bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption and scattering in nonlinear optical polymeric systems
Authors: Skumanich Andrew , Jurich Mark , Swalen J. D.
Published: July 2002

Fundamental absorption measurements were made on organic films which are candidates for nonlinear optical materials applications, in order to gain insight into the loss mechanisms. Photothermal deflection spectroscopy was used to obtain the full absorption spectrum and to explain observed variations in waveguide attenuation. The absorption data provide a complete characterization of the optical spectrum and allow us to select optimal polymer-chromophore systems. The variations in the attenuation at 832 nm are due to the chromophore absorption, while for wavelengths longer than 1 μm, the C—H vibrational overtones and combination bands of the polymer dominate the spectrum and are likely to be the main source of waveguide attenuation.
Bilayer embodiment of the bipolar spin switch
Authors: Johnson Mark
Published: July 2002

The ‘‘bipolar spin switch’’ is a novel, three-terminal, ferromagnet-paramagnet-ferromagnet trilayer device. A bilayer embodiment of this device is presented. Spin and charge flow in a ferromagnet-paramagnet model system is discussed and design criteria are developed. Experimental data from a prototype device is qualitatively and quantitatively consistent with the model.
Anisotropic excitation spectra of GaAs/AlGaAs quantum wells grown on vicinal plane substrates
Authors: Wentink D. J. , Dawson P. , Foxon C. T.
Published: July 2002

We report measurements of the photoluminescence excitation spectra of a series of GaAs/AlGaAs quantum well samples grown on vicinal plane substrates with differing off-cut angles. When the plane of polarization of the exciting light is changed we have observed a clear variation in the ratio of the strength of the n=1 light and heavy hole exciton transitions in samples grown on vicinal plane substrates. This behavior is attributed to anisotropic scattering at steps in the heterointerface.
Comparison of the physical properties of GaN thin films deposited on (0001) and (011̄2) sapphire substrates
Authors: Sun Chien-Jen , Razeghi Manijeh
Published: July 2002

A direct comparison of the physical properties of GaN thin films is made as a function of the choice of substrate orientations. Gallium nitride single crystals were grown on (0001) and (011̄2) sapphire substrates by metalorganic chemical vapor deposition. Better crystallinity with fine ridgelike facets is obtained on the (011̄2) sapphire. Also lower carrier concentration and higher mobilities indicate both lower nitrogen vacancies and less oxygen incorporation on the (011̄2) sapphire. The results of this study show better physical properties of GaN thin films achieved on (011̄2) sapphire.
Study of the interface-state formation at different temperatures
Authors: El-Hdiy Abdelillah
Published: July 2002

The interface-state formation caused by a negative gate bias in a polycrystalline silicon gate-oxide-semiconductor capacitor versus injecting temperature in the range of 77–400 K is studied. It is found that this interface-state generation is temperature independent in this range of temperatures, which indicates that the motion of the mobile species (as the hydrogen-related species) through the SiO2 toward the Si/SiO2 interface seems unlikely. One assumes that this no thermal activation agrees with the break of Si—Si or Si—O distorted bonds at the Si/SiO2 interface caused by injecting hot electrons during stress.
Role of stacking faults as misfit dislocation sources and nonradiative recombination centers in II-VI heterostructures and devices
Authors: Guha S. , DePuydt J. M. , Qiu J. , Hofler G. E. , Haase M. A. , Wu B. J. , Cheng H.
Published: July 2002

We have investigated the role of stacking faults in high quality ZnSxSe1−x heterostructures and a ZnSxSe1−x/CdxZn1−xSe based II-VI blue-green quantum well laser structure grown on GaAs substrates. We find that these stacking faults, which originate at the epilayer/substrate interface during the initial stages of the growth, act as sources for misfit dislocation formation in the quantum well region of ZnSxSe1−x/CdxZn1−xSe based devices. We have analyzed the formation mechanism of these dislocations. We also show through cathodoluminescence microscopy that these stacking faults act as nonradiative recombination centers which therefore reduce the luminescence of these devices.
Observation of group delay dispersion as a function of the pulse width in a mode locked Ti:sapphire laser
Authors: Zhang Zhigang , Yagi Takashi
Published: July 2002

The group delay dispersion which needs to be compensated in a mode locked Ti:sapphire laser was found to be a function of the resultant pulse width. This is attributed to the frequency chirp induced by self-phase modulation.
Surface diffusion length of Ga adatoms on (1̄1̄1̄)B surfaces during molecular beam epitaxy
Authors: Nomura Y. , Morishita Y. , Goto S. , Katayama Y. , Isu T.
Published: July 2002

The spatial variation of the growth rate on mesa-etched GaAs (1̄1̄1̄)B substrates during molecular beam epitaxy of GaAs is measured from the period of the reflection high-energy electron diffraction (RHEED) intensity oscillation using in situ scanning microprobe RHEED. The surface diffusion length of Ga adatoms on the (1̄1̄1̄)B surface is determined from the spatial variation of the growth rate. The surface diffusion length on the (1̄1̄1̄)B surface increases as the substrate temperature is raised or the arsenic pressure is decreased. The typical value of the diffusion length is about 10 μm at a substrate temperature of 580 °C and an arsenic pressure of 5.7×10−4 Pa, which is an order of magnitude larger than that on the (100) surface along the [011] direction. The activation energy of the surface diffusion length changes with the surface reconstruction. Anisotropic diffusion, as reported for the (100) surface, is not observed on the (1̄1̄1̄)B surface.
Pyrolytic laser assisted chemical vapor deposition of Al from dimethylethylamine-alane: Characterization and a new two-step writing process
Authors: Han Jaesung , Jensen Klavs F. , Senzaki Yoshihide , Gladfelter Wayne L.
Published: July 2002

