate the density profile, the group delay can be defined in two different ways: (i) from the derivative of the absolute phase of one of the two probing waves or (ii) by calculating the phase difference between the two probing waves. We explain how ultrafast sweep operations (down to 10 μs) significantly reduce the influence of plasma turbulence on the phase measurements. We also point out the importance of carefully filtering the signal detected in order to keep only information coming from the reflection at the cutoff and get rid of parasitic reflections. It is shown that the phase difference technique does not completely remove fluctuations such as long radial correlation magnetohydrodynamic perturbations. The density profiles determined by the heterodyne reflectometer are in good agreement with the measurements from the other diagnostics of Tore Supra.
Phase-amplitude crosstalk in intensity modulated near infrared spectroscopy
Authors: Alford K. , Wickramasinghe Y.
Published: July 2002

Near infrared spectroscopy (NIRS) instruments that rely on phase sensitive detection suffer from what is called “phase-amplitude crosstalk,” i.e., the phase measured is dependent on the average light intensity entering the detector. Changes in detector rise time with input light intensity is the accepted explanation of this phenomenon. It is concluded here that an additional simple mechanism can cause phase-amplitude errors, particularly if the ratio of the ac component of the detected signal to the dc component is low. It is shown that the form of the phase distortion encountered during the development of a new phase sensitive NIR instrument can be modeled by assuming the presence of a synchronous interfering signal, due to rf coupling, at the detector output. This modeling allows a required margin between the detected signal of interest, i.e., the signal from the tissue and the interfering signal to be set in order to achieve a measured phase accuracy necessary to derive sufficiently accurate clinical parameters.
Photoluminescent properties of Er-doped GaP deposited on Si
Authors: Wang X. Z. , Wessels B. W.
Published: July 2002

Er-doped GaP layers were deposited on Si (111) substrates using atmospheric pressure metalorganic vapor phase epitaxy. A strong characteristic Er3+ intra-4f-shell emission at 0.80 eV (1.54 μm) is observed over the temperature range of 12–300 K. The integrated intensity of the 0.80 eV emission is only weakly temperature dependent, decreasing less than 50% as temperature increases from 12 to 300 K. These results indicate that Er-doped GaP thin films deposited on Si are suitable as a material for integrated optoelectronic applications.
Dynamics of transgranular voids in metallic thin films under electromigration conditions
Authors: Maroudas Dimitrios
Published: July 2002

A theoretical analysis is presented for the morphological evolution and stability of transgranular wedge-shaped voids in conductor lines with bamboo grain structure under an applied electric field. A linear stability analysis identifies void shapes that become unstable with respect to the growth of perturbations with wavelengths longer than a critical wavelength. The analysis predicts a critical wavelength that is consistent with experimental observations in unpassivated aluminum lines. The predicted changes in shape during the morphological evolution of the void surfaces are in agreement with the experimental observations.
Ionized-cluster-beam deposition and electrical bistability of C60–tetracyanoquinodimethane thin films
Authors: Gao H. J. , Xue Z. Q. , Wang K. Z. , Wu Q. D. , Pang S.
Published: July 2002

We report an ionized-cluster-beam (ICB) deposition and the electrical bistability of C60–tetracyanoquinodimethane (TCNQ) thin films. The films are fabricated by using an ionized-cluster-beam deposition method in a high vacuum system. The as-deposited films were characterized by transmission electron microscopy and optical absorption spectroscopy, which verified the formation of the charge-transfer complex system in C60–TCNQ thin films and the microstructure of these thin films. The structure and the electrical property of the ICB deposited Ag-TCNQ thin films are also presented. The possible conductive mechanism of these ICB deposited thin films is discussed in the letter.
Observation of lateral confinement effect in Ge quantum wires self-aligned at step edges on Si(100)
Authors: Sunamura H. , Usami N. , Shiraki Y. , Fukatsu S.
Published: July 2002

Spontaneous formation of step-edge Ge quantum wires and lateral quantum confinement effect are clearly observed in Si submonolayer-Ge/Si heterostructures grown on Si(100). By depositing submonolayer equivalent Ge (Q), the Ge atoms are found to line up in a wire-like fashion at the 〈01̄1〉 step edges as revealed by plan-view transmission electron microscopy. Photoluminescence (PL) peak energy shift with Q due to lateral quantum confinement of holes is clearly observed, and PL features characteristic of the quasi-one-dimensional system are also addressed.
La1.85Sr0.15CuO4−δ bicrystal grain boundary Josephson junctions
Authors: Beck A. , Froehlich O. M. , Koelle D. , Gross R. , Sato H. , Naito M.
Published: July 2002

