bluuebunny/crossref_metadata_2025 huggingface dataset:
A computer-controlled arbitrary flow wave form generator for physiological studies
Authors: Eriksson Anders , Persson Hans W. , Lindström Kjell
Published: July 2002

The ability to reproduce wave forms of the human arterial blood flow in vitro is essential for the study of vascular haemodynamics. The arbitrary flow generator described in this article is designed for this application. The flow generator consists of a progressive cavity pump driven by a computer-controlled servo motor. The chosen pump type offers advantages such as good linearity, high load tolerance and mechanical robustness. The flow generator is capable of producing noninterruptible standard and physiological wave forms. Standard wave forms include sinus, triangular, square and pulse. Three physiological wave forms are predefined: carotid, femoral and aorta. The performance of the flow generator is evaluated in detail. Constant flow up to 5 l/min is generated with high accuracy (<1%) and low ripple (−50 dB). The output is virtually insensitive to an increased flow resistance. A back pressure of 0.5 bar results in an output loss of less than 2.5 ml/min. The flow generator has an output bandwidth (−3 dB) of 11 Hz and a maximum slew rate of 2210 ml/s2. No significant difference (p<0.001) was found in the motor regulation during load. The achieved performance makes the flow generator very suitable for its application.
Apparatus for temperature programmed desorption studies of thin films
Authors: Chalamala Babu R. , Uebelhoer David , Reuss Robert H.
Published: July 2002

A versatile analytical system based on thermal desorption spectroscopy of thin films is presented. We have found the apparatus to be a useful tool for measuring the desorption characteristics of trapped gases in physical vapor deposited thin films and in determining the thermal stability of multi-component thin film compounds and multilayer structures. The system was also found to be a convenient tool for determining outgassing properties of thin film and thick film materials for vacuum microelectronic device applications.
A simple circuit for driving light-emitting diodes with low supply voltage
Authors: Liao Tai-Shan , Chang Chun-Ming
Published: July 2002

A low-cost and simple circuit for driving light-emitting diodes with low supply voltage was demonstrated. By using only three transistors, a light emitting diode was drived with supplied voltage as low as 0.85 voltage.
Effect of chemical and ion-beam etching on the atomic structure of interfaces in YBa2Cu3O7/PrBa2Cu3O7 Josephson junctions
Authors: Jia C. L. , Faley M. I. , Poppe U. , Urban K.
Published: July 2002

The atomic structure of the interfaces of Josephson junctions formed by epitaxial YBa2Cu3O7/PrBa2Cu3O7/YBa2Cu3O7 triple-layer films was investigated by high-resolution transmission electron microscopy. The samples were fabricated by sputter deposition on surfaces which were etched ex situ either chemically, using a nonaqueous Br-ethanol solution, or by an Ar ion beam. In the interfaces produced after ion etching a thin intermediate layer with a thickness of a few nanometers was observed. The main part of this layer consists of cubic PrBa2Cu3O7 or YBa2Cu3O7 which is cation disordered. The interfaces formed during deposition on Br-ethanol-etched surfaces did not contain such an intermediate layer but exhibited high structural perfection similar to that of interfaces produced in situ. These observations permit a qualitative explanation of the difference in the electrical properties of junctions produced by these two techniques.
Breaking up of misfit dislocations in GaAs/In0.3Ga0.7As/GaAs heterostructure
Authors: Wu J. , Li W. , Fan T. W. , Wang Z. G. , Duan X. F.
Published: July 2002

The effect of GaAs cap layer with different thicknesses in the GaAs/In0.3Ga0.7As/GaAs heterostructure on misfit dislocation is investigated with transmission electron microscopy, and it is found that lines of misfit dislocation break up and move out of the structure when the GaAs cap layer thickness exceeds a certain amount. The breaking up and moving out of misfit dislocations, initially confined in the (001) substrate/InGaAs epilayer interface, occur mainly along the [110] direction on the interface in the structure.
Nanometer-scale magnetic MnAs particles in GaAs grown by molecular beam epitaxy
Authors: De Boeck J. , Oesterholt R. , Van Esch A. , Bender H. , Bruynseraede C. , Van Hoof C. , Borghs G.
Published: July 2002

Spherical MnAs ferromagnetic particles with controllable diameters (5–30 nm) are embedded in a high quality GaAs matrix. The particles are formed in a two step process consisting of the epitaxy of a homogeneous Ga1−xMnxAs layer at low temperatures using molecular beam epitaxy followed by phase separation upon annealing. During the annealing step, the excess arsenic in the as-grown film forms magnetic MnAs precipitates with the Mn from the Ga1−xMnxAs lattice. Structural and room-temperature magnetic properties of the heterogeneous GaAs:MnAs films are described. The magnetic MnAs rich layers can be incorporated into semiconductor heterostructures as demonstrated by growing (GaAs/AlAs) multiple quantum well structures in combination with GaAs:MnAs layers.
Spin reversal in ultra-thin magnetic films with fourfold anisotropy
Authors: Chui S. T.
Published: July 2002

We discuss the coercive behaviour of systems with fourfold symmetry. In agreement with recent experiments, in simulation studies we found a two step process with two coercive fields for the x and the y magnetizations. A new physical picture for the spin reversal is obtained. Dipolar effects are found to be very important.
Evidence of ferroelectricity weakening in the polycrystalline PbTiO3 thin films
Authors: Sun Li , Chen Yan-Feng , Ma Wen-Hui , Wang Lian-Wei , Yu Tao , Zhang Ming-Sheng , Ming Nai-Ben
Published: July 2002