We describe pyrolytic laser assisted chemical vapor deposition of Al from dimethylethylamine-alane with 514-nm radiation from an Ar ion laser. High purity Al lines with resistivity close to bulk Al are reported for a range of operating conditions. The relationship between operating parameters and materials properties of the deposited lines is delineated. Results from deposition on different substrates, Pt, Au, W, and Si, provide insight into thermal and nucleation effects in the laser writing process. Based on the observed nucleation behavior, we demonstrate a two-step fast writing process involving fast laser nucleation of lines, followed by selective chemical vapor deposition of Al on the nucleated pattern.
Overcoming the pseudomorphic critical thickness limit using compliant substrates
Authors: Chua C. L. , Hsu W. Y. , Lin C. H. , Christenson G. , Lo Y. H.
Published: July 2002

We demonstrated the high-quality molecular beam epitaxy growth of exceedingly thick In0.14Ga0.86As pseudomorphic layers on thin, free-standing, compliant GaAs substrates. We first fabricated 800-Å-thick compliant platforms before growing a lattice-mismatched layer on the platform. The layer we grew exceeds its usual critical thickness by about twenty times without strain relaxation. X-ray analysis confirms a shift in the InGaAs peaks grown on the compliant substrate, indicating an unrelaxed strain of 0.9%. Moreover, atomic force microscope profiles verify that layers grown on compliant substrates are much smoother than layers grown on a plain substrate.
Time-resolved optical characterization of InGaAs/GaAs quantum dots
Authors: Wang G. , Fafard S. , Leonard D. , Bowers J. E. , Merz J. L. , Petroff P. M.
Published: July 2002

We report on the optical characterization of the strained InGaAs/GaAs quantum dots (QDs). The temperature dependence of the photoluminescence (PL) indicates that the onset energy of the thermal quenching in ∼20-nm-diam QDs is enhanced by a factor of ∼2 as compared to a quantum well (QW), due to the additional confinement. At low temperature, an increased carrier lifetime is observed for the QDs as compared to a reference QW (880 vs 330 ps). The carrier lifetime in the QDs was found to be independent of the temperature for T<30 K. In addition to this different dynamics of the localized excitons, we find that in the steady state PL and PL excitation, there is virtually no overlap between the emission and the absorption energies.
BeTe/ZnSe graded band gap ohmic contacts to p-ZnSe
Authors: Mensz P. M.
Published: July 2002

BeTe is not a very well known wide-band semiconductor. Due to the close lattice match to GaAs and ZnSe and p-type as-grown character, BexZn1−xTexSe1−x graded band-gap layers appear an ideal candidate for ohmic contact to p-type ZnSe based semiconductors. These contacts allow for an implementation of epitaxial structures of II-VI compound diode lasers entirely lattice matched to the GaAs substrate. The numerical calculations predict contact resistivity of BexZn1−xTexSe1−x graded gap contacts lower than ρc=10−4 Ω cm2 at acceptor doping level 1×1018 cm−3, which corresponds to a voltage drop across the contact layer of less than 0.1 V during lasing operation.
High quality crystalline YBa2Cu3O7−δ films on thin silicon substrates
Authors: Haakenaasen R. , Fork D. K. , Golovchenko J. A.
Published: July 2002

YBa2Cu3O7−δ(001) (YBCO) films with near perfect crystallinity have been grown epitaxially on Si(100) using two intermediate buffer layers: Yttrium-stabilized zirconia (YSZ) and CeO2. All layers were grown by an in situ pulsed laser deposition process. The new films have Rutherford backscattering spectroscopy minimum yields as low as 5%, compared to 12% for YBCO films deposited directly on YSZ. The superconducting onset is above 90 K with a transition width ΔT of 1 K. After film deposition the Si substrate could be etched from the back to give a circular, 2-mm- diam, 4000 Å uniformly thick Si membrane with 300 Å YSZ, 80 Å CeO2, and 1500–3000 Å YBCO on top.
Origin of the green/blue luminescence from nanocrystalline silicon
Authors: Tamura Hideki , Rückschloss Markus , Wirschem Thomas , Vepřek Stan
Published: July 2002

It is shown that the green/blue luminescence which is found in oxidized nanocrystalline and ‘‘porous’’ silicon is associated with the presence of OH groups adsorbed on structural defects in the SiO2 network.
Study of the two-dimensional–three-dimensional growth mode transition in metalorganic vapor phase epitaxy of GaInP/InP quantum-sized structures
Authors: Carlsson N. , Seifert W. , Petersson A. , Castrillo P. , Pistol M. E. , Samuelson L.
Published: July 2002

Ga0.5In0.5P/InP quantum-sized structures, grown by metalorganic vapor phase epitaxy, have been optically characterized by photoluminescence, cathodoluminescence, and photoluminescence excitation spectroscopy. Additional structural information has been obtained by atomic force microscopy. We find that the two-dimensional layer-by-layer growth mode is limited to the growth of 1-ML-thick and, in part, 2-ML-thick quantum wells. The transition towards three-dimensional Stranski–Krastanow island growth occurs before the second monolayer of InP is completed. To further study the dynamics of the island formation, growth interruptions were introduced between the InP deposition and the subsequent growth of the upper GaInP barrier. The two types of coherent islands show a quantum confinement in vertical direction, corresponding to about 2- and 3-ML-thick and about 9- and 10-ML-thick InP strained quantum wells.
Comparison of optical nonlinearities in piezoelectric strained [111]- and [001]-grown (In,Ga)As/(Al,Ga)As quantum wells
Authors: Livingstone M. , Galbraith I. , Wherrett B. S.
Published: July 2002