We have investigated La1.85Sr0.15CuO4−δ grain boundary Josephson junctions (GBJs) fabricated on [001] tilt SrTiO3 bicrystals with misorientation angles of 24° and 36.8°. The resistive transition and the current–voltage characteristics of the GBJs are found to be close to those predicted by the resistively shunted junction (RSJ) model. Their product of the critical current density, Jc, and the normal resistance times area ρn, scales proportional to about (Jc)q with q=0.5–0.6 similar to what is found for YBa2Cu3O7−δ  GBJs. By analyzing Fiske resonances the Swihart velocity of the GBJs and the ab-plane London penetration depth of La1.85Sr0.15CuO4−δ are obtained to c̄=2.5×106 m/s and λL=200 nm at 4.2 K, respectively.
Effect of hydrogen passivation on Be-doped AlGaAs/GaAs quantum wells
Authors: Buyanova I. A. , Ferreira A. C. , Holtz P. O. , Monemar B. , Campman K. , Merz J. L. , Gossard A. C.
Published: July 2002

Hydrogen passivation of high quality AlGaAs/GaAs:Be quantum wells (QWs) is studied by photoluminescence (PL) spectroscopy. The interaction of hydrogen with both Be dopants and QW interfaces is analyzed. The efficiency of Be acceptor passivation by hydrogen is shown to be dependent on the Be concentration. By comparing the PL spectra of the QW structures with different doping level, before and after hydrogenation, a partial (up to 50%) deactivation of Be atoms is revealed. It is also shown that a prolonged hydrogenation strongly degrades the interface sharpness, presumably due to hydrogen-enhanced intermixing.
Electrical properties of boron nitride thin films grown by neutralized nitrogen ion assisted vapor deposition
Authors: Lu Ming , Bousetta A. , Bensaoula A. , Waters K. , Schultz J. A.
Published: July 2002

Boron nitride (BN) thin films (containing mixed cBN/hBN phase) have been deposited on Si(100) substrates using neutralized nitrogen beam and electron beam evaporation of boron. All as-deposited BN films were p type with a room-temperature carrier concentration in the range of 5×1016 to 1×1017 cm−3. The Mg-doped BN films showed carrier concentrations in the range of 1.2×1018 cm−3 to 5.2×1018 cm−3 when the Mg cell temperature was varied from 250 to 500 °C. The films were analyzed for both majority elements (B and N) and dopant/impurity (Si, Mg, Fe, etc.) incorporation using secondary ion mass spectroscopy and mass spectroscopy of recoiled ions (MRSI). MRSI is shown to be superior for dopant characterization of boron nitride thin films.
High-T
c superconductor/normal-metal/superconductor edge junctions and SQUIDs with integrated groundplanes
Authors: Hunt B. D. , Forrester M. G. , Talvacchio J. , McCambridge J. D. , Young R. M.
Published: July 2002

Epitaxial, high-Tc superconductor/normal-metal/superconductor (SNS) edge-geometry weak links and superconducting quantum interference devices (SQUIDs) have been fabricated with integrated YBa2Cu3O7 (YBCO) groundplanes and SrTiO3 insulators, using a process which incorporates six epitaxial layers. The SNS edge junctions were produced using off-axis sputtered films and Co-doped-YBCO normal metal interlayers. These devices show excellent performance with typical critical current-resistance (IcRn) products of 500–800 μV for 100–150 Å thick normal metal layers at 65 K, and 1-σ critical current density ( Jc) spreads as small as 12%. SNS SQUIDs incorporating groundplanes exhibit voltage modulation of up to 130 μV at 65 K and 40 μV at 77 K. SQUID inductance measurements indicate microstrip inductance values of 1 pH per square at 65 K.
Detection of current-induced vacancies in thin aluminum–copper lines using positrons
Authors: Asoka-Kumar P. , O’Brien K. , Lynn K. G. , Simpson P. J. , Rodbell K. P.
Published: July 2002

In situ depth-resolved positron annihilation spectroscopy (PAS) is used to show dynamic formation of vacancies in 1 μm×1 μm Al-0.5 wt % Cu lines under current flow. We show that the number of vacancies in these lines increases when a dc current (8×104 A/cm2) is applied. This increase in vacancy concentration is substantially greater than that due to thermal vacancy generation alone (4×1018 cm−3 versus 3×1017 cm−3). Isothermal measurements (with no current flow) yield a vacancy formation energy of 0.60±0.02 eV. These results show that PAS can be used to examine the initial stages of interconnect damage due to electromigration.
Native-oxide laterally confined whispering-gallery mode laser with vertical emission
Authors: Deng H. , Deng Q. , Deppe D. G.
Published: July 2002

Data are presented demonstrating whispering-gallery mode lasing in a diode with native-oxide lateral confinement and vertical emission. The vertical emission is obtained from distributed Bragg reflectors that confine the mode in the normal direction, and that results in a real wave vector component normal to the crystal surface external to the cavity.
Low resistivity copper germanide on (100) Si for contacts and interconnections
Authors: Borek M. A. , Oktyabrsky S. , Aboelfotoh M. O. , Narayan J.
Published: July 2002