Studies on the polycrystalline PbTiO3 thin films deposited on (001) redoped n-Si substrates by metalorganic chemical vapor deposition (MOCVD) under reduced pressure indicated the evidence of ferroelectricity weakening [Appl. Phys. Lett. 65, 1906 (1994)] when the grain size of PbTiO3 was down to the scale of ∼1100 Å. X-ray diffraction patterns demonstrated that the perovskite unit cells have a contraction along the c-axis direction. All transverse optical phonon modes in the films were observed shift downward in the Raman spectrum measured at 300 K. And the spontaneous polarization of PbTiO3 is determined to be 52 μC/cm2 by using Si substrates as the bottom electrodes. The three related experimental observations are conjected to be attributable to the finite size effect and the surface conditions in the ferroelectric thin film.
Anomalous second harmonic generation in BaTiO3 thin films
Authors: Rotter L. D. , Kaiser D. L. , Vaudin M. D.
Published: July 2002

We present data on second harmonic generation in as-grown BaTiO3 thin films, including the dependence of the second harmonic intensity on the incidence angle and on the polarization direction of the fundamental beam. The data indicate that the effective second harmonic generation coefficient in these films is unrelated to the crystallographic orientation of the BaTiO3 as determined by x-ray diffraction. We suggest as an alternative explanation electric-field-induced second harmonic generation.
Observation of two-dimensional photonic band behavior in the visible
Authors: Lin H.-B. , Tonucci R. J. , Campillo A. J.
Published: July 2002

Nanochannel glasses, containing regular hexagonal closed packed arrays of parallel air cylinders with periods varying from 0.25 to 0.19 μm, are shown to display photonic band-gap behavior throughout the visible that is consistent with previously published theory. Immersing the glasses in liquids of various refractive indices allowed observation of the buildup of the bands as a function of cylinder dielectric constant.
High power InAsSb/InAsSbP double heterostructure laser for continuous wave operation at 3.6 μm
Authors: Popov Andrei , Sherstnev Victor , Yakovlev Yury , Mücke Robert , Werle Peter
Published: July 2002

High continuous wave (cw) optical output power of 10 mW has been achieved from an InAsSb diode laser at 82 K in the 3.6 μm spectral region. The threshold current has been as low as 37 mA at 82 K with a characteristic temperature of 23 K. It could be shown that in multimode operation of the InAsSb-laser the cw optical output power for a single lasing mode is limited to about 2 mW.
Tunneling current noise in thin gate oxides
Authors: Alers G. B. , Krisch K. S. , Monroe D. , Weir B. E. , Chang A. M.
Published: July 2002

We have examined fluctuations in the tunneling current of 3.5 nm SiO2 barriers for voltages in the direct tunneling regime. We find a 1/f power law for the spectral density of the fluctuations where f is the frequency. This 1/f noise can be attributed to fluctuations of a trap assisted tunneling current through the oxide that causes current noise but is not evident in the I–V curves. We suggest that this noise may be a more sensitive probe of trap assisted tunneling and degradation in thin oxides than other measures. At voltages above a threshold of 2.5 V, we observe the reversible onset of non-Gaussian current transients in the noise. The onset of these current transients can be related to a transition in the spacial uniformity of the tunneling current density that may result in eventual breakdown of the oxide.
Temperature-dependent study of optical gain in (Zn,Cd)Se/ZnSe multiple-quantum-well structures
Authors: Kreller F. , Puls J. , Henneberger F.
Published: July 2002

The role of excitons in the laser action of ternary II–VI quantum wells is investigated. As a consequence of an increased stability due to localization on alloy disorder, bi-excitons are dominant up to temperatures clearly above 100 K. Steady-state pump-probe measurements reveal a bi-excitonic gain as large as 2.5×104 cm−1. Above 200 K, the threshold density grows steeply and the laser emission turns into electron–hole plasma recombination.
Current self-oscillations in photoexcited type-II GaAs-AlAs superlattices
Authors: Hosoda M. , Mimura H. , Ohtani N. , Tominaga K. , Watanabe T. , Fujiwara K. , Grahn H. T.
Published: July 2002

Self-oscillations of the photocurrent have been observed in type-II GaAs-AlAs superlattices. In addition to the fundamental frequency, several higher harmonics are present. The frequency of the oscillations can be tuned for a fixed carrier density from 15 to 120 MHz by simply changing the applied bias. The frequency distribution within a certain voltage range can be varied by changing the density of photoexcited carriers. For larger carrier densities, higher frequencies are observed in a different voltage range. This system could therefore be used as a high-frequency oscillator, which can be controlled by two external parameters, the applied voltage and the light intensity.
Light-induced expansion of fiber tips in near-field scanning optical microscopy
Authors: Lienau Ch. , Richter A. , Elsaesser T.
Published: July 2002