The nonlinear absorption and refractive index spectra for [111]- and [001]-grown (In,Ga)As/(Al,Ga)As single strained-layer quantum-well structures are calculated, for different photogenerated carrier densities. Screening of the strain-induced electric field by photocarriers that have escaped from the [111]-grown quantum well is shown to produce a similar magnitude of nonlinear refractive cross section to that of the conventional [001]-grown quantum well.
Chemical vapor deposition of heteroepitaxial Si1−x−yGexCy films on (100)Si substrates
Authors: Atzmon Z. , Bair A. E. , Jaquez E. J. , Mayer J. W. , Chandrasekhar D. , Smith David J. , Hervig R. L. , Robinson McD.
Published: July 2002

Thin heteroepitaxial films of Si1−x−yGexCy have been grown on (100)Si substrates using atmospheric pressure chemical vapor deposition at 625 °C. The crystallinity, composition, and microstructure of the SiGeC films were characterized using Rutherford backscattering spectrometry, secondary-ion-mass spectrometry, and cross-sectional transmission electron microscopy. The crystallinity of the films was very sensitive to the flow rate of C2H4 which served as the C source. Films with up to 2% C were epitaxial with good crystallinity and very few interfacial defects. Between 800 and 900 sccm of 10% C2H4 in He, the C content increased dramatically from 2% to 10% and the as-grown films changed from crystalline to amorphous. In order to establish deposition conditions for the crystalline-amorphous phase transformation, one SiGeC film was deposited as the 10% C2H4 flow was increased linearly from 500 to 1500 sccm during growth. When the C content reached ∼4%, the film developed considerable stacking defects and disorder, and at around 11% C, the film became amorphous.
High quality self-nucleated AlxGa1−x N layers on (00.1) sapphire by low-pressure metalorganic chemical vapor deposition
Authors: Wickenden D. K. , Bargeron C. B. , Bryden W. A. , Miragliotta J. , Kistenmacher T. J.
Published: July 2002

High quality AlxGa1−xN alloy films with x<0.4 have been prepared on self-nucleated (00.1) sapphire substrates by low-pressure metalorganic chemical vapor deposition. It has been shown that the lattice constant of the films varies linearly with alloy composition x (Vegard’s law is obeyed) and that homogeneous and inhomogeneous strain and alloy clustering are minimized in these self-nucleated AlxGa1−xN films. Consistent with their reduced strain and chemical uniformity, the derived optical band gaps of these AlxGa1−xN films also show a linear dependence on alloy composition x, yielding a bowing parameter b≊0 eV.
Potential of CCDs for UV and x-ray plasma diagnostics (invited)
Authors: Janesick James R. , Elliott Tom , Marsh Harry H. , Collins Stewart , McCarthy James K. , Blouke Morley M.
Published: July 2002

A program is under way to develop charge-coupled device (CCD) sensors for space-based x-ray astronomy imaging spectrometers. To date, laboratory line emission spectra have been acquired throughout the range of 277 to 8000 eV (carbon through copper Kα emission) and CCD sensitivity has been demonstrated throughout the range of 1.1 through 8000 eV. Image resolution is excellent, limited almost entirely by the 15-μm pixel size. These results are presented and specialized techniques are described which permit such low energy response, high spectral resolution, and efficient charge collection. Finally, analysis is presented of one particular CCD characteristic which currently limits UV and x-ray performance: charge diffusion.
Stress effects in thermal cycling of copper (magnesium) thin films
Authors: Toomey J. J. , Hymes S. , Murarka S. P.
Published: July 2002

The stress in the thin films of pure Cu and Cu (Mg) alloy deposited on oxidized silicon substrates has been measured in the range of room temperature to 500 °C by the use of an in situ laser based curvature measuring tool. Results indicate that small amounts of Mg (∼2–3 at. %) in copper changes the stress-temperature behavior significantly when compared with that of pure copper. The alloy, after the first annealing cycle which apparently equilibrates the film, shows a predominantly elastic stress-temperature behavior up to ∼400 °C compared to pure Cu, which shows a departure from linearity at ∼200 °C. Rutherford backscattering and x-ray diffraction studies were also carried out to elucidate the role of Mg in strengthening the copper film. The results indicate that additions of Mg to Cu increase the mechanical strength of the film and that such behavior is retained in several (subsequent) thermal cycles to 400 °C.
Design and performance of an electron cyclotron resonance plasma source for standard molecular beam epitaxy equipment
Authors: Sitar Z. , Paisley M. J. , Smith D. K. , Davis R. F.
Published: July 2002