We have deposited thin films of Cu3Ge on (100) Si by the sequential e-beam deposition of an amorphous Ge layer followed by a polycrystalline Cu layer. The films were then annealed in flowing N2 for 30 min over a temperature range of 150–600 °C to induce complete recrystallization of the Cu–Ge films. Films of Cu3Ge form a smooth, atomically sharp interface with (100) Si over a wide range of anneal temperatures, as determined by transmission electron microscopy, indicating the lack of any chemical reactions, i.e., compound formation, at the film/substrate interface. We have observed by secondary ion mass spectrometry, that a substantial amount of Si has diffused into the Cu–Ge in varying amounts that exhibit strong dependence on the anneal temperature. Despite the large amount of Si that has diffused into the films, the Cu3Ge maintains its low resistivity (ρ=10–15 μΩ cm) up to an anneal temperature of 600 °C, at which point the film completely loses its structural integrity, i.e., it becomes discontinuous with the formation of complex compounds. These results show that Cu3Ge represents a potential candidate material for contact and metallization for next generation Si-based semiconductor devices.
Determination of minority carrier lifetimes in CuInSe2 thin films
Authors: Puech K. , Zott S. , Leo K. , Ruckh M. , Schock H.-W.
Published: July 2002

We present detailed measurements of carrier lifetimes in CuInSe2 thin films. Time-resolved photoluminescence measurements are performed on polycrystalline samples grown with a lateral gradient in the Cu/In ratio, thus enabling continuous evaluation of lifetime dependence on film composition. Luminescence at two distinct spectral positions is observed: A high energy emission, attributed to free carrier or free exciton recombination (depending on composition), that decays extremely fast with a lifetime of tens of picoseconds to a few ns; a lower energy emission from defect related recombination that decays much more slowly, with typical lifetimes greater than tens of nanoseconds. We compare these results with results from CuInGaSe2 films. Radiative coefficients and minority carrier lifetimes are obtained from the data.
Frequency shifts of cantilevers vibrating in various media
Authors: Weigert Stefan , Dreier Markus , Hegner Martin
Published: July 2002

A simple model is presented for rectangular cantilevers when vibrating in various media. The mass of the surrounding medium affected by the motion of the lever is calculated. It depends on the dimensions of the lever, on the excited mode, and on the density of the medium. Although the viscosity of the media is not taken into account, the resulting predictions for the resonance frequencies agree well with experimental data obtained for levers in air and water up to the seventh harmonic.
Andreev reflections at interfaces between δ-doped GaAs and superconducting Al films
Authors: Taboryski R. , Clausen T. , Hansen J. Bindslev , Skov J. L. , Kutchinsky J. , So/rensen C. B. , Lindelof P. E.
Published: July 2002

By placing several Si δ-doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ-doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.
Time resolved study of self-assembled InAs quantum dots
Authors: Yu Haiping , Lycett Sam , Roberts Christine , Murray Ray
Published: July 2002

We have investigated the exciton kinetics in self-assembled InAs quantum dots and ultrathin quantum wells grown by molecular beam epitaxy on (001) oriented GaAs substrates. At low temperatures, the photoluminescence decay time of the quantum wells increases almost linearly while the decay time of the quantum dot system is independent of temperature. However, above 50 K there is a linear increase in the decay time of the dots which may be due to electrons escaping into the wetting layer or occupation of nonradiative exciton states. Under the conditions of high injection, relaxation from the excited states has a time constant of about 500 ps.
Growth, transport, and magnetic properties of Pr0.67Ca0.33MnO3 thin films
Authors: Singh S. K. , Palmer S. B. , McK. Paul D. , Lees M. R.
Published: July 2002

We have grown Pr0.67Ca0.33MnO3 thin films on LaAlO3 using pulsed laser deposition. Below 50 K, a field induced insulator-metal transition results in changes in resistivity of at least 6 orders of magnitude. The field induced conducting state is metastable at low temperature. The temperature dependence of the resistivity exhibits considerable hysteresis in a field of 40 kOe but becomes reversible in a field of 80 kOe.
Fabrication of in-plane aligned a-axis oriented YBa2Cu3O7−x trilayer Josephson junctions
Authors: Takeuchi I. , Warburton P. A. , Trajanovic Z. , Lobb C. J. , Dong Z. W. , Venkatesan T. , Bari M. A. , Booij W. E. , Tarte E. J. , Blamire M. G.
Published: July 2002

We have fabricated vertical YBa 2Cu3O7−x/PrBa 2Cu3O7−x/YBa 2Cu3O7−x (YBCO/PBCO/YBCO) Josephson junctions using in-plane aligned a-axis oriented YBCO multilayers on (100) LaSrGaO4(LSGO). The Tc’s of the device electrodes are typically greater than 80 K. Josephson coupling is observed for barrier thickness up to 800 Å. Strong nonlinear I–V characteristics arising from the properties of PBCO are observed for thicker barrier devices. Chip to chip spread in the device characteristics may be intrinsically due to the PBCO barrier.
Fabrication of small diamond tips for scanning probe microscopy application
Authors: Oesterschulze E. , Scholz W. , Mihalcea Ch. , Albert D. , Sobisch B. , Kulisch W.
Published: July 2002