The emission profiles of laser diodes working at 780 nm and 1300 nm are studied by near-field scanning optical microscopy. As the near-field probe is scanned across the laser mirror facet, the laser emission induces a transient expansion of the probe tip which is monitored using shear force microscopy. The thermal expansion of the tips reaches absolute values of up to 100 nm per mW of emitted laser power. A fully metallized near-field probe tip is shown to serve as a local bolometer with a spatial resolution of better than 1 μm.
Oscillations of interlayer coupling and magneto-optics in sputtered Co–Al/Cu multilayers
Authors: Zhou Shi-Ming , Chen Liang-Yao , Zheng Wei-Ming , Zhu Wei-Rong , Wang Yu , Wang Ya-Dong , Zheng Yu-Xiang , Jin Qing-Yuan , Qian You-Hua
Published: July 2002

We have prepared [Co90Al10/Cu]30 multilayers by ion beam sputtering and studied the behavior of interlayer coupling, magnetoresistance, and the magneto-optical Kerr effect. For Co–Al/Cu multilayers, the in-plane saturation field, coercive force HC and magnetoresistance ratio ΔR/R have been observed to oscillate with a period of about 0.9 nm, as a function of Cu spacer layer thickness. Furthermore, polar Kerr rotation θk in visible wavelength range also oscillates with the same period. The magneto-optical activities may be attributed to changes in the optical constants with the magnetic state.
Mechanical resonance behavior of near-field optical microscope probes
Authors: Froehlich Fred F. , Milster Tom D.
Published: July 2002

The mechanical resonance behavior of near-field optical microscope probes is examined with a simple experiment on a flat pyrex sample. While our tapered-fiber probe is locked on the second resonance for servo control, the vibration characteristics around the first resonance are investigated. We find that the overwhelming cause of decreased vibration amplitude as the tip approaches the sample is an increase in damping presumably due to a fluidlike layer on the sample. A small additional effect is also observed that could be due to force derivatives.
Synthesis of GaN nanocrystals by sequential ion implantation
Authors: Wolk J. A. , Yu K. M. , Bourret-Courchesne E. D. , Johnson E.
Published: July 2002

We have synthesized GaN nanocrystals by sequential implantation of Ga and N ions into a sapphire substrate followed by a postimplantation anneal. The nanocrystals have been identified as the wurtzite phase α-GaN structure by transmission electron microscopy. We also found that the nanocrystals are aligned with the sapphire following the relationship: (0001)sapphire∥(0001)GaN and (112̄0)sapphire∥(112̄0)GaN. The use of a sapphire substrate allows for the measurements of optical properties, and near band-edge luminescence and the yellow band are observed in photoluminescence spectroscopy.
Effect of nonhydrostaticity on the pressure induced phase transformation of rhombohedral boron nitride
Authors: Taniguchi T. , Sato T. , Utsumi W. , Kikegawa T. , Shimomura O.
Published: July 2002

An in situ x-ray diffraction study of room-temperature compression of rhombohedral boron nitride (rBN) was performed up to 10 GPa. Although no phase transformation of rBN was observed in a liquid pressure transmitting medium, the structure of rBN changed to become disordered within the layered stacking sequence at less than 1 GPa with the solid-state pressure transmitting medium. Further transformation to the wurtzite structure (wBN) was observed at 6 GPa and was unquenchable upon the release of pressure at room temperature. The orientation relationship of the phase transformation of rBN to wBN was compared with that of hexagonal BN to wBN.
Reduction of dislocation density in mismatched SiGe/Si using a low-temperature Si buffer layer
Authors: Linder K. K. , Zhang F. C. , Rieh J.-S. , Bhattacharya P. , Houghton D.
Published: July 2002

The reduction of the dislocation density in relaxed SiGe/Si heterostructures using a low-temperature Si(LT-Si) buffer has been investigated. We have shown that a 0.1 μm LT-Si buffer reduces the threading dislocation density in mismatched Si0.85Ge0.15/Si epitaxial layers as low as ∼104 cm−2. Samples were grown by both gas-source molecular beam epitaxy and ultrahigh vacuum chemical vapor deposition.
Metal–oxide–semiconductor capacitors formed by oxidation of polycrystalline silicon on SiC
Authors: Tan J. , Das M. K. , Cooper J. A. , Melloch M. R.
Published: July 2002

A method to form SiO2/SiC metal–oxide–semiconductor structures by oxidation of a thin polycrystalline silicon (polysilicon) layer deposited on SiC is demonstrated. The oxidation time used is sufficient to oxidize all the polysilicon while short enough at 1050 °C to insure insignificant oxidation of the underlying SiC. Since the oxidation of SiC is highly anisotropic, this method allows uniform oxide formation on a nonplanar SiC surface. The SiO2/SiC interface quality is comparable to that obtained with thermal oxidation.
Low-power photocurrent nonlinearity in quantum well infrared detectors
Authors: Mermelstein C. , Schneider H. , Sa’ar A. , Schönbein C. , Walther M. , Bihlmann G.
Published: July 2002

We report on the intensity dependence of the responsivity in quantum well infrared photodetectors (QWIP). A strong reduction of the responsivity is observed already at small excitation powers for a QWIP with N=4 periods. This nonlinearity is caused by a partial screening of the electric field across the main part of the active region. The intensity dependence is analyzed using a phenomenological approach, which allows us to calculate the nonlinearity from the measured dark current and responsivity. Applying this approach to a background-limited 50 period QWIP, we find that the nonlinearity is already present below 1 mW cm−2, which is less than the thermal background.
High rate dry etching of Ni0.8Fe0.2 and NiFeCo
Authors: Jung K. B. , Lambers E. S. , Childress J. R. , Pearton S. J. , Jenson M. , Hurst A. T.
Published: July 2002