An electron cyclotron resonance plasma source which can be inserted into a 2.25-in.-diam. sleeve designed for standard molecular beam epitaxy effusion cells has been designed and commissioned. This source uses a coaxial microwave line structure, with an external tuner and small electromagnets for the production of the resonance and extraction magnetic fields. It also provides the same source-to-substrate distance as other sources which eliminates wall collisions and resulting recombination of active species. The source exhibits stable and continuous operation over four decades of pressure and gridless plasma beam extraction. The low-pressure characteristics of the plasma beam for nitrogen (hydrogen) include an ion current density of 0.2 (1.0) mA/cm2, an ion density of 4×109 (5×1010) cm−3, an electron temperature of 120 000 (70 000) K over a 3-in. wafer located 20 cm from the source. This source has been successfully used to grow thin films of GaN and AlN and layered structures of these two materials on SiC and sapphire substrates at temperatures as low as 400 °C.
New valve design for high-pressure sapphire tubes for NMR measurements
Authors: Horváth István T. , Ponce Emilio C.
Published: July 2002

A simple titanium valve for high-pressure sapphire tubes is designed and tested for the application of NMR measurements.
An experimental apparatus for measuring surface resistance in the submillimeter-wavelength region
Authors: Cook Jerry D. , Zwart John W. , Long Kenwyn J. , Heinen Vernon O. , Stankiewicz Norbert
Published: July 2002

A simple comparative technique to characterize surface electrical resistance of small metallic samples at submillimeter wavelengths is presented. A quasioptical hemispherical resonator, fed by an optically pumped far-infrared laser is used, with the sample serving as the plane mirror. The cavity developed combines a small beam size at the sample, enabling measurements on samples with widths as small as 5 mm, with a large quality factor Q, so that surface resistance losses are readily measurable. This cavity uses a simple new geometry that feeds the cavity through a single small coupling hole that also serves as a system of monitoring energy storage in the cavity by means of a single external beamsplitter. An alternate approach of inserting an internal beamsplitter into the resonator was found to be unacceptable due to excessive losses and alignment problems.
Bimodal terminal velocities using the falling needle viscometer
Authors: Thiessen David B. , Krantz William B.
Published: July 2002

The falling needle viscometer is a particularly useful instrument for measuring the shear viscosity when small sample volumes are necessitated. Under some conditions this instrument can exhibit bimodal terminal velocity observations which precludes a straightforward analysis to obtain the viscosity. This phenomenon is explained via an analysis for the fluid mechanics and associated entropy production of a needle falling asymmetrically with respect to the centerline of the instrument. This analysis provides a rationale for using only the smaller terminal velocities, thereby permitting an unambiguous data analysis.
A simple ac susceptibility measuring system at low temperatures
Authors: Soumura T. , Tsutsui K. , Azuma T. , Saito S. , Maeda T.
Published: July 2002

An automated ac susceptibility measuring system based on an ac mutual inductance bridge is described. A personal computer and a digital-analog converter are used to control the amplitude and phase of the compensatory signal which balances the bridge. An analog multiplier and a variable-resistance photocoupler are used in the amplitude and phase control circuits, respectively. The magnetic moment of a sample for a 1-μV output voltage is 3.8×10−6 emu/μV. Sufficient sensitivity was confirmed for an application to oxide superconductors.
The effect of strain on the interdiffusion in InGaAs/GaAs quantum wells
Authors: Ryu Sang-Wan , Kim In , Choe Byung-Doo , Jeong Weon Guk
Published: July 2002

The effect of strain on the cation interdiffusion in InGaAs/GaAs quantum wells is described. It is found that the Fick’s diffusion equation does not properly describe the interdiffusion in the heterostructure with strained layers. It is believed that the strain changes crystal defect concentration and thus diffusivity is influenced by strain. Diffusion equation including the strain effect is formulated and solved numerically. The experimental photoluminescence peak shifts as a function of annealing time are well-fitted by this analysis and useful parameters such as diffusivity describing InGaAs/GaAs quantum well interdiffusion are extracted.
Robust high-temperature sapphire cell for metal vapors
Authors: Neuman J. A. , Wang P. , Gallagher A.
Published: July 2002

We describe a simple vacuum cell that successfully contains high-temperature and high-density alkali-metal and alkaline-earth vapors. The transparent cell, made from sapphire parts joined by melted glass powders, operates without a buffer gas. Temperatures over 700 °C and metal-vapor densities greater than 1017 cm−3 have been achieved. Such an optical cell is useful for many atomic physics experiments where collisions with buffer gas atoms are problematic or where a spatially uniform distribution of high-density, corrosive atoms is desired.
Study of forward characteristics of a cryogenic temperature sensor diode
Authors: Bose Mohua , Ota S. B.
Published: July 2002

The current dependence of the forward voltage of a cryogenic silicon diode temperature sensor (CRYO Industries of America, Inc.) is reported here in the temperature range 10–300 K. The current values are varied from 10 nA to 0.1 A. The sensitivity of the diode is observed to increase for lower current values in the high temperature range. The rapid increase of the forward voltage at low temperatures is observed to be suppressed considerably by reducing the current. This study helps us to conclude that for all practical purposes such commercial diodes (prescribed for a particular current value of 10 μA) can be operated for a wider range of current values, with suitable calibration data.
Double-modulation electro-optic sampling for pump-and-probe ultrafast correlation measurements
Authors: Frolov S. V. , Vardeny Z. V.
Published: July 2002

We describe a novel electro-optic double-modulation (DM) sampling technique for ultrafast transient spectroscopy, which is characterized by a superior signal-to-noise ratio compared to that of a regular single-modulation technique. DM is achieved by a combined effect of a radio-frequency modulation, which eliminates most of the low-frequency noise, and an audio-frequency modulation, which makes use of a high-performance, low-frequency lock-in amplifier. The DM sensitivity is comparable to that of the more sophisticated schemes involving electrical mixing and the A–B noise reduction method. We show that the DM technique offers superior performance in two-beam transient pump-and-probe correlation measurements compared to the regular single frequency modulation technique and is an ideal scheme for three-beam picosecond correlation measurements.
Timing control of an intense picosecond pulse laser to the SPring-8 synchrotron radiation pulses
Authors: Tanaka Yoshihito , Hara Toru , Kitamura Hideo , Ishikawa Tetsuya
Published: July 2002