A process for the fabrication of diamond cantilevers integrated on a silicon wafer is presented. At one end the cantilevers possess a tip with a small radius of curvature thus allowing their use in scanning probe microscopy applications. The influence of various procedures to enhance diamond nucleation of the properties of the tips is investigated. Ultrasonic pretreatment with 1 μm diamond paste and subsequent hot-filament chemical-vapor deposition turned out to yield the best results. Micro-Raman measurements show the tips to consist of stress-free diamond up to their very apex.
Linear arc discharge source for large area plasma processing
Authors: Bárdoš L. , Baránková H. , Berg S.
Published: July 2002

A linearly scalable plasma source based on the radio frequency generated hot hollow cathode discharge between two parallel plates with a magnetic field perpendicular to the plates near the outlet of the cathode is introduced. The magnetic field facilitates and confines the hollow cathode discharge which leads to a high power density and a high cathode wall temperature. The geometry and location of hot zones is directly controlled by magnetic field. The linear arc discharge (LAD) source exhibits similar features as the cylindrical radio frequency hollow cathode plasma jet. Experiments indicate a metastable assisted growth of TiN films. LAD source extends abilities of the radio frequency hollow cathode plasma jet to the large area processing.
Emission mechanisms and band filling effects in GaAs–AlGaAs V-groove quantum wires
Authors: Tribe W. R. , Steer M. J. , Mowbray D. J. , Skolnick M. S. , Forshaw A. N. , Roberts J. S. , Hill G. , Pate M. A. , Whitehouse C. R. , Williams G. M.
Published: July 2002

We present a study of emission mechanisms and band filling effects in GaAs–AlGaAs V-groove quantum wires. A comparison of surface emitted photoluminescence (PL) and electroluminescence (EL) spectra from a p-i-n junction structure shows enhanced quantum wire emission in EL compared to PL. This behavior is discussed in terms of enhanced carrier capture by the quantum wire for electrical injection into the structure. With increasing forward bias current the quantum wire EL spectra exhibit subband filling and saturation effects. In addition to the ground state transition, at least three excited state related transitions are observed.
Thermal decomposition of ultrathin oxide layers on Si(111) surfaces mediated by surface Si transport
Authors: Watanabe Heiji , Fujita Ken , Ichikawa Masakazu
Published: July 2002

Thermal decomposition of ultrathin oxide layers (less than 1 nm thick) on Si(111) surfaces was studied by using scanning reflection electron microscopy and scanning tunneling microscopy. Void formation, where the diameter and density of the voids depend on the oxide film thickness, occurred on terraces randomly and independently of the buried steps at SiO2/Si(111) interfaces. Decomposition of the oxide layers caused by the void growth produced atomic-height holes on exposed Si surfaces. The surface roughness produced by the holes after thermal decomposition increased with the thickness of the oxide layers. The surface mass transport of Si adatoms to form volatile SiO products explains these experimental results.
Ga segregation in DyBa2Cu3O7−δ/PrBa2Cu3−xGaxO7−δ/DyBa2Cu3O7−δ ramp-type Josephson junctions
Authors: Verbist K. , Lebedev O. I. , Van Tendeloo G. , Verhoeven M. A. J. , Rijnders A. J. H. M. , Blank D. H. A. , Rogalla H.
Published: July 2002

Ramp-type Josephson junctions with highly doped PrBa2Cu3−xGaxO7−δ barrier layers (x=0.7, 1.0) have been investigated by high-resolution electron microscopy. A Ga-rich intergrowth and Ga diffusion in the ion-milled SrTiO3 substrate are observed. The Ga segregation is responsible for the deviating electrical behavior as expected from extrapolation of low doping levels (x=0.1–0.4). The Ga diffusion in the ion-milled substrate, and possibly in the base electrode, may hamper Josephson junction fabrication using substituted REBa2Cu3O7−δ materials (RE=rare earth).
Very low dark current metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN epitaxial layers
Authors: Carrano J. C. , Grudowski P. A. , Eiting C. J. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report a very low dark current (∼57 pA at 10 V reverse bias) metal–semiconductor–metal photodetectors fabricated on GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition. The photodetectors exhibit the typical sharp band-edge cutoff, with good responsivity. There is indication of a photoconductive gain mechanism. We also performed a Medici simulation to establish an effective area for current density calculations.
Cryogenic picosecond sampling using fiber-coupled photoconductive switches
Authors: Verghese S. , Zamdmer N. , Hu Qing , Förster A.
Published: July 2002

A photoconductive sampling technique was developed that is well suited to probing cryogenic devices such as mesoscopic devices. Low-temperature-grown GaAs photoconductive switches were embedded in a coplanar waveguide and illuminated with pigtailed optical fibers. The chip was conduction cooled in a liquid helium cryostat. The fiber-coupled switches were designed to minimize external infrared-radiation loading and to obtain efficient optical to electrical conversion. Measurements at 300, 77, and 4.2 K demonstrate very low thermal loading and a temporal resolution of 4.7 ps—arising from dispersion of the femtosecond pulses (λ=810nm) in the optical fiber.
Stable vortex configurations in superconducting 2×2 antidot clusters
Authors: Puig T. , Rosseel E. , Baert M. , Van Bael M. J. , Moshchalkov V. V. , Bruynseraede Y.
Published: July 2002