A Cl2/Ar plasma chemistry operated under electron cyclotron resonance (ECR) conditions is found to produce etch rates for NiFe and NiFeCo of ⩾3000 Å min−1 at ⩽80 °C. The etch rates are proportional to ion density and average ion energy over a fairly wide range of conditions. Under the same conditions, fluorine or methane/hydrogen plasma chemistries produce rates lower than the Ar sputter rate. The high ion current under ECR conditions appears to balance NiClx, FeClx, and CoClx etch product formation with efficient ion-assisted desorption, and prevents formation of the usual chlorinated selvedge layer that requires elevated ion etching conditions. Post Cl2-etch removal of surface residues is performed with an in situ H2 plasma exposure.
Formation of nanoscale ferromagnetic MnAs crystallites in low-temperature grown GaAs
Authors: Wellmann P. J. , Garcia J. M. , Feng J.-L. , Petroff P. M.
Published: July 2002

We report the formation of nanosize ferromagnetic MnAs crystallites imbedded in low-temperature grown GaAs using Mn+ ion implantation and subsequent annealing. The structural and magnetic properties of the crystallites have been characterized by transmission electron microscopy, electron beam induced x-ray fluorescence, and superconducting quantum interference device magnetometry. After an optimized thermal annealing at 750 °C, MnAs crystallites of 50 nm in size are formed. These nanomagnets show room temperature ferromagnetism.
Organic–inorganic heterojunction light emitting diodes based on poly(p-phenylene vinylene)/cadmium sulfide thin films
Authors: Deepak Kumar N. , Joshi Mukesh P. , Friend Christopher S. , Prasad Paras N. , Burzynski Ryszard
Published: July 2002

We report on a solution processed novel electroluminescent heterostructure device consisting of an inorganic semiconductor, CdS, and a polymer emitter, poly(p-phenylene vinylene) (PPV). This configuration provides the advantage of excellent charge transporting properties of an inorganic semiconductor and the high luminescence quantum efficiency of an organic emitter. The electroluminescence spectrum obtained from this hetrostructure device is similar to the emission spectrum of pure PPV. Even with aluminum electrode, we have achieved stable electroluminescence at very low threshold voltage (3 V) and overall quantum efficiency of about 1% in these devices. The device emitted bright electroluminescence radiation (150 cd/m2) at a driving voltage of 10 V under the forward bias condition.
Generation of pulse-modulated induction thermal plasma at atmospheric pressure
Authors: Ishigaki Takamasa , Fan Xiaobao , Sakuta Tadahiro , Banjo Toshiyuki , Shibuya Yukihito
Published: July 2002

The radio frequency induction thermal plasma of sufficiently high electric power for materials processing has been successfully generated with a pulsemodulated operating condition. A solid-state amplifier, which supplies the electric power with a nominal frequency of 1 MHz, was employed for the pulsing plasma generation. The Ar–H2 plasma was generated at a high power level of 17 kW at atmospheric pressure. Typically, the plasma remained stable until the pulse duty factor went down to 30%, when the period of the high power level was 5 ms and the low power level was about 6 kW.
A sonochemical approach to the surface synthesis of cadmium sulfide nanoparticles on submicron silica
Authors: Arul Dhas N. , Gedanken A.
Published: July 2002

A sonochemical procedure for the preparation of cadmium sulfide (CdS) nanoparticles coated on submicron spherical silica particles has been described. Ultrasonic irradiation of a slurry of silica microspheres, cadmium sulfate, and thiourea in an aqueous medium for 3 h under ambient air yields cadmium sulfide–silica (CSS) composite. Heating the initial amorphous CSS nanocomposite at 150 °C for 1 h under a N2 atmosphere yields diffraction peaks assignable to the CdS phase. The transmission electron microscopy image of CSS shows that the CdS nanoparticles homogeneously coated on the silica (SiO2) carrier. The infrared spectroscopy illustrated the structural changes that occur when the amorphous SiO2 is coated with CdS nanoparticles ultrasonically. The photoluminescence spectrum of the CdS and CSS shows a broadband with a maximum centered around 560 nm, which is similar to that of quantum CdS particles. The sharp onset of a CSS emission band compared to that of the CdS emission band in the red region indicates that the SiO2 carrier provides a medium for controlling the aggregation of CdS nanoparticles.
Demonstration of Si homojunction far-infrared detectors
Authors: Perera A. G. U. , Shen W. Z. , Liu H. C. , Buchanan M. , Tanner M. O. , Wang K. L.
Published: July 2002

A 48 μm cutoff wavelength (λc) Si far-infrared (FIR) detector is demonstrated. Internal photoemission over a Si interfacial work-function of a homojunction consisting of molecular beam epitaxy grown multilayers (p+ emitter layers and intrinsic layers) is employed. The detector shows high responsivity over a wide wavelength range with a peak responsivity of 12.3±0.1 A/W at 27.5 μm and detectivity D* of 6.6×1010 cmHz/W. The λc and bias dependent quantum efficiency agree well with theory. Based on the experimental results and the model, Si FIR detectors (40–200 μm) with high performance and tailorable λcs can be realized using higher emitter layer doping concentrations.
MeV He+ ion induced delamination of diamond films
Authors: Som T. , Bhargava S. , Malhotra M. , Bist H. D. , Kulkarni V. N. , Kumar Satyendra
Published: July 2002