We have developed a control system to synchronize intense picosecond laser pulses to the hard x-ray synchrotron radiation (SR) pulses of SPring-8. A regeneratively amplified mode-locked Ti:sapphire laser is synchronized to 40 ps SR pulses by locking the laser to the radio frequency of the ring. The synchronization of the pulses is monitored by detecting both beams simultaneously on a gold photocathode of a streak camera. This method enabled us to make a precise measurement of the time interval between the beams, even if the trigger of the streak camera drifts. Synchronization between the laser and the SR pulses has been achieved with a precision of ±2 ps for some hours. The stable timing control ensures the possibility of making two-photon excitation and pump–probe experiments with time resolution of a few tens of ps (limited by the pulse duration of the SR). We have used this system to show that closing undulator gaps in the storage ring shifts the arrival time of the SR pulses, in accord with expectations for the increased power loss.
Aerodynamic laser-heated contactless furnace for neutron scattering experiments at elevated temperatures
Authors: Landron Claude , Hennet Louis , Coutures Jean-Pierre , Jenkins Tudor , Alétru Chantal , Greaves Neville , Soper Alan , Derbyshire Gareth
Published: July 2002

Conventional radiative furnaces require sample containment that encourages contamination at elevated temperatures and generally need windows which restrict the entrance and exit solid angles required for diffraction and scattering measurements. We describe a contactless windowless furnace based on aerodynamic levitation and laser heating which has been designed for high temperature neutron scattering experiments. Data from initial experiments are reported for crystalline and amorphous oxides at temperatures up to 1900 °C, using the spallation neutron source ISIS together with our laser-heated aerodynamic levitator. Accurate reproduction of thermal expansion coefficients and radial distribution functions have been obtained, demonstrating the utility of aerodynamic levitation methods for neutron scattering methods.
YBa2Cu3O7 step-edge dc SQUID with coplanar control lines
Authors: Hasegawa H. , Tarutani Y. , Fukazawa T. , Kabasawa U. , Takagi K.
Published: July 2002

A dc superconducting quantum interference device (SQUID) with coplanar control lines has been fabricated from a single layer of YBa2Cu3O7 thin film. Step-edge junctions were formed using resist/Si double-mask layers. Current–voltage characteristics of this SQUID at 77 K show Shapiro steps under microwave irradiation and are qualitatively consistent with the resistively shunted junction (RSJ) model. The spacing between the closed loop and the coplanar control lines in the tested SQUIDs ranged from 0.2 to 1.5 μm. The measured output voltage-control current characteristics showed that the experimentally derived mutual inductances were a function of the spacing, in agreement with the calculated results. The typical mutual inductance for a spacing of 1.0 μm was ∼0.5 pH, which is adequate for detecting signals of flux-transfer logic gates in which the typical logic amplitude is ∼0.1 mA. A coplanar structure is thus useful for logic gate applications.
Band line-up transition in AlxGa1−xAs/In0.5Ga0.5P from capacitance-voltage analysis
Authors: Kim Il-Jong , Cho Yong-Hoon , Kim Kwan-Shik , Choe Byung-Doo , Lim H.
Published: July 2002

The conduction-band discontinuity ΔEc of AlxGa1−xAs/In0.5Ga0.5P heterojunctions grown by liquid phase epitaxy on GaAs substrate was studied using the capacitance–voltage (C–V) characterization technique. The C–V measurements were made on a series of samples with x ranging from zero to about 0.3. The carrier profiles for the samples with x=0 and x=0.06 give ΔEc values of 90 and 40 meV, respectively, showing the type I (straddling) band line-up. For x=0.18 and 0.29, the values of ΔEc were 45 and 110 meV, respectively, with the carrier profile characteristic of the type II (staggered) band line-up. From these results, ΔEc of the heterojunction is found to vanish at about x = 0.12. This agrees well with our previous result determined from the photoluminescence measurements.
Enhancement of electron emission efficiency of Mo tips by diamondlike carbon coatings
Authors: Chuang F. Y. , Sun C. Y. , Cheng H. F. , Huang C. M. , Lin I. N.
Published: July 2002

The effect of diamondlike carbon (DLC) films coated by pulsed laser deposition technique on the electron emission characteristics of Mo tips is examined. Turn-on voltage (V0) was lowered from 40 V for Mo tips to 22 V for DLC coated Mo tips and maximum anode current (IA) was increased from ∼44 μA for Mo tips to ∼2.0 mA for DLC coated Mo tips. Maximum anode current (IA) for the DLC coated Mo tips, however, decreased during operation. Raman spectroscopy and selected area diffraction (SAD) in transmission electron microscopy (TEM) revealed that the degradation of electron emission behavior can be ascribed to the conversion of sp3-bonds, characteristic for diamond, to sp2-bonds, characteristics for graphite. The transformation of the structure is assumed to be induced by the local heat from the DLC coatings.
Thermal stability of the silicon doping of GaAs grown on (111)A oriented substrates
Authors: Pavesi L. , Henini M.
Published: July 2002