The vortex state of superconducting 2×2 antidot clusters has been studied by transport measurements. Characteristic oscillations have been observed in the magnetoresistance and superconducting-normal phase boundary at specific values of the magnetic flux Φ coming from the formation of stable vortex configurations. In analogy with semiconducting 2×2 quantum dot systems, a bistable diagonal state, perspective for single flux quantum logic applications, has been identified at half-flux filling, Φ/Φ0=0.5, where Φ0=h/2e is the flux quantum.
Voltage tunable SiGe photodetector: A novel tool for crypted optical communications through wavelength mixing
Authors: Masini G. , Colace L. , Galluzzi F. , Assanto G. , Pearsall T. P. , Presting H.
Published: July 2002

A novel tandem SiGe–Si device is demonstrated which, based on different photoresponses in two back-to-back diodes, exhibits a voltage tunable spectral responsivity. This feature, in conjunction with a wavelength dependence in the current direction, allows the efficient decryption of a signal when substantial noise is added to distinct carrier frequencies. Time dependent encryption schemes are also possible via a modulated voltage bias.
Structure and magnetism of the Fe/GaAs interface
Authors: Filipe A. , Schuhl A. , Galtier P.
Published: July 2002

We study the magnetic properties of Fe thin films epitaxially grown on GaAs (001) for a large range of substrate temperature. Magnetization deficiency has been observed and studied. Its dependence on both thickness and temperature clearly show the existence of a nearly half-magnetized phase at the interface, covered by “as-bulk” Fe. Furthermore, reflection high-energy electron diffraction studies show a transition between two bcc structures with different crystalline parameters. Transmission electron microscopy confirms the formation of this interfacial phase, for which the compound Fe3Ga2−xAsx seems to be the best candidate.
Quantum transport through atomic wires
Authors: Wan C. C. , Mozos José-Luis , Taraschi Gianni , Wang Jian , Guo Hong
Published: July 2002

We have investigated quantum transport through long wires in which a section consists of one or several Al atoms in a chain. The self-consistent ground state electronic potential is obtained using the first principles ab initio method and the conductance is calculated by solving a three-dimensional quantum scattering problem. We have observed quantized conductance when there are two or more Al atoms in the chain. Resistance is calculated for these wires at the Fermi level.
Reduced bias growth of pure-phase cubic boron nitride
Authors: Litvinov Dmitri , Clarke Roy
Published: July 2002

We report results on an improved growth process for cubic boron nitride (c-BN) films. The films are deposited on a dc-biased silicon substrate using ion-assisted sputtering. First, we grow a BN template layer at a bias voltage which maximizes the sp3 content. After this template layer attains a thickness of ∼500 Å, corresponding to the coalescence of the mosaiclike grain structure, we find that we can reduce the substrate bias to about 50% of its initial value while sustaining pure phase c-BN growth. The reduction in nitrogen ion energy results in a dramatic increase in the growth rate as well as significantly improved film quality.
Determination of interface layer strain of Si/SiO2 interfaces by reflectance difference spectroscopy
Authors: Yang Z. , Chen Y. H. , Ho Jacob Y. L. , Liu W. K. , Fang X. M. , McCann P. J.
Published: July 2002

Detailed studies of the often-observed resonance feature near 3.4 eV in the reflectance difference spectrum of Si/SiO2 interfaces and Si surfaces show that the resonance is due to the intrinsic local-field effect, and that its energy position coincides with the E1 energy of bulk Si. Using the energy position of the resonance of the pseudomorphically grown Si/CaF2 interfaces as a reference point, the strain-induced resonance energy shift of the Si interface layer at several Si/SiO2 interfaces are obtained and the strain in these layers is determined. The results show that the interface layers are highly strained with an equivalent hydrostatic pressure of 0.79 GPa, but still maintain a high degree of order.
Mode selective polymer channel waveguide defined by the photoinduced change in birefringence
Authors: Watanabe Osamu , Tsuchimori Masaaki , Okada Akane , Ito Hiroshi
Published: July 2002

A transverse electric (TE) mode selective polymer waveguide was fabricated by a photobleaching process using a urethane–urea copolymer. The output power of the TE modes through the waveguide was higher than that of the transverse magnetic (TM) modes, when the TE and TM modes were excited equally, the TE mode to TM mode extinction ratio being 27 dB. Mode selectivity was explained by the following relation of the refractive index: the refractive index for the TE modes in the core region was higher than that in the cladding region, while the refractive index for the TM modes in the core was lower than that in the cladding. This relation was attained by the photoinduced change in birefringence of the polymer layer.
Demonstration of waveguide couplers fabricated using microtransfer molding
Authors: Zhao Xiao-Mei , Smith Stephen P. , Waldman Samuel J. , Whitesides George M. , Prentiss Mara
Published: July 2002