We report room temperature delamination of chemically vapor deposited diamond films on a Si substrate due to 1.5 MeV He+ ion bombardment. The delamination of films has been studied by scanning electron microscopy. The threshold fluence at which the delamination takes place has been determined in situ by monitoring the hydrogen signal with the help of elastic recoil detection analysis. Micro Raman measurements show the presence of residual stress (1.19 GPa) in the as-prepared films and its relaxation at the peripheri of the delaminated regions. It is proposed that enhancement of the residual stress during He+ ion bombardment leads to stress saturation conditions which result in the delamination of the brittle diamond films from the film/substrate interface. These findings lead to the possibility of creating ion-beam induced channels in diamond films for device isolation by suitable choice of film/substrate combination.
Electron and hole tunneling in a moderate density quantum dot ensemble with shallow confinement potentials
Authors: Huffaker D. L. , Deppe D. G.
Published: July 2002

Data are presented on InAlGaAs quantum dot ensembles that possess different degrees of lateral tunnel coupling for electrons and holes. Atomic force microscope images show that even for moderate densities (∼4.8×1010 cm−2), but with shallow potentials, many dots of an ensemble can be in such close proximity that tunneling is expected for both electrons and holes. On average, however, holes remain confined while electrons freely tunnel. Even with electron tunneling, single dot emission is observed for small area light emitting diodes.
Optical determination of molecule diffusion coefficients in polymer films
Authors: Podgorsek R. P. , Franke H.
Published: July 2002

We present a method using surface-plasmon leaky mode spectroscopy for the analysis of the vapor–molecule diffusion process in polymer films leading to the diffusion coefficients of the molecules. The solution of the diffusion differential equation in the case of thin films yields a molecule concentration profile. Linearity between refractive index increase and molecule concentration gives the same variation for the index increase. Thus, a theoretical expression for the measured reflectivity can be derived using the transfer-matrix formalism for layered media, and theoretical fits to measured reflectivity data finally yield the corresponding diffusion coefficients. For the polymer Teflon®AF1600 diffusion coefficients for benzene and toluene are determined to 4.9×10−10 and 2.7×10−10 cm2/s, respectively.
Spectroscopic characterization of the evolution of self-assembled CdSe quantum dots
Authors: Kim J. C. , Rho H. , Smith L. M. , Jackson Howard E. , Lee S. , Dobrowolska M. , Merz J. L. , Furdyna J. K.
Published: July 2002

We have investigated the evolution of molecular beam epitaxy (MBE)-grown, CdSe self-assembled quantum dots on ZnSe surfaces using microphotoluminescence techniques. Bare CdSe dots at room temperature undergo Ostwald ripening over a time scale measured in days. At the elevated temperatures maintained for MBE growth and dot formation, ripening is expected to progress much faster. Capping the dots with a thin ZnSe layer “freezes” the ripening, allowing one to sample different stages of the dot evolution and subsequent characterization. We have grown eleven samples, each with a different time interval, or growth interruption, between dot formation and capping; the growth interruption times ranging from 0 to 300 s, during which the samples were kept at 300 °C. Using microphotoluminescence spectroscopy, we have resolved the sharp emission peaks due to individual dots in each sample and, by analyzing the ensemble characteristics, have identified a new regime in the evolution of CdSe dots.
Injection lasers with vertically aligned InP/GaInP quantum dots: Dependence of the threshold current on temperature and dot size
Authors: Riedl T. , Fehrenbacher E. , Hangleiter A. , Zundel M. K. , Eberl K.
Published: July 2002

We report on threefold-stacked vertically aligned InP/GaInP quantum dot injection lasers emitting in the visible part of the spectrum (690–705 nm) with a low threshold current density of jth=172 A/cm2 at 90 K showing a thermally activated increase towards higher temperatures. We derived an activation energy for this behavior, which is found to be just one half of the energetic distance between the dot transition energy and the wetting layer band gap. Thus, we identify thermal evaporation of carriers out of the dots and into the wetting layer states as the process responsible for the increase in the threshold current. The nonradiative carrier lifetime in the wetting layer (τnrWL) is estimated to be approximately 250–400 ps.
Epitaxial MnAs/GaAs/MnAs trilayer magnetic heterostructures
Authors: Tanaka M. , Saito K. , Nishinaga T.
Published: July 2002

We have successfully grown MnAs/GaAs/MnAs ferromagnet/semiconductor trilayer heterostructures on GaAs(111)B substrates by molecular beam epitaxy. The epitaxial orientations of MnAs and GaAs are (0001) and (111), respectively, as expected. It was found that epitaxial monocrystalline GaAs can be grown on the As-rich (3×2)-(0001) MnAs surface. Cross-sectional images by transmission electron microscopy showed that the trilayers are formed as intended with fairly smooth and atomically abrupt interfaces. Double-step features were observed in magnetization characteristics due to the difference in coercive force between the top and bottom MnAs layers. The interlayer coupling was small when the thickness of the GaAs spacer layer was 5–10 nm.
Experimental observation of two-dimensional electron gas in the CdS quantum wells of CdS/ZnSe heterostructures
Authors: Kažukauskas V. , Grün M. , Petillon St. , Storzum A. , Klingshirn C.
Published: July 2002