The thermal stability of the silicon doping of (111)A GaAs, grown by molecular beam epitaxy under different As overpressures, is studied by electrical and photoluminescence measurements. Annealing treatments were performed at 1028 K for 8 hr in As-poor conditions. It is found that the silicon doping of (111)A GaAs is unstable; annealing turns the doping from p type to n type in p-type doped samples and increases the compensation in n-type doped samples. The role of the point defects in the process is discussed.
In situ, atomic force microscope studies of the evolution of InAs three-dimensional islands on GaAs(001)
Authors: Kobayashi N. P. , Ramachandran T. R. , Chen P. , Madhukar A.
Published: July 2002

The size evolution of molecular beam epitaxy-grown strained InAs three-dimensional (3D) islands on GaAs(001) is examined using in situ ultrahigh vacuum atomic force microscopy. Remarkably, just after the initiation of well-formed 3D islands at ∼1.57 ML InAs deposition, the lateral size dispersion and average value are found to first increase drastically with the smallest amount (∼0.05 ML) of additional InAs deposition and then decrease and saturate, indicating the onset of a natural tendency for size equalization, including through loss of material from the initially formed largest islands. These observations are found to be consistent with the previously suggested island-separation dependent influence of the evolving island-induced substrate strain fields on the adatom migration and incorporation/detachment kinetics that control the evolution of the islands.
Fabrication of GaAs coupled quantum wires on V-grooved substrate
Authors: Komori Kazuhiro , Wang Xue-Lun , Ogura Mutsuo , Matsuhata Hirofumi , Imanishi Hideki
Published: July 2002

We have fabricated coupled quantum wires, consisting of two GaAs quantum wires as small as 5 nm thick and 30 nm in width (15 nm in effective width) separated by a 1–3 nm thick AlGaAs barrier, using flow rate modulation epitaxy (FME) on a V-grooved substrate and observed its optical characteristics. A clear energy splitting between the fundamental coupled state and the higher-order coupled state of the coupled quantum wires was observed from the room-temperature photoluminescence. Furthermore, the splitting energy increases as the thickness of barrier decreases from 3 to 1 nm, which is predicted theoretically for coupled quantum systems.
InGaAs quantum wires grown by low pressure metalorganic chemical vapor deposition on InP V-grooves
Authors: Kappelt M. , Grundmann M. , Krost A. , Türck V. , Bimberg D.
Published: July 2002

Single InGaAs quantum wires were fabricated by low pressure metal organic chemical vapor deposition on V-grooved InP substrates. For substrate patterning a new wet chemical etching process that leads to high quality V-grooves with {111}A facets was used. The growth parameters of the InP buffer layer have a strong impact on the quantum wire formation. Scanning electron microscopy, photoluminescence, and spatially resolved cathodoluminescence experiments have been performed to characterize the structures. The crescent shaped InGaAs quantum wires have dimensions of about 13 nm height and 100 nm width. The wire luminescence is found to be at λ=1575 nm (FWHM=17 meV).
Nonequilibrium electron distributions and phonon dynamics in wurtzite GaN
Authors: Tsen K. T. , Joshi R. P. , Ferry D. K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoç H.
Published: July 2002

We report experimental measurements of nonequilibrium electron distributions as well as phonon dynamics in wurtzite GaN by using subpicosecond time-resolved Raman spectroscopy. Our experimental results have demonstrated that for electron densities n≥5×1017 cm−3, the nonequilibrium electron distributions in wurtzite GaN can be very well described by Fermi–Dirac distribution functions with the temperature of electrons substantially higher than that of the lattice. The population relaxation time of longitudinal optical phonons was directly measured to be τ≂5±1 ps at T=25 K.
Spatial distribution of the luminescence in GaN thin films
Authors: Ponce F. A. , Bour D. P. , Götz W. , Wright P. J.
Published: July 2002

The spatial dependence of the luminescence intensities at the band edge (364 nm) and at the ‘‘yellow’’ defect-band (centered at 560 nm) regions for epitaxial GaN films have been studied using cathodoluminescence microscopy at room temperature. The films were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates and were not intentionally doped. Significant nonuniformities in the band-to-band and in the yellow band emissions were observed. Yellow luminescence in small crystallites appears to originate from extended defects inside the grains and at low-angle grain boundaries. The size of band-to-band emission sites correlates with low-angle grain sizes observed by transmission electron microscopy.
The origin of blue and ultraviolet emission from porous GaP
Authors: Meijerink A. , Bol A. A. , Kelly J. J.
Published: July 2002

The luminescence properties of porous n-type GaP are reported. Apart from the orange emission of bulk GaP, emissions in the blue and ultraviolet are observed. Evidence is presented to show that this blueshifted emission is not due to quantum confinement effects, as previously suggested, but to gallium oxide on the surface of the porous material.
Sticking coefficient and growth rate during Al chemical vapor deposition
Authors: Karpov I. , Gladfelter W. , Franciosi A.
Published: July 2002

Studies of aluminum chemical vapor deposition (CVD) from dimethylethylamine alane on GaAs(100) 2×4 surfaces were used to identify the high-temperature, flux-limited growth regime and determine the effective sticking coefficient α. Following a short induction period, α was found to achieve a largely temperature-independent steady-state value (α=0.13±0.04), substantially lower than expected based on current CVD models. We propose that steric effects—previously ignored in CVD—play a role in determining the upper limit of α and therefore the maximum achievable growth rate.
Effect of Si doping on the dislocation structure of GaN grown on the A-face of sapphire
Authors: Ruvimov Sergei , Liliental-Weber Zuzanna , Suski Tadeusz , Ager Joel W. , Washburn Jack , Krueger Joachim , Kisielowski Christian , Weber Eicke R. , Amano H. , Akasaki I.
Published: July 2002