Arrays of single-mode optical waveguides, couplers, and interferometers were fabricated from a UV curable prepolymer using microtransfer molding. The coupling between the waveguides was reproducible and consistent across the array and was controlled after fabrication by additional UV exposure.
Logarithmic normal distribution of particle size from a luminescence line-shape analysis in porous silicon
Authors: Yorikawa H. , Muramatsu S.
Published: July 2002

Photoluminescence spectra of porous silicon prepared with electro-chemical etching, were analyzed using a simple model based on a statistical distribution of particle size. It was found that the size of nanoparticles, obeys the logarithmic normal distribution in all the porous silicon investigated here. This model also explains the dependence of the photoluminescence spectrum on the excitation energy.
Chaotic instability of currents in a reverse biased multilayered structure
Authors: Lukin Konstantin A. , Cerdeira Hilda A. , Colavita Alberto A.
Published: July 2002

A new principle to generate chaotic signals using the phenomenon of charge avalanche multiplication and internal feedback in multilayered semiconductor structures is suggested. Linear and nonlinear theories for the self-oscillations are developed and existence of the chaotic regime with fast decay of correlations is proven.
Does a dipole layer at the p–i interface reduce the built-in voltage of amorphous silicon p–i–n solar cells?
Authors: Nuruddin A. , Abelson J. R.
Published: July 2002

The open-circuit voltage of amorphous silicon p–i–n solar cells is 0.1–0.3 V less than the total Fermi level shift in the p- and n-type layers. It was hypothesized that a dipole layer at the p–i interface reduces the potential drop across the i-layer. We determine the electrostatic potential profile using an in situ Kelvin probe during incremental depositions of p-type a-Si,C:H and undoped a-Si:H layers by direct current reactive magnetron sputtering. We confirm the existence of a dipole layer, but which produces a potential loss of only ∼20 mV. Thus, most of the “missing” voltage in solar cells must have other origins.
Low temperature photo-oxidation of silicon using a xenon excimer lamp
Authors: Zhang Jun-Ying , Boyd Ian W.
Published: July 2002

Low temperature (250 °C) photo-oxidation of silicon initiated by a Xe2* excimer lamp operating at a wavelength of 172 nm has been investigated. The induced reaction rate of 0.1 nm/min is 90 times greater than thermal oxidation at 612 °C and more than three times greater than that previously obtained at 350 °C using a low pressure mercury lamp. It was found to be strongly dependent upon oxygen pressure with the highest rates being achievable below 10 mbar. Ellipsometry, Fourier transform infrared spectroscopy, capacitance–voltage, and current–voltage measurements have been employed to characterize the oxide films and designate them as high quality.
A work-hardening based model of the strain relief in multilayer graded-buffer structures
Authors: González D. , Araújo D. , Aragón G. , Garcı́a R.
Published: July 2002

This paper describes the modeling of the dislocation distribution and of the strain relief in linearly and step-graded structures, based on work-hardening considerations. The model considers the energy variation in an InGaAs/GaAs system upon introducing a new dislocation into the interfacial fixed array of misfit dislocations. An analytical expression for the strain relaxation in graded-buffer structures is proposed. Transmission electron microscopy observation confirms the model predictions and reveals that the saturation state of relaxation is reached in the buried layers.
Growth of strain-relaxed Ge films on Si(001) surfaces
Authors: Sakai Akira , Tatsumi Toru , Aoyama Keiko
Published: July 2002

We have grown thin Ge films that were fully strain relaxed and had smooth surfaces on Si(001) surfaces without buffer layers by ultrahigh vacuum chemical vapor deposition. The procedure consists of layer-by-layer Ge growth with hydrogen-surfactant mediation and high-temperature (∼700 °C) post-growth annealing for strain relaxation. The key step is the formation of a thin (less than 1 nm thick) capping Si or SiGe layer on the layered Ge film before the annealing. This capping layer effectively suppresses clustering of Ge during the annealing, even at high temperatures. Cross-sectional transmission electron microscopy of annealed samples having a 20-nm-thick Ge film clearly revealed a periodic array of 90° full-edge dislocations with a Burgers vector of a/2〈110〉 type confined at the Ge/Si interface. This dislocation structure leads to efficient strain relaxation in the Ge film, which was also confirmed by x-ray diffraction measurement.
GaN growth on Si(111) substrate using oxidized AlAs as an intermediate layer
Authors: Kobayashi N. P. , Kobayashi J. T. , Dapkus P. D. , Choi W.-J. , Bond A. E. , Zhang X. , Rich D. H.
Published: July 2002

We have demonstrated that GaN can be grown epitaxially by atmospheric pressure metalorganic chemical vapor deposition on an aluminum oxide compound layer utilized as an intermediate layer between GaN and a Si(111). X-ray diffraction measurement indicates that single-crystal hexagonal GaN with its basal plane parallel to the Si(111) plane is grown. Using a scanning electron microscope, the macroscopic evolution of GaN grown on the AlOx/Si(111) substrate is found to be similar to that of GaN grown on a sapphire(0001) substrate. Cathodoluminescence (CL) spectrum shows a unique emission that consists of several peaks with the intensity comparable to that of the near-band-edge emission. Unique characteristics in CL spectrum are discussed in terms of a possible oxygen contamination of GaN grown on the AlOx/Si(111) substrate.
Inelastic nuclear resonant scattering with sub-meV energy resolution
Authors: Toellner T. S. , Hu M. Y. , Sturhahn W. , Quast K. , Alp E. E.
Published: July 2002