We report the observation of a two-dimensional electron gas (2DEG) in the CdS quantum wells of CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy. The CdS layers are about 4 nm thick; the thicknesses of the ZnSe-doped layers and the growth temperatures of the heterostructures were varied in order to investigate their influence on the 2DEG parameters. The effective sheet concentration of the 2DEGs reached up to 1.5×1013 cm−2. The maximal Hall mobilities were nevertheless still inferior or equal to 380 cm2/V s. Possible scattering mechanisms that cause such low values are discussed. Most probably, it is scattering by stacking faults or by interface roughness.
Optical studies of electric fields in poly(2-methoxy-5-ethyl (2′-hexyloxy) para-phenylene vinylene) light-emitting diodes
Authors: Giebeler C. , Whitelegg S. A. , Campbell A. J. , Liess M. , Martin S. J. , Lane P. A. , Bradley D. D. C. , Webster G. , Burn P. L.
Published: July 2002

We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade.
Order–disorder transition in epitaxial ZnSnP2
Authors: Seryogin G. A. , Nikishin S. A. , Temkin H. , Mintairov A. M. , Merz J. L. , Holtz M.
Published: July 2002

We report on the growth of ZnSnP2 on GaAs(100) substrates by gas source molecular beam epitaxy. Samples were grown in the temperature range of 300–360 °C. A small change in the Sn/Zn flux ratio at constant substrate temperature was found to result in a transition from a lattice mismatched, Δa/a∼0.4%–0.7%, disordered crystal structure to a lattice matched, ordered chalcopyrite structure. Infrared reflectance and Raman measurements were used to monitor this phase transition. Formation of the two different crystal modifications is discussed in terms of vapor–solid and vapor–liquid–solid growth modes.
Adsorption of Zn on the GaAs(001)-(2×4) surface
Authors: Ohtake Akihiro , Hanada Takashi , Yasuda Tetsuji , Yao Takafumi
Published: July 2002

Adsorption of Zn on the GaAs(001)-(2×4) surface has been studied by reflection high-energy electron diffraction (RHEED) and total-reflection-angle x-ray spectroscopy in real time. Adsorption sites of Zn atoms on the (2×4) surface have been determined using rocking-curve analysis of RHEED. The exposure of the (2×4) surface to a Zn beam results in the preferred adsorption of Zn at Ga-vacancy sites in the missing dimer trenches of the (2×4) unit cell. The amount of adsorbed Zn atoms under a Zn flux has been estimated to be ∼0.50 and ∼0.20 monolayers at 200 and 250 °C, respectively.
Molecular beam epitaxial growth of InGaAsN:Sb/GaAs quantum wells for long-wavelength semiconductor lasers
Authors: Yang X. , Jurkovic M. J. , Heroux J. B. , Wang W. I.
Published: July 2002

InGaAsN:Sb/GaAs quantum wells (QWs) were grown by solid-source molecular beam epitaxy using a N2 radio-frequency plasma source. Photoluminescence reveals an enhancement in the optical properties of InGaAsN/GaAs QWs by the introduction of Sb flux during growth. X-ray diffraction and reflection high-energy electron diffraction analyses indicate that Sb acts as a surfactant. This technique was used to improve the performance of long-wavelength InGaAsN laser diodes. A low-threshold current density of 520 A/cm2 was achieved for an InGaAsN:Sb/GaAs single quantum well 1.2 μm laser diode at room temperature under pulsed operation.
Comparison of mobility and hole current activation energy in the space charge trap-limited regime in a starburst amine
Authors: Staudigel J. , Stössel M. , Steuber F. , Simmerer J.
Published: July 2002

Using two complementary methods, we have investigated the individual contribution of the space charge-limited hole transport in vapor-deposited films of 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (m-MTDATA) to the temperature behavior of organic light-emitting diodes. In single-layer indium tin oxide/m-MTDATA/Ag structures, we have measured the activation energies of the current density and of the hole mobility as a function of the applied electric field. Both activation energies obtained under steady-state and pulsed conditions are comparable, which confirms that the temperature behavior of the current density is predominantly governed by the hole mobility.
Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect
Authors: Nagengast D. G. , van Gogh A. T. M. , Kooij E. S. , Dam B. , Griessen R.
Published: July 2002

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of Y1−yMgyHx, films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating MgH2 is formed together with cubic YH3−δ. In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.
Photocurrent self-oscillations in a spatially direct GaAs/AlGaAs superlattice
Authors: Tomlinson A. M. , Fox A. M. , Cunningham J. E. , Jan W. Y.
Published: July 2002

We report the observation of self-sustained photocurrent oscillations in a weakly coupled spatially direct intrinsic GaAs/Al0.3Ga0.7As superlattice. The oscillations are attributed to the dynamics of unstable electric field domains related to the Γ−Γ e1-e2 tunneling resonance. The observed oscillation period is much longer than the carrier recombination time and the interwell tunneling time. The nonlinear behavior shows a complex dependence on the optical power and the external bias.
Improved spectrometric performance of CdTe radiation detectors in a p-i-n design
Authors: Niraula Madan , Mochizuki Daisuke , Aoki Toru , Hatanaka Yoshinori , Tomita Yasuhiro , Nihashi Tokuaki
Published: July 2002