Transmission electron microscopy, x-ray diffraction, low-temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si-doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three-dimensional dislocation network and a strain gradient along the c axis of the layer.
Mechanisms of recombination in GaN photodetectors
Authors: Binet F. , Duboz J. Y. , Rosencher E. , Scholz F. , Härle V.
Published: July 2002

Steady-state and transient responses of a nonintentionally doped GaN photodetector are investigated. The kinetics of the photoresponse demonstrate the existence of deep levels in the gap, acting as recombination centers with an acceptor character. The photoresponse displays two competing processes: a bimolecular recombination, dominating at high optical power range, and a monomolecular recombination involving long response times. The observed persistent photoconductivity and the huge photoconductive gain are due to the small electron capture cross section and a much faster hole capture rate.
Differential microscopy by conventional electron off-axis holography
Authors: Tanji Takayoshi , Ru Qingxin , Tonomura Akira
Published: July 2002

Differential microscopy is realized by conventional off-axis electron holography with an electron biprism behind the specimen. Two phase images reconstructed from two holograms which are obtained with slightly different potentials of the electron biprism are utilized to make a one-dimensional differential image. Polystyrene latex particles which are charged by electron irradiation are used to demonstrate that the differential image is independent of the distortion of a reference wave.
Carrier transport in a photorefractive multiple quantum well device
Authors: Canoglu Ergun , Yang Ching-Mei , Garmire Elsa , Mahgerefteh Daniel , Partovi A. , Chiu T. H. , Zydzik G. J.
Published: July 2002

Using picosecond time-resolved four-wave mixing we have investigated the turn-on and turn-off mechanisms in a photorefractive multiple quantum well spatial light modulator with Stark geometry. We show that holes contribute to the turn-on time as a slower rise (∼300 ps) of the diffraction efficiency. Furthermore, the diffraction grating is dominated by either electrons or holes, depending on the applied voltage polarity relative to the diffraction of illumination. The grating decays in 0.4–8.5 ns, depending on the grating spacing. We show that the decay is due to lateral transport of carriers at the multiple quantum well/dielectric interfaces, where low-temperature grown layers can increase the diffraction efficiency and device resolution.
Planar light-emitting devices fabricated with luminescent electrochemical polyblends
Authors: Yu G. , Pei Q. , Heeger A. J.
Published: July 2002

Light emitting devices were fabricated in planar (surface cell) configurations with low operating voltage. Polymer blends composed of a red or green color luminescent polymer and solid state electrolyte were used as the active materials. Under external bias, electrochemical doping occurs in the vicinity of the contact, and a dynamic p-n junction forms between the electrodes. Light emission is observed in the junction area at bias voltages greater than 2 V, reaching 102cd/m2 at voltage well below 5 V. The planar configuration allows display devices to be hybridized with integrated circuits on a silicon wafer. On transparent substrates, such as glass or plastic films, light emission can be viewed from both sides of the device. Emissive displays in the planar surface cell configuration can be fabricated using roll-to-roll processing at room temperature without need for vacuum equipment.
The effect of carbon on the valence band offset of compressively strained Si1−x−yGexCy/(100) Si heterojunctions
Authors: Chang C. L. , St. Amour A. , Sturm J. C.
Published: July 2002

Capacitance–voltage measurements have been used to study the effect of carbon on the valence band offset of compressively strained Si1−x−yGexCy/(100) Si heterojunctions grown by rapid thermal chemical vapor deposition with substitutional C levels from 0% to 2.5%. The valence band offset between Si1−x−yGexCy and unstrained (100) Si decreases at a rate of 20–26 meV per % C. Our work indicates that the change in the bandgap of Si1−x−yGexCy as carbon is added is entirely accommmodated in the valence band.
Electric field effect on luminescence efficiency in 8-hydroxyquinoline aluminum (Alq3) thin films
Authors: Stampor W. , Kalinowski J. , Di Marco P. , Fattori V.
Published: July 2002

Electric field-induced luminescence quenching in thin films made from common organic electroluminescent material of aluminum (III) 8-hydroxyquinoline (Alq3) is reported. The dependence of luminescence quenching on excitation wavelength and electric field is attributed to field-assisted hopping separation of charge in localized excited states. The effect extrapolated to high electric fields can reduce the luminescence yield by as much as 60% limiting electroluminescence quantum efficiency in high-field-driven light emitting diodes based on the Alq3 emitter.
High photoluminescence efficiency of InGaAs/GaAs quantum dots self-formed by atomic layer epitaxy technique
Authors: Mukai Kohki , Ohtsuka Nobuyuki , Sugawara Mitsuru
Published: July 2002

We investigated the photoluminescence (PL) intensity of InGaAs/GaAs quantum dots self-formed by atomic layer epitaxy (ALE) technique as a function of temperature. We report that the reduction of PL intensity following a temperature increase was two-orders smaller in the ALE-grown dots than in the dots grown by molecular beam epitaxy via Stranski–Krastanov mode. Measuring the radiative lifetimes and evaluating the temperature dependence of PL intensity, we conclude that the nonradiative channel outside the dots caused the PL reduction.
Scanning tunneling microscopic and spectroscopic characterization of diamond film prepared by capacitively coupled radio frequency CH3OH plasma with OH radical injection
Authors: Ito Masafumi , Murata Kazuya , Aiso Koukichi , Hori Masaru , Goto Toshio , Hiramatsu Mineo
Published: July 2002