With an undulator-based synchrotron source and a tunable x-ray monochromator, we produce an x-ray beam with a flux of 4×108 photons/s in an energy bandwidth of 920±110 μeV (ΔE/E≈6.2×10−8) at a photon energy of 14.4 keV. The tunability of the monochromator allows for a measurement of lattice excitations in α-Fe using the technique of inelastic nuclear resonant scattering from Fe57. The phonon density-of-states are extracted from the measurement and compared to the calculated phonon density-of-states derived from neutron-scattering measurements.
Transport in gold cluster structures defined by electron-beam lithography
Authors: Clarke L. , Wybourne M. N. , Yan Mingdi , Cai S. X. , Keana J. F. W.
Published: July 2002

The near-room temperature current-voltage (I-V) characteristics of small structures made from the metal-cluster material Au55[P(C6H5)3]12Cl6 were studied. It is shown that these electron-beam defined structures have highly nonlinear characteristics with features, including a threshold voltage and scaling behavior, which are consistent with Coulomb charging of individual Au55 cores in a disordered array. Applied radio frequency signals introduce plateaus in the I-V characteristics, which demonstrates the presence of coherent tunneling in these cluster systems.
Ultralow-threshold (50 A/cm2) strained single-quantum-well GaInAsSb/AlGaAsSb lasers emitting at 2.05 μm
Authors: Turner G. W. , Choi H. K. , Manfra M. J.
Published: July 2002

Strained single-quantum-well, broadened-waveguide GaInAsSb/AlGaAsSb diode lasers have exhibited room-temperature threshold current densities as low as 50 A/cm2, one of the lowest values reported for diode lasers at room temperature. These lasers, grown by molecular beam epitaxy, have emission wavelengths of ∼2.05 μm, characteristic temperature of 65 K, internal quantum efficiency of 95%, and internal loss coefficient of 7 cm−1. Single-ended cw power of 1 W is obtained for a 100-μm aperture.
A Fullerene derivative as an electron beam resist for nanolithography
Authors: Robinson A. P. G. , Palmer R. E. , Tada T. , Kanayama T. , Preece J. A.
Published: July 2002

We have explored the application of chemical derivatives of C60 as high-resolution electron beam resists. Facile spin coating was used to produce ∼100-nm-thick films of a C60 tris adduct (three functional groups) on Si surfaces. We find that these films function as high-resolution negative resists for electron beam lithography using monochlorobenzene as a developer. The film has a sensitivity of ∼1 mC/cm2 for 20 keV electrons, an order of magnitude higher than that of C60 itself, and the dry-etch durability is much better than that of conventional novolac based electron beam resists. Features with widths of 20 nm were produced.
Demonstration of cascade process in InAs/GaInSb/AlSb mid-infrared light emitting devices
Authors: Dupont E. , McCaffrey J. P. , Liu H. C. , Buchanan M. , Yang Rui Q. , Lin C.-H. , Zhang D. , Pei S. S.
Published: July 2002

We demonstrate the cascade process in mid-infrared electroluminescent InAs/GaInSb/AlSb multi-quantum-well devices. We report the proportional relation between the emitted optical power and the number of periods. This observed scaling is associated with the sequential transport of electrons from one active region to the next situated downstream in potential energy through the injection region. Deviations from this exact scaling are correlated with the variation of the wafer-to-wafer structural quality.
Modeling the fluorescent lifetime of Y2O3:Eu
Authors: Ranson R. M. , Evangelou E. , Thomas C. B.
Published: July 2002

Phosphor thermography, relying on the temperature dependence of the decay time of photoluminescence from suitable phosphors, provides remote measurement of the temperature of components. Such a phosphor is yttrium oxide doped with europium (Y2O3:Eu). Associated with this phosphor is also a rise time. Demonstrated is that the rise time is also temperature dependent, as a result of known electronic transitions within the Eu ions. For the phosphor Y2O3:Eu (3.4 at.%), the rise time is an activated process in the temperature region between 25 and 850 °C. Faster than the decay time, the rise time offers the opportunity for measurement of higher velocity components.
Observation of persistent photoluminescence in porous silicon: Evidence of surface emission
Authors: Fan J. C. , Chen C. H. , Chen Y. F.
Published: July 2002