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150 °C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25 °C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.
Sub-THz spectroscopic system using a multimode laser diode and photoconductive antenna
Authors: Morikawa O. , Tonouchi M. , Hangyo M.
Published: July 2002

A system for measuring transmittance of electromagnetic waves in the sub-THz region is proposed. The electromagnetic radiation is generated by the excitation of a photoconductive antenna with a commercially available multimode laser diode. The spectral coverage of the radiation is increased by defocusing the light spot on the photoconductive antenna. Transmitted radiation is detected by a hot-electron bolometer through a Martin–Puplett-type interferometer. Transmittance is measured for n-type Si wafers with various doping levels. The carrier densities calculated from the transmittance agree well with those obtained from the dc conductivity measurement.
Magnetic electrodes for spin-polarized injection into InAs
Authors: Meier G. , Matsuyama T.
Published: July 2002

Tailored magnetic electrodes are an important prerequisite to inject and detect charge carriers into a semiconductor with defined spins. Especially, if a gate electrode is used to tune the polarization of the carriers by the Rashba effect, magnetic electrodes providing simultaneously a high degree of polarization and a low stray field are important. We have simulated magnetization and hysteresis curves of permalloy electrode configurations and verified our theoretical results by magnetic-force microscopy for electrodes prepared on p-type InAs single crystals. This semiconductor exhibits a strong, gate-voltage dependent Rashba effect and therefore is a candidate for the realization of the spin transistor.
Synchrotron radiation-induced surface-conductivity of SiO2 for modification of plasma charging
Authors: Cismaru C. , Shohet J. L. , McVittie J. P.
Published: July 2002

In this work, we investigate the electrical surface conductivity that is temporarily induced in SiO2 by exposure to monochromatic vacuum-ultraviolet synchrotron radiation for modification of plasma charging. Special preprocessed test structures were exposed to controlled fluxes of monochromatic synchrotron radiation in the range of 500–3000 Å (approx. 4–25 eV), the energy band of most plasma vacuum-ultraviolet radiation. The highest oxide surface conductivity is achieved during irradiation by photons with energies between 15 and 18 eV. This enhanced oxide surface conductivity holds the potential to discharge high-aspect ratio structures that charge up during plasma processing due to electron shading, and thus minimize plasma-processing-induced damage to semiconductor devices.
Three-dimensional photonic band gap structure of a polymer-metal composite
Authors: Zhou Ji , Zhou Y. , Ng S. L. , Zhang H. X. , Que W. X. , Lam Y. L. , Chan Y. C. , Kam C. H.
Published: July 2002

A three-dimensional photonic band gap structure based on self-assembled crystals of polystyrene microspheres was fabricated by filling the pores with metallic silver. An almost complete stop band at 580–600 nm is observed in the optical transmission spectra. In comparison with pure polystyrene colloid crystals, the absorption band of Bragg diffraction in the composite was much more intense and broader, due to an enlargement of the contrast between the spheres and the background. A shift to shorter wavelengths in the band occurred because of a decrease in the average refractive index.
Thermal conductivity study of porous low-k dielectric materials
Authors: Hu Chuan , Morgen Michael , Ho Paul S. , Jain Anurag , Gill William N. , Plawsky Joel L. , Wayner Peter C.
Published: July 2002

An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.
Guided self-assembly of Au nanocluster arrays electronically coupled to semiconductor device layers
Authors: Liu Jia , Lee Takhee , Janes D. B. , Walsh B. L. , Melloch M. R. , Woodall J. M. , Reifenberger R. , Andres R. P.
Published: July 2002

We report the controlled deposition of close-packed monolayer arrays of ∼5-nm-diam Au clusters within patterned regions on GaAs device layers, thus demonstrating guided self-assembly on a substrate which can provide interesting semiconductor device characteristics. Uniform nanometer scale ordering of the clusters is achieved by a chemical self-assembly process, while micron scale patterning is provided by a soft lithographic technique. Scanning tunneling microscope imaging and current–voltage spectroscopy indicate the Au nanoclusters are strongly coupled electronically into the underlying semiconductor substrate while exhibiting only weak electronic coupling in the lateral plane.
Fourier transform spectroscopy of BaO: New ground-state constants from the A 1Σ+–X 1Σ+ chemiluminescence
Authors: Li Hongzhi , Focsa Cristian , Pinchemel Bernard , Le Roy Robert J. , Bernath P. F.
Published: July 2002

The A 1Σ+–X 1Σ+ emission spectrum of BaO excited in a Broida-type oven was revisited using a Fourier transform spectrometer. Chemiluminescence was observed from excited BaO molecules generated from the Ba+N2O reaction, and 82 vibrational bands of 138BaO were measured in the range 8900–21 000 cm−1 at a resolution of 0.04 cm−1. Vibrational levels were observed up to v″=20 of the ground state and up to v′=11 for the excited state. In addition, 72 bands from the minor isotopomers 137BaO, 136BaO, and 135BaO were detected with v″ up to 14 and v′ up to 4. Over 15 500 lines with J″ up to 119 were measured with a precision of about 0.005 cm−1, which shows that the Broida oven is an excellent source for high-resolution emission spectroscopy. Our main goal here is to obtain an optimum set of molecular constants for the ground electronic state. To avoid complications due to the well-known perturbations for vA′⩾1 of the A 1Σ+ state, our unified combined-isotopomer analysis used a novel combination of a Dunham expansion description of the ground state with independent band-constants for data involving A-state level vA′=0, while fitting to an independent term value for each observed A-state vA′⩾1 vibration–rotation level of each isotopomer. Significantly improved ground-state constants and A-state vA′=0 band constants are obtained, together with term values for some 1372 higher-v′ A-state levels.
Effects of phase-bunching in strongly turbulent plasmas
Authors: Skjæraasen O. , Robinson P. A. , Melatos A.
Published: July 2002

The effects of phase bunching on the collisionless dissipation of nonlinear wave fields is explored, with emphasis on situations relevant to strong turbulence applications. It is argued that in a homogeneous, steady-state plasma, there is no preferred phase of the electric field experienced by particles as they enter a wave packet. However, an initially phase-uniform ensemble of particles will generally be phase-bunched after interacting with a wave packet. This can lead to a dramatically intensified interaction with subsequent packets encountered by the particles. Numerical calculations reveal that the local wave dissipation can increase by orders of magnitude if the transiting particles have been phase-bunched prior to entering a wave packet. The wave particle interactions, called transit-time dissipation, comprise Landau damping and a nonresonant type of damping. The nonresonant damping causes a redistribution of field energy within a wave packet. This effect is particularly strong in phase-bunched systems. These results may force modifications to previous treatments of strong turbulence which have assumed isotropy and homogeneity, and employed standard Landau damping.
Melting and crystallization in thin film of n-alkanes: A molecular dynamics simulation
Authors: Shimizu Thoru , Yamamoto Takashi
Published: July 2002

Recent discovery of the anomalous crystallization in n-alkanes and n-alcohols at the free surfaces of melts has revealed the necessity for much deeper understanding of the molecular mechanism of crystallization. With the aid of large-scale molecular simulation, and by use of a simplified molecular model of bead spring, the molecular processes of melting and crystallization in a thin film of n-alkane are directly observed. It is found that melting of the thin film is greatly influenced by its surface state. The film is more stable when the surface chains are lying perpendicular to the surface, on which the chain ends have marked preference. Crystallization by both rapid and slow cooling of the melt is shown to give rise to a formation of monolayers on the free surfaces. By slow stepwise cooling, the layer-by-layer growth of stacked lamellae is clearly reproduced. Detailed inspection of the molecular processes involved shows that there are considerable differences in the molecular mechanisms of monolayer formation and 3D crystallization. Furthermore, the molecular mobility and diffusivity in the surface monolayer are discussed. It is shown that longitudinal displacements of the chains along their axes are much larger in the monolayer than in the crystals, and that the calculated rate of transverse diffusion in the monolayer shows good comparison with experimental values. The interchanges of the molecules between the melt and the surface monolayers are observed directly; dynamical stability of the surface monolayer is clearly demonstrated.
Anatomy of μc-Si thin films by plasma enhanced chemical vapor deposition: An investigation by spectroscopic ellipsometry
Authors: Losurdo M. , Rizzoli R. , Summonte C. , Cicala G. , Capezzuto P. , Bruno G.
Published: July 2002

A detailed analysis of the anatomy of microcrystalline (μc-Si) films deposited by plasma enhanced chemical vapor deposition from both SiF4–H2 and SiH4–H2 mixtures is performed by spectroscopic ellipsometry (SE). Specifically, the μc-Si film anatomy consists of an interface layer at the substrate/μc-Si bulk layer, a bulk μc-Si layer, and a surface porous layer. All these layers have their own microstructures, which need to be highlighted, since it is this overall anatomy which determines the optical properties of μc-Si films. The ability of SE to discriminate the complex microstructure of μc-Si thin films is emphasized also by the comparison with the x-ray diffraction data which cannot provide unambiguous information regarding the distribution of the crystalline and the amorphous phases along the μc-Si film thickness. Through the description of the μc-Si film anatomy, information on the effect of the growth precursors (SiF4 or SiH4) and of the substrate (c-Si or Corning glass) on the growth dynamics can be obtained. The key role of the F-atoms density and, therefore, of the etching-to-deposition competition on the growth mechanism and film microstructure is highlighted.
Structure and dynamics of nonaqueous mixtures of dipolar liquids. II. Molecular dynamics simulations
Authors: Venables Dean S. , Schmuttenmaer Charles A.
Published: July 2002

Molecular dynamics simulations have been used to study mixtures of acetone/methanol, acetonitrile/methanol, and acetone/acetonitrile over their entire composition range. Using the effective pair potentials of the neat liquids, the simulations reproduce much of the experimental spectra presented in the previous paper [D. S. Venables, A. Chiu, and C. A. Schmuttenmaer, J. Chem. Phys. 113, 3243 (2000)]. In addition to the total dipole spectra, autocorrelation functions and their corresponding spectra were calculated for the single dipole moment as well as the translational and rotational velocities of each component in the mixtures. Radial and spatial distribution functions, hydrogen bonding, and methanol chain formation in the mixtures were also analyzed. The red-shift of the high frequency librational band is attributed to methanol chains breaking up into shorter segments, and to hydrogen bonding between methanol and co-solvent molecules. Methanol molecules have a strong tendency to reside in chains, even at low methanol concentrations, and hydrogen bonding is the primary determinant of structure in the mixtures.