Electronic structure at a surface of diamond film synthesized by a capacitively coupled radio frequency CH3OH plasma chemical vapor deposition with OH radical injection has been investigated. The electronic structure was characterized by using scanning tunneling microscopy and scanning tunneling spectroscopy in an ultrahigh vacuum condition. As a result, the electronic structure was identified as an n-type electronic structure at the surface, which was considerably different from those of boron-doped diamond films and amorphous carbon films.
Self-limiting growth of GaAs surfaces on nonplanar substrates
Authors: Biasiol G. , Reinhardt F. , Gustafsson A. , Kapon E.
Published: July 2002

The growth of GaAs epitaxial layers by organometallic chemical vapor deposition on top of V-grooved substrates is found to exhibit a self-limiting behavior. As in the case of the self-limiting growth of AlGaAs on similar patterned substrates, the self-limiting GaAs profile exhibits characteristic crystallographic nanofacets. However, these facets are considerably broader than in typical, self-limiting AlGaAs profiles obtained at similar growth temperatures. Atomic force microscopy in air reveals the three-dimensional structure of the self-limiting GaAs surfaces, showing monolayer steps on the central (100) nanofacets and quasiperiodic modulation caused by step bunching on the side {311}A nanofacets. The width of the self-limiting GaAs V grooves can be reduced to less than 10 nm at sufficiently low growth temperatures, thus providing useful templates for growing, e.g., self ordered InGaAs/GaAs quantum wires.
Microprobe x-ray fluorescence with the use of point-focusing diffractors
Authors: Chen Z. W. , Wittry D. B.
Published: July 2002

A toroidal point-focusing mica crystal diffractor was used to focus monochromatic x rays from a microfocus x-ray source operated at 0.1 mA and 30 kV. The Cu Kα x-ray focal spot of 50 μm×85 μm had 1.6×104 photons/s/μm2. Microprobe x-ray fluorescence (MXRF) spectra were recorded with a Si(Li) energy dispersive detector for bulk specimens of GaAs, Si, and Muscovite. Low background due to monochromatic excitation resulted in predicted detection limits as low as 2 ppm for a measurement time of 500 s. Laboratory MXRF systems based on point-focusing diffractors were shown to provide lower detection limits, larger working distance, and higher theoretical intensity than systems using capillary optics.
Photoreflectance study of the substrate-film interface of GaAs homoepitaxial structures with different in situ substrate surface cleaning processes
Authors: López-López Máximo , Meléndez-Lira Miguel , Goto Shigeo
Published: July 2002

The substrate-film interface of GaAs homoepitaxial structures prepared by molecular beam epitaxy, employing three different in situ treatments for the substrate surface preparation: (1) cleaning by hydrogen radicals (H*), (2) exposure to trisdimethylaminoarsine (TDMAAs), and (3) the usual thermal cleaning under an arsenic flux have been studied. The concentrations of interfacial residual impurities of C and O were measured by secondary ion mass spectroscopy (SIMS). These impurities pin the Fermi level producing an interfacial electric field, whose strength was evaluated by photoreflectance spectroscopy. For semi-insulating substrates, the usual thermal cleaning process resulted in very high concentrations of C (2×1019 atoms/cm3) and O (1.3×1018 atoms/cm3) at the interface, producing a large electric field (1.3×106 V/m). The impurities were drastically diminished to below the SIMS detection limit by using the H* cleaning, and as a result the electric field was effectively reduced. On the other hand, we observed higher concentrations of impurities, and larger interfacial electric fields on Si-doped substrates.
Switching fields and magnetostatic interactions of thin film magnetic nanoelements
Authors: Kirk K. J. , Chapman J. N. , Wilkinson C. D. W.
Published: July 2002

Switching fields of magnetic elements with nanometric dimensions have been investigated by Lorentz microscopy using a transmission electron microscope. Acicular elements of Co and Ni80Fe20 were fabricated by electron beam lithography and lift-off techniques. They were 1.6–3.5 μm long, 200 nm wide, and 20–50 nm thick, with flat rectangular ends or triangular pointed ends, and were patterned in linear arrays with center-to-center spacing ranging from 7 μm to 250 nm. Switching fields and reversal behavior of the elements were found to depend strongly on the shape of the ends and, in a closely packed array, on element separation, thereby providing a way of controlling their magnetic properties.
Quantized conductance in a heterostructurally defined Ga0.25In0.75As/InP quantum wire
Authors: Ramvall P. , Carlsson N. , Maximov I. , Omling P. , Samuelson L. , Seifert W. , Wang Q. , Lourdudoss S.
Published: July 2002

We report on the observation of quantized conductance up to 10 K in epitaxially regrown, heterostructurally defined, 100-nm-wide Ga0.25In0.75As/InP quantum wires. In addition to the plateaus at integer steps of 2e2/h, we observe plateaus at 0.2(2e2/h), 0.7(2e2/h), and 1.5(2e2/h), indicating spin polarization at zero magnetic field. Of these, the first two plateaus appear to evolve into one at around 0.5(2e2/h) when the sample is subjected to a magnetic field parallel to the quantum wire. The observation of quantized conductance is made possible by the substantial improvement in the quality of the interface by regrowth.