We report on the observation of persistent photoluminescence (PPL) in oxidized porous silicon. The PPL decay can be well described by a stretched-exponential function, and its decay rate is not sensitive to the change of temperature. We point out that the PPL behavior can be interpreted in terms of the picture that the emission arises from the excited surface complexes, which is produced by capture of photocarriers tunneling from the nearest shallow trap in the nanocrystalline silicon. To explore the microscopic origin of the surface compounds, we performed infrared absorption, and found that the PPL intensity correlates well with Si–OH vibration mode. Further evidence is provided by the recent theoretical calculation showing that the Si–OH complex can emit the photon energy in the range observed here. We thus provide concrete evidence to support the fact that the PL signal of porous silicon does contain surface emission.
Multimillion-atom molecular dynamics simulation of atomic level stresses in Si(111)/Si3N4(0001) nanopixels
Authors: Bachlechner Martina E. , Omeltchenko Andrey , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya , Ebbsjö Ingvar , Madhukar Anupam , Messina Paul
Published: July 2002

Ten million atom multiresolution molecular-dynamics simulations are performed on parallel computers to determine atomic-level stress distributions in a 54 nm nanopixel on a 0.1 μm silicon substrate. Effects of surfaces, edges, and lattice mismatch at the Si(111)/Si3N4(0001) interface on the stress distributions are investigated. Stresses are found to be highly inhomogeneous in the nanopixel. The top surface of silicon nitride has a compressive stress of +3 GPa and the stress is tensile, −1 GPa, in silicon below the interface.
Identification of Ag-acceptor related photoluminescence in Ag111 doped CdTe
Authors: Hamann J. , Burchard A. , Deicher M. , Filz T. , Ostheimer V. , Schmitz C. , Wolf H. , Wichert Th. ,   The ISOLDE Collaboration
Published: July 2002

Bridgman-grown, nominally undoped CdTe crystals were doped with Ag by implanting radioactive Ag111. Photoluminescence spectra of the crystals show a donor-acceptor pair (DAP) line at 1.491 eV. The decrease of the intensity of this line with a half life of T1/2=(7.2±0.4) d is in good agreement with the half life of the β− decay of Ag111 to Cd111 of 7.45 d. This decrease is not caused by the aging behavior of Ag which was reported in the literature. The data show that the involved acceptor defect contains exactly one Ag atom and confirm the earlier assignment of the acceptor to the AgCd defect. Based on the DAP line at 1.491 eV, the spectra did not reveal a contamination of the CdTe crystals by stable Ag.
Experimental verification of super resolution in nonlinear inverse scattering
Authors: Chen Fu-Chiarng , Chew Weng Cho
Published: July 2002

This letter presents an experimental verification of the super-resolution phenomenon in a nonlinear inverse scattering algorithm, namely the distorted Born iterative method, by using an experimental setup based on a recently developed time-domain ultra-wide-band radar imaging system at the University of Illinois at Urbana-Champaign. The experimental result also demonstrates that the distorted Born iterative method can recover information on the backside of a penetrable scatterer even though scattering data are collected only from limited viewing angles on the front side of the scatterer.
Microprobe spectroscopy of localized exciton states in II–VI quantum wells
Authors: Cingolani R. , Sogawa F. , Arakawa Y. , Vanzetti L. , Sorba L. , Franciosi A.
Published: July 2002

We have investigated exciton localization in Zn1−xCdxSe/ZnSe quantum wells by microprobe, spatially resolved luminescence. We found that compositional fluctuations in Cd-rich (x=0.23) quantum wells induce the formation of a tail of states in the gap, where a continuum of localized exciton states forms. On the other hand, shallow quantum wells (x=0.11) exhibit sharp lines due to excitons localized at monolayer well-width fluctuations extending over a few hundred nanometers. The observed results are consistent with the occurrence of localized exciton lasing in deep ZnCdSe quantum wells.
Significantly enhanced creep resistance in low-melting-point solders through nanoscale oxide dispersions
Authors: Mavoori H. , Jin S.
Published: July 2002

Nanosized, nonreacting, and noncoarsening oxide dispersoids have been incorporated into solder alloys to create an improved solder structure with an ultrafine grain size of ∼2000–5000 Å and significantly enhanced mechanical properties. As much as three orders of magnitude reduction in the steady-state creep rate has been achieved. These solders also exhibit improved (4–5 times higher) tensile strength at low strain rates and improved ductility under high strain rate deformation. It is demonstrated that with a dispersion of TiO2 particles, the Pb–Sn eutectic solder with a low melting point of 183 °C can be made more creep resistant than the 80Au–20Sn eutectic solder with a much higher melting point of 278 °C.
Highly anharmonic potential modulation in lateral superlattices fabricated using epitaxial InGaAs stressors
Authors: Luyken R. J. , Lorke A. , Song A. M. , Streibl M. , Kotthaus J. P. , Kadow C. , English J. H. , Gossard A. C.
Published: July 2002

A coherently strained stressor structure is used to create a one-dimensional periodic potential in the two-dimensional electron gas at a AlGaAs/GaAs heterointerface. We demonstrate that from magnetotransport the Fourier coefficients of the conduction band modulation can be determined. In contrast to conventional electrostatic patterning, “hard” potential modulation with dominant contributions of higher harmonics is achieved. In the regime of narrow stressor-stressor distance, the strain-induced potential modulation can be calculated analytically from elasticity theory. The calculated magnetoresistance which can be derived from the stressor-induced potential is in good agreement with the experimental